ENVIRONMENTAL SOIL SCIENCE

INVITED LECTURES

A Training Course For the Non-Soils Specialist

BRISBANE, AUSTRALIA 9-11 FEBRUARY 1993

Australian Society of Soil Science Inc., (Queensland Branch)

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I.F. Fergus K.J. Coughlan

Editors

Published by the Australian Society of Soil Science Incorporated (Queensland Branch). C/~ Department of Primary Industries, Meiers Road, Indooroopilly, Queensland 4068, Australia.

Copyright © Australian Society of Soil Science (Queensland Branch) 1993.

Printed in Brisbane, Australia by Print People.

Citation of this publication should take the form:

Fergus, I.F. and Coughlan, K.J. (1993). 'Environmental Soil Science' (Australian Society of Soil Science Incorporated, Queensland Branch: Brisbane, Australia).

Individual chapters should be cited:

Author(s) Name(s). Title of Paper. In 'Environmental Soil Science'. (Eds. I.F. Fergus and K.J. Coughlan). pp. 00-000. (Australian Society of Soil Science Incorporated, Queensland Branch: Brisbane, Australia).

Note from the publisher:

1. Articles in this publication are protected by copyright. Material from the publication may be used providing both the author and publisher are acknowledged.

2. To purchase this publication, contact:

The Convenor, ASSSI (Queensland Branch) Environmental Soil Science Course C/- Natural Resource Management Department of Primary Industries Meiers Road, Indooroopilly, ald. Australia 4068.

National Library of Australia Cataloguing in Publication Data.

Title: 'Environmental Soil Science'

ISBN 0 9587460 52

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PRESIDENT'S FOREWORD

Deterioration in the condition of the earth's biosphere, particularly over the past twenty years or so, is of major concern. Consequently, the need to understand clearly the effects of human activities on the environment, and to make appropriate responses, is urgent. This is irrespective of whether the adverse impacts are from industrial, urban, rural, recreational, extractive or depositional pursuits.

Many professionals now find that their planning must include a study of the effects of a proposed activity on the environment. The breadth of study required is continually increasing, and it is most difficult to keep abreast of the wide range of topics that need to be considered.

One vital topic is the impact of a planned activity on the soil, or indeed the effect of the soil on the activity. One way or the other, the soil will interact, and the implications must be investigated. This requires an understanding of the nature and behaviour of soils, including their chemical, physical and biological properties. The non-soils specialist may well find a need for an understanding of soil science that goes beyond either basic or present levels of training.

Recognising this need, the Queensland Branch of the Australian Society of Soil Science resolved to present a short course presenting some basic principles of Environmental Soil Science, and at the same time provide a reference book on the subject for future use. An Organising Committee ably convened by Mr Ted Gardner (with full membership listed elsewhere in this Publication) devoted considerable effort to constructing the course, and is to be commended for its efforts. Fifteen lectures, prepared by a total of 24 eminent specialists, will be presented and discussed, and there will be a practical session to consider the contribution of Soil Science to Environmental Impact Statements. Areas covered in the lectures include legislative aspects, basic soil properties and their measurement, causes and effects of soil erosion, movement of elements and compounds into and through soils to aquatic systems, and effluent disposal to soils. The lectures were prepared on an honorary basis by some very busy people, and the Queensland Branch thanks them for their fine contributions.

We trust that the participants, who come from a very wide range of disciplines indeed, will find the course of considerable benefit not only as background information, but also by direct relevance and application to their diverse areas of planning. If the course plays a role in helping the preservation of our soils for future generations, this will adequately reward all those involved in its preparation and presentation. As with past courses presented by the Society, the book of papers will be available for sale afterwards, and we trust this will be of use to those not fortunate enough to attend in person.

G.E. Rayment President

Australian Society of Soil Science Inc. (Queensland Branch) 1992/93

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TRAINING COURSE ON ENVIRONMENTAL SOIL SCIENCE FOR THE NON·SOILS SPECIALIST

ORGANISING COMMITTEE

E.A. Gardner (Convenor)

G.E. Rayment I.R. Phillips L.C. Bell M.E. Probert D~E. Baker (Treasurer) I. F. Fergus (Managing· Editor) K.J. Coughlan (Co-Editor)

WORD PROCESSING AND FORMATTING

Jackie Wakefield

The Committee expresses its thanks to Jackie for her skilful and enthusiastic work with the word processor, which contributed very significantly to the preparation of this book.

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ENVIRONMENTAL SOIL SCIENCE

A Training Course for the Non-Soils Specialist

9-11 February, 1993

CONTENTS Page

SETTING THE SCENE - LEGISLATION AND PLANNING

Environmental Law in Queensland M. Ricketts . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Planning Issues Affecting Agricultural Land M.A Capelin ......................... 11

SOIL CHARACTERISTICS AND INTERACTIONS WITH ENVIRONMENTAL HAZARDS

Basic Properties of Soils

Application of Soil Science Concepts to Real Soils

L.C. Bell. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33

B.J. Bridge & M.E. Probert ............... 55

Erosion Processes and Contaminant Transport R.J. Loch & D.M. Silburn . . . . . . . . . . . . . . . .. 77

Erosion Control in Civil Constructions P.N. Truong . . . . . . . . . . . . . . . . . . . . . . . .. 107

Solutes and their Transport through Soils I.R. Phillips. . . . . . . . . . . . . . . . . . . . . . . . .. 133

Nitrogen and Phosphorus and the Process of Eutrophication in Aquatic Systems D. W. Connell & AH. Arthington . . . . . . . . . .. 161

Environmental Chemistry of Organic Chemical and Biocides M. W. Silvey, B. W. Simpson & P.J. Silvey . . .. 1 77

Heavy Metals: Toxicity, Sources, Chemistry and Loading Rates G.E. Rayment & G.A Barry . ............. 211

Guidelines for Investigation and Treatment of Possible Contaminated Land R. Sadler & P. Imray. . . . . . . . . . . . . . . . . .. 237

Hydrology and Chemistry of Landfills W.E. Razzell ........................ 269

Land Disposal of Effluent from Intensive Rural Industries E.A Gardner, M.A. Gilbert & R.J. Shaw . . . .. 283

Chemistry, Treatment and Disposal of Municipal Sewage Effluent and Sludge P.O. Beavers . . . . . . . . . . . . . . . . . . . . . . .. 319

SOIL SCIENCE AND ENVIRONMENTAL IMPACT STATEMENTS

The Review of EIS Documents R.J. Barley. . . . . . . . . . . . . . . . . . . . . . . . .. 357

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ABSTRACT

ENVIRONMENTAL LAW IN QUEENSLAND

M. Ricketts Biologist and Bush Lawyer

Queensland has embarked on an ambitious program of environmental law reform, encompassing aspects from approval procedures through management issues and, finally, site remediation and waste disposal. This reform is being effected through changes to legislation, policy and procedure. Thus, any analysis of environmental law in Queensland can only outline the skeletal framework of the new regime of environmental management.

However, the general trend is clear. The onus is clearly shifting to the proponent to plan and manage their projects within the confines of ecological sustainability with less regulatory prescription of how that is done. The other side of this shift to self­management is a marked increase in accountability, both through more open processes and also greatly increased penalties.

1 INTERNATIONAL AND NATIONAL INFLUENCES

Environmental law, both in Queensland and the rest of the World, is in a state of flux. The public is demanding that Governments "fix" the environmental consequences of our modern life-style. Similarly, business is struggling to meet these new expectations at a time when most are more concerned about financial rather than global survival. Laws everywhere are trying to set the framework of Ecologically Sustainable Development yet no one has claimed to have solved the dilemma of balancing present requirements against future needs in a data "vacuum" and an economic system that still hasn't resolved how to incorporate environmental factors into cost-benefit analysis.

Increasingly, international treaties are effecting our laws. The Rio Declaration, Agenda 21, Montreal Protocol, Basel Convention, CITES and many other agreements are evidence of a growing awareness that our species has but one habitat and degradation of that habitat recognises no national boundaries. While this trend in law has been around for quite sometime, it has taken the inescapable issues of ozone depletion and climate change to galvanise public political support for global measures.

The main legal consequence of this supra-national approach is the increasing role of the Australian Government in environmental matters. Traditionally and constitutionally, the environment is an issue for the States. Specific triggers such as Foreign Investment Review Board approvals, areas of the National Estate, National Economic Zones, and specific Commonwealth territory used to be the only avenues for federal involvement. Since the High Court decision precipitated by the Commonwealth's rejection of the proposed Franklin below Gordon Dam in the World Heritage listed south west of Tasmania, there is now clear power for the Commonwealth to have jurisdiction to ensure implementation of International Treaties lawfully entered into by the Australian Commonwealth.

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Sometimes this power is used co-operatively as in the case of marine oil spills where each State and the Commonwealth have enacted mirror legislation to fulfil international obligations. This has the added bonus of achieving consistency. In the case of ozone­depleting substances, each State has headed in the same general direction but with significant differences in legislation and enforcement. At other times, the Commonwealth has moved in direct opposition to the State as was the case when Canberra ordered a cessation of logging the World Heritage listed Wet Tropics.

Since this flexing of constitutional muscle, relations between the two layers of government have improved. There is a growing appreciation of the adverse consequence of having ad hoc decision-making and confused environmental responsibilities. This desire for predictibility has come from business, the public and government and has resulted in the InterGovernmental Agreement on the Environment (IGAE). Those interested in the agreement should be able to get copies from the Premier's Department.

This agreement is between the Commonwealth, States and Local Authorities designed to clarify the environmental responsibilities of the various levels of government. Basically it states that if the parties behave in certain prescribed ways, other parties will not interfere, even though they have the legal power to do so. For example, there is a prescribed methodology for Impact Assessment. If a State uses this "accredited" method, the Federal Government will not require a second Impact Assessment under their powers.

The Agreement also requires a certain degree of standardisation across Australia in environmental data collection and emission levels. There will be legislation to establish a National Environmental Protection Authority made of representatives of all States, the Northern Territory and the Commonwealth. This body will resolve issues such as appropriate emission standards through a two thirds majority vote. It remains to be seen how effective it will be. One thing is certain however, the broad direction of environmental law in Australia can be deduced from successful initiatives elsewhere and, similarly, Queensland is picking the eyes from successful statutes interstate. The flavour may be different but the diets are converging.

2 QUEENSLAND

Like elsewhere, Queensland's environmental legislation is changing rapidly. In addition, many environmental initiatives are being implemented through policy or administrative procedure. For example, the Environmental Policy of the Department of Minerals and Energy has no specific legislative framework yet provides a complex system of environmental management and requires very substantial bonds as guarantees of environmental compliance. Similarly, the Department of Primary Industries is pursuing many of its environment initiatives through the educative/ peer pressure techniques of Land Care and Integrated Catchment Management. Thus, it is no longer sufficient to use legislation as a complete guide to environmental legal requirements.

It is also difficult to adequately define environmental legislation. There are 9 Acts that deal, in some minor or major way, with the environmental management of water. There is Planning, Land Use and Impact Assessment legislation (e.g. State Development and Public Works Act 1971, Local Government (Planning and Environment) Act 1992 ),

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that dealing with the conservation and management of natural and man-made resources (e.g Heritage Act 1992, Nature Conservation Act 1992), that endeavouring to control the ecological consequence of the use of natural resources (e.g. the Mineral Resources Act 1989, Forestry Act 1959, Water Resources Act 1989, Fisheries Act 1989) and that controlling harmful human impact and emissions (e.g. Clean Air Act 1971, Noise Abatement Act, Contaminated Land Act 1991).

There is also the added complication of site or industry specific Acts such as those that cover the mining at Weipa or the emission of noise at Fig Tree Pocket in Brisbane. Rather than cover all areas of "environmental legislation", we will concentrate on the majm Acts and the new directions of legislative reform in Queensland. I will leave the legislative and administrative control of resource development for another day. It IS growing exceedingly complex and, as mentioned, must be discussed in the context of various policy initiatives.

3 LEGISLATIVE AND ADMINISTRATIVE REFORMS

The recommendation of the Commission of Inquiry into Fraser Island and the Great Sandy Region was that an independent Environment Protection Agency was not desirable in Queensland. Accordingly, the Government has developed a model of environmental management that is decentralised throughout State Government departments and local authorities with a 'lead agency' role for the Department of Environment and Heritage. That role includes co-ordination of environmental management with audit and State of Environment reporting requirements.

Queensland has a strong third tier of government in comparison to some other States. It is the focus of local land use planning. In the past, State and regional concerns were not well integrated with those of the Local Authority. The result meant that State regulators simply added more layers of approval processes on top of the planning approvals of Local Authorities. Also, most State approvals were not co-ordinated either so proponents were faced with a maze of approvals mechanisms even within the same Department. Often the various approval bodies had no inter-action with each other, with the exception of the bigger proposals.

While the proponent desired concurrent approvals in most cases, the reality was closer to a loosely connected chain with most links unaware of the implications of their particular deliberations. Clearly, putting an applicant through sequential approval hoops only to stumble at the last, is poor administration and costs all parties dearly. Worse still, there was incredible pressure on those final approvals to be favourable, given that so much time and money had ah'eady been invested by proponent and regulators.

4 INTEGRATED DEVELOPMENT APPROVAL SYSTEM

The new approach was outlined by the Premier in his "Leading State - Economic Development Policy" of April 1992 and is now known by yet another of the acronyms that dog our lives, IDAS or the Integrated Development Approval System. The system is aiming to consolidate State and Local land use approvals. The model involves a set of clear standards or criteria to enable the proponent to assess their chances before they have

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invested major sums, bought an inappropriate site or developed such internal inertia that the project is resistant to change. The provision of "up-front" criteria will contrast with the present opaque systems and shall enable the public to be aware how environmental planning will be implemented and thus facilitate their participation. It is hoped this and other measures will reduce the level of disputation after the decision on a proposal is made.

The reform of the planning system will encourage the "one stop shop" approach through the Local Authority with referral, 'behind the scenes', to State bodies only when necessary for issues of State or regional importance. Those Local Authorities will be acting within the constraints set by State Planning Policies or other criteria such as the Environment Protection Policies developed through the Environment Protection Act or Coastal Management Plans. Where the approval lies within State responsibilities, a whole of Government view will be co-ordinated by the appropriate Department. This will be determined, in the case of major developments, by the Co-ordinator General's Office within the Premier's Department.

As the Local Authorities will be taking on new work and responsibilities, they will be resourced to competently address this expanded role. It is expected that reform of Impact Assessment will lead to better integration of that process into on-going environmental management through approval or licensing conditions. Enforcement and monitoring will become crucial as Queensland moves towards a more performance-based planning system. This will complete a feed-back loop between planning and management that has been somewhat tenuous in the past because of the change in jurisdiction and the separation into two disjunct processes of what should be a continuum.

Thus, for all but the major projects, State authorities will move from an approval function into an audit role to ensure the criteria are being followed and there is consistent decision­making across the State. The recent Nature Conservation Act and Coastal Protection Act, which is in its final consultation, will provide planning criteria with their respective foci of rare, endangered and littoral habitats. Similarly, State Planning Policies may be used to address issues such as noise or odour that have not been given sufficient weight by planners in the past, resulting in incompatible land uses and a constant source of complaint and conflict.

5 IMPACT ASSESSMENT

It is expected that new legislation on Impact Assessment will be developed during the term of this Parliament. It is expected to be a refinement of the present system and will bind the Crown in all but the mining sector. If the trend established elsewhere is continued, Impact Assessment will pay more heed to the social and economic consequences of a proposal. This is consistent with the ideal of Ecologically Sustainable Development which mandates the integration of the three determinants.

The normal development by the private sector is presently mainly controlled by the Local Government (Planning and Environment) Act with its schedule of triggers for environment impact statements (EIS). Thus, impact assessment is at the time of application for a change of land use and is site-specific. It does not cover non-land use proposals such as

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the marketing of a product with environmental consequences. The Local Government (Planning and Environment) Act (1992) also allows a Local Authority to develop its own schedule of triggers for EIS's and can also require an EIS before considering an application.

Unfortunately, this means the EIS does not get presented to Council until the public objection period is over and thus the public has little way of judging whether to object to the environmental consequences of the proposal. At present, there appears to be only a tenuous connection between the EIS or subsequent approval conditions or emission licensing. Nor, sadly, is there a commitment to audit EIS predictions and subsequent conditions to establish the accuracy and efficacy of the process. Experience elsewhere suggests that EISs have generally underestimated impact but, in Queensland, we are still operating without adequate feedback mechanisms.

At present, the State is required to take into account environmental effects by Section 29 of the State Development and Public Works Organisation Act (1971). This has never had any specific triggers developed for Impact Assessment procedures and the responsibility for co-ordination has been delegated from the Co-ordinator-General to the Director­General of the Department of Environment and Heritage. There is no consistency of process across Government about how this provision is satisfied. Some roads are subject to EIS '5, others not, programs to clearfell native forest for plantation are assessed internally with no formal public comment, some stream pumping licences are subject to leaving a residual ecological flow, others not. Clearly, this is a major challenge to Government to provide predictibility and accountability for its own environmental decision-making. Avenues such as the Judicial Review Act (1992) and the proposed Administrative Appeals Tribunal and Freedom of Information Act will be used increasingly by members of the public, conservation groups and the business community to bring environmental accountability to the public sector.

In keeping with most new legislation, resource use and management legislation is relying increasingly on policy and administrative procedure to handle environmental matters. This is, to an extent, sensible, as most legislation is written in terms of providing legal power rather than legal responsibilities. Any state· agency has obviously the power to do a full impact assessment of its policies or specific proposals but rarely is there a requiiement for that to happen. It is hoped that the new State Planning and Environment Policies will provide similar predictibility and accountability for the public sector planning and management that already exists in the private sector and these new procedures will have to comply with the procedures required under the InterGovernmental Agreement on the Environment.

6 EMISSION MANAGEMENT

The current Acts covering traditional "pollution" are the Clean Air, Clean Water, Noise Abatement and Litter Acts. They use a licensing model to control the emission of wastes that are known or suspected of leading to more serious or permanent degradation such as sewage disposal. For those problems that are more transitory or quality of life issues such as noise, there is a complaint-driven approach to control. These Acts have proved to be unenforceable due to an unfortunate combination of legal problems, lack of political will

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and an extensive list of exemptions that render enforcement unjust. These Acts will be replaced by the environment protection legislation in the near future so I will concentrate on it and its place in a new system of environmental management for Queensland.

The Environment Protection Act's Drafting Instructions been approved by Cabinet and aim at achieving Ecologically Sustainable Development. More than that, the Act will fit into a jigsaw of legislative and administrative reforms of the environmental and planning spheres. This 'cradle to grave' system begins with devolution of increased power to Local Authorities and more efficient planning, impact assessment and approval processes. Then comes a suite of legislation dealing with on-going management of the environment, primarily the Environment Protection, Coastal Management, and Nature Conservation Acts. Finally, there is the 'graveyard' of past folly, ignorance and mismanagement, the Contaminated Land Act which shall provide an inventory of sites and provide clean up powers.

The Environment Protection Legislation itself, will not be a great surprise to those familiar with the legislation of other States. It will mandate a consultative and predictable process for developing a legally-enforceable strategy, known as an Environmental Protection Policy (EPP) , to address environmental issues. Again, it is hoped that this will facilitate the development of a social consensus on how best to deal with an issue and reduce future conflict. This will become increasingly important as we move from the point sources to the diffuse sources of contamination that require a lifestyle or social change rather than better technology. It should also encourage a sense of ownership of the appropriate strategy amongst those responsible for the problem.

The Act's scope is somewhat flexible and can be altered by the Government of the day depending on the departmental administrative responsibilities and the nature of the environmental problem. The Department of Environment and Heritage has initially targeted pollution through the replacement of the out-dated and ineffectual Clean Air, Clean Water and Noise Abatement Acts with State-wide Environment Protection Policies on Air, Water, Noise and Waste. It will bind the Crown. Some other Departments have indicated a desire to use this consultative process and Environmental Protection Policies to address their own particular environmental responsibilities such as transport noise. In such cases, the Department of Environment and Heritage would ensure the standards set are consistent across Government and also perform an audit role of the environmental performance of the Policy.

Environment Protection Policies, as sub-ordinate legislation, will remain current for 7 years unless new data or a national consensus developed through the Intergovernmental Agreement on the Environment compels an earlier review. It is hoped that this time frame will allow a reasonable planning interval for business. There is a possibility of using economic instruments such as allowing 'banking' of emission credits to further smooth the investment peaks in new plant or processes. Such a system or other such as transferable rights would be developed through the EPP process with the two rounds of consultation so these economic mechanisms will not lie in the initial Act and no government policy has yet been developed on them.

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Environment Protection Policies will be the key mechanism for setting standards and management strategies. They could contain offences and penalties specific to that EPP. Policies may be State-wide or simply apply to a region or specific environment such as the Gladstone airshed, if the issues are sufficiently unique to require that targeted approach. The EPPs or sections thereof, can be delegated to appropriate authorities such as the Department of Resource Industries for the control of the impacts of mining with the Department of Environment and Heritage maintaining an audit role. To ensure each EPP is well drafted and targeted, an Impact Assessment of the ecological, economic and social consequences will be prepared for each Policy.

The first Environment Protection Policies will be Air, Water, Noise and Waste Management and the recent Inter-governmental Agreement on the Environment indicates that there will be an effort to ensure standards adopted are consistent with those set in other states (as they often are now through NHMRC and ANZECC standards). Those Policies will be developed after the legislation so the current Acts will apply until about mid 1993. After those have been declared, other more specific Policies will be developed on a needs basis, such as Transport Noise or Litter. They can be regional in scope. For example, the assimilative capacity of a particular environment may require special management. The Brisbane airshed has the physical characteristics necessary for a "Los Angeles" style photochemical smog problem. It is not inconceivable that, in the future, the Brisbane airshed may require measures not necessary elsewhere in the State.

In addition to the EPP process, the Act will contain the heads of power for various proven mechanisms of environmental management, such as Licensing, Enforcement, Financial Assurances, Clean Up, Appeals and State of Environment Reporting provisions. Many of these will be familiar and it is Queensland's intention to maintain consistency with other States where that is consistent with Queensland Government policy.

Licensing will be substantially different to that in Queensland at present. It is intended that self-management will be encouraged but that the transparency and accountability will be similarly increased. Proponents will be expected to provide sufficient data to enable a decision on licensing. In some major cases, that will amount to a full Impact Statement but usually would be details of the waste stream, ecological effects and minimisation strategies. Commitments made by the proponent can then be incorporated into the licence conditions. One licence will cover all emissions under this Act and the licence is for 'life of project', though conditions can be varied to correspond with new data, revised standards or changes to the licensee's waste stream.

Those businesses which cannot comply with licence conditions or the general provisions of an EPP, will be required to develop an Environment Management Plan to detail their program to achieve compliance. The Plan will need to be approved by the licensing authority and, with the exception of commercial secrets, shall be publicly available. This will reduce the problem of specific sectors calling for exemptions when standards change. Some industry organisations are already looking at developing generic Plans to maintain the legendary 'level playing field' within their industry. If an individual business needs to vary the plan the onus is on it to approach the regulating authority for an amendment. Financial assurances can be required to ensure the Plan is followed and non-compliance with an Environmental Management Plan is an offence.

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The advantage is that the solution is developed by the offender. This ensures that the business concerned has examined the ramifications of what is in effect a social contract and 'owns' the Plan psychologically. It is possible that this process could be extended to all licensees if the Environment Management Plans are successful. Again, it would shift the responsibility for self-management and regulation back onto the proponent and ease the burden on the resources of the government. By keeping those conditions within the general parameters of an EPP and in the public (and competitors) gaze there is less likelihood of a 'captured' bureaucracy.

In addition, annual compliance statements, signed by the Chief Executive Officer, will be required of licensees. While it is not proposed to prosecute for licence non-compliance on the basis of this or other self-monitoring data, the information will be invaluable in designing cost-effective auditing. Also, the wilful provision of false information is an offence. In a similar manner, exception reporting and self audits will be exempt from legal proceedings unless they are furnished as supporting evidence by the licensee. Such exemption will be dependent on certain requirements such as prompt disclosure and the development of an approved Environment Management Plan to address the problem.

Mandatory environmental auditing can be required as a response to a particular problem or perceived high level of risk. There is no proposal to accredit auditors in the Victorian model. Accountability is ensured by requiring the accuracy of the audit to be attested to by the CEO. There is also the power to refuse to accept an audit. Again, we are attempting to sheet home responsibility to the highest level of the business. There is no protection afforded to this data other than commercial secrets and compulsory audits will be public information.

The enforcement provisions will follow the standard three tiered scales of severity. Penalties will also be brought into line with those determined by the National Environmental Protection Authority with expected million dollar fines for those in the black hats. The Corporate Officer Liability will be strict, not absolute and is not directed solely· at management but at the person most responsible at the highest corporate level. That person will have the defence of "due diligence" available. In addition, there are clean up provisions, procedures for emergencies and malfunctions and short term exemptions are fairly standard, The current timetable would see an Act ready to present to Parliament in the autumn session with the Environment Protection Policies to follow.

7 CONTAMINATED LAND ACT

The final reform in environmental management is that of the Contaminated Land Act (1991). The rationale behind the Act is that contamination is relative to use and complete remediation is not desirable or necessary in many cases where use can be without threat to the environment or health. The first stage involved amendment to the town planning process to ensure that there was an alarm trigger when a land use change is proposed for potentially contaminated land. The second phase is the compellation of a list of conta;minated sites throughout Queensland. The register is being compiled and details will be attached to the Titles Register. Owners of land that has been used for a prescribed purpose (attached as a schedule to the Act) or is known to be contaminated have until the end of 1992 to register details. Land subsequently contaminated must be registered within 30 days.

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Sites will be classified as Probable, ConfIrmed, Restricted and Released. A site used for a prescribed purpose becomes Probable. ScientifIc assessment would follow and, either Released and or become Confirmed. If specific uses are permitted with the assessed level of contamination, a Restricted Site with attached uses is declared. Complete remediation results in a Former Site. Neither Former or Released Sites are listed on the, Titles Register. An owner is required to notify potential purchaser of contamination or unsatisfied notices.

The polluter pays principle guides liabilities with the owner and, fInally, the Local Authority incuning liability if the polluter can not be located. Appeals are permissible if the contamination was in accord with the legal and accepted practice of the time, contamination is historic and it is unreasonable to recover costs or if the person is unable to pay. There are cases where the State will share costs.

8 CONSERVATION

The new laws in this area are the Nature Conservation Act, the Heritage Act and the proposed Coastal Management Act. As mentioned there have also been amendments to other Acts to better address emerging environmental priorities. The Nature Conservation Act has up-dated the management of reserved areas such as W orId Heritage Areas and National Parks, including a new type of Park involving aboriginal ownership and management. There is also a greater emphasis on off-reserve conservation such as the mechanism for voluntary agreements for conservation management of private title. These will be known as Nature Refuges and will be attached to the title to be binding on future owners.

Conservation Plans can be developed to address the needs of endangered species, critical habitats or areas of major interest. They are binding and legally enforceable. In addition, there will be a schedule of rare or endangered fauna and flora known as the Nature Conservation Register. Species thus listed can trigger State Government involvement in the management issues of that species including resumption of crucial habitats or imposition of an Interim Conservation Order until a Conservation Plan can be implemented.

The Coastal Protection Act had not been fInalised at time of writing but general trends are evident. The Act is intended to up-date the Beach Protection Act, the Canals Act, and parts of the Harbours Act both in area of jurisdiction but also to allow a wider consideration of issues than the old Acts. There will be the ability to declare Coastal Management Plans over specifIc areas that will be integrated into the IDAS system and Local Authority Strategic Plans. Those Plans can require restoration as well as particular development constraints. They will also enable specifIc conditions as part of are-zoning approval.

A Coastal Management Authority is proposed but, unlike the present Beach Protection Authority, it will not be an approval body but simply advisory through its research and education functions. Its proposed composition is mainly government but with members from the conservation and development lobbies. There is also an ability to form Regional Advisory Committees and all Plans will be open for public comment. All areas covered

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presently as Erosion Prone Areas will be automatically covered by the new Act as Protection Areas. Coastal Management Control Districts will be declared to set landward limits to development or specific management requirements. If no specific designation is established, the Act will cover land below high water.

The Queensland Heritage Act will establish a Register to record places, agreements, areas and orders and permits. No development on those properties can take place without approval but registration can be challenged. All sites currently listed in the Heritage Buildings Protection Act 1990 will be placed on the new register with rights of appeal within 60 days to the Heritage Council, an independent representative panel. Agreements may be made between the owner and the Government to maintain the heritage features of that particular property.

As you can see the situation is both fluid and increasingly complex. Also, the increased use of policy and administrative procedure to address environmental responsibilities will make it more difficult to track down specific requirements. The good news is that the governments are attempting to place more and more constraints "up front" in the form of plans or written proscriptive policies so that proponents know which sites are likely contenders for their particular form of development. The national standardisation will help to reduce the occurrence of "pollution havens" and lend some uniformity to processes such as Impact Assessment. Environmental management is becoming more open, predictable and accountable. It will also place a greater burden on both public and private sectors to justify and monitor their proposals.

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PLANNING ISSUES AFFECTING AGRICULTURAL LAND

M.A. Capelin SEQ 2001, Brisbane, Qld, 4000.

ABSTRACT

The diversity of land use in rural areas of Queensland demands a rang#{ of planning approaches to meet physical, economic and environmental requirements. The supply of land is directly influenced by planning measures, which should aim to protect land resources, retain allotments of viable size, and give support to sound management practices and nature conservation.

Issues affecting land use include conversion to non-agricultural uses, conflict over land management practices, the possibility of management leading to degradation, and fragmentation by sub-division. To facilitate planning, land must be classified, and the paper discusses various options and schemes that have been used for this, including the definitions of good quality agricultural land.

Land use planning is the responsibility of various levels of Government. It is required by Government agencies, industries and individuals for planning, environmental impact assessment, and to describe natural resources. Information needs to be presented in an accessible form, and at an appropriate level of detail. There are serious problems in achieving this in Queensland.

1 INTRODUCTION

Whereas the genesis of land use planning in the cities of Europe was to manage the location and types of land use to improve the health and environmental quality of urban areas, the need for rural land use planning has also long been recognised. The purposes of rural land use planning may be different to those in urban areas and relate more to the protection and management of resources and the natural environment, but the overall aim of improving the community's long term health and environmental quality is consistent with those early aims.

Over 91 per cent (158 million hectares) of Queensland's total area is managed for some form of agricultural production, with over three million hectares planted for crop production and over four million hectares sown to improved pasture for animal grazing. The remaining agricultural land is managed under natural or modified vegetation cover for animal production. The gross value of agricultural production contributes over 30 per cent of the State's Gross Domestic Production.

Cropping land uses are dominated by wheat (973 000 ha), grain sorghum (570000 ha), green forage (443 000 ha), barley (343000 ha), sugar cane (288 000 ha) and sunflower seed (148 000 ha); and animal production is dominated by wool, meat (cattle, sheep, pigs and poultry), milk, eggs and honey. Apart from these major products, a great range of other agricultural enterprises ranging from temperate and tropical fruit crops to deer farming are applied to the State's rural lands.

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This diversity of agricultural land-use in rural areas demands a range of planning approaches to meet their physical, economic and environmental requirements. Of the inputs required for agricultural production (land, labour, technology and capital), only the supply of land is directly influenced by planning measures. Land must not only be physically suitable for a particular form of agriculture but must be available in adequate sized parcels and be used for a purpose which does not preclude development for agricultural production. Outputs and side effects generated by the use of land for agriculture may have negative spill-over effects on neighbouring properties. These include noise, dust, chemical spray drift and soil erosion.

Thus, planning approaches need to control allotment sizes, determine appropriate land-uses, influence land management practices and recognise that adjacent uses must be controlled or managed to avoid conflicts resulting from spill-over effects. Planning approaches used in agricultural areas should aim to achieve the following:

protection of land resources suitable for agriculture; supply of allotments to form holdings of sufficient size to allow economic agricultural operations; support for land management practices which are ecologically and economically sustainable; protection of nature conservation resources; protection of adjoining uses from adverse spill-over effects resulting from agricultural operations; and resolution of conflict between incompatible adjoining land-uses.

This paper explores some of the issues affecting agricultural land in Queensland and the planning solutions currently being implemented. The focus of the paper is on the need for and use of soils information to effectively implement these solutions.

2 LAND USE ISSUES IN AGRICULTURAL AREAS

Issues affecting agricultural land which have been identified by rural planning projects in recent years and which are considered in this paper are:

Conversion of land to non-agricultural uses Conflict over land management practices

• Land management and land degradation Fragmentation of rural land by subdivision DefInition and identifIcation of good quality agricultural land

2.1 Conversion of land to non-agricultural uses

Agricultural land is acknowledged by Governments at all levels as an important national resource because of its existing and potential contribution to national income, global food production, rural communities and an attractive rural landscape. There is general agreement on the need for good quality agricultural land, as a finite and diminishing resource, to be conserved and managed for the longer term benefit of agricultural industries of significance to the state and the nation in the public interest (Hallsworth 1978, Smit et at 1987, Christensen et at 1988, EWG 1992).

Urban development pressures are currently a major contributor to the alienation and reduced productivity of rural lands and are strongly driven by population growth. For example· population growth in South East Queensland is expected to add approximately 450,000 people

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to the 1991 population by the year 2001. This additional population will occupy approximately 25,000 ha. Put another way, each year 2,500 ha of bushland, agricultural land and other rural land will be consumed for housing and other urban purposes. Over the 18 months to March 1992, 3,400 ha of land in SEQ planning schemes were rezoned from Rural Zones for other purposes, whilst over the same period the area zoned for Residential P4TPoses increased by 1,400 ha and the area zoned for Rural Residential purposes increased by 2,660 ha (DHLGP, 1992).

As a result of these pressures, rural and good quality agricultural land is under constant demand for development purposes. Between 1980 and 1990,3,882 ha of assigned cane land were converted to non-rural uses; 615 ha were purchased by public agencies; and a further 13,743 ha were converted to other rural uses (Coleman and Edwards 1991). In Brisbane City between 1975 and 1988, the area under cropping declined by 400 ha (30%) to 800 ha (Puttock and Ritchie 1990).

2.2 Conflict over land management practices

Intensive agriculture is not dissimilar from other forms of industry in the generation of noise, dust, odour and other wastes from production processes. All rural land holders have an obligation to the community to manage their land to maintain its productive capacity, to safeguard the health and safety of employees and neighbours, and to protect the environment. Many of these obligations are formalised in legislation such as the Workplace, Health and Safety Act, Clean Waters Act, Clean Air Act and Noise Abatement Act. However .conflict between agricultural uses and uses such as residential, conservation and industrial is not unusual. Common sources of conflict are listed in Table 1, the most frequent being spray drift, fires and noise.

Table 1 Sources of land management conflict in urban-rural fringe areas

Impacts on farmers

Vandalism

Dog attacks on stock

Urban run off onto farmland

Theft of produce and stock

illegal rubbish dumping

Modified farm practices

Production loss near boundary

Competition for groundwater

Septic waste contamination

Fires

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Impacts on residents

Dust

Smoke and ash

Pesticide spray drift

Animal and spray odours

Machinery noise

Animal waste disposal

Farm machinery on roads

Scare or hail cannons

24 hour operations

It is important to distinguish between farming practices which have the potential to cause harm to the environment and people through land degradation, water pollution or injury, and those practices which generate odours, smoke and dust which do no harm apart from reducing the amenity of non-farming residents. Management practices which degrade the environment or endanger health must be modified , but the latter practices should be tolerated as essential components of food production.

Many farm outputs do not harm the environment nor threaten human health and are normally accepted by the farming community as inevitable by-products of farming. However they may be a source of nuisance and conflict to residential neighbours not accustomed to and less tolerant of their presence.

Most farmers have felt secure in their occupation of rural land and their right to manage their land as they saw fit in order to earn a living from their labour and skills while maintaining the productivity of land and water resources. However in recent years, the advent of the environmental movement, rural residential living and general fragmentation of rural land has brought a large number of new settlers into rural areas who are not dependent on rural production for their income and with a strong desire to maintain their perception of a pristine rural environment. Pressures on farmers to modify or eliminate certain practices have led to less efficient farming practices, increased costs, reduced productivity, and, in some cases, the cessation of farming.

In many cases where urban development is encroaching onto rural areas, two distinct planning issues arise. One is the management of existing conflicts caused by a mix of land uses; the other is the prevention of future conflicts likely to be caused by expanding residential areas or changes in agricultural management practices such as' 24 hour harvesting or greater use of agricultural chemicals.

Existing conflicts are best managed by negotiation between neighbouring groups to agree on acceptable practices. Such agreements may be difficult to achieve and expert mediation may be necessary. Potential conflicts are more likely to be avoided by separating potential conflicting uses through planning controls which restrict the range of allowable uses in neighbouring areas or by implementing buffers between conflicting uses.

At the same time, diversification of farm enterprises into compatible uses such as farm tourism may be encouraged. Such activities as bed and breakfast accommodation and farm holidays are compatible with most farm enterprises, provide opportunities for additional farm income and assist in improving understanding between the farming and urban communities.

The most common role for planning authorities in managing this form of conflict is to zone land for specific purposes so that potentially conflicting uses are separated. In rural areas this involves excluding residential development or locating clusters of housing in areas remote from land used for or suitable for agricultural production; and requiring buffer areas between residential developments and agricultural production areas.

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2.3 Land management and degradation

The land resource is generally under pressure from agricultural practices which, in some localities, are responsible for degradation of the resource. These processes threaten both the long term productivity and availability of the land resource and the quality of water resources through sedimentation and the movement of nutrients and pollutants.

Land resources are valuable and non-replaceable. The future economic viability of rural and urban communities, and the supply of food, fibre and wood to local and export markets depends on the careful management of the land. Land-use choices and land management strategies are required which encourage the maintenance of the land in productive condition, the conservative use of surface and groundwater resources, and prevent soil eroSIOn, deforestation and other forms of degradation such as salinity and water pollution.

The role for planning in the achievement of sustainable agricultural production is to influence land-use through strategies which locate agricultural enterprises on the most productive land, protect associated water resources, control the clearing of vegetation on non-agricultural land, and encourage the adoption by land holders of sound management practices which are consistent with the principles of ecologically sustainable development. The planning system should also provide areas of land available to a producer which are sufficiently large to implement sustainable management practices. Grazing holdings for example, need to be sufficiently large to allow flexibility in management, the ability to destock pastures to allow regeneration, and the opportunity to implement a burning regime if that is considered desirable to maintain pasture condition and control weeds.

Government policy initiatives to address land degradation issues include the draft Natiopal Strategy for Ecologically Sustainable Development (June 1992); the Queensland Integrated Catchment Management Strategy (October 1991); and the Queensland Decade of Land Care Plan (March 1992).

2.4 Fragmentation of rural land by subdivision

Fragmentation of the rural land resource into small parcels and the use of these parcels for residential purposes has two detrimental effects on agricultural industries. Firstly, the incidence of conflict over agricultural management practices is increased by the mix of residential and agricultural land uses. The boundary effects of small allotments around the edge of an agricultural operation can severely limit the ability of agricultural producers to conduct normal management practices when the amenity of scattered domestic dwellings is to be protected. Secondly, the ability of agricultural industries to restructure in response to changing economic and market. conditions is severely limited in areas of small allotment size and mixed uses. These conditions result in increased land prices and holdings which may consist of a number of allotments spread over a large area interspersed by residential uses.

Changes in seasonal conditions and market prices result in fluctuations in income received from the sale of rural commodities. Diversification of production is the key to the survival of farms dependent on fluctuating commodity prices. High value crops such as sub-tropical fruit crops may be profitable on areas as small as 10 hectares, while sugar-cane and broad­acre crops such as wheat, barley and sunflower require approximately 70 hectares.

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Subdivision controls need to provide for holdings sufficient in area to allow diversification and provide income in below average years so that long-term profitability based on proven farming systems is possible.

The median holding sizes for farming activities in South East Queensland are shown in Figure 1.

Nurseries

Orchards

Plantations

Vegetables

Potatoes

Sugar

Grain

Dairy

Beef & Grain

Beef

o 50 100 150 200 250 300

hectares

Figure 1 Median area of agricultural holdings in South East Queensland

In urban fringe areas, where fragmentation into small allotments and encroachment of urban uses into agricultural areas has resulted in an intimate mix of land uses, there are particular planning problems. While land converted to or unequivocally zoned for urban uses is clearly unavailable for agriculture, the availability of other urban fringe areas for agriculture is more difficult to determine. Whilst it may not be appropriate to insist on the continued farming use of a small allotment surrounded by residential uses, the continued use of holdings of sub­economic size for agricultural production may be appropriate by share farming, part-time farming or amalgamation with neighbouring holdings.

Amalgamated of allotments to form viable holdings is common practice in many agricultural industries; and the use of these small allotments for hobby (part-time) farming can contribute significantly to local agricultural production. Alternatively, more intensive agricultural uses such as nurseries or flower production can utilise the smaller allotments.

For example, of the good quality agricultural land in Redland Shire, only 41 % or 850 ha is on allotments greater than 2 ha in area. However this area supports over 100 establishments with annual value of agricultural operations of over $20,000 per annum. These are dominated by nurseries, vegetable farms and orchards with median holding sizes of 4-7 ha. (ABS, 1992). There is a need to develop criteria to identify areas which, while physically suitable for agriculture, are not available for agricultural purposes due to tenure, subdivision, institutional or land use constraints. Separate local planning policies may be required for these fragmented farming areas which facilitate the restructuring of rural industries through amalgamation of titles, or allocate land to non-agricultural uses of community value. Hobby (non-commercial) farming should not be encouraged where it may result in the loss of agricultural land.

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2.5 Definition and identification of good quality agricultural land

2.5.1 Definition

The measures adopted by governments to control the rate of conversion of farmland vary with the seriousness of the problem and their willingness to intervene, but they all depend on a definition of what constitutes 'important agricultural lands'. Smit et al (1987) contend that the concept of 'importance' embraces three fundamental criteria: suitability of land, uniqueness, and demands on the land relative to its productive capacity. The determination of each of these criteria depends to some extent on appropriate land resource data. These data are essential for effective land-use planning and the resultant preservation of agricultural land.

A wide variety of terms and definitions are currently used in planning schemes which refer to areas of agricultural land. The most common terms are good quality agricultural land and prime agricultural land.

Prime agricultural land refers to the 'first grade or best quality' (Macquarie Dictionary). Good quality agricultural land is a broader definition and includes land with a greater range and degree of limitations within the bounds of land suitable for sustainable agricultural use. On the basis that the prosperity of established agricultural industries is dependent on a range of agricultural lands (not just prime lands), the term good quality agricultural land is used to define those lands which require identification and planning attention.

In Planning Guidelines for the Identification of Good Quality Agricultural Land issued by the Department of Primary Industries {DPI) and Department of Housing, Local Government and Planning (DHLGP) in association with State Planning Policy 1/92, good quality agricultural land is defined as:

... land which is capable of sustainable use for agriculture with a reasonable level of inputs and without causing degradation of land or other natural resources. (DPI & DHLGP,1992).

Within this definition the terms agriculture and sustainable have the following meaning:

Agriculture is the use of land for crop or animal production.

Sustainability is the ability of the (agricultural) system to maintain its productivity with no net decline over many decades, even if subjected to stress or perturbation. (Hamblin, 1991).

Stress is a regular or continuous, relatively small and predictable disturbance such as seasonal waterlogging, from which a system fully recovers.· A perturbation is an irregular, infrequent, relatively large and unpredictable disturbance such as a severe drought or new disease from which recovery may be complete butoccurs over a longer time. (Hamblin, 1991).

The relationship between prime and good quality agricultural land is shown in Figure 2 in the context of agricultural land classifications currently used in Queensland. A description of the

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agricultural land classes used by the DPI is given in Table 2.

Table 2

CLASS

Agricultural Land Classes

DESCRIPTION GOOD QUALITY AGRICULTURAL LAND

Class Crop Land - Land that is suitable for current and potential All Crop Land is good A crops with limitations to production which range from none to quality agricultural land.

moderate levels.

Class B

Class C

Class D

Limited Crop Land - Land that is marginal for current and potential crops due to severe limitations; and suitable for pastures. Engineering and/or agronomic improvements may be required before the land is considered suitable for cropping.

Pasture Land - Land that is suitable only for improved or native pasture due to limitations which preclude continuous cultivation for crop production; but some areas may tolerate a short period of ground disturbance for pasture establishment.

Non-Agricultural Land - Land not suitable for agricultural uses due to extreme limitations. This may be undisturbed land with significant habitat, conservation and/or catchment values or land with very steep slopes, shallow soils, rock outcrop or poor drainage.

Limited Crop Land is good quality agricultural land where required for particular crops of local significance.

Pasture Land may be good quality agricultural land where pastoral industries are the predominant primary industries.

None

Source: Planning Guidelines - the Identification of Good Quality Agricultural Land (DPIIDHLGP)

Figure 2

SUITABLE

FOR

CULTIVATION

MARGINAL

FOR

CULTIVATION

IMPROVED

PASTURE

NATURAL

PASTURE

NON­

AGRICULTURAL

AGRICULTURAL LAND LAND CAPABILITY

CLASSIFICA nON CLASSIFICA nON

~-~ O·~1Q

PRIME AGRICULTURAL LAND

LAND SUITABILITY

FOR CROPPING

(1· 5)

GOOD QUALITY AGRICULTURAL LAND THROUGHOUT THE STATE

GOOD QUALITY AGRICULTURAL LAND BY LOCAL DEFINITION

Source: AgricullUrailand draH policy paper. SEa 2001

Agricultural land classifications in use in Queensland

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2.5.2 Identification

The identification of good quality agricultural land has been carried out by explorers, settlers, pastoralists and farmers since the earliest days of European settlement, when areas such as New Farm were used for food production for the new colony. The first areas recogn;ised as good quality agricultural land were alluvial soils supporting rainforest. As settlement spread after the 1860's, other areas with agricultural potential were recognised.

In more recent times, more comprehensive surveys of the land resources of the state have been undertaken by the CSIRO and DPI to document soil properties for research into productivity and land degradation, and to provide information for land use planning. More detailed studies have examined particular sub-regions, catchments, local government areas and industries for a range of purposes.

A framework for the protection of agricultural land requires the identification of good quality agricultural land at two levels of detail. One is at a strategic planning scale where the land resource areas to be protected need to be clearly indicated. This information must show the areas of good quality agricultural land which are required and available for agricultural use. Concepts of physical suitability, demand and availability should be included in this process of identification. Conventional soil and land evaluation studies provide an assessment of physical suitability; but the assessment of demand for agricultural land is more complex involving consideration of production trends, market demand for produce, processor requirements, and the emergence of new crops.

The identification of good quality agricultural land at a more detailed level is appropriate for development approvals or property planning at the individual property level. Detailed information collection for this purpose should be the responsibility of development proponents or property owners, but processes for checking the accuracy of information should also be in place. The collection and checking of detailed land resource information may result in adjustments to the broad-scale strategic planning information.

3 PLANNING SOLUTIONS

3.1 Planning framework

The planning system in Queensland is established under the LocalGovernment (Planning and Environment) Act. As the title implies, this Act gives local government much of the responsibility for planning, but State government also plays a significant role.

Planning schemes are planning instruments which provide a comprehensive basis for both 'forward planning' and 'development control'. The Act specifies that schemes shall consist of:

(a) ... provisions for the regulation, implementation and administration of the planning scheme, (b) zoning maps and any regulatory maps, (c) a strategic plan, (d) a development control plan (if any), (e) any amendment ... in respect of the planning scheme.

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The main 'forward planning' components are the strategic plans and development control plans; and the main 'development control' components are the zoning maps and scheme provisions which qualify how land in each zone can be used.

Responsibility for the preparation, implementation and enforcement of planning schemes covering rural and urban lands rests with local government. However the State government must approve the schemes before they take legal effect. Similarly proposed amendments to schemes such as rezonings follow the same process of state government ratification.

The DHLGP has the responsibility for advising the State government on the approval of schemes; advising local governments on technical planning matters generally; and coordinating the State government's overall response to local government on planning issues.

Recently there has been recognition of the need to provide a broader planning context for these locally generated planning schemes so that regional and state-wide interests can be incorporated into planning decisions. Accordingly the State government has decided to prepare a series of State Planning Policies on matters of state significance (see below); and regional planning guidance is being prepared in several parts of Queensland by the State government, local government and the community working together.

A hierarchy of policies is therefore evolving, reflecting the three main communities of interest: state, regional and local. In consequence, 'forward planning' in Queensland should improve considerably, providing the basis for more consistent decisions on land use and development proposals.

The key parts of the planning framework for the protection of agricultural land in Queensland consist of State Planning Policy 1/92 titled Development and the Conservation of Agricultural Land which was released in December 1992; and planning schemes prepared by individual local authorities of which the key components are the strategic plans and development control plans.

3.2 State Planning Policy

The intention of the state planning policy is to provide guidance to local governments on the exercise of their planning powers in relation to the protection of good quality agricultural land. These powers include the preparation of planning schemes, the determination of proposed amendments to schemes, and the determination of subdivision and development applications in agricultural areas. The state planning policy does not advocate the 'down zoning' of land to protect agricultural land resources. In this sense, no loss of existing land uses rights is proposed.

The policy advises that development on and adjacent to areas of good quality agricultural land should be restricted in accordance with four broad principles:

1. Such land should not be built on unless there is an overriding need for the development in terms of public benefit and no other site is suitable for the particular purpose;

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2. The State government will not support the use of such land for rural residential developments when equally viable alternative locations exist;

3. The fact that existing farm units and smallholdings are not agriculturally viable does not in itself justify their further subdivision or rezoning for non-agricultural purposes; and subdivision provisions and policies should encourage amalgamation of titles where this would enhance farm viability;

4. Planning provisions should aim to minimise instances of incompatible uses locating adjacent to agricultural operations; and measures to ameliorate potential conflicts should be devised.

The determination of ' oveniding need' is the responsibility of the local government in the first instance on the basis of the state planning policy, the strategic plan and submissions by development proponents. In almost all parts of the state there are adequate areas of non­agricultural land available for urban expansion but in some local areas a clear choice between agriculture and urban uses is necessary.

3.3 Strategic plans

Strategic plans are forward looking plans covering the whole local government area and comprise the following parts:

1. a map or series of maps depicting preferred dominant land uses; 2. a statement of objectives in respect of each of the preferred dominant land uses; and 3. criteria for the implementation of the plan.

All new planning schemes or reviews of existing schemes must include a strategic plan which sets out the objectives and preferred land use distribution over the next 10 years. Strategic plans should clearly designate areas for agricultural use and areas for urban development. Agricultural areas are those areas identified by land suitability studies and located on holdings which may be· managed either alone or in association with other holdings as agricultural enterprises. Urban development areas should be restricted to land required for urban uses determined by population growth and located on land of low agricultural qUality. Specific planning studies in urban fringe areas which are not required for urban development and are also fragmented into very small holdings may be necessary to determine appropriate land uses.

Appropriate strategic plan provisions for areas designated as preferred for agricultural land uses may be as follows:

Good Quality Agricultural Land:

These areas are shown on the strategic plan map and designate the areas of good quality agricultural land in the shire based on an evaluation of the requirements of agricultural and related industries of the shire; and an assessment of the suitability of land resources to provide these requirements. The designated areas may include land of lower agricultural quality important in maintaining the integrity of rural enterprises which utilise a range of land types. These areas are to be protected for agricultural use by only permitting subdivision and use which would not prejudice the efficient operation of those uses.

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Objective:

To protect good quality agricultural land for sustainable primary industry use.

Implementation:

(a) The land designated as good quality agricultural land on the strategic plan map includes all major areas of land in the shire suitable and available for sustainable agricultural production.

(b) Rezoning proposals in the areas designated as good quality agricultural land from a Rural zone to a zone allowing uses which would adversely affect or cause the loss of such land will not normally be approved. .

(c) Subdivision of land in the areas designated as good quality agricultural land will only be permitted if the Council is satisfied that the allotment(s) created will enhance the efficient, productive, agricultural use of the land and will not constrain normal agricultural management practices.

(d) Where non-rural development is approved in accordance with the Strategic Plan adjacent to an area designated as good quality agricultural land, Council will require an appropriate buffer area which uses natural features or is incorporated within the development to minimise conflicts over agricultural practices; and to demarcate the boundary between agricultural and development uses.

The strategic plan provides the greatest opportunity for local governments, the public, farming and other rural interest, together with the State government to agree on longtenn objectives for rural lands. The plan's policies and strategies should reflect the economic, social and environmental needs of the broad community.

3.4 Development Control Plans (DCP)

In contrast to broad based strategic plans, DCPs deal with a localised area or a specific issue which warrants specific implementation guidance. DCPs are optional and are prepared only where a local government considers it necessary.

In rural areas, strategic plans usually provide an adequate basis for controlling development but, in some circumstances, a more detailed local focus is required. Some recent DCPs dealing with rural land issues are listed in Table 3. The DCPs prepared for Beaudesert, Cambooya, Fitzroy, Wondai, Mareeba and Pine Rivers (Whiteside) Shires are concerned with the encroachment of rural-residential land-uses into agricultural areas and provide local authorities with better controls over the location and quality of these developments.

Another group of plans prepared by Cairns City, Gatton Shire and Toowoomba City is concerned with the development of steep hillside lands. These plans place restrictions on subdivision, location of structures and tree clearing to maintain slope stability, control bushfires and protect landscape features. The plans prepared by Mackay City and Redland Shire are for rural areas adjacent to expanding urban areas to facilitate the orderly conversion of agricultural land to urban uses. The other plans prepared in the Pine Rivers Shire are intended to protect a water supply reservoir (Lake Samsonvale) and local waterways (Kallangur).

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The DCP approach provides planning authorities with a flexible means of approaching planning problems which do not occur throughout a local authority area. This approach is appropriate for those local authorities who wish to prepare a general purpose strategic plan and place specific controls in a separate and detailed DCP.

Table 3 Some DCPs prepared in the rural areas of Queensland

Local authority DCP No. Date Title

Beaudesert DCP-1 09/02/85 Mount Tamborine Cairns City DCP-4 26/01/85 Hillside Land Cambooya DCP-1 21/05/88 Rural-residential Development Fitzroy DCP-1 23/05/87 Rural-residential Gatton DCP-2 21/05/88 Escarpment Area Mackay City DCP-1 12/03/88 South Mackay Mareeba DCP-1 27 / 07 /85 Kuranda and Environs Pine Rivers DCP-4 27/02/82 Whiteside

DCP-5 14/05/88 Lake Samsonvale DCP-9 14/05/88 Kallangur Waterways Study

Redland DCP-1 20/02/88 Local Development Toowoomba City DCP-3 10 / 06/89 Escarpment Area Wondai DCP-1 25/02/84 Rural Subdivision

3.5 Land use and subdivision controls

3.5.1 Zoning

The approaches to rural planning in Queensland are . generally based on the principle of agricultural zoning. This approach includes measures to allow the use of rural lands for

. normal rural land-uses and prevent or restrict certain developments which· are considered incompatible. In addition, measures are available to plan the subdivision of rural land to safeguard existing rural land-uses. The particular planning instruments adopted in each area are at the discretion of each local government. However DHLGP provides guidelines which recommend preferred instruments for use in particular areas.

Planning measures to control land use in Queensland may be described as 'uncompensated regulatory measures' (Willis, 1983). The planning system regulates land use and development in particular areas in the overall public interest. When the planning system confers particular use rights on certain areas of land after careful consideration of community needs, compensation is not generally payable to landholders whose development rights are limited. Individuals who believe that their interest in premises is 'injuriously affected' by a planning scheme provision may make a claim for compensation to the local government under S. 3.5 of the Local Government (Planning and Environment) Act within 3 years of the provision taking effect. In a similar way betterment is not obtained directly from landholders whose development rights are expanded, although it may be considered that the community benefits through economic growth, public infrastructure and employment which may flow from the development process.

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Planning schemes in Queensland which date from the 1970's have placed rural areas into a range or rural zones. Each planning scheme defines the uses which are permitted, require consent, or are prohibited in each rural zone. Generally, uses other than agriculture, animal husbandry, dwelling construction and related rural uses either require council consent or are prohibited. For example Jondaryan Shire Council permits the following activities in its Rural "B" zone:

Agriculture Animal husbandry Duplex dwellings Dwelling-houses Recreation

Development rights for non-rural uses can be obtained by changing the zone in which an allotment is located from a rural zone to an appropriate zone in which a desired development use is permitted. This rezoning process is subject to public scrutiny and rezonings require the consent·of the Governor-in-Council. Nevertheless, this means of control.over land-use is effective only if rezoning decisions are taken within a broader policy framework such as a strategic plan and appropriate controls over subdivision. With the advent of state planning policy 1/92, the rezoning of land in areas identified as good quality agricultural land and which is also designated for rural uses on strategic plans is more difficult to achieve.

3.5.2 Subdivision

The economics of viable holdings is not strictly a planning issue, but subdivision policies should discourage the fragmentation of the land resours:;e into small uneconomic holdings. Subdivision controls need to provide flexibility for land owners to achieve production efficiencies through adjustment in holding sizes in response to economic or market conditions, provided that complementary land use controls can prevent the intrusion of conflicting uses into agricultural areas.

The basis for subdivision policy in rural areas should be to provide allotments for the formation and restructuring of holdings while discouraging the creation of allotments for uses which would conflict with dominant rural activities.

The effectiveness of subdivision controls in maintaining allotment sizes in rural areas is weakened in most areas by generous discretionary subdivision provisions. These provisions allow councils to approve the subdivision of small allotments in rural areas for a range of purposes. Most commonly these allotments are approved to allow the construction of a dwelling for family members, but other pmposes such as rural homesites are also included.

Discretionary· provisions in planning schemes are normally included to allow consideration of special circumstances which cannot be covered by a general policy. By definition, such provisions should only be used in exceptional circumstances. There is wide acknowledgment that these subdivision provisions particularly those which allow for subdivision for family purposes, are commonly used by applicants to avoid the rezoning process with resulting fragmentation of rural land and unserviced ribbon development. Whereas some by-laws restrict 'family' allotment approval to those who are involved in rural production, many of

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these allotments are subsequently sold-off to the open market.

In Maroochy Shire for example, it is estimated that between March 1988 and October 1990, approximately two thirds of subdivisions affecting 2,800 ha of rural land were approved under discretionary subdivision provisions. In Redland Shire, where the minimum subdivision size in agricultural areas is 16-20 ha, only 91 ha (4.4%) of good quality agricultural land remains on allotments over 12 ha in area indicating that widespread subdivision into allotments below the minimum size has been allowed.

The desire of aging farmers to retire on a portion of their farm and dispose of the balance area may be met by allowing the subdivision of an allotment for such a pmpose although it would be. desirable that such an allotment was located on land of low agricultural quality.

. Group title subdivision may also· be appropriate so that management of the common area remains the responsibility of the allotment landholders who are engaged in farm production. The approval of multiple dwellings on a single allotment may also be appropriate in some circumstances. However the practice of allowing the subdivision of allotments for other family members who are not engaged in farm activities is inappropriate and only leads to unnecessary fragmentation of agricultural land.

4 INFORMATION REQUIREMENTS

The information needs of planning encompass inventories of biological and other natural resources such as air, soil, water, minerals and building materials; social and cultural information on population structure, composition and health; and economic information on industries, income and employment.

Within this smorgasbord of information, land is the terrestrial surface of the earth characterised by distinctive attributes that include the lithology of underlying rocks; landform and surface features including soils, vegetation and water; and the climate within both the lithosphere and the atmosphere. Land resource information is defined as:

Biophysical data on one or more attributes of natural land resources including soils, landform and vegetation and any interpretation of this information for the assessment of land suitability, land constraints, and land management. (QGIWG, 1991)

4.1 Land use planning and land resource information

Land use planning based on appropriate land resource information has important benefits for the resolution of problems in both urban and rural areas. These benefits include locating uses on land best suited for them; protecting areas which should be retained for specific uses; and identifying hazard areas which require special management. The major pmposes for which land resource information is required are the mapagement of land use conflict, land use planning, planning of engineering works, environmental impact assessment and property planning and management.

A recent survey of users of land resource information has indicated that information is most in demand for strategic / regional planning, environmental impact assessment and detailed property planning (Table 4). Over 300 replies were received to a questionnaire distributed

25

by DPI to Federal, State and Local Governments, education, business, professional and community groups. Of these, 18% nominated strategic or regional planning, 17.5% nominated environmental impact assessment, and 16% nominated property planning as activities requiring access to adequate land resource information.

Table 4 Response to a survey of needs for land resource information

ACTIVITY

Strategic/regional planning Environmental impact assessment Property planning Development control Infrastructure planning Research/teaching Community participation

% OF ALL REPLIES

18 17.5 16 14 12.5 8 7

(Source: Report on Land Resource Infonnation for Planning)

4.2 Forms of information: maps, reports, digital data.

SCALE OF MAPPING

1:100000 1: 10 000 1: 10 000 1: 10 000 1: 10 000 1:100 000 1: 10 000

Land resource information comes in a wide range of forms appropriate for different purposes and usually relates to either areas or points. Areal data are presented as maps of soils, land attributes and interpretations for land suitability or capability. Point data from inspection or sample sites are stored on databases and may be presented in reports or in tabular form.

Information has traditionally been stored as 'hard copy' in reports, on maps and field books but is increasingly stored as 'soft copy' in digital forms. This includes map boundary information which, once digitized into a geographic information system (GIS), provides flexibility to combine similar, compatible data and present tailor-made maps for users. Provided that care is taken to respect the accuracy and reliability of different data sets, combinations of attributes such as slope, soil erodibility or stability, soil chemical or physical properties, vegetative cover etc can be combined with land use, cadastre or tenure information to satisfy specific requirements.

Different levels of land resource information are necessary to satisfy different planning needs. The broadest level is that required for strategic or regional planning. Mapping scales required at this level vary with land use intensity from 1: 500,000 for western pastoral lands; 1: 250,000 and 1: 100,000 for pastoral/cropping lands; to 1 :50,000 mapping necessary in coastal and intenSively used areas.

For decision making at the property level for development control, environmental impact assessment and site planning, information is needed on the land resources of individual sites. This level of information is unlikely to be available in published maps and needs to be collected by detailed site descriptions specific to individual land use proposals.

26

These different levels of information reflect responsibilities for data collection. In general terms, data collection for strategic or regional planning provides major public benefits through better planning and decision making and is the responsibility of and funded by public agencies. On the other hand, data collection at the detailed site or property level is usually related to development approval for private benefit and is appropriately the responsibility of the proponent or landholder. Thus whereas State or local governments should fund the collection of base data for the preparation of planning schemes, data collection to support development applications should be funded by proponents.

4.3 Goals for the collection of land resource information.

Historically, the collection and storage of land resource information has been by public agencies for scientific purposes and the documentation of resources to plan state-sponsored development such as irrigation and land settlement schemes. In most cases the agencies have been their own clients and little attention has been paid to the needs of the wider community for information. The result has been that information is available for only limited areas and in forms that are specific in application.

Published land resource surveys in Queensland have been undertaken by DPI, CSIRO and private consultants. Although the State is covered by the 1 :2,000;000 Atlas of Australian Soils, there is no complete, uniform coverage of land resource maps at a scale appropriate for strategic planning.

The increased demand for better management of natural resources has highlighted the need for information by all sectors of the community. This has resulted in the development of goals for a state-wide coverage of land resource information. These goals accept that the responsibility for data collection for strategic planning purposes rests with the public sector. These goals, determined by a State government working group (QGIWG, 1991), are as follows. Areas referred to are shown in Figure 3:

Mapping of selected coastal and near-coastal areas at 1 :50,000 scale Mapping of central and southern cropping/pastoral areas at 1:250,000 scale with 1:100,000 scale mapping in key areas; Mapping of western and northern areas at 1 :500,000 scale.

Surveys at these suggested scales should provide a base for planning and an inventory of resources on which more detailed studies can be based. Maps of land resources interpreted for land suitability at an adequate scale for strategic planning are available only for some coastal and sub-coastal areas. Where land resource surveys have been completed in the extensive cropping and grazing lands west of the Great Dividing Range, map scales are 1: 250,000 or smaller. The most comprehensive coverage exists in the south west of the state where the arid lands have been documented at a scale of 1 :500,000. These scales are satisfactory for far-western areas but are too small for more closely settled agricultural areas such as the Darling Downs. Figure 4 shows current progress towards state-wide goals by DPI surveys.

The major gaps in information are along the coastal strip south of Sarina, particularly in the south-east comer from Maryborough to the border; the cropping/pastoral lands of the

27

hinterland; and the north-west tropics. These hinterland and inland areas are largely covered by land system surveys of CSIRO at 1:500,000 scale, but their information requires expert interpretation for application to land use planning issues in these areas.

The lack of information for the coastal strip is of particular concern as this area is under intense pressure for development in response to population and tourism growth. In the absence of a comprehensive coverage, data must be collected in an uncoordinated ad hoc program for specific projects and small areas. The danger with this approach is that such data is never integrated into a total database' for use in strategic or regional planning and similar data may be collected for different purposes over the same area.

At current funding, staffing and rates of progress, it is estimated that the state-wide goals outlined above will be achieved in 35 years. It is of concern that by 2025 most of the land use and natural resource planning issues requiring this information may be already determined and that mistakes made due to inadequate information may be difficult if not impossible to correct.

4.4 Good quality agricultural land

A current planning issue dependent on land resource information for successful resolution is the conservation of agricultural land. The key documents in achieving a rational approach to this issue are the state planning policy 1/92 referred to previously and strategic plans of individual local governments.

It is made quite clear in the policy that, whereas the powers to protect land resources from inappropriate uses are held by local government and the Minister responsible for Housing Local Government and Planning, the exercise of these powers is dependent on the correct identification of the location and extent of 'Good Quality Agricultural Land'. The classification of agricultural land in Table 2 will be used throughout the State to standardise this approach.

With the paucity of available land resource information recognised previously, particularly in the coastal areas of the state, strategies for the identification of good quality agricultural land are necessary so that planning schemes can give effect to the state planning policy. These strategies are being developed and have been published as Planning Guidelines for local government and proponents of development in agricultural areas. They are, based on the collection of information at two levels to address the needs of both strategic planning and development assessment'(DPI and DHLGP 1993).

At the strategic planning level, it is essential that areas of good quality agricultural land are designated and that appropriate objectives for the protection of these areas by zoning and subdivision controls are stated. Information has· been collated in the Planning Guidelines which indicates the best available land resource information for each local government area and interprets this information to define the areas of agricultural' land which should be protected. In many cases, this information does not meet the standards set by the goals set out previously, and in some cases, only 1:2,000,000 scale information is available. These deficiencies will only be addressed in the short term by the allocation of additional funding to land resource surveys.

28

Where development is proposed which may result in agricultural land defined in a strategic plan being taken out of production, a detailed assessment of land resources may be necessary. The onus is placed on the proponent to show that the proposal either will not impact adversely on the agricultural land resource or is of oveniding community benefit and cannot be located on alternative land of low agricultural qUality.

To ensure that an adequate level of information is supplied by the proponent to local government, the Planning Guidelines specify standards for data collection. The DPI accepts responsibility for training proponents, consultants and local government staff in the collection, interpretation and assessment of land resource data for this purpose.

Figure 3

WI

PROPOSED LAND RESOURCE ASSESSMENT GOALS FOR QUEENSLAND

LEGEND

5:SJ Existing ,mop~'ing .

~ 1 :250 000 (1 : 100 000 in key oreos)

.1:50000

- Includes cooslol cnd esluarine fringcs

Land Resource Assessment Goals for Queensland

29

.,

DEPARTMENT OF PRIMARY INDUSTRIES

LAND RESOURCE MAPPING (JULY 1991)

LEGEND

~ IN PROCRESS

m PLANNED

m COMPLETEO

Figure 4 DPI Land Resource Mapping, July 1991

5 CONCLUSION

11

Land use planning and management isa major responsibility of various levels of Government and land resource information is one of the fundamental requirements for the planning process.

It is required by a large number of government agencies, industries and individuals for strategic planning, environmental impact assessment and infrastructure planning; and without it, the following natural resource issues cannot be adequately· addressed:

conservation of agricultural land; vegetation and habitat protection;

• viable living areas for rural enterprises; and • geotechnical constraints to urban development.

30

To be of maximum use, infonnation needs to be presented in an accessible fonn, meet minimum standards, and be at a level of detail appropriate to the purpose for which it is required. Scales of infonnation required for strategic and regional planning have been adopted for different regions of the State from 1:500,000 scale in western areas to 1:50,000 in coastal areas. Data collection at this level is the responsibility of public sector agencies. At the detailed level appropriate for project planning and assessment, standards may be set by public agencies, but private proponents and landholders are responsible for data collection.

There are serious deficiencies in the availability of broad scale infonnation necessary for addressing strategic issues such as the SEQ 2001 Project and Cape York Peninsular Land Use Study. At current rates of survey, these deficiencies will continue until at least 2025. There is a need for additional funding from all levels of government to close these infonnation gaps and to develop a priority setting process for the allocation of scarce resources which takes into account the range of land resource infonnation needs across the state.

6 REFERENCES

Australian Bureau of Statistics (1992). Median values of selected agricultural activities, Brisbane and Moreton Region, 1990-91. Infonnation consultancy to the SEQ .2001 Project. ABS, Brisbane.

Christiansen, G. R, Budd, W. W., Reganold, J. P. and Steiner, F. R (1988). Farmland protection in Washington State: an analysis. Journal of Soil and Water Conservation 43 (5): 411-415.

Coleman, G. and Edwards, L.D. (1991). Management of arable land resources. Proceedings of Australian Society of Sugar Cane Technologists pp17-25.

Department of Housing, Local Government and Planning (1993). Zoning applications for South East Queensland. Unpublished report to the State Planning Policy Division. DHLGP, Brisbane. .

Department of Primary Industries & Department of Housing, Local Government and Planning (1992). Planning Guidelines: The Identification of Good Quality Agricultural Land. DPI, Brisbane.

Environment Working Group (1992). Agricultural Land Draft Policy Paper. SEQ 2001 Project, Department of Housing Local Government and Planning, Brisbane.

Hallsworth, E. G. (1978). Purposes and requirements of land resource survey and evaluation. Report No.3. Commonwealth and State Government Collaborative Soil Conservation Study 1975-77. Australian Government Publishing Service, Canberra.

Hamblin, A. (1991). Sustainability: Physical and Biological Considerations for Australian Environments. Working Paper No.WP/19/89. Bureau of Rural Resources, Department of Primary Industries and Energy, Canberra,

31

Puttock, L. and Ritchie, M. (1990). Agricultural Land. Ch 7 in Environmental Climate Assessment. Background discussion Paper No.6. The Brisbane Plan - a City Strategy. Brisbane City Council, Brisbane.

Queensland Government Inter-Departmental Working Group (1991). Report on Land Resource Information for Planning. Land Resources Branch, Department of Primary Industries, Brisbane.

Smit, B., Ludlow, L., Johnston, T. and Flaherty, M. (1987). Identifying important agricultural lands: a critique. The Canadian Geographer, 31 (4): 356-365.

Willis, I. (1983). Rural land use - US control measures. Australian Planner April/May: 35-37.

7 ACKNOWLEDGMENTS

The assistance of Jeremy Harle, Department of Housing Local Government and Planning; and the permission of the Director, SEQ 2001 to use material gathered for the Agricultural Land Project is gratefully acknowledged.

32

BASIC PROPERTIES OF SOILS

L.C. Bell

Department of Agriculture, University of Queensland, St. Lucia, Qld. 4067'

ABSTRACT

Most humans are supported by soils, through the vegetation that grows in them. Soils also support roads and buildings, and are used for waste disposal This paper provides an introduction to the composition of soils and the processes occurring in them, to help an understanding of how soils behave under their various uses.

The factors affecting the development of the soil profile are discussed, and the components that make up the soil mass described. These may be either inorganic or organic, and there may be reactions between them.

Soil attributes that affect use as a plant growth medium include physical support, nutrient, water and oxygen supplies, and freedom from inhibiting factors.· Soil attributes that affect use for either waste disposal or load support are also considered.

Important chemical processes in soils include cation and anion exchange, adsorption/desorption, 11J,ineralisation/immobilisation and dissolution/precipitation. Important physical processes are water and air retention and movement.

1 INTRODUCTION

Soil is the thin veneer of unconsolidated material covering the earth's land surface and is the medium in which most plants grow. Soils are complex biogeochemical materials which are formed and continually modified by the effect, over time, of climate, topography, vegetation, man and other biota acting upon the underlying parent materials.

Much of the human population on earth is supported by soils tlrrough the dependence of vegetation on these materials for nutrients, water and anchorage. Soils also provide support for roads and buildings and are used for waste disposal. When used effectively in waste management, soils protect groundwater by filtering toxic chemicals and disease organisms from waste water.

To understand how soils behave, as either the growing medium for plants, support for man­made structures or a biochemical filter, one must know something about the composition of soils and the processes occurring in these materials. This paper provides a brief introduction to these topics.

33

2 FORMATION OF SOIL

2.1 Soil Forming Processes

Soils form from rocks and sediments by the combined effects of climate, plants, animals, topography and time, with the rate of soil formation varying from 1 to 300 years for Imm of soil. The chemical, physical and biological soil-forming processes (weathering) act upon the parent material to produce a wide variety of soils with markedly different properties. The soils formed are characterized by the formation of a profile of horizons lying parallel to the surface; the surface, subsoil and decomposing parent material horizons are designated the A, Band C horizons respectively.

One way of understanding soil .formation is to consider soils as being formed by transformations, trans locations and homogenisation. Transformations involve breakdown of minerals (e.g. feldspars, pyroxenes and amphiboles) in the parent rocks and of organic matter. The breakdown products may be lost tlrrough leaching or be synthesised into new compounds (e.g. layer alumino-silicates, iron and aluminium oxyhydroxides, carbonates and humus).

Transloeations may occur within or out of the soil or as accessions to the profile. Movement within the soil profile occurs by material dissolving or being suspended in the soil water and moving with it either downward or laterally. Suspended material is often deposited in the subsoil forming characteristic B horizon accumulations of clay or iron oxides. Accessions may occur tlrrough accumulation of organic matter in the surface of the soil or through salts being added to the profile by rainfall or a rising water table.

Acting against these differentiating processes is the process of homogenisation which results from the physical movement of soil by worms, termites, burrowing animals and humans.

2.2 Soil Profile

As indicated earlier, the soil forming processes produce a profile consisting of several horizons, and these differ from those above or below in one or more soil properties such as thickness, colour, texture (relative proportion of sand, silt and clay), structure (arrangement of soil particles into aggregates),· consistence (resistance to deformation) and chemical composition. In some soils, the boundaries between horizons merge gradually over several centimetres and can be difficult to observe without close examination. The letters A, Band C are used to designate the major horizons, but these can be further subdivided.

The 0 or organic horizon consists of a surface layer of plant materials in various stages of decomposition. It is poorly developed or absent in disturbed soils but is common in forest soils.

The A horizon or surface soil is the zone of most biological activity and root proliferation. It iscommoniy dark resulting from an accumulation of organic matter and is the zone of maximum eluviation or leaching involving weathering of minerals and translocation of clay­sized'material to the subsoil (B horizon). In some soils the horizon can be divided into two sections, viz. an Ai horizon of 5 to 30cm thick, which is darkened by organic matter, and a . paler A2 horizon underneath of 5 to 50 cm or more. The A2 horizon in some cases may be

34

white (bleached horizon).

The B horizon, which may vary in depth from lOcm to in excess of 2m, is the subsoil and is characterized by lower biological activity, generally lower levels of organic matter, higher content of layer alumino-silicate clay minerals and iron and aluminium oxyhydroxides and higher bulk density than the A horizon. Additionally the structure (aggregation of particles) is different from the surface horizon, and red and yellow colours may be more pronounced as a result of the accumulation of iron compounds. In comparison with the surface horizon, the B horizon may have lower levels of available nutrients, higher levels of soluble salts and poorer drainage. In some cases the B) horizon can be divided into an upper B) horizon, which is transitional to the A horizon, and the underlying B2 horizon.

The C horizon includes the weathered, consolidated or unconsolidated layers of parent material below the B horizon which are unaffected by biological soil-forming processes.

2.3 SoH Classification and Mapping

Because of the wide range of parent materials and the different intensities of soil weathering in various parts of the world, there is almost an infinite variety of soils. To study soils and to use soils effectively, clearly some sort of classification is necessary. Classification enables one to transfer information on soil from one area to another and to help one to predict the suitability of different soils for various uses. A description of International and Australian soil classifications is given in the paper by Bridge and Probert in this volume.

The use of information on different soils is aided by the recording of information on their distribution across the landscape. The most common method of showing the distribution of soils is by the preparation of maps which depict the various areas of land which are dominated by the same soil class or a similar grouping of soil classes in the case of compound soil map units. The kind of soil mapping unit used depends on the purpose of the soil survey and the scale of mapping.

Another approach is to produce maps of land units which include other elements of the environment as well as soil (e.g. vegetation and topography). Considerable information on soil properties can be stored in computers, and, by combining these data with a geographic reference input, maps of soils based on a given physical or chemical property can be produced (e.g. a map showing the distribution of soils with various permeability classes).

Further information on soil classification and mapping in Australia is given in the text by Gunn et al. (1988) and by Thompson (1988).

3 SOIL COMPONENTS

Soils consist of solid, liquid and gaseous phases, with the solid phase making up approximately 50% by volume in the surface but often increasing to approximately 60% in the subsoil. The ratio of air to water in the remaining volume of soil (pore space) fluctuates widely for a given soil. The solid phase of soil is a complex assemblage of inorganic (mineral) and organic constituents.

35

3.1 Inorganic Components

The inorganic component consists of particles of sand, silt and clay (Table 1) which are clustered together to form aggregates (peds). The relative proportion of each of these size fractions determines the texture of the soil (Figure 1).

Table 1

Size fraction

Coarse sand

Fine sand

Silt

Clay

Figure 1

Some characteristics of particle size fractions in soils

Particle diameter (mm)

0.2-2.0

0.02-0.2

0.002-0.02

<0.002

l 70 I

I I

/ 60 I

Mineralogy

Primary minerals (particularly quartz)

J, Secondary minerals (particularly layer alumino-silicates and Fe and AI oxyhydroxides)

Clay

-0 ~

(\

~~ 'J1~

\

'\ C!ay loam

30 -Sandy clay loam

201--------< ,80

Silt loam

70 60 40 30

.. Per cent sand

A soil textural triangle showing how the proportion of sand, silt and clay particles determines the textural class of a soil

36

The inorganic components of soil consist of both residual coarse-grained primary minerals from the parent rock and fine-grained secondary minerals which have formed from the decomposition products of the primary minerals (Table 2). The resistant mineral quartz dominates the sand and silt fraction. In young soils, feldspars, hornblende, augite and micas may also be present, and these predominantly occur in the sand fraction, and to a lesser extent the silt fraction; in highly weathered soils, there may be little of these minerals. The secondary layer alumino-silicates dominate the clay-size fraction, while other secondary minerals, the iron and aluminium oxyhydroxides, tend to be concentrated in the silt and clay fractions.

The layer alumino-silicates consist of a combination of a silica tetrahedral layer with an aluminium octohedral layer (i.e. a 1: 1 alumino-silicate such as kaolinite) or an aluminium octahedral layer sandwiched between two silica tetrahedral sheets (i.e. a 2: 1 alumino-silicate such as montmorillonite). The layer silicates tend to be plate-shaped rather than spherical, and individual crystals (platelets) are held together by hydrogen bonding (1: 1 silicates) or coulombic attraction (2: 1 silicates) to form stacks of crystals or domains. The nature of the bonding between the individual platelets has a major influence on the physical behaviour of soil.

Both the physical and chemical properties of a soil are influenced by the proportion of the clay-sized fraction in the soil, and this is related to the mineralogy (and hence surface reactivity) and to the large specific surface area (surface area per unit weight) of the fraction. One can easily demonstrate that the specific surface increases markedly as particle diameter decreases. Thus the external specific surface of the largest particles in the coarse sand and clay fractions may be 0.002 and 80m2g- 1 respectively. Additionally, however, many layer silicates, which occur in the clay-size fraction (e.g. montmorillonite), have an internal surface area between the platelets which is accessible to the soil solution, and the specific surface for these minerals can be as high as 800m2g-1•

As will be discussed later (Section 5), the layer silicates have a net negative surface charge density and this property, coupled with their high specific surface, results in them having a marked effect on the physico-chemical properties of soils.

3.2 Organic Components

A characteristic of all soils is the presence of organic matter which is responsible for the darkening of the surface horizon, where the levels range from 1 to 10% but typically decline to much lower values in the subsoil « 1 %). Despite its generally low levels, organic matter in soils has a major effect on soil properties. It consists of both non-living and living components .

The non-living component (residues and humus) serves as a reservoir of plant nutrients (particularly nitrogen, phosphorus and sulphur), is important in developing and maintaining soil structure, and the negative charge on its colloidal component is important in the retention of cations and water.

37

Table 2 Primary and secondary minerals which occur in soils

Mineral Group

Silicates -

Oxides

Sulphides

.Phosphates

Silicates -

Oxides, hydrous oxides and hydroxides (Oxyhydroxides)

Sulphates

Phosphates

Sulphides

Chlorides

Silica

Feldspars

Micas

Amphiboles

Pyroxenes

Orthosilicates

Borosilicate

Silica

Layer alumino­minerals

Examples Formula

Primary minerals

Quartz

Orthoclase, microcline Plagioclase

Muscovite Biotite

Hornblende

Augite

Olivine Gamet Zircon

Tounnaline

Hematite Magnetitie llmenite Rutile Anatase

Pyrite

Apatite

Si~

KalSi30 S

NaAlSi30 r CaA4SizOs

K(AlzS4;)A4°ZO<°H). Kz(AlzS~g,Fe)~ZO<OH)4

Caz(Mg, Fe,Al>s(Si,Al>S0zz{OH)2

(Ca,NaXMg,Fe,AlXSi,Al)Z06

(Mg,FehSiO•

F~i301Z ZrSiO.

NaF~B~(Al3Si~Z7(OH).

Fez03 F~O. FeTi03 Ti~ TiOz

FeSz

Cas(PO~(F ,CI,OH)

Secondary M"merals

Chalcedony Opal

Kaolinite halloysite Montmorillonite Hydrous mica Vermiculite Chlorite Allophane

Hematite Goethite Lepidoaocite Amorphous iron oxides Gibbsite Boehmite, diaspore

Calcite Dolomite .

Gypsum Jarosite

Apatite Variscite Vivianite Octacalcium phosphate

Pyrite

Halite

38

Si~

SiOPlzO).

Si..Al.ou,(°H>s M" o~i,(Al3AM&I.~ZO<OH). (K,Na,Ca}z(Si,Al¥AI.Mg,Fe).O,JOH). M" oJSi,AI>S<Mg,Fe,Al)~ZO<OH).2H20 (Si,AI>S<Mg,Fe,Al)~,JOH) •. (Mg,Fe,AlMOH)6

Alz°3·XSi02·~O

Fez03 FeOOH FeOOH

Fez03·nHZO Al(OHl:! AlOOH

eaco. (Ca,Mg)C°3

CaSO.·2HzO KF~(SOJZ<OH)6

Cas(PO~(F,CI,OH)

(Al,Fe)POJ2HzO FeZ\.",Fe3+.(POJz{OH).<HP>a.,. CaaHz(POJ~O)s

FeSz

NaCl

The living component, the soil organisms, is important in many processes important to soil fertility, e.g. release of nutrients from residues and humus by mineralisation, the fixation of nitrogen by Rhizobium bacteria on legume roots and the enhanced nutrient uptake by roots resulting from the presence of mycorrhizal fungi.

3.2.1· Non-living components

The non-living organic component of soils includes partially decomposed plant and animal residues and stable soil humus and is by far the major organic matter component. The residue component of the non-living portion is variable in content but is usually a minor component (often < 1 to 5%).

The main residue component is plant material which contains carbohydrates (sugars, starches, hemicellulose and cellulose), lignins, proteins, fats and waxes. The sugars and starches and some of the hemicelluloses and the proteins are rapidly decomposed, whereas cellulose and some of the hemicellulose and proteins are decomposed more slowly. Lignins and some of the waxes and tannins are the most resistant. Compounds very resistant to microbial action are formed either through modification of compounds in the original plant tissue or by microbial synthesis; collectively these compounds comprise the soil humus.

Humus consists of a heterogeneous mixture of complex organic compounds which can be classified into nonhumic and humic compounds. The nonhumic substances, which make up 20 to 30% of humus, are less complex and less resistant to microbial attack. They' are characterized by ring-type structures including polyphenols and polyquinones formed by decomposition, synthesis and polymerisation. These compounds are amorphous and dark in colour, have high to very high molecular weights (up to several thousands) and have no sharply defined physical or chemical properties. Traditionally the humic substances are classified into three groups, which are arbitrary rather than discrete chemical subdivisions, viz.

(1) fulvicacid - lowest in molecular weight and lightest in colour, soluble in acid and alkali and most susceptible to acid attack.

(2) humic acid - higher in molecular weight and colour, soluble in alkali but not in acid and medium in resistance to microbial attack.

(3) humin - highest in molecular weight, darkest in colour, insoluble in acid and alkali and most resistant to microbial attack.

Humus displays the typical properties of hydrophilic colloids. It has a high specific surface area and, coupled with a surface negative charge developed by ionization of functional groups (-COOH carboxylic, -OH hydroxy or phenolic), generates a cation exchange capacity (100-600 c mol (+) kg-I) which exceeds that of inorganic components in soils. Its high specific surface also is responsible for it making a significant contribution to the water holding capacity of soils.

39

3.2.2 Living components

The living components in soil include plant roots, fungi, bacteria, algae, protozoa, insects, earth worms and burrowing animals. The biological activity in the soil is dominated by the input of carbon from decaying plant material and the nature of the environmental conditions (temperature, water content etc.) affecting general biological activity. The plant carbon is a source of energy for a variety of organisms, particularly the bacteria and fungi, which break' down plant and animal tissue and convert it to the stable end product of decomposition, soil humus, with the loss of energy and CO2.

3.3 Interrelationship between Inorganic and Organic Components

In surface soils a large proportion of the organic matter is intimately associated with the mineral constituents, particularly those in the clay-sized fraction. The bonding of soil humus to minerals occurs via a number of mechanisms including coulombic attraction (through exchangeable cations to negatively charged surfaces or by anion exchange to positively charged sites), covalent bonding to iron, aluminium and silicon at crystal edges, H bonding and van der Waals forces. This intimate bonding between humus and the clay-size minerals in soil is one of the major factors which contributes to aggregate stability.

4 SOIL ATTRffiUTES AFFECTING USAGE

As indicated previously, one is generally interested in soils as either a plant growth medium, a filter for waste materials 'or support for roads and buildings. In each case, there are certain attributes of soils which influence their suitability for each of the various uses.

4.1 Plant Growth Medium

4.1.1 Support

Plant roots anchored in the soil enable growing plants to remain upright. The nature of the soil fabric, particularly its bulk density, affects root extension. Bulk density is a measure of the relative amount of pores (voids) and solid particles (Section 6). The lower the porosity in a soil,the higher is its density, and the more difficult it is for roots to ramify throughout the soil.

4.1.2 Source of nutrients

Plants require at least 16 essential elements to enable them to grow and complete their life cycle. Carbon, hydrogen and oxygen, which are combined in the photosynthetic reaction and obtained from air and water, make up about 90% of the dry matter of plants. The remaining 13 elements are obtained primarily from the soil. Nitrogen, phosphorus, potassium, calcium, magnesium and sulphur are required in relatively large amounts (macronutrients), whereas iron, manganese, copper, zinc, boron, molybdenum and chlorine are needed in only trace amounts (micronutrients).

Both the inorganic and organic components of soils are a source of the essential elements. Greater than 90% of all the nitrogen and sulphur in soils is in organic combination, and, thus

40

the soil humus is the most important source of these nutrients for plants. The inorganic constituents of soils are the dominant sources of most of the other nutrients.

Not all of the total amount of a nutrient element in a soil is available to plants. Elements such as nitrogen and sulphur in humus are unavailable to plants until they are released to the soil solution by microbial decomposition (mineralisation). Other elements become accessible to plant roots through the chemical processes of solublization of minerals, ion exchange or desorption which releases the nutrients in ionic form to the soil solution (Section 5).

4.1.3 Source of water

To produce one kilogram of dry plant matter, plants must take up from several hundred to several thousand kilograms of water. Most of this water passes through plants (transpiration) and is responsible for providing a continuous supply of nutrient elements from the soil solution. Thus the ability of a plant to grow in a given soil is very much dependent on the soirs capacity to provide a non-limiting supply of water to the plant roots.

Only a proportion of the water which falls on a soil as rainfall or irrigation is held by the soil; some is lost as surface runoff, and a portion of that which infiltrates the soil surface moves through the profile under gravity and passes beyond the root zone. Not all the water which is held by soil against gravity is available to plant roots. Productive soils have a high "available" water capacity; the process of water retention is further discussed in Section 6.

4.1.4 Oxygen requirement

Plant roots need oxygen to enable them to function efficiently in water and essenti~ element uptake. Oxygen is needed for the respiration process which releases energy required for important biochemical processes involved in uptake. As oxygen is used up by roots, it must be replaced by diffusion down from the soil surface. Carbon dioxide is produced by the respiration of roots and microbes and, in high concentrations, can be toxic to roots. Thus if a large proportion of the pores in a soil are filled with water (waterlogged condition) Or if there is only a small proportion of the soil volume having large pores (> 60 pm) capable of allowing rapid gas exchange, then plant growth will be restricted.

4.1.5 Freedom from inhibiting factors

Chemical factors

Plant growth is inhibited if the roots are exposed to excessive levels of acidity, alkalinity, salinity or toxic elements. The optimum pH range of the root zone for plant growth depends on the plant species, but, below about 4.5 to 5.0, growth can be retarded by aluminium and/or manganese toxicity and, above a pH of about 8.0, reduction in growth can occur through unavailability of phosphorus and micronutrients such as iron, manganese, copper and zinc.

Plants vary in their tolerance to salinity, but few can tolerate electrical conductivity (EC) levels in excess of 8mScm-1 (saturation extract) without yield reduction, and few survive at EC values greater than 2OmScm-1•

41

The most common metal toxicities in soil are those due to aluminium and manganese at pH values less than about 5.0; However, in mine wastes and in soils contaminated by wastes containing metals, plant growth can also be limited by toxic concentrations of copper, zinc, lead, nickel and chromium, particularly if the medium is acid. The availability of metals rapidly decreases with increasing pH, and the probability of a metal toxicity is much reduced at pH values greater than 7.

Physical factors

A suitable root zone for plants should have a good available water capacity but be sufficiently well drained so as not to affect root growth through lack of aeration. Additionally the soil should not provide mechanical impedance to the expanding root system. All of these properties are a function of the pore size distribution in the soil and its stability. This distribution, in turn, is determined by the primary particle size analysis and the aggregation of these particles.

Instability of aggregates on wetting through slaking and dispersion can lead to the formation of a crust on the soil surface after drying. Such crusts can prevent or reduce seedling emergence. Mechanical impedance to root ramification can limit plant growth in soils which have been compacted as a result of traffic by heavy equipment, particularly when the soil is moist and most vulnerable to compression.

Microbiological factors

Micro-organisms directly or indirectly affect plant growth through their influence on nutrient uptake and cycling and nitrogen fixation. In some soils, it is necessary to inoculate seed of legumes for the appropriate strain of Rhizobium bacteria which is necessary for nitrogen fixation but which may be missing in the soil.

In soils contaminated by toxic wastes and in mine wastes, there is often a dearth of mycorrhiza and other beneficial micro-organisms. Seed inoculation with mycorrhiza is not yet a practical proposition, but the use of a small amount of fresh soil on a contaminated site will often supply the suite of beneficial microbes necessary for good plant growth.

4.2 Waste Disposal

Soils are used for the disposal of a variety of wastes including sewage effluents and sludges, municipal garbage and food processing wastes. To be an effective filter, soil must have a high capacity to immobilize metals and organic compounds (adsorption) and to break down organic matter through microbial degradation. Soils high in clay· have high adsorption capacities but generally low infiltration rates, whereas sandy soils have high infiltration and permeability rates but low immobilization capacities. Thus a compromise often has to be reached between permeability and immobilization capacity.

When garbage is placed in a dump, it is important that the material at the base of the . landfill be sufficiently impermeable to prevent leachate from reaching the groundwater, as discussed by Razzell in this volume.

42

4.3 Load Support

A soil's strength determines its ability to support a load which can stress a soil in tlrree ways. Shear strength is the resistance of soil to shearing (sliding) stress; compressive strength is the resistance to compressive stress; and tensile strength is the resistance to pulling stress.

Soil strength is determined by frictional resistance which is determined by the specific surface of the soil particles, their shape and the water content of the soil. Soil strength generally decreases with increasing water content because water films reduces the contact between individual soil particles. A small increase in water content of a dry soil can increase cohesion between particles, however, and thus cause an initial increase in soil strength. In general, coarse-textured soils have higher soil strength than those containing appreciable amounts of clay-size particles.

5 CHEMICAL PROCESSES IN SOILS

Irrespective of whether one is interested in a soil as a growing medium for plants or as a filter for wastes, it is essential to have an understanding of the chemical processes which affect the concentration of ions and molecules in the soil solution. The soil solution can be considered to be in quasi-equilibrium with the solid phase, and its composition can be altered by the processes of solubilization/precipitation, ion exchange, adsorption/desorption and mineralization/immobilization (Figure 2).

Plant roots take up ions from the soil solution. This solution generally contains a low concentration of ions and, in the case of nutrient elements, must be replenished constantly by various solution - solid phase processes to supply enough nutrient for the satisfactory growth of plants. For example, in the case of calcium, magnesium and potassium, from <1 to 5% of the total amount of these elements available to plants (soluble plus exchangeable forms) occurs in the soil solution.

The various processes shown in Figure 2 are also responsible for buffering· the soil solution composition when fertilizers or wastes are added to soils.

5.1 Ion Exchange

5.1.1 Charge development

Soil possesses a net surface charge which is most commonly negative but, in the subsoils of highly weathered soils, may be positive. This electrostatic charge can be measured on most components of soil, but the major contributors are the layer alumino-silicates in the clay fraction and soil humus primarily as a result of their surface charge density and high specific surface. Surface charge can develop as a result of tlrree types of processes, viz. (1) isomorphous replacement, (2) ionization of functional groups, and (3) specific adsorption of IOns.

Isomorphous replacement is responsible for most of the negative charge possessed by the clay-sized layer alumino-silicates and arises as a result of charge imbalances within the crystal structure. The charge may arise through the substitution, during the crystal formation, of AI3-!-

43

for Si4+ in the silica tetrahedral layer or Mg2+ or Fe2+ for Al3+ in the aluminium octahedral layers (Section 3.1.). This charge is called permanent negative charge and is expressed at the surface of the crystals.

Ionization of hydroxyl groups at the crystal edges of the clay-sized aluminium silicates and of iron, aluminium and manganese oxyhydroxides can also produce charge which can be negative or positive depending on pH; the amount of negative charge increases with increasing pH (Figure 3) and with ionic strength of the soil solution.

EXCHANGBABLB FORMS

ADSORBBD FORMS

PLANT

PRIMARY MINERALS

ORGANIC MA'ITER

Figure 2 Simplified representation of nutrient and non-nutrient dynamics in soil

44

+ 0

'" I/OH2 on "i/H on I/OH

:lIro ," M , M , M

/1~H2 H+ /I"OH

H+ /I"OH 2

where M = Fe, AI, Si or Mn

Figure 3 Schematic representation of surface charge development on the edges of layer silicates and of iron, aluminium and manganese oxyhydroxides.

Ionization of carboxyl (- COOH), phenolic (- OH), enolic (- OH) and imide (- NH) groups on soil humus also generates surface charge which is always negative and increases with increasing pH and ionic strength of the soil solution. The major contribution to negative charge generation in organic matter is from carboxyl ionization.

R - COOH~R - COO- + H+ (1)

Although isomorphous replacement and ionization of functional groups are the . main nechanisms for generation of surface charge in soils, small changes in charge can occur through the specific adsorption (chemical bonding) of anions or cations added to soil Thus phosphate will increase surface charge through covalent bonding, whereas metallic cations such as copper and zinc will increase positive charge.

5.1.2 Cation and anion exchange capacity

The amount of electrostatic charge developed on soils can be quantified in terms of the cation exchange capacity (CEC) or anion exchange capacity (AEC). The exchange capacity of a soil is a reflection of its mineralogy and organic matter content. Table 3 lists the cation exchange capacities for some of the more important soil minerals and organic matter. As indicated previously, most soils have a net negative charge, and CEC values of> 40 c mol (+) kg-1 are considered high, values of 15-30 c mol (+) kg-1 .are medium, while values < 5 cmol (+) kg-1

are very low. Soils with very high CEC values tend to have high contents. of montmorillonitic minerals and/or organic matter, whereas those with very low CEC values are either sands or highly weathered soils of the subtropics and tropics which are low in the 2: 1 layer silicates. It is generally only in the subsoils of highly weathered soils that an AEC exists. This capacity is related to the relatively high concentration of iron and aluminium oxyhydroxides, which gives positive charge under acidic conditions, and to the absence of negatively charged organic matter at depth; most commonly the AEC is <5 cmol (-) kg-1 and is balanced by N03-, cr and SO/. The net result of the CEC in soils is that cations are held by electrostatic or coulombic attraction to the solid phase surface, but that there is rapid and continual exchange of ions between the solid and solution phases. The relative affinity of ions for the surface depends on their valence and hydrated radius and follows the lyotropic senes -

45

Table 3 Cation exchange capacity and specific surface area for soil components

Mineral type Surface area (m2g-1) CEC (cmol(+)/kg) * Hydrous oxides of Fe and AI (pH 8.0) 25-50 0.5-1

Kaolinites 10-20 2-15

Clay micas (illites) 90-130 20-40

Smectites 750-800 60-120

Vermiculites 750-800 120-200

Allophanes 500 .. 700 50-100

Organic matter 500-800 100-600

* The unit of cmol(+)/kg is equivalent to meq 100g-1 which was formerly used for CEC.

The ions held on the surface are exchangeable and can be replaced by other ions adding to the soil in fertilizer or waste (Figure 2). Cation exchange is important in soils, because it helps retain the basic nutrient cations against leaching; the ions held on the exchange are available to plants as the exchange is reversible.

The exchangeable cations can be divided into two groups, viz. (1) the basic cations Ca2+, Mg2+, K+and Na+ and (2) the acid cations H+ and AI3+ (and associated ions AIOH2+ and AI(OH)t). The base saturation is equal to (I: basic cations/CEC) x 100, whereas the exchange acidity is (H+ + AI ions/CEC) x 100.

For soils with pH values above 7, there is little exchange acidity, and the relative abundance of basic cations in surface soil is Ca2+ > Mg2+ »K+ »>Na+. With depth in the profile, Ca2+ usually declines, Mg2+ and Na+ increase, and K+ shows no particular trend. In some soils (naturally sodic soils or· those affected by salt addition), exchangeable Na+ can be high, and this produces undesirable effects such as crusting and low infiltration related to the effect of the ion in increasing dispersion of the soil colloids. An exchangeable sodium percentage (100 exchangeable Na+/CEC) of as low as 6% can cause discernible crusting in soils, whereas the effects at values in excess of 15% can be severe.

When the pH of soil is less than about 5.5, layer silicates become unstable releasing aluminium ions which are attracted to exchange sites. In very acid soils (PH < 4.0), aluminium ions may make up more than half of the CEC. Below a pH of about 5.5, plant growth may be restricted by toxicities of aluminium and/or manganese and deficiencies of calcium or magnesium. Additionally H+ per se can be toxic particularly at pH values less than about 4.0.

46

To improve plant growth in very acid soils, it is thus important to raise the pH through the use of liming agents. The amount of lime required will depend on many factors including the buffering capacity of the soil and the neutralizing capacity of the liming agent. A major contributor to the generation of H+ ions in soil and hence the buffering capacity is aluminium which hydrolyses in a series of reactions to produce H+.

5.2 Adsorption/Desorption

The process of adsorption refers to the accumulation of an ion or molecule on a surface. Desorption is the reverse of this process. The forces operating may be chemical or physical.

Ions can be held on soil surfaces by coulombic or electrostatic bonding (non-specific adsorption) or by the formation of chemical (covalent) bonds (specific adsorption). Non­specific adsorption is the process by which exchangeable cations and anions are held in soils, and this process ,has already been discussed (Section 5.1).

Specific adsorption differs from non-specific adsorption in that ions involved in this process tend to be adsorbed on soil surfaces in amounts far in excess of that predicted from the relative proportion of the ion in solution. Thus, if an equimolar solution of chloride (nonspecific ally adsorbed) and phosphate (specifically adsorbed) is added to a soil, most of the phosphate adsorbs on soil surfaces, whereas most of the chloride remains in solution (Figure 4). Specifically adsorbed ions are held more strongly on the soil surface but can be des orbed particularly by an ion which forms a stronger chemical bond with the surface than the ion in question. The relative bonding energies for some important anions in soils follow the order-

phosphate > arsenate> selenite = molybdate = borate> sulphate> chloride = nitrate

The pH at which maximum adsorption of a specifically adsorbed anion occurs is at the p~ of the anion's conjugate acid.

Metallic cations tend to specifically adsorb on soil surfaces, and the bonding energIes normally follow the sequence-

Pb > Cu > Zn > Ni > Cd = Co > Mn

The pH at which maximum adsorption is reached tends to be equal to the pKa of the cation. Adsorption of all metallic cations increases with increasing pH. Thus one method of ameliorating a soil contaminated with heavy metals is to raise the pH by liming.

The adsorption of organic compounds such as pesticides on soil surfaces can also occur by physical adsorption processes such as hydrogen bonding and van der Waals forces. The individual· bonding energy is not as strong as that with covalent bonding, but these forces are additive, and, for molecules that may have many points of contact with the soil surface, the total forces of attraction can be high.

The site of specific adsorption of anions can be the surface of iron, aluminium and manganese oxyhydroxides and the edges of layer alumino-silicates (Figure 4). Cations can be specifically

47

adsorbed on the same sites (Figure 5) but, additionally, can form a special type of bonding (chelate or ring structure) with several carboxyl groups on soil humus.

Figure 4

Figure 5

/. -0

'" H+ M-O - - - -- .- - - cr / H

-0

"'M-O OH

/ ""/ -0, . /p~ 'M-O ~O-

/ -0

" Non-specific adsorption of chloride and specific adsorption of phosphate to an oxbydroxidesurface (M can be Fe, AI, Mn or Si)

Non-specific adsorption. of potassium and specific adsorption of copper to an oxyhydroxide surface (M can be Fe, AI, Mn or Si)

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5.3 MineralisationlImmobilization

The breakdown of organic matter in the form of humus residues, dead microbes and root exudates to release nutrients such as nitrogen, sulphur and phosphorus is termed mineralization and is largely a microbial process as is immobilization, the reverse process. The mineralization/immobilization process is carried out by a wide range of heterotrophic micro-organisms, mainly fungi and bacteria.

If the decomposing organic material contains more nitrogen than the micro-organisms need for cell growth, the excess is excreted as NH/, and net mineralisation occurs. This will generally occur when the nitrogen concentration of the organic matter is greater than about 2.5% and the C/N ratio less than about 15. If the decomposing organic material contains less nitrogen than the micro-organisms need, they will take up NH/ (and N03- if present) and net immobilization occurs, i.e. if the N% is less than 1.2% or C/N ratio greater than about 33.

The rate of net mineralisation or immobilization is very much dependent on environmental factors influencing the activity of micro-organisms, viz . .temperature (optimum about 40°C), moisture (optimum from field capacity to saturation) and pH (acidity can inhibit the process).

5.4 Dissolution/Precipitation

The dissolution of minerals, a weathering process, releases potassium, calcium and magnesium and other metals from such minerals as feldspars, ferromagnesian silicates and micas (Table 2). In the highly weathered soils of the tropics, most of these minerals may have completely dissolved in the upper part of the soil profile. In most soils, dissolution of oxyhydroxides and layer silicates supply most of the soluble iron, aluminium and manganese. In semiarid environments, soils may contain secondary calcium sulphate and carbonate; and the solubility of these minerals,will control 'the solution level of calcium and sulphate.

Addition of fertilizers or wastes to soils may result' in the solubility products of various compounds being exceeded, resulting in precipitation. Additionally an induced change in the pH of soil such as by the addition of lime may reduce the concentration of some ions (e.g. metallic cations) through precipitation. Knowledge of the solubility product constants for compounds allows one to calculate whether or not such a compound will persist in soil and, if so, what concentration (or activity) of ions it will support in the soil solution.

6 PHYSICAL PROCESSES IN SOILS

As indicated previously, the individual particles in soil (inorganic and organic) are bound together to a greater or lesser degree to form aggregates. The gas and liquid phases in soils occupy both the inter-aggregate and intra-aggregate pores. The smaller or capillary pores (particularly those within aggregates) effectively hold water by surface tension, but the rate of movement of water through these pores is slow. Larger or macropores do not hold water as readily against gravity, and thus allow soils to drain freely. Because . the diffusion of oxygen through water is about 1/10000 of that through air, macropores are important in ensuring effective gaseous exchange between the zone surrounding plant roots and the above­ground atmosphere. There is no marked dividing line between capillary and macropores, but an approximate division is a diameter of 60Jlm.

49

The pore size distribution in a soil determines the water and oxygen supply to plant roots, and these aspects will be considered in more detail below.

6.1 Water Retention

The supply of water to plants is determined by the water storage capacity of the soil, its ability to be replenished from surface applied water, its internal drainage, and the depth and distribution of the root system.

Ma.ss Relations

Figure 6 Schematic diagram of the soil as a three phase system. V and M denote volume and mass whereas subscripts s,w,a,t, refer to soil solids, water, air and total soil respectively

In discussing the water holding characteristics of soils, it is useful to refer to a schematic diagram of soil as a three phase system (Figure 6). One can now define the following -

Water content by weight of soil solids (6g)

Water content per volume of soil (6v)

where Vt = Vs + Vp

=

=

Mass of water (Mw~ ·1 Mass of oven dry soil ~ g g

Volume of water (V w>­Volume of soil (Vt)

(2)

The equivalent depth of water (D) stored in the root zone of depth (Z) be readily calculated from the volumetric water content thus -

D=6v xZcm (4)

50

It can be shown that

= (5)

where BD is the bulk density of the soil and pH20 is the density of water.

Bulk density (dry) = Mass. of oven tcv soil (Ms) g cm-3

Volume, of SOl t) (6)

Note that the bulk density of mineral soils may range from 0.9 to 1.8 g cm-3, whereas the absolute density of soil particles (AD) is about 2.6g cm-3•

The total porosity (f) of a soil is the volume that is available to be filled with air arid water, VIZ.

f =

= 1 _ BD AD

(7)

,;, (8)

The fractional air-filled void space (E) corresponding to a given volumetric water content (av)

is given by

(9)

As a rule of thumb, an air-filled void space of 0.10 cm3 cm-3 is considered as the minimum necessary for adequate aeration for plant roots. .'

,:. - . . ~

Field capacity and permanent wilting point represent the approximate upper and lower limit of water that is held in soils and vvhich is available for plants. The magnitude of these lirilits depends on the total porosity, pore size distrib~tion and ~~:ture.

Immediately after substantial rainfallill- irrigation, a soil;s pore space will be filled withwater (saturation), but the macropores in soils drai1freadily. The drainage often slows after 24-hours, and the water content at this time is arbitrarily considered to be the field capacity. In practice, intermediate-sized pores continue to drain under, the influence of gravity. An approximation to the field capacity value obtained in the field after 24 hours is to me~ure the water content of an undisturbed soil core on a pressure plate at 0.1 bar suction.

As the water content of a soil decreases through drainage, evaporation and plant extraction, a point is reached where the remaining water is held in films and small capillary pores with such energy that roots have difficulty extracting it This water content is the permanent wilting point and can be determined by measurement of the water content of soil exposed to a 15 bar suction on a pressure plate. The difference between this' value and the field capacity represents the available water capacity. The available water capacity tends to be highest for intermediate textured soils, although the degree of aggregation of the soil is also important (Table 4).

51

It is relevant to note here that all the water held in the available water range is not equally available to plants. As soil dries around plant roots, water film become discontinuous, and the rate of conduction of water to be root slows.

Table 4 Influence of field texture and structure on the water retained between -0.1 bar and -15 bar for a range of Australian soils (after Gardner 1988)

Water retained mm/lOcm Field texture class

Well structured soils Structureless soils

Sands 16 10

Sandy loams 27 12

Loams 24 13

Clay loams 19 13

Light clays 17 12

Medium-heavy clays 12 11

Self mulching clays 20 15

6.2 Water Movement

Water moves in soil in three ways, viz. saturated flow, unsaturated flow and vapour movement. The flow of liquid water is due to a gradient in matric potential from one zone to another in soil, the direction of flow being from a zone of higher moisture potential to one of lower moisture potential. Saturated flow occurs when the soil pores are completely filled with water, whereas unsaturated flow occurs when the pores are only partially filled with water. In soils, water vapour moves very slowly primarily because the differences in vapour concentration are small; even at permanent wilting point, the relative humidity in the soils is near 98%. Vapour movement in soils is negligible relative to liquid movement.

The infiltration rate of water into a soil is initially high for all soils if they are dry but, once the soils are wet, the rate depends on the distribution, continuity and stability of the macropores. Very permeable soils may have infiltration rates as high as 1O-2cm sec-l, whereas soils of low permeability will have rates of lO-scm sec-lor less.

The infiltration rate of a soil can only be maintained if the system of macropores is maintained. The zone where this system is most likely to collapse is at the surface of the soil. Wet aggregates at the soil surface are very prone to disruption by falling raindrops, and the smaller units tend to clog the macropores. Soils high in exchangeable sodium are particularly prone to surface sealing, which results from aggregate breakdown as a result of dispersion of the clay colloid. In swelling soils, the infiltration rate maybe initially high as water enters large cracks, but the rate rapidly slows as inter- and intra-crystalline swelling involving layer alumino-silicates closes these cracks.

52

Once water has entered the surface of a soil, the flow rate depends on the driving force (potential gradient) and the ease of flow through the medium (hydraulic conductivity), and can be expressed by Darcy's Law -

Q = K ~ dx

where Q is the amount of water passing per unit cross-sectional area per unit time, K is the hydraulic conductivity, and d'l'/dx is the change in potential per unit distance along the direction of flow. For saturated soils, K can be considered to be essentially constant and dependent on the size and configuration of the pores whereas, for unsaturated soils, K decreases with water content.

The hydraulic conductivity of saturated soils is most influenced by texture and structure. Sandy soils have higher hydraulic conductivities than finer-textured soils. Soils, which have an aggregate structure which is stable on wetting, also conduct water more rapidly than 'those in which the aggregates break down into smaller units which clog existing pores.

In unsaturated soils, water movement is slower than in saturated soils since most of the water movement is via capillary pores. At high potentia1.1evels (high water contents), hydraulic conductivity is higher in coarse-textured soils than in clay soils, whereas the opposite is true at low potential values (low water contents). This relationship is observed because the dominance of macro-pores in coarse-textured soils benefits saturated flow, whereas the predominance of capillary pores in clay soils contributes to more unsaturated flow than in sands.

6.3 Compression

Plant roots can only grow in soils if they can (1) elongate and expand within the available macropore space, (2) elastically deform the soil or (3) shear the soil both ahead and behind the root tip. A simple test to measure soil strength involves measuring the force required to push a metal probe (penetrometer) into the soil. This test shows that root growth is significantly reduced when impedances reach 2 - 3 MPa (20-30 bars). High mechanical impedances can be generated in soils deformed by excessive loads as a result of traffic by farm or other machinery. In the surface soil, compaction is usually confined to wheel tracks, whereas plough pans can build up over a long period of time and provide a uniform barrier to root growth.

Compaction of soil by traffic can also reduce root growth by reducing macroporosity and gaseous exchange. As indicated previously, an air-filled porosity of 0.10 cm3 cm-3 is considered a minimum for adequate aeration for plant roots.

7 CONCLUSION

This paper on the basic properties of soils has necessarily been of an intrOductory nature to cater for those without previous training in soil science. Several of the aspects discussed here are brought out again by subsequent lecturers in this course; this repetition is needed for their

53

coherent presentations, and one hopes this will help emphasise the importance of these most significant soil properties. For those wishing to gain a more in-depth understanding of the topics covered in this paper, texts such as those by Brady (1990) or Wild (1988) should be consulted in the first instance. A large number of specialist texts on soil mineralogy, chemistry, physics and biology is also available.

8 REFERENCES

Brady, N.C. (1990). The Nature and Properties of Soils. Tenth Edition.. Macmillan Publishing Company, New York.

Gardner, E.A. (1988). Soil water. In 'Understanding Soils and Soil Data' (Ed. I.F. Fergus), pp. 153-185. (Australian Society of Soil Science Incorporated, Queensland Branch: Brisbane).

Gunn, R.H., Beattie, I.A., Reid, R.E., and van de Graaff, R.H.M. (Eds.) (1988). Australian Soil and Land Survey Handbook: Guidelines for Conducting Surveys. (Inkata Press, Melbourne).

Thompson, C.H. (1988). Soil distribution: perception and portrayaL In 'Understanding· Soils and Soil Data' (Ed. I.F. Fergus), pp. 29-39 (Australian Society of Soil Science Incorporated, Queensland Branch: Brisbane).

Wild, A. (Ed.) (1988). Russell's Soil Conditions and Plant Growth. (Longman Scientific and Technical, Harlow).

54

APPLICATION OF SOIL SCIENCE CONCEPTS TO REAL SOILS

B.J. Bridgel and M.E. Proberf

1 CSIRO Division of Soils, Queensland Wheat Research Institute, PO Box 2282, Toowoomba, Qld, 4350

2 CSIRO Division of Tropical Crops and Pastures, Cunningham Laboratory, St Lucia, Qld, 4067

ABSTRACT

This paper outlines the various soil classification schemes that have been used for Australian soils. These are the Great Soil Groups, the Northcote Factual Key, and Isbell's new classification system. The relationship between these schemes and overseas ones, such as Soil Taxonomy (USA) and the World Soil Map (FAO-UNESCO) are examined. A brief summary of the Unified Soil Classification used by engineers is presented.

The pedological description and associated physico-chemical data are presented for four contrasting soils. The interpretations of this information is described in some detail and applied to various land use options, including agriculture, engineering structures and waste disposal.

1 INTRODUCTION

Soil science has its own jargon; at least as much as any other discipline, and maybe more. Our aim in this paper is to give an outline of what you will encounter if you go seeking information about soils. We will show how such information is reported, what is reported, and how one can use it to predict the behaviour of solutes and water in soils and the suitability of a soil for a particular land use.

2 SOIL CLASSIFICATION

Perhaps the first surprise is that there is no universally accepted world-wide system of soil classification. One reason for this is that soils tend to be local and therefore a particular regIOn or country is more concerned about its own soils than those of the rest of the world. However, even within Australia there are several different classification systems in use.

The purpose of classifying any objects (including soils) is to organise knowledge and facilitate communication. There is an expectation that units which are similarly classified (and named) share similar properties, so that the use of a name conveys information about the unit. In the case of soils, it may be possible to infer something about the uses to which a particularly named soil may be put, what it is unsuited for, or special management needs that exist. A classification system is a prerequisite for producing a soil map, which is devised to best represent the natural occurrence of soils in the landscape. However, it is very important to understand the difference between named soil types (or other taxonomic

55

units) and soil mapping units. Map units are of necessity 'impure' in the sense that they almost always contain soils outside the range permitted by definition for a specific taxonomic unit. Thus a soil map indicates what soil (or group of soils) is likely to occur in a given area, but it cannot reveal exactly what soil will be found at a given location.

Our knowledge of soils is growing all the time, particularly in regions where little was known a few decades ago. In addition, more analytical data is being provided for all our soils. It is this increasing knowledge that has required new systems for classifying soils. You are referred to the papers of Moore et al. (1983) and Isbell (1988) for detailed information on soil classification and its complexity. These papers are the basis of the material presented here, which of necessity must be brief.

In Australia, two soil classification schemes have been in use. These are the Great Soil Group classification (Stace et al., 1968) and "A Factual Key for the Recognition of Australian Soils" by Northcote (1979). Now a third one is being introduced and developed by R.F. Isbell of CSIRO Division of Soils (Technical Report 1/1992, unpublished). Two overseas classifications have gained a degree of international recognition, so they are often encountered in publications directed to an international readership. One of these is the 'World Soil Map Legend' (FAO-UNESCO, 1974) which was designed to facilitate the mapping of the soils of the world at a small scale (1:5,000,000). Of all the schemes we shall mention, this is the only one which was conceived from the start as a classification to encompass soils on a global scale. The second is the US Soil Taxonomy (Soil Survey Staff 1975); this was devised originally for the soils of the United States of America, but with considerable input from Western Europe. It has become a de facto international classification. All of these schemes classify the soil profile. Beyond that, there are pronounced differences.

3 CURRENT AUSTRALIAN CLASSIFICATION SYSTEMS

3.1 The Great Soil Group Scheme

The evolution of this scheme can be traced from the paper by Prescott (1931), through the publication by Stephens (1953), and culminating in "A Handbook of Australian Soils" (Stace et al. 1968). The latter work contains general descriptions of 43 Great Soil Groups, together with a large body of laboratory data.

Although Stephens' "Manual of Australian Soils" and the "Handbook" are nominally many level hierarchical classifications, in practice they have been used only at the Great Soil Group level. In essence, the scheme depends on there being 'modal classes', but the central concepts of these classes have never been defined. Furthermore, the classes cannot have clear and unambiguous boundaries; no key has been constructed so that identification is highly subjective.

Its main advantage was in the fact that it was widely used for many years, so that many people were familiar with the main properties of many of the groups. However, a serious disadvantage was its failure to cater for large numbers of Australian soil profiles. Without updating its original database and modifying or expanding the scheme, it was doomed to become obsolete.

56

3.2 Northcote Factual Key classification

This was the system under which the soils of Australia were mapped at a scale of 1:2,000,000. This "Atlas of Australian Soils" project (Northcote et al. 1960-1968) also resulted in the coining of some largely descriptive class names for what in the Key are represented as simple code symbols. The Factual Key was complemented by the publication of "A Description of Australian soils" (Northcote et al. 1975) which provided descriptive data for the principal profile forms, and further formalised some descriptive class names. The latter did not always conform with those used earlier in the Atlas map sheets.

The Key is a bifurcating hierarchical scheme with five categorical levels. These are divisions, subdivisions, sections, classes and principal profile forms. All classes are mutually exclusive and all the keying attributes can be determined in the field. These are all morphological, with the exception of pH and presence of carbonate as determined by a simple field kit.

The nomenclature is a code that tends to be confusing to those who are unfamiliar with the scheme. The alphabetic characters in the code are mnemonic and do have consistent meanings, but the numeric digits do not. Emphasis is given to surface and near surface horizons, which can lead to classification difficulties in disturbed soils.

Non-identification or mis-identification is virtually impossible because of the exclusive nature of the largely binary division. However, some new soil profile descriptions need to have new classes created for them.

Table 1 is a small extract of the large table given by Moore et al. (1983) to provide approximate correlations between the Great Soil Groups of Stace et al. (1968), the Principal Profile Forms or classes of Northcote (1979), the Great Groups of "Soil Taxonomy" (Soil Survey Staff 1975) and the soil units of the 'World Soil Map' (FAO­UNESCO, 1974). It should be noted that a revised legend has been published for the latter (FAO-UNESCO 1988). The point to be stressed in this comparison isthat there are no direct equivalents between the various classifications.

3.3 A new classification system for Australian soils

An Australia-wide committee has been working on a new soil classification for several years (R.F. Isbell CSIRO Division of Soils Technical Report 1/1992, unpublished). This new scheme is now at its "Second Approximation". The Technical Committee on Soil Conservation recommended to the Standing Committee on Soil Conservation (SCSC) in July 1991 that all States should formally adopt the new soil classification system. In response the SCSC noted that 'all States and Territories will test the new Australian Soil Classification System'.

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Table 1 Approximate correlations between various soil classification schemes. Those in boid are expected to be the most common correlatives. Extract from table of Moore et aL (1983).

Great Soil Group Factual Key Soil World Soil Map Taxonomy

Black earth Ug 5.1, Ug 5.4 Pellustert Pellic Vertisol . Chromustert Chrumic Vertisol Terrert

Solodised solonetz Dr 2; Dr 3; Db 1; Db 2; Natrustalf Orthic Solonetz and solodic. soils Dy 2; Dy 3; Dy 5; Natrixeralf Albic Luvisol

Dd 1: Dd 1.33; Dd 1.43 Paleustalf Solodic Planosol Haplustalf

Krasnozem Uf 5.21; On 3.11; Paleustalf Eutric Nitosol· On 4.11; Gn 4.31 Acrohumox Humic Nitosol

Palehumult Humic Ferralsol Haplohumox Rhodic Ferralsol Acrorthox Humic Acrisol Eutrustox

Yellow podzolic Gn 2.31; Gn 2.71; Gn 3.84; Haplustalf Albic Luvisol soils Dy 2.21; Dy 2.22; Dy 2.31; Haplustult Orthic Acrisol

Dy 2.41; Dy 3.21; Dy 3.31; Paleustalf Ferric Acrisol Dy 3.41; Dy 3.61; Dy5.41; Haploxeralf Dy 5.81 Paleustult

The new classification has an hierarchical framework of order, suborder, great group, subgroup and family. It has borrowed numerous concepts from other classification schemes. A change from previous Australian schemes is the use of laboratory data at some levels in a number of orders. A diagrammatic view of the scheme at the order level is shown in Figure 1. The nomenclature can be recognised by the fact that all orders end in -OSOL.

•. Isbell (unpublished) provides comments for e.ach of the orders in terms of relations to the other Australian classification, that is, Great Soil Groups and the Factual Key.

4 ENGINEERING CLASSIFICATIONS

The classification of soils as engineering materials has evolved over a long period of time, particularly through the efforts of Atterberg (1913) and Casagrande (1947). Engineering classifications are pragmatic, deal only with the soil material of interest, and have rio generic or profile application. There are two schemes in current use.

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Figure 1

ALL SOILS

Organic soil material

Negligible pedological organization

Weak pedological organization

Bs, Bh, or Bhs horizons

Clay ;:>: 35 %,cracks, slickensides

Prolonged seasonal saturation

Strong texture-contrast

Sodie B horizon pH ~5.5 in B horizon

r , SODOSOLS CHROMOSOLS

Lacking strong texture-contrast

.. -

..

OR GANOSOLS

RU DOSOLS

TE NOSOLS

PO DOSOLS

VE RTOSOLS

BY DROSOLS

pH Bh

< 5.5 in orizon

KUROSO LS

Calcareous Structured, high Structured Massive B horizon throughout iron B horizon B horizon

r r

CALCAROSOLS FERROSOLS DERMOSOLS

Man-made soils

r OTHER SOILS - Please reCer any examples oC Ihese soils

to the Author to allow revision oC the present scheme.

KAND OSOLS

... A NTHROPOSOLS

Schematic summary of the orders in 'A Classification System for Australian Soils' (Isbell unpubl.).

The first scheme is one based on visual inspection and simple identification tests and is presented in the Earth Manual of the US Bureau of Reclamation (1960). In this scheme, the first major division is on particle size assessment, that is, gravels, sands, silts and clays. Gravels and sands are subdivided according to the presence of 'fines' and grading. Silts and clays are subdivided according to dry strength, plasticity and dilatancy (swelling) and organic content. These tests can give a qualitative assessment of the soil materials behaviour under load.

The second scheme is the "Unified Soil Classification" based on the work of Casagrande (1947). It was adopted in 1952 by the US Bureau of Reclamation and is used by engineers in Australia. It is similar to the scheme based on visual inspection, but is based on laboratory data. The first major division is on particle size sieve analysis into gravels, sands, silts and clays. Gravels and sands are then subdivided on the content of 'fines' and particle size grading. Silts and clays are subdivided according to their liquid and plastic limits (Atterberg limits) while organic soils are classified separately.

The subdivision of silts and clays according to the Atterberg limits is gone into quite thoroughly, various formulas and graphs being given in conjunction with the classification

59

table. The emphasis is on predicting the behaviour of saturated clays under loading for supporting structure, building dams and being held by retaining walls.

Both classification schemes are presented by Kezdi (1974), the details being provided in his tables 13 and 14 respectively.

5 SOIL PROFILE DESCRIPTIONS

In any published soil survey, a handbook or manual accompanies the soil map and contains a great deal of geographical information (such as climate, topography and land use) about the area that has been surveyed. It also contains a series of descriptions of soil profiles that are the type sites for the mapping units, or a range of profiles within the mapping units. The profile descriptions are what the pedologist sees when the soil is examined using an auger, excavated pit, coring rig or convenient road cutting. There is some jargon concerning texture and structure, but this is not difficult to understand. You are referred to McDonald et al. (1990) for details on how to describe a soil.

Before continuing, you are advised to refer to the exemplar descriptions for four contrasting soil profiles and associated data that have been reproduced from "A Handbook of Australian Soils", and follow through the headings of the first example.

When describing a soil, the pedologist takes note of a number of geographic features about its location:

(i) Position and elevation. These are recorded so that the site can be located again, and also related to the nearest climatic information.

(ii) Topography. This is of great importance in defining the drainage and aeration of a soil and whether there is soil movement down a slope. Soils at the top of a slope tend to be well drained and more leached than those further down the slope. Soils at the bottom of a slope tend to be wetter and less aerated. These conditions affect the colour of soil, particularly those that contain iron minerals. Micro-relief, particularly gilgai, indicates phenomena such as swelling and shrinking.

(iii) Climate. Temperature, rainfall and evaporation all affect soil profile development through leaching and weathering. Production of clay minerals and their movement through processes of illuviation lead to a wide variety of soil classes.

(iv) Parent material. The parent rock or weathering products such as colluvium and alluvium provide the mineral fabric for the leaching and weathering processes to act on. Soil developed on acidic rocks such as granite tend to be sandy or light textured, while those developed on basic rocks such as basalt tend to be clayey or heavy textured.

(v) Profile drainage. This is estimated by the pedologist on the basis of experience and visual evidence obtained from the soil colour. Sandy soils tend to drain quickly unless impeded by a deeper clay horizon, while clay soils tend to drain slowly unless well structured to depth in the profile. Poor drainage creates anaerobic conditions and reducing conditions in the subsoil during wet conditions and results in 'gleying' and mottling.

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(vi) Native vegetation. The native vegetation community integrates the properties of the soil profile and the local climate. It reflects the nutrient status, water availability and aeration status of the soil. Large trees tend to grow on well watered deep soil profiles that are well drained. Ti-tree scrub grows on waterlogged soils with greatly impeded drainage. Some of the best grassland occurs on the fertile black cracking clays in a semi-arid environment. Stunted trees and thin grassland occur on soils of poor nutrient status in dry climates, culminating in desert scrub at the extreme. Vegetation communities in a district closely follow soil boundaries and are used by pedologists to define those boundaries on aerial photographs.

(vii) Land use. Where land has been cleared, the land use broadly reflects the type of soil being farmed. Cropping soils usually have good nutrient status and good water availability. Soils used for extensive grazing tend to be poorer and/or drier. Land use can be modified by farming. practices such as fallowing, fertilising and irrigation.

The soil profile description reports on the morphological properties of a sequence· of soil horizons (layers) which are defined in terms of depth from the soil surface. These may be at fixed depth intervals unless there is a clear change in properties.

(i) Colour. The colour of selected soil clods or fragments is determined using a Munsell colour chart which has a set of reference colour chips for comparison. Colour is affected by water content and this is recorded if possible, together with mottling and other variations.

(ii) Texture. This is estimated by manipulating the soil between the fingers and thumb. Sandy or light textured soils have little cohesiveness and do not roll into threads of 3 mm diameter. Loam soils will roll into threads, but these lack plasticity and will fracture if bent. Clay or heavy textured soils form threads easily and these can be bent into a ring. Water content is critical for determining texture, the soil under manipulation should be moist but not wet.

(iii) Structure. This is estimated by gently breaking a piece of the undisturbed soil, either by hand or dropping to the ground, and looking at the size and shape of the aggregates that resist this treatment. Examination with hand lens may be necessary. Soils can be weakly or strongly structured, the latter having easily discernible aggregates of similar size. Aggregate shape is important, with six main shapes being described.

Crumb. Distinctly rounded aggregates, often loosely bound together. Common in top soils that are high in organic matter.

Blocky. Roughly cubic aggregates with distinct faces and sharp edges. Common in the A horizon of clay soils.

Subangular blocky. As above, but without sharp edges and the 'comers knocked off'.

Parallelipipedal. Aggregates with distinct faces and sharp edges, but with the angles between faces not at 900. Common in swelling clay soils below the A

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horizon, extreme examples being the lozenge shaped aggregates found in slickensides at depths of 1 m and below.

Platy. Aggregates whose vertical height is much less than horizontal width. Sometimes found in cultivated soils where shear strain has modified the structure.

Columnar. Aggregates whose vertical height is much greater than horizontal width ('prismatic'). Often found in clay B horizons where exchangeable sodium is high.

(iv) Concretions. These can be carbonate, gypsum or iron and manganese nodules, either hard or soft. They are formed by the leaching of salts or weathering products from the upper horizon(s) and precipitation deeper in the profile. They are common in semi-arid or arid soils, and a distinct concretionary layer can indicate the seasonal depth of wetting under native vegetation.

(v) Biological activity. Plant roots, insect ,and earthworm channels are recorded. Such biological activity can affect infiltration and water storage in the soil.

6 PHYSICO-CHEMICAL DATA

Although the pedologist is highly skilled in recognising different soils and those factors that will affect their productivity, quantitative information is needed to confirm. and support the field descriptions. Selected profiles are analysed in the laboratory for their physical and chemical properties, which can be interpreted by non-pedologists for their own use. Reported data may include pH, salinity, total nutrients,· exchange capacity, particle size, mineralogy, and water retention characteristics.

(i) pH. This is a measure of the acidity or alkalinity of the soiL It is generally considered that for agricultural crops the desirable pH is neutral to slightly acid (pH = 6-7). Extremely acid soils can have toxic levels of AI andMn, while extremely alkaline soils can have poor availability of plant nutrients such as Zn and P. This is an important consideration in the disposal of acid mine drainage and alkaline slime ponds.

The pH of a soil is best measured on a saturation extract, but this requires hand mixing and suction filtration which is a slow and laborious procedure. Usually a 1:5 soil water suspension is measured directly with a glass-reference electrode assembly. The pH value thus obtained can be affected by salt content and is often unstable. Use of a 0.01 M solution of CaCl2 avoids such problems. In many soils the pH in CaCl2 may be 0.5-1.0 less than in water.

(ii) Air dry water content. .. This is recorded as a correction for weighing out samples of mineral soil for further analysis. It is related to clay content and can range from <1 % for sandy soils to over 12% for soils with >60% clay.

(iii) Soluble salt content. This gives an indication of the salinity of the soil, which must be low for good plant growth. Unfortunately, the critical limit of soluble salt above which crops are affected varies with soil texture. A swelling clay soil with a high saturated water content of 0.75 g g-l would have a critical limit of 0.2%, while a sandy soil with a

62

saturated water content of 0.25 g g-l would have a critical limit of 0.05%. It is much better to use the electrical conductivity (EC) of an extract of the saturated soil (the 'saturation extract') where the corresponding critical limit, applicable to all soils, is EC = 4 dS mol. The reader is referred to Richards et al. (1954) for comprehensive details. The electrical conductivity is sometimes multiplied by an appropriate factor and reported as total soluble salt content.

(iv) Chloride content. Sodium chloride (NaCl) is the world's most common soluble salt. The chloride ion (Cn is easily determined, so that a comparison between NaCI and total soluble salt content estimates the content of other salts in the soil. The most common of these are calcium and magnesium carbonates, bicarbonates and sulphates, with potassium occurring to a lesser extent. Chlorides are the dominant salts in leached profiles, while carbonates and sulphates dominate in little leached profiles and many reach sufficiently high concentrations that calcium carbonate or gypsum is precipitated.

(v) Los,S on ignition. This is an easily performed test that is related to, but not a good estimate of, the organic matter in the soil. It is unlikely to be determined now that better methods are available for determining soil organic matter.

(vi) CaC03 content. Presence of CaC03 in the soil indicates a little or mildly leached profile. An appearance of significant CaC03 at a particular depth in the profile indicates the depth of seasonal wetting in a semi-arid environment, there being little wetting below that depth. A change in land use, for example, from virgin grassland or woodland to a crop-fallow system, can significantly increase the depth of seasonal wetting. However, translocation of CaC03 downwards under such a new regime would be slow because of its low solubility.

(vii) Organic matter. Organic matter is considered a very important constituent of the soil for the following reasons.

(a) It can impart a good structure to the soil by means of organic-clay bonds. Such bonding can decrease the stickiness of clay soils and increase their friability.

(b) It can increase the exchange capacity of sandy soils, and

(c) It provides a source of plant nutrients, particularly nitrogen.

It is generally considered that a minimum organic matter content of 3% is required for good structure and nutrient supply in the surface horizon, while 5% is considered desirable. Structural problems become severe if the organic matter content falls below 1.5%, commonly resulting in hard setting and surface crusting with poor infiltration.

Modern practice reports figures for organic carbon rather than organic matter. The conversion is OM = 1.72 x organic C (Moody and Bruce 1988).

(viii) Nutrient content: Nitrogen, phosphorus, potassium and sulphur. These elements are important for plant nutrition, but the total element contents reported are not a good indication of the availability of these elements; they provide information on the

63

nature of the soil organic matter (C:N ratio) and the nutrient content of the parent material. Measures of extractable (available) phosphorus and exchangeable potassium are interpretable in terms of plant available nutrients, but beware that different extractants may be used for "available" soil P ~ The most important deficiencies in Australian soils are nitrogen and phosphorus and the reader is referred to Ladd and Russell (1983), Probert (1983), and Williams and Raupach (1983) for details concerning Australian soils.

(ix) Particle size. The mineral fraction of the soil is graded into five particle size classes, and we use the international standards in Australia. These are gravel (G) > 2 mm diameter, coarse sand (SC)0.2-2mm diameter, fine sand (FS) 0.02-0.2 mm diameter, silt (Si or Z) 0.002-0.02 mm diameter and clay (C) < 0.002 mm diameter. The USA standard puts the division between silt and fine sand at 0.05 mm diameter.

Particle size controls of soil's texture. Two simple triangular texture diagrams are shown in Figure 2 (Kezdi, 1974) which illustrate how this works. Note that there is a change from loam texture to clay texture at 30% clay content. At lower clay contents, the mineral skeleton of the soil is rigid, for it is dominated by the large sand particles (usually quartz) which are in contact with each other. Above 30% clay content, the sand grains are not in contact and the mineral skeleton of the soil is dominated by the small clay particles. The latter have properties of plasticity, stickiness and swelling. The silt content does not become important until it exceeds 50%. Silty soils are cohesive, but lack the plasticity of clays.

Figure 2

o lOa Olmm>d>0006mm (%)

100 BO 60 M 20 ° ,0 ~-r-""~--r-----;r--;--.,,.-~--,-----. 100

Lean Clay and stlt

Triangular texture diagrams (Kezdi, 1974)

64

The clay content is important for plant nutrition, for clay minerals consist of crystalline plates that are negatively charged and retain adsorbed cations on their surfaces, such as Ca++, Mg++, K+, NH;, Zn++, etc. This capacity for adsorbing cations, called the 'exchange capacity', is greatly dependent on clay mineralogy as well as clay content.

(x) Cation exchange capacity (CEC). As mentioned above, the exchange capacity of a soil is of great importance in retaining cationic plant nutrients. The exchange capacity depends on the clay content, clay mineralogy and organic matter content. Typical values are listed as follows (Norrish and Pickering, 1983):

Organic matter 50-300 me% (cmol[i+]kg- l )

Smectites (Montmorillonite) 100-150 me% Illites (mica types) 10-30 me% Kaolinites 1-10 me% Interstratified (K-S) 10-80 me%

For a high cation exchange capacity, a soil has to have a significant content of smectitic clay. This clay has a three-layer crystalline structure and exhibits a high degree of swelling when wet. Soils with predominantly kaolinitic clays of two-layer crystalline structure have a low CEC and show little swelling. They need significant amounts of organic matter to have reasonable exchange capacities. Soils with CEC's <10% are quite poor at holding cationic plant nutrients.

(xi) Exchangeable cations. The routinely measured exchangeable cations are Ca++, Mg++, K+ and Na+. The first three are important for good plant growth, while the ratio of exchangeable Na+ to Ca++ and Mg++ is of great importance in soil structure. Exchangeable Ca++ has the important effect of flocculating or "clumping together" individual clay particles, imparting a stable structure to the soil through interparticle and interaggregate bonds (Emerson, 1983). This structure creates pore space that facilitates the growth of plant roots and the movement of water and air through the soil.

The level of exchangeable Na+ in the soil above which physical problems appear is quite low. It is usually agreed that an exchangeable sodium percentage (ESP = exch Na+/CEC x 100%) greater than 15% is undesirable in all soils (Richards et al. 1954). Australian work points to a lower limit of 6% for clay soils where the clay content >30% (Northcote and Skene 1972; Loveday and Bridge 1983).

It would be expected that the sum of the exchangeable cations would approximately equal the total cation exchange capacity. In neutral to alkaline soils, this is so. In acid soils the summation should include the exchangeable acidity which is not always measured. However, particularly in soils where iron and aluminium sesquioxides are present, the traditional methods of measuring CEC, using concentrated salt solutions at pH 7 or higher, yield unrealistic results. Such soils are said to have variable charge clays, and modern methods have been developed to determine CEC's appropriate to the field conditions. Meanwhile, older data for such acidic soils show CEC to be considerably greater than L cations, and the reported CEC is highly questionable. To calculate ESP's for these soils it is best to use L cations in place of CEC.

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(xii) Available phosphorus. In these examples, the phosphorus extracted with a O.5M solution of sodium bicarbonate adjusted to pH = 8.5 is reported (Colwell, 1983). However in other reports different extractants may have been used and it is not a simple matter to make comparisons between "available P" measured with differing methodologies. These tests .. are used as a guide for fertiliser requirements, being much better than total phosphorus.

(xiii) Silicate analysis. This is carried out on the total soil fraction and indicates the minerals in the soil that are residual from the parent rock. It also gives another estimate of total (but not available) nutrients. The most common minerals involve the elements silicon, aluminium and iron. X-ray fluorescence or spectrographic methods of analysis are customarily used.

(iv) Mineralogical analysis of clay fraction. This is carried out using X-ray diffraction methods. The most common clay minerals are smectite, illite, kaolinite, K-S interstratified, vermiculite and quartz. Others include aluminium minerals such as gibbsite and iron minerals such as haematite, which are often found in highly weathered soils developed on basalt. Mineralogical analyses are not always available because of the specialised X-ray equipment needed.

(xv) Soil moisture characteristics and bulk density. The water retained at a suction of 0.1 bar is measured on natural aggregates of soil using a porous suction plate, and is an estimate of the . 'field capacity' or 'drained upper limit' of the soil. The 15 bar water content is measured on a ground and sieved sample of soil using a pressure membrane apparatus, and estimates the 'wilting point' or 'lower limit' of water in the soil. The difference between the two water contents is an estimate of the 'available water' for plant growth. In practice, other factors such as salinity, root density and evapotranspiration rate can alter this value. For a discussion on plant available water capacity, the reader is referred to Gardner (1988).

Bulk density is the weight of dry soil per unit volume and is measured by means of a 10 cm diameter thin-walled sampling tube. It is used to convert soil moisture contents measured by drying and weighing to volumetric values. Bulk densities greater than 1.6 g cm-3 are considered to hinder the growth of plant roots.

7 INTERPRETATION OF PROFILE DESCRIPTIONS AND PHYSICO .. CHEMICAL DATA

Four contrasting soil profiles have been selected from "A Handbook of Australian Soils" (Stace et al., 1968), and the appropriate data are reproduced from that source in the

. booklet of supplementary information for this course. These profiles represent large areas of similar soils, are derived from different parent materials and climates, and illustrate a wide range of soil characteristics. The geographical data, soil profile description and physico-chemical data for each soil will be discussed and then used to comment on the suitability of the soil for projected land uses.

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7.1 First example: Profile B560 (pp 120-3) Great Soil Group - Black Earth Principal Profile Form - Ug 5.16

This is an example of a mildly leached dark soil with a uniformly textured profile.

Examining the geographic data for this soil gives us the following useful information:

(i) The soil is situated west of the Great Dividing Range where rainfall is 660 mm per year and less than half the potential evaporation. This, in combination with the very slow profile drainage, would indicate that the profile is not highly leached.

(ii) The basalt parent material indicates that the soil would be of clay texture and of high fertility. This is reflected by its use for grain cropping.

(iii) The gilgai micro-relief indicates that the soil swells and shrinks considerably; the different vegetation on the puff and shelf reflecting differences in profile hydrology.

This description refers to the profile sampled on the shelf of the linear gilgai, the soil on the adjacent mound being described separately. We see that the soil is dark coloured, heavy textured and strongly structured to at least 3 m depth. The top 10 cm has a granular (crumb) to fine blocky structure with a thin brittle crust, the structure becoming coarser and the colour darker with depth. At 60 cm, the structure becomes parallelepipedal and carbonate concretions appear. At 90 cm, mottling appears and there are many carbonate concretions, indicating the seasonal depth of wetting. Plant roots are numerous in the top 60 cm and extend to 90 cm. Faunal channels extend to 175 cm depth. The profile continues to be heavy textured and strongly structured to the depth of sampling at 315 cm, a change of parent material being noted at 300 cm.

Looking at the physico-chemical analyses, we can make the following observations:

(i) The pH is neutral at the surface, becoming more alkaline with depth.

(ii) Soluble salts are very low to 30 cm depth, moderate to 83 cm depth and quite high below that depth. This confirms that the seasonal depth of wetting only extends to ",90 cm.

(iii) The chloride profile shows that about half the salt content to 83 cm depth is NaCI, decreasing to < 10% of the total salts below the depth. This is consistent with the observed carbonate and gypsum in the profile.

(iv) Organic matter levels are high (> 3%) to at least 45 cm depth. This contributes to the strong structure of the soil and its natural fertility.

(v) Total nitrogen levels are high in the upper 30 cm or so of the profile, contributing to the high fertility of this soil. Exchangeable potassium and available P in the surface soil layers are adequate for crop nutrition.

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(vi) The particle size analysis shows the high clay content of this soil, which increases from 38% at the smface to 56% at 45 cm depth and below. There is very little . coarse sand present.

(vii) The exchange capacity of this soil is high, allowing it to retain cationic nutrients very well. The exchangeable cations are dominated by Ca++ and Mg++, with Na+ becoming important below 30 cm depth. At 83 cm, the soil is quite sodic, with an ESP >20%. This is balanced by an increasing salt content and is not important as long as this section of the profile is not brought to the smface.

(viii) The mineral fraction of the soil is dominated by silicates and aluminates, with some iron oxides. The clay fraction is mostly montrnorillanite (smectite) which agrees with its swelling properties and high exchange capacity.

The usefulness of these black earths for grain cropping has been known for a long time. Their fine granular smface structure, high fertility and high water holding capacity make them very suitable for this purpose. However, after many years of cultivation two problems have appeared.

The first is soil erosion. Normally, these soils are cracked when dry and storm rainfall can run down the cracks and hence infiltrate into the profile. Eventually the cracks close because of swelling and the very slow drainage through the profile cannot cope with prolonged rainfall. Runoff· occurs and flows down the pediment slope. Where there is native vegetation the soil smface is protected, resulting in little erosion. If the soil is bare, raindrop action forms a smface crust, further reducing infiltration and increasing runoff. The result is rilling and gullying. The soil that is lost is from the A horizon is the richest in organic matter and nutrients. Erosion can be controlled by engineering works such as contour banks and grassed waterways, and can be greatly reduced in situ by maintaining smface cover. Zero-till and stubble retention methods, double cropping and the use of pasture leys all help in this regard.

The second problem is fertility decline. Eventually the high initial levels of nitrogen in the soil decrease as the organic matter is mineralised and used by crops. When it is 'mined out', crop yields and grain quality fall. Fertilisers become necessary, increasing costs of proouction, or leguminous pasture leys grown in rotation with crops.

There are a number of engineering problems associated with black earths. Firstly, their swelling properties and plasticity (classifying theIll as 'fat clays') make good foundation design for structures imperative. Deep piling and 'rafting' techniques for buildings are common. They are also expensive. Differential soil movement, as indicated by the gilgai micro-relief, can slowly damage roads, cause telegraph poles to lean and snap expensive opticaitransmission cables unless good (and expensive) design criteria are used. Roadworks are a particular problem. In flatter areas, engineers have tended to build roads up above the general level of the landscape so that they remain trafficable during prolonged wet weather. In keeping with good standard practice, culverts are put in at drainage lines, the latter often being shallow depressions. Unfortunately, during wet weather when there is considerable runoff, the road acts a dam, concentrating water into the culverts. Large flows occur through these which scour into the soil on the downstream

68

side, forming large gullies. The latter can be several kilometres long and can threaten structures such as railway lines, farm buildings, silos, etc. The problem can be alleviated by putting in many more culverts (expensive), but perhaps the best solution is to build the roadway at the same level as the landscape and allow the water to gently flow across the road for its whole length. The road may not be trafficable for a short time, but this may be a small price to pay for landscape stability.

In terms of waste and effluent disposal, the high exchange capacity of the black earths make them very suitable for absorbing cationic wastes, particularly NH;. Because of the very slow drainage when wet, any effluent disposal scheme would need permanent vegetative cover on the soil, so that the soil will dry out and crack. A perennial pasture would be suitable. Irrigation with effluent would be episodic and rotate around a number of paddocks, only filling the crack volume at each application. Continuous application, as occurs with household septic systems, would keep the cracks closed and infiltration rates very low.

7.2 Second example: Profile B551 (pp 302-5) Great Soil Group - Krasnozem Principal Profile Form - Gn 2.11

This is an example of soil dominated by sesquioxides with a gradationally textured profile. The geographical data associated with the profile description again gives us useful information.

(i) The soil is situated on a plateau in the Great Dividing Range, where rainfall is summer dominant and about two-thirds of the yearly evaporation. The free drainage would indicate that the profile would be subject to periodic leaching.

(ii) The clay laterite on basalt parent material indicates that this soil is not a good example of a krasnozem, which would normally form directly on basalt. However the influence of the basalt would result in a soil that is inherently fertile, and this is reflected by the native forest vegetation which contains both Eucalypt and non­Eucalypt species.

From the profile description we can see that the soil. is reddish coloured, medium textured and moderately structured to at least 3 m depth. The top 30 cm has a loam texture with a fine crumb to blocky structure. The texture becomes heavier with increasing depth, becoming a clay loam at 40 cm and a light clay at 60 cm, hence its classification as a gradational profile in the Northcote Key. By 90 cm depth, the structure becomes completely blocky, with smooth faces and interlocked peds being prominent at 150 cm depth. Ferruginous fragments are found throughout the profile, indicating a high sesquioxide content. The predominantly red colour from oxidised iron compounds indicate the good aeration right throughout the profile.

The following observations can be made from the physico-chemical analyses:

(i) The pH of the profile is mildly acid throughout and this has consequences in the determination of exchange capacity.

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(ii) Soluble salts are very low throughout the profile and chloride is not measurable. The profile is obviously highly leached.

(iii) Organic matter levels are very high at the smface and remain high to 30 cm depth. These levels contribute to the crumb structure and loam texture observed in the top 30 cm.

(iv) Nitrogen levels are high in the top 30 cm of soil, contributing greatly to its fertility, and these are associated with· the high levels of organic matter. Available phosphorus is moderately high, whilst exchangeable potassium is adequate (0.2 me %).

(v) The particle size analysis shows that the clay content of this soil is only 25% in the top 30 cm, hence its loam texture. The clay content increases to 50% at 60 cm depth, reaching 65% at 150 cm. There is a corresponding decrease in silt and fine sand fractions; very little coarse sand is present.

(vi) The exchange capacity appears high in the top 30 cm of the profile, but it is three times the sum of the exchangeable cations. This is a variable charge soil, and the exchange capacity should be measured at the soil pH, not at the standard pH of 7. Looking at the sum of cations, we see that the exchange capacity is around 17 me% at the surface, decreasing to 7-8 me% at depth. Calcium and magnesium are dominant, with low levels of sodium. Note that exchangeable potassium is quite low. The higher (estimated) CEC at the smface is associated with the high levels of organic matter.

(vii) The mineral fraction of the soil is dominated by silicates, aluminates and iron oxide. Note the high levels of the latter at 20%. The clay minerals are kaolin and chloratised vermiculite with significant amounts of gibbsite (Al), haematite (Fe), goethite (Al) and magnetite (Fe). This high sesquioxide content accounts for the soil's red colour, moderate structure and variable charge.

Krasnozems are used for a wide range of farming activities such as dairy pasture, horticulture, grain legumes and cereals. Their occurrence in well watered areas, fine stable structure, high initial fertility and free draining profiles make them one of the best soils in the country. However, after many years of cultivation, severe problems have appeared. These are associated with organic matter decline.

(i) Nutrient deficiencies. Most of the nutrients in Krasnozems are associated with the organic matter content. As this decreases, deficiencies of nitrogen, phosphorus and potassium develop and the exchange capacity decreases. These nutrients can be replaced with fertilisers but the lower exchange capacity means that K+ is not held strongly in the soil and is easily leached out. Phosphorus is a different problem. The high sesquioxide content gives these soils a large capacity to sorb phosphorus and large applications of phosphatic fertiliser may be required to correct P deficiency where this affects arable crops or pasture legumes.

70

(ii) Structural degradation. Loss of organic matter means loss of structural bonds, particularly in the top 30 cm of soil. The crumb structure disappears and the soil becomes more massive with large blocky structural units. Infiltration declines and erosion starts to occur from runoff, necessitating engineering earthworks. Under repeated wheel loadings (especially when wet) the subsoil below the depth of cultivation becomes compacted, restricting root growth and generally exacerbating the problem.

On these soils, zero till methods have little effect. It seems that massive additions of organic matter are needed to bring about structural and fertility improvement. Recent work has shown that this can be achieved with a vigorous Kikuyu pasture, preferably with additions of fertiliser and earthworms. The grass stolons are quite efficient at breaking up the large blocky structural units and creating a fine surface structure, while the earthworms burrow into and loosen up the compacted zone.

Some shrinkage occurs in Krasnozems, particularly in prolonged dry weather, so large structures must be supported by good foundations such as deep piles. The engineering problems in these soil are nowhere near as great as they are in the black earths, despite their high clay content at depth. This is because of the different mineralogy between the soils, Krasnozems having no smectitic clay minerals.

The high hydraulic conductivity of the Krasnozems makes them very attractive for effluent disposal. Continuous application, as in household septic systems, works very well. However, their low exchange capacity and free drainage means that pollutants such as nitrate can be leached through the profiles and enter groundwater. A vigorous pasture may be required to take up the nitrate as it is applied, or if leaching is still a problem, tree plantations with deep root development may be better interceptors. The acidity of the

_profile would need to be watched if nitrate leaching occurs. Phosphates would not be a problem because of the fixing properties of the sesquioxide minerals.

7.3 Third example: Profile B569 (pp 330-2) Great Soil Group- Yellow Podsolic Principal Profile Form - Gn 2.74

This is an example of a leached soil with a strongly differentiated profile.

The geographical data gives us the following useful information:

(i) The soil is situated in hilly terrain in the coastal lowlands, where rainfall is summer dominant and exceeds yearly evaporation. This profile would be subject to continuous leaching.

(ii) The sandy alluvium parent material underlain by sandstone indicates that this profile would be dominated by the sand fraction. Under continuous leaching, clay-forming minerals would be weathered and illuviated down the profile. Any clay enrichment deeper in the profile would impede drainage and cause waterlogging.

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(iii) The native vegetation of poor Eucalyptus, Puitenaea, Casuarina and Xanthorrhoea indicate a profile of low fertility. This is reflected by the land use of grazing on improved pastures established by correcting multiple nutrient deficiencies.

Turning our attention to the profile description, we see that the soil is yellow-brown coloured, weakly structured and has a sharp texture change at 75 cm depth. The top 15 cm is dark coloured and has a loamy sand texture. This gradually changes to a light yellowish-brown sandy clay at 50 cm depth, with a sharp change to a yellow/red mottled medium clay at 75 cm depth. There is further change to a light grey medium clay at 150 cm, which becomes more sandy below 300 cm. The yellow and grey colours indicate periodic waterlogging in the profile, the iron compounds being in the reduced form.

From the physico-chemical analyses, the following observations can be made:

(i) The pH is acid in the light textured A horizon, becoming strongly acid in the B horizon.

(ii) Soluble salts and chlorides are very low throughout the profile, which confirms that the profile is highly leached.

(iii) Organic matter levels are moderate at the soil surface, decreasing rapidly to a low level at 20 cm depth.

(iv) Total nutrient levels are low except in the surface soil where organic matter is present. The available P present would be expected to be non-limiting for improved pastures.

(v) The particle size analysis shows the low clay content in the A horizon and two large increases in clay content at 50 cm and 75 cm depths. These confirm the texture changes noted in the profile description. Coarse sand is dominant in the A horizon and present throughout the profile, and silt is low. The increases in clay content at 50 cm and 75 cm depth are mostly at the expense of the fine sand fraction.

(vi) The exchange capacity of this soil is very low, particularly in the 0-75 cm section of the profile. As noted for the Krasnozem, CEC > sum of cations, indicating that the clay in this soil also has a variable pH-dependent charge. The dominant exchangeable cation is calcium from 0-40 cm, with magnesium becoming dominant at depths >40 cm.

(vii) The mineral fraction of the soil is nearly all silicates in the A horizon, with some aluminates and iron oxide present in the medium clay B horizon. The clay minerals are dominated by kaolin, with significant chloritized vermiculite in the A horizon and some rutile throughout the profile.

Despite the high rainfall environment, these soils were of little use to agriculture because of their low inherent fertility and periodic waterlogging. With the correction of nutrient deficiencies some land has been cleared and put under improved pastures for grazing. The most common land use in Pinus plantations for softwood timber. The high leaching

72

environment means that periodic additions of fertiliser with trace elements are required for sustained production.

The yellow podzolics pose no outstanding problems for engineering pmposes. When exposed, the B horizon tends to erode rapidly and during prolonged wet periods interflow occurs within the sandy A horizon, giving rise to seepage problems in cuttings and drainage channels. The yellow colour and texture contrast profiles of these soils, particularly if the structure is weak, generally indicate that periodic waterlogging occurs and drainage is impeded. This can create problems in effluent disposal, particularly in high rainfall areas. Their low exchange capacity means that they are also poor adsorbers of cationic pollutants.

One proposed use of the example Yellow Podzolic is the disposal of sewage sludge amongst the Pinus plantations. The application of the organic sludge has no effect on the infiltration capacity of the A horizon, and the pine trees would deal with any nitrate quite happily. However, there are questions concerning the fate of heavy metal pollutants. There is little CEC in the profile to retain them, and downslope interflow within the sandy A horizon during prolonged wet periods could carry heavy metals into the local streams. These could lead to contamination of local water supplies,estuarine fish habitats and the sea coast. This is being investigated.

7.4 Fourth example: Profile B558 (pp 163-7) Great Soil Group - Solodized Solonetz Principal Profile Form - Dy 5.43

This is an example of a mildly leached soil with a strongly differentiated profile and a sodic B horizon. The geographical data for this soils gives us the following useful information.

(i) The soil is situated on gently rolling terrain west of the Great Dividing Range in a semi-arid environment. The rainfall is summer dominant and less than half the potential evaporation, indicating that the soil would not be highly leached. This is supported by the observation of impeded drainage in the sub-soil.

(ii) The sandy alluvium parent material overlaying quartzose sandstone indicate that this profile would be dominated by sand. With a free draining A horizon, some weathering and leaching would be expected, with illuviation of clay down the profile. The native vegetation of Callitris, Eucalyptus, Casuarina and Acacia spp. indicate a soil of low fertility.

Looking at the profile description, we see that the soil has a brown coloured light textured (sandy) structureless A horizon, with a sharp change at 60 cm to a sandy clay B horizon of columnar structure and yellow/grey mottling. Below 100 cm, the texture becomes lighter with depth, a fine sand being present at 270 cm. The yellow and grey colours in the B horizon indicate periodic waterlogging, with iron compounds being in the reduced form.

73

The physico-chemical analyses tell us the following:

(i) The pH of the soil is strongly acid at the soil surtace, becoming mildly acid just above the B horizon. The top of the B horizon is mildly alkaline, becoming more alkaline with depth.

(ii) Soluble salts are very low and chloride is absent in the A horizon. More soluble salts occur in the B horizon but levels are low and only half of them are chlorides.

(iii) Organic matter levels are very low except for the immediate soil surface.

(iv) Total nutrient levels are very low throughout the profile. Exchangeable potassium concentrations are very low in this soil.

(v) The particle size analysis shows a dominance of fine sand throughout the profile, which comes from the parent material. The abrupt change in clay content at 60 cm depth confirms the sharp texture change recorded in the morphological description.

(vi) The exchange capacity of the soil is very low, particularly in the A horizon. CEC > sum of cations in the acid A horizon, indicating that the clay minerals have variable charge. The alkaline B horizon shows better agreement. The exchangeable cations are dominated by magnesium and sodium, with the ESP being around 50% in the B horizon. It would be expected that the B horizon would be dispersive.

These soils are of little use for agriculture because of their low fertility and low water holding capacity in a semi-arid environment. This is reflected by the present land use, which is controlled logging of cypress pine. Some land has been cleared for grazing under improved pasture, using low application of fertiliser, but costs have been high and production disappointing, particularly for sheep and wool.

The poor structure and high sodicity in the B horizons of the Solodized Solonetz soils pose a number of engineering problems. During wet periods, the structureless A horizon becomes waterlogged· because of the impeded drainage in the B horizon, losses all cohesion, and the landscape becomes impossible to vehicular traffic. Very large vehicles of high clearance may be able to find traction on the top of the sandy clay columns in the B horizon, pushing the 'spewy' A horizon aside and leaving large ruts behind. The latter then set hard when dry. The sodic B horizon is highly dispersive and quite unsuitable for road building, becoming greasy when wet by rain of low electrolyte content, and readily eroding. The high sodicity also causes problems in farm dams and other earthworks.

For the same reasons, these soils are not suitable for effluent disposal. Their limited infiltration capacity means that large areas would be needed and disposal would have to be rotated from area to area. There would be poor retention of cationic pollutants because of the low exchange capacity.

74

8 CONCLUSIONS

We hope that these four examples of different soil profiles give some insig.ht into how soil descriptions can be interpreted and used. The pedologist's observations and descriptions of colour, texture and structure can be used to qualitatively assess the hydrology of the soil profile. The physico-chemical data can give insights into the fertility of the soil, its stability and its powers to absorb applied nutrients or pollutants. This information is useful in assessing the suitability of soil for a particular land use, whether it be agriculture, roadworks, engineering structure, effluent disposal, etc.

9 . REFERENCES

Atterberg, A. (1913). Die Plastizitaet und Bindigkeit liefemde Bestandteile der Tone, Int. Mitteil. Boden Kunde, 3. .

Casagrande, A. (1947). Classification and Identification of Soils. Tans. Amer. Soc. Civil Engrs.,113.

Colwell, J.D. (1983). Fertiliser Requirements. In 'Soils: An Australian Viewpoint', ch. 50, pp. 795-815, (CSIRO/Academic Press, Melbourne).

Emerson, W.W. (1983). Inter-particle bonding. In 'Soils: An Australian Viewpoint', Ch. 31, pp. 477-498, (CSIRO/Academic Press, Melbourne).

FAO-UNESCO (1974). 'Soil Map of the World, 1:5,000,000'. Vol. 1. Legend (FAO, Rome) .

. FAO-UNESCO (1988). 'Soil Map of the World: Revised Legend'. World Soil Resources Report 60. (FAO, Rome).

Gardner, E.A. (1988). Soil Water. In 'Understanding Soils and Soil Data'. (Ed. I.F. Fergus). pp. 153-185 (Aust. Soc. Soil Sci. Inc., Qld Branch, Brisbane).

Isbell, R.F. (1988). Soil Classification. In 'Understanding Soils and Soil Data'. (Ed. I.F. Fergus). pp. 13-27. (Aust. Soc. Soil Sci. Inc., Qld Branch, Brisbane).

Kezdi, A. (1974). 'Handbook of Soil Mechanics, Vol. 1. Soil Physics'. Ch. 4, pp. 97-108.

Ladd, J.N. and Russell, J.S. (1983). Soil Nitrogen. In 'Soils: An Australian Viewpoint', Ch. 37, pp. 589-610.

Loveday, J. and Bridge, B.J. (1983). Management.of Salt-Mfected Soils. In 'Soils: An Australian Viewpoint', Ch. 53, pp. 843-856.

McDonald, R.C., Isbell, R.F., Speight, J.G., Walker, J. and Hopkins, M.S. (1990). 'Australian Soil and Land Survey Field Handbook', 2nd Edition (Inkata Press, Melbourne).

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Moody, P.W. and Bruce, RC. (1988). Soil Testing and Fertiliser Recommendations. In 'Understanding Soils and Soil Data'. (Ed. I.F. Fergus), p. 78, (Aust. Soc. Soil Sci. Inc., Qld Branch, Brisbane).

Moore, A.W., Isbell, R.F. and Northcote, K.R. (1983). Classification of Australian Soils. In 'Soils: An Australian Viewpoint', Ch. 20, pp. 253-266.

Norrish, K. and Pickering, J.G. (1983). Clay Minerals. In 'Soils: An Australian Viewpoint', Ch. 22, pp. 281-308.'

Northcote, K.H. (1979). 'A Factual Key for the Recognition of Australian Soils', 4th Edition. (Rellim Technical Publications, Glenside, S.A.).

Northcote, K.H. and Skene, T.K.M. (1972). Australian soils with saline and sodic properties. CSIRO Aust. Div. Soils, Soil Pub!. No. 27.

Northcote, K.H., Hubble, G.D., Isbell,. RF., Thompson, c.R. and Bettenay, E. (1975). 'Description of Australian Soils' (CSIRO, Melbourne) .

. Northcote, K.H., with Beckmann, G.G., Bettenay, E., Churchward, H.M., Van Dijk, D.C., Dimmock, G.M., Hubble, G.D.,. Isbell, . RF., McArthur, W.M., Murtha, G.G., Nicholls, K.D., Paton, T.R, Thompson, C.H., Webb, AA and Wright, M.J. (1960-68). 'Atlas. of Australian Soils'. Sheets 1-'10, with explanatory notes. (CSIRO and Melbourne University Press, Melbourne).

Prescott, J.A. (1931). The soils of Australia in relation to vegetation and climate. CSIRO Aust. Bulletin No. 52.

Probert, M.E. (1983). Organic Phosphorus and Sulphur. In 'Soils: An Australian Viewpoint', Ch. 36, pp. 583-588. .

, . Richards, L.A. (Editor) (1954).'Diagnosis ; and Improvement of Saline and Alkali Sol1s'.

USDA Agriculture Handbook No. 60. (US Government Printer, Washington D.C.)i .

Soil Survey Staff (1975). 'Soil Taxonomy: A Basic System of Soil Classification~or .' Making and Interpreting Soil Surveys'. USDA Agriculture Handbook No. 436. GJS.

Government Printer; Washington D~C.)~ , i

~ . \

; . Sike, H.C.T., Hubble, G.D., Brewer, R" NortttCote, K.H., Sleeman, J.R, Mulcahy,M.J.' . and Hallsworth, E.G. (1968). ' A Baridbobk of Australian Soils' (Rellim Tec~al.

Publications, Glenside, S.A). .' . t .

S~phens, C.G. (1953). ' A Manual of Austrlilian~Soils'. (CSIRO, Melbourne).

U$ Bureau of Reclamation. (1960).'Earlh Mandal' (USBR, Denver, Colorado) . . ~ , ); ". ~

williams; C.H. and Raupach, M. (1983{ Plant{ Nutrients in Australian Soils. In 'Soits:. An Australian Viewpoint', Ch. 49, pp. 777-794.

76

EROSION PROCESSES AND CONTAMINANT TRANSPORT

R.J. Loch and D.M. Silburn Natural Resource Management, QDPI, Toowoomba, Qld, 4350.

ABSTRACT

The main processes operating in soil erosion are raindrop splash, shallow unconcentrated overland flows, rilling, and guUying. The erosive agents operating in each are discussed, as is their likely response to environmentalfactors, and the interrelationships between the processes.

The characteristics of particles moved by erosive mechanisms are detailed with particular reference to particle movement as either suspended or bed-load sediment, deposition, and selective transport. Methods for measuring sediment properties, and ways of expressing the measurements in meaningful terms, are outlined.

Factors affecting erosion and deposition by water from upland areas include rainfaU and runoff, rainfaU intensity, surface cover, slope and slope length, and soil erodibility, aU compounded by temporal variation.

Movement of contaminants (including nutrient elements, heavy metals and pesticides) during erosive processes is detailed, and iUustrated by problems of spreading sewage sludge in a forested area. Predictive models for erosion and contaminant transport have a valuable role in future understanding of the subject.

1 EROSION PROCESSES

A range of processes of erosion is commonly recognised. There are benefits in distinguishing between processes, as each has its distinctive erosive agents, and distinctive responses to environmental factors. Understanding of the processes operating on research areas can be crucial to obtaining relevant parameters for modelling and planning. As well, differing erosion processes may require different management strategies for amelioration.

1.1 Splash

Rates of splash transport are highest when the soil surface is not covered by water, and decline rapidly between 0 and 2 mm water depth (Moss and Green 1983). Thus widespread splash occurs early in a rainfall event, before runoff develops. During runoff, splash will be restricted to areas of soil that (due to surface roughness) project above overland flow. The response to water depth also means that splash .causes net movement of particles from exposed areas to areas of the soil surface covered by water, and hence, is a source of detached particles for other forms of sediment transport. Using simulated rain, Moss and Green (1983) noted that 5.1 mm diameter raindrops supplied nearly half the sediment transported in shallow overland flow, but that 2.7 mm drops produced about 6 %. As natural rain at 50 mm/h would have less than 5 % of drops greater than 5 mm, and only 50 % of drops >2.7 mm (Wischmeier and Smith 1958), splash is unlikely to be a major source of

77

detached particles, except possibly under conditions of high soil cohesion, when detachment by flow is extremely limited. However, raindrop impact can be an appreciable source of clay­sized particles in runoff (Loch and Donnollan 1988), with the kinetic energy of drop impacts being particularly important in dispersing clay held in aggregates on the soil surface. Figure 1 illustrates the clear relationship between the concentration of fine particles in sediment and surface exposure.

Figure 1

'"" 1.2 ~

0 01 '--"

0

E E

N 0 0 0 0.6 0

....... c Q)

E 0

""0 Q)

(f) 0 0 . 40 60 80

% surface exposed to rain

Linear relationships (r = 0.89, P<O.OS) between sediment <0.02 mm and the proportion of the soil surface exposed to rain

Particles are typically moved distances of 0.1-1.0 m by splash. Net downslope movements are generally insignificant unless slopes are extremely steep, as the net downslope component of raindrop energy is small, and factors such as wind may have greater influence on .the direction of net splash transport.

1.2 Transport in shallow, unconcentrated, overland flows

Referred to as interrill, rainflow, or RIFT (Rainfall Induced Flow Transport), this refers tathe entrainment and transport of sediment by.shallow (typically 0-10 mm depth) and non-incised overland flow impacted by rainfall.

In shallow flows that, in the absence of rainfall, were unable to entrain and transport sediment, Moss et al. (1979) showed that sediment is entrained and transported due to the combined action of raindrops and flow. When a drop impacts the flow, the result;ing turbulence ejects particles up into the flo~, and the particles remain in the flow for a period of time, during which the particle travel~ some distance in the direction of flow. KinI1ell (1990) developed the equation:

c (p,d) = 6Rd ~d Dpd (n d3 hyl (1)

where p and d refer to diameters of particles and drops respectively, c is sedim.ent concentration, R is the intensity of raindrops of diameter d, t is the time that particles take to settle to the bed after being ejected into the flow, D is the mass of particles lifted into'the

78

flow by a drop impact, and h is the depth of water flow.

Subsequently, Kinnell (pers. comm.) simplified equation 1 to the expression:

c (s,d) =1.9 ks I f[h,r] (2)

that deals with soils of non-uniform particle sizes (denoted by s) and rainfall of a range of drop sizes (denoted by r), and rainfall intensity I. In equation 2, ~ is effectively an interrill erodibility, and equates to (t}xI Dpd ) in equation 1, whereas f[h,r] describes interactions between slope, water depth, and drop energy.

Interrill erosion rates are sensitive to flow depths, reaching a maximum and then decreasing rapidly for depths greater than 2-3 drop diameters (Moss and Green 1983). Hence, with increasing slope lengths, interrill erosion rates could be expected to reach a maximum and then decline as water depths increased downslope.

1.3 Rilling

Rills are generally defined as flow channels that can be obliterated by tillage, so that size of channels is the major distinction between rills and gullies. Dimensions of rill channels can very greatly, but typically rills would be either shallow (e.g., to the depth of tillage), or narrow and more deeply incised.

On rough surfaces, overland flow invariably concentrates into a number of· preferred flow paths (Emmett 1978). It would be simple, but incorrect, to define all lines of concentrated flow as rills, as rills are generally regarded as being areas of active erosion. Fenneman (1908) suggested that there were two physically distinct types of overland flow channel. Where there was net erosion, channels were incised, but where there was net deposition, channels were shallow and lined with their own deposits of sediment. Consistent with this, Loch and Donnollan (1983a) found that clearly incised rills, sometimes having visible knickpoints, carried· 4.7 -4.8 times the coarse sediment of broad, flat flow lines that' could be equated with rain-flow transport. Similarly, Meyer et al. (1975) reported a 3-fold increase in sediment concentrations associated with rill development.

Such large and consistent increases in sediment transport capacity suggest a marked and well­defined change in the nature of flow in overland flow channels. Flow velocities are certairuy higher in rills, with rain-flow and rill velocities of 0.10-0.20 and 0.34-0.49 ms-1 respectively being recorded in rainulator studies on a cracking clay soil on 4% slope (Maroulis, unpub.). As well, Moss et al. (1982) found that above a characteristic discharge (which was a function of bed slope), secondary flow circulations developed in rill flow and caused the incision of rill channels.

The concept of a critical discharge (or some critical value of flow erosivity) for rill initiation is widely accepted, but useful estimates of critical values for a range of soils are not readily available. The WEPP project has generated data for US soils (Elliot et al. 1989) to be used in the WEPP model, which predicts detachment by rill flow as:

(3)

79

where Df is detachment rate, Kr is a rill erodibility factor, 't and 'tc are shear and critical shear forces respectively, G is the rill sediment load and Tc the rill sediment transport capacity.

Critical shear/streampower/unit streampower values are controlled partly by soil cohesion, and partly by the competence of the flow to carry the sizes of particles available for transport.

For practical purposes, it could be expected that tilled cracking clay soils and any other soils that retained relatively coarse aggregation when wet would have very low values of critical shear for rill initiation, provided they were dry prior to the runoff event. If soils are wet for several days prior to runoff, critical values for rill initiation will be much higher (Govers et al. 1990, Govers and Loch in press), as soil cohesion (both between and within aggregates) will be much higher.

There are several important consequences of the incision of rills. Firstly, changes in soil cohesion with depth can strongly influence rill development. Tilled soils generally represent a loose, tilled layer overlying a layer of much greater strength, so that rills tend to be confined to the loose, tilled layer of soil. As discharge increases, the extra flow is initially accommodated by a slight increase in flow depth, followed by widening of the rill (Loch and Donnollan 1989). Consequently, flow depth remains relatively constant, transport capacity per unit width of rill does not vary greatly, and rill sediment concentrations are relatively constant across a range of discharges (Loch and Donnollan 1983a).

Interactions with soil strength profiles can also cause a distinctive temporal pattern of rill erosion rates. If, under experimental conditions, a fixed discharge is applied for some time, sediment available for transport is rapidly depleted as the rills incise to a non-erodible layer. This variation through time in rates of rill erosion can create difficulties in estimating the importance of rilling. (The shorter the erosion event, the more important rilling is likely to be.) Data from rainulator plots on Vertisols on the eastern Darling Downs, Queensland suggest that for relatively short runoff events, rilling contributes approximately 65% of eroded sediment (Loch and Donnollan 1983a), which is a similar estimate to those from a range of studies on other soils (Govers and Poesen 1988). However, for field catchments on the eastern Darling Downs, Freebairn and Wockner (1986) estimated the contribution of rills to total erosion to be closer to 50%.

A further consequence of rill incision is that sediment carried by rills is largely derived from sub-smface layers. This may be important if, e.g., rills incise into a contaminated or nutrient­rich layer. Conversely, other materials of concern may be concentrated at the soil smface, e.g., pesticides applied as sprays can accumulate at the smface of direct-drilled soils. As well, aggregate sizes at depth in a soil may. be coarser than at the smface, especially after the smface has been rainfall wet.

1.4 Gully erosion

Gullies are distinguished from rills by a general definition that classes gullies as being too large to be obliterated by tillage. In agricultural land, gullies are the form of erosion that most rapidly renders land unsuitable . for cropping, as gullies can prevent the use of agricultural machinery, even though the average depth of soil may still be sufficient to support crop growth. Deeply incised flow lines such as gullies are also of particular concern

80

where noxious or toxic materials are buried.

Similar to rills, gully development is controlled by thresholds. Rather than specify some measure of flow erosivity, geomorphologists have identified thresholds of slope and catchment area to be exceeded for gullies to form. Because gullies incise to much greater depths than rills, and hence, erode very cohesive material that cannot be detached by rills, much larger quantities and erosivities of flow are required to form gullies than for rills.

Gully development in valley floors can be the result of long periods of sediment deposition and steepening of the valley, but the actual o~et of gullying may be associated with an unusually intense rainfall/runoff event. There is evidence that some catchments may undergo periods of stability and sediment accumulation, followed by periods of gullying (Lang 1984). In agricultural/pastoral areas, depletion of surface vegetative cover and reduction in evapotranspiration can increase ,runoff rates, giving an effective increase in catchment area, thus encouraging gully development. Changes in climate that lead to increased runoff rates would similarly increase gullying. Culverts and various road drainage structures, by lowering the base level of a valley floor at those points, also initiate gullies, and the conditions for gully development may exist for some time before a major storm provides the trigger for gully development.

Again, like rills, gullies can undergo periods of active incision and development, and then gradually reach equilibrium if a new and stable bed gradient is established. During active gully expansion, rates of erosion can be high. Piest et al. (1975) reported one gully that eroded headward approximately 45 m in 7 years, and removed 3 500 tof soil,equivalent to 16.7 t/ha/yr from the 30 ha agricultural catchment studied. Sheet and rill erosion removed 66.7 t/ha/yr during the same period. However, from a nearby grazed catchment, soil losses from sheet and rill erosion were only 0.75 t/ha/yr, and for such catchments; gully erosion

, would be the major form of sediment removal.

1.5 Interrelationships of erosion processes within landscapes

Of the processes discussed, a general distinction between raindrop and flow dominated processes is possible. Within a catchment, raindrop dominated processes of erosion will occur wherever water depths are favourable. Areas of concentrated flow, by' concentrating runoff and increasing velocities, act to reduce overland flow depths on their contributing areas. Even if not actively eroding, rills and gullies can provide effective transport of sediment transported to them by raindrop-dominated processes.

Erosion resulting from raindrop impacts is generally reasonably consistent throughout a rainfall event, whereas rill and gully erosion show greater temporal variations associated with channel development and subsequent stabilisation. both within rainfall events, and, for gullies, over much larger time scales.

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2 SEDIMENT PROPERTIES, TRANSPORT MECHANISMS, DEPOSmON AND SELECTIVE TRANSPORT

2.1 Suspended and bedload sediment

The particles transported by erosive mechanisms vary considerably - in size, density, shape, and roughness. These differences determine not only the manner and rates of transport, but also the distances over which particles may be carried by a particular event. The sediment properties listed above control the velcicities at which particles settle in water, with settling velocities being greatest for large, dense, smooth, round particles.

Depending on settling velocity, particles ejected into overland flow may remain in the flow rather than settle to the bottom. This occurs when flow turbulence is sufficient to counterbalance the tendency of the particles to settle. Such particles are described as being in suspension, and are referred to as suspended load. Because flow turbulence can vary, particles that are suspended under some conditions may not be so under others. Under all conditions, particles <0.002 mm (such as dispersed clay) will be suspended load, and particles 0.02-0.002 mm would also commonly function as suspended sediment.

. .

If particles carried in flow do not remain in suspension, they will regularly settle to the bed (of the flow line) or to the soil surface, to be re-entrained if sufficient lift force is applied by flow turbulence (including raindrop impacts) or impacts of other particles. Such particles are referred to as bedload, and typically move by saltation, rolling, or sliding. In s()me instances, all bedload sediment may be referred to as contact load, but some authors use contact load to refer specifically to particles moved by rolling and sliding. Bedload particles move in a bedload layer, close to the bed and regularly exchanging particles with it. Individual bedload particles move by a series of quick steps with relatively long intermediate rest periods. This means that bedload sediment will move at a lower velocity than the flow that transports it, and hence, eventual deposition is inevitable. Suspended sediment, however, can move considerable distances in runoff.

While more evenly distributed than· bedload, suspended load is also not evenly distributed throughout the flow, with concentrations being lower near the surface and banks. In larger, deeper channels, the variation in total sediment load with depth and width creates considerable difficulty in obtaining representative samples of average sediment concentrations.

Clearly, bedload particles will comprise the coarser, denser fractions of sediment. A range of equations has been developed to describe both the threshold conditions for movement of different particle sizes, and the potential transport rates of bedload sediment. Typically they include measures of flow erosivity and of particle size and density.

The relative proportions of bed and suspended sediment can vary greatly, depending on the material being eroded, and on the turbulence of the erosive flows that occur. However. for many soils, as much as 80% of the eroded sediment is bedload, made up of aggregates and sand grains (Young 1980, Loch and Donnollan 1983b, 1988). Other materials vary greatly. Some sewage sludges can be quite coarse (Loch et al. 1992), but up to 50% of feedlot _ manures can be made up of particles that settle very slowly (Lott et al. in press).

82

2.2 Deposition, selective transport, delivery ratios, and sediment traps

Deposition is the process of particles settling out of overland flow. Under some conditions, deposition and re-entrainment occur continuously, so that there is little or no net deposition. Under other conditions, there may be considerable net deposition.

The rates at which different classes of sediment particles are deposited depend on both their settling velocities, and on the distance over which particles settle. Bedload, because it is both close to the bed and has high settling velocities is particularly susceptible to deposition. The constant entrainment, deposition, and re-entrainment of bedload sediments typically leads to sediment being enriched in those fractions that are less prone to deposition, a phenomenon referred to as selective transport. At the same time, the selective deposition of the most easily deposited sediment (usually the coarsest sizes) can form a protective layer or armour on the bed that reduces rates of entrainment. Where sediment is only being entrained from the bed, rates of entrainment will eventually be controlled by the rates of detachment of the coarse particles that dominate the bed, and size distributions of sediment will approach those of the original bed material (Moss et al. 1979). However, where not all sediment is entrained from the bed (with some being entrained, e.g., from exposed roughness elements or from erosion of channel banks), there is potential for selective transport of finer fractions of sediment to persist throughout an erosion event (Loch et al. 1988).

Net deposition occurs when the capacity of overland flow to transport sediment is reduced. Within catchments, deposition is commonly associated with reductions in land slope, or with the presence of vegetation that reduces the velocity and transporting capacity of-runoff. As flow transport capacity declines, the sediment most readily deposited will be removed, and the remaining sediment will become progressively finer. (Finer sediment is generally equated with slower settling velocities.) As the sediment originally entrained from soils may contain up. to 80% bedload, the proportion of sediment originally entrained that actually leaves a catchment can be quite small, is usually restricted to the suspended sediment, and is referred to as a delivery ratio. As progressively larger catchments are considered, gradients of flow lines decrease and deposition further reduces delivery ratios.

Impoundments can also encourage deposition. For water. storage dams, sediment deposition can lead to serious losses of storage capacity, and there have been extreme instances of moderately large dams being filled with sediment in a single runoff event.

In a: number of situations, impoundments are deliberately created to cause deposition and to prevent high concentrations of sediment leaving an area. (This is common practice in many mines.) Sediment traps are generally fairly shallow to increase deposition, and are designed so that the water remains in the impoundment long enough for the sediment to settle out, so that flow through the sedimentation area is not so turbulent that deposition is impeded. Design of sediment traps must be based on the settling velocity distribution of the sediment that is to be trapped. In some instances, data on settling velocities can identify the proportion of sediment that can be settled economically. For example, Figure 2 shows a settling velocity distribution of feedlot manure, of which it is only practical to settle out the faster-settling 50%. Dramatic increases in settling time would only slightly increase the quantity of sediment deposited. For example, for a settling basin 1.8 m deep and the material shown in Fig. 2, a settling time of 10 min would settle out slightly more than 55% of sediment. Trebling the settling time would only settle out a further 6% of sediment. Barfield et al.

83

(1981) give considerable detail on design and perfonnance of settling basins, and would be a useful starting reference for further reading.

Although sediment traps· provide a means of preventing high sediment concentrations from leaving an area, it is worth noting that they are, at best, temporary methods of controlling impacts of erosion. Through time, continued deposition of sediment can lead to losses of capacity in flow lines or impoundments, creating the potential for future instability.

Figure 2

-I-' o c

100

80

~ 20

1000 2000 . 3000 Settling time (seconds)

Proportions of fresh cattle faeces vs settling times, for a 1.8m settling column, data from Lott et aL (in prep.)

2.3 Measuring sediment properties

Probably the major requirement is to define what information is required. It is possible to measure sediment size distributions by sieving, and with care this analysis can be extended to particles as fine as 0.035 mm. However, a measure of size gives no definite information on either settling velocities or particle densities.

Settling velocities are best measured' directly, as the measurement integrates the effects on settling velocity of size, density, shape, and roughness. Results may be presented as distributions of velocities, or, alternatively, as equivalent sand size distributions. These are calculated by equating each settling velocity class with the diameter of sand particle that would have the same settling velocity. Settling columns provide straightforward data, with accuracy of the measurements being affected by the size of the sample used, and by the length and diameter of the settling tube (Hairsine and McTainsh 1986). Control of water temperature is essential.

Sediment densities refer to the density, in water, of the (wet) sediment of interest. Wet densities can be difficult to measure directly, as particles at saturation can have layers of water of varying thickness adhering to them, and assemblages of small, saturated particles may also, due to surface tension, hold considerable water between the particles. Therefore, . it is simpler to estimate an effective density, based on comparison of settling velocities and

84

actual size distributions. By assuming a range of densities for the actual size distribution, a range of equivalent sand size distributions can be calculated and compared with the equivalent sand size distribution based on settling velocity measurements. By that process, it is possible to find a density such that the two size distributions are similar (Figures 3(a) and (b». This procedure attributes all the change in settling velocity of a known size fraction to variations in density, and neglects effects of shape and roughness.

100

c 80 o

s::. .... ... Gl C

;;::

o~--~--~~~~---

0.00 2.00 4.00

Sediment size (mm)

Figure 3a: Comparison of actual size distribution (solid line) with equivalent sand size distribution from settling velocities (Inverell soil)

o~--~~~~~~---

0.00 2.00 4.00

Sediment size (mm)

Figure 3b: Comparison of equivalent sand sizes derived from settling velocities (dashed line) or from actual size with an assumed density of 1.5 Mg m-3

(Inverell soil)

For soils, Loch and Rosewell (1992) produced a relationship between content of sand >0.02 mm and wet density (Fig. 4) that allows accurate prediction of wet densities fromthe content of sand >0.02 mm. '

. , ' ,.' . .

When measuring sediment properties, there are several important concerns:

(i) the sediment, if sampled from an eroding area, should NOT be dried and subsequently re-wet for measurement. Both the drying and the re-wettingcan greatly alter size distributions.

(ii) if a dry sample of soil (or whatever th~ material qf interest may be) has been taken, the wetting treatment applied to that sample to generate "sediment" MUST be as close as possible to the wetting that occurs under field conditions. (Immersion wetting is seldom acceptable!)

(iii) Sediment samples, if taken wet, do not generally store well for more than several hours, especially if from soils with a' component of fine particles. There is a tendency for the sediment to settle in a cohesive lump in the bottom of the sample container.

85

Figure 4

2.6

>-4J . iii

~2.2 "0

4J C Q)

E ~ 1.8 o (f)

0

1.4 o Percent sand 0.02' mm

Relationship (Y =1.46 + .05(1.03)X, R2 = 0.94) between wet sediment density and, percent sand >0.02mm

3 FACTORS AFFECTING EROSION, DEPOSITION AND SEDIMENT YIELD

In steeper, upland segments of the landscape net erosion occurs; that is, soil detachment exceeds deposition and there is a net loss of soil. In slope segments or channels lower in the landscape, net deposition can occur (Fig. 5), often due to a reduction in slope or increase in hydraulic roughness. Total soil lost from these segments is less than the total of sediment input from above and detachment within the segment, and localised deposits of sediment (silt fans etc.) are often observed. In this section we discuss environmental and land management factors that affect upland erosion prior to large scale net deposition. Discussion of upland erosion will be restricted to sheet and rill water erosion, not including gully or tunnel erosion.

3.1 Erosion by water from upland areas

3.1.1 Effects 0/ rain/aU and runoff

Soil erosion results when soil is exposed to the erosive powers of rainfall energy and flowing water, operating by the various process~ described above. Runoff and rainfall are therefore the climatic driving forces of erosio1l,. Until recently, rainfall was seen, somewhat inaccurately, as a major direct agent of soil erosion by water:

' ... the splash or impact effect of tpe raindrop which we now know to be the first and most important stage in the erosion process.' (Hudson 1971, pp. 38).

'. '

'The fundamental cause of soil erosion is that rain acts upon the soil, and the study of erosion can be divided into how it will be affected by different kinds of rain, 'and how it will vary for different conditions of soil.' (Hudson 1971, pp. 45)

86

Figure 5

Land slope 6% Channel slope 0.3%

cIe~!tlon along channel

no flume.

Soil movement and deposition within a contour bay catchment

Considerable research effort was spent on determining relationships between rainfall indexes, usually based on rainfall amount, intensity and/or energy, and measured soil erosion (Wischmeier and Smith 1958, Hudson 1971, Lal 1976c). The most commonly used of these indexes in Australia and the USA is the rainfall erosivity of Wischmeier and Smith (1958), calculated for each rain-storm as the product of total energy and maximum 30 minute intensity (EI30). Wischmeier and Smith (1958) found that event E130 explained 70 to 89% of variation in event soil losses from bare plots, for five US soils. E130 values for individual rainfall events are added to give total values, e.g. monthly or annual.

The mean annual rainfall erosivity provides a useful indication of the climatic· potential for upland erosion, providing a basis for inter-regional comparisons (Freebairn et al. 1992). Basically, the greater the product of total rainfall and rainfall intensity, the greater the erosion potential. Figure 6 shows the trend for increased annual average erosion with increasing mean annual E130, for similar soils and cropping systems, along a north-easterly transect through the wheat cropping areas of eastern Australia (Freebairn 1984). Maps of the spatial distribution of mean annual E130, tables of monthly distribution of E130 and equations relating E130 to rainfall intensity-frequency-duration data are available (Queensland - Rosenthal and White 1980, New South Wales - Rosewell and Turner (1992), Australia - McFarlane and Clinnick 1984).

On an event basis (rather than average annual basis) rainfall is a less useful predictor of erosion, due to the influences of other factors. Physical relationships between rainfall and erosion do exist - more intense rainfall is more likely to cause runoff; larger storm amounts are more likely fill the soil water deficit and cause runoff - hence the relationships found between event rainfall indexes and soil erosion (eg. Wischmeier and Smith 1958, Freebairn and Wockner 1986). However, these analysis only include events where runoff and erosion occurred. The majority of rain does not become runoff; average annual runoff is typically 5-15 % of rainfall for inland cropping and pasture areas, but may be as high as 50% on bare, degraded rangeland (Silburn et al. 1992). For individual rainfall events there is often no runoff and therefore no erosion, while on occasions over 80% of rain may run off; soil water content is an important factor in determining whether rainfall will produce runoff (Yule, 1987,

87

Fig.7). Thus the relationship between rainfall and upland erosion on an event basis is complex and indirect, due to the influence of other factors apart from the amount and intensity of rain. Event erosion is more directly related to runoff than rainfall (Freebairn and Woekner 1986) and can be best understood (and predicted) by considering the hydrology of the system (ie. soil water balance and occurrence, depth and rate of runoff).

60

50 • Greenmount

... cis z= :6 40 en en 0 • Greenwood ..J

..J 30 0 en ..J

:§ 20 z z <

10 • Texas (USA)

• Gunnedah

0 .Wagga Wa ga

75 100 125 150 175 200 225

EROSION INDEX (metric unit)

Figure 6 Relation between soil loss and erosion index

80 VERY WET PROFILE

- 60 ~ -CoL CoL 0 40 Z :;) ex:

20

o----~----~----~~--20 40 60 80

SURFACE COVER (%)

Figure 7 Effects of soil water content and cover on event runoff on a clay soil

88

Land use, particularly the soil water deficit created by plants and the cover afforded by plants and plant residues, has a major effect on the water balance and runoff (and therefore the erosion potential) and on erosion itself. Wockner and Freebairn (1991) discuss the effect of the pattern of plant water use (summer crop verses winter crop) on the monthly distribution of runoff and erosion (Fig. 8).

Figure 8

250 <E-- fallow ~ ~ summer ... crop

200 _ Soil Movement ., ..e ~ 150 ... c ..,

100 e .., > 0 50 ~ := 0 0 en

e 250 winter --it> -cI( fallow .. ..s

crop

200 It: 0 c 150 :::l ~

] 100 0

f-< 50

0 A S 0 N D F !vi A !vi J

Month

Total monthly rainfall erosivity (EI3o), runoff and soil erosion.for a summer crop-winter fallow system (upper figure) and winter crop-summer fallow system (lower figure)

3.1.2 Temporal Variation in Soil erosion

Annual rainfall in Queensland is highly variable. Typical coefficients of variation (CV) are: 25 % Darling Downs, 28 % south east Qld, 35 % central Qld and Maranoa, 42 % Charleville, 47 % northern Qld (Willcocks and Young 1991). This variability is reflected, and increased, in annual runoff and soil erosion, with CV of 100 % or more. The large year to year variation in measured erosion on the Darling Downs (where annual rainfall has less variability) is illustrated by two consecutive four year periods (1980-83, 1984-87) which averaged 78 t/ha and 14 t/ha respectively (Wockner and Freebairn 1991, Fig. 9).

Embedded in the variable annual rainfall sequences are large and/or high intensity rainfall events of comparatively low frequency. Over a 14 year period, Wockner and Freebaim (1991) found that, out of 81 rainfall events that produced runoff, six storms caused 70 % of the total soil erosion. Also, large erosion events occurred both in 'wet' and 'dry' years. Edwards (1987) found that 10 % of runoff events produced 90 % of the total soil loss at long­term sites throughout the cropping regions of NSW. Thus the challenge to management of erosion and its off-site impacts is to develop strategies that control erosion during the infrequent large events. Designing for short return periods and accepting failure for larger events will be an ineffective strategy (Freebaim and Wockner 1986).

89

Temporal variability and the importance of sporadic large events creates a number of problems. Monitoring must continue for many years if the aim is to representatively sample the climate and in particular to determine mean annual erosion. Soil loss models based on empirical mean annual data (such as the USLE, discussed latter), and land use policy-making based on use of mean annual data or model, will be subject to large uncertainties. Large variability also affects perceptions and management of erosion. Erosion (and off-site water quality) is ignored when little is observed for several years, followed by a flurry of interest, and possibly unnecessary public 'outrage', because so much erosion is seen in a brief period.

3.1.3 Effects of land use

If runoff and rainfall are the driving forces for erosion, and set the erosion potential, cover and, under some circumstance soil strength, are major controlling factors that determine "how much of this potential is achieved.

Cover

Land use and management" practices are found to have a large effect on rates of upland soil erosion over a wide range of environments. Edwards (1987), in summarising 4500 plot-years of record from six centres in NSW, found this was a major feature of the data and attributed it to the ground surface conditions created by the land use, especially the proportion of ground cover. Major soil erosion events were almost entirely confined to periods when ground cover was low. For cultivated agriculture, Freebaim et aI. (1992) concluded "cover reduces soil erosion more than any other factor in tillage management, a conclusion borne out by many catchment and simulated rainfall studies (Aveyard et al. 1983, Freebaim and Wockner 1986, McFarlane et al. 1989, Sallaway et al. 1990)". "

In pasture and grazing lands, the control of upland erosion afforded by cover is, if anything, even greater than in crop land (Gilmour 1968, Lang and McCaffrey 1984, Ciesiolka 1987, Gardener et al. 1990, Silbum et al. 1992). This is because, in general, high cover levels are associated with greater soil water use, lower animal stocking rates and less soil detachment by the animals and with improved infiltration and soil pores due to root and insect activity. The more permanent nature of pasture cover means that it can improve soil propertie~ in addition to its effect as a mechanical barrier (Gilmour 1968, Connolly, Ciesiolka and SilbUm, unpubl.). Studies in US rangelands indicate that the direct contribution ofrainfall interception and soil protection by grass and shrub canopy to reducing runoff and erosion is small compared with the contribution of ground surface cover and good soil structure (Simanton et al. 1991). ""

In forests, where cover is high, upland erosion is low. Erosion is more related to establishment and early growth phases of new plantations (Gilmour 1968, Cassells 1984), roads and disturbed areas; Gilmour (1968) measured 28 and 240 times more erosion frotn. a bare fire break than fro~ eucalypt or pine forests, respectively. However, in open woodlands in drier or seasonally dry environments, higher erosion rates have been measured under living woodlands than under well managed pastures on cleared or killed woodland (Gardener et al. 1990). When water is limiting, trees irilribit grass growth, and if animal numbers arenot reduced accordingly, cover levels will be lower and erosion higher in the woodland. Leaf litter from trees is an important contribution to cover in forests and woodlands. .

90

Critical cover levels required to reduce soil erosion to acceptable levels have been quoted as: over 30 % (Freebaim et al. 1992),40% (Silburn et al. 1992, Gardener et al. 1990),50-70% (Lang and McCaffrey 1984); or as dry weight: 2-4 t/ha (Lal 1976b), 7 t/ha (in forestry, Gilmour 1968). Regarding cover type, the value of cover for controlling erosion is roughly: living near-ground > dead near-ground > living canopy > no cover. Surface rocks have similar effects to other forms of cover; increased infiltration with increases in surface rock cover have been found for a range of soil textures (Rawls et al. 1989).

Example from cropping on cracking clay soils.

Soil erosion rates for a range of agricultural land uses on a set of contiguous hillslope catchments on a black cracking clay are shown in Figure 10 (Wockner and Freebaim 1991, additional data G. Wockner peTS.comm.). The land use practises are summarised in terms of the mean cover (projected cover of plants or plant residues) - the mean of cover measurements after each runoff event - and therefore mainly reflect the cover during the summer rainfall season. Erosion is clearly related to the cover provided by each land use. There is a ten fold increase in erosion with 20 % cover compared with 45% cover. Cover of 40% or more is needed to reduce erosion to an acceptable level.

Cover affects erosion during individual rainfall events by reducing the amount of runoff, the rate of runoff (for instance the peak discharge) and the concentration of sediment in the runoff. These effects vary from·event to event depending on conditions on the catchment and the size of the event (for instance, the depth, duration and intensity of rain). Cover can appreciably reduce the amount of runoff for dry to moist soil conditions but for very wet conditions runoff is similar for all cover levels (Figure 10 Yule 1987, adapted from Freebairn and Wockner 1986). As more runoff is likely from wetter catchment conditions, total runoff over time is less affected by cover for the annual fallow-crop land uses (FigUre 7). Runoff generally occurs during the wetter summer periods when most of the land is in fallow and the only covet is afforded by crop residues. ·However, pasture, which uses water through most of the year, produced half as much runoff as any of the crop land uses because it maintained a high soil moisture deficit much of the time.

Management of Soil strength

In land uses such as agriculture and horticulture where tillage is used, soil strength may vary due to tillage. Freshly tilled soil has lower bulk density and strength than consolidated soil, though consolidation and resistance to rilling increase with time (or accumulated rainfall, we$ng and drying etc.) since tillage; differences may be evident even after 1.5-25 months (Foster et al. 1982). Ciesiolka and Smith (1992) studied erosion from pineapples grown on gravelly clay loam on steep slopes and found that, although pineapple leaves provide little soil contact cover for the first half of their growth cycle, soil erosion declined substantially during this period, because of increased soil strength and development of a surface armour of stones. Ciesiolka et al. (1991) found that strength and bulk density of the surface soil steadily increased up to 37 months after tillage on a loamy sand; erosion rate was reduced 10 fold after 15 months consolidation.

However, use of natural consolidation to control erosion has certain limitations. On self mulching soils, Loch and Donnollan (1989) found that, in the absence of stubble, rill erosion was similar for zero, fine and rough ti.11ed conditions - these soils effectively till themselves. Also, considerable erosion can occur in the time taken for consolidation to develop. Ciesiolka

91

and Smith (1992) found that 85 - 100 t/ha of soil was lost before consolidation and armouring became effective; they recommended that mechanical compaction be used to increase soil strength early in the pineapple growth cycle. Also with the consolidation of the soil surface, there is a marked decline in potential infiltration rates due to crusting. Ciesiolka (unpubl.) measured final infiltration rates under rain of 60 mm/hr on dry freshly tilled gravelly clay loam compared to 5-10 mm/hr (and much earlier runoff initiation) when the soil had consolidated over 5 or more months. Consolidated bare soil would generally produce large amounts of runoff, possibly of the order of 50% of rain (Silburn et al. 1992). The combination of cover (to maintain infiltration rates), and consolidation (to increase resistance to erosion), is preferred.

3.1.4 Effect of Slope and Slope Length

Slope steepness

In general, erosion increases as slopes become steeper, due to increases in runoff velocity and to gravitational effects on particles. Slope steepness is recognised as one of the most important factors influencing erosion rates.

Accurate prediction of responses of erosion to slope steepness has, however, been difficult. An apparent expectation that responses of erosion to slope and slope length would be similar for both single events and annual average erosion may have contributed to the problem. As well, the potential for interactions between slope steepness and length have only recently been recognised, possibly because much of the original data came from relatively short plots. The presence or absence of rill initiation could be a major factor influencing responses to slope.

Cropland erosion data for several locations were used in developing the Universal Soil Loss Equation relationship:

A= 0.43 + 0.30 s + 0.043 r where s = slope steepness in percent; A = annual soil loss

The data included sites with slopes from 3-18% and from 5-25%.

In the WEPP model, slope length is related to gradient changes because· slopes of greater length lose more soil as they become steeper. However, available data generally refer to slopes of 20% or less, reflecting the major emphasis on agricultural land. The Revised USLE does incorporate data on steeper slopes than did the USLE, and predicts less response to very steep slopes than did the USLE. For rehabilitation purposes, the Revised USLE would be recommended, and the review of slope steepness effects for disturbed land by McIsaac el al. (1987) could be useful. They· recommended an equation:

S = (12 ± 7) sin e + B

where S is a steepness factor to be used to account for effects of slope on erosion, e is slope angle, and B an intercept chosen so that S = 1 at 9% slope.

Slope shape can also be important, with research and modelling on some mined areas indicating that concave slopes will erode less than straight or convex slopes.

92

Figure "9

Figure 10

140

120

-jg 100 ~ c 80 0

-iii 0 60 '-W

--0 40 (IJ

20

o

[:}!~0~=]1 78 tlha II 14 tlha II 34 tlha

Mean 1976-91

76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91

Year

Measured annual soil erosion from a wheat-bare summer fallow catchment on clay soil on the Darling Downs

80

E'70 .§.

'0 c: (jJ '" "" "5 '" c:

.:E 50 c: c

'" ::&

.!.4O co

~ tl 30 .3

0 0 en

g20 c:

.:E c: c '" 10 ::&

0

Greenmount (W&F91 - 11 years 1978/19 to 1988/89) Av. Cover all storms

o

o

o o

• Bore Fallow oWh£t

o

• SummEirCrop -Inemp Stubble 0 Wheat

• Stubble Mulch - Wheat • Zerotill- Wheat

+--.....j--:---c--+--I---+---l--t---+--+-......-!.Lf'osture

0 IO 20 30 40 50 70 80 90

Mean Cover ("10)

• Soil loss (t/ho)

o Runoff (mm)

Erosion and runoff as functions of plant cover for a range of land uses on cracking clay soil at Greenmount, Darling Downs

93

Slope length

There is a general expectation that as slope length increases, erosion per unit area will also increase, as a result of increasing quantities and erosivities of overland flow. This is confirmed by some empirical studies (Wischmeier and Smith 1978, Ciesiolka et al. 1991, Ciesiolka and Smith 1992) but not by others (Smith et al. 1953). Of the slope length data from 14 sites used in development of the USLE (Wischmeier and Smith 1978) the exponent of length ranged from 0.0 (no effect) to 0.9 (large effect); at two sites there was no response (Foster 1982). Many of the empirical relationships in the USLE have been updated for the Revised USLE, and the slope length relationship in the Revised USLE is definitely to be preferred for steeper slopes.

Hydrologically there are two main effects of increasing slope length: (a) the catchment"area and therefore discharge increases, and (b) there is a greater probability of the flow becoming concentrated and, if soil strength is low relative to flow erosivity, ofrilling occurring. These effects lead to two explanations of the effect of slope length on soil erosion.

Firstly, there is an effect due to rilling. Where soil is easily detached, no effect of discharge (or length) is found for either rain flow and rill erosion, but there are significant differences in the erosion rates between the two types of erosion (Loch and Donnollan 1983a). When discharge (or distance down slope under a particular runoff rate) exceeds a critical value, rilling is initiated. Thus for a particular runoff event erosion rates from short slope lengths will be much smaller than for long slope lengths, but the effect is a step rather than a steady trend. Because the critical discharge for rill initiation will not be reached at the same downslope distance in all runoff events, soil erosion averaged over a large number of events will appear to increase more gradually with slope length.

In cases where rilling is initiated at low discharges, there are conditions (where a surface layer of very low strength overlies soil of considerably greater strength) where slope length will have little effect on rates of rill erosion. (Under such conditions, rills will tend to widen as further incision is prevented by the layer of consolidated soil below the tilled layer; flow depths will remain relatively constant, as will sediment transport capacity per unit width of rill and sediment concentrations.) Many tilled soils would fit this description, though only self-mulching Vertisols would fit it consistently. (Soils that do not self-mulch would undergo occasional consolidation in the absence of tillage.) Field data for cracking clay soils in the U.S. (Smith et al. 1954) confirm the lack of effect of slope length on erosion rates suggested above.

The second explanation for the effect (or lack of effect) of slope length on soil erosion is related to the interaction of detachment and transport capacities. When the rate of detachment of soil particles is low relative to the available transport capacity (the detachment limiting case) erosion may increase with distance downslope, especially if the erosive power increases with increasing flow depth down the slope. The contrasting case is where soil is easily detached and fills the available transport capacity at a short distance down the slope. If transport capacity and contributing area increased at the same rate with slope length, erosion (per unit area) would be constant. This would occur if there were no change in infiltration down the slope, the catchment were rectangular, and transport capacity per unit runoff rate did not change due to greater flow depth or development of more flow concentrations.

94

3.1.5 Soil Erodibility

Soil erodibility describes soil susceptibility to erosion. In the broadest sense, some soils will erode more than others under the same conditions, due to differences in:

* infiltration capacity (and hence the amount and rates of runoff generated), * cohesion or strength (resistance to detachment by raindrops and overland flow),

and; once soil is detached,

*

*

the 'transportability' of the detached sediment particles (the ease with which flow can keep particles moving) the 'depositability' of the detached sediment particles (the ease with which particles settle out of the flow).

The quantitative definition of erodibility varies depending on the model, mathematical or notional, used. Erodibility is often defined for the soil in its most erosion prone state and all factors that decrease erosion (such as cover and soil consolidation) are considered separately.

Some definitions lump all factors into a single erodibility term. For instance, the DSLE (Wischmeier and Smith 1978) basically defines soil erodibility as the averageanIiual erosion rate (under a specific set of soil and slope conditions) per unit average annual rainfall erosivity. Thus the definition of the erodibility term is based on measured rates of soil erosion. As such data are available for only a few soils (about five in A~stralia, LOCh and Rosewell 1992) methods have been developed to estimate erodibility trom other more commonly measured soil properties. Equations and nomographs have been developed to estimate erodibilty, using soil particle sizes, organic matter, structure and permeability classes for smface soils in the USA (Wischmeier and Smith 1978), and using physical and chemical properties for subsoils (Romkens et al. 1977). Some properties of Australian soils are quite different from those in the USA studies, the soils often being older, with more clay and less silt. Loch and Rosewell (1992) give erodibility values for some Australian cropping soils, and provide practical methods for incorporating effects of soil aggregation (common in clay soils) and particle density into the estimation of erodibility.

Other models divide erosion prediction according to sub-processes and landscape scales, and therefore have separate parameters for some of these components. An example is the CREAMS erosion model (KniseI1980). Hydrology is dealt with separately, and input to the model for sediment size and settling velocity by size class, and detachment and transport parameters for both intenill and concentrated flow are needed. Thus more parameters are needed than for simpler models. However many of these parameters can be measured directly in the laboratory and the remainder can be derived using erosion data from a small number of suitable events (Loch et al. 1989), rather than using long term erosion data. Loch et al. (1989) and Silburn and Loch (1991) found that \\lith the CREAMS model once differences in site conditions (slope, catchment area, etc.), sediment characteristics (size and density of each size class) and hydrology were input into the model, the key parameters of detachment and transport were not different between four clay soils.

4 CONTAMINANT TRANSPORT

In terms of effects of runoff on aquatic ecosystems, it should be noted that high concentrations of sediment can have direct detrimental effects irrespective of any other

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pollutant that the runoff may carry.

Contaminants carried by runoff include nutrients (either directly eroded from soils, or dissolved from fertilisers or other soil amendments) and a range of other organic and inorganic compounds (with sources ranging from pesticides to industrial effluent),

Gallant and Moore (1992) note that "The key chemical characteristics governing contaminant transport in the environment are water solubility, adsorption, and persistence. The solubility of pesticides varies greatly, ranging from virtually insoluble .... to soluble. Generally, the greater the water solubility, the more potential there is for pesticides to leach from the soil, either moving to the water table or being removed in surface runoff... ... Adsorption, which results in physico-chemical interaction or bonding of pesticides with soil retards leaching by holding pesticides in the active surface soil where most rapid breakdown occurs. Persistence is the ability of the pesticide to resist degradation, and is usually described by the half-life ... " Although their comments dealt specifically with pesticides, they can be generalised to most potential contaminants.

Potential contaminants vary greatly in their solubility, adsorption, and persistence, and this can greatly affect the management strategies that could be required to minimise environmental problems. As an example, the potential for spreading of sewage sludge in a forest area can be considered.

Firstly, some solutes in the sludge are slowly-soluble, and will continue to be released from the sludge for 1-2 years. These solutes include phosphates and heavy metals, which are strongly adsorbed by many soils. If the sludge is spread on the soil surface, there is potential for these contaminants to move in surface runoff, and reach aquatic ecosystems during the following 1-2 years. But, if the sludge is incorporated into the soil, phosphates and heavy metals leached out of the sludge will be adsorbed by the soil, and immobilised. (Erosion of the soil, however, would then mobilise those adsorbed materials and, because the heavy metals in particular would be largely adsorbed by the fine clay particles, there would be potential for these to be carried considerable distances as part of the suspended load.) Because the heavy metals are not degraded or in any way destroyed, there will be a finite limit to the amount of sewage sludge that can be added to any area of soil.

In contrast, N03- and NH4+ in the sludge are highly soluble, and are largely removed from the sludge in the first rainfall event to cause significant leaching. If the sludge is spread onthe soil surface, the N03- and NH/ that move into aquatic ecosystems would be rapidly degraded, though possibly causing some short-term problems. If the sludge is incorporated into the soil, however, N03- andNH/ will not be greatly adsorbed, and may leach into the groundwater where their residence time may be quite long. There may be potential for soil conditions to cause denitrification and gaseous losses of N to the atmosphere, and hence, the persistence of N03' and NH/ in the soiVgroundwater system would be a major consideration.

One point that emerges from the above example of sewage sludge disposal is that the location of the source of contaminant and the subsequent pathways of contaminant movement can be quite important. In some instances, there is potential to greatly reduce specific problems,

Where erosion causes movement of contaminants originally resident in soils, there are commonly both dissolved and adsorbed components. Adsorption of contaminants to clay and organic matter particles means that the contaminants are carried dominantly by suspended

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sediment. Selective transport of suspended sediment can result in concentrations of contaminants in sediment being higher than in the soil from which they came. These increases in concentrations are referred to as enrichment ratios, and are sometimes used to estimate the amounts of nutrients or other contaminants removed with eroded sediment.

A range of models have been developed for modelling movement of contaminants within the landscape. These are reviewed in detail by Gallant and Moore (1992), who note that the single greatest difficulty in applying models of contaminant movement to Australian conditions is the need for suitable databases, especially of soil properties. In the short term, many of these models will give no more than relative responses to alternative management strategies.

5 PREDICTIVE MODELS OF EROSION

A wide range of models, varying in complexity and purpose, is available for prediction of soil erosion. These use data from the relatively few sites and time periods where erosion has been measured. The more useful ones are those with a generic structure representing the processes and input parameters that describe site specific conditions; they can be used to estimate erosion for a much wider range of combinations of conditions than have been studied empirically.

Some important capabilities required of soil erosion modelling are to:·

*

*

*

*

predict the longterm soil loss from a soil profile (fundamental to the sustaina.bility or otherwise of productivity);

enable comparison of alternative management practices, including structures and land shaping, in controlling soil erosion;

predict the effects of environmental factors (climate, soil, slope etc) on erosion, so that appropriate land use or capability can be defined;

predict off-site impacts (siltation of infrastructure, water quality etc.). These types of applications entail modelling larger. and often more complex, catchments. Modelling the sediment sizes being transported also becomes more important.

In practice no single model provides all the required capabilities. A range of models is required for erosion prediction and the choice of model depends on the aims, scope. time scale and landscape scale of the particular problem. A few examples of uses and appropriate types of models are: .

*

*

Decision support systems. land capability assessment (Thomas et ai, 1992). broad scale inventory (Littleboy et al. 1991):- USLE (Wischmeier and Smith 1978). systems models such as PERFECT (Littleboy et al. 1989);

Effect of erosion on productivity/sustainability (Littleboyet at 1992b). comparison of land uses over long periods:- Systems models e.g. PERFECT.

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* Design of structures, landfOlnlS, land shaping and bench/batter layouts etc. for erosion and off-site sediment control on agricultural, horticultural, mine and construction sites:- multi-scale erosion deposition models e.g. CREAMS erosion (Knisel 1980) with multi-dimensional runoff models such as KINCON (Connolly et al. 1990) or ANSWERS (Beasley et al. 1980) to predict storm runoff inputs;

* Effects of land use on off-site sediment load, including predicting inputs to sedimentation ponds and water bodies:- as above, also more broad-scale spatial models such as AGNPS (Young et al. 1989);

* Interpretation of experimental data in terms of physically meaningful parameters, generalising results from experimental studies, integration across scales:- process models (e.g. CREAMS, Rose 1985).

5.1 Soil loss models

5.1.1 Importance of hydrology

Sediment yield is the product of sediment concentration and volume of runoff for a time period. Sediment concentration is itself determined in part by hydrologic factors. Thus modelling hydrology is an important prerequisite to modelling soil erosion and deposition. All erosion models either contain a hydrologic component (embedded in the model or explicitly defined), or require hydrologic inputs.

5.1.2 The Universal Soil Loss Equation (USLE)

(Wischmeier and Smith 1978) is a statistical summary of annual average soil loss data from plot studies in the United States. It was intended for predicting the long term average soil loss for specified cover and management on a given field. It predicts upland (overland flow and rill) erosion only and does not take into account net deposition. Wischmeier and Smith (1978) state that 'the USLE is not recommended for prediction of specific soil loss events'. They give a· clear statistical interpretation that a prediction for a specific event or year is really an estimate of the mean soil loss for a large numbers of years (events) with the same annual (event) EI30 and C factor, with a wide range of combinations of within year (event) distributions contributing to the mean value. Only this statistical interpretation is valid. Therefore there is no valid method of deriving parameters for the USLE except from long term erosion data although reasonable estimates can be made for pragmatic purposes.

With all its limitations, the USLE is still a useful predictive model for land use planning, particularly when used on a comparative basis. The USLE cover-soil loss relationship has consistently proven reasonably correct throughout the world. Freebairn et al. (1989) found that measured reductions in erosion due to cover on clay soils were greater than those predicted by the USLE. Galletly (1985) showed that, if the USLE had been available during the early expansion of cultivation into the Lockyer Valley, it would have given warning of high erosion rates that subsequently removed about 0.5m of soil from some 1O,OOOha of sloping lands.

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5.1.3 Systems Models

PERFECT (Littleboy et al. 1992) simulate the plant-soil-water-management system through time (e.g. 100 years). Simulating the soil water balance is a key feature of these models. They are generally one dimensional, representing a single hydrologic-soil-crop unit and use a time step of one day. Systems models can extrapolate through time, giving long term averages and probabilities useful in decision support systems. Because of their integrated nature, systems models are useful for studying effects of erosion on other components of the system (e.g. crop yield; Littleboy 1992b, Thomas et al. 1992) and vice versa (e.g. effect of management on soil loss). However as they generally contain a large number of empirical components, local data are needed for validation and to provide parameter values.

Erosion is only a part of the system represented and the erosion component is usually a simple event soil loss equation such as MUSLE (Williams 1975) or the simpler models of Rose (1985). Systems models keep track of many system state variables (e.g. cover) and predict outputs such as daily runoff that are useful inputs to the erosion components. Erosion models for use in systems models should be developed to use these 'known' values. In systems models using a daily time step, accurate prediction of runoff rates (e.g. peak) and rainfall energy (e.g. EI3o) is difficult; this limits the accuracy of prediction of event soil loss. However, good predictions of long term erosion and the probability distribution of annual

. erosion can be obtained. Being based on a water balance, systems models can simulate the complex interactions of rainfall patterns and crop water use patterns (Figure 8) and temporal variability in erosion (Figure 9) reasonable well.

5.1.4 Process Based Models

CREAMS describe the system mathematically, as do systems models. However, the scope of the model is different, with greater emphasis on details of particular processes. Other parts of the system may be ignored altogether. Time scales are necessarily shorter, modelling single events with a time step in minutes and with spatial detail increased, including two or three dimensions. A greater range of scales of processes is often included, for example, from soil aggregate breakdown causing surface sealing and supplying sediment, to routing of runoff and sediment in second and third order channels. For practical reasons it is not possible to include all processes, scales, dimensions etc. The model will be most complete in aspects relating to the intended (limited) use of the particular model. The user must choose a model with strengths in areas of importance for the case at hand.

Process models can extrapolate across landscape scales and are a powerful tool for exploring effects of management options in complex situations. Because they involve less distortion and lumping in time and space, it is more likely that parameters for process models can be measured directly or be derived from event erosion data, as described in the section on soil erodibility. Hydraulically complex but regular systems such as constructed landforms (mine spoil bench/batter structures - see Silburn et al. 1990, bed/furrow and contour bay/waterway cropping systems - see Connolly et al. 1990) are particularly suited to modelling with CREAMS in combination with storm hydrology/ hydraulic models such as KINCON (Connolly et al. 1990) or ANSWERS (Beasley et al. 1980).

99

5.2 Simple vs Complex Models?

The accuracy of model predictions is determined by model structure and the inputs and parameters used. As the structure of a model is made more physically realistic, the model becomes more complex and requires more inputs and parameters. However, the more physically realistic model has the advantage that its parameters are more measurable. Simple models may appear easier to use as fewer parameters are required, but the complexity is simply shifted to the parameters, making their derivation more difficult. For instance, the USLE is a simple model mathematically; however there is a large component of the model embodied in the written reference material. When key processes are not considered, or are lumped together, data from studies of these processes cannot easily be used to derive the lumped parameter values. Simplifications can lead to complications! The more physically realistic model also has the advantage of being applicable over a wider range of situations. However, as more complex models often require more effort to use, prudent choice of models to suit the requirements of the application is important.

5.3 Model Parameters

Models for soil loss prediction based on a wide range of approaches, from statistical summaries of data through to detailed erosion-deposition process models, are available. All have one common limitation: how does the user obtain the parameter values that will give the best predictive accuracy?

Recent research on soil erosion modelling reviewed by Silburn and Loch (1990) has shown that ~good predictions can be obtained using parameters measured at small scales provided the model is a sufficiently physically realistic representation of the processes operating at the scale of interest and that these are the processes operating at the scale of measurement. Successful examples of this approach were given for hydrologic modelling on cultivated and native pasture catchments and for modelling erosion on a range of clay soils. Rainfall simulator plots of 1m2 gave useful parameters for modelling the hydrology of field catchments using a modified ANSWERS model. Data from rainulator plots (up to 90m2) gave good estimates of parameters for modelling hillslope erosion using four soil loss equations and the CREAMS erosion-deposition model. With the CREAMS model, 'soil erodibility' differences between three clay soils were fully accounted for by differences in hydrologic characteristics and sediment properties alone. None of the soil loss models account for differences in soil erosion due to the occurrence verses non-occurrence of rills except by changes to the soil erodibility parameters. When processes are 'lumped' in a model the parameters are distorted and less use can be made of measured physical properties. Thus, in the past, little use was made of the large amount of process data obtained from laboratory analysis and rainfall simulator studies. .

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Wischmeier, W.H. and Smith, D.D. (1978). Predicting rainfall erosion losses - a guide to conservation planning. U.S. Department of Agriculture, Ag. Handbook No. 537.

Wockner, G.W. and Freebairn, D.M. (1991). Water balance and erosion study on the eastern Darling Downs - An update. Aust. J. Soil & Water Conservation 4: 41-47.

Young, RA. (1980). Characteristics of eroded sediment. Trans. ASAE 23: 1139-1146.

Young, RA., Onstad, c.A., Bosch, D.D. and Anderson, W.P. (1989). AGNPS, a non-point source pollution model for evaluating agricultural watersheds. J. Soil and Water Conserv. 44: 168-173.

Yule, D.F. (1987). Water management of vertisols in the semi-arid tropics. In "Management of vertisols under semi-arid conditions", Proc. First Regional Seminar on Management of Vertisols under Semi-arid Conditions, Nairobi, Kenya, 1-6 Dec. 1986, Inter. Board of Soil Research and Management Inc. (IBSRAM), pp. 107-123.

106

EROSION CONTROL IN CIVIL CONSTRUCTIONS

P.N. Truong Natural Resource Management, QDPI, Indooroopilly, Qld. 4068.

ABSTRACT

With increases in population and living standards, large areas of lands are converted each year to housing estates, industrial sites and public utilities. As the community become more concerned with the deterioration of the environment, there is an increasing demandfor bio­engineering techniques to control erosion and land stabilisation in civil constructions, particularly in urban environments.

Although engineering and non-engineering techniques are discussed, the emphasis of this chapter is on bio-engineering materials and methods. These materials are currently available in Queensland and these techniques have been successfully used under a range of climatic conditions.

1 INTRODUCTION

With increases in population and living standard, large areas of lands are converted each year to urban and industrial uses such as housing estates, shopping centres, schools, factories and highways. During the period of conversion land is disturbed and often exposed to water and wind erosion.

The amount of erosion depends on the soil types, slope, gradient, climatic factors and construction methods. On-site erosion can delay work and increase cost while offsite effects such as air and water pollution can cause significant environmental damage. Environmental damages include gullied slopes, washout streets and underground utilities, buried lawn, clogged drainage ditches and storm water drains, pollution to streams, beaches and coastal water.

Many forms of erosion occur at civil construction sites which need to be stabilised: Sheet and rill erosion, gully and tunnel erosion and also lands lip (mass movement) can occur simultaneously on one site. These erosion forms are due to many factors but in civil works they are caused mainly by the concentration of runoff water, steep and long slopes and unconsolidated ground. They often occur at high rates over a very short period of time during or after construction.

Erosion control in civil engineering works is needed for structural, environmental and aesthetic reasons. Land stabilisation performs an important engineering function because of its direct influence on the soil, both at the surface by protecting and restraining the soil and at depth by increasing the shear strength of the soil mass. Both State and local authorities now have strict statutory requirements to reduce/prevent these environmental damages. With certain methods, for example the use of vegetation, land stabilisation can reduce the visual impact of the engineering works and enhance the quality of landscapes.

107

2 OBJECTIVES AND PRINCIPLES

The main objectives in soil conservation in civil construction are to prevent controllable erosion and to minimise offsite sediment damage. Soil conservation measures that prevent or minimise the effects of concentrated flow, reduce slope gradient and slope length or provide protective ground cover are the keys to effective erosion and sediment controL

In the short term the aim is to reduce or minimise the damaging effects of these forces on the bare surface but the long term aim is to 'strengthen both the surface and the soil mass against these erosive forces. In the case of wind erosion, velocity at the soil/air interface is critical while for water erosion both raindrop impact and velocity and volume of runoff water are important. To be effective both engineering (constructed) and non engineering (surface stabilisers, geotextiles etc.) measures are needed but also there is an increasing demand for the vegetative measure - bio-engineering techniques - in civil constructions not only for aesthetic reasons but also because it is very cost effective in most situations. As most engineering measures are well documented (see the first three references, also Coppin and Richards 1990; Quilty et al. 1978), the emphasis of this chapter is on bio-engineering techniques. However those measures that can complement or enhance the bio-engineering techniques are also discussed. The materials and technology presented here are currently available and particularly applicable under Queensland conditions.

3 PLANNING CONSIDERATIONS

3.1 Types of measures

At any major construction site both temporary and long term land stabilisation measures are required. In areas where final landscaping is to be done after the construction (e.g. car parks in shopping centres), only short term measures are needed. On areas where slope stabilisation is required before construction can proceed (e.g. retaining walls), long term or permanent measures are used. The materials and methods described later in this chapter can provide both short term and long term stabilisation depending on the cost, topography and urgency of the operation.

3.2 Locality

The location of the sites often dictates the techniques and also the cost of the measures needed. In urban areas, due to the high population density, high land value and often restricted space, the operation is more difficult, and needs to be effective and to provide fast results. However these sites are often smaller in size and close to power and water, and clients can accept higher unit cost so more intensive and advanced techniques can be used.

In non urban areas, the problem is not as pressing, temporary measures are not often required and low cost and low maintenance techniques are more acceptable.

3.3 Compatibility

Cost is only one factor in co~idering the techniques/measures needed for each site. The measures need to be compatible with the local environment. While large scale engineering measures are more acceptable on industrial and non urban lands, bio-engineering techniques are often preferred in urban areas.

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4 PREDICTION OF SOIL EROSION

Prediction of potential soil movement is an integral part of the planning and design of suitable control measures. For smaller sites, over-estimation is usually adopted as a safety factor but for larger sites a good estimate of potential soil movement is important· to minimise cost.

The Universal Soil Loss Equation (USLE) (Wischmeier and Smith 1965) is the most commonly used method in Australia in which rainfall, soil erodibility, topography, cover and practice factors are taken into consideration. Information on erosion processes and more details on prediction of potential soil movement is discussed by Loch and Silbum elsewhere in this book.

5· TECHNIQUES AND MATERIALS

Every site differs in topography, soil type and in the activity to which it is subjected. Therefore control measures must be tailored for each individual site. In general these measures fall into three categories:

• Surface protection of exposed surface • Control of runoff water • Trapping of sediments

In general the last two categories require constructed or engineering structures while non engineering and bio-engineering techniques are needed for the first category.

5.1 Land shaping

Land shaping refers to reshaping of the ground surface by excavating and filling to obtain desired gradients determined by topographic survey and proposed developmentlayout. It may be designed to provide more suitable sites for buildings, service facilities or associated land uses or to improve surface drainage.

The general guidelines are:

•

•

•

•

Where possible, development design should utilise existing topography and natural features to avoid extreme land modifications. Shaping only those areas which are to go under immediate construction, as opposed to shaping the entire site, is a great help in erosion control. .

Topsoil should be stripped and stockpiled for respreading on all exposed areas when final shaping has been completed (Topsoil storage see Section 5.4.2).

Timber, logs, rubbish and other vegetative matter which will interfere with land shaping or will affect the stability of fill should be removed.

Isolated large rocks should not be left in fill. They may be a major threat to batter stability if the fill becomes saturated with water, and could initiate mass movement.

109

5.1.1 Batter construction

Batters are exposed earth (rock) smface, usually with steep slope gradient. They are created by earth (rock) excavation (cut batter) or by deposition (fill batter) of excavated materials.

Batters represent a special and severe case of land shaping. While the surface may be protected by one of the methods described below, the resistance of the batter to erosion will be determined primarily by the engineering design.

Batters must be designed to satisfy stability criteria, with allowance for future maintenance. For stable soils, a batter slope of 2 horizontal length to 1 vertical length (2H: 1 V) is likely to be satisfactory, but such a batter can only be mown by hand. If the embankment is to be mown by a conventional tractor, a batter of 5H:1V is desirable for safety. A 3H:1V slope is about the steepest on which tractors and maintenance equipment can operate efficiently. Examination of existing batters in the locality will give a good indication of suitable slopes. Rounding of the top and toe of batters helps control erosion and permits easier mowing.

Cut batters. Diversion banks to carry water from the face of the slope should be built at the top of slope prior to cutting operations. Steepness of slope will depend on the soil and design. Cut slopes of 3H: 1 V or flatter are desirable for erosion control and stability. As the batter is excavated, serrated cuts may be necessary to hold topsoil and assist with the establishment of vegetation. Temporary toe drainage should be maintained as the work progresses, with the permanent toe drainage installed when final depth is reached. Subsoil drainage should be provided, where necessary, to intercept seepage that would adversely affect slope stability. Cut slopes 10m or more in vertical height should be benched to provide access for planting and maintenance. The benches should keep water from flowing down the face of the slope. They should be sloped toward the toe of the upper bank and should convey water to a suitable outlet (Figure 1.1).

Fill batters. Permanent toe drainage should be installed at an early stage and discharged to a suitable outlet. The filling should start at the lowest point. At the completion of each work . period, or at the onset of rain, a windrow of suitably compacted material should' be constructed along the edge of the batter, to prevent drainage water from passing down the' fill slope. Topsoil should be removed from the area to be filled. The existing soil should be scarified to improve bonding with the fin material. '

Fill material should be free of sod, roots, or other material that will decompose or prevent proper compaction. Fill material should be spread and compacted in a series of horizontal layers (usually 15 to 20cm thick) to attain the designed compaction.

Slopes of 3H: 1 V or flatter are desirable' for establishing and maintaining vegetation. Fills 10m or more in vertical height should be benched to provide access for planting and maintenance (Figure 1.2).

Terraced batters. This type of batter sqould be considered on slopes greater than 3H:IV, where the vertical height exceeds 5m. The width of each terrace or bench should satisfy stability requirements, with a minimum of width. of 2m. Each bench should be sloped inwards and graded longitudinally for drainage (Figure 1.3).

110

1. ,

NORMAL CUT OR FILL B,UTERS

...-----'-----ROUNOED SHOULDER

CATCH DRAIN

~y....~y....~--~----TOPSOIL

¥,..!-------;...----COMPACTED SUBSOIL - SURFACE MAY REQUIRE SCARIFYING

,2. STEEP CUT BATTERS

~:y.,~~~ ____ TdPSOIL

ROCKY OR HIGH~Y COMPACTED SUBSOIL SERRATED BY GRADER BLADE

3. VERY STEEP CUT BATTERS HANO PLANT -GROUNO'COVER PLANTS

WIRE NETTING. -BUSHES . --------t-----______ -CREEPERS

BRUSHWOOD. [7~~~§m~~~~:m:~~ PLASTIC NETTING OR TIMBER SLATS

- KIKUYU GRASS

POSITIONED BEFr~~~~~~~ia§§~~~~~~~ lOPSOIL POURED· "-TOPSOIL FROM TOP OF . BATTER NATURALLY

'~~~~COMPACTED SUBSOIL

SUPPORT POSTS ---~~""'""ORIVEN WELL INTO

COMPACTEO SOIL

4. VERY L.ARGE CUT OR FILL ~.A:rTERS ".

Figure 1

CATCH ORAIN

...------,!~~~~~-'-WIDTH OEPE NOS ON ACCESS REQUIREO

.~~~~~---~TERRACE

,u,~-'--------__ CA'TCH ORAIN

Batters . construction and top soiling . techniques (Soil Conservation Authority of Victoria (Garvin et al 1979)

111

Vertical batters. In certain situations (e.g. when cutting through rock or firmly consolid.ated subsoil), a near-vertical batter with no vegetative cover may be the best solution. In these cases, provision of a wider hori:wntal area between the batter and the road (or other construction at the bottom of the batter) should be considered. When slopes are prone to massive slumping, very steep batters may also be the only way to control the situation. Barriers may have to be erected to minimise the consequences of collapse in critical areas.

For small batters in sensitive areas, the use of retaining walls may provide an economic and aesthetic solution.

5.2 Engineering or constructed structures

As mentioned above, these structures are designed mainly to control runoff water and to trap the sediment load .. Their general design and standards are welLlisted in most engineering and soil conservation handbooks and they are beyond the scope of this paper. Only the commonly used structures, particularly those needed to complement the non engineering and bio­engineering techniques are mentioned here.

5.2.1 Runoff control structures

• Diversion channels/diversion banks. A diversion channel is an earth channel with a minor ridge on its lower side constructed across the slope. It is designed to protect slopes or development works below it by intercepting surface water and diverting it to a stable outlet at a non-erosive velocity.

•

Where flows are too large to be contained by a simple channel, a diversion bank is constructed. The operating principle is the same, but it comprises a larger channel behind a substantial bank which is pushed up in the course of excavating this channel. Height of the bank and channel cross-section depend on the size of flows that must be contained.

Diversion banks may be temporary or permanent structures. They are used:

across slopes to reduce slope length into non-erodible lengths;

above batters, borrow pits, gully heads and similar areas to protect them from erosive concentrations of runoff;

at the base of cut or fill slopes to carry sediment-laden flows to sediment traps;

to divert runoff away from buildings and exposed construction sites.

Diversion banks must have stable outlets to minimise gully erosion.

Level Spreader. A level spreader is an outlet constructed on the contour line. It converts a concentrated flow of runoff into sheet flow, and discharges this at a non­erosive velocity onto an undisturbed area stabilised by vegetation.

112

Level spreaders may be used as outlets for diversion channels, where stonn runoff has been intercepted and diverted to stable areas. They should be used only where the spreader can be constructed on undisturbed soil. The area directly below the level spreader should be level to ensure even spreading of water.

• Perimeter bank. This is a temporary banier of compacted soil or hay bales located around the perimeter of construction sites or other disturbed areas. It prevents sediment-laden runoff from leaving a construction site or disturbed area, and prevents off-site runoff from entering it. Stonn runoff prevented from entering a disturbed site by a perimeter bank should be directed to a stable disposal area.

5.2.2 Sediment trapping structures

• Gravel outlet. The gravel outlet is an . auxiliary gravel structure installed in conjunction with and as part of a diversion channel, perimeter bank, or other structure designed to temporarily pond sediment-laden surface runoff. It may be formed solely from gravel, or may incorporate a core of straw bales. It provides a means of draining stonn runoff while retaining the sediment.

• Sediment trap. Sediment traps are temporary de-silting structures designed for use during the construction period. They are designed to trap sediment in runoff before it enters stonnwater pipes or channels, which receive runoff from only a small catchment. The trap operates by slowing or stopping runoff water ,at some point on its route, so causing it to deposit its· sediment load.

A trap generally will be required at or near the final point of discharge from the construction site. Consideration also should be given to the trapping of sediment at other points on the site drainage system, so as to prevent the blocking of the main site drains.

The need for traps in the completed works should be avoided, if at all possible. Sediment traps are useless unless maintained, and the need fora sediment trap in completed works would suggest that soil has not been appropriately stabilised.

The followings are descriptions and illustrations of small temporary sediment traps recommended by the Soil Conservation Authority of Victoria.

•

•

Straw bales (Figure 2.1). Straw bales are used to detain small quantities of sediment from bare areas. Bales must be securely anchored to the ground, with ends tightly abutted. Roughening of soil or embedding bales to a depth of 75mm may be necessary. Crushed rock (50mm) is satisfactory for the outlet. Inspection after each runoff event is necessary. The structure should be removed when the bare area has been stabilised (Figure 2.1).

Small earthworks (Figure 2.2). These are an inexpensive means of collecting sediment and could easily be extended if required. Allowance should be made for sediment removal; the sediment should be deposited in a suitable area and in such a manner that it will not erode. The structure should be removed and the area compacted when the construction area has stabilised. Regular maintenance is required after each rainfall producing runoff.

113

~ ~

.j:::>..

STRAW BALES STAKED TO THE GROUND

l '- . • ::i;'·p4.fJ.!? "JJf'" '~~'::';o!i'(~":I" ( {: g~ • .t~~.;:~ ."(.=. ,.".

J~~ M •• : -('6 .. _ - I"'\:l.r l ...... "'" .

t fi1-:,,,. i(;~if;!~r;t'4' , 't·::~~J.~;.~·~1: .• {i~,O-- CRUSHE!) ROCK .oUTLET ~ : ~'~ .. :J:"': 't. , TO HALF HEIGHT

., OF BALES

1. IMPROVISED FROM LOCAL MATERIALS

I \

i \. EXCAVATION

jCl--Y SMALL DIAMETER • ~\\I\\I,) ~\ 'OVERFLOW' WATER LEVEL. -

DRAINAGE PIP~E . ty 1'1 __ UNDISTURBED, VEGETATED f 1---\-.\ /'~'I. \ '~ OVERFLOW

'1 1 I UNDISTURBED VEGETATED BANK BUILT FROM ~ J: :\~ AREA-MINIMUM GRADIENT

. r \':\~ EXCAVATED MATERIAL ~ 1 WIDE AS POSSIBLE

2. SMALL EARTHWORKS

PERFORATED BAFFLE TO REOUCE WATER VELOCJTY /

/

4. BRUSHWOOD BANK

5. SILT

~ ;~~~~DF;:~'I~ENT --------9"USHWDOO

TRENCH

~I , ~'I~~~" W~!~RIC

r---::::----------~~~I'i.~N~~~t:PEO

FENCE COVERED WITH FILTER FABRIC REINFORCED WITH FINE MESH NOTING

6. CHECK DAMS

---------------------TRAPPED SEOIMEN T

}--------------.,..> Sf.nIES OF CHECK

PORTABLE STEEL TANK FITTED WITH BAFFLES

(REMOVABLE FOR CLEANING)

DAM 5 IN GUllY OR WATERCOVRSE .

3. PORTABLE TANKS

Figure 2 Temporary small sediment traps (Soil Conservation Authority, Victoria)

• Portable tanks (Figure 2.3). In built-up or congested areas, the portable tank may offer a practical solution. The size of the tank is based on a judgement decision, with due consideration being given to the soil type, area exposed, slope and the anticipated rainfall. Provisions should be made for the regular cleaning out of sediment.

• Brushwood banks (Figure 2.4). Brushwood banks are easily constructed using trenches or similar materials to support the filter fabric, which is buried in a cut-off trench on the upstream side and tied to the ground on the downstream side of the bank. The runoff water should be made to pass through the filter fabric, leaving most of the soil particles behind. Hessian is suitable only for small jobs of short duration, since it tends to deteriorate after thirty days.

• Silt fences (Figure 2.5). Silt fences are similar to straw bales, except that a strong fence about 500mm high is used to support the filter fabric. The filter fabric is buried in a cut-off trench, to prevent silt from escaping underneath the fence. A full width of filter membrane should be used. When the deposited sediment reaches the level of the top of the fence, the membrane may be cut above the level of sediment and supported on a new fence. The deposited sediment is then stabilised by vegetation.

• Check dams (Figure 2.6). Check dams should be located in a straight section of the watercourse. They may be constructed from either semipervious or impervious materials such as brush, logs, timber, rock, concrete, sheet piling, sandbags, etc. Care should be taken in their use, since the dams may cause downstream scouring. Care must be taken to prevent failure caused by the weight of the water or sediments undermining or side cutting, or downstream scouring.

In addition to these small structures, others such as filter dams and sediment and retention basins are suitable for the principal discharge point of major construction sites.

• Filter dams and weirs. Filter dams built of pervious materials such as straw bales, washed stone or gravel, gabions, or sandbags filled with gravel or stone may be placed across minor drainage lines to trap sediment. In grassed channels they should be embedded at least lOOmm in the soil to prevent water tunnelling beneath them. Hay bales should be stacked to secure them, while stone-filled sandbags should be stacked in an interlocking fashion.

On larger drainage lines, timber and hay or timber and stone weirs, comprising a core of baled hay or stone contained by timber form work or a log barrier on either side, will be more secure. These structures are very effective temporary or permanent sediment traps for perennial streams, and are particularly applicable where work is to be done in the stream itself.

In all the above cases, the check structures should be designed with adequate storage and outflow capacity to secure them against failure, which may result in more damage than would have occurred if the structure had not been used in the first place.

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• Sediment and retention basins. Sediment basins are installed prior to development activity on a new area and should remain in place until such activity has been completed and the land has been stabilised. They may be sited to trap runoff from individual construction locations such as industrial, commercial or school development sites, or they may be located on floodways below urban subdivisions.

A sediment basin does not replace on-site control measures such as perimeter banks, diversions, and sediment traps at stormwater inlets. These measures filter only a portion of the sediment carried In runoff. The sediment basin is a final check to trap a significant part of the remaining sediment, before runoff discharges into stormwater mains or enters streams.

5.3 Non-engineering measures

The main effect of non engineering measures is the protection of exposed and erodible soil surfaces. Most of these measures provide short term protection and are used in conjunction with other measures. However, some measures can provide permanent protection such as concrete blocks and pavers.

5.3.1 Surface roughening

Temporary protection can be obtained by roughening the soil surface to reduce runoff and subsequent erosion.

Track marks left by dozer runs up and down the slope (along the line of steepest grade) can be very effective. Surface can be roughened by an excavator with a toothed bucket or a chisel plough, scarifier or ripper. On low and long slopes only intermittent lines are required.

5.3.2 Chemical surface stabilisers

The use of chemical surface stabilisers is quite common in urban and industrial situations. Most chemical stabilisers are soil binders, providing a thin surface crust. The advantages of these methods are that they provide instant protection, and are suitable for temporary stabilisation while the construction progresses. They are generally most effective against dust control at industrial sites, and sand blasting and movement on canal estate developments. These sites can be stabilised vegetatively later.

The main disadvantage of these chemicals is that the crust must remain intact to be effective. Therefore no traffic of any kind is permitted. In addition specialised machinery is generally needed for their application.

Some of these products are designed to be used on their own as temporary surface stabilisers, others to be used in conjunction with the vegetative method (Table 1).

116

Table 1 Some of the chemical stabilisers available in Australia.

TRADENAMES (or Common Names)

Bitumen (Anionic emulsion)

E.P. Powder (Earth Protection)

Soil LOK

Crustex

Geotech 804 ) )

Geotech IT )

RATE

10,000 - 50,000 L/ha (1:1 mixture)

250 kg/ha

2,500 - 25,000 L/ha

1,000 - 4,500 L/ha

2,000 - 6,000 L/ha

SOLUBILITY

Water soluble

Water soluble

Water soluble

Water insoluble

Water insoluble

5.3.3 Geotextiles

A wide range of geotextiles is now available commercially in Australia (Table 2). In general they fall into three groups:

• Filter membranes. These are made of strong synthetic material such as fibre glass and non biodegradable synthetics and generally have the property of being permeable to water but not to soil particles. They are mainly used as a drain lining in dispersible soil, as an underlay over the subsoil which is then covered with topsoil, as a lining to a drain barrier or sediment basin or to form a silt fence (Figure 2.5).

•

Some of the filter membranes available in Australia are:

Geotex - woven polypropylene cloth Rheem Polyfabric - woven polyethylene Akren polyethylene fabric - woven polyethylene Sarlon Polyweave (R, HR and F) - woven polypropylene Belton Geotextile - woven polypropylene Polyfelt Geotextile - non woven polymer resin Terran Geotextile - woven polypropylene and polyster

Mattings. (Fibrous netting materials). Mattings have increasingly become an essential tool in controlling water erosion in the laJ1d stabilisation field. There are two types, one for surface laying and the other to be· buried under the topsoil.

The three main roles of the surface laid mattings are considered to be:

117

•

Soil erosion control through surface cover which increases water infiltration, reduces surface runoff and minimises soil movement.

Promoting vegetation establishment by modifying the micro-climate of the soil surface through partial shading, moderating surface temperature (lower in summer and higher in winter), reducing water evaporation, improving soil organic content and protecting seedling from wind blast.

Long term protection in 'area where vegetation establishment is difficult, such as shallow soil and steep slopes. On any particular site generally the combination of vegetation and geotextile material provides a composite solution to the soil erosion problem.

In view of the above roles the ideal matting should provide:

High percentage surface cover to protect soil surface from erosive rainfall and runoff.

Thick cover to intercept splashed soil particles.

Spongy texture with open space throughout the material to allow light penetration and seedling emergence.

Highly absorptive and flexible when wetted so the matting remains a good contact with the soil surface it is protecting.

Mattings are made from a wide range of materials from biodegradable natural fibres to permanent synthetics. From the above discussion the surface mattings made from natural fibres are generally more effective than the synthetic ones (Table 2).

The aim of the buried type of matting is to hold the topsoil on steep slopes. They are normally made of bulky and spongy synthetic materials. The matting is pinned and covered with topsoil. This matting is not suitable in areas with high intensity rainfall as it is not very effective in retaining the top soil under heavy rain.

Cellular grid or pocket fabric: These so called soil reinforcement products are designed to hold the topsoil in place by trapping it in small compartments. These soil reinforcement products are recommended for very steep slopes where keeping topsoil in place is very difficult. Two· products available in Australia are Terra Lock and GrassCel.

Methods of installation of all these products are provided with the materials.

118

Table 2 Some of the more commonly used mattings in Australia.

TRADE NAMES PRODUCT DESCRIPTION

. Surface laid products:

Natural Fibre:

Dekowe 700 and 900 (Belton Ind.) Open mesh long lasting woven coconut fibres.

Enviromat Wood wool enclosed in fine polypropylene netting

,Fibermulch Coconut fibres with polypropylene backing

Geojute Open mesh woven jute mesh

Jutemaster Fine jute fibres stitched on to hessian backing

Organimat Sugarcane bagasse enclosed in fine polypropylene netting

Terramat Fine coconut fibre stitched on to polypropylene backing

Soil Saver Medium mesh of woven jute yam

North American green Straw/coconut fibre erosion control blank

Synthetic:

Rh~m Multimesh

Rheem' Oystershade

Rheem Shademesh

Rheem 'Windguard

Sarlon Conwed Net

Sarlon Shade mesh

North American Green

Buried product:

Enkamat

Woven polypropylene - bio-degradable -

Knitted polypropylene open mesh - bio­degradable

Woven polyester with different mesh size - non bio-degradable

Woven polypropylene medium mesh, bio.;. degradable "

Fine polypropylene netting used to cover loose mulch

[..

Woven polyester with different mesh size - non bio-degradable

.Recycled nylon - flexible channel liners

Fused nylon monofilament spongy mesh

Methods of installation are usually provided with each product.

119

5.3.4 Mukhing

Mulching is the use of plant residue or other suitable material to cover the soil surface. Mulch has the same effect on soil erosion control and vegetation establishment as matting. In fact matting is a mulch substitute which is normally more expensive but commonly used when mulch is not available, or is unsuitable for steep slopes.

Mulch materials include sugarcane bagasse, a mixture of sawdust and planer shavings, shredded newspaper, hay, straw, paper pulp and fibreglass.

Mulch should be spread evenly with maximum depth of 50mm. Spreading can be done by hand for small areas but mechanical mulching is normally used for larger areas. Straw mulching is most economical and is most commonly used in Australia. Where long term protection is needed fibre glass mulching can be used. However due to its high cost, fibreglass is recommended only for areas that need special and long term protection.

Mulch is normally kept in place by spraying with a bitumen emulsion, or by covering with fine netting material following spreading.

5.4 Bio-engineering measures

Due to both the aesthetic effect and to the community concern about the environment, (particularly thegr,eenhouse effect), bio-engineering is becoming a preferred option of land stabilisation, especially in the urban environment. Vegetation alone will not eliminate all the problems of erosion and sedimentation, but when used in conjunction with good engineering practices, these problems will be greatly reduced.

Vegetation has two main roles in land stabilisation, soil surface protection and soil mass reinforcement. In terms of soil surface protection, vegetation is the best defence that can be provided to protect the exposed soil against erosion. The main advantages are:

Interception and retention of a proportion of rainfall, plus reducing the quantity and kinetic energy of the rain reaching the soil surface.

Improvement of soil permeability, thus reducing the quantity of runoff.

Retardation of overland flow of water, and reduction of wind velocity at the soil-air interface.

Stabilisation of sloping land by root reinforcement. The root system of deep rooting species binds the soil and increases its shear strength.

Providing vegetative barriers which can act as very efficient water spreaders or can be used as sediment filter strips.

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5.4.1 Minimising the damage

Before construction operations begins, most sites have some form of drainage system and are vegetated and generally stable. The most elementary, and generally the cheapest, control measure is to keep the disturbance of the site to the minimum necessary to carry out the work immediately in hand. Some site clearing will almost certainly be necessary, but it is worth going to considerable trouble to preserve vegetation in critical areas.

Figure 3 shows some measures which can reduce the impact of construction works.

5.4.2 Topsoil stripping and stockpiling

The humus and nutrients necessary to sustain plant growth are mainly concentrated in the top few centimetres of the soil. The possibility of maintaining or regenerating grass (particularly) on the construction site depends, therefore, on retaining the topsoil. If this is lost, grassing will depend on expensive soil treatment.

The best way of preserving topsoil is to leave it where it is, protecting it from traffic and avoiding siting stockpiles on it. Where earthworks are to be carried out, however, the topsoil should first be removed and stockpiled for later use. In some barren areas, of course, the topsoil will be so poor and thin that this will not be worth doing.

Before stripping topsoil, it is helpful to reduce vegetative cover by~grazing, burning, or mowing (preferably with removal of mown material), as excessive vegetative growth makes topsoil removal difficult. Also, large quantities of green matter in stockpiles promote chemical and biological degradation of seeds, runners, rhizomes etc. which may otherwise be a source of regrowth when topsoil is respread.

Grazing if possible is a preferred approach for this reduction in vegetative cover. It allows retention of a good quantity of seed and vegetative parts from which plants can re-establish. Burning may often be a more practical approach, but it destroys much of the residual seed, although root stock material will usually survive.

To ensure maximum preservation of plant material for revegetation, and to minimise subsoil inversion, ideally topsoil should be removed in two parts. The organic layer (usually about 50mm deep) is removed and stockpiled first. This stockpile should be a low, flat mound, if the aim is to ensure maximum survival of vegetative material (runners, root stock etc.). It can be stored as a higher mound, built to the natural angle of repose of the soil, if seed viability is the primary consideration. The remaining topsoil should be removed subsequently and stockpiled in a separate mound. The stockpiles should be free from traffic and should be . located away from drainage areas.

The topsoil stockpile itself needs to be protected against wind and water erosion. For large stockpiles sowing of a fast growing species is the most economical method (millet in summer and annual rye in winter). For small stockpiles, covering with mattings is very practical and effective.

121

Figure 3

1.

2.

3.

DRAINAGE LINE OR STREAM

'-------- LARGE LOGS

'------''TREE STUMPS RETAINEO

L"::. ~F 5'REA:(] BUFFER ZONE

RETAIN EXISTING RETAIN AS MANY VEGETATION AS "TREES AS POSSIBLE -BUFFER ZONE RETAIN GRASS FILTER TO STREAM STRIPS ALDNG CONTOURS

Rm'N"~ VEGETATION AS ~DlSTURBED AREA OF FILTER STRIP CONSTRUCTION SITE

Erosion and sedimentation control by utilisation of natural features (Soil Conservation Authority of Victoria)

122

5.4.3 Site preparation

Subsoil Treatment. Exposed subsoil should be heavily topdressed using a mixed fertiliser with a high phosphorus content. The rate of fertiliser application should be four to five times tl).e recommended rate for normal pasture establishment (e.g. 600 to 800. kgha-1 of 12N:22P or similar fertiliser.

The purpose of this heavy application is to establish and maintain a high level of phosphate and nitrogen in the subsoil, so that plant roots will be encouraged to grow down into this l~yer. With deep penetration of roots the soil is effectively bound, and long term persistence . . of plant cover is more assured. Other soil conditioners such as gypsum or lime are also n~eded for sodic (high in soluble and exchangeable Na) and acid subsoils respectively. The r¢commended rate is 5-10 t ha-1• !

The site should subsequently be ripped on the cOIltOur to a minimum depth of 300mm. This ripping, carried out prior to topsoiling, provides a key for the topsoil and improves infiltration of water into the subsoil. It will also aid root penetration and improve plant growth and persistence.

Topsoil Treatment. Topsoil should be analysed before using. For best results, it should have the following characteristics: pH between 5.5 and 7.5, Chloride less than 500 mgkg-1 and electrical conductivity (EC) less than 0.5 dSm-1•

High acidity and salinity levels are detrimental to plant growth particularly during the establishment phase. Most Queensland topsoils have pH between 5.5 and 7.0. A pH lower than 5.0 may lead to heavy metal toxicity such as Al and Mn and a pH higher than 7.5 often results in micronutrient deficiencies (Cu and Zn) and may be associated with high Na. Extreme pH levels may also indicate contamination such as old dumps of industrial waste. Soils with higher chloride contents and higher ECvalues than the above limits are considered saline and usually have excess Na content. Saline soil should not be used.

5.4.4 Topsoil spreading and compacting

Topsoil is respread in the reverse sequence to its removal, so that the surface soil, containing any seed or vegetative parts, is returned to the surface. It should be evenly spread over the whole slope and firmly compacted to a depth between 75 to l00mm. If possible topsoil should be tapered on moderate slopes from a thickness of l00mrn at the top of the slope to 75mm at the bottom, to allow for downwards movement. Greater depths of topsoil are wasteful, ineffective and may cause mass movement.

Compacting can be done by dozer tracts. On low slopes the first few runs can be done across the slope but the final runs should be up and down the slope so horizontal track marks can trap seeds, fertilizer and water. On short and '§teep slopes compacting can be done by pressing the back of an excavation bucket on the topsoil or by dragging a heavy chain be~ween two dozers across the slope.

It is essential that this phase of the operation be carefully conducted. Should excessive exposure of subsoil occur, or the site become under or over compacted by unsuitable

123

equipment or by operational techniques, plant growth will be impaired.

Figure 1 shows tops oiling techniques recommended by the Soil Conservation Authority of Victoria.

5.4.5 PlJlnt establishment methods

Plants can be established either by veg~tative cuttings and turf planting or seed sowing:

• Vegetative planting. This can be done by spreading and discing the cuttings into the topsoil, by placing them in prepared furrows, or with the use of a sprig planter. These methods are applicable only on gentle slopes. Grass turf is used for instant cover for both surface protection and aesthetic reasons. It is most often used on expensive real estate developments but it is also recommended sometimes for highly unstable and difficult to stabilise areas where good maintenance facilities (water, fertilising and weed control) are available. The three types of turf material currently available in south east Queensland are: green couch, blue couch and Kikuyu.

• Seeds sowing. When topography permits, sowing with conventional agricultural machineries such as combine seeder or seed broadcaster is most economical. On steep slopes hydroseeding and hydromulching methods of sowing are used. In hydroseeding a combination of seeds and fertilisers and water (at required rates) is mixed and pumped out at high pressure onto the batters. The high water pressure provides a compacting effect as it imbeds the seeds into. the topsoil. The rate of application should be between 30,000-35,000 L/ha. In hydromulching, in addition to the seeds and fertilisers rates mentioned above, mulching material such as paper pulp (shredded newspaper at 5 t ha-1) or wood fibre (a paper mill product at 1.5 t ha-1) and an emulsion are also included. The emulsion can be a bituminous slow curing anionic emulsion, a modified co-polymer dispersion or a polymer binder. All these should be miscible in water.

5.4.6 Seeds mixtures and rates

For successful long term stabilisation seed mixtures should contain species which provide primary, secondary and sometimes tertiary covers.

•

•

•

The primary cover is needed to provide early protection to the surface and als,? to supply mulch for the establishment of the secondary and tertiary covers. A fast growing cereal crop is often used; it provides fast germination and establishment for initial cover then dies off at the end of the growing season.

The secondary cover is a perennial grass which will provide long term surface protection. The ideal species for the secondary cover should be hardy ,drought and frost tolerant, self seeding, relatively free of disease and pest problems, and have low nutritional requirement and require minimal maintenance.

The tertiary cover or tree cover is needed for steep slopes stabilisation. One important consideration is that whenever possible species native to the region, or proven

124

introduced species, should be used as they are well adapted to the area. This also reduces the risk of introducing a potential weed to the region.

Table 3 provides some guidelines to the selection of planting materials for Queensland conditions. The actual rates used can be varied to suit local requirements.

Table 3 Seed mixtures and rates suitable for land stabilisation in Queensland.

REGION COVER SEASON SPECIES RATES

North and central . Primary . All year - French or 20-25 kg/ha Queensland Japanese millet

Secondary All year - Rhodes grass 8 kg/ha - Giant Bermuda 3 kg/ha - Green couch 5 kg/hg - Indian couch 5 kg/ha (if available)

Tertiary * All year - Acacia 500 g/ha - Local spp 1.2 kg/ha

South and south Primary Summer - French or 20-25 kg/ha east Queensland (September-April) Japanese millet

Winter - Oats, annual 15-20 kg/ha (May-August) rye or Fescue

Secondary All year As for north & 3 kg/ha central Qld plus Kikuyu (if preferred)

Tertiary * All year As for north & central Qld

* Selection from Table 4.

The Main Roads Division, Queensland Department of Transport recommends the following shrubs and trees (Table 4).

125

Table 4 Tree and shrub species suitable for batter stabilisation in Queensland.

SPECIES

Acacia con currens + Acacia fimbriata + Acacia saligna Bauhinia galpinii Callistemon salignus Callistemon viminalis Callitris columellaris Casuarina cunninghamiana Casuarina glauca C asuarina torulosa Eucalyptus intermedia Eucalyptus tereticornis Eucalyptus torelliana Ficus hilii Melaleuca leucadendra (broad leaf) Melaleuca quinquenervia

COMMON NAMES

Wattle Brisbane Wattle Golden Wreath Wattle Red Bauhinia White Bottlebrush Weeping Bottlebrush Bribie Island Pine River Sheoak Swamp Sheoak Rose or Forest Sheoak Red or Pink Bloodwood Blue Gum Cadaga Hills Fig Broadleaved Tea Tree Paper Barked Tea Tree

+ ;These two species can be readily established by direct seeding. However they are not very hardy and often die off after four or five years.

Trees can be established by direct seeding (seeds are mixed with grass seeds and others) at sowing or by planting from tubes. Tubing generally gives a higher success rate but it is more expensive and is often recommended for small areas only or when the seeds are not available. Due to the high operating cost, the use of high quality seeds is strongly recommended.

5.4.7 Vegetative barriers

The use of vegetative barriers in erosion control is not new, but due to the lack of suitable species,·its application has been very limited. Recently Vetiver grass (Vetiveria zizanioides) has shown to be particularly suitable for this purpose. Overseas studies have shown. that Vetiver, when planted as hedges on the contour, spreads concentrated runoff water and traps its sediment load. Therefore potentially the planting of this species can replace or supplement some engineering measures mentioned above, particularly on steep slopes. . ...

5.4.8 Fertiliser types and rates of application at sowing

Weed competition is an important aspect which has to be considered in the determination of the mix and quantity of fertiliser used at sowing. The aim is to promote the establishment and growth of the sown species, hence only a relatively low fertiliser rate is needed initially

126

for the surface. Complete NPK mix with relatively low N is recommended at 80-100 kgha-1•

High N fertiliser tends to promote weeds, especially broad-leaved species which will compete for light and water with the sown species. Follow up fertilising after establishinent is most essential. The use of slow release fertilisers is recommended if available at reasonable cost.

5.4.9 Maintenance

A good maintenance program is most essential for any stabilisation work as the seeds are .. sown into a very hostile environment. The high cost of site preparation and planting would be wasted if post planting maintenance were inadequate.

Watering: This is most critical following germination. The frequency of watering and the quantity of water required varies with the season, soil depth and slope, and should be estimated from evaporation. Only low pressure sprays should be used as high pressure jets can wash away the seeds and mulches. Extra water may be needed in summer for slopes facing west.

Slashing: Slashing is recommended to reduce the excessive growth of the primary cover and also to remove immature seed heads. This is particularly important for northern and central Queensland areas and for the summer planting in southern Queensland, as regrowth can compete strongly for light and water with the secondary and tertiary cover species.during the following summer.

Slashing is not necessary when the primary cover is poorly established. Subsequent slashing may be necessary to encourage the spreading of some summer growing grasses. Slashing is not practical after trees have emerged.

Fertil~ing: Complete NPK is recommended at 50 kgha-1 in spring and autumn for the first two to three years.

Weed control: Weed control is often needed in urban areas mainly for aesthetic reasons. However, if badly infested with weeds, especially broad leaf species, selective chemical weed control is recommended particularly during the establishment phase.

5.4.10 Costs and Services

Site preparation constitutes the major part of the cost. The actual cost of this operation depends largely on the availability of machinery, topography and topsoil. Maintenance is the second major component of the total cost.

The planting, seed and fertiliser cost is rela1;ively minor in comparison to the land shaping and tops oiling costs. For example the 1990 cost of the following methods of planting are:

Broadcasting (seed and fertiliser) Hydroseeding (seed and fertiliser mixture) Hydromulching (seed and fertiliser, mulch and bitumen) Strawmulching (hydroseeding, straw mulching, bitumen)

127

$0.05 - $0.08m-2

$0.12 - $0.18m-2

$0.43 - $0.55m-2

$0.55 - $0.63m-2

If geotextile products are used the cost varies with the materials and total area of application and it ranges from $0.80 to $3.50m-2 including installation. The cost of chemical surface stabilisers varied between $0.20 - $0.30m-2 for temporary protection (3-6 months) to more than $1.00m-2 for long term protection.

In Queensland sowing is mostly conducted by private contractors and it is imperative to check the quality of their equipments and work. Experience and appropriate machinery for the job are most essential. Unsuitable equipment can ruin the best preparation and highest quality planting materials. If possible check the results of their previous work and talk to their customers before engaging their services.

6 SOME CASE STUDIES

Application of the principles and techniques mentioned above is illustrated in some of the following case studies.

6.1 Dust control and wind erosion

Wind erosion per se is generally not a major problem in Queensland, except in the far south west. However dust storms at constructions sites and sand blasting at some canal estates on the coast often cause major concerns.

The solution to the problem is surface cover and measures that can reduce the wind velocity particularly at the soil surface. There are a few options available: watering, mulches, chemical surface stabilisers, mattings and vegetative cover (grass, shrub and tree planting). The cost would vary according to the area involved, topography, available machinery and material costs, but generally for a typical construction site or a standard ocean front estate, the establishment of a vegetative cover is the most cost effective method. Chemical surface stabilisers could be considered for areas not subject to traffic, which would destroy the fragile surface seal.

For a temporary protection (up to 12 months) fast growing crops such as millet, oat, annual rye can be established very quickly by hydromulching. Higher rates (80-100% more than recommended rates) of seeds and fertilisers should be used. Watering is essential to ensure early establishment. For long term protection, secondary cover species such as couch and Rhodes grasses should be included in the seed mixture. The planting of shrubs and trees in strategic areas should also be included for long term protection.

6.2 Stabilising low and long slopes

Where watering is available, the most economical method of stabilising these areas is hydroseeding onto a fairly rough surface. Dozer tracks, scarifiers or chisel ploughs can be used on larger areas.

On flat areas seed broadcasting can be used but the surface needs to be rolled after the operation. Where watering is not available, hydromulching is recommended as the seeds, (with the help of the mulch), will adhere better to the soil surface.

128

On long slopes with uneven topography temporary level spreaders or graded contour banks should be considered to reduce concentrated flow. Natural drainage lines should be identified and protected with some mattings on poor and shallow soils.

The whole area should be protected with a diversion bank on the top and a toe drain at the bottom of the slope. Extra care should be taken to protect the outlets of these structures.

6.3 Cut steep batters

Serrated cuts on slope exceeding 3H: 1 V gradient are recommended particularly on erodible soils. The most important operations here are the spreading and the compacting of topsoil on to the cut surface. Topsoil can be pushed down from the top or spread by an excavator. Topsoil should be compacted after every 50 or 75mm of topsoil. Several operations are needed before the desired depth is attained. On long batters with short slope length (up to 5m) such as road batters, the use of a heavy chain is most economical. An excavator with a large bucket is recommended for longer slopes. On sites where subsoil is very poor or impermeable, application of fertilisers or other soil conditioners should be considered.

The whole area should be watered first prior to sowing by the hydromulching method. In critical areas such as those with rocky or impermeable subsoils, the use of straw mulching or geotextile should be considered.

Protection from run-on water is very important.

6.4 Fill steep batter

The most important operation here is the compaction of the fill materials. The original soil surface should be ripped to at least 500mm and the compaction should be carned out after the spreading of every 200-300mm of fill.

The quality of the fill materials is also essential. Soil tests should be carned out to avoid the use of saline or highly acidic or alkaline materials. Application of fertilisers before tops oiling is very important and other soil conditioners should be considered where applicable.

Before spreading topsoil the fill surface should be watered. This would ensure a higher subsoil moisture levei for plant growth, and also would facilitate the bonding between subsoil and topsoil. Topsoil should be spread and compacted as in cut batters. Pre-sowing watering and sowing by hydromulching are recommended. On erosion-prone areas such as old drainage lines, or in areas with rocky subsoil, the use of geotextiles or straw mulching is advisable.

Again protection from run-on water and the provision of a toe drain are essential.

6.5 Irrigation channel banks

Grassing of irrigation channel banks (above the water line) is probably one of the most difficult stabilisation tasks. These banks are highly compacted to prevent leakage and to retain the capacity of the channels, so that topsoiling is normally impractical. Therefore

129

plants must be established on highly compacted subsoil.

To create a more favourable environment for plant establishment, gypsum is needed in most cases to improve water infiltration particularly on sodic subsoil. Gypsum at the rate of 8-10 tha-1 is first applied to the slope (preferably by hydromulching equipment), then the bank surface is roughened up with a toothed harrow before the hydromulching operation starts (seeds and fertilisers at double the normal rates). The whole area is then strawmulched and covered with bitumen spray to keep the straw in place. In very erodible areas where grassing is difficult and slow, fibreglass mulching can be used.

The seed mixture used on these banks has to be carefully selected to avoid the risk of introducing weed seeds into irrigation water. If rainfall is not enough, watering is required until the grasses are well established.

Protection from run-on water in the early stage of establishment is essential.

REFERENCES

Anon (1978). Erosion control manual. Colorado Department of Highways. US Department of Transport.

Anon (1980)_ Soil Conservation Handbook. QDPI Soil Conservation Branch.

Anon (1981). Erosion and sediment control. Guide for Hawaii. USDA and Soil Conservation Service, Honolulu, Hawaii.

Coppin, N.J. and Richards, LG. (1990). Use of vegetation m civil engmeenng. Butterworths, London.

Garvin, R.J., Knight, M.R. and Richmond. (1979). Guidelines for minimising soil erosion and sedimentation from construction sites in Victoria. Soil Conservation Authority of Victoria.

Quilty, I.A., Hunt, I.S. and Hicks, R.W. (1978). Urban erosion and sediment control. Soil Conservation Service of NSW. Technical Handbook No.2.

Wischmeier, W.H. and Smith, D.D. (1965). Predicting rainfall - erosion losses from crop land east of the Rocky Mountains - Guide for selection of practices for soil and water conservation. USDA Agricultural Handbook No. 282.

130

FURTHER READINGS

Bache, D.H. and MacAskill, LA. (1984). Vegetation in civil and landscape engineering. Granada, London.

Charm an, P.E.V. and Mmphy, B.W. (1991). Soils, their properties and management. Sydney University Press.

Nunn, RM. (1981). Dust abatement in the goldfields of Western Australia. W.A. Department of Conservation and Environment Bulletin No. 86.

Zallar, S.A. Soil Stabilisation and Revegetation Manual. Victoria Soil Conservation Authority.

131

132

SOLUTES AND THEIR TRANSPORT THROUGH SOILS

ABSTRACT

I.R. Phillips

Faculty of Environmental Sciences Griffith University Nathan QLD 4111

The movement of chemicals can significantly affect the composition and quality of soil In recent years, the movement of chemicals into the soil environment has caused significant. degradation of this imporatnt resource. It is imperative therefore, that today's environmental scientists and engineers understand, and be aware of, the principal mechanisms controlling the movement of chemicals in soil.

This manuscript outlines the principal mechanisms controlling the movement of chemical in soiL These mechanisms are convection (which includes Darcy's Law and its application), dispersion (which involves molecular diffusion and hydrodynamic dispersion), ion exchange and ion exclusion. Descriptions of these mechanisms are highlighted using a number of relevant examples, and by applying simple mathematical relationships to estimate the rates of movement of reactive and non-reactive chemicals in soil

In addition, factors affecting ion exchange rates are discussed. These factors include the effects of clay mineralogy, soil structure, and exchange kinetics.

1 INTRODUCTION

Mobile chemicals have been present in the environment for millions of years due to processes such as rock weathering and organic matter decomposition. These processes have produced a wide range of soil types, and have influenced the chemical composition of surface waters and groundwaters. Human activities continue to influence water quality due to fertilizer applications and poor management techniques for waste disposal.

The study of solute (dissolved chemicals) mgvement in soil is essential for maintaining the long-term productivity and usability of soil. this knowledge of solute movement can be used for maintaining soil fertility through minimising the loss of nutrients from the root zone, for preventing soil salinity, and for minimizing soil and groundwater pollution through the introduction of agricultural (phosphorus and nitrogen) and industrial (heavy metals, acid mine drainage, cyanide, organo-chlorines, and other organic chemicals) contaminants.

In this manuscript, the important physical and chemical factors affecting the rate of movement of chemicals through soil will be outlined. In particular, these will include:

the principal physical mechanisms such as convection, diffusion and hydrodynamic dispersion;

133

the principal chemical mechanisms such as ion exchange and ion exclusion;

the use of breakthrough curves to describe these physical and chemical mechanisms;

ion exchange kinetics;

equations describing solute movement.

2 PRINCIPLE MECHANISMS AFFECTING SOLUTE. MOVEMENT

When chemicals are applied to the soil surface, they are washed into the soil with the infiltrating water, where their rate of movement is dependant an a number of important mechanisms, these being:

convection; dispersion; ion exchange; ion exclusion.

These mechanisms result in the added chemical either remaining in the soil water or being attracted to the electrical surface charge of the soil colloids (e.g. clay minerals and organic matter). Chemicals which remain in the soil water make up the soil solution, and examples of the range of soil solutions which can be found in the soils are presented in Table 1. This Table shows the major chemicals present in the soil solution include Ca2+, Mg2+, K+, Na+, cr, N03-, SO/", PO/" and heavy metals, to name a few.

Table 1 Examples of the ionic composition of soil solutions

Chemical Agricultural Parameter soil

(mg L-1)

pH 6.3 Ca2+ 20 M~+ 5 K+ 20 Na+ 9 Fetota' 0 NH4+ 5 N03- 50 cr 18 SOl" 38

Landfill leachate (mg L-1)

7.5 80 250 1300 1400 16 1200 1 1800 3

134

Acid mine drainage (mg L-1)

2.6

1270

4550

Fertiliser industry (mg L-1)

9.4 5 5 420 165

1100 420

300

These chemicals can arrive at the soil smface due to processes such as fertilizer applications, organic matter decomposition, rainfall accessions,' oxidation of metal sulphides, erosion/deposition, animal excreta and in industrial/household wastes. Once at the smface, these chemicals can move through the soil in either the vertical or horizontal directions with the soil water. The downward movement of chemicals with the drainage water is more commonly referred to as leaching, and the processes affecting leaching rates are the focus of this manuscript.

2.1 Breakthrough Curves To Describe Solute Movement

For a particular ion, the relative importance of convection, dispersion, exchange and exclusion during leaching can be quantitatively described using breakthrough curves (BTC)~ ... These curves relate the concentration of the ion in the leachate to the cumulative volume of flow, or as it is more commonly referred pore volume, where a pore volume is defined as the volume of space -occupied by air or water between the soil particles.

REMEMBER: Total volume Pore Volume

= =

Volume of sqlids + Volume of water + Volume of air . Volume of water + Volume of air *. .

For saturated soil the pore space will be filled with water, and this volume is equal to one pore volume of solution. Consequently, any water additions to the soil surface will cause an equal amount of water to pass out of the bottom of the soil depth.

In the following discussion, the physical and chemical mechanisms affecting solute movement are described with the aid of BTC (Figure 1). The BTC in Figure 1 are for chemicals applied to the soil smface as a band, or as is commonly referred, a solute pulse. A solute pulse could represent an application of fertilizer or a spillage of industrial chemicals.

Figure 1

(a)

0.5 1.0 ,1.5 2.0

Cumulative leachale.(pote votumes)

Breakthrough curves for che~cals involving (a) piston flow, (b) non­reactive flow, (c) preferential flow, (d) flow with ion exchange, and (e) flow with ion exclusion. [These mechanisms are discussed in detail in the text.]

135

2.2 Physical Mechanisms Affecting Solute Movement

The more important physical mechanisms affecting solute movement in soil are convection and dispersion, and these mechanisms are strongly dependant on the movement of water in soiL Before discussing these mechanisms, the movement of water in soil will be briefly outlined.

2.2.1 Darcy's law

The flow of water through soil was first studied experimentally more than a century ago (in 1856) by a French engineer Henry Darcy. He found that the quantity of water passing through a column of soil over a specified time depended on several factors, these being:

cross-sectional area of the soil column; length of the soil column; height of water above the soil column or hydraulic head; the hydraulic conductivity, or the ease with which water flowed through the soiL

Darcy showed that the volume of water flowing through a column of soil could be described using the formula:

V

where V A t L HA HB K

= [1]

= volume of water passing through column (m3)

= cross-sectional area of soil column (m2)

= time (s) = length of column (m) = hydraulic head at inflow end of column (m) = hydraulic head at outflow end of column (m) = hydraulic conductivity which indicates the ability of the soil to transmit

water (m S-l)

The rate at which water passes through a soil column, commonly referred to as the soil water flux density, is defined as the volume of water passing through a unit cross-sectional area per unit time, and is calculated as:

q = v I (A t)

where q =

Substituting Equation [2] into Equation [1] gives:

q = V I (A t) = K (HA - H B) I L

where Equation [4] is the well-known Darcy Law.

136

[2]

[3] [4]

In one-dimensional flow (e.g. horizontally or vertically), Equation [4] takes the differential form:

q

where q K dH/dz

- sign

=

= = =

- K dHldz [51

soil water flux (m S-l)

the hydraulic conductivity (m S-l)

difference in hydraulic head over distance L, or hydraulic gradient (m m-1)

shows that the water moves from high to low potential

Equation [5] shows that water flow in soil occurs in the direction of, and at a rate proportional to, the hydraulic gradient, and is also dependant on the ability of the soil to transmit water, or the hydraulic conductivity.

The hydraulic conductivity, K, is strongly dependant on the soil physical properties such as porosity, aggregate stability and soil texture. Some examples of K under conditions of saturated soil water flow (Kw) are presented in Table 2.

Table 2

Soil Texture

Clay Loam Fine sand Coarse sand Gravel

Some examples of saturated hydraulic conductivity for ditTerent materials

10-7

10-6 _ 10-5

10-5

10-3

10-2

The hydraulic gradient, dH/dz, is simply the change in hydraulic head, H, between two points separated by a distance of z metres, It is found that as the hydraulic gradient increases, the rate of soil water flow increases. This will be discussed with the aid of examples below where Darcy's Law is used to determine how water moves in soil for horizontal and vertical water flow conditions.

Application of Darcy's Law in saturated soil - vertical soil water flow

Vertical soil water flow into a saturated soil occurs due to the presence of gravitational and pressure heads. Figure 2 shows a uniform, saturated vertical column of soil, with a water reservoir at both ends of the column. This Figure shows that water enters the column at the top of the column, indicated as Point A, and exits the column at the bottom at Point B.

The total hydraulic head at Points A and B is made up of a pressure head and a gravitatiomil head. Consequently, water will flow in the direction from where the hydraulic head is greatest to where it is least.

137

The hydraulic heads and the soil water flux (q) are calculated in the following example.

Let: column length L = 0.2m column surface area A = 0.1 m2

height of inflow water = o m above the column height of outflow water = o m below the column reference level = o m below the column Ksat = 0.001 m S-l

L HTA

HgA

-- -- -- -- -- - ......... ~~~-I _REF

Figure 2 Vertical water flow in a water-saturated soil column

Now, the total hydraulic head at the inflow end, HTA, equals the sum of the pressure head (I\A) and the gravitational head (HgA), or:

The total hydraulic head at the outflow end, HTB, equals the sum of the pressure head (HpB)

and the gravitational head (HgB)' or:

Therefore, using Equation [3]:

q = VIAt = and determining HTA and HTB as:

138

= Om + 0.2 m =

= Om+Om =

then,

= 0.2 m - 0 m =

Therefore, the soil water flux, q, is:

q = = = =

V/At Ksat (HTA - HTB) / L 0.001 m S-I * 0.2 m /0.2 m 0.001 m S-I

0.2 m

Om

0.2 m.

EXAMPLE 1 - Application of Darcy's Law To Horizontal Soil Water Flow

Consider horizontal soil water flow in a saturated column of soil as shown in the accompanying diagram. Calculate the saturated hydraulic conductivity in units of m S-I if the discharge rate at the end of the column is 1.62 cm3 min- I and the internal cross-sectional area of the column is 20 cm2•

27cm 12cm

1« 50cm

Solution

(1) Firstly, assume both the discharge rate, q, and the hydraulic gradient, dH/dz are constant with time.

(2) Secondly, convert the discharge rate from units of cm3 min- I to m S-I, and the surface area from cm2 to m2•

139

Now, 1 cm-3 = 1 minute 20 cm2

= =

10-6 m3

60s 20*10-4 m2

So discharge rate in m3 S-l is:

= =

1.62*10-6 m3 I 60s 2.7*10-8 m3 S-l

(3) Thirdly, calculate soil water flux, q:

Since q

Then q

= =

= =

discharge rate I area volume per unit time I area in units of m3 m-2 S-l

1.62*10-6 m3 I (60 s * 20*10-4 m2)

1.35*10-5 m S-l

(4) Fourthly, calculate the hydraulic gradient between points A and B.

Now the height of water above point A = 27 cm or 0.27 m, while the height of water above point B = 12 cm or 0.12 m, and the distance between point A and point B = 50 cm or 0.5 m. Also, the reference level is taken as the same height as points A and B in the soil column. So,

at point A, at point B,

HTA = ~A + HgA = 0.27 + 0 = 0.27 m, while HTB = ~B + HgB = 0.12 + 0 = 0.12 m

The gravitational head at point A=B=O because we are taking our reference level as mid-point in the soil column.

Now, dz =

= distance between point A and point B 0.5 m

dH/dz = (0.27-0.12) m 10.5 m 0.15m I 0.5 m =

= 0.3

Thus, according to Darcy's Law:

~at = = =

q I (dH/dz) (1.35*10-5 m S-l) 10.3 4.5*10-5 m S-l

140

2.2.2 Convection

The principle mechanism responsible for the movement of solutes through soil is convection in the moving soil water, and leaching of solutes by mass flow alone is commonly termed convective transport. The relationship between the convective flux, J, and the solution ion concentration, c, can be described using Darcy's Law, q, as:

J = cq = c (K dH/dz) [6]

Since q is the volume of water flowing through a unit area per unit time (m3 m-2 S-1), and c is the mass of solute per unit volume of solution (mol m-3), then J is the mass of solute passing through a unit surface area per unit time (mol m-2 S-1).

If a band of solute is applied evenly to the surface of a water-saturated soil column and then leached with water, the rate of leaching can be calculated as:

v

where v

=

= = =

velocity of flow (m S-1)

Darcy flux (m S-1)

volumetric water content (m3 m-3)

By re-arranging Equations [6] and [7], the flux, J, can be calculated as:

J =

[7]

[8]

When no mixing occurs between the solute and the leaching water, the thickness of the band and its concentration will remain constant as it moves through the soil. Therefore, the band of solute will be displaced from the column after the addition of one pore volume of water. This pattern of flow is commonly termed piston flow and is illustrated as Curve A in Figure 1.

Piston flow is commonly observed during immiscible displacement when one solution displaces another solution with no mixing between the two solutions such as observed between oil and water.

We can also obtain an estimate of the average residence time, t, of the chemical in a layer of thickness, L, by:

t = L I v [9]

EXAMPLE 2 - Solute Transport By Convective Flow

Consider the case where a soluble chemical has been applied on the soil surface at a concentration of 5 mol Ll. The properties of this chemical are such that it does not interact with the soil, plant or atmospheric environment. If the annual rainfall is 1500 mm and the annual evapotranspiration is 1250 mm, the watertable is 20 m below the surface, and the

141

unsaturated soil water zone above the watertable has a constant volumetric water content of 0.25 m3 m-3, estimate (a) the rate of leaching of the chemical in the unsaturated zone; (b) the time for the pollutant to reach the watertable; and (c) the solute flux.

Solution

(a) Assume that the chemical is transported by convection alone. The rate of leaching is calculated using Equation [7]:

v = =

(1500-1250) mm year-I 10.25 1000 mm year-lor 1 m year-I.

(b) To estimate the residence time we use Equation [9]:

t = =

20 m /1 m year-I

20 years.

Therefore, much of the applied chemical can be expected to reach the groundwater and enter the watertable after about 20 years .

. (c) To calculate the solute flux use Equation [8]:

J = v 6v c

where v = 1 m year-I

6v = 0.25 m3 m-3

c = 5 mol L-I or 5000 mol m3

. J .. = 1 m year-I * 0.25 m3 m-3 * 5000 mol m-3

= 1250 mol m-2 year-I

Thus, for every 1 m2, 1250 moles of chemical will pass through a depth of 1 m per year.

2.2.3 Dispersion

Piston flow is seldom encountered in the field since many chemicals are soluble in water which allows mixing between the leaching solution and the solute pulse. In the absence of chemical interactions with the soil colloids, the concentration pattern of solute leaching can be approximately described by the normal or bell shaped curve (Curve B in Figure 1). The shape of this breakthrough curve (BTC) indicates that a degree of mixing between the solute and leaching water has occurred. Solute leaching which involves some mixing is referred to as miscible displacement, and this mixing is a result of:

molecular diffusion; hydrodynamic dispersion.

142

Molecular diffusion

All chemicals in the soil solution possess thennal energy which causes them to move in a random manner throughout the soil solution. Where concentration gradients exist in the soil solution, the chemicals in solution will move from zones of higher concentration to zones of lower concentration until an equilibrium is attained by the process of diffusion. This motion (or Brownian Movement) tends to cause solutes to be unifonnly distributed throughout the soil, and the rate at which diffusion occurs depends on the concentration difference between two volumes of solution.

In bulk solution, the rate at which an ion diffuses from one point to another may be described by Fick's First Law, in which the amount of ion crossing a unit sectional area per unit time (F) is given by:

where

F

F D c z

NOTE:

=

= = = =

- D dc/dz

flow of solute per unit time across a unit area (mol m·2 S-l)

apparent diffusion coefficient (m2 S-l)

concentration of the ion in solution (mol L-1)

[10]

distance in the direction of net movement of the diffusing ion (m)

The negative sign in Equation [10] indicates that diffusion occurs from areas of high concentrations to areas of low concentration.

The rate at which solutes diffuse through soil decreases with time as adjacent volumes of solution attain chemical equilibrium. The rate of change in ion concentration over time is given by the relationship, dc/dt, and the rate of change in the concentration gradient with distance is given by d2c/dz2• This rate d2c/dz2 can be combined with the diffusion coefficient, D, to give Fick's Second Law of Diffusion:

dc/dt = [11]

which states that the rate of change in concentration over time is equal to the spatial rate of change in concentration gradient over distance multiplied by the diffusion coefficient.

We can show how Equation [11] is derived using the conservation of mass principle whereby:

mass of solute entering a volume of soil (F) = mass of solute exiting + mass of solute stored in a volume of soil (F +.l\F)

Consider a volume ,of soil given below with an area of xy (m2) and a thickness of Az (m).

143

=='~ F+AF 7

The mass of solute entering the soil volume during the time interval, L\t (s) is given by: F x y ~t.

The mass of solute exiting the soil volume during the time interval, L\t (s) is given by: (F + ~F) x Y L\t.

So, the mass of solute remaining in the soil volume during time ~t must equal: F x y ~t - (F + M) x y L\t,

or -M x y L\t.

Now, this mass of solute has been retained by a volume of soil with dimensions x y llz. So the change in the mass of solute in this volume of soil in time L\t, i.e. the change in solute concentration, L\c, with time, L\t, can be determined as: (Remember: mass / volume is concentration)

L\c L\c L\c / ~t

= = =

- (L\F x y L\t) / (x y L\z) -M~t/~ -M/~

which can be written in differential fmm as:

dc/dt = - dF/dz

Substituting Equation [10] for F above gives:

dc/dt = - d/dz (- D dc/dz)

dc/dt = D d2c/dz2, which is Equation [11].

Values for the diffusion coefficient, D, reported in the literature have generally been determined by measuring the rate of ion diffusion in large volumes of solution. These values of D are generally much higher than those values observed in soil systems because when studying the diffusion of solutes through soil, a number of geometric, physical and chemical properties of the porous media must be considered.

144

These properties include:

cross-sectional area available for diffusion; diffusion path length or tortuosity; mobility of solutes close to the electrically charged soil surface.

(i) Reduction in cross-sectional area

The cross-sectional area available for diffusion of chemicals in soil is largely dependant on the soil water content as diffusion is much faster across a water medium than an air medium (except when volatile chemicals are involved). Thus, as the soil becomes wetter, the area available for diffusion increases, which in turn speeds up the rate of diffusion. It should also be remembered that values of D in soil differ from those obtained in bulk solutions because the soil water occupies only a fraction of the total soil volume. In soil then, the movement of the diffusing chemicals is often impeded by soil particles, while this does not occur in bulk solutions. The effect of water content on the rate of ion diffusion is shown in Figure 3 and some values of D for various solutes as a function of water content is given in Table 3.

Table 3

Ion

Na+ Na+ er er pot pot

Diffusion coefficients for Na+, er and P043- in relation to soil water content in a sandy clay loam

Water content (m3 m-3)

0.2 0.4 0.2 0.4 0.2 0.4

Diffusion coefficient (m2 S-l)

0.2 x 10-10

2.2 x 10-10

2.4 x 10-10

9.0 x 10-10

0.3 x 10-13

3.3 x 10-13

(ii) Tortuosity of Diffusion Path

Due to the geometric nature of soil systems, the diffusion path length of solutes in soil is much greater than in bulk solution due to the tortuous and winding paths the diffusing ion must follow in soil. This tortuous path is strongly dependant on soil water content because diffusion of solutes occurs primarily in solution. Consequently, as the soil water content decreases, the value of D decreases (see Figure 3).

145

(a) Wet soil

.". ... ------- .... -.",.---_ .... .... --- ... ..... .-

AU)( $ of chemical

/ I

I I

I I • I I I , \ \ ,

" ..... - .... ' ... _---_..... " 't Soil particles

(- \ Water film _/

---+-- OiHuslon pat~

--..,.---------

"', , ,

.­...... ;

\ \ \ \ , I I I I

I I

/

(b) Dry soil

Figure 3

Awe of chemical

"",..--- ..... , '" , .- \

I I , I I

Schematic representation showing effects of water content on ion diffusion (a) wet soil, (b) dry soil. [NOTE: in dry soil, water films are thin and so are not joined in many places; therefore, cross-sectional area available for diffusion is reduced compared to wet soil]

(iii) Surface Charge

Soil colloids such as clays and organic matter possess an electrical surface charge (discussed in detail below). This has a number of important effects on the diffusion rates of solutes. For example, if the surface charge is negative the mobility of positively charged solutes (cations) close to the colloid surface will be much lower than their mobility some distance away from the surface due to electrostatic attraction, while negatively charged solutes (anions) will tend to be repelled from the clay colloid surface and their diffusion rates will tend to be higher, as seen in Table 3 for cr compared to Na+.

146

A further complicating effect of the surface charge is that at low water contents, the mobility of anions may be impeded due to exclusion from entering narrow soil pores whose electric double layers overlap (Figure 4). Ion exclusion (discussed later) results in solutes of the same charge as the colloid surface being repelled. When these solutes move through narrow pores, the diffuse double layer (DDL) of two colloids may overlap, thus inhibiting entry of the ion into the soil pore l .

Wide pore

Figure 4

. . 1 DOL .. , ..... -------,

------------~>-----~---. I Narrow ~ __________ ~- ~re

DOL

Effect of diffuse double layer (DDL) on ion diffusion path [NOTE: excluded ion unable to enter narrow pore due to electrostatic repulsion by overlapping DDLs, and this effect more likely in fine-textured soils at low. water contents as the DDL will comprise a higher proportion of the soil water]

Hydrodynamic dispersion

Hydrodynamic dispersion arises from the heterogeneous nature of pore water flow velocities. The main causes of this heterogeneity are (Figure 5):

frictional drag close to the solid surface; varying flow velocities in pores of different size; tortuous flow paths in aggregated media; density differences between the solutions either side of the interlace, especially if the denser solution is on top in a vertical column. The denser solution fingers downwards giving a very uneven solute front.

IThe electrical charge of soil colloids is balanced· by ions of opposite charge. These ions tend to diffuse back to the bulk soil solution due to diffusion gradients, thereby forming a diffuse cloud of ions close to the colloid surface. The electrical charge of the colloid plus the cloud of balancing ions comprise the diffuse double layer.

147

Hydrodynamic dispersion is often found to be greater in soils possessing cracks and large channels (due to old root channels, worm holes,shrinkage cracks in swelling soils, etc.) under saturated or near saturated flow conditions. This is because much of the infiltrating water is by-passing the soil matrix and flowing down these cracks. These cracks and channels are commonly referred to as preferential pathways. The BTC for a band of cheinicalapplied to the surface of a soil column containing preferential pathways and leached under saturated flow conditions is shown in Figure 1 as Curve C.

This BTC shows the maximum solute concentration in solution to emerge from the soil column prior to one pore volume of leachate being collected, and arises from preferential leaching through the cracks and large channels. Furthennore, this BTC shows pronounced "tailing" when compared to curve B which describes leaching in the absence of preferential pathways; This "tailing" has been explained on the basis that two distinct water regions eXIst in the soil; one region termed mobile where solute movement involves convection and dispersion, and another region termed immobile or stagnant where solute movement occurs primarily by molecular diffusion. The presence of immobile water regions can have a significanttimpact on the rates of cation exchange, as is discussed later.

With much of the water moving in the mobile region (i.e. the cracks and channels), solute located in'the smaller soil pore spaces can be by-passed. Once the solute concentration in the large ppres is reduced to below that in the immobile water region, diffusion of solute from the immobile region towards the mobile region will result. Consequently, the tailing observed in the BTC can be explained by the slow diffusion of solute between the immobile and mobile soit water regions.

Field dataIeported by Rose et al (1983) for nitrate leaching through a soil profile containing a number of large cracks showed that after the main peak of nitrogen fertilizer had passed out of the soilprofile~ a smaller-peak was observed some time later. The occurrence of this small concentration peak waS explained by the diffusion of nitrate from the immobile water region once the .nitrate concentration in the mobile region became very small.

During unsaturated soil water flow however, the influence of immobile water on solute. movemembecomes negligible as much of the water flow occurs within the smaller pores. In unsatltrated soil,pore water velocities can be as low 10-6 m s"l, and the influence of hydrodYJl8lllic dispersion on solute spreading is negligible, with the major mechanism causing spreading\l being molecular diffusion. Under these conditions, it has been found that the amount of dispersion can be approximated by the product of the volumetric water content and the diffUsion coefficient of a particular ion in water; i.e., Bv D, where Bv is determined experimentally and D is obtained from physical chemistry reference texts.

148

(a)

(b)

(c)

(d)

Figure 5

1I111t11111fJIIJltfllfl! 8 •• o •

---0----...... ---____ o • 8 .; •

~"'~~~~~~~~~ Heavy displacing ?" solution

Less~dense displacing solution

Mechanisms contributing to hydrodynamic dispersion showing solute particles at time t (0) and at time t+ at (.): (a) frictional drag close to solid surface, (b) varying flow velocities in different sized pores, (c) tortuous flow paths .in aggregated media, and (d) density differences between solutions

149

The combined influence of convection, molecular diffusion and hydrodynamic dispersion on the leaching of a band of solute through soil can be described as:

dc/dt =

where =

[12]

the dispersion coefficient (m2 S-I) and accounts for the combined processes of molecular diffusion and hydrodynamic dispersion

The first term on the r.h.s. of Equation [12] describes solute movement by diffusion, and the second term describes solute movement due to mass flow of water.

2.3 Chemical Mechanisms Affecting Solute Transport

2.3.1 Ion exchange

Soil colloids possess an electrical charge which is balanced by an equal amount of charge of opposite sign. In most situations, these solutes are referred to as exchangeable ions, and since soils are commonly net negatively charged (but can possess significant positive charge), cations are the major type of exchangeable ions retained by the charged colloid surface.

Exchangeable cations are attracted to the charged surfaces by electrostatic (or coulombic) forces, i.e. the postively charged cation is drawn towards the negatively charged colloid surface. As these cations are readily exchangeable with cations in solution, they are referred to as exchangeable cations, and the process of cation exchange is an important mechanism reducing the rate at which cations are leached in soil. This process can be explained according to the following reaction:

2 soil-Na+ + 1 solution-Ca2+ .,. 1 Soil-Ca2+ + 2 solution-Na+

The above reaction demonstrates that the application of a band of solute to the surface of a soil column results in a transfer of cations between the solution and exchange phases. Cations retained by the soil colloids are therefore left behind instead of leaching with the drainage water. Consequently, the rate of leaching of exchangeable cations can be much slower than that predicted by Equation [12] due to retardation as discussed below.

Cation exchange also changes the pattern of solute movement as shown by Curve D in Figure 1. Due to cation exchange, the cation solution concentration becomes skewed; on the advancing interface the concentration increases rapidly to a maximum value, while on the des orbing interface, the concentration decreases gradually, producing a long tail. The length of the tail is dependant on the amount of adsorption, while the shape is influenced by the different flow velocities which may occur as the solute moves through soil.

The general mathematical equation describing solute movement in the presence of cation exchange is:

[13]

150

where = = =

the amount of cation adsorbed per weight of soil (mol kg-l) soil bulk density (kg m-3)

proportion of pores conducting water (m3 m-3) [NOTE: n is porosity during saturated conditions]

The partitioning of cations between the solution and exchange phases can be represented by an exchange isotherm, the simplest of which is.a linear exchange isotherm (Figure 6). The exchange isotherm is obtained by plotting S, the amount of cation adsorbed per unit mass of soil (mol kg-l) against c, the concentration of the ion in the equilibrium solution (mol L-l).

Solution concentralion (me/mL)

Figure 6 Linear exchange isotherm [NOTE: the slope equals "k" the partition coefficient]

In the case of a linear exchange isotherm, the relationship can be described mathematically by the following:

S =

where k =

kc [14]

the partition coefficient (L kg-l), and equals the slope of the linear exchange isotherm

Substitution of Equation [14] into Equation [13] yields:

dc/dt + pJn d(kc )/dt = dc/dt + pJ<Jn dc/dt = (1 + pJ<Jn) dc/dt

R dc/dt

Ds (d2c/dz2) - v (dc/dz) Ds (d2c/dz2) - v (dc/dz) = Ds (d2c/dz2) - v (dc/dz)

= Ds (d2c/dr) - v (dc/dz)

151

[15]

where R R

= =

the retardation factor, and is defined as: 1 + (Pb kin)

Calculating cation leaching rates

[16]

To calculate the velocity of an exchangeable cation (v*) through soil, the average pore water velocity, v, is divided by the retardation factor, R. Thus, it follows that:

.. v = .. v =

v I R v I (1 + Pbk/n)

[17] [18]

This simple approach can be used to adequately predict the rate at which reactive and non­reactive solutes move through soil relative to the average pore water velocity, as outlined below".

(i) Calculating Average Pore Water Flow

When water is added to the soil surface, it will infiltrate the soil surface and move downwards throug.};l the soil profile. We showed earlier (Equation [7]) that the average pore water velocity, v, can be calculated by:

v =

v =

= =

average pore water velocity (m S-l)

Darcy flux (m S-l)

volumetric water content of the soil (m3 m-3)

(ii) Leaching rates for non-reactive chemicals

As the water moves downwards it takes dissolved chemicals with it. If the dissolved chemicals do not interact with the soil colloids (e.g. cr and N03- in soil with predominantly negative surface charge), then the rate of chemical leaching will be the same as that for water. In this respect, the value of the exchange coefficient, k, is zero, and Equation [18] becomes:

.. v = v I (1 + 0)

= v

(iii) Leaching rates for reactive chemicals

In the case where k has a value greater than zero, the value of v* will be less than 1, indicating that the cation is moving at a slower rate than the average pore water velocity due to processes such as cation exchange. For example, the exchange isotherm for Ca2+ may have given a value for k of 0.5 L kg-1• For a soil with a bulk density of 1200 kg m-3 and a porosity of 0.4 m3 m-3, then using Equation [18] we obtain:

152

* v / (1 + (0.5 x 1200 / 0.4) v = = v / (1 + (1500 x 10-3 m3 m-3)

= V / (1 + 1.5)

= V / 2.5

= 0.4 V

Therefore, rate of Ca2+ movement through this soil is 0.4 times the average pore water velocity_

REMEMBER: units for kpb/n = = =

(L kg-i) x (kg m-3) / (m3 m-3)

L m-3

10-3 m3 m-3 (as 1 L = 10-3 m3)

EXAMPLE 3 - Rate of Movement of Solutes in Soil

Potassium chloride fertilizer is applied to the surface of a soil, and allowed to leach through the soil profile_ Given the following information:

average pore water velocity, v, is 0.01 cm S-l porosity, n, is 0.2 bulk density, Pb' is 1.2 g cm-3

exchange coefficient for potassium, k, is 0.7 mL g-l

calculate the retardation factor for potassium and chloride, and the average velocity of the potassium and chloride ions, in rates of cm sol.

Solution

Given that the average pore water velocity, v, is 0.01 em S-l, and we know that:

* v = v/R,and

R = (1 + Pbk/n).

For chloride,

k=OsoR=1.

Therefore, * v = v =

For potassium,

k = 0.7 mL g-l, so

R = = =

1 + (1.2*0.7)/0.2 1 + 4.2 5.2

0.01 cm sol.

(g cm-3 * mL g-l) units cancel out

153

Therefore, * v

2.3.2 Ion Exclusion

= =

0.01/5.2 0.0019 cm S-I.

The majority of soils possess negative surface charge, and this causes negatively charged ions or anions in the soil water to be repelled and excluded from the volume of water immediately adjacent to the charged particle surface. Anions thus have a smaller effective water volume available for transport than say the exchangeable cations, and the process of exclusion results in anions being located some distance away from the charged surface in the bulk soil solution (Figure 7).

Figur.ee7

DDL DDL I J I I I .. I I I I I I

'-+~ ___ ~. I Water available to, excluded ion

~-----~)o Total water content

Schematic representation of ion exclusion

In the':obulk soil solution, soil water flow is generally faster than that closer to the charged surface'due to a reduction in the frictional drag forces. This, in combination with concentration gradients, can result in anions moving through the soil at velocities faster than the average pore water velocity.

The shape of the BTC (Curve E; Figure 1) is very similar to that of Curve B for a non­reactive ion not experiencing exclusion or adsorption, except that its maximum concentration occurs prior to one pore volume of leachate passing through the column.

An approximate expression for the volume of soil water from which anions are excluded can be represented as:

Tic = a I SQRT(b t3 c) [19]

154

where T c b f3 a

= = = = =

amount of anion repelled per unit surface area (mol cm-2)

concentration of equlibrium solution (mol mL- l )

balancing cation valency constant (1.06xlQ15 cm mor l )

a factor which depends on the ratio of the valences of the attracted and repelled ions (q=2 for NaCI and q=1.46 for CaCI2)

The term on the r.h.s. of Equation [19] represents the volume of solution per unit surface area (i.e. distance) from which the anion would be excluded, or the effective thickness of the diffuse double layer (DDL).

Equation [19] also shows that the magnitude of anion exclusion increases as the cation valence and solution concentration decreases, which is consistent with the effect of these. parameters on the diffuse double layer thickness.

Also, not only does anion exclusion increase the rate of anion movement in soil, but also the rate of cation movement since an equivalent amount of postive charge must accompany the negatively charged anions in the soil solution to maintain electroneutrality ..

3 EXCHANGE RATES

So far the movement of chemicals in soil has been discussed assuming that exchange between cations in solution and those in the electric double layer is instantaneous.~ . For particular soil types and cations the rate of cation exchange is not instantaneous, but can take some period of time ranging from hours to days.

The major cause for the slow rate of reaction between the soil and cation is due to the time it takes for the cation to diffuse from the soil solution to the exchange site. The rate of diffusion has generally been found to be greatest in soils containing significant amounts of 2:1 layer clay minerals such as vermiculite, and in well-aggregated soils.

3.1 Exchange In 2:1 Layer Minerals

Consider two soils (Figure 8), one kaolinite dominated, the other vermiculite dominated. Kaolinite is a 1: 1 layer clay mineral and has no interlayer spacing; thus exchange occurs predominantly on the planar surfaces which are in direct contact with the soil solution and the exchanging cations. Under this condition, exchange is rapid, and assumed instantaneous.

However, in the vermiculite soil, a high proportion of the exchange sites may be located in the interlayer spacing, so cations have to diffuse from the bulk solution to these exchange sites. This diffusion can take an extended period of time depending on the diffusion pathlength. Consequently, exchange equilibrium would not be attained instantaneously in this soil, but would take a period of time (e.g. hours).

155

(a) Kaolinite [NOTE: exchange sites are readily available to solution ions]

(b) Vermiculite [NOTE: interlayer sites available to small ions (e.g. K+) not to larger ions (e.g. Ca2+). Time will be required for K+ to diffuse from solution to the interlayer sites]

Figure 8 Effects of clay mineralogy on rates of exchange; (a) 1:1 layer mineral (kaolinite) and (b) 2:1 layer mineral (vermiculite)

Lack of equilibrium is most noticeable with the small cations such as potassium (K+) and ammonium (NH/) as these cations can fit into the small interlayer spacings, while the larger cations such as calcium (Ca2+) cannot. Consequently, exchange in the interlayer spacings generally involve K+ and NH/, while Ca2+ and Mg2+ are generally confined to the planar sites (except in the case of 2: 1 swelling clay minerals such as montmorillonite).

3.2 Exchange In Aggregated Soil

Experimental work has shown that exchange equilibrium does not occur instantaneously in aggregated media. This has been attributed to the time required for the exchanging cation to diffuse to the exchange sites within the aggregates (Figure 9). The structure of the soil aggregate indicates that time is required for the cation to diffuse to the exchange sites, particularly to those sites on clay particles located in the centre of the aggregate, and within

156

the organic matter.

(a) Soil aggregate

(b) Organic matter

Soil partiCle surface

indicates readily available exchange sites

indicates diffusion paths for ions to exchange sites

organic matter

clay domains

Figure 9 Ion diffusion within (i) a soil aggregate, and (b) soil organic matter [NOTE: organic matter resembles a plate of spaghetti, with the exchange sites located within this amorphous material]

3.3 Relationship Between Pore Water Flow Velocity and Diffusion Rates

From the above discussion, it is apparent that in soils where diffusion may be a rate-limiting process in cation exchange reactions, the rate at which water drains through the soil profile will be a major factor determining whether exchange equilibrium is attained.

We can estimate whether the rate of water flow is slow enough to allow diffusion of the cations into the soil aggregates or interlayer spacings by evaluating the microscopic Pec1et number (Pe). The Pe is the ratio of the time scale for diffusion of cations into the aggregate compared to the time scale for their transport over a similar distance by convection (i.e. with water), and is defined as:

157

Pe = [14]

where Pe = Peelet number (dimensionless as a ratio) v = pore water velocity (m S-l)

I = diffusion length (m) Do = molecular diffusion coefficient in bulk solution (m.2 S-l)

A Pe of <1 indicates the time scale for diffusion is less than that for advection and therefore sufficient time for diffusion of the cations into the soil aggregates or interlayer spacings exists.

EXAMPLE 4 - Use of Peelet Number in Exchange Equilibrium Studies

In an experiment investigating K+ transport in a soil with significant amounts of 2: 1 layer clay minerals, it was observed that diffusion was not a rate-limiting process for attaining exchange equilibrium. Using the following experimental information, explain the reason for this behaviour.

the bulk diffusion co-efficient for K+ was 10-9 m2 S-l;

soil aggregates were <0.35 mm in diameter; average pore-water velocity was 2.6 * 10-6 m S-l.

Solution

To determine why, calculate Pe using the following information.

The soil aggregates were 0.35 mm in diameter or less, consequently, the maximum diffusion length was 0.175 mm or 1.75 * 10-4 m (aggregate radius as this is distance to centre of aggregate). The maximum pore-water velocity through the soil was 2.6 * 10-6 m S-l, and the bulk diffusion coefficient for K+ was approximately 10-9 m2 S-l.

Thus, the Peelet number is:

Pe = =

1.75*10-4 * 2.6*10-6 /10-9

0.46

Since Pe was <1 sufficient time for diffusion of K+ into the aggregates was available in the experiment.

At higher flow rates, say 10-5 m s-l, sufficient time would not have been available and exchange equilibrium would not have been achieved.

3.4 Exchange Kinetics

Exchange kinetics is concerned with the rate at which . a chemical reaction comes to equilibrium. It has been reported (e.g. Ogwada and Sparks, 1986) that exchange rate coefficients can be calculated from first-order kinetic equations as derived below.

158

Apparent Exchange Rate Coefficient (ka')

where Kt

Ko ~

= = =

=

amount of K+ on soil at time t (mol kg-I) amount of K+ on soil at equilibrium (mol kg-I) absolute velocity coefficient of the exchange process (S-l)

Separating variables and re-arranging Equation [21] gives:

=

[21]

[22]

REMEMBER: can re-write Kt in terms of Ko as Kt = Kt x KjKo' which allows Equation [22] to be derived.

Integration of the form dx/(l +x) gives In(l +x), so integration of Equation [22] gives:

In (1 - K/Ka> = (as ~ Ko is a constant, integration of 1<a Ko dt gives 1<a Ko t).

Expressed in terms of base 10 logarithms gives:

where = =

=

= k't a

apparent exchange cofficient (1<a ~) I 2.303 slope obtained by plotting log(1-KlKo) against time t

[23]

[24]

[NOTE: divide by 2.303 as this is a factor for converting In values to log values, e.g. log 10 = 1, while In 10 = 2.303]

A similar derIvation can be used to obtain the apparent desorption rate coefficient, ~', from equation [19] below:

[25]

4 REFERENCES

Ogwada, R.A., and Sparks, D.L. (1986). Kinetics of ion exchange on clay minerals and soil: 1. Evaluation of methods. Soil Science Society of America Journal 50: 1158-1162.

Rose, C.W., Chichester, F.W., and Phillips, 1. (1983). Nitrogen-labelled nitrate transport in a soil with fissured shale substratum. Journal of Environmental Quality 12: 249-252.

159

160

ABSTRACT

NITROGEN AND PHOSPHORUS AND mE PROCESS OF EUTROPHICATION IN AQUATIC SYSTEMS

D.W. Connell l and A.H. Arthington 2

1 Government Chemical Laboratory, Archertield, Qld. 4108

2 Centre for Catchment and In-Stream Research, Griffith University, Nathan, Qld. 4111

Aquatic areas can become enriched with plant nutrients from ferlilised lands, sewage, erosion and so on. This can lead to excessive growth of blue-green algae and other phytoplankton as weU as rooted and floating aquatic plants. In lakes and dams the water mass can become stratified during summer leading'to poor water quality conditions in the bottom waters due to accumulation of decaying organic matter. Surfac~ waters can containia«e quantities of alg~, causing problems for water supply authorities and others using the water. In recent years the toxic blue-grein algae have accentuated problems caused by nutrient enrichment.

1 INTRODUCTION

The enrichment of aquatic areas with plant nutrients is an important process in aquatic pollution, and a significant aspect of this is eutrophication. Eutrophication was described by Weber in 1907 when he introduced the terms oligotrophic, mesotrophic, and eutrophic (Hutchinson 1969). These terms describe the natural eutrophication process as a sequence from a clear lake to a bog by enrichment with plant nutrients and increased plant growth. However, they may also be applied to eutrophication arising from human activities, which is called cultural eutrophication.

In the natural eutrophication process, plant detritus, salts and silt from a catchment are transported in runoff water and deposited in the water body over geological time. This leads to nutrient enrichment, sedimentation, infilling, and increased biomass. Figure 1 illustrates in general terms how eutrophication is related to aging. The final stage of the process results in the formation of bogs, swamps, and the extinction of the water body.

Water bodies with little flushing, such as lakes, dams and enclosed seas, become eutrophic through nutrient enrichment over an extended time scale, as described above. This follows the generally accepted eutrophication pattern. However, nutrient enrichment also occurs in situations where infilling and increased sedimentation leading to the formation of swamps is less likely, due to comparatively rapid water movement and flushing. This situation

.. arises in streams; estuapes, the continental 'shelf, and the open seas: Nevertheless, these water bodies may snow many of the characteristics of eutrophication and are often referred to in terms of trophic status. Nutrient enrichment and cultural eutrophication can be greatly accelerated by human activities.

161

Figure 1

Nutrients and Plant Growth

0: EXTINCTION .,.. ... --- ,.,---LLI ,'-"" I- "'" ",'" ",,,,,,,

~ .. ," PosSIble Pol"" ,. ::E ~ I 01 EOflcllmen' ,,"

'::: I al Olllo,enl ,. U ~ I Ini'loI 51a9l' /

~ "c; ~ /. / '';/ e ~ ~, :5 ~ /

"t I II.. I Eul,ophicollon

;~ ~ /' ~~ ill1 ,/ ~ ! ~~------------~--------------

AGE ~

Hypothetical curve of the course of eutrophication in a water body. The broken lines show the possible course of accelerated eutrophication when enrichment from pollution occurs.

Eutrophication can cause a number of important problems in water use. An increase in the pORulations of plants can lead to a decrease in the dissolved oxygen content of the water, 01'1· plant death and decomposition by microbial action. This decreases the suitability of the water as a habitat for many species of fish and other organisms. The increase in turbidity and colour which occurs during eutrophication renders the water unsuitable for domestic use or difficult to treat to a suitable standard for this purpose. Odours are also produced by many of the algal growths which create problems in domestic use. Blooms and pulses of aquatic plants become more frequent and, if toxic, lead to the death of fish and other aquatic organisms, and also of terrestrial organisms using the water; Floating aquatic plants and algal scums can render a water body unsuitable for recreation and water sports and also cause navigation problems (Connell 1981).

2 CHARACTERISTICS OF NUTRIENT BEHAVIOUR

Primary production of plant matter through photosynthesis occurs principally in the upper layers of water bodies, the photic zone, where the availability of sunlight is greatest. This process can be represented by:

CO2 + H20 + P04ii + N03- ---> carbohydrate and protein + O2 I I

plant matter

Important sources of nutrients in many streams are natural and waste animal and vegetable matter. The degradation of natural plant and animal matter, as well as organic wastes produces nutrient salts and consumes oxygen (dissolved oxygen in water) by the respiration process as indicated below:

162

carbohydrate + protein + fat (natural animal and plant matter and organic waste) +02 ---->

(with low dissolved oxygen) CH4 + CO2 + NH4 + + N02- + organic P

or

(with high dissolved oxygen) CO2 + H20 + N03- + P04"'

A generalized diagram showing the pattern of nutrient movement occurring in aquatic areas is shown in Figure 2. If the water body is oligotrophic the nutrient concentrations, biomass of living aquatic plants and dead plant matter are present in low amounts. Thus their impact on dissolved oxygen, general water quality and other characteristics is minimal. However, if nutrient concentration is high the growth of aquatic plants and the resultant production of oxygen will be high in the upper water layers (the epilimnion). When this mass of plant matter dies and falls to the bottom, microbiological degradation occurs through respiration with a depletion of the dissolved oxygen in the lower water layers. In addition, some undegraded plant matter will be added to the bottom sediments, while a proportion of the degradation products are plant nutrients and can be reused in plant growth. The overall characteristics of oligotrophic and eutrophic areas can be assessed in terms of the characteristics shown in Table 1.

Table 1 Measures of eutrophication in a water body

1. Nutrient and associated ion concentrations in the water

2. Total dissolved solids (specific conductance)

3. Dissolved oxygen status

4. Standing crop (biomass)

5. Primary production

6. Production\biomass ratio

7. Transparency of the water

8. Species diversity and types present

9. Lake mOIphometry

10. Sediment core analysis

11. Algal bioassays

163

Figure 2

WATER SURFACE

EPILIMNION plant aquatiC}(PhYtoPlanklon

C03 + H30 +. )0 . allached nutrient salts PholosyntheSJs plants benthic plants)

D"pentillg 011 I "".1'" t:iu:ul.'ion D".lh

I HYPOLIMNION Respit2/ion CO 2 + HaO + ~Iant utilising

nutrient salts ~issolv"d oltygen

t plant fragments

t

+ O2

A generalized diagrammatic representation of some major chemical processes occurring in a stratified aquatic area.

Changes in the chemical characteristics of the water, perhaps due to evaporation, addition of run-off water from the catchment and so on, can result in the release of nutrients from sediments. Low dissolved oxygen concentrations in water allow the release into the water mass of phosphorus, iron and manganese absorbed onto sediments. An example of this situation is shown in Figure 3. These figures are derived from a eutrophic lagoon, Braidwood Lagoon in New South Wales, subject to heavy algal growth (blooms) during the summer months (May 1972). Studies have indicated that generally in early summer, higher temperatures or some other environmental factor results in increased bacterial activity in the bottom sediments which are rich in organic matter. This leads to a reduction of dissolved oxygen, which allows the release of phosphorus and an increase in carbon dioxide dissolved in the water. These conditions may stimulate an algal bloom .

.l J

:E A-A-

2

Figure 3

-,... 1 \ I \ I 1

I \ b 6i-...... I \I' U\ ~ .:;

I \1 \ u c:

I t \ ...... 4

I ..J

I ~ .. I ...

I ,\ z I ;(

1\ I I /.1 \ a: 2 I \ " v\

/L \

o III I I \ I Dec Apt Jun

Jul Dec Apr Oct Feb 69 70

68 69 MONTHS MONTHS

The left graph shows the concentration of total nitrogen (broken line) and total inorganic phosphorus (solid line) as related to rainfall (right graph) at Braidwood Lagoon, NSW. The horizontal bars show periods of algal blooms. (Source: May 1972.)

164

Jun 7(1.

3 EUTROPHICATION AND WATER MOVEMENT

Vertical movement of water is influenced by such factors as the surface to bottom temperature profile, winds, morphology and water inflow. Seasonal and other regular environmental changes can .lead to corresponding patterns of vertical water movement. This leads to reasonably predictable patterns of thermal stratification and vertical mixing, allowing water bodies to be classified according to this (see Bayly and Williams, 1973). These authors report that the most common type of lake in Australia and New Zealand is the warm monomictic type, ie. complete circulation occurs once every year, in winter, at temperatures above 4°C.

Stratification of waterbodies often occurs during summer when the epilimnion is warmer and less dense than the hypolimnion. Thus the epilimnion tends to float undisturbed at the surface. At the onset of winter the upper layers decrease in temperature and, when the density is equal to that of the lower waters, mixing occurs. Thus nutrient rich waters are circulated to the upper layer and a bloom of algae may occur.

If there is little vertical movement of water in a lake or similar aquatic area the lower layers of water can be isolated from the oxygenated upper layers and thus be more readily deoxygenated. This can lead to the release of nutrients from sediments which may stimulate eutrophic conditions. The processes of photosynthesis operating in the epilimnion and hypolimnion lead to distinctive changes in the vertical patterns of some important water components. With oligotrophic water bodies little vertical change is usually found but with eutrophic waterbodies the death of animals and. J)lants and the demand on dissolved oxygen in the bottom waters leads to the profiles shown in Figure 4. In periods of low or zero flow many Australian streams are reduced to stagnant pools or lagoons. These bodies may develop conditions similar to those observed in eutrophic standing waterbodies. For example, Connell et al. (1981) have found that elevated nutrient concentrations develop in the estuary of the Fitzroy River, Queensland, during the low flow winter period.

Figure 4

OLIGOTROPHIC WATER BODY

SURF~A7CE ________ ~~

plant nutrients (NO; & PO!)

BOTTOM ..

dissolved oxygen

CONCENTRATION~

EUTROPHIC WATER BODY

dissolved oxygen

CONCENTRATION~

Generalized. patterns for surface to bottom distributions of dissolved. oxygen and plant nutrients for oligotrophic and eutrophic water bodies.

165

Flowing streams subject to nutrient enrichment can exhibit distinctive patterns of chemical and ecological behaviour. For example, in a flowing stream subject to a point source organic effluent discharge from a sewage treatment plant, there are distinctive and predictable chemical and ecological responses, shown in summary form in Figure 5. Firstly, the BOD (biological oxygen demand) increases abruptly and there is a corresponding decrease in the dissolved oxygen concentration, forming a 'dissolved oxygen sag' (Figure 5A). In response to decay and the gradation in the availability of oxygen downstream from the discharge, various forms of nitrogen and phosphorus occur in peak: and then declining concentrations (Figure 5B). The ecological response to these conditions includes a rapid increase in the biomass of bacteria as they decompose the organic matter, and the prolific growth of Protozoa and algae as well as sewage fungus (Figure 5C).

Figure 5

EffecUi of Reduced Dissolved Oxygen and Associated Factors

A

------.!~-------

B

...... -

............. ,.-, .... //" ......

........ / " _.,.......... / "-... ... AI.GAE

// "'" "'" BACTERIA - - --/" r-a••• ------

\.., " .. -..... ------------------

c

/........... -............. PROTOZOA .~.~.~ . ."...." ---- -- .-...... : ... : .................. .

o

DISTANCE ~

DOWNSTREAM

Diagrammatic representation of the effects of an organic discharge on the downstream section of a stream: (A) and (B) chemical changes; (C) microorganisms; and (D) other organisms. [Compiled from Hynes (1960). Reprinted with permission from Liverpool University Press.]

166

Conditions in the stream below a sewage treatment plant become unfavourable for normal clean-water animals such as insects and Crustacea, but a few tolerant species thrive in the presence of the organic matter, especially the Tubificid worms and Chironomid larvae (blood-worms, Diptera; Figure 5D). Both of these groups are able to survive at very low dissolved oxygen concentrations because they have haemoglobin in their body fluids, whereas most aquatic invertebrates do not. The haemoglobin gives these organisms their bright red colour in polluted waters, and both can be regarded as biological indicators of organic enrichment in freshwater systems.

With increasing distance downstream from the discharge, chemical and physical conditions improve and may return to the levels normally found in clean-water streams. However, the rate and extent of recovery will depend on the severity of the dissolved oxygen sag, the degree of nutrient enrichment, and whether or not any toxic substances such as residual chlorine or metals are present in the discharge. In Australia, domestic sewage released to waterways after treatment may contain low concentrations of such toxicants, and their adverse effects combined with the other impacts described above will tend to severely reduce aquatic biodiversity.

The release of organic wastes containing phosphates and nitrates also has chemical and ecological effects in estuaries, shallow littoral areas and in the oceans. However, they are not as readily detected due to the patterns of water movement and the normally high dissolved oxygen levels in these waterbodies. Nevertheless, similar ecological changes occur. There is usually a reduction in the number of species and diversity in heavily polluted areas and an increase in biomass of some tolerant invertebrate species, such as certain polychaete worms which are the marine equivalent of the Tubificid worms in freshwater streams and the benthos of lakes.

The effects of nutrient enrichment on coral reefs have received little attention compared to the effects of enrichment in freshwater systems (Hawker and Connell 1989). The recorded effects include increased primary productivity and biomass in both phytoplankton and benthic algae, and localized dissolved oxygen depletion, affecting coral nutrition, growth and ultimately, their survival. The diversity of coral species generally decreases due mainly to competition with algae for space and light, which is reduced by the dense phytoplankton levels present in overlying water (Hawker and Connell 1989). The return towards oligotrophy is relatively slow because the nutrients cycle between the sediments, benthic organisms and the overlying water.

4 SOURCES OF NUTRIENTS

A convenient way to divide discharges of nutrient containing wastewaters is to categorise them as point source and diffuse. Point source discharges are those arising from a specific location whereas diffuse discharges arise from many dispersed sources, ego via eroded soil and sediment derived from cultivated land.

4.1 Sewage

The discharge of untreated or primary treated sewage into an aquatic area causes deoxygenation and during this process nutrient salts are released. Secondarily treated

167

sewage has had the biochemical oxygen demand removed during treatment but contains the same nutrient salts as would be released by untreated sewage. Of course it does not cause deoxygenation on discharge. So untreated, primarily and secondarily treated sewage all contribute nutrients to aquatic areas. The most important nutrients are in the form NH/, N03-, and P041ii which occur substantially as a result of the degradation of proteinous matter and detergents.

4.2 Detergents

The builders used in detergents to enhance the actIvIty of the surfactant are usually polyphosphates. Phosphate occurs in crude sewage and survives treatment in a sewerage plant. Phosphates in detergents are a major source of phosphate in sewage.

4.3 Water Run-off

Storm water run-off in agricultural and urban areas can contain significant concentrations of nutrients. Overflow from septic tanks, household wastes from sinks, laundries and baths are major contributors to the nutrient content. Australian soils are deficient in phosphorus and exhibit an ability to retain phosphorus. Campbell (1978) has evaluated phosphorus outputs from a number of stream catchments in Victoria as related to land use patterns.

4.4 Point and Diffuse Sources of Nutrients

Recently an extensive investigation of nutrient pollution in the Murray-Darling system has been completed (Gutteridge, Haskins and Davey 1992). An overall summation of their findings is shown in Table 2. The diffuse sources of nutrients were identified as forest, pasture and crops and the point sources were irrigation, urban stormwater, municipal sewage treatment plants and fish farms. The proportions from point and diffuse sources exhibit variation related to rainfall but clearly, point sources, which are more easily controlled, comprise a significant proportion of the total.

Table 2 Total point and diffuse source nutrient inputs to streams in the Murray-Darling Basin.

Nutrient Loads (tonnes/year)

Category

Point Sources

Diffuse Sources

Ratio Point/Diffuse Sources

Dry Year

Total Total P N

650 3900

250 1600

2.6 2.4

From: Gutteridge, Haskins and Davey (1992).

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Average Year Wet Year

Total Total Total Total P N P N

750 4400 900 5300

950 6700 4300 28000

0.8 0.7 0.2 0.2

5 NUTRIENTS AND PLANT GROWTH

If the growth of algal cells is not limited by any environmental or nutrient factor then population growth occurs according to an exponential function.

Exponential growth of this kind cannot· be maintained for very long by algae due to limitations of various kinds. For example, elements, such as carbon, nitrogen, sulfur, hydrogen, and oxygen are needed to construct plant tissue, particularly proteins and carbohydrates. In addition to these major elements, phosphorus, iron, magnesium, sodium, and a variety of other elements are needed to construct other vital components.

Table 3 Essential elements in plant tissue compared with that in river water

ELEMENT

Oxygen Hydrogen

Carbon Silicon

Nitrogen Calcium

Potassium Phosphorus Magnesium

Sulfur

DEMAND BY PLANTS/SUPPLY IN WATER (APPROX)

1 1

5000 2000

30000 <1000 1300

80000 <1000 <1000

Source: Vallentyne (1973).

In most aquatic areas, carbon and oxygen are readily available from carbon dioxide in the atmosphere and hydrogen and oxygen can also be readily obtained from water. On the other hand, the other elements mentioned above are usually obtained from dissolved salts in the water or sediments. However, these substances are not always available in the quantities required to maintain maximum growth. For example, Table 3 shows a comparison of the relative quantities of the elements required for plant growth with their occurrence in river water. Ratios for N and P indicate that these elements are in comparatively short supply compared with all the other elements and that phosphorus is likely to be less available than nitrogen. This situation is generally applicable to aquatic areas; thus these elements are often growth limiting and an addition of them to a water body will stimulate plant growth. Of course, these data are generalized and, in individual cases, other elements or combinations of elements may be limiting.

In addition to essential nutrients and elements, all algae require a suitable water temperature and sufficient quantities of light. Water temperatures and light are normally

169

more than adequate for supporting algal growth during at least a part of the year in temperate and tropical areas. In such waters growth will be dependent primarily on the availability of nutrients, and the extent to which phytoplankton can utilise the light which penetrates the water column. The complex interplay of these growth factors and environmental conditions determines both the biomass of algae and the composition of the phytoplankton community (Oliver, 1992).

In a turbulent water body, algal cells may be canied out of the illuminated upper water layer into the deeper" dark layers of the water column, resulting in a reduction in the mean light intensity to which they are exposed. Eventually growth may become light limited and at some depth will finally cease. The mixing depth is determined by the interaction of turbulence induced by water flow and wind, and the stability of the water column due to thermal stratification. When water flow and turbulence decrease, as in weir pools and in areas of rivers with low discharge, thermal stratification of the water column can occur, resulting in a reduction in mixing depth and a more favourable light environment for algae. This situation, coupled with high concentrations of nutrients, may be sufficient to trigger algal blooms, such as the massive blooms which developed in the Darling River in late 1991.

The average algal cell has a composlhon of 16 nitrogen to 1 phosphorus by atoms (Redfield 1958); thus at molar N:P ratios above about 16:1 phosphorus is likely to become limiting because nitrogen is available in excess compared to the average requirements of the cells. If the molar N:P ratio is lower than 16: 1 then nitrogen may become limiting. However, each algal species will have optimum ratios for the nutrients that it requires. In waters with very low N:P ratios, where nitrogen is expected to become limiting, the phytoplankton may become dominated by nitrogen fixing Cyanobacteria (blue-green algae) which can fix nitrogen from atmospheric sources.

In many inland waters such as lakes and water storages, a strong correlation has been demonstrated between summer phytoplankton biomass levels, measured as the total chlorophyll concentration, and the total phosphorus concentration at the start of the growing season (Vollenweider 1968; Dillon and Rigler 1974; Ferris and Tyler 1985). The development of quantitative relationships between nutrient 'loads' (quantities) and algal biomass is an essential part of managing troublesome algal blooms. From these relationships, water managers can develop criteria for phosphorus loadings in particular waterbodies.

Phosphorus loading models have been used successfully in a range of aquatic systems to predict the reduction in phytoplankton biomass resulting from a decrease in the phosphorus load (Oliver 1992). However, in Australian waters the basic relationships appear to be quite different from those developed in the northern hemisphere.

Research groups are working on improving our understanding of algal growth under different conditions, taking into consideration such factors as the high suspended sediment load which is common in many Australian inland waters (Ferris and Tyler 1985) and the influence of other factors such as turbulent mixing and losses of biomass due to zooplankton grazing.

170

Phosphorus loading models are based on the total phosphorus level in the system, but it is actually the dissolved fonn of phosphorus (orthophosphate) which is available for uptake by phytoplankton. This bioavailable fonn of phosphorus varies between different aquatic systems, particularly in relation to the concentration of suspended sediment matter (Oliver, 1992). In highly turbid waterways, such as the Murray-Darling system, a major proportion of the total phosphorus is bound to the suspended particles and is not biologically available.

This discussion of algal growth indicates that the management of algal blooms requires firstly, a sound knowledge of the environmental conditions which stimulate and maintain algal growth, and secondly, a quantitative basis for relating algal growth and biomass to these environmental conditions (Oliver 1992). When these two sets of information are available, water managers may be able to manipulate environmental conditions such as turbulence and nutrient concentration to control or reduce algal growth. Management issues are discussed in Section 6.

6 ALGAE AND CYANOBACTERIA

6.1 Types of Algae

Microscopic organisms termed 'algae' are comprised of two major groups which are not related although they are rather similar in appearance. The Cyanobacteria, or 'blue-green algae' are allied to the true bacteria, and have similar characteristics; for instance, they lack a nuclear membrane and many other structures found in the true algae.'

There are 12 major groups of true algae most of which are found in freshwater environments although two phyla are mostly marine. The important freshwater groups are the green algae (Chlorophyta) including desmids, the diatoms (Bacillariophyta), the dinoflagellates (Dinophyta), the Chrysophyta (golden-brown algae) and the euglenoids (Euglenophyta). Many occur all over the world but there are some fonns found only in Australian waters.

Algae and Cyanobacteria vary in size and shape from minute, single-celled fonns <5 JIm to species visible without the use of magnification. There is immense variety in the shapes and textures of algal cells, from simple globular fonns to spindle and star-shaped species, and various colonial fonns composed into filaments or clumps. It is necessary to use a high powered microscope and an identification key to separate phytoplankton communities into genera and species.

6.2 Blue-Green Algae

The Cyanobacteria have characteristics which enable them to exploit the environment of lakes and storages and achieve large populations and high biomass in circumstances where other algae are less successful. These characteristics include the existence of overwintering forms (akinetes), the capacity for 'luxury' uptake of nutrients, mechanisms for buoyancy regulation and mucilage production.

Cyanobacteria can assimilate higher levels of nutrients than they require when ill

171

conditions of high nutrient concentration. This 'luxury' uptake allows population growth to continue during periods when nutrients are not readily available in the water column. The formation of gas vacuoles allows Cyanobacteria to regulate their buoyancy and take advantage of favourable light environments and nutrient concentrations at various times of the 24 hour diel cycle. Cyanobacteria produce a mucilage which provides protection and the mucilage together with the buoyancy of the colonies contributes to the formation of large surface scums. It is these surface scums that are the most visible evidence of algal blooms.

6.3 Effects of Blue-Green Algae

The development of algal blooms is considered to be undesirable from a number of perspectives (Johnstone, 1992). In ecological terms, an algal bloom is often a symptom of degraded conditions where a nutrient rich or eutrophic system is dominated by episodic occurrences of high populations of a limited range of species. The various adverse ecological consequences of eutroppication have been outlined above, such as deoxygenation of the waterbody and fish kills. From a human perspective, algal blooms are undesirable because they can cause taste and odour problems and physical effects in water supply systems. Some species also have the potential to cause toxic contamination of potable water supplies. Threats to human and stock health posed by toxic Cyanobacteria blooms are presently the major concern in Australia.

Prolific algal growth, including the growth of Cyanobacteria, can cause the physical obstruction of fixtures such as spray nozzles, filters, valves and pipes in water treatment and supply plants. Some of these problems can be avoided by changing the position or type of offtake to avoid intake or accumulation of algae. This approach would benefit from a better understanding of the spatial dynamics of algal communities in water storages (Johnstone, 1992). .

The existence of toxic blue-green algae in water supplies has been known for some time. For example reports of stock deaths were attributed to blooms of Nodularia in Lake Alexandrina as early as 1878. It is only in recent times that the potential for these toxins to affect humans through use of contaminated water supplies has been fully recqgnised. Where contact with Cyanobacteria IS likely without any ingestion of the cells, or water, skin irritants are likely to constitute a significant health risk, causing rashes and lesions. Some people repeatedly develqp hay-fever like symptoms such as rhinitis, conjunctivitis and asthma after swimming in water containing Cyanobacteria. Gastroenteritis (caused by lipopolysaccharide endotoxins) may develop following the oral ingestion of Cyanobacteria during swimming; the symptoms include abdominal pains, nausea, headaches, diarrhoea and vomiting.

The most serious human health effects are due to two classes of toxins, the liver toxins (hepatotoxins) and those affecting the nervous system (neurotoxins). In general these classes of toxin are associated with different taxonomic groups of the Cyanobacteria. The genera Microcystis and Oscillatoria produce cyclic peptide hepatotoxins known as microcystins but there is evidence that the normally neurotoxic Anabaena can also contain hepatotoxins. Toxicity of the various types is variable, some toxins being only moderately toxic compared to botulism and tetanus, but highly toxic compared to substances such as

172

strychnine or sodium cyanide. Cylindrospermopsis produces the most toxic hepatotoxin examined to date. The 1979 outbreak of liver toxicity in a population of aboriginals living on Palm Island has been attributed to this toxin. It is believed that the toxin was released into the local water supply when algal blooms were treated with copper sulphate (Hunter, 1991).

Cases of acute exposure of animals to Cyanobacteria neurotoxins are reported to have caused death due to respiratory failure in less than 15 minutes. Anatoxins include neuromuscular blocking gents affecting acetylcholine receptors at nerve synapses (Keevil, 1991). The symptoms of toxic effect are decreased movement, collapse, exaggerated breathing, convulsions and death. Aphanotoxins include saxitoxin and neosaxitoxin and are the least toxic of the neurotoxins. They inhibit nerve conduction by acting as sodium channel blocking agents in nerve cells.

Stock losses associated with consumption of toxin contaminated water have been reported on numerous occasions. If no other source of water is available, stock will drink from water with thick surface accumulations of Cyanobacteria. There is also anecdotal evidence that chronic effects can include a reduction in milk production from dairy cows. Concern has been expressed regarding the possibility of toxins or their residues being accumulated in meat products obtained from stock that have consumed Cyanobacteria. Likewise: there are questions concerning the health threat posed from the accumulation of Cyanobacteria on fruit and vegetables irrigated with contaminated water. These issues have not been researched and the risk at this stage is unknown.

6.4 Control and Treatment of Algal Blooms

The objectives of control and treatment are to reduce the occurrence and severity of Cyanobacterial and other algal blooms and to provide methods for detoxifying contaminated water. The range of measures to minimise the frequency and severity of cyanobacterial blooms includes controls on nutrient inputs and loadings, controls on physical environmental parameters and modifications to the biology of aquatic systems (Johnstone 1992).

The most obvious fonnof nutrient control is to target major point sources of nutrients and attempt to reduce their contribution. The success of such measures in reducing the potential for algal blooms will depend upon the significance of those sources to the total pool of available nutrients and other factors that affect nutrient dynamics (eg. oxygen status of sediments). Diffuse sources of nutrients are less amenable to control but are likely to contribute a significant proportion of the total nutrient input to many water bodies (see Table 2). Riparian vegetation may reduce the transport of both nutrients and sediment to streams but the characteristics of effective buffer strips need to be explored. There is also the possibility of collecting and treating diffuse runoff in wetland systems that are capable of removing available nutrients. The most effective control of diffuse nutrient discharge is to reduce the source through changed land management practices.

The physical environment can be modified to make conditions less suitable for cyanobacterial growth, ego by altering the light and thermal regime in a water body and the degree of turbulence. There are some constraints on the extent to which such changes

173

can be made and also questions about other impacts of these changes. For small water storages it is possible to exclude light by providing a roof or cover over the storage. For larger bodies of water the light regime can be modified by increasing either the turbidity or colour of the water. Such modifications are undesirable in many instances and would degrade the value and potential beneficial uses of the water.

Water temperatures can be managed to some extent by altering the mix of inflows, location of off takes, and by artificial mixing of storages. The flow rate of rivers will also have an effect on the temperature regime, and additional turbulence will generally reduce the formation of pools of warm, calm, stratified water. Turbulence is amenable to modification in both rivers and water storages. In rivers turbulence can be modified as a function of flow rate and flow management, although exactly how to achieve the desired results is the subject of considerable debate. For water storages artificial destratification can create sufficient turbulence to have an impact on the growth of Cyanobacteria.

There is some potential for controlling the development of cyanobacterial blooms through manipulation of biological interactions such as fish feeding and the intensity of grazing by zooplankton. Biomanipulation has the potential to reduce some problems of algal blooms but there is a need for considerable research into food chain dynamics and other aspects of aquatic ecology before biomanipulation becomes a feasible proposition on a general scale. Other biologically based solutions include the use of aquatic vegetation and wetlands to act as nutrient sinks and filters, including the use of riparian buffer strips, as discussed above. Research and development work is required to explore the opportunities and potential benef}~s in these areas.

It is inevitable that algal blooms will occur in water systems and that in order to meet short term water requirements the algae will need to be removed. The common approach is to dose the water with an algicide (usually copper sulphate) which will kill most of the algal population. There are a number of issues regarding the use of such biocides. Firstly, biocides tend to be non-specific, and they can therefore inflict considerable damage by harming potentially beneficial organisms. The persistence of biocides varies; some will degrade rapidly, others will remain unchanged for long periods. Copper is a persistent chemical that often accumulates in the sediments of storages which· are dosed for algal control. Copper based compounds are not the only algicides available, but they are the only ones registered for use in potable waters in Australia. '

Processes that remove algal cells intact (eg. filtration, flocculation by lime or alum) do not usually liberate toxins from the cells. The liberation of toxins can pose several problems. Physical removal of the cells will no longer remove toxins from the water. The toxins are no longer associated with the cells, therefore any assay of potential toxin levels must be made directly on the toxin itself. It is more difficult to concentrate free toxins for analysis than toxins contained in algal cells. The need for a capacity to measure toxins at low levels in water becomes very important if the water is treated with algicides or the bloom is decaying naturally.

Some forms of water treatment can remove cyanobacterial toxins from water. The entry of intact cells into reticulation systems can be restricted by selective withdrawal of water from depths where algal counts are low. Some reduction of cell counts can also be

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achieved by using oil control booms or water sprays to keep smface scums of Cyanobacteria away from offtake points. Some filter systems can trap cells and prevent them passing further down the system. Such controls are of limited use if cells have ruptured and liberated toxins. Activated carbon has been shown to remove toxic compounds from water.

7 CONCLUSIONS

Nutrient problems are likely to continue in Australian waterways for some time in the foreseeable future. Even if all nutrient discharges could be halted, the large reservoirs of nutrients in aquatic sediments ensure that nutrient levels in overlying waters will follow current trends. Over time the nutrient levels in sediments would be expected to decline due to processes such as denitrification, physical movement and dispersal and covering by fresh sediments. High nutrient levels and other environmental conditions in aquatic areas can stimulate massive growths of algae and aquatic plants. Problems with toxic blue­green algae are of most concern to water users. Most State governments have responded to the apparent increase in toxic blue-green algal blooms by setting up task forces with representation from the various government departments which are involved. These task forces are addressing contingency planning for occurrences of blooms, water quality standards, and testing and research.

8 REFERENCES

Bayly, I.A.E. and Williams, W.D. (1973). Inland Waters and Their Ecology, Longman Australia, Hawthorn.

Campbell, I.e. (1968). Inputs and outputs of water and phosphorus from four Victorian catchments. Australian Journal of Marine and Freshwater Research, 29: 577~581.

Connell, D.W., Bycroft, B.F., Miller, G.J. and Lather, P. (1981). Effects of a barrage on flushing and water quality in the Fitzroy River Estuary, Queensland. Australian Journal of Marine and Freshwater Research, 32: 57-63.

Connell, D.W. (1981). "Water Pollution - Causes and Effects in Australia and New Zealand", University of Queensland Press, St Lucia, pp. 42-60.

Dillon, P.J. and Rigler, F.H. (1974). The phosphorus-chlorophyll relationship in lakes. Limnology and Oceanography, 19: 767-773.

Ferris, J.M. and Tyler, P.A. (1985). Chlorophyll-phosphorus relationships in Lake Burragorang, New South Wales, and some other Southern Hemisphere lakes. Australian Journal of Marine and Freshwater Research, 36: 157-168.

Gutteridge, Haskins and Davey (1992). An investigation of nutrient pollution in the Murray-Darling River system. Report by Gutteridge, Haskins and Davey, Pty Ltd, for Murray Darling Basin Commission, Canberra City, p. 18-8.

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Hawker, D.W and Connell, D.W. (1989). An evaluation of the tolerance of corals to nutrients and related water quality characteristics. International Journal of Environmental Studies, 34: 179-188.

Hunter, P.R. (1991). Human illness associated with Cyanobacteria (blue-green algae). Public Health Laboratory Services Microbiology Digest, 8: 96-100.

Hutchinson, G.E. (1960). Eutrophication, Past and Present. In "Eutrophication: Causes, Consequences", National Academy of Sciences, Washington, DC. pp. 17-28.

Johnstone, P. (1992). Current status of knowledge on key issues. In: Cyano-Fighter, July 1992: 6-13.

Keevil, C.W. (1991). Toxicology and detection of Cyanobacteria (blue-green algal) toxins. Public Health Laboratory Services Microbiology Digest, 8: 91-95.

May, V. (1972). Blue green algal blooms at Braidwood, New South Wales. Science Bulletin No 82, NSW Department of Agriculture.

Oliver, R.L. (1992). Phytoplankton growth in waters of the Murray-Darling Basin. In: Seminar on Algal Problems in Water Supplies. Gutteridge, Haskins and Davey Pty Ltd, Feb. 1992, pp. 6.

Redfield, A.C. (1958). The biological control of chemical factors in the environment. American Science, 46: 205-222.

Vollenweider, R.A. (1968). Scientific fundamentals of the eutrophication of lakes and flowing waters, with particular reference to nitrogen and phosphorus as factors in eutrophication. Organisation for Economic Co-operation and Development Technical Report DAS/CSI/68.27.

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ENVIRONMENTAL CHEMISTRY OF ORGANIC CHEMICALS AND BIOCIDES

M.W. Silvey\ B.W. Simpson2 and P.J. Silvey3

1 Standards & Inspection, QDPI, Brisbane, Qld, 4000. 2 Agricultural Chemistry, QDPI, Meiers Road, Indooroopilly, Brisbane, Qld, 4068. 3 Dept. of Chemical Engineering, University of Qld, St. Lucia, Brisbane, Qld, 4067.

ABSTRACT

Science and technology can no longer be isolated from the influence of socio-political issues that relate to health and the environment. Concern about the human influence on the environment is a global issue and it is mainly due to the exploding population. However the community has expressed fears, via the media, about risks associated with the use of synthetic chemicals, particularly the biocides. This review attempts to place the risks associated with modern chemicals in perspective and explain some accepted misperceptions about the part which chemicals play in our lives. Chemistry of life is highlighted and natural environmental chemicals and synthetics are compared. The possible pathways of biocides in the environment are illustrated with an example of a pesticide impact study. FinaUy, the current and some future methods of handling the remediation of organic chemicals are reviewed. With the exception of a few localised incidences, Australians enjoy safe and healthy food, water and environment. As always our modern society should be alert and practice preventive measures to ensure that future safeguards on our environment are in place and obvious to the community.

1 PREAMBLE

Chemicals conjure up different meanings to different people. It is the very interpretation of this term which is currently causing most people a great deal of confusion and unjustified anxiety. Chemists remind us that everything around us is made up of chemicals. Our environment, the earth itself, is a package of chemical compounds - some very simple and others highly complex. Blends of these chemicals make up our environment - the living and the inanimate, the natural and the synthetic.

But there is a loose and more popular definition of chemicals. This is the one promulgated by the newspapers, magazines, radio and television which seem to carry public debate forward. Chemical is interpreted by the general public to mean all synthetic matter which is deemed to be toxic to the environment and to humans. 'Chemical- free' substances are considered ·to be free of synthetic but not of natural chemicals. These contradictions in terms and misperceptions are quite acceptable by the media and hence the public. There is a certain level of complexity which is beyond the capacity of the media. Inevitably this means the general public is misinformed on these and other modern day issues.

The very reason for this discussion is in response to the perceived threat of synthetic chemicals on the health of our global environment. Organic chemicals and some of their toxic components, the biocides, will be highlighted because they are considered by most people to be especially hazardous. We intend to summarise the part that both natural and,

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in particular, the human-made synthetic chemicals play in our modem life. Our principal aim is to place current chemical issues in perspective and in context. We also need to examine the risks associated with synthetic chemicals, especially pesticides, to help you decide their relative importance when compared with other exposures. Bold statements will be made for the sake of brevity. Their validation may be found by the discerning reader in numerous sources of reputable scientific literature.

2 INTRODUCTION

Although science, through technology, influences every aspect of our practical life its effect at the intellectual level seems minimal. Modem technology has dragged the global community through at least four' Ages' over the past 60 years. The Nuclear Age which began in the 1930's, was followed by the Electronic Age, then the New Chemical Age and now, in the 1990's, to the Environmental Age (Crone 1990; Naisbitt and Aburdene 1990). The Environmental Age heralds the single and largest problem facing the world - the ever­increasing human population (World Bank 1992).

The effects of human population pressure on our health and environment are experienced and seen from different viewpoints or multiple perspectives (Silvey 1992). As an individual your perspective is influenced by personal experiences, knowledge and attitudes. As a member of an organisation you tend to adopt the corporate viewpoint and if you are also a scientist you may even have an objective perspective. For example, as an individual living in Australia, you may be exposed to a daily litany of media rhetoric on issues ranging from global warming to chemical residues in our bread. As a member of a company or Department you note that wastes and pollution are generally within set guidelines and regulations. But as a scientist you may believe that public concern about chemical pollution is grossly exaggerated. A person living in a poor and undeveloped country, such as Sub-Saharan Africa, would not share your perceptions about the environment nor your priorities on environmental restrictions. While we experience our so-called environmental problems through the media, the African paupers actually experience their own kind of environmental problems on a daily basis. Their problems are not industrial-based pollution but immediate life-threatening ravages of disease, insufficient quality, variety and quantity of food, insufficient clean drinking water, soil erosion and nutrient losses, natural disasters, loss of vegetation and wildlife habitats and social instability. Human concerns are all relative.

In our modern society synthetic organic chemicals seem to take the brunt of blame for the perceived assaults on the environment and public health. Reports of leaking underground fuel tanks, leaching of nitrates and nitrites into groundwater reservoirs, increasing air pollution in large cities and seemingly frequent contamination of our food and animal feeds with pesticide residues, industrial chemicals and bacterial toxins are common. These events or situations are not observable but people believe they occur even though abundant evidence shows that most chemicals are dissipated or degraded in our environment (Gare 1992). As we will elaborate later, the earth and its environment is perhaps more resilient, powerful and unremitting than we tend to believe.

A relevant observation is that most trained scientists are cautious people when discussing their own area of expertise. They are loathe to make unqualified statements SInce

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knowledge is always imperfect. This reticence may be interpreted by the public as hiding the truth. The public is hindered by the general lack of basic scientific knowledge in the community. This latter statement is not meant to be arrogant or denigrating. It is simply that most people cannot keep up, let alone cope, with the rapid changes occurring in all facets of science and technology. In such a situation we all find difficulty in differentiating between scientific truths and myths. More importantly we tend to lose sight of the true perspective of life or, to put it colloquially, the 'big picture'. Most of us still have a myopic anthropocentric view of the world.

The review below aims to equip you with a basic understanding of the nature, toxicity, distribution, disposal, remediation and relative risks of environmental biocides. We will

. attempt to present an objective viewpoint but may stray into some subjective opinions. We trust the reader will be better informed to make critical reviews of other opinions and of course to seek further information from a variety of sources.

3 REVIEW OF THE BASICS

3.1 Chemistry and Life

We learned that chemistry was the study of the composition and structure of matter. And matter, containing energy, could be solids, liquids and gases which are made up of unique elements and their compounds. Vaguely we are reminded that chemistry could be divided into:

•

•

•

•

•

Physical chemistry - concerned with the laws, expressed mathematically, of chemical changes,

Inorganic chemistry - a study of the characteristic of non-carbon elemtmts and their compounds,

Organic chemistry - a study of carbon-based compounds,

Biochemistry - a study of compounds and chemical changes associated with living processes,

Nuclear chemistry - the study of atomic nuclei and radioisotopes.

Life as we know it is the dynamic interaction of natural chemicals at both the macro and micro levels. In fact, life of all living systems on earth seems to follow an intricate cyclie;al pattern. Life begins with chemical, bonding, followed by successive growth and . development phases during which time useful chemicals are selectively acquired from the surrounding environment. At the same time, unwanted or waste (generated by the system) chemicals are discarded. Finally, for various reasons, the chemical infrastructure disintegrates and disperses. This process is being constantly modified by the ever­changing surrounding ecosystems and pressures of natural competition, some destructive and others synergistic. This pattern of events could· be applied to all chemical structures be they living or inanimate.

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Why such chemical creativity in all its forms is developed and modified to be ultimately "destroyed" in death or major upheaval is beyond the scope of mere mortals like ourselves. Despite the seemingly impressive technical knowledge and some understanding of biochemistry, biotechnology and ecological competition, humans have only a limited knowledge of the capacity and the dynamic power of our environment.

3.2 Environment

What do we mean when we refer to the word environment? The environment is the dynamic composition of three major systems:

• Physical system (geosphere) of land, air and water, • Biological system (biosphere) of plants, animals and microorganisms • Social system (sociosphere) involving politics, economics and culture.

All of the environmental components are reactive, interdependent and are constantly changing. Soils are mainly formed from the interaction of physical and biological systems.

Inclusion of the social system indicates the human influence on the use and management· of the other systems. In fact a new interdisciplinary study called environmental science has emerged. Besides tracing the evolution of the biosphere this science examines the way our resources could be repaired and managed by humans so all organisms could flourish more or}ess together in a sustainable manner.

There are two types of environment; the external and internal environments (Jones et ai. 1990). The external environment is considered to be the abiotic conditions, both physical and chemical, which have major impact on life of individual organisms. The internal environment refers primarily to the chemical structure and functions which control the well-being of an individual plant or animal. We will refer mainly to the wider aspect of the environment with regard to synthetic chemicals.

3.3 Natural Chemicals

Virtually all the chemicals that make up the geosphere and biosphere are natural. There are 92 elements that form the crust of the earth. Of these ten account for 99.9%, by weight, of all chemicals with oxygen (46.6%), silicon (27.7%), aluminium (6.5%) and iron (5.0%) making the bulk.

Only 11 elements are common in all living organisms. In the human, for example, oxygen (65.5%), carbon (18.5%), hydrogen (9.5%) and nitrogen (3.3%) make up the bulk of the body. Since two-thirds of the body is water, the chemistry of life is primarily the chemistry of water.

The vast proportion of chemicals that the human and other living organisms are exposed to occur naturally; in the air, food, water and living environment. All plant and animal foods consumed by humans are composed of differing amounts and various types of natural chemicals. These include carbohydrates, fats, proteins, vitamins, minerals and

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water which are required for growth, reproduction and maintenance of health. Plants also need chemical nutrients to grow and flourish.

Some of the natural chemicals found in both plants (e.g. prussic acid) and animals (e.g. antibiotics) help protect them against pest attack and disease. As you might expect some of these natural pesticides (chemicals which control pests) or biotoxins are known to cause human/animal illness and even death if consumed in large doses. In fact some investigators (Ames et aZ. 1987; Ames and Gold 1989) claim health risks from natural chemicals in foods are greater than those from human-added chemical residues (e.g. synthetic pesticides).

Suffice to say humans and other animals have learned, perhaps by trial and error, to avoid or moderate what they consume to ensure a healthy state. Most living organisms are rarely exposed to toxic levels of natural biocides in either their food or environment.

3.4 Synthetic Chemicals

The discovery and large scale production of synthetic chemicals over the past 70 years have resulted in the modem world in which we live. Humans, perhaps more than any other organisms, are exposed to a wide variety albeit relatively small amounts of synthetic chemicals (compared to natural chemicals) on a daily basis. Nevertheless when we learn that some 70,000 chemicals are used daily in agriculture, consumer products and industry we tend to heed media reports of their possible harmful effects (BMA 1990). Selinger (1991) described a vast array of synthetic chemicals that are found in our modem laundry, kitchen, dining room, boudoir, toilet, medicine cabinet, garden and public places. Domestic chemicals are taken for granted and few would consider that they are potentially toxic if not used as intended. As expected synthetic chemical residues find their way into the atmosphere, water supplies, soil and food. These residues may come from fertilisers, pesticides and as pollutants from burnt fuel, sewage and industrial wastes. Our processed foods incorporate additives which serve to enhance flavour, texture, colour, nutrient content or prolong storage life. Synthetic chemicals are part of the daily life of most humans.

Synthetic organic chemicals are still claimed to have objectionable effects on the environment. These can be broadly grouped into four classes (Crone 1990) as summarised in Table 1.

Although we could not function without synthetic chemicals, their perceived overuse has led to a state of public anxiety, more aptly described as 'chemophobia'. As we shall discuss later, synthetic chemicals are not any more toxic than the natural chemicals that humans have already lived with for so long. Some chemicals require better and safer methods of use and waste disposal.

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Table 1 Types of synthetic organic chemicals and their perceived objectionable etTects on the environment (adapted from Crone 1990)

Chemical class

Structural:

Bulk: plastics

Fibres

Pesticides:

Process:

Industrial

Domestic

Drugs:

Description

Used for implements, furniture, plumbing

Used in textiles, carpets, ropes

Control pests such as insects, weeds, fungus and snails

. Used to modify or facilitate a process without forming a significant portion of the end product

Variety of uses, with some substituting for natural products

Chemicals for the treatment of disease or which affect body functions

Examples

Perspex, PVC, polystyrene, plastic containers, polythene bags

Rayon, nylon, terylene, fishing line

Parathion, carbamates, 2,4,5-T, metaldehyde, used containers

Wetting agents, mould release agents, PCB, PBB, sterilants, enzymes, dioxin, additives

Detergents, polishes, disinfectants, cosmetics

Antibiotics, antidepressants, aspirin, salbutamol

4 SYNTHETIC CHEMICALS AND THE ENVIRONMENT

Objectionable Effects

Aesthetic (visual), possibly economic (damage to useful plants and animals)

Aesthetic, economic

Health (deleterious to humans and non-target organisms)

Health, economic'

Health, economic

Health (environmental amounts negligible)

We note that increasing amounts of synthetic chemicals are being produq(xl, used and distributed throughout the world. Crone (1990) argued that since the elements of synthetic chemicals originated from the environment by drastic alterations, their 'redistribution represents a return to the environment after use. He suggested that the problem of increasing amounts of synthetic chemicals in our environment was mainly attributed· to the economics of distribution and collection. Crone reasoned that raw material was collected in bulk:, converted to synthetic chemicals and made into articles or reformulated for use.

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The products were distributed to the consumer, be they organisations or individuals. All parties found this process to be economically favourable when profits were made from purchased products.

A major concern now is what happens to wastes and residues generated through manufacturing or when discarded by the consumers throughout the urban and rural areas. Ideally waste should be collected and destroyed or converted into harmless products which could be assimilated by the environment. Until recently no or little profit was made from wastes or residues. The degree of environmental pollution is still a product of the economics of production and waste control together with effective legislation. It is technologically possible to reduce or eliminate environmental pollution from synthetic organic chemicals.

4.1 Environmental Pollutants

Environmental pollution, such as land denudation, smoggy air and contaminated water supplies, has been mainly caused by the increased use of fuel for energy, metals mining and their manufactured products and the concentration of human natural waste in populated areas. The evidence is readily seen in many areas throughout the globe and the large volume of literature produced about this topic might also be listed as pollution.

During the last fifty years, the production and use of synthetic chemicals have also resulted in environmental pollution but in different ways. The most obvious form and voluminous amount of synthetic chemical pollution are from non-toxic structural chemicals. Plastic bags and containers, household items, furniture, tools and textiles ,with paper, form the bulk of garbage that fill our dumps. The more subtle form of synthetic chemical pollution is the slow and less dramatic accumulation of residues. An unknown quantity is distributed by people into the abiotic and biotic environments through natural processes. Of major public concern are the residues which have resulted from purposely applied chemicals, such as non-biodegradable pesticides, or from the dumping of intractable biocides such as polychlorinated and poly brominated biphenyls (PCBs and PBBs) and dioxin (wastes from chemical plants and an infamous impurity from the herbicide 2,4,5-T) which contaminate abandoned plants or dump sites. If not contained the intractable biocides, especially some of the chlorinated hydrocarbons, could lead to and, in some cases, have resulted in contamination of soil and water compartments of the environment.

4.2 Biocides in Food

Another concern is the presence of biocides (especially cancer-causing pesticides) in our food supply. The human diet contains an enormous background of natural chemicals, such as natural plant pesticides and the products of cooking. Half of these natural biocides in normal food plants are carcinogenic (Ames et ai. 1987; Ames and Gold 1989).

Most people are exposed to some 2000mg/day of burnt material in the diet and about 1500mg/day of natural pesticides in plants (Gold et ai. 1992). The total amount of synthetic pesticide residues in food is in the order of O.09mg/day. So 99.99 percent of the pesticides ingested is from natural pesticides. Humans are generally immune from such

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low doses of both synthetic and natural biocides quoted above. Even at higher doses the immune response is inducible as evidenced by insect resistance to pesticides. Despite these and many other findings the public tend to believe that synthetic chemicals are toxic despite the fact that naturally occurring chemicals are toxic at the same dose. As we will discuss later, the toxicology of natural and synthetic toxins are basically the same. Gold et al. (1990) concluded that natural dietary carcinogens are not important in human cancer and so the relatively low-dose exposures to synthetic chemicals are even less significant to human health.

5 CHEMICALS AND TOXICITY

5.1 When Does a Chemical Become a Biocide?

As we mentioned at the onset everything is made of chemicals. All chemicals can be harmless or harmful to living organisms; it mainly depends upon the amount of exposure to the chemical. The oft quoted Swiss physician, Paracelsus (1493-1541), said "All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy."

Too little of a particular chemical may cause a deficiency condition while an excess could cause toxicity. For example, the trace element, zinc, is required by the human for its body's production of enzymes, nucleic acids, white blood cells, nerve tissues, skin and so on. A deficiency of zinc (less than 1Omg/adult/day) is harmful and over time can cause anaemia, skin disprders, impotence and depressed immune response. Too much intake of zinc ( toxic levels) cause abdominal pain, anaemia again, fever and bleeding in the stomach. Fortunately our internal environment (like the external environment) is very tolerant and forgiving for a wide range of chemical doses.

Therefore all chemicals are potentially harmful when an excess of a certain amount of that chemical enters the body of a living organism. Note that a toxic level of a particular chemical does not necessarily cause death.

5.2 Nature of toxicity

Toxicology is a study of how chemicals can adversely affect living organisms. As expected in any new science (only sixty .years old) the more that is known the more complex the topic becomes.

For practical purposes a chemical, becomes a toxin or a biocide at and above a certain critical dose but only after it gets onto or into the body of a living organism. This critical dose is called the threshold dose aitd the value for animals or plants depends on:

•

•

•

the chemical substance (characteristic of the structural, process, pesticide or drug chemical)

the type of living organism (e.g. human, rabbit, rat or fish and plants),

the size of the organism (either weight or body surface area),

184

• how the chemical enters the organism (i.e. ingested, inhaled, absorbed or injected),

• how long or often the organism has been exposed to the chemical (seconds, minutes, hours, days, months or years)

and

• the purity of the chemical.

Ideally, . the correct expression of toxicity of any chemical should include the name of the biocide, . its effect and concentration, the type and size of the organism, route of entry and time of' exposure. For example, if a 70kg adult male inhaled, over a two hour period, 60,OOOpIlm of the fumigant/insecticide methyl bromide he would show severe respiratory distress and most likely suffer a fatal heart attack. In the real world such an event rarely occurs out most people will continue to accept just the name or class of a synthetic chemical:to infer toxicity to all living beings.

5.3 Measuring Toxicity to Human and other Animals

Can we know with certainty whether a particular chemical is safe? As you can see from the above example the answer is not quite that simple or clear cut. We do not have complete information on all the 750,000 chemical that are now in use in homes, industry and agriculture. Most of these chemicals are either harmless or human'exposure to them is insignificant under normal circumstances.

Of course there are some chemicals which are more toxic than others and they are loosely classified under the term of biocide. A number of factors must be considered before even a crude threshold value of a chemical can be given. Scientists usually present predictive values with statistical standard deviations. For practical purposes, expressions of toxicity have large safety margins built into the reported value. Some of the main measurements are briefly mentioned below (Crowther, Silvey and Hughes 1991).

Toxicity cannot be measured easily on humans so animals with similar metabolism to humans have been used to estimate the lethal dose of a particular chemical. Test animals have usually included rats, mice, rabbits, birds, dogs and fish. As expected, differences between the biology of human and test animals lead to inconsistencies and errors in measuring the toxicity of all products that can affect human health. Mortality and morbidity records of humans exposed to biocides are also used to establish toxicity values.

The main measures of toxicity are LDso and LCso'

LDso of a chemical is the estimated lethal dose which when administered to a group of test animals will kill 50% of them. The unit of measurement is mg per kg bodyweight and this value measures the toxicity by oral intake or through skin absorption.

LCso of a chemical is the estimated lethal concentration in air or water which will kill 50% of a group of test animals. The unit of measurement is mg per cubic metre (usually

185

measured over four hours of exposure). This value measures the toxicity of a chemical by inhalation.

Toxicity varies with the route of entry. In general, oral toxicity is greater than dermal toxicity but the effects of toxicity are faster when a biocide is inhaled (LC values are mostly much lower than LD values). The most common routes in accidental pesticide poisoning are in the order of skin absorption, inhalation and by mouth.

The smaller the LD50 or Le50 value is, the more toxic the substance. This concept is illustrated in Table 2 which presents probable oral dose of an active chemical constituent estimated to kill a 70 kg adult male.

Table 2 Probable lethal oral doses of Biocides

LD50 of active constituent

(mg/kg)

<5

50

500

5000

15000

Estimated lethal amount of pure active ingredient for a 70 kg adult male

a few drops

1 teaspoon

30g or 2 tablespoons

500g or 500 mL

> 1000g

Commercial chemical formulations are rarely pure active ingredient so the lethal doses are proportionately higher.

All substances can be hazardous in the right amount, but there are great differences in toxicity between substances. Some are extremely dangerous, others are fairly harmless, and many fall between these extremes. Table 3 lists the toxicities of some common substances.

Another measurement commonly reported to indicate safety levels of chemical residues that may be consumed in food by people is the Acceptable Daily Intake (ADI). This value is the threshold dose (expressed in mg per kilogram bodyweight) of a particular chemical which, if ingested on a daily basis, will not adversely affect the life time health of the consumer (animal). A safety factor, usually in the order of 100 (range: 10-1000) is built into the ADI figure.

All chemicals can be used safely provided the right amount is used, the appropriate purpose is respected (i.e. used according to the label) and, if necessary, wear the correct protective gear.

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Table 3 Toxicities of Some Common Substances

Toxicity Class

Biotoxin

Supertoxic

Highly toxic

Moderately toxic

Substance

botulin

atropine

amphetamine methomyl strychnine

nicotine dieldrin caffeine paraquat aspmn profenofos 2,4-D

Slightly toxic DDT

Harmless

penicillin table salt glyphosate

trifl uralin ethyl alcohol

5.4 Toxic Effects of Chemicals

Product Use or Source

Bacterial toxin

Plant toxin

Drug stimulant Insecticide Plant alkaloid

Tobacco toxin Insecticide Coffee beans Herbicide Analgesic Insecticide Herbicide

Insecticide Antibiotic Condiment Herbicide

Herbicide Alcohol

Oral LDso (Rats)

(mg/kg) 0.01

1

5 13 16

50 54 150 150 350 358 450

500 1000 3300 4300

10000 14000

Once inside the body chemical substances are either excreted, stored or metabolised. Excretion is mostly via the urine and faeces. Storage of an assimilated substance may occur in most tissues and organs including the blood, fat, skeleton and teeth. Metabolism of assimilated chemicals takes place mainly in the liver where the products of metabolism (either harmless or harmful) can then be excreted or stored. These normal functions in our internal environment are upset if toxicities (or deficiencies) occur. If a person or animal is poisoned various signs and symptoms may give clues as to the extent of the InJury.

The main toxic effects of a biocide range from:

• sub-acute (no observable signs),

• acute (signs seen very quickly after poisoning; minutes, hours, days),

187

to

• chronic (signs and symptoms delayed; months, years).

Sub-acute effects of toxicity may have symptoms (effects reported by the patient or shown in tests) but to the observer the person is normal and perhaps coping with the poison.

Injuries from acute biocidal poisonings are usually a result of a single and sudden event or accident. Typical signs of acute poisonings include headaches, blurred vision, sweating, rapid pulse, heart palpitations, vomiting, fits, breathing difficulties and possibly death.

The most common effects and the type which concern most people are the chronic poisonings resulting from multiple doses over days, weeks, and sometimes years. Chronic effects can be due to accumulation of the chemical (e.g. organochlorines and heavy metals) or its effects (e.g. organophosphates and asbestos). High dose responses show up as malfunctions of various body systems such as disorders of the skin (some 30 percent of all chronic pesticide toxicities) nervous system, blood, heart, liver and reproductive organs. Low dose responses are insidious and include the dreaded cancers and mutations from damage to cell genetic material.

As can be surmised, when the dose of a biocide increases the severity of the toxic effect increases. No chemical is so toxic that it cannot be used safely, providing the hazard is reduced by taking extreme precautions. But there is none so safe that it cannot be used without minimum precautions. A safe lifetime exposure to a hazardous chemical is one that keeps exposure to below the NOEL or No Observable Effect Level.

The National Health and Medical Research Council of Australia and many other organisations here and overseas conduct research and update massive databases on the toxicity of most synthetic and natural chemicals. Guidelines on their safe use, purpose and disposal have been legislated and published on a regular basis.

6 THE FATE OF CHEMICALS IN THE ENVIRONMENT

Since all chemicals have different characteristics and properties their behaviour in equally variable environments make the study and prediction of the fate of released chemicals a very complicated subject. Only generalisations could be made in this section with some specific examples concerning biocidal properties of some pesticides.

The major routes that synthetic chemicals are released into the environment are shown in Figure 1.

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Figure; 1

BIOCIDE -:e" Chemical remains - at original site

., .. "ow, p"""" I Ch~l," • (Toxic or harmless),\.~ •• _o:.. Perm1e.,;on Soil CimPlex

"""_, RemobiDsBtion

t Perme.tlon Displaced by change

ENVIRONMENT ;.--in surrounding conditions

Biotic / '-Abiotic • Aquatic!T errestrial

Flora Fauna Microbes

• Air • Soil

Surface sediments BOhom sediments

• Water

Solar energy ECOSYSTEM Climate

Major routes for release of chemicals into the environment (modified from Crone 1990)

In our modem society, most of the chemicals released, discharged or disposed of in the environment are controlled by regulations and dispersed through planned facilities to minimise contamination. With such a variety of synthetic chemicals in daily use not all the methods of disposal are effective or suitable for all chemical wastes. We can also assume that a few chemical producers, consumers or regulators do commit errors, misuse or even abuse the systems and procedures that are in place to promote responsible and safe use and disposal of hazardous chemicals. So a factory might disdharge toxic gases into the air by incomplete incineration or dispose of oily liquids into sewer, rivers or a pit in the" ground without consideration of the consequences of further contamination through redistribution by leaching. Similarly chemicals may be dispersed from the home through burniiIg;aerosols, dumping on the land and flushing down the drain.

Agricultural activity has been singled out for special attention because farm chemicals are usually applied widely throughout the countryside and the suspected pesticide residues in food products. The subsequent fate of organic and inorganic nutrients and synthetic pesticides in the environment depend upon their chemical and physical properties. Figure 2 illustrates the possible pathways of rural chemicals (or any other chemical for that matter) in the environment.

Figure 2

Storage /system /' Incineration '- :

~H::~:~ :::..- '~~~'~/'I· ==A=I=R==1.1

~ Rural'" ,.-l=.' ~~~ . .... ---... kroperty~r= i HumanWaste

l WATER 1 ,

.,. Sewage.",... ..".,.,.- l : r-- Sewage ,-------, system -- system

HOUSEHOLD

_. solid waste _' , ~ ___ ... • garbage : :

The possible pathways of rural chemicals to the environment (adapted from Crone 1990)

189

The activity of a chemical dispersed into the environment is dictated by its stability, reactivity and its binding capacity to soil or other components. Besides being assimilated by the local flora and fauna, chemicals may be degraded by microorganisms, bound to particles and organic matter in the soil, sediments or water for different lengths of time and be displaced to contaminate the wider environment. Crone (1990) observed that, in general, chemically reactive biocides displayed acute toxic behaviour in the environment. These chemicals are usually short-lived and so do not present a long-term pollution problem. Organophosphorus insecticides, phosgene and methyl isocyanates are some examples of less stable' biocides. The more stable and chronically toxic biocides tend to cause long-term pollution problems because of their persistence and resistance to degradation. Some of the chronically toxic biocides, now banned, include the chlorinated hydrocarbons insecticides such DDT, BHC, aldrin and dieldrin, PCBs from electrical insulators and PBBs from fire retardants.

In the next section the impact of pesticides in the rural context will be used to illustrate the fate of biocides in the environment.

7 EFFECT OF PESTICIDES IN THE ENVIRONMENT

7.1 Impact of Pesticide Toxicity

Perhaps one of the most emotive and scientifically challenging aspects of the impact of biocides in the environment is the issue of pesticide toxicity.

The argument about whether a pesticide should be used in the environment is usually a non-issue to the community unless there is some identifiable but fundamental signal which suggests an unacceptable impact. Some of the more obvious signals include odour, colour (stain in water or soil), taint (water or food) or toxicity (to bird or fish)? Pesticide toxicity tends to concern both the general public and the scientific community and these incidents are of particular interest to the media.

For example, the impact of aerial spraying and fish kills clearly shows how pesticide usage draws media attention. This type of incident is effectively and comprehensively covered by journalists together with vivid photographs or film. These reports are regularly re-used in all forms of media to evoke public outrage about unacceptable environmental pollution and health issues. Sometimes these reported incidents do not relate to the suspected chemical or actual circumstances. Of course some fish kills occur as a direct result of biocide exposure. Several biocides are known fish toxins and so strict precautions have been proscribed and should be observed. Some still argue that such environmental biocides should be banned or at least restricted where such non-target environmental damage could result.

Excessive media attention places pressure on industries to evaluate and justify their usage of pesticides. For example, most attention has been focused on the cotton industry, which depends on significant use of pesticides for its viability. One of the key insecticides, endosulfan, is particularly toxic to fish and has been linked with fish kills on several occasions. Because endosulfan is so effective for controlling cotton insect pests, the industry has directed considerable effort in promoting its safe and responsible use practices

190

to minimise any adverse environmental impact.

Table 4~ lists some of the technical properties of several insecticides used in the cotton industryy The LC50 values show the relative toxicity of these compounds to fish. The lower the LC value, the more toxic the insecticide.

Table 4;; Toxicity and Persistence Properties of Some Cotton Pesticides (from Batley and Peterson 1992)

Pesticide Annual average usage (g/ha)

Chlorpyrifos 15

Cypermethrin 18

Deltamethrin 6

Endosulfan 2570

Fenvalerate 39

Methomyl 14

Monocrotophos 108

Parathion 113

Profenofos 263

Sulprofos 81

Thiodicarb 53

LC50 (ug/L) (Fish)

122

2.8

1

1.4

76

3400

23000

1500

80

800

2550

Half life (water)

2.7 weeks

50 weeks

4 hours

2.7 days

17 days

20 weeks

2 weeks

6 hours

20 days

The immediate reaction to the data in Table 4 is to ask why we should tolerate the use of such toxic chemicals because if large quantities of these insecticides are used, eventually all the fish would be killed. However if an LC50 value for a particular insecticide is examined:in relation to its real impact on fish we see a different story. For example, the synthetic pyrethroid insecticide, deltamethrin is listed as being very toxic to fish. However,this same chemical is used in Asia to control mosquitos in rice fields where fish continue to live without adverse effect (Hamilton, pers. comm.). Other field studies showed that fish populations in rivers were healthy despite aerial sprays of pyretilloid insecticides onto adjacent forests (see pages 229-232 Lahey 1985). How can these reports be believed if the LC50 data are correct?

The answer to this question is that LC50 toxicity data is based on laboratory tests using pure water and often under controlled exposure conditions. These artificial conditions include a constant supply and flow of newly-treated water through test cages and removal of fish excrement from the immediate environment. Such tests are used to obtain the most accurate assessment of direct effect on the fish species. The findings are of value in assessing potential environmental impacts provided they are used with other properties (e.g

191

octanoVwater partition coefficient, log kow) to understand how the compound may behave in the real environmental situation.

In the case of the pyrethroids, their low water solubility means that the compound would be strongly attracted to organic matter and soiVsediment particles rather than being free or biologically more available in the water. This example demonstrates the complexity of such environmental tolerance and the problems associated with isolated data being either misused and mis-understood.

7.2 Mode of Transport

Pests are controlled by applying selected biocides directly to plants, animals or structures or by mixing them with soil, water and even air. Apart from accidental spills or ilTesponsible dumping, most biocides enter the environment via some form of controlled application. Aerial spraying hand-held and machine-operated ground rigs all attempt to deposit the chemical on the intended target such as crops and parcels of land.

Fertiliser, usually in the granular form, can be applied and deposited accurately on the land with little· off-target losses. However pesticide application, usually in a liquid formulation, is less precise, despite considerable effort to optimise spray application efficiency (Banks et al. 1990).

Spray application technology has in minimised spray drift. This involves optnmsmg biological efficacy of the pesticide by controlling spray droplet size and by developing more precise and more effective spray equipment. Despite these advances off-target movement of such compounds remains a key environmental issue for other secondary reasons.

7.3 Pesticide Persistence

Only a few pesticides (e.g. pyrethroids for insects) are used for instant fast-knockdown biological control. Most pesticides are required for longer term protection or for indirect systemic (affords protection from within) action. Hence most agricultural pesticides must have some level of persistence and stability in the intended environment or host. For example, if long term weed or insect control is required, it is inappropriate to apply a compound which will last only one day or until the first shower of rain. Whether a compound last in the environment for two days or two years is not an environmental issue unless the compound is no longer required (changed situation) or the compound is capable of moving to another location or environmental compartment.

The unintended movement of biocides from their area of use is a real problem complicated by a range of pathways such as from:

• plants to soil • soil to plants • plants to animals • soil to animals • soil to groundwater

192

• soil to surface water • water to terrestrial animals • water to aquatic biota

A thorough understanding of the chemical and biological properties of biocides IS

necessary to assess and solve the mechanism of off-target movement and problems of environmental persistence.

Movement of biocides from soil to aquatic systems is another area that demands considerable knowledge. Biocides which dissolve in water easily can mobilise in solution either in simple runoff to rivers and streams or through the soil profile into the groundwater. For example, pesticides could contaminate aquatic systems by the following ways (Simpson, Rayment and Spann 1992):

In surface water through

(a) Direct contamination: • Normal use

- Direct application to control aquatic weeds and other pests.

Relocation of pesticides to non-target sites from spray drift, evaporation/redeposition and aerosol formation.

- Mosquito eradication programmes. • Misuse

Careless disposal following rinsing of pesticide containers, spray equipment, mixing vats, and empty containers.

(b) Indirect contamination: • By water movement

- Pesticides in solution in run-off water from rural lands, roadsides, home­gardens, parks and gardens, etc.

- Pesticides adsorbed on eroded soil particles and sewage sludge eroding from sites similar to above.

- Inflow of contaminated groundwaters to surface waterways.

In groundwater through

(a) Direct contamination: • Normal use

- No legitimate case exists. • Misuse

- Direct (accidental or otherwise) disposal of pesticide products in wells or bores.

- Back-flow from pipes/tubes.

193

(b) Indirect contamination: • Normal use

- Inadvertent leaching of pesticides to groundwater after routine applications in agriculture and for other purposes.

• Misuse - Leaching to groundwater through uses not in accord with good practice

(e.g. uSe of unapproved chemicals in vicinity of boreholes).

- Leaching of pesticides from uncontrolled land-fills which have known groundwater linkages.

Fortunately, the majority of biocides are relatively non-polar (e.g. organochlorine insecticides) or ionic (e.g. the herbicides, glyphosate or paraquat). Such properties mean that these chemical compounds will strongly bind and persist onto the soil particles especially if the soil is not disturbed or covered with a protective stubble. These bound biocides will stay in place until finally degraded.

The stability of biocides in the environment is measured in terms of their decay rates or half-lives. The half-life of a biocide is the time required for the disintegration of one-half of the amount of that chemical. These half-lives are average values and are subject to fluctuations in accordance with Poisson statistics. For example, the organochlorine DDT has a half-life of approximately 3 to 15 years in soil whereas others, such as some organophosphates, are relatively short lived with half-lives in soil ranging from half a day to 5 years.

Most find difficulty in accepting that half-life values are not precise figures because of many qualifying factors or conditions. The long lasting herbicide atrazine, is used in minimal-till agriculture and has a reported half-life in soil of about 40 days. Recent studies under Queensland conditions (Hargreaves et al. 1993) showed that the half-life of atrazine varied from 23 to 149 days depending on the soil type. Table 5 shows that this variation is linked to soil pH and clay content.

Another example relevant to most people in the use of the organophosphate insecticide, chlorpyrifos, widely used in agriculture and under homes. It has a reported half-life in soil varying from less than 10 days to greater than 120 days, depending on variables such as temperature, soil moisture and pH. In some instances, increased concentration can also lead to increased half-life as required by the home-owner in barrier treatment for termite control. So there are different mechanisms involved in the pesticide degradation processes in protected area such as under buildings or slabs.

The stability of a biocide in soil is therefore closely linked with its unique behaviour in a particular environmental. The longer a biocide persists, the greater the chance that it becomes an unwanted problem and the greater the possibility off-site contamination through movement. With increased understanding of the mechanisms involved and improved management systems being adopted by pesticide users, the less chance of off­site biocide movement.

194

Table 5 Half-life of Atrazine in a Range of Soils

Half-life pH Clay (%) Soil Description (days)

Red-brown earth (a) 23 5.3 25

Red-brown earth (b) 32 6.2 22

Krasnozem 45 6.0 56

Brigalow -(grey clay) 51 8.0 46

Grey cracking clay 51 7.8 55

MywybiUa (cracking clay) 72 7.6 69

Coolibah alluvial 73 7.9 55

Brigalow plains R04 74 8.3 43

Brigalow-Belah brown earth 83 8.4 43

Rolling Downs soil 84 8.2 53

Brigalow-Belah 90 8.3 48

Waco (cracking clay) (a) 127 8.1 70

Waco (cracking clay) (b) 149 8.7 71

7.4 Control of Biocides in the Environment

Despite popular belief, there are strict controls in place covering the sale and use of biocides, particularly pesticides and veterinary drugs.

In Queensland, the Agricultural Standards Act, the Chemical Usage (Agricultural and Veterinary) Control Act and the Agricultural Chemicals Distribution Control Act are administered by the Department of Primary Industries to control the responsible use of agricultural chemicals. In addition, other Acts, such as the Clean Waters Act, the Contaminated Land Act and the new Environment Protection Legislation (now being developed), are administered by the Department of Environment and Heritage to further protect the environment from chemical contamination.

All pesticides and veterinary products are registered by a national registration board administered by the Federal governments and supported by State governments and other representatives. In the registration process human and animal health as well as environment safety are considered. All requests for registration must be supported with details of environmental behaviour including environmental fate, persistence and toxicology for both the parent compound and any metabolites. Such supporting data must also take into consideration the aspects of bioaccumulation; now also a marketing factor as exemplified by traces of organochlorin in export cattle. Since organochlorins have low water solubility (non-polar property) they can accumulate in fatty tissues of animals when

195

accidentally ingested through contaminated soil or feed (Noble 1990). Similarly many aquatic animals, particularly crustaceans, can bioaccumulate environmental contaminants from water and sediments. All registered pesticides and veterinary drugs are recorded onto a computer database (e.g. INFOPEST from Department of Primary Industries, Queensland).

There is also a legal obligation that any pesticide or veterinary drug must be used in accordance with the label recommendations. These include the dose or applications rate of the pesticide, the target crop or target animal and the withholding period (i.e. the minimum interval between last application and sale). The withholding period of importance from both a safety point of view and marketing requirement in that it ensures the product has been used according to the label and that any residues would be below a maximum acceptable limit at time of sale.

Maximum residue limit of a biocide (MRL) is the maximum permissible residue level of a particular compound on or in a food or fibre set at both International and National forums. At the international scene most nations accept the standard set by the Food and Agriculture Organisation of the United Nations (FAa) and the World Health Organisation (WHO). This alignment with international standards forces nations to adopt responsible uses and minimises problems with world trade. In Australia MRLs are set by the National Health and Medical Research Council. The MRL value of a biocide in a product is basically a calculated trade standard to suggest possibility of misuse. Unlike ADI (Acceptable Daily Intake), MRL values are not health standards although the figures represent a 100 to a 1000 fold human safety margin.

7.5 Assessment of Environmental Impact

In relatively simple cases, such as the discharge of a particular biocide near or into a river an assessment of the likely environmental impact can be made with reasonable accuracy. Such a study still requires good technical infonnation and a careful study of the location to predict both short and long-term effect.

For more complex situations where there are multiple inputs over a range of environmental matrices, such as a large catchment site (Simpson, Rayment and Spann 1992) much more information and effort are required to assess quantities of real and potential inputs.

Before undertaking any comprehensive study on the likely environmental impacts of biocides in a selected area, a structured approach is required to ensure that findings are factual and unbiased. The following key points. should be carefully considered:

• Determine the boundaries of the area being studied

• Record any streams or rivers in the catchment area and whether their source or end-point is within the study area.

• Note the land use activities in the area.

196

• List the type of land use in relation to the likely use of biocide (e.g. agricultural chemical use by primary producers, mosquito control by Local Government or treatment plants by industry).

• Determine what biocides are registered or available for use for any likely activities by interrogating registration data bases, interviewing produce agents and consulting with chemical companies.

• For each registered biocide use, obtain details on application rates and application frequency.

• Estimate what biocides are actually used within each land use activity by interviewing key representatives of each activity, with users such as spray applicators, advisers, consultants, extension officers with field responsibilities and interested community members within the study area.

From the information obtained the total quantities of each biocide used per year by each activity can be calculated for the study area. Reliable data on environmental and toxicological properties of each biocide used are essential to estimate the potential environmental impact for each compound. Such potential impact is closely linked to the amounts used, the activity and location.

The results of such an environmental impact study can form the basis of targeted research, monitoring program and improved strategies to manage catchmerintreau Comparison with other published studies and the use of simulation models can complement local studies. In some cases, mathematical models (Moore 1992) can handle the multiple variables that influence fate and persistence of organic compounds in different compartments of the environment. Such models are now sufficiently refined in the United States to predict the potential environmental behaviour of every new compound submitted for registration.

Governments have to respond to the community concerns regarding real or potential environmental contamination. In order to make sound and responsible decisions concerning the environment and health, meaningful, accurate and up-to-date data are required. Lack of reliable factual data lead to irresponsible decisions based on emotive pressure or simply a need to show some action.

7.6 Interpretation of Analytical Data

Tables of analytical results really mean very little unless they are interpreted by a person with full knowledge of how and when the samples were taken and analysed and the toxicity, chemical or biological properties of the compounds involved.

Before takillg· samples from the environment, the reason why the samples are being taken must be clear. The literature is full of analytical environmental data which of little value because of obtuse goals.

197

7.6.1 Reliable Soil Samples

When taking soil samples for the purpose of detennining environmental orgamc contaminants the following questions should be considered:

• What is the real need for the analysis?

• Do I need an average value to describe a site?

• Is there a possibility for point-source contamination (e.g. an old dip or dump site)?

• How mobile is the suspect contaminant (would it still be where it was applied)?

• Has the soil been disturbed by cultivation, relocation or eroded?

• What is known about the stability of the compound?

• Do I want to know what is in the top 10 cm of the surface layer or the average for a deeper profile?

• Should I mix a series of samples and take subsamples or is it of more value to have individual sample results?

. The above decisions clearly influence the analytical result and its usefulness. The analytical result may be precise but may bear no relationship to how representative a ~ample it is. A simple analytical result can be totally misleading unless it is accompanied by adequate sample description.

7.6.2 Reliable Water Samples

As with soils, a representative sample is a prerequisite for meaningful analytical results. Unfortunately water samples are often taken according to some planned protocol based on convenience (monitoring surveys) rather than under optimal (technical) conditions.

Environmental water samples have the added complexity of suspended soil particles. Before taking any environmental water samples, consider the following points:

•

•

•

•

•

What is the real need for the analysis?

In a river system, should I be sampling before or after a wet season?

Is the contaminant more likely to be present during peak flow?

Is a result required for a water plus suspended solids sample or just the water phase?

If filtering is required should this be carried out in the field or in the laboratory?

198

• Do I need an average analytical value for the river system or would a specific point-source sample be more meaningful (e.g. suspected discharge point or known agricultural concern)?

• How much volume should be taken for the water sample (different analyses can require larger volumes)?

• Is there a need to use special sampling containers or sampling equipment?

Contaminant are a special problem in water samples. Apart from the problems with actual sampling, any water sample taken for organic biocide analysis will be totally useless if it has been stored in a plastic container. Glass or Teflon are preferred to avoid losses and contamination caused by plasticisers. When sampling groundwater, care must be taken to avoid plastic in pumps, hoses and bore casings. Lubricants from pumping equipment can also produce contamination problems.

7.7 Analysis and Interpretation

Most major analytical laboratories use modem, highly senslove analytical equipment capable of analysing a wide range of organic contaminants. Detection limits, the lowest concentrations that the laboratory can confidently report, have been progressively reduced due to the enhanced capabilities of the analytical equipment and the on-going pressure to have lower and lower detection limits.

Whilst such technical improvements are important to the development of analytical chemistry, the ability to find lower and lower residues presents some real problems for those trying to interpret analytical data. What is the significance of 0.0001 mg/kg of a pesticide in soil? Is it real or is it from contaminated equipment? Even if the value is true, is there any use in filling up reports will such data if they have no health or environmental significance?

Such sophisticated analytical capabilities combined with the complexity of environmental variability and the demands of appropriate sampling protocols highlights the need for specialist environmental scientists. Misuse and misunderstanding of analytical data will always occur but irresponsible emotive misuse must be strongly challenged and rejected.

8 CONTROL OF ENVIRONMENTAL BIOCIDES

As previously discussed the production of chemical waste and its safe management are now of global as well as local importance. While hazardous chemicals may have been an economic problem in the past it is also an opportunity for business and research organisations. Developing new technology and controls, identifying the source and types of rural waste, its production and its disposal or reuse are all part of ecologically sustainable development program in developed countries. Governments are also involved in research, monitoring, legislation, codes of practice and risk communication regarding effective potentially hazardous waste management.

199

The proper disposal of unwanted biocides (especially pesticides) and their containers is of critical importance not only to the rural industry but also the community at large throughout Australia. The implication of pesticide residues in our food, water, soil, vegetation and livestock and people will continue to be a controversial issue.

Solutions to this problem now have a nationally focus but much of the practical solutions are being dealt with, with varying degrees of success at the local levels. The discussion below will only highlight some current methods dealing with biocides and not all are satisfactory, new nor universally adopted or approved by all agencies. Since much has been mentioned by previous authors, except for a brief summary for controlling pesticide wastes, we propose to concentrate on bioremediation.

8.1 Nature of Pesticide Waste and Problems

Pesticide wastes and unwanted contaminated containers are constantly being generated from the manufacturing source to the farm and indeed home-user (FAO, 1985). This problem is exacerbated by the:

•

•

•

•

•

and

•

ever increasing volume of production and use of pesticides,

wide range of associated hazards,

highly toxic potential of some pesticides,

wide variety of off-targets,

routine problem of farmers confronting wastes and disposal problems which are time consuming,

the occasional effects of misuse and error are not visually obvious.

The major sources of waste are a result of:

• • • • • • • • • and •

leaks and spills, storage of banned chemicals, chemicals which are no longer effective (pest resistance), rejected by the farmer, amongst stockpile of chemicals in the storage shed, wide range in farming enterprises, crops or stock, expired products, containers decayed, fear of handling concentrates,

Ignorance.

200

The two major problems are disposal of waste pesticides and used containers. The moral duty of the user of biocides is to ensure that unused/waste pesticides and empty containers are disposed of safely and responsibly and also attempt to render all pesticides totally harmless to all life forms. Guidelines have been published by various government groups.

The main forms of pesticide waste include rinsate, excess diluted pesticide and concentrates. The main options for disposal of pesticides and other biocides (the notes in brackets) are summarised as follows (FAO, 1985):

(a) Physical Methods - Fixation in concrete (leaching, incompatibility, poor matrix) - Adsorption in clay, organic matter or activated charcoal - Incineration

low temperature (400C, small amounts, usually inadequate, incomplete and risk of noxious gases being produced)

. high temperature (large amounts, expensive, few available)

- Photodegradation (on impervious soil, concrete, corrugated iron; simple, cheap, convenient; suitable for dilute formulations, plus oxidation and microbial activity; risky)

(b) Chemical Detoxification Methods - Use hydrolytic, oxidative and reductive reagents

. acids, alkali, hypochlorites

. iodine, sulphur, cyanides, acetone, acetates (Chemical methods are currently unsuitable on a large scale and can create further disposal problems).

(c) Bioremediation (microbiological) Methods (see section below) - Land disposal

Cultivated land (soil, clay adsorption, microbiological degradation, volatilisation, ultra-violet light)

Disposal pits

- Composting using inoculum

with sewage with food processing wastes with manures

- Genetically altered microbes

201

Options for the disposal of empty biocide containers include:

• Sell/send to commercial drum recyclers

• Triple rinse and add the rinsates to the spray tank

• Render rinsed containers unusable by puncturing and crushing

• If burying containers on the property, choose a site well away from houses, crops, stock, surface water supplies and bores

• Bury over 50cm deep

• Disposal at a tip approved by the local authority IS preferred (do not burn containers as by-products may be more toxic)

9 BIOREMEDIA TION

Bioremediation is a process that uses microorganisms such as bacteria, protozoa or fungi to degrade harmful chemicals into less toxic or non toxic compounds. In order to reproduce and function properly, microorganisms require major nutrients, such as nitrogen, phosphorus, sulphur and trace elements as well as carbon and energy. The energy is, in most cases, provided by the oxidation or fermentation of organic compounds found in the environment. In the ideal situation microorganisms will use an organic chemical contaminant as a carbon and energy source and transform it into harmless substances consisting mainly of carbon dioxide, water and fatty acids. A well-documented example of this is the breakdown of petroleum hydrocarbons (Compeau et al. 1991). This natural process is encouraged in bioremediation by promoting the growth of microorganisms that can degrade the contaminants and convert them to non-toxic byproducts.

When a population of microorganisms is exposed to contaminants, it will evolve to develop an increased ability to degrade those substances. Under enrichment, new strains of bacteria develop that can use the contaminant as a primary energy source. Enrichment for biodegradation is possible even for the complex synthetic chemicals (such as pesticides and solvents) often found on hazardous waste sites.

Conventional bioremediation processes such as the treatment of domestic waste have been successful because of the broad range of microbial species capable of degradation. However when the components of hazardous waste are synthetic, recalcitrant and toxic, the range of potential degradative microbes is greatly reduced. As will be discussed later, genetic manipulations may offer a solution.

9.1 Chemicals and Sites Suitable for Bioremediation

Examples of some major organic chemicals that may be suitable for biodegradation include:

202

• chlorophenols (e.g. PCP, Frick et al. 1988) • PAHs (e.g. creosote, Wang et al. 1990) • organochlorines (e.g. 2,4-D, Fogarty & Tuovinen 1991) • hydrocarbons (Morgan & Watkinson 1990) • benzene, toluene,xylene • alkanes & alkenes (e.g. fuel oil, Frankenberger et al. 1990) • chlorinated solvents (e.g. chloroform) • nitrogen heterocyclics (e.g. pyridine) • polychlorinated biphenyls (e.g. trichlorobiphenyl) • monochlorinated aromatic compounds (e.g. chlorobenzene) • chlorinated aliphatics (e.g. trichloroethylene, Broholm et al. 1991)

With the exception of the aerobic degradation of P AHs and nitrogen heterocyclics, all the other classes of chemicals mentioned above can be biodegraded under both aerobic and anaerobic conditions.

Bioremediation has been successfully used to decontaminate some of the following range of environmental chemical wastes sites:

• · , • • •

• • • • • • •

9.2

Groundwaters (Klecka et al. 1990) Wood preserving sites (Frick et al. 1988) Leaking storage tanks (Litchfield 1991) Contaminated land/soil (Ellis et al. 1990) Fuel spill sites

- above ground (Song et al. 1990) - underground (Frankenberger et al. 1989)

Compost (Fogarty 1991) Refinery sludges Leachates Sediments Plant effluents TNT (trinitrotoluene) (Fernando 1990) Nuclear waste (Ashley, 1990)

Persistent Chemicals

Some biodegradable organic compounds may still persist in the environment because of unfavourable conditions. Since successful degradation requires defined conditions for microbial activity, biodegradation may be hindered or prevented by a number of factors including the following:

•

•

the immediate micro-environments may be too alkaline, acidic or toxic to allow the growth of microbes,

the pollutant may not be in a form that is readily available to a diverse range of microbes (for instance, oil trapped in a matrix of clay),

203

• nutrients necessary for survival may be lacking,

• the level of water activity may be unfavourable,

• the environment may lack oxygen, nitrate or sulphate necessary to use the chemical as an energy source,

and

• the number and type of microorganisms may not be adequate for degradation.

For maximum efficiency of the bioremediation process, information is required on types of microorganisms present, their nutrients needs and optimum growing conditions. Laboratory-scale trials are setup using samples from the site, and possible deficiencies in the system are determined. For example, if the availability of oxygen is inadequate the site can be purged with air or tilled by mechanical means; certain nutrients or water may have to be added to stimulate growth; if the concentration of the contaminant is prohibitively toxic it can be diluted by adding non-contaminated soil.

9.3 Advantages of Bioremediation

Bioremediation has a number of advantages which make it an attractive option for decontamination; for example:

• it is a highly effective treatment method with little or no consequential harm to the environment. The degradative microbial population dies off when the contaminant is degraded. The residues can be monitored and are usually harmless products,

• applications are wide ranging and include in-situ treatment, eliminating the need for transport of toxic waste and so reduce the risk of public exposure,

• it can be used effectively in conjunction with physical chemical treatment

• it is usually less expensive than other technologies such as incineration (ranging from one third to one half of the cost).

9.4 Disadvantages of Bioremediation

Some of the limitations associated with bioremediation are as follows:

•

•

there is a perceived distrust of microbial processes and great concern over the release of genetically engineered organisms; field applications are often hindered by regulatory uncertainties.

there are limitations due to the heterogeneous nature of wastes and the waste site, e.g. a site may contain a -complex mixture of contaminants and may also contain contaminated concrete and debris,

204

• waste storage will be required periodically and it is therefore land and management intensive,

• our experience with bioremediation is limited. Very few full-scale aerobic biological systems designed to remove hazardous organic chemicals are in operation today,

• toxic by-products may be formed in some cases and, because end- points or extent of biodegradability are not always predictable, incubation sites must be carefully monitored.

9.S Current Bioremediation Techniques in Use

There are three broad categories of bioremediation methods in use based on the site of treatment; namely, in-situ, above-ground and a combination of chemical and microbial processes.

(a) In-situ Treatment. The basic assumption is that there are indigenous microorganisms in groundwater and sub­surface/surface soils. Some of these organisms may be use the contaminants as a nutrient substrate and can degrade these compounds naturally. The digestion usually ceases when some nutrients become depleted but with addition of these limiting nutrients (usually oxygen, nitrogen or phosphorus) the process Can proceed to completion.

The two major sites for in-situ treatment are groundwater and sub-surface and surface soils.

• Groundwater and sub-surface soil bioremediation: Oxygen can be added to groundwater by extracting the water and then reinjecting it through wells or trenches after charging with oxygen. Sub-surface drains may be used to deliver nutrients and oxygen to depths of about 10 metres. Contaminants above the watertable can be biodegraded if the soil is relatively porous and permeable to water and air. A nutrient solution can be sprayed directly onto the surface and venting wells can be installed throughout the contaminated area to deliver oxygen directly into the soil. .

This method of in-situ bioremediation has been used to clean up sites contaminated with diesel fuel, petrol and aviation fuel, leaking underground storage tanks and soils contaminated with chlorinated solvents.

• Surface soil bioremediation: Simple tilling of surface soil to provide aeration with the addition of water and nutrients will stimulate bacterial growth. Naturally­occurring microorganisms can degrade contaminants under controlled conditions and prevent volatilisation of the chemical into the air. Bioremediation was a technique used in the 1989 Alaskan oil-spill when nutrients were applied to beaches contaminated with oil. It was discovered that the spraying of hot water onto the beach had reduced the degradation process because beneficial microbes were killed.

205

(b) Above Ground Treatment. Above-ground bioreactors (AGB) are designed to treat contaminants in either liqUid or solid phases. This method is more economical for soils which have already been excavated.

•

•

•

•

Liquid or slurry phase: Soil is mixed in a bioreactor with water and microbial broth which is suspended in a nutrient solution or fixed on a solid substrate such as activated carbon or diatomaceous earth. The inoculum can come from the contaminated materials, an activated sludge treatment plant, pure cultures or, in the near future, genetically-engineered microbes (GEM). The AGB lend themselves to the use of GEM because of the enclosed reactor design. The mixture may be operated aerobically or anaerobically or a series of both with nutrients added as required. The pH and temperature are controlled.

Solid-phase: In solid-phase treatment, the excavated soil is placed in a lined enclosure before water and nutrients are percolated through the mound. The leachate and run-off are collected and a vacuum system may be installed to extract any volatile contaminants.

Composting: Composting, used by farmers, can also be used for treating soils containing hazardous organic compounds, including pesticides (Fogarty et al, 1991). The contaminated soil is combined with highly biodegradable material such as woodchips and decomposition is allowed to occur under controlled conditions independent of the soil medium. Bulking material (sawdust or straw) may be further added to improve porosity of the compost substrate.

Landfarming: Landfarming has been used extensively by the petroleum industry and differs from the composting method in that the natural soil is the main medium. Loading rate, moisture and fertiliser application are required to accelerate the process. A further ideal pre-requisite is a duplex soil with a thick claypan to minimise groundwater contamination from any leachates.

(c) Combined Treatment In some cases bioremediation will only be effective in combination with chemical treatment, since many hazardous waste sites contain complex mixtures of organic and inorganic chemicals that are not readily degraded. For example, a chemical dehalogenation process can remove chlorine from polychlorinated biphenyl (PCB). Then the subsequent biological treatment, which is more effective after dechlorination, can be used to complete the soil restoration.

9.6 Selection Criteria for Bioremediation

The prime objectives for bioremediation are reliable, effective and predictable outcome of the waste degradation process. This is achieved by. a biological process which, in most cases, functions as a mixed culture system in a complex environmental matrix.

In the identification, evaluation and selection of bioremedial alternatives, an objective assessment of each treatment method should be made to ascertain its technological and

206

non-technological feasibility. This evaluation must involve extensive sampling and laboratory analysis in order to determine the nature and extent of the contamination and the environmental characteristics of the site. Questions such as the following should be considered to select the most appropriate bioremedial process:

• Are the chemicals potentially biodegradable?

• Are any contaminants potentially toxic to the microbial degradation process?

• Is another type of treatment necessary before bioremediation can be used?

• Can the technology meet cleanup goals for the site?

• What are the microbiological characteristics of the environment at the site? (e.g. do aerobic or anaerobic organisms predominate?).·

• Is the environment appropriate for bioremediation or must conditions be adjusted (e.g. change pH, remove toxic metals or change moisture content).

• What are the microbiological needs of the site? (e.g. would nutrients or other special bacterial species need to be added?)

• What are the regulatory, socio-political and business implications?

9.7 Use of Genetically Engineered Microorganisms(GEM)

It is possible to insert a genetic code for degradation of a specific contaminant from one microorganism into another. This technique could then expand the range of characteristics of the recipient microorganism. Geneticists are attempting to develop bacteria that can break. down several components of crude oil, degrade chlorinated aromatic compounds such as aldrin, and also to remove metals from mines and wastewater. At present the use of GEM is limited to laboratory studies but the potential for commercial-scale application is enormous.

9.S Potential Applications

The future of bioremediation methods will depend on the need and urgency of hazardous chemical control. Some of the predicted applications include:

•

•

•

•

Waste minimisation by, for example, process stream bioreactors to reduce the by­products before they become mixed with other waste streams.

Off-the-shelf bioreactors for the treatment of agricultural waste,

Heavy metal removal from groundwater or wastewater.

Removal of sulphur compounds from mine waste.

207

Effective development· and use of bioremediation will hinge on the integration of knowledge and skills of many different disciplines; especially between microbiologists and engineers.

10 CONCLUSIONS

• Environmental orgamc chemicals are scientifically, technologically and sociologically complex issues.

• Concerns about chemical issues must be placed in perspectiv~, in the wider context and in relative terms. .

• There is a need to separate myths from proven facts and this chapter attempts to help the reader make better value judgements and. risk assessments concerning various environmental chemical issues.

• All chemicals behave differently in the environment.

• The life of a chemical in the environment is dictated by its chemical stability and its tendency to bind to other environmental components.

• Toxicity ofa chemical is related largely to its dose; risk of toxicity is related to its hazard potential and exposure.

• All our environmental problems have solutions.

• The reader, should be able to seek further infonnation and ,be a more discerning inquirer.

• Adverse hazardous chemical incidents are relatively rare but like all accidents, they inevitably occur from time to time. The challenge t6 the whole community is to constantly play an active part in reducing both the risk and hazard of accidents and the misuses and abuses of any harmful chemicals.

• In general the environment has a large buffering capacity to the influence of human manipulation but the human beings should adjust their activities to reduce local and regional pollution.

11 REFERENCES

Ames, B.N., Magaw, R. and Gold, L.S. (1987). Ranking of possible carcinogenic hazards. Science. 236: 271.

Ames, B.N. and Gold, L.S. (1989). Pesticides, risks and applesauce. Science. 244: 755.

Ashley, N.V. and Roach, D.J.W. (1990). Review of biotechnology applications to nuclear waste treatment. J. Chem. Tech. Biotech. 49: 381-394.

208

Batley, G.E. and Peterson, S.M. (1992). environment. CRDC Workshop: Environment, Goondiwindi.

Fate of cotton pesticides in the riverine Impact of Pesticides on the Riverine

British Medical Association, (1990). The BMA Guide to Living with Risk, Penguin Books, London.

Broho1m, K., Christensen, T.R. and Jensen, B.K. (1991). Laboratory feasibility studies on biological in situ treatment of a sandy soil contaminated with chlorinated aliphatics. Env. Technol. 12: 279-289.

Banks, A., Broadley, R, Collinge, M. and Middleton, K. (1990). Pesticide Application Manual - 2nd edition. Department of Primary Industries, Information Series QI89003.

Compeau, G.C., Mahaffey, W.D. and Patras, L. (1991). Full-scale bioremediation of contaminated soil and water. p.91 In Environmental biotechnology for waste treatment, G.S. Sayler et al. (eds), Plenum Press: New York.

Crone, H.D. (1990). Chemicals & society - a guide to the new chemical age. Cambridge University Press, Sydney.

Crowther, D., Silvey, M. W. and Hughes, P. (1991). Pesticide use and safety workshop manual. Department of Primary Industries, Queensland.

Ellis, B., Balba, M.T. and Theile, P. (1990). Bioremediation of oil contaminated land. Env. Technol. II: 43-455.

FAO (1985) Guidelines for the Disposal of Waste Pesticide and Pesticide Containers on the Farm.

Fernando, T., Bumpus, lA. and Aust, S.D. (1990). Biodegradation of TNT (2,4,6-trinitrotolvene) by Phanerochaete chrysosporium. Appl. Env. Microbiol 56: 1666-1671.

Fogarty, A.M. and Tuovinen, O.H. (1991). Microbiological degradation of pesticides in yard waste composting. Microb. Revs. 55: 225-233.

Frankenberger, W.T. Jr, Emerson, K.D. and Turner, D.W. (1989). Env. Mgmt 13: 325-332. In Situ bioremediation of an underground diesel fuel spill: a case history.

Frick, T.D., Crawford, RL., Martinson, M., Chres and , T. and Bateson, G. (1988). Microbiological Cleanup of Groundwater contaminated by Pentachlorophenol p.173 In G.S. Omenn ed. Environmental Biotechnology Reducing Risks for Environmental Chemicals Through Biotechnology, Plenum Press, New York.

Gare, W.W.(Editor) (1992). Reviews of Environmental Contamination and Toxicology. Springer-Verlag, New York Inc., 123: pp.vii-viii.

Gold, L.S., Slone, T.H., Stem, B.R, Manley, N.B. and Ames, B.N. (1992). Rodent carcinogens: Setting priorities.. Science. 258: 261.

209

Jones, G., Robertson, A., Forbes, J. and Hollier, G. (1990). Environmental Science.­Collins Reference Dictionary. William Collins Sons and Company Limited, Glasgow.

Klecka, G.M., Davis, J.W., Gray, D.R. and Madsen, SOS. (1990). Ground-Water 28: 534-543 .. Natural bioremediation of organic contaminants in ground· water: Cliffs-Dow Superfund Site.

Lahey, J.P. (1985). The Pyretbroid Insecticides. (Taylor and Francis, London and Philadelphia) .

Litchfield, C.D. (1991). Practices, potential and pitfalls in the application of biotechnology to environmental problems. p. 147 In Environmental Biotechnology for Waste Treatment, G.S. Sayler et al (ed.), Plenum Press: New York.

Moore, I.D. (1992). Modelling the fate· of pesticides in the environment. Australian National University Workshop Proceedings, ISBN 086740 400 O.

Morgan, P. and Watkinson, R.J. (1990). Assessment of the potential for in situ biotreatment of hydrocarbon-contaminated soils Water Sci. Technol. 22: 63-68.

Naisbitt,1. and Aburdene, P. (1990). Megatrends 2000. Pan Books. London.

Noble, A. (1990). .1!he ·relationship between organochlorine residues in animal feeds and residues in tissues, milk and eggs: a review. Aust. J. Exp. Agric. 30, 145-54.

Schonborn, W. (1986). Historical development and ecological fundamentals. In W. Schonborn (ed.). Microbial Degradations. Biotechnology, Vol 8, VCH, Weinheim, Germany, p23.

Selinger, B. (1991). Chemistry in the marketplace. Harcourt Brace Jovanovich, Publishers: Sydney.

Silvey, M.W. (1992). Perspective on pest control in sustainable agricultural systems. In 'Proceedings of the second international symposium on integrated land use management for tropical agriculture' Brisbane, Queensland: Australia, September, 1992, Queensland Department of Primary Industries; Queensland QC92009.

Simpson, B.W., Rayment, G.E. and Spann, K.P. (1992). Pesticise Use Study -Pumicestone Passage Catchment. Department of Primary Industries

Song, H.G., Wang, X. and Bartha, R. (1990). Bioremediation potential of terrestrial fuel spills. Appl. Env. Microbial 56: 652-656.

Wang, X., Yu, X. and Bartha R. (1990). Effect of bioremediation on polycyclic aromatic hydrocarbon residues in soil. Env. Sci. Technol. 24: 1086-1089.

World Bank. (1992). Development Report 1992. Development and the Environment. Oxford University Press: New York.

210

HEAVY METALS: TOXICITY, SOURCES, CHEMISTRY AND LOADING RATES

ABSTRACT

G.E. Rayment and G.A. Barry

Agricultural Chemistry, Land Management Division, QDPI, Indooroopilly, Qld, 4068.

Heavy metals are an unordered group of metals, metalloids and non-metals which have become important environmental contaminants. At lower concentrations some of these heavy metals are essential plant or animal nutrients. All have been mobilised in the environment by human activities.

This paper describes the human toxicity of several of the heavy metals. It covers the natural and anthropogenic sources of the heavy metals including their chemistry in soils and the environment generally. Most emphasis is given to arsenic, cadmium, chromium, copper, lead, mercury, selenium and zinc. General principles relevant to methodology and loading rates of heavy metals to soils are discussed.

1 INTRODUCTION

Chemical elements in the modern periodic table are arranged by 'increasing atomic number in horizontal rows (periods) and in vertical columns termed groups or families (eg. Petrucci 1972). A number of groups have characteristic family names (eg. alkali metals, noble gases, transition metals, the lanthanides or rare earths, the radioactive actinide metals) but there is no grouping termed heavy metals. The elements shaded in Figure 1 represent an imprecice grouping of 'heavy metals' or 'trace metals'. The grouping includes non-ferrous metals and toxic metalloids, the latter having chemical properties intermediate between those of metals and non-metals. All have an atomic density >6 g cm-3 (Alloway 1990a).

Equally ill-defined is the term 'pollution'. This can relate to energy patterns, radiation levels, physical or chemical constituents, or living organisms introduced into the environment as a consequence of direct or indirect human activities. These -- including toxic heavy metals -­become pollutants when present in sufficient quantities to cause recognisable toxic effects or a diminution of amenity and the quality of life (Livett 1988).

Heavy metals are important for economic and environmental reasons. Many are used in heavy industry and in highly technical applications including electronics. Some are physiologically essential at low concentrations for plants and/or animals (eg. cobalt, copper, chromium, molybdenum, nickel, selenium, zinc), while many are significant pollutants of soils, human foods and ecosystems throughout the world. Heavy metals can be transformed from one chemical form to another through chemical and bio-chemical reactions but are not destroyed. That is, they need to be used and disposed of wisely. This paper provides an insight into the heavy metals identified in Figure 1, with particular attention given to toxic heavy metals causing most environmental concern at global scale. Those elements are -- in alphabetical order -- arsenic, cadmium, lead and mercury.

211

lA

Li

Na

K

Rb

Cs

Fr

2A

8e

Mg 38

Ca Sc

Sr Y

8a La *

Ra Ac #

* Lanthanide series:

# Actinide series:

48

Ti

Zr

Hf

Ku

~ He 3A 4A 5A 6A 7A

8 C N 0 F Ne

Al Si P S CI Ar 58 68 78 - 88 18 28 -V 8r Kr

Nb I Xe

Ta At Rn

Ha

I Ce I Pr I Nd I Pm I Sm I Eu I Gd I Tb I Dy I Ho I Er -I Tm I Yb I Lu I

IThlhl u 1~lfulbl~I~lal&I~I~I~I~1

Figure 1 The modern Periodic Table of elements with most biologically-essential and environmentally-toxic heavy metals and metalloids highlighted by shading.

2 TOXICITY

The case against heavy metals as toxic environmental contaminants has strengthened over time. For example, the harmful effects of lead were recognised by the Greeks by at least the second century Be (Waldron 1973). Toxicologists studying cases of human poisoning from heavy metals observed that visible clinical symptoms were likely only in cases of high exposure either at an occupational level or following gross contamination of a localised site. Well known Japanese examples of the latter are mercury toxicity from fish consumption at Minemata and ltai-itai disease from consumption of excess cadmium.

At lower but still unacceptable levels of exposure - typically from consumption of certain foods -the symptoms may be confined to the physiological or biochemical level (Hutton 1987). Learning difficulties in children and abnormal behaviour patterns are recognised symptoms of lead poisoning in children (Lansdown 1979), whereas there is evidence for the association of cadmium, chromium, copper, selenium and zinc in cardiovascular disease (Shaper 1979). Nickel, chromium and cadmium are potential human carcinogens (McGrath and Smith 1990, Tanenaka et al. 1983). Table 1 provides summary infonnation on metabolic factors and provisional tolerable weekly intakes associated with arsenic, cadmium, lead and mercury. Hutton (1987) gives more infonnation on human health concerns associated with arsenic, cadmium, lead and mercury. .

212

The toxicity of particular heavy metals to both plants and animals varies with chemical form. For example, organo-arsenic compounds are less toxic than inorganic compounds (O'Neill 1990) whereas organic mercury (eg. methyl mercury) is more toxic than inorganic mercury (Anon 1980, Hutchinson and Meema 1987). In general, water soluble forms of the heavy metals have greater toxicity -- including higher rates of absorption following ingestion -- than do sparingly soluble or insoluble compounds.

3 SOURCES OF HEAVY METALS

Heavy metals occur in air, water, minerals, soils, biota, fertilisers, other manufactured goods and waste products at quite variable concentrations. Moreover, global trade in most heavy metals has increased markedly in the last half century (Table 2).

3.1 Atmospheric Sources

The majority of heavy metals in the atmosphere are present in particulate matter (dust), in aerosols, and occasionally as vapours (eg. mercury). Thermally-driven air currents lift the metals whereas winds are mostly responsible for horizontal movement.

The life of aerosols in air varies with particle size. The smallest particles (0.001-0.08 pm) typically survive for < 1 hour because they coagulate to form larger particles. In the range 0.08-1.0 pm the life time is 4-40 days, whereas particles >1.0 pm survive from minutes to days (Fergusson 1990). It follows that the physical and chemical characteristics of atmospheric emissions, combined with the prevailing atmospheric conditions, will determine the likely zone of contamination from point-source emissions.

Well known examples of atmospheric pollution by heavy metals include arsenic from a variety of sources (Buat-Menard 1987) and lead from motor vehicle emissions (Farmer 1987). The lead-zinc smelter at Port Pirie in South Australia is known to have contaminated soils (Merry 1988) and sediments (Tiller et al. 1989) well distant from the source of emissions. In 1976 in the United States alone, some 217000 metric tonnes oflead were released to the atmosphere from the combustion of leaded petroL Other examples of heavy metal emissions are in Tables 2 and 3.

3.2 Fertilisers, pesticides, manures and sludges

Copper, zinc and molybdenum are deliberately applied to cropping and pastoral lands known or perceived to be deficient in these nutrients. Typical application rates (of element) are 2, 2 and 0.1kg/ha, respectively, every three to four years or so. Fungicides containing copper, zinc and manganese as active ingredients also have regular usage, particularly in fruit­growing areas. For example, in the Pumices tone Passage Catchment north of Brisbane, annual applications of copper hydroxide and copper oxychloride are estimated at 16300 and 13 000 kg, respectively (Simpson et al. 1992). Rayment and Brooks (1974) provided early evidence of accumulations of copper and zinc in the surface horizons of soil from long­established citrus orchards in Queensland. Arsenic from herbicides used for Harrisia cactus

. control in Central Queensland (over 1 000 t of arsenic pentoxide from 1952-62; Cuddihy and Fergus 1974) and from pesticides used in cattle dips still persists at points in some Queensland soils.

213

Table 1 Key metabolic factors following environmental exposure to heavy metals [adapted from Hutton (1987) and Rayment (1991)]

Factor Lead Mercury Cadmium Arsenic

Key entry Ingestion, Ingestion, Ingestion, Ingestion pathway inhalation inhalation of inhalation; ego

metal vapour tobacco

Gastrointestinal approx 10 approx 80 of approx 5 * > 80 absorption (%) vapour; 7 of

ingested Hg2+

Organs bone, teeth kidneys, brain, kidney, liver keratinous accumulating liver tissue

Major routes of urine (75-80%) urine & faeces urine urine excretion

Biological half- approx 20 y approx 35 to 90 > 10 Y 10-30 h life days

Acute toxic neurological & Internal: Internal: edema, gastro-effects in other central damage to gastro- irritation, humans nervous system intestines, renal intestinal muscle spasms,

and gastro- failure, disturbance coma intestin<u circulatory Inhalation:

, " disturbances collapse chemical Inhalation: pneumonitis bronchial irritation, pneumonitis

Chronic toxic impaired inflammation of renal fatigue, skin effects in fertility, mouth, tremors, dysfunction, damage, non-humans chromosomal irritability - bone-changes, cirrhotic portal

abnormalities, anxiety-shyness, tumors hypertension kidney damage, skin lesions

Provisional Adults: 0.0033 (organic 0.007 0.015 tolerable weekly 0.05 mercury) (WHO 1989) (inorganic intake (mg/kg Children: (WHO 1989) arsenic) body weight) 0.025 (WHO 1989)

(WHO 1986)

* Individuals with low iron store or on a calcium deficient diet may absorb as much as 20% (WHO 1985). This is an example that values for absorption of heavy metals can vary widely for a variety of reasons.

214

Manufactured fertilisers and alternative sources of plant nutrients including sewage sludge and animal manure contain heavy metals as contaminants. Swaine (1962) collated much of the world's early literature on the "trace element" content of fertilisers. Commonly, phosphatic fertilisers contain higher concentrations and a greater variety of heavy metal impurities than do fertilisers supplying nitrogen and potassium. For example, Rayment et al. (1989) reported concentrations of cadmium in phosphatic fertilisers, expressed as mg Cd per kg of P, of 413 ± 39.7. Total cadmium concentrations in other fertilisers (excluding trace element fertilisers) and in most soil amendments (limestone, dolomite) were low. Further examples are given in Table 4, together with data for contaminant heavy metals in sewage sludge and animal manures. The location and method used to produce sewage sludge, the source of feeds given to animals, and the moisture content affect the concentrations of heavy metals present in particular waste products.

Because sewage sludges are commonly applied to land at rates of 5 t/ha or more, particular attention must be given to heavy metal concentrations on a batch to batch basis, particularly if sourced from industrial regions. Typically, these sewage sludges contain higher concentrations of contaminants than do sewage sludges obtained using the same treatment process but sourced from predominantly residential areas.

3.3 Soils, Sediments and Waters

The earth's crust, common minerals and soils contain variable concentrations of heavy metals (Table 5). In un-polluted soil, parent matenal, the extent of weathering,'Pr:9fi1e depth, organic matter content, pH, the degree of aeration, cation exchange capacity- (CEC) and other soil properties affect their concentration and distribution. For example, Table 6 provides summary details on the effects of soil acidity and aeration on the mobility of particular heavy metals. Those with low mobility irrespective of pH and oxidation state are particularly persistent in soils. For reasons such as these, parent material is an umeliable guide to the heavy metal content of soils, particularly under highly weathered conditions.

Heavy metals are present in all aquatic ecosystems, sourced from natural emISSIons, weathering of minerals, and the decomposition of organic matter. In the oceans, there are strong linear relations (Bruland 1980) between cadmium, nitrate and phosphate, corresponding to a Cd:N:P atom ratio of 3.47 x 10"4: 15.2: 1. Zinc is strongly correlated with silicate (1 atom of Zn for 18 700 atoms of Si), nickel correlates with a combination of phosphate and silicate, while copper displays evidence of deep-water scavenging. Despite long residence times [approximately 50,000 years for cadmium (Boyle et al. 1976), 6 000 to 40 000 years for nickel (Bruland 1980) and 830-5 000 years for copper (Bruland 1980)] biological scavenging for these heavy metals appears to prevent them from polluting surface oceans.

215

Table 2 Examples (x 103 tty) of heavy metal production and recent global emissions to soils [from Alloway (1990a)]

Heavy metal Year Global emissions to

1930 1950 1980-1985 soil, 1980's

Cadmium 1.3 6 15-19 22

Chromium 560 2270 11 248-9940 896

Copper 1611 2650 7660-8 114 954

Lead 1696 1670 3580 * 796

Manganese 3491 5800 26720 1670

Mercury 3.8 4.9 7.1-6.8 8.3

Nickel 22 144 759-778 325

Tin 179 172 251-194

Zinc 1394 1970 5229-6024 1372

* Data from Chilvers and Peterson (1987)

Table 3 Examples ofinputs of toxic heavy metals into the atmosphere.from natural sources and industrial emissions [from Pacyna (1987) and Fergusson (1990)]

Source Arsenic Cadmium Lead Mercury Selenium

Natural inputs

Volcanic 300-800 30-800 100-9600 10-1700 (ng/kg dust)

Windblown 0.5-2 0.002-1.7 0.4-70 0.6 (ng/kg dust)

Forest fires 0.5-4.4 0.03-2 1.1-78 (ng/kg dust)

V;ietation 3.5 2.7-36 21-280 (n g dust)

seaJray 0.1-0.6 0.001-0.003 0.001-0.09 (n g)

European emissions (1979)

Coal combustion 461 146 1676 221 (t/year)

Oil combustion 217 110 1159 (t/year)

216

Table 4 Examples of typical ranges (mglkg) for heavy metal contaminants in fertilisers, sewage sludge and farmyard manure (from Alloway 1990a)

Metal Fertiliser Sewage sludge Farmyard ComKx°sted manure re se

PhosQhate Nitrate

Arsenic 2-1200 2-120 3-30 3-25 2-50

Cadmium 0.1-170 0.05-8.5 <1-3400 0.1-0.8 0.1-100

Chromium 66-245 3-19 8-41000 1-55 1.8-410

Cobalt 1-12 5-12 1-260 0.3-24

Copper 1-300 50-8000 2-172 13-3580

Lead 7-225 2-27 29-3600 1.1-27 1.3-2240

Manganese 40-2000 60-3900 30-970

Mercury 0.01-1.2 0.3-2.9 0.1-55 0.01-0.36 0.09-21

Molybdenum 0.1-60 1-7 1-40 0.05-3

Nickel 7-38 7-34 6-5300 2-30 1-280

Selenium 0.5-25 1-10 2.4

Zinc 50·1450 1-42 90-49000 15-566 82-5894

Table 5 Average abundance (mglkg) of total heavy metals in the earth's crust, common minerals and in typical soils [developed from Plant and Raiswell (1983), Thornton (1983) and Fergusson (1990)]

>·"\:"·,·,,,·~t·:

Element Earth's Basalt Granite Sandstone Shale Limestone Soil crust range

Antimony 0.2 0.2 0.2 1 0.2-10

Arsenic 1.5-1.8 1.5-2 1.5-2 1-2 15 1.7-2.5 0.1-40

Bismuth 0.05-0.17 0.03-0.15 0.07-0.1 0.18 0.1-0.4

Cadmium 0.11-0.2 . 0.13-0.2 0.09-0.2 0.2 0.1 0.01-2

Cobalt 25 50 1-5 0.3 20 4 1-40

Chromium 100 200-220 4-20 35 100-120 10 5-1000

Copper 55 90-100 10-15 2 50 4-15 2-100

Indium 0.049 0.058 0.04 0.2-0.5

Lead 12.5-14 3-6 18-24 7-12 20 8-9 2-300

Manganese 950 2200 500 850 1100 850

Mercury 0.05-0.08 0.01-0.05 0.085 0.03-0.05 0.09-0.5 0.05 0.01-0.5

Molybdenum 1.5 1-1.5 1.4-2 0.2 3 1 2

Nickel 75 140-150 0.5-8 2 50-70 12-20 5-500 Selenium 0.05 0.05 0.05 0.05 0.6 0.08 0.01-1.2

Thallium 0.45-0.6 0.08-0.1 0.75-1.1 0.82 0.3 0.1-0.8 Zinc 70 100-110 40 16 100 20-25 20

217

Boyle et al. (1976) suggest that a cadmium concentration of approximately 0.07 JIg/L could be indicative of world rivers. This contrasts with reported concentrations (Anon 1986) as high as 30 and 12 JIg Cd/L in water samples from streams in the Townsville region. Maximum concentrations of cadmium in other Queensland coastal streams provided in the same report suggest that undesirable cadmium fluxes are occurring, probably sourced from industrial wastes and domestic sewage.

Based on heavy metal data from the Mississippi delta (Presley et al. 1980), maximum concentrations occur at low river flows. There were corresponding enrichments in particulate cadmium, copper, manganese and zinc, apparently associated with increased organic matter and/or swface coatings on fine grained particles. It is clear that particulate matter is of overwhelming importance to the fluxes of heavy metals in freshwater systems, accounting for around approximately 90% of river metal loads in the Mississippi River (Presley et al. 1980). Moreover, these particulates appear to settle relatively quickly upon entering the ocean.

Table 6 Relative mobilities of heavy metals at different states of oxidation and acidification with non-limiting moisture, ignoring surface effects of soil colloids as well as interactions with organic phases (adapted from Plant and Raiswell 1983)

Relative Soil acidity Soil oxidation state mobilities

Acidic Neutral/alkaline Oxidising Reducing

Very high Mo, Se

High Mo, Se, Zn, Cu, Mo, Se, Zn Co, Ni, Hg

Medium As,Cd As,Cd Cu, Co, Ni, Hg, As,Cd

Low Pb, Bi, Sb, Tl Pb, Bi, Sb,Tl Ph, Bi, TI

Very low to Cr Cr Mn,Cr Cr, Mo, Se, Zn, immobile Co, Cu, Ni, Hg,

As, Cd, Pb, Bi, Sb, Tl

4 CHEMISTRY

The approximate atomic weight, atomic radius and ionic radius of the heavy metals plus -­for comparison -- a few important soil constituents are summarised in Table 7. The similarity in ionic size of divalent calcium and divalent cadmium is probably responsible for interactions which occur between the two elements. Chemical facts on some of the more important heavy metals follow.

218

Table 7 Atomic weights and atomic and ionic radii for heavy metals and some common soil cations [extracted from Masterton and Slowinski (1978)]

Element Atomic mass Atomic radius (A) Ionic radius (A) and (valance §)

Aluminium 26.98 1.43 0.50 (+3)

Antimony 121.75 1.41

Arsenic 74.92 1.21

Bismuth 208.98 1.46

Cadmium 112.40 1.49 0.97 (+2)

Calcium 40.08 1.97 0.99 (+2)

Chromium 52.00 1.25 0.64 (+3)

Copper 63.55 1.28 0.96 (+1)

Indium 114.84 1.62 0.81 (+3)

Lead 207.2 1.75

Magnesium 24.31 1.60 0.65 (+2)

Manganese 54.94 1.29 0.80 (+2)

Mercury 200.59 1.55 1.10 (+2)

Molybdenum 95.94 1.36

~ickel 58.7 1.24 0.69 (+2)

Potassium 39.10 1.10

Sodium 22.99 1.86 0.95 (+1)

Selenium 78.96 1.17 1.98 (-2)

Thallium 204.37 1.71 0.95 (+3)

Zinc 65.38 1.33 0.74 (+2)

§ not all valance states are included

4.1 Arsenic

Arsenic occurs at elevated concentrations in nature in zones of sulfide mineralisation. Moreover, the most common arsenic containing mineral is arsenopyrite, FeAsS (O'Neill 1990), although commercial production is typically as a by-product of copper, lead, gold and silver production. Some commonly encountered species of arsenic in the environment, in addition to this are presented in Figure 2. In oxygenated natural waters arsenic is usually present as the arsenate ion (HAs(j-) with trivalent arsemc (the most toxic species to humans) as H3As03 favoured under less-oxidising conditions. Methanogenic bacteria in soils and stream sediments can promote the formation of trimethylarsine, with monomethylarsonic acid and dimethyl arsinic acids as intermediates (Batley and Low 1989).

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As a non-metal arsenic forms covalent compounds. Many of its chemical characteristics are similar to phosphorus: ego both form oxyanions (arsenate and phosphate anions are examples). However, arsenic can exist at oxidation states of +3 and +5, and ligands other than oxygen atoms form stable species that are not found with phosphorus.

The oxides As20 3 and As30 s are water soluble but the sulphides -- particularly As2S3 -- are relatively insoluble (Ure and Berrow 1982). Both arsenite and arsenate are formed from the oxidation of sulphides while conversion of arsenite to arsenate can occur in alkaline soils under the influence of ferric oxide. Soil sorption of arsenate is similar to phosphate: it is strongly adsorbed by both hydrated iron and aluminium oxides, by poorly crystalline iron and aluminium components of soils, and by organic matter (Ure and Berrow 1982, O'Nei111990). In general, the sorption of As(V) for both iron and aluminium hydroxides is greater than As(III).

In soils, phosphate and anionic species of arsenic compete for the same adsorption sites but phosphate binds more strongly.

The stronger adsorption exhibited by phosphate ions is due to their smaller size compared to corresponding arsenate species, and because they have higher charge densities than arsenite species. That is, higher concentrations of both arsenite and arsenate are required to displace phosphate ions from soil surfaces. The reverse also applies: ie. phosphate additions to arsenic contaminated soil will tend to remobilise adsorbed arsenite and arsenate ions prior to subsequent resorption deeper in the soil profile.

4.2 Cadmium

Cadmium is the 67th element in order of abundance. This small amount is widely distributed in sulphide and oxide deposits, generally in association with much higher concentrations of zinc (Aylett 1979).

The divalent cation (Cd2+) is the predominant oxidation state of cadmium in nature.

Cadmium is extensively associated with collodial and particulate matter, with soluble speciation in oxic soil solutions confined predominantly to (in decreasing order) the free Cd2+ ion, CdS04° and CdCI!+. In oxic, alkaline soils the predominant species are Cd2+, CdCI!+, CdS04° and CdHC031+. The proportion of organically-bound cadmium in soil solution, even following sewage sludge addition, is relatively small (Alloway 1990b). Amounts of electrically neutral CdS04° and CdCI2° species are likely to increase beyond pH 6.5.

Cadmium tends to be more mobile in soils -- and therefore more available to plants -- than are many other heavy metals, including lead and copper (Table 6). Moreover, adsorption processes rather than precipitation appear to control the distribution of cadmium between soil­bound and soluble forms at the concentrations normally encountered in soils, including the majority of polluted soils (Alloway 1990b).

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Arsenic (III)

Arsenic (V)

Monomethylarsonic acid

Dimethylarsinic acid

Arsenobetaine

Arsenocholine

Trimethylarsine

As =0 I

OH

o II

HO - As - OH I

OH

o II

CH3 - As - OH I

OH

o II

CH3 - As - OH I

CH3

[(CH3)]3 As+ CH2 COO-

Figure 2 Commonly encountered species of arsenic [from Batley and Low (1989)]

Cadmium adsorption by soil particles is recognised as a rapid process but is strongly pH dependent. Experimental data reviewed by Alloway (1990b) and others suggest the following:

•

•

•

•

•

Adsorption by sandy and loamy soils can increase by a factor of three for every pH increase between 4 and 7.7.

Sorption continues to increase markedly up to pH 8.

Soils with either high contents of hydrous iron oxides or organic matter probably adsorb more cadmium than do those with large quantities of 2: 1 lattice clays, despite high cation exchange capacities typically associated with such clays.

Hydrous manganese oxides will increasingly adsorb cadmium as soil pH increases, but the process is reversible when pH falls.

Competition with other metal ions such as calcium, chromium, cobalt, copper, nickel and lead can inhibit adsorption of cadmium.

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- -• Du~ to its low tendency to continue to react with soil particles, cadmium has greater persistence in soils than zinc> cobalt> nickel (Barrow 1991).

• Solid phase cadmium sulphide (CdS) can form in anoxic soils and sediments but the development of soil acidification following subsequent oxidation can result in increased plant availability of cadmium.

4.3 Chromium

Chromium is closely associated with magnesium and nickel in igneous rocks. It occurs commonly in +3 and +6 oxidation states in the environment. Of these, Cr3+ is the most stable and least toxic to humans (McGrath and Smith 1990).

In some podzolic soils, the chromium contents of the A horizons are lower than the B horizons, suggesting the possibility of leaching of chromium from the upper horizon where wheathering is most intense (Ure and Berrow 1982). In soils derived from basic rocks, much of the naturally-occurring chromium is present in chromite, magnetite, and ilmenite. These minerals are resistant to weathering and accumulate in the sand-size fractions of soils. In contrast, the chromium in soils naturally low in this heavy metal tends to concentrate in the clay-size fractions.

Chromium(VI) exists as an anion in soils. Chromate is in pH-dependent equilibrium with other forms of chromium(VI) such as HCr04- and dichromate (Cr20/), with crO/ the predominant form at pH > 6. In the presence of organic matter, and particularly in acidic soils, there is rather rapid reduction of chromium(VI) to chromium(III). Moreover, chromium(III) is much less mobile than chromium(VI) and adsorbs to soil particles more strongly (McGrath and Smith 1990). The adsorption and solubility of chromium(ill) in soil is similar to that of aluminium(III), in response to changes in pH and phosphorus status.

As chromium(VI) anion species are reduced by organic matter, the chromium present in sewage sludge exhibits the chemistry of chromium(III) compounds.

4.4 Copper

Chalcopyrite (CuFeS2) is a mineral used for the commercial extraction of copper. It is also present in many other types or rocks and is subsequently released (eg. from silicates, sulphides and oxides) when rocks decompose on weathering (Ure and Berrow 1982). There are some stable Cu(I) ions but the predominant oxidation state is the Cu2+ ion (Baker 1990). In soils, copper is associated with organic matter, oxides of iron and manganese, silicate clays, and some other minerals.

Copper forms plant-available [Cu(H20)J2+ in acid soils and Cu(OH)2° under neutral and alkaline conditions. Typically, however, the Cu2+ ion is the dominant species in acidic soil solutions. Other ionic species over a range of soil solution pH values include CuS040,

Cu(OH)2°, CuC03°, Cu+, CuClo, Cu(CI2r. and numerous organic complexes (Baker 1990).

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Copper is specifically 'fixed' or adsorbed by many soils, making copper one of the least mobile heavy metals in soils.

Organic matter is invariably the dominant factor controlling copper retention by soils.

4.5 Lead

Rocks containing small amounts of lead (10-100 mglkg) are quite prevalent. Due to its similar ionic size to potassium, Pb2+ is able to replace potassium in silicates, and particularly in potassium feldspars and biotite. In general, the lead content of soil-forming minerals increases in the sequence from ultrabasic to granitic rocks (Ure and Berrow 1982). Two oxidation states of lead are stable in the environment, namely Pb2+ and Pb4+. There is an extensive chemistry of Pb4+ compounds, especially tetra-alkyl and tetra-aryl compounds (Davies 1990).

Lead is recovered commercially from galena (PbS) and, to a lesser extent, cerrusite (PbC03),

angle site (PbS04) and some others. Often, the ore bodies are also enriched in zinc or zinc­copper, plus other heavy elements such as antimony, arsenic, bismuth, cadmium, tin, gallium, indium, germanium and tellurium (WHO 1985).

Like many heavy metals, lead is more enriched in organic soils than in mineral soils. It also tends to be enriched in mineral soils high in illite and montmorillonite clays. This reflects lead's tendency to be higher in soils relative to the parent material.

The great majority of inorganic lead salts are insoluble in water. Although this contributes to lead's very slow movement in most soils, lead tends to accumulate slightly in poorly drained soils relative to their well-drained counte:rparts. In non-calcareous soils there is evidence that lead concentrations are regulated by Pb(OH)2' Pb3(P04)2 and Pbs(P04hOH. In calcareous soils PbC03 is important. Soil solution lead is mainly in the cationic form with some organic complexation. Divalent lead as lead sulphate (PbS04) generally dominates in soils affected by lead emissions from motor vehicles . (Davies 1990).

In urban areas, levels of lead generally decrease with increasing distance from sources of contamination. For example, paint may be an important source of lead contamination in proximity to old buildings, with surface layers (eg. 0-2 cm) being most affected. Indeed, the tendency for lead to be highest in surface horizins is a common occurrence in almost all soils. This reflects the reluctance of lead to leach in soils, including those amended with sewage sludge. That is, most soils have large capacities to immobilise lead.

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4.6 Mercury

Mercury occurs in nature principally as cinnabar (HgS) and as complex sulphides with zinc, iron, and other metals. It may also replace heavy elements such as barium, bismuth, cadmium, gold, lead, silver and zinc in other minerals (Ure and Berrow 1982).

Oxygen and dry air do not react with mercury appreciably at room temperature in the absence of UV radiation. However, it reacts with ozone at room temperature to give mercuric oxide (HgO). It also reacts -- particularly on heating -- with sulphur, selenium and tellurium but not with nitrogen, phosphorus, arsenic, carbon and germanium (Aylett 1973).

Adsorption processes dominate the retention of mercury by soil. Over the soil pH range 5.5 to 4, organic matter is mainly responsible for mercury retention. Adsorption of mercury by clay minerals is strong in soils of neutral pH.

Mercury readily forms both monovalent (Hg+) and divalent (Hg2+) cation species. In addition to redox (Eh) potential, pH and chloride ion concentrations are key parameters controlling the speciation of mercury in the soil solution (Steinnes 1990). It rarely occurs in the free ionic form in soils due to complex formation. In acidic solutions Hg2+ normally occurs as the neutral HgClt complex or attached to organic matter. Under alkaline conditions the Hg(OH)2° complex plus. strong complexes with humic matter can be expected.

No monomeric mercury(n organoderivatives have ever been described (Aylett 1973). However, there are very many organomercury(II) compounds. Indeed, the immobilisation of mercury in soils is a consequence of its ability to form stable complexes with organic matter, and to its adsorption onto iron oxides, clay minerals, and soil colloids. Mercury vapour is retained by soil as organic complexes (Ure and Berrow 1982). In general, top-soils and surface horizons have the highest concentrations of mercury. Uniform distribution throughout the entire soil profile would suggest upwards migration from lower zones of mercury contamination, primarily in the vapour phase.

Caution: The vapour pressure of metal mercury at room temperature (0.1 mg/m3) is about 200 times the maximum allowed concentration. Because long exposure to low concentrations of mercury produces adverse cumulative effects, containers of mercury must be kept well sealed and rooms well ventilated.

4.7 Selenium

Like arsenic, selenium concentrations are closely related in igneous rocks to those of sulphur, which it can replace in sulphides. Selenides and selenites are found but selenate is rare. The S:Se content in the lithosphere is about 6 000. The ability of selenium to enter the crystal lattice of molybdenum is relevant to the occurrence together of high selenium and high

224

molybdenum (Ure and Berrow 1982).

Generally the weathering and oxidation of selenium-containing sulphide minerals yields selenite (SeO/). Only rarely is there sufficient oxidation potential to form selenate (SeOt). This latter form is more readily absorbed by plants than selenite. As it is also less-firmly held by soil particles, it is more likely to be lost by leaching. However, in a recent study, selenate movement in a clay loam did not extend beyond 100 mm (Whelan and Barrow 1991). Volatile selenium is recognised as a loss mechanism from soil.

The oxidation states of selenium are 0, II, IV and VI, all of which occur in soils under particular conditions (Neal 1990). Selenium(IV), which may exist as selenite (SeO/") or biselenite (HSe03-) depending on pH, and hexavalent selenate (SeOt) are the aqueous oxygenated forms of selenium. They are primarily responsible for the reactivity of this element in soils. Dimethylselenide -- an organoselenide [Se(II)] -- is one of the simplest and perhaps most environmentally significant organic selenium compounds of soils.

Adsorption reactions represent an important mechanism whereby normally soluble selenium species are attenuated by soil particles, affecting their progress through the soil profile. Soil pH is particularly important in this regard, although 1: 1 clay minerals such as kaolinite may have higher sorption capacities than 2:1 clay minerals such as montmorillonite (Neal 1990). Moreover, selenite adsorption by iron oxide is extensive, rapid, and decreasing with pH between 3 and 8.

,,~, ... , ...... '

Anion adsorption in soils generally follows the sequence phosphate > arsenate' ~ selenite ~ silicate ::> sulphate ~ selenate > nitrate > chloride

4.8 Zinc

Zinc substitutes for ferrous iron, magnesium and other ions with an ionic radius about 0.8 A in silicate and oxide minerals. It also occurs as sphalerite (ZnS) in some rocks (Ure and Berrow 1982). As a Group lIB metal it can lose two electrons (but no more) to form dipositive ions. As with cadmium and mercury, zinc is relatively low melting and volatile (Alyett 1973). Zinc hydroxide [Zn(OH)J is amphoteric while zinc carbonate (ZnC03)

decomposes at the relatively low temperature of 300'C. This compares with 400°C for magnesium carbonate and 90(J'C for calcium carbonate.

In addition to parent material, the amount of zinc in unpolluted soils is controlled by clay content, cation exchange capacity, iron and aluminium oxides, organic matter and pH.

In general, increasing quantities of clay, cation exchange sites, oxides and organic matter correlate positively with total zinc concentrations (Ure and Berrow 1982).

225

The free Zn2+ ion does exist in soil solution but it precipitates when the solubility products of compounds with its reaction partners are attained. Precipitates form with hydroxides, carbonates, phosphates, sulphides, molybdates, and with several other anions, including humates, fulvates and other organic ligands (Kiekens 1990).

In addition to the above, zinc can exist in soil as:

• adsorbed and exchangeable zinc in collodial fractions; and

• as secondary minerals and insoluble complexes in the solid phase.

Chemical equilibra for zinc in soil cause the solubility of zinc to increase at decreasing pH values. Conversely, zinc is more strongly adsorbed at alkaline pH values. Below pH 7.7 Zn2+ predominates, while ZnOH+ is more prevalent above pH 7.7. Above a pH of about 9.1, the neutral species Zn(OH)2 is predominant (Kiekens 1990). Note that computer-based, chemical equilibrium models such as GEOCHEM can be used to approximate the ionic species of heavy metals -- and other elements -- in soil solutions (Sposito 1983).

Naturally low concentrations of soil zinc in acid peats, podzols and gley podzols are attributed to leaching. The more usual tendency, however, is for zinc concentrations to be highest in surface soils.

5 METHODOLOGY

Chemical· analysis of heavy metals is carried out for several reasons, viz:

(a) to assess the degree of soil or crop pollution;

(b) to measure the geographical extent of natural contamination or pollution; and

(c) to predict the likely effects on human, plant and animal health.

It follows that the method of analysis should be chosen with the purpose of the analytical data in mind.

The measurement of the total elemental concentration in soils provides an assessment of the base-line or background situation. Total concentrations also provide unequivocal evidence of changes in soil composition of heavy metals produced by elution, pollution, plant uptake or agricultural practices. In contrast, total concentrations do not generally provide a reliable guide to the biological effectiveness, toxicity or mobility of the metal under study. Moreover, data on total metal concentrations of soil are of limited value to meaningless if one or more of the following apply:

• the sample is not truly representative of the area or soil;

• the depth of sampling is uncertain or unknown;

• the moisture status of the sample· at time of reporting is not recorded;

226

• the sample is contaminated at any time prior to or during analysis; and

• the units used for reporting the result are omitted.

The apparent differences in total heavy metal concentrations attributable to soil sampling and soil heterogeneity can be quite large. Sampling depth and the number of sub-samples should be in accord with established protocols or otherwise reflect known past uses of a site and the likely movement of heavy metals.

There are many techniques available to measure total concentrations (Ure 1990) with several giving less than the true result. Digestion procedures using strong acids such as hydrofluoric (HF), nitric (HN03) and perchloric (HCI04) are common. In conjunction with extended heating, these acids -- usually in some recognised combination -- transform the heavy metals from liquid, organic and/or mineral phases to a common solution phase suitable for subsequent chemical analysis. Widely used analytical finishes include atomic absorption spectrometry (AAS), inductively-coupled plasma atomic emission spectrometry (ICP AES), and anodic stripping voltammetry (ASV). More recently, the use of ICP-mass spectrometry (ICPMS) has enabled analysts to achieve much lower detection limits using quite small sample sizes. X-ray fluorescence spectrometry is a non-destructive technique suitable for the analysis of many heavy metal pollutants, particularly when present at elevated concentrations.

Prediction of heavy metal bio-availability to plants, etc. commonly involves use of one or more soil extracting solutions calibrated empirically against plant growth or heavy metal content in the plant (or animal). Soil extractants used for this purpose include dilute acids, complexing agents and inorganic salts in conjunction with different soil:extractant ratios, ionic strengths, and extraction times. Although a number of well established extraction procedures with some useful predictive powers exist, many are specific to one element and crop and may be restricted in use to particular soil types (Ure 1990). Techniques based on the measurement of heavy metal species in soil solution offer good prospects for success in predicting heavy metal bio-availability.

Because of the ubiquitous nature of most heavy metals including their occurrence in air, it is essential to avoid accidental contamination during sampling, during transit of samples to the laboratory, and during subsequent laboratory operations. When heavy metals at trace to ultra-trace concentrations are expected, sample handling and subsequent chemical analyses are best conducted in pre-tested laminar-flow hoods and laboratory clean rooms.

6 HEA VY METAL LOADING RATES TO SOILS

The first principle to recognise when considering safe loading rates of heavy metals to soils is that the quality of the soil resource base must not be degraded beyond acceptable limits. That is, the agronomic, ecological and economic sustainability of the area should not be prejudiced in the immediate or long term. Soil has a major advantage in this regard. It is easy to collect and relatively cheap to monitor for impacts of land and water management.

Concurrent with accelerating use and redistribution of heavy metals world-wide, local authorities have been forced to seek alternative means of dealing with ever increasing volumes of waste from sewage plants (eg. Brodie 1991). There is a growing public

227

perception that land disposal of treated sewage and associated sludge is preferable to ocean discharge or incineration. However, this practice is not without risks with heavy metal contamination of land being one of these.

While elements such as nitrogen and phosphorus have several pathways for removal from soil, similar flexibility does not exist for heavy metals. Firstly, they are non-degradable in the environment. Secondly, their removal from soil via plant material is inconsequential due to comparatively low levels of uptake relative to plant macro-nutrients. Thirdly, and as a consequence of their lack of mobility in most soils (Table 6), contaminant heavy metals tend to remain close to their points of entry, typically in the top 10-15 cm of soil profiles.

To overcome concerns about heavy metal pollution of soils and plants in relation to human and animal health, food standards for heavy metals (Anon 1990; Rayment 1991) and guidelines for the protection of people and the environment have been developed. Most soils­related research to date has dealt with conditions for disposal of sewage sludge [ego Awad et al. (1989)]. However, the principles involved could be applied to any other metal-containing material disposed on agricultural or other rural lands.

The land disposal of solid wastes is complicated. Soil type is a major variable, as is the rate of solubilization and subsequent mobilisation of heavy metals present in the waste. The potential to contaminate food crops as well as ground and surface waters are additional considerations. Particular care is necessary as residence times for many heavy metals in soils range from 100 - 3000 years (Alloway 1990a).

Before a decision is taken to dispose of significant quantities of waste material onto agricultural lands; there must be strong grounds, in addition to public acceptance and human health considerations, to ensure that:

(a) the land on which it is applied is not irreversibly contaminated or otherwise physically, chemically or economically damaged;

(b) there is negligible likelihood that toxic chemicals etc. will enter surrounding surface waters or leach to the ground water; and

(c) noxious substances and/or pathogens contained in the waste material do not result in violative residues in edible tissues or pose health risks to livestock, native fauna and flora, etc.

Rate of application, frequency of application, period of application and odour all have an important bearing on the likely acceptability of land application of waste materials. Other controlling factors include soil properties, the type and concentration of constituents in the material -- either alone or in combination, their expected rate of decomposition, and the assimilative capacity of the soiVplant system involved. Moreover, the assimilative capacity is usually obtained from a critical soil level. In· essence, a safe metal loading rate to soil is one which does not exceed a critical level separating acceptable from unacceptable crop quality or some other measure of environmental acceptability.

228

Soil pH is an important property, and in general, metals become more available as the pH decreases (molybdenum and selenium are exceptions). Under acidic conditions, soils have a lower adsorption capacity for most heavy metals (Table 6). Therefore, uptake by plants growing in acidic soils is favoured and leaching potential is increased. At alkaline pH values, metals tend to precipitate in carbonate and hydroxide forms, rendering them less soluble, less mobile and thus less available to plants. Accordingly, the metal loading from a waste material applied at a certain rate and known to be quite safe on alkaline soils may cause toxicity or accumulation problems for plants on acidic· soils. . Hence knowledge and manipulation of soil pH is an important management option for minimising metal uptake by plants. and/or leaching through soils.

Soil properties controlling plant availability of heavy metals include pH, organic matter content, cation exchange capacity and specific adsorption.

As a note of caution, the method of analysis affects the apparent soil pH. Measurements made in a suspension of soil and a neutral salt such as CaCl2 -- a common practice in southern Australia -- can be up to one pH unit lower than pH determined in a soiVwater extract (In Queensland, soil pH is commonly determined in a 1:5 soiVwater extract, following equilibration for 1 hour.) In highly weathered soils the difference . between the two pH methods is usually < 0.5 of a pH unit.

The ability of a soil to hold or retain macro-nutrient cations (calcium, magnesium, potassium) and sodium against loss by leaching is called its cation exchange capacity (CEC). Moreover, soils with a low CEC «5 m. equiv./l00g) such as sandy and acidic soils have a low capacity to retain cations including cationic-heavy metals. Some international acceptance criteria for heavy metal loadings on agricultural soils vary with soil CEC, with highest loadings permitted on soils with CEC in excess of 15 m. equiv./l00g. However, a major limitation in using CEC as a basis for heavy metal loadings to soils is its method of analysis.

No single method for CEC is suitable for all soil types (Rayment and Higginson 1992), while the method selected for use controls the result obtained. In view of the limited research carried out on the relationship between CEC and plant uptake and the conflicting results for CEC, there are suggestions that the present practice of using CEC as a basis for establishing metal-loading limits to soils should be abandoned (Page et al. 1987). Alternatively, if CEC is used as a guide to heavy metal loadings, then the method of analysis must be defined in the standard.

As mentioned earlier, organic matter has a major influence on soil chemical properties and the forms of heavy metals in soils. Moreover, the type of organic matter added to soils can regulateJhe plant availability of some heavy metals (Kuntze 1986). Readily decomposable materials such as manures or roots may increase heavy metal solubility. In contrast, stabilised peat or equivalent decreases solubility and may cause metal deficiencies in plants and animals.

The ecological consequences of heavy metal pollution of soils is related largely to both heavy metal mobility and solubility. Current evidence indicates that the rate at which a plant root

229

absorbs heavy metals such as cadmium and zinc depends on the activity of the free-ion form of the metal in solution at the root surface (Page et al. 1987). Since the movement of heavy metals within soils is mainly in the solution phase, then the chemical factors that control the equilibrium of metals between solid and solution phase will influence the mobility of heavy metals. Adsorption/desmption processes are important in this regard .. Guidelines (eg. Awad et al. 1989) that set limits for the content of heavy metals in sewage sludge and the maximum permissible loadings per year are important. This is because the amount and composition of waste material applied to soils influences the composition of heavy metals in the soil solution.

There are several different mechanisms involved in the adsorption of metal ions by soils. These include CEC, specific adsorption, organic complexation and co-precipitation. While the extent of adsorption of metal ions by soils can be quantitatively measured by isotherms such as Langmuir and Freundlich, it can be very difficult to determine which of the above mechanisms is controlling the retention or adsorption of the metals.

A further complication is that heavy metal adsmption is affected by the adsorbent or solution composition in which adsmption is being measured. Homann and Zasoski (1987) studied the effects of solution composition on cadmium sorption by sewage sludge amended American forest soils and found that dissolved calcium strongly depressed cadmium sorption whereas dissolved sulphate had little effect. Current studies being undertaken with highly weathered soils of south-east Queensland suggest a similar influence of calcium competition on cadmium sorption (G.A. Barry, unpublished data). This has significant implications when determining or modelling the metal adsmption properties of soils.

The assimilative capacity of a soil has a major bearing. on what may be considered as an acceptable loading of waste material. It depends on processes· such as·adsmption, dispersion, immobilisation and decomposition which occur in the upper zone of the soil-plant system. By examining waste constituents and soil properties as already discussed, it is possible to determine what is referred to as the land limiting constituent (L.L.C.).

The Land Limiting Constituent is defined as the constituent requiring the largest land area commensurate with its rate of supply.

It follows that the L.L.C. approach assists in identifying which metals are critical. In this way, acceptaole rates of waste material can be applied to ;land such that the maximum acceptable curttulative loading in a soil is not exceeded.

Another consideration is the potential for a site, where waste material has been applied, to be registered as contaminated within the provisions of the Contaminated' Land Act 1991. Ideally, heavy metal loadings should not exceed the investigation threshold limits for contaminants stated in the 1991 Draft Guidelines for Assessment of Contaminated Land in Queensland. These are based on total metal levels and a sampling depth of 20 cm (Anon 1991). Ultimately, acceptable heavy metal loading rates to soils requires knowledge of the likely risks to humans, livestock, wildlife and the environment. This necessitates knowledge of the potential for transfer of heavy metals to animals, crops and waters.

230

Several methods have been used in different countries to estimate the maximum cumulative soil cadmium loading based on the predicted increase in dietary cadmium. Some of these were considered when the US EPA proposed the regulations on . land application of sludge (US EPA 1979). Food standards, which set legal limits for maximum permitted concentrations of heavy metals in a variety of food types (eg. Anon 1990), may serve as surrogates for maximum loading rates in areas where food crops and animals are grown or produced.

7 REFERENCES

Alloway, B.J. (Ed.) (1990a). "Heavy Metals in Soils". (Blackie and John Wiley & Sons, Inc., New York).

Alloway, BJ. (1990b). Cadmium. Ch. 6. In: "Heavy Metals in Soils". ed. B.J. Alloway. (Blackie and John Wiley & Sons, Inc., New York).

Anon (1980). Recommended Health-based Limits in Occupational Exposure to Heavy Metals, report of a WHO study group. WHO Technical Report Series, 647. (WHO, Geneva).

Anon (1986). The Water Quality Council of Queensland - Data Supplement to the Fourteenth Annual Report, 30th June, 1986.

Anon (1990). "Food Standard Code". National Health and Medical Research CounciL (Australian Government Publishing Service, Canberra).

Anon (1991). "Draft Guidelines for Assessment of Contaminated Land in Queensland". (Chemical Hazards and Emergency Management Unit, March 1991).

Awad~ A.S., Ross, A.D. and Lawrie, R.A. (1989). Guidelines for the use of sewage sludge on agricultural land, 2nd edition. (NSW Agriculture and Fisheries, Sydney).

Aylett, B.J. (1973). "The Chemistry of Zinc, Cadmium and Mercury". Pergamon Texts in Inorganic Chemistry Vol. 18. (Pergamon Press, Oxford).

Aylett, BJ. (1979). The chemistry and bioinorganic chemistry of cadmium. Ch.1. In: "The Chemistry, Biochemistry and Biology of Cadmium". ed. M. Webb. (Elsevier/North­Holland Biomedical Press, Oxford).

Baker, D.E. (1990). Copper. Ch.8. In: "Heavy Metals in Soils". ed. B.J. Alloway. (Blackie and John Wiley & Sons, Inc., New York).

Barrow, N.J. (1991). Comparison of the behaviour of Cd, Co, Ni, Zn and Hg in soil. Ch. 13. In: "Soil Science and the Environment". (Australian Society of Soil Science Inc.,W A Branch).

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Batley, G.E. and Low, G.C.-K. (1989). Applications of high~performance liquid chromatography to trace element speciation studies. Ch. 6. In: "Trace Element Speciation: Analytical Methods and Problems". ed. G.E. Batley. (CRC Press, Inc., Florida).

Boyle, E.A., Sclater, F. and Edmond, J.M. (1976). On the marine geochemistry of cadmium. Nature 263, 42-4.

Brodie, J. (1991). Urban impact on the Great Barrier Reef. In: "Land Use Patterns and Nutrient Loading of the Great Barrier Reef Region". ed. D. Yellowlees. pp.18-26. (James Cook University of North Queensland, Townsville).

Bruland, K.W. (1980). Oceanographic distributions of cadmium, zinc, nickel, and copper in the North Pacific. Earth and Planetary Science Letters 47: 176-98.

Buat-Menard, P. (Rapporteur) (1987). Group report: Arsenic. Ch. 4 In: "Lead, Mercury, Cadmium and Arsenic in the Enviroriment". Scope 31. eds. T.e. Hutchinson and K.M. Meema. (John Wiley & Sons, Brisbane).

Chilvers, D.e. and Peterson, P.J. (1987). Global cycling of arsenic. Ch. 17. In: "Lead, Mercury, Cadmium and Arsenic in the Environment". Scope 31. eds. T.C. Hutchinson and K.M. Meema. (John Wiley & Sons, Brisbane).

Cuddihy, W.L. and Fergus, LF. (1974). Arsenic toxicity in· the Collinsville Basin, Queensland. Australian Soil Science Conference, Working Papers, Melbourne. Victoria. pp. 4-12 - 4-15.

Davies, B.E. (1990). Lead. Ch. 9. In: "Heavy Metals in Soils". ed. B.J. Alloway. (Blackie and John Wiley & Sons, Inc., New York).

Farmer, P. (1987). "Lead Pollution from Motor Vehicles 1974-86: A Select Bibliography". (Elsevier Science Publishing Co. Inc., New York).

Fergusson, J.E. (1990). 'The Heavy Elements: Chemistry, Environmental Impact and Health Effects". Pergamon Press, Sydney).

Homann, P.S. and Zasoski, R.J. (1987). Solution composition effects on cadmium sorption by forest soil profiles. Journal Environmental Quality 16: 429-33.

Hutchinson, T.e. and Meema, K,M. (Eds.) (1987). "Lead, Mercury, Cadmium and Arsenic in the Environment". Scope 31. (John Wiley & Sons, Brisbane).

Hutton, H. (1987). Human health concerns of lead, mercury, cadmium, and arsenic. Ch. 6. In: "Lead, Mercury, Cadmium and Arsenic in the Environment". Scope 31. eds. T.e. Hutchinson and K.M. Meema. (John Wiley & Sons, Brisbane).

Kiekens (1990). Zinc. Ch. 13. In: "Heavy Metals in Soils". ed. BJ. Alloway. (Blackie and John Wiley & Sons, Inc., New York).

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Kuntze, H. (1986). Geological and pedological factors affecting the heavy metal load capacity of soils. In: "Factors Infhlencing Sludge Utilisation Practices in Europe". eds. R.D. Davis, H. Haeni and P. L'Hermite. (Elsevier Applied Science Publishers, London).

Lansdown, R. (1979). Moderately raised blood lead levels in children. Proceedings of the Royal Society. London. 205: 145-51.

Livett, E.A. (1988). Geochemical monitoring of atmospheric heavy metal pollution: theory and applications. Advances in Ecological Research 18: 65-177.

Masterton, W.L. and Slowinski, E.J. (1978). "Chemical Principles with Qualitative Analysis". (W.B. Saunders Company, Philadelphia).

Merry, R.H. (1988). Investigations on cadmium in South Australia: rainfall, soils, cereals, pastures and soil-plant relations. pp. 62-79 In: "Cadmium Accumulations in Australian Agriculture". Bureau of Rural ResourcesProceedings No.2. (Australian Government Publishing Service, Canberra).

McGrath, S.P. and Smith, S. (1990). Chromium and nickel. Ch. 7. In: "Heavy Metals in Soils". ed. BJ. Alloway. (Blackie and John Wiley & Sons, Inc., New York).

Neal, R.H. (1990). Selenium. Ch. 12. In: "Heavy Metals in Soils". ed. B.J. Alloway. (Blackie and John Wiley & Sons, Inc., New York).

O'Neill, P. (1990). Arsenic. Ch.5. In: "Heavy Metals in Soils". ed. B.J. Alloway. (Blackie and John Wiley & Sons, Inc., New York).

Pacyna, J.M. (1987). Atmospheric emissions of arsenic, cadmium, lead and mercury from high temperature processes in power generation and industry. Ch. 7. In: "Lead, Mercury, Cadmium and Arsenic in the Environment". Scope 31. eds. T.e. Hutchinson and K.M. Meema. (John Wiley & Sons, Brisbane).

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Petrucci, R.H. (1972). "General Chemistry, principles and modem applications". (Macmillan Publishing Co., Inc., New York and Collier Macmillan Publishers, London).

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Tiller, K.G., Merry, R.H., Zarcinas, B.A. and Ward, T.J. (1989). Regional geochemistry of . metal-contaminated surficial sediments and seagrasses in the Upper Spencer Gulf, South Australia. Estuarine, Coastal and Shelf Science 28: 473-93.

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Ure, A.M. and Berrow, M.L. (1982). The elemental constituents of soils. Ch.3. In: "Environmental Chemistry Volume 2, A review of the literature published up to the mid-1980". Snr. rep. H.J.M. Bowen. (The Royal Society of Chemistry, London).

USEPA (1979). Criteria for the classification of solid waste disposal facilities and practices. Federal Register 44: 53438-64.

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Waldron, H.A. (1973). Lead poisoning in the ancient world. Medical History, London. 17: 391-9.

Whelan, B.R. and Barrow, N.J. (1991). Selenium in the soil- where does it go. CH. 14. In: "Soil Science and the Environment". (Australian Society of Soil Science Inc., WA Branch).

WHO (1985). Review of potentially harmful substances - cadmium, lead and tin. Reports and Studies No. 22. IMO/FAO/UNESCO/WMO/WHO/lAEA/UN/UNEP Joint Group of Experts on the Scientific Aspects of Marine Pollution. (WHO, Geneva).

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236

GUIDELINES FOR INVESTIGATION AND TREATMENT OF POSSIBLE CONTAMINATED LAND

R Sadler and P Imrar

1 ,Government Chemical Laboratory, PO Box 594, Archertield QLD 4108 2 Queensland Health, Brisbane QLD 4000

ABSTRACT

The paper appears in two sections. The first part deals with existing legislation in Queensland through the Contaminated Land Act, and considers various definitions, the Contaminated Sites Register, land uses with a high potential for contamination, legal liability, a1uJ appeals. Requirements for conducting a site investigation are described, with reference to site location, description and history. The site investigation aims to characterise contaminants present, and to facilitate design of a management or clean-up program, implications of which are discussed.

The second part of the paper deals with the mechanics of conducting an investigation and methodology for treatment and remediation of contaminated sites. Sampling is a· vital but difficult task, and various sampling patterns are reviewed. Sampling equipment ranges /romsimple hand tools to truck-mounted boring rigs.

Laboratory analytical problems of sample preservation and choice of appropriate instrumental techniques are detailed; these depend on the nature of the sample collected. Quality control and expression of results are critical to adequate laboratory information. Analytical chemistry underpins the investigation of contaminated land~

1 BRIEF OVERVIEW OF LEGISLATION IN QUEENSLAND

In Queensland the main piece of legislation providing for control of contaminated sites is now the Contaminated Land Act, which was proclaimed from the 1 January 1992. However, the first legislation to address the issue of contaminated land was the Local Government (Planning and Environment) Act, which was amended in March 1991 to make site assessments necessary where land that had been used for prescribed purposes was to be rezoned.

The Local Government (Planning and Environment) Act was amended such that under Section 8.3A "Site Contamination Report" was called for by the local authority where land which had been used for a prescribed purpose was to be rezoned. This was the· first step in the Queensland Government's program to control inappropriate use of land which may have been contaminated by previous land uses. This Act has just been further amended to extend these provisions from rezoning to also cover subdivision of land and change of consent use. The trigger for the site assessment here is that the local authority receives a rezoning, subdivision or change of consent use application, for land which has been used for a prescribed purpose. The Site Contamination Report issued on the land by the State Government (Department of Environment and Heritage) is based on investigations conducted by environmental consultants. These investigations assess the site history, and

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by appropriate' sampling and analysis, the degree of chemical contamination of the land.

1.1 Contaminated Land Act (1991)

This was the second stage in the Government's strategy for control of contaminated land. The purpose of this Act was to put in place' a system to identify and manage land which has been contaminated and to prevent further contamination of land. The objectives of the Act were:

• to, define contaminated land,

• to prevent further contamination of land,

• to identify all contaminated land in Queensland,

• to establish a register of contaminated land,

• to provide public access to that information,

• ',to enable assessment and, if necessary,' remediation of contaminated land,

• ,to put in place mechanisms for site-specific assessment and management,

• to provide for recovery of costs where sites'must be assessed and cleaned up by the Government,

• to ensUre that any restrictions placed over contaminated land are maintained, and

• to establish, if the Minister so desires, an Advisory Council to advise the Minister on matters related to contaminated land.

Key definitions from the legislation are:-

"Contaminated land" means land, a building or structure on the land, or matter in or on land, that, in the, opinion of the Director is affected by a hazardous substance so that it is, or causes other land, water or air to be, a hazard to human health or the environment.

"Hazardous substance" means a substance that because of its quantity ,concentration, acute or chronic toxic effects, carcinogenicity, teratogenicity, mutagenicity,' corrosiveness, flammability, or physical, chemical or infectious characteristics, may pose a hazard to human health or the environment when improperly treated, stored, disposed of or otherwise managed.

The Act prohibits the contamination of land and provides that contaminated materials or soils may not be moved off contaminated sites, or disposed of, without approval by the Director. Further details of the contaminated land act are available in the publication 'A Guide to the Contaminated Land Act'.

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1.2 Notification

Where land was contaminated prior to proclamation of this Act, any person, owner or occupier of land, local authority, Government Department or Statutory Authority must notify the Director of the existence of land they believe to· be contaminated or which is, or has been, used for a prescribed purpose by the 1st January 1993, or within 30 days of becoming aware of the likelihood of the land being contaminated (whichever is the later). Land that becomes contaminated after 1 January 1992 must be notified within 30 days of that person's becoming aware of contamination.

1.3 The Contaminated Sites Register

Mter land is notified it is classified in the Contaminated Sites Register as a possible site, probable site, a confirmed site, a restricted site, a former site or a released site. There is a definition for each of these classifications provided in the Contaminated Land Act.

When a site is classified, details of the entry to the Contaminated Sites Register are provided to the owner of the property·and to the relevant local authority.

Public access is available to any part of the Register apart from that where sites are classified as "possible". Land may be classified as possible site, if the land or its locality is reported to be contaminated and cannot be given some other classification. Such cases would be where the real property description was not known, or where the basis of the report has been very weak (eg. an anonymous phone call). ":'';1

When a person is selling land which has been classified in the Contaminated Sites Register as a probable site, a confirmed site, or restricted site then the seller must give written notice to thebuyer, setting out the classification of the land and particulars of any current or unsatisfied notices issued under the Contaminated Land Act in relation to the land.

1.4 Prescribed Purposes

In the Contaminated Land Regulations is a list of land uses (Schedule I) which have been found, both in Australia and overseas, to have a high potential for contamination of land. Such uses include service stations, gas works, tanneries, cattle dips and landfills. Where land has been used for a prescribed purpose, or is being used for a prescribed purpose, these sites are entered into· the Contaminated Sites Register as probable sites. In the case of many industrial sites there would be no further action taken and the site would remain in the Register within the probable category. The importance of the link back to the Local Government (Planning and Environment) Act provisions comes into play with these sites when itis proposed to rezone, subdivide, or change the consent use of this land.

1.5 Liability

The Act is based on the "polluter pays" principle. The polluter or the person who committed the pollution would be the first person in line for bearing costs associated with assessing and cleaning up contaminated sites.

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The owner is held liable if, at the time the land was acquired by the owner, the land was recorded in the Contaminated Sites Register as a confinned site, a restricted site or a probable site, or the contamination occurred after the acquisition of the land by the owner.

There are circumstances where the local authority can be held . liable and these are in cases where the contamination arose because of an approval or action of a local authority that they should have known would result in contamination of the land; or the land has been recorded in the contaminated sites register as a restricted site and after the recording the local authority has given approval for the use of or activity on the land, contrary to restriction; or the land has been recorded in the Contaminated Sites Register and after the recording:

(a) the land is being, or has been, used for a purpose prescribed in the regulations; and

(b) the local authority has permitted a use of, or activity on, the land that resulted in a hazard to human health or the environment.

Notices to assess, remediate or validate remedial work can be issued under the Contaminated Land Act to the person believed responsible or liable for the site.

1.6 Appeals

The Act enables any person, or local authority, to whom a notice is served, or Ministerial direction given, which requires expenditure for either investigation and/or remediation of contaminated land to appeal to the Planning and Environment Court. The conduct of the appeal is in accordance with the Rules of Court made under the Local Government (Planning and Environment) Act 1990.

The Court can affirm a notice or direction, set aside a notice or direction,substitute its own notice or direction, or amend the notice or direction. A notice or direction made by the Court is to be given effect by all persons concerned.

1.7 National Approach To Contaminated Land

The Contaminated Land Act provides a systematic framework for identification and registration of contaminated sites, and for assessment, clean-up, cost allocation and general management of contaminated land. The Act provides for a site-specific approach as proposed in the, National Guidelines for Assessment and Management of Contaminated Land (ANZECC/NH&MRC). This is scientifically based and depends on site-specific health and environmental risk assessment and takes account of present and proposed land use. The fundamental goal of contaminated site management is to implement socially acceptable and cost-effective management strategies which mitigate threats to, and provide protection for, public health and the environment.

The Contaminated Land Act provides a strong legislative basis for identification and management of contaminated sites in Queensland and with the provisions of the Local Government (Planning and Environment) Act requiring Site Contamination Reports when land which has been used for a prescribed purpose is the subject of rezoning, subdivision

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or change of consent-use application, situations such as residential development over contaminated land should be avoided.

2 REQUIREMENTS FOR CONDUCTING A SITE INVESTIGATION

Procedures for investigating possibly contaminated sites are described in the "Guidelines for the Assessment of Contaminated Land in Queensland" Jan 1 1992.

The situations in which assessment of land will most commonly take place are:

(a) where change of land use is proposed (eg. rezoning, subdivision, change of consent use);

(b) when land on the Contaminated Sites Register is to be sold. This is not a legal requirement but often a potential purchaser would want an accurate appraisal of a site they were considering buying;

(c) when the owner of a 'probable' site, so classified because it has been used for a prescribed purpose, wants the site reclassified.

There will be occasions when a notice to conduct a site investigation is issued under the Contaminated Land Act. This will occur when a site is believed to present a risk to human health or the environment.

Often investigation is a staged process where a preliminary investigation is done first and if this indicates the site is not contaminated that may be all that is required. When the preliminary investigation shows the site has contaminates above investigation threshold levels then further investigation and health and environment risk assessments are needed.

The following issues should be addressed during this investigation:

2.1 Stage 1 - Preliminary investigation

2.1.1 Site location and description

It is essential that the location of the site and the significant features involved in its contamination history are accurately and clearly identified. There are two aspects of this:

, • maps, plans or diagrams should be used to clearly identify the location of the site

in relation to its surrounds (eg street access, neighbouring property boundaries, etc) and identify significant internal features (eg sample sites, buildings etc).

• the real property description of each parcel of land must be determined. This should ideally be provided in the Lot-on-Plan form, and the land title volume and folio number for each parcel should also be supplied.

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2.1.2 Site history

This should take into account:

• present and past land uses and on-site activities.

• processes canied out on the site and their location.

• waste disposal practices and chemical spills.

• earthmoving activities canied out on the site and the source of any fill imported to.the site.

2.1.3 Site inspection

A basic description of local geology, soil type, topography, proximity to surface waters and groundwater and adjacent land uses should be given. An inspection should be canied out to confirm site history and identify possible further areas of contamination indicated by disturbed or discoloured soil, disturbed or affected vegetation, presence of chemical containers, holding tanks etc, chemical odour or quality of surface water. Should the site history and inspection indicate no chemical contaminants are likely to be present, no further investigations may be necessary. Details of the procedures are described subsequently.

2.2 Stage 'IT· Investigation

When preliminary (Stage I) investigation reveals contaminants at levels above investigation threshold values, a Stage II investigation would generally be done.

The decision on how to proceed in the second stage . of investigation will require professional judgement as the best approach will vary from site to site, and be dependent on the findings of the preliminary investigation.

The procedures for sampling, analysis and presentation of data apply to Stage II investigations also.

2.3 Human· Exposure and Environmental Effects Assessments

When the sampling and analytical work is complete then assessments can be made of potential human exposure and environmental impact. Protocols for health risk assessment are described in the proceedings of the National Workshop on Health Risk Assessment and Management of Contaminated Land (November, 1991). If the current or proposed land use would result in unacceptable levels of human exPosure or unacceptable environmental effects then management or clean-up options must be developed.

The aims of the site investigation are:

• to characterise contaminants present, their concentrations and distribution;

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• to facilitate design of the management or clean up program.

Remediation proposals should be agreed to by the Department of Environment and Heritage. Clean-up objectives and validation requirements, as well as proposed methods of remediation, should be approved before work is commenced. If off-site disposal of contaminated soil or other materials is proposed, approval must be obtained under the Contaminated Land Act.

2.4 Soil Criteria

Four levels of contaminant concentration may be recognised:-

• background levels - ie. the range of values for a substance found by a national testing program (excluding very high values from mineralised areas).

• investigation thresholds - ie. the concentration of a contaminant above which further investigation and assessment is required.

• acceptable levels

• unacceptable levels ~ ie. levels with potential for adverse effects on health.

The question is often asked whether there should be a set of "acceptable levels" and. these applied to all sites·. or whether there should be site specific assessments. The Contaminated Land Act and Guidelines for the assessment of contaminated land in Queensland provide for site-specific assessments. This is consistent with the approach in the ANZECC/NHMRC Guidelines and consequently there is no list of acceptable levels.

2.5 Investigation Thresholds

The investigation thresholds are set to include a margin of safety and no unacceptable risk is expected from soil contaminated at these levels.

The investigation thresholds are used (a) to clear suspected, possible contaminated sites where testing does not confirm suspicion of contamination, and (b) to trigger further investigation and assessment of sites where testing finds levels of contaminants above the investigation thresholds.

Finding contaminant levels above the investigation thresholds does not automatically imply the site is a risk to human health or the environment. Site-specific risk assessment is needed to determine what risks, if any, the site presents ..

2.6 Site Specific Assessment

While clean-up to background levels may seem desirable, it is not generally justified on the basis of risk assessment, and it would impose higher costs than a less stringent clean­up target.

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Risk assessment need not be quantitative; often qualitative or semi-quantitative approaches are sufficient.

It is easy to explain different health-based "acceptable levels" in relation to different land uses. For example, on an industrial site risk assessment would be based on adults at the site during normal working hours but in a residential situation the risk assessment would be conducted for a two year old child who spends all its time at the site. The lower body weight and higher soil ingestion rate for a child compared to an adult will mean "acceptable levels" on an industrial site will be at least 10 times higher than on residential land; other differences in absorption of contaminants and skin contact may also be factors in deriving varying "acceptable levels".

Health-based levels would also vary depending on the size of the contaminated area. Contamination on a single residential block versus contamination over whole suburbs (eg around a smelter) will involve fewer exposure routes and hence a higher level of contaminants could be tolerated.

Environmental risk will also vary depending on the situation and size of the site. Sites located such that surface water and groundwater may become contaminated present a higher risk than sites isolated from water bodies. Again considerations of the size of the contaminated area in relation to receptor species range will influence risk. Contamination in an area used by endangered species may demand more stringent criteria than in other locations. Setting acceptable levels in soil for protection of surface and groundwater needs site-specific assessment and considerations of soil type, contaminant mobility, local topography and hydrogeology.

2.7 Setting Clean-up Standards

Often investigation thresholds or background levels are used as the target levels for remediation. For small sites this may be justified in that the cost of doing site-specific risk assessment outweighs the costs of cleaning-up to these levels. Land developers may also prefer this approach in order to be able to claim land is really "clean" rather than "acceptable for the proposed use",

However, in cases of large contaminated areas and industrial sites continuing in industrial use, the value of site-specific risk assessment to determine acceptable levels is appreciated and utilised. Site management or partial clean-up to mitigate unacceptable risks allows the most cost-effective solutions.

Conducting risk assessment to set acceptable criteria is beyond the scope of this paper. References by Paustenbach (1989) and the Proceedings of the National Workshop on Health Risk Assessment and Management of Contaminated land (August 1991) provide more information on the topic.

The relationship between the various soil criteria is shown in Figure 1.

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range increasing concentration -)

Background Level

Investigation Level

a 1 2 3 4 b

Response levels

Proposed Land Use 1. Residential 2. Horticultural 3. Recreational 4. Commercial/lndustrial

Environmental Protection

a. Groundwater b. plant

(Not to scale - Figure from EI Saadi and Langley, 1991)

Figure 1 Relationships between soil criteria

Possible overt health effects

3 THE MECHANICS OF INVESTIGATION AND TREATMENT METHODOLOGY

3.1 The Soil Profile

It is a common concern of scientists working in other areas of environmental study, that their medium is of non-homogeneous composition. Any substance introduced for example into a stratified lake would not be expected to distribute itself equally between the epilimnion and hypolimnion. On the other hand, the soil profile consists of a number of layers or horizons (see Figure 2) .

Figure 2

. :~. ~ ~:;;;o~·;.~~::~:· ~":: : ~ sandy loam : •

• : :: : : : : : eo ... : •• eO

.:.- :

B. Horizon {subsoil}, I

columnar clay

•••• "1:.' .•.•.•.• r;.:. ... r-: ..... ::'" oc. ~ C,. Horizon, day ~ t:;:; WIth fragments :..:...:..:

8 "Of bedrO~k . =:7:

The soil profile. Major horizons are shown in the diagram on the left and details appear in the diagram on the right.

245

Contamination can affect the behaviour of soil compone~ts within this profile. Acid wastes mobilize endogenous reserves of calcium and magnesium carbonates in the A horizon as well as causing relocation of iron and clay" minerals to the B horizon. Frequently, the colour of the contaminated horizon changes as a result and this may be of use in terms of locating the contaminated soil. Alkaline contaminants on the other hand have the opposite effect, conserving carbonates of calcium and magnesium as well as iron and clay minerals in the upper horizons. Buried wastes are also brought to the surface with evaporation water, particularly in areas of low rainfall which receive little artificial irrigation.

Few investigators seem to consider that by taking a sample say lOcm deep they may well be including material from more than one horizon - material of different chemical composition which cannot be expected to have similar physical .or chemical properties as regards the contaminant. By taking material from different parts of one site, one or more horizons may be absent from one sampling point. It is one thing to say - "the top IOcm were sampled ...... ". What if the B2 horizon was exposed at one point and the Ai at another? Was the contamination deposited on the surface before the Ai horizon was lost? If so, the two sampling points will not be equivalent at all, even though they are both "surface soil". If the contamination was deposited after partial loss of the Ai horizon, can we really expect leach tests or contaminant speciation to be equivalent at the two sites? It is thus most important to note, not only where one is sampling but what one is sampling.

The importance of depth sampling varies from site to site. A decision as to whether to undertake depth sampling will be governed by two factors: (1) Is there any chance of contact with contaminants at depth? This would include considerations such as whether development/use of the site includes excavation, tilling etc. of the site, which would lead to human contact; or whether the roots of plants which may contribute to the food chain and are capable of taking up the contaminant would penetrate a sub-surface contaminated zone (2) Is the contamination observed on the surface actually a result of contaminants which have been carried upwards from a buried waste, by evaporation water? In any case, surface sampling usually precedes depth sampling and it is unwise to proceed with depth sampling unless a picture of surface contamination has been built up.

3.2 Planning The Sampling Operation

Once it has been decided from which parts of the site samples are to be collected, it is necessary to develop a strategy of sampling. The overall strategy developed will depend upon the waste deposition on the site. Basically, three questions must be answered (a) Is the waste deposition known to be restricted to a particular area, or is the current investigation limited to a specific area? (b) Is there any information on how the contamination is likely to be distributed laterally (in discrete areas vs generally over the site)? and (c) To what depth should sampling be extended?

On some occasions, a pilot study of the area may be conducted as a prelude to a full investigation of the site. Many investigations particularly of smaller properties will involve the whole area. In such cases, there is probably no need· for a pilot study and the main sampling program can proceed from the outset. On the other hand, properties with larger area may not be contaminated throughout. The initial strategy adopted will depend

246

upon historical information. Two examples are given in Figures 3 and 4, in which a limited sampling program is used to act as a pilot study for the detailed examination of the site.

A

Cattle Dip .. : .• ···~·~· .. ·I- - -I • ••••• L--______ ---' Holding Pen

~B

Figure 3

........... .- ........... .

Dip Contents Pumped Out Here

•••••• • •••••••••• : •• C • \ :. . : ...... :

-...... • e •. = .. ... .. .. . . . .. . .. -.

Pilot sampling around an old cattle dip.

The property in Figure 3 is a farm, with only one area suspected of contamination. A disused cattle dip, dating from the 1930's (and used up until the late 1960's) is in one corner of the property. Animals passed through the dip in the direction of the arrows and were placed in a holding pen to one side of the dip. It is also known (as a general fact regarding cattle dips) that spent contents were disposed of by pumping out over the' ground. There is no available information however as to the location of the holding pen and the site of disposal of the dip contents. One approach would be to sample the entire area surrounding the dip in a very detailed manner but this would be both labour intensive and would involve an excessive amount of analysis. For this reason initially four samples (indicated by A,B,C,D) are taken around the dip. Each sampling point is located half way along the side of the dip and two metres out from the dip. It is apparent from the results that contamination is present only in samples C and D and hence a detailed sampling of these areas is subsequently carried out.

247

•

• •

•

Figure 4 Pilot sampling of gold mine tailings dump.

The second example, (Figure 4) is a park, in which gold-mine tailings (contaminated with mercury and arsenic) are known to have been dumped. The initial sample (indicated by a solid hexagon) indicated seriotis contamination by the two elements, but no information is available as to the extent· of dumping which has taken place. Again, the entire area could be intensively sampled but the exercise would not be cost-effective. Before intensive sampling takes place, a pilot program is conducted to establish the extent of the contamination. A series of samples is taken at right angles to each other, the sampling points being separated by at least 10 m. Each sample point is analysed until the level of contaminants decreases to background values. Through these investigations some idea of the perimeter of the contamination is attained and an intensive sampling of the contaminated area can be. undertaken.

Once the decision has been made to proceed with an intensive sampling, consideration must be given to the pattern of sampling which is to be employed. Usually, a detailed sampling program will be governed by the information gained from historical data, pilot sampling etc. If no such information is available, an externally generated sampling strategy must be employed. There are two basic patterns of externally generated sampling strategies, viz random and grid. Within each are a number of sub-classes or means of achieving the desired result. In general, grid sampling is employed when there are expected to be fairly large areas of contamination.

248

contamination

-x- -2x-

• • • • • • • • • • • • • • ~ • •

• • • • • • •

§ contamination

Figure 5 Square vs triangular grid sampling.

Figure 5 shows an extreme example of grid sampling. The contamination is located in parallel bands and two grid sampling strategies are shown: A square grid and a triangular grid. Note how a square grid. with points x units apart is required to detect the waste whilst an offset (triangular grid) with points 2 x units apart will satisfactorily cover the area. Some 64 samples are required with the square grid, compared to only 32 with the triangular grid. Although the above is an extreme example, both mathematical and geometric proofs can be devised to show the advantages of a triangular sampling grid. (cf Parkhurst, 1984)

Once a decision has been made to proceed with a grid sampling program, it is necessary to determine the spatial separation of grid points. This decision will be governed by a number of factors which are discussed below:

(a) The time available for the program. In general, it is preferable to commence with a fairly wide spacing of grid and proceed, if necessary to a more narrow grid. There may be instances however, where intense public concern or time available for the decision demands results be produced as rapidly as possible. In such cases, closely spaced grids must be used for sampling programs.

(b) The area likely to be contaminated. Clearly, the grid chosen must fit within the contaminated area. It would be inappropriate to sample an areaAO m x 40 musing a 40 m grid spacing.

(c) The variation expected in contaminants. An area onto which the contents of a cattle dip pumped will probably show highest contamination near the location of the pump outlet, with a progressive decrease, the further one moves away from this point. Hence, a fairly close grid spacing is required in order to determine at what point the level of contaminant becomes acceptable. On the other hand, a gasworks site, on which there has been a history of tar burials will show considerable levels

249

of P AH throughout the area and little would be gained by arranging sampling for this parameter on a closely spatial grid.

(d) The ultimate use to which the result will be put. In the context of this paper, sampling will be primarily used to determine to what extent (if any) the site is contamina~d. If a subsequent remediation phase becomes necessary however, it will be important to consider the economics of this process. Where a relatively cheap operation will suffice (eg. removal of the soil to some other site), there is probably no great need to accurately define the extent of the contamination. On the hand however, if some highly 'sophisticated and expensive process is involved (eg. supercri;rical fluid extraction of the soil followed by high temperature incineration of the waste), then removal of contaminated soil must be minimised and hence sampling will have to be carried out on a very tightly spaced grid.

(e) The pattern of contamination expected/encountered. Irrespective of the levels of contamination . observed within the polluted area, the affected regions may be distributed in vanous ways about the site .

...

.. ~ contamination

Figure 6 Sampling of ditTerent contaminated Sites using identical grid patterns.

Figure 6 shows two contaminated areas which have been sampled on similar offset grid patterns. In one case, the entire site is contaminated and the sampling pattern chosen will adequately describe the situation. The other site however, consists of a number of large, discrete areas of contamination and has not been adequately sampled at all.

250

Figure 7

®. ® @ ® b @ ,., @ ,. ,. ., "!. ." • ,.

• • • • • • @ • ® • • • • • @ ® @ @ ® @ .~. ,. ":.' .~ ., .,.

• • • • " @ ~ @ ~ ~® ®. • • ® '! .,. @ ~ ., ~., ~.,

• .. " • • @ ® @ ® @. • ® ,., ~., .~. • ., ." • • • • •

$ ® ~ @ • • @@ ., ,. @ ..... . ... ®@ ~. 0 @ @ . ,. "'!" "!" • • ,. ,- ..... • ... ¢ID .® ® ® !) • • .... ... .. .. ' .,

IUnl contamination

Random vs triangular grid patterns for sampling areas with isolated areas of contamination.

Figure 7 shows how even a very close grid sampling program can miss small discrete contaminated areas.

3.3 Composite Sampling

Composite sampling is a technique frequently used in pollution analysis of water and air. It is particularly useful in sampling the average output from a premises. For example, if the average BOD in a plant effluent is set at a certain level over a 24 hOUT period, a 500 mL sample might be withdrawn every 6 hOUTS and stored under refrigeration. At the end of the 24 hOUT period, the samples are pooled and submitted for analysis.

The technique is of less use in soil sampling because soil is a less miscible matrix than water or air and a discharge is not involved. In addition, a high concentration of a single sample can be diluted in the composite. Where composite sampling of a site is used, compositing should be done in the laboratory, not in the field. The laboratory should composite sub samples of the material collected. If analysis suggests one or more of the original samples to be contaminated then the soils may be checked individually. To test compliance or non compliance with a fixed criterion, the criterion is divided by the number of samples. The reasons for this are set out below:

Say that six samples are composited for a parameter where the maximum acceptable level is 400 and the detection limit is 10

CASE A: Results: -600; 10; 10; 10; 10; 10 Mean:- 108 (n=6) criterion = 400 = 67

n 6

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CASE B: Results:-300;250;380;260;310;390 Mean:- 315 (n=6) criterion = 400 = 67

n 6

CASE C: Resu1ts:-390;20;30; 10;50; 10 Mean:- 85 (n=6) criterion = 400 = 67

n 6

Note that one high result (Case A) in excess of or just below (Case C) the criterion is still flagged by this technique. A series of high results is similarly identified (Case B). It can be seen that this procedure highlights possible areas of contamination, which would be missed by comparing the arithmetic mean with the criterion. The advantages of compositing in the laboratory can be readily discerned from the above discussion.

The samples would be highlighted as requiring individual analysis. As the individual samples would still be available, it would be possible to carry out such analysis. This situation would not prevail had the compositing taken place in the field.

In some instances, where spot contamination is a distinct possibility, sampling at individual points on a site may lead to erroneous results. Composite sampling at points in these instances may provide ca better picture of the contamination scenario. An example of composite sampling at·apaiticular point is given in Figure 8.

Figure 8

1.4m 1.4m

1.4m 1.4m

tit Sample Point, dictated by program

• Additional Sampling Points

Composite sampling at points to avoid spurious effects caused by spot contamination.

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Effectively, a square with a side of 1m is drawn around the sampling point, samples being collected both at the designated sampling point itself and also at the four additional points. Equal volumes of soil are collected from each point (with the aid of a measure), and the results composited as described above.

3.4 Execution of Sampling

Random sampling is generally easy to accomplish, irrespective of the topography. On flat terrain, grid sampling positions are easily established with the aid of a measuring tape. Without specialized surveying equipment however, grid patterns are impossible to establish on sloping ground. Two options are available (a) the land may be surveyed prior to the sampling exercise and the positions pre-determined (b) samples may be taken in approximate locations, and the points marked for subsequent identification on a cadastral map by a surveyor.

As mentioned above, the distance between-sampling points on· a grid pattern is dictated by a number of factors. The term "contaminated area" need not pertain to the entire site, depending upon the contaminant. For example, in the case of the gasworks site mentioned previously, the contaminated area as regards PAH's would constitute the entire site, whereas the contaminated area as regards sulfides, would comprise only the area around the old purifiers.

Generally, a wider grid will be used on a large area and more samples will need to be taken from a site where contaminants are distributed in a patchy' f~shion than in a uniform one. Table 1 provides a guide to such sampling. . .

Table 1 Suggested Sampling Strategy for Contaminated Sites+.

Area (ha) Grid Size Number of. Samples

> 20 100 50

2 - 20 50 30

0.1 - 2 30 25

Up to 0.1 15 6

+ Where more specific advice is not available. It is not possible to make a specific recommendation as regards randomly distributed contaminants, as each site will have to be considered on a case by case basis.

3.5 Equipment for Soil Sampling

It should be stated at the outset, that there is no ideal method of sampling soil. Basically, there are three types of soil sampling techniques used at contaminated sites:- Manual surface grab sampling, manual shallow subsurface coring and subsurface coring with heavy equipment.

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3.5.1 Manual Surface Grab Sampling

The majority of the sampling performed for investigations of contaminated land will be included in this classification. Grab samples are taken (with the aid of metal or plastics grabs or scoops) from the upper layers of the A horizon. It is general in such work to exclude the ~ and Aa layers. Metal trowels are now available which bear depth graduations on their blades. This enables a more precise definition of the origin of the sample. Normally, the quantity of soil adhering to the trowel is minimal and hence the risk of carryover from one sample to another is negligible. If however, the soil is wet or contains some contaminant which is likely to adhere to the trowel, then the trowel must be cleaned (or a fresh tool used) before any further sampling on the same site is attempted. Where there are areas of obvious contamination on site, it is advisable to work from the less contaminated to more contaminated sites' in the sampling operations. Before proceeding to another site, all sampling equipment should be thoroughly cleaned.

3.5.2 Manual Shallow Sub-surface Coring

Samples of this kind are generally removed with either hand held augers, metal push tubes or split barrel devices. Their use is generally restricted to the top 1-2 m of soil. Augers, while useful for removing material from the uppermost part of the A horizon, suffer from problems of 'down-hole sloughing'. (That is, material from around the periphery of the hole already drilled falling into the auger as it attempts to dig further). Thus an auger with barrel 10 cm long may progress 10 cm into the soil with the first entry. The second entry may appear to remove an equal volume of soil but the hole may progress only another 2 cm into the ground. The problem is worst in the case of dry sandy soils and least noticeable in the case of wet clay soils. Metal push tubes and split barrel devices are of more use in sampling to relatively shallow depths.

Sampling of soils for volatile components presents a particular problem and a split barrel apparatus may be used for this type of investigation. Three inserts of equal length are placed end to end within the spilt barrel sampler, the apparatus closed and forced into the ground. Once the sample has been taken, the sampler is withdrawn, opened and the three inserts removed. They are immediately capped and forwarded to the laboratory for analysis.

All of the above methods are restricted to sampling in the A and upper portions of the B horizon. They are adequate for the majority of sub-surface investigations which will be required on contaminated sites., There may however be occasion to penetrate further, particularly in cases where surface contamination results from a buried waste or where remediation of the upper horizons may be affected by buried contaminants.

3.5.3 Subsurface Coring with Heavy Equipment

Sampling of this kind usually requires truck-mounted or sled-mounted boring rigs. They are used for penetrations greater than 1-2 m. Included amongst these units are:

(a) Solid and hollow stem boring. Both methods are used to access deep soil for sampling by some other means. Solid stem augers afford some control with soil

254

which is somewhat tight and saturated. The auger is subsequently removed and the sampling device (usually a tube type or split barrel sampler) forced further into the soil to obtain the specimen. A hollow stem auger has the advantage that once the desired depth is reached, the sampling device may be passed through the hollow core of the drill, the latter device acting effectively as a casing which prevents 'down-hole sloughing'.

(b) Bucket-type Auguring. This technique is used for boring through heterogeneous material (eg. a former landfill site). An examination of the sub-stratum can then be made, layer by layer. as the device penetrates. Samples may be collected of each layer brought to the surface.

(c) Mud rotary drilling. This method does have some application in the field of subsurface soil sampling, provided the composition of the drilling mud is known and it does not cause any chemical change in the soil particles brought to the surface. The method is particularly valuable at contaminated sites where noxious or flammable fumes are likely to be evolved from the substratum.

(d) Air rotary drilling. This method is only of use when the investigator is certain that there will be no volatile constituents of any interest to the study in the soil samples. Because there is considerable mixing of samples as well as the possibility for sloughing during the sampling operation, care should be taken in the interpretation of results.

3.6 Interfacing with the Laboratory

In order to maximize the efficiency of sampling and expedite the gathering of data, it is important that liaison be maintained with the chosen analytical laboratory. It is recommended that only laboratories holding current NATA certification for the parameters in question be used. The sampler should ascertain at what rate the laboratory is capable of processing samples and adjust the sampling program accordingly. In this direction, it is important, at the outset, to inform the laboratory what detection limits are sought. This will, in turn be dictated by the purpose of the sampling exercise. If it is desired to sample soils for compliance or non-compliance with criteria set as regards contaminated soil, then detection limits are relatively high and the laboratory may be able to utilize quicker, less accurate procedures. If however, an accurate costing of some expensive cleanup procedure for the contaminated soil is required, then a more time-consuming analytical technique may have to be employed.

It is also important that the laboratory receive adequate notification as to the arrival of the .samples. Each sample should be unambiguously numbered and be adequately described on a submission form which accompanies or precedes the samples. The submission form should state.

(a) the name of the sampler (b) the date of sampling (c) the place sampled (d) the type of sample (grab/composite)

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(e) whether the sample is intended for legal purposes (f) whether the sample is urgent or not (g) the identity of the sample (h) the analytes requested (i) whether any analyte is suspected of being of high concentration U) any factor which the sampler believes may have a bearing on the results.

3.7 Types of Analysis

Analysis of contaminated sites may involve a number of matrices:-

(a) soil (b) soil water (c) groundwater (d) leachate generated from contaminated soil in situ (e) leachate artificially produced in the laboratory from contaminated soil.

Whilst the finish in each case is chosen from the same range of techniques, sample pre­treatment varies from one matrix to another. Moreover, it is particularly important to appreciate what information is being sought before embarking upon an analytical program. Take, for example, a site contaminated with heavy metals. Soil (or other matrices) could be analysed for heavy metal content by some technique such as XRF, or the soil could be extracted by some suitable reagent and the resulting solution analysed by a technique such as atomic absorption. (M). or inductively coupled plasma spectrometry (ICP). It is most unlikely that . the results so obtained would be equivalent to those from analysis for total heavy metal content. Equally, it is important to consider the relevance of the two types of results in the context of the study. Total heavy metals indicates the amount which would be available under a worst case scenario. It is highly unlikely that such a situation would prevail and an extractable heavy metal value could be expected to provide a more accurate assessment of the environmentally significant content of the element. The problem which arises is the choice of extractant, since the biological effects of many contaminants are ill understood.

It must be remembered that· once the sample has been removed from its parent matrix, analysis has effectively commenced. The next step in the analytical process is transport of the sample to the laboratory. Table 2 (based on AS2031.1 - 1986), lists conditions for sample storage an<;i preservation. The list pertains to liquid samples and some of the conditions of AS2031.1 have been relaxed in view of the relatively high concentrations of contaminants which are of interest in these studies. Soil samples' generally do not require the same degree of care as do liquid samples, because of the immiscible nature of the matrix. Nevertheless, problems can arise with volatile samples. If transported even in a sealed bag, a significant loss of volatile components from a contaminated soil can occur.

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Table 2 Preservation conditions for contaminated water streams.

DETERMINAND CONTAINER NORMAL VOLUME PRESERVATION MAXIMUM REQUffiED FOR A PROCEDURE HOLDING SINGLE SAMPLE PERIOD

Metals Acid-washed 200mL Acidify 28 days plus Arsenic polyethylene with concentrated

nitric acid to pH 1-2

Metal or Polyethylene 200 mL Freeze upon 24 hours Arsenic collection Speciation

Chloride Polyethylene 100 mL None required 6 months or glass

Fluoride Polyethylene SoomL None required 28 days

Phenolics Glass 1000 mL Acidify to pH 4 21 days with orthophosphoric acid. A<;ld 19 copper sUlfaI2 per litre.

Trace Solvent washed 1000 mL None required 24· hours Organics glass with teflon Store at 4°C

lined-lids. Solid phase extraction (SPE) may be used as an alternative.

Total Glass 100 mL Store at 4°C Analyse Organic within Carbon (TOC) 6 hours

Cyanide Polyethylene SOO mL Add sodium or glass Hydroxide

solution to p~12

On anival at the laboratory, some form of sample preparation must take place. In the experience of the GCL, non-uniform preparation is the single most significant cause of failure to repeat analysis between two laboratories. The procedure used for soils is taken from AS1289 - 1976 and samples are air dried at a temperature not exceeding 50°C. They are subsequently ground in a tungsten carbide mill prior to extraction. As noted above, there are a number of options available as regards the extraction phase of soil analysis. Inorganic analysis of contaminated sites usually involves heavy metal determination. A considerable variety of procedures is available for extraction of contaminated soils, prior

257

to determination of the heavy metal content by AA or ICP (Olszowy, 1992). The two most satisfactory appear to be US EPA 3050 (a nitric/hydrochloric/peroxide method) and US EPA 200.2 (an aqua regia digest).

One should be aware of artifacts caused by the extraction and preparation procedures. For example, samples ground in a tungsten carbide mill will be contaminated with cobalt, amongst other elements.

Table 3 shows the results of arsenic determination on standard soils using US EPA 3050 digests. Recoveries of arsenic in all cases are less than 100%, indicating some fraction of arsenic is not satisfactorily extracted by the procedure.

Table 3 Arsenic level in standard soils. +

SRM CERTIFIED US EPA 3050 VALUE

NBS 2604 23 21 Accuracy (%) -9

NBS 4355 94 77 Accuracy (%) -18

,;.~~

GXR-l 427 380 Accuracy (%) -11

GXR-2 25 22 Accuracy (%) -12

GXR-3 3970 3730 Accuracy (%) -6

+ data from Olszowy 1992.

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Table 4 Techniques of Instrumental Inorganic Analysis

Method

X Ray Fluorescence Spectrometry

Inductively Coupled Plasma Spectrometry

Atomic Absorption Spectrometry

Anodic Stripping Voltammetry

Ion Chromatography

Reporting Limit

1-10 mg kg-1

0.1 - 50 mg kg-1

1-10 mg kg-1 (flame) 0.01 - 1 mg kg-1 (carbon rod) 0.01 - 1 mg kg-1

(hydride generation)

0.1-100 mg kg-1

10-100 mg kg-1

(detection limits vary from element to element)

Details

Best for analysis of solid samples. Capable also of semiquantitative analysis. Accurate within ±5%.

Best for analysis of liquid samples. Can analyse a variety of elements simultaneously.

Best for analysis of liquid samples. Can do only one element at a time. Available in flame/carbon rod/cold vapour or hydride generated forms.

Used in speciation studies. Prone to interference by overlapping elements.

Mostly used for anion separation, but also increasing use with cation separation. Problems can be encountered with over-lapping elements.

Semivolatile organic components of soils are extracted by the use of suitable solvents. Generally, total organics rather than available organics are studied and the emphasis is thus upon fairly exhaustive extraction procedures. The usual method of extraction is a dichloromethane based procedure described in US EPA method No 3550.

More recently, supercritical fluid extraction has been used in place of the classical liquid extraction techniques. (Hawthorne, 1990). The technique has the following advantages over the more commonly used procedure (a) Only small volumes of solvent are used (b) The use of chlorinated organic solvents is avoided (c) The process is relatively quick, only approximately an hour being required per extraction (d) Because the process takes place at low temperatures, loss of labile components is avoided (e) The process can be automated, unlike the US EPA method which is labour intensive, particularly in the concentration step. Volatile organic components of soils are extracted by purge and trap techniques.

Table 4 lists instrumental procedures for determination of inorganic components whilst Table 5 lists instrumental procedures for organic analysis of soils.

259

Table 5 Instrumental organic analysis.

(a) Gas Chromatography: Requires a volatile sample or one capable of forming volatile derivatives. May be fitted with a variety of detectors. Quality of results depends on type of detector, column, carrier gas flow rate etc. Results based upon retention times alone are equivocal. Samples may require considerable workup prior to analysis. Positive identification of components is provided by units interfaced with mass spectrometers or Fourier transform infrared spectrometers.

(b) High Performance Liquid Chromatography: Can handle liquid samples and instrument may be fitted with a variety of detectors. DitTerent columns may be fitted, to allow separation by reverse phase, ion exchange, gel permeation or other forms of chromatography. With a given column, results obtained depend on type of detector, flow rate, solvent used etc. Results based on elution times are equivocal. For positive identification, units interfaced with mass spectrometers or diode array detectors are available.

(c) Fourier Transform Infrared Spectrometry: Of use for identification of relatively pure compounds. First generation instruments could determine composition only of pure substances, but second generation instruments otTer subtraction capability, allowing study of solutions, mixtures.

(d) Photoionization Detectors (not interfaced with a gas chromatograph): Useful for detecting vapours of a single substance whose composition is known and to which the detector will respond (e.g. they could be used for studying chloroform contaminated soil). As they have no resolving power, they are of no use with mixtures and hence could not be used to study soil contaminated with say chloroform and hexane.

3.7.1 Analysis of Soil Water, Groundwater and Leachates.

The methodology for sampling these three matrices is beyond the scope of this paper. Soil water is taken as refening to the water existing naturally within the upper horizons of the zone of aeration and groundwater to the water within the zone of saturation (Longwell and Flint, 1962). Leachate refers to any stream of water produced by passage of natural water through a contaminated zone. At or near contaminated sites, the soil and groundwater are often affected by leachates. Further details on sampling groundwater are available in an AWRC publication. (Water Resources Management Committee, 1991).

Under normal situations, samples are preserved and transported to the laboratory in accordance with AS2031.1 - 1986. Sampling of organic components in water presents a particular problem because of the large volumes of sample which must be transported to

260

the laboratory, the fragility of the prescribed containers, the need for refrigeration and the relatively short time period allowed. For this reason, an alternative method of sampling is desirable. Adsorption of organics onto various solid phases with subsequent elution at the laboratory is an option which satisfies the above criteria. (Junk, 1987). SeeTable 6.

Table 6 Features of solid phase adsorbents

ADSORBENT

Activated Carbon

Styrene divinylbenzene polymers

Diphenylphenylene oxide

CI8 bonded silicas

PROBLEMS

Irreversible adsorption . resulting in poor recoveries with some species (e.g. chlorinated hydrocarbons); heterogeneous nature of activated carbons.

Less satisfactory for soluble hydrophilic compounds; problems with trace contaminants in resin requiring extensive cleaning.

DESORPTlON STEP

Solvent washing.

Solvent washing; thermal desorption in specific cases.

Low recoveries··for phenols; Thermal desorption. losses during drying of adsorbent.

Low recoveries of phenols and certain other species particularly at high sample pH.

Solvent washing; supercritical fluid extraction.

Although the technique is generally used for sampling of organics in water, there is no reason why it cannot be used (given a suitable adsorbent) for sampling of inorganics. Such an approach.has been proven satisfactory using 8-hydroxyquinoline in a fluidised-bed (rather than a packed bed) apparatus. (Hofstraat et aZ, 1991.)

Although most studies of sub-surface water movement pertain to downward percolation of water, there is also an upward movement through evapotranspiration. Thus, there is a pathway for movement of contaminants upwards as well as downwards. Plants can readily take up contaminants (particularly heavy metals from soils) and accumulate them in their roots or foliage. (Thornton, 1986). Interactions exist and plants growing on high levels of copper are capable of enhanced cadmium uptake. Thus, toxic levels of cadmium have been found in the leaves of lettuce growing on high copper soils, even though the soils

261

contained an acceptable concentration of cadmium. (Sadler et aI, 1990). In arid areas where the soil is well below field capacity most of the time, evaporation water moves to the surface, canying with it dissolved species from buried contaminants. (Smith and Bell, 1986; Sadler et al, 1990).

3.7.2 Leachate ArtijiciaUy Produced in the Laboratory

From the foregoing section, it is obvious that leachates constitute an important aspect of contaminated site investigation. Indeed, they may well be the most immediate item of concern regarding the site. It follows that some method of simulating the leaching process would provide valuable data in investigation of contaminated sites. For this reason, there has been considerable interest in laboratory methods which predict the leaching process. Two types of leaching test have been devised - batch and column. In the former, a known ratio of contap'linated soil ,and extractant are agitated for' a set period of time. The leachate is then separated from the parent material and analysed for the desired components. In the latter, a column of the contaminated material is irrigated with the, leaching solution and the concentration of the contaminant monitored in the effluent. Both methods have ,their advantages and disadvantages. (Lowenbach et al, 1977; Jackson et aI, 1984). Basically, batch procedures are easier to conduct, are more reproducible, lend themselves better to studies on impermeable media and do not suffer compressibility problems. Their disadvantages are that they are hard to interpret in terms of the real-life situation and do not take account of prolonged leaching of wastes. The results obtained from batch leach procedures, cannot be readily interpreted as steady state or rate related data. ;>!J~~;

Column tests if properly designed provide data representative of the field situation and can be interpreted in terms of the rate of pollutant release to the environment. The test is relatively inexpensive as it uses commonly available pieces of equipment, allows for recirculation of the eluate and can be used in attenuation stUdies of soils. Disadvantages are that column flow rates are often slow and may even .decrease as a result of compression of material during the course of the experiment.! (These problems may be at least partially solved by mixing the contaminated soil with sand). Unless the column is very carefullyrpacked, channelling of the eluent may occur1( giving an under-estimate of the actual leacfring potential. The emphasis with these tests j must be on proper design if any approxima:tiort of field conditions is to be obtained and the abovementioned factors explain why cqlumn studies often suffer from poor reproducibility.

. .~

US EPA'methM 625/$9/022 (US Federal Register, 1989) de$,cribes a variety of leach test procedures, forl stabilistxI/solidified wastes. Of the procedur~s described, all but two are batch proceduJts. : Mdst importantly however~ the methods ~e not designed for use with contaminated .~oil~ and the most commonly used (the TdLP 'prOCedure) uses various acetate buffers\as extractants. Whilst this is a suitable concept for testing materials placed in a landfill, it has little relationship to contaminated sites. : This nas not prevented the blind adoption of thi~ technique for testing contaminated \soiIs. :Obviously, soils are generally leached by tainwater, which has a higher pH thad the acetate buffers used in these tests. Not only will this over-estimate leachibility of most' heavy metals, but it will under-estimate: the loss of arsenic which decreases with pH. Column leaching tests appropriate for'contaminated soil are prescribed by some countries (Bauw et al, 1991).

262

Inorganic analysis of liquid samples involves generally similar procedures to those described for soil. X-ray fluorescence spectrometry is generally not used as much because of the necessity to dry samples prior to analysis. Organic analysis also makes use of a similar range of equipment. Ionic constituents of soilwaters, groundwater and leachates lend themselves to separation and determination by capillary electrophoresis, which is becoming an important tool in investigatory work.

3.7.3 Quality Assurance Procedures

The entire program of analysis must be accompanied by a suitable quality assurance program. Procedures required for this type of work generally fall into three categories:-

(a) Ensuring proper calibration of instruments.

This phase is generally satisfactorily carried out by most laboratories as regards checking response of instruments to standards. Unfortunately the amount of work carried on by many laboratories goes little further than this. It is relatively simple for an instrument to respond satisfactorily to a standard of a pure compound and only gross flaws in the equipment will prevent a linear response over a suitable range. Effectively, this is only checking the detection system. All analytical chemistry depends upon two phases though -viz separation and detection. A gas chromatograph may give proportional peaks for increasing amounts of DDT, but it is also necessary to ensure that the DDT is adequately separated from other organochlorines which could be expected to be present in the sample. Similarly, arsenic can be determined by XRF but there. is partial interference from lead which has a line in the same region of the spectrum. These two situations provide examples of the two different approaches which have to be used by analytical chemists in order to provide proper calibration of equipment. In the first (gas chromatograph), a suitable column plus optimal conditions of oven temperature, carrier gas flow rate etc must be employed in order to achieve satisfactory resolution. In the second (XRF) , because no simple manipulation of the apparatus will remove the lead line from the arsenic region, a correction factor has to be applied to the determination.

(b) Recovery of spiked samples and external standards~

Satisfactory response of detectors and separation of. components by the instrument constitutes a check on the finish stage of the analysis. As no environmental analysis likely to be associated with contaminated site investigation involves direct introduction of a sample into the instrument, some check is required of the entire process. This is generally provided by adding a standard at the extraction step and checking for recovery at the end of the procedure. There are two approaches to this technique. It may be desired to check that recovery of a family of compounds e.g. phenols is satisfactory. In this case, a surrogate standard is used in all extractions. This standard would be a compound which is not found in the sample but has similar molecular properties. Thus, say m-cresol was absent from the sample, it would make an ideal surrogate standard. On the other hand, it may be desired to check for quantitative recovery of a particular phenol, say 2,4,6-trichlorophenol. In this case, say that the method suggests that the sample contains x mg kg-1 of 2,4,6-trichlorophenol. Then a known quantity of 2,4,6-trichlorophenol (equivalent say to y mg kg-1 in the original sample) is added at the extraction phase. The final

263

concentration should, within limits of experimental error be x+y mg kg-I. During initial workup of the technique, pure samples of various phenols may be carried through part of the procedure, but note the importance of including matrix extraction as described above for proper evaluation of the method.

(c) Extraction of standard (certified) reference materials.

With many types of analysis, even addition of standards as described in the previous section, does not provide conclusive proof of a method's viability as regards the species in question. Say that a water sample is being analysed for cadmium content. The sample could be spiked with cadmium nitrate, with a near to 100% recovery. This may not be of any relevance to the study unless it can be proven that the cadmium was in this form in the original sample. It is generally more reassuring to use a standard (certified) reference material (SRMOr CRM) - i.e. a sample of natural soil or water for which a mean value for the substance(s) in question has been established through consensus analysis. A wide variety of SRM's is available for soil and water and a laboratory should regularly check its technique by extraction and analysis of these. The results are most conveniently plotted on charts showing a mean value with a range of one standard deviation above and below the mean.

There are several miscellaneous aspects of quality assurance procedures. For example, it would be desirable to perform each analysis in duplicate or even triplicate. Most laboratories however do not have this capacity and the best that can be done is routine replication of every nth sawple where n ":f 10. Other useful aspects of quality control procedures include checking replication of work between more than one operator, verification of results by more than one equivalent method and participation in properly designed interlaboratory test programs. Guidelines for establishment of such programs are laid down in AS 2850-1986. It is important to note that sampling comprises part of the analysis and quality assurance procedures should be employed, particularly with procedures such as solid phase extraction. These are detailed in a draft Australian Standard.

3.7.4 Units for Results

Soil results are generally expressed as mass of determinand per unit mass of sample. Thus results appear as mg kg-lor J..lg kg-I. It will be seen that these are equivalent to parts per million (ppm) and parts per billion (ppb) respectively. Occasionally, when contamination is very high (> 10,000 mg kg,I), the results may be expressed as % mass of sample.

Results for water samples are usually expressed as mass of determinand per unit volume of sample. Thus, results appear as mg L\ J..lg L-I or ng L-I, It will be seen that these are equivalent to parts per million (ppm), parts per billion (ppb) or parts per trillion (ppt). Sometimes, particularly when it is necessary to determine (for treatment purposes) the ionic composition in terms of equivalents, the mass of the constituent ion is divided by its equivalent weight. The results appear as milliequivalents per litre (expressed as m-equiv L-1). This form. of expression is usually restricted to milliequivalents, as only major anions and cations are of interest.

264

Leachate results are usually expressed as concentration of the determinand appearing in the solution, generated under specified conditions. Occasionally, they may be as % of the total mass available for leaching.

3.8 Disposal of Contaminated Material

There are basically two types of remediation. In one, the contaminant is remediated in the soil matrix where it occurs. In the other type, the contaminants are separated from the soil and the extracted contaminants subjected to further treatment, or dumped.

Thus soil may be treated in situ, excavated and removed, or the contaminant extracted and destroyed; Complete discussion of the methodology required is given by Ropper, 1990 (general techniques); Freeman, 1989 (incineration, pyrolysis vitrification and molten salt); Yu, 1990 (supercritical fluid extraction); Mueller et aI, 1989 (biodegradation of PAR's); Sims et aI, 1986 (in situ treatment techniques) etc. From the above it is obvious that two factors have a significant bearing on the success of process: Firstly, it is absolutely necessary to have an accurate estimate of the level of contamination. This means analytical data regarding the site should be verified tltrough referee analysis by another laboratory, which holds NATA accreditation for the determination required. Inappropriate remediation techniques have been employed in certain instances as a result of incorrect analysis. Apart from wasting large sums of money, use of incorrect remediation methodology has the capacity to increase the volume of contaminated material, particularly if soil dilution is chosen as an option. Secondly, once the operation is completed, there must< be a second audit of the site. This audit must take int8iaccount the intrinsic levels of contaminants in the soil as well as the stability of any fixation product formed.

A similar array of techniques is available for treatment of contaminated water from such sites with the obvious exceptions of in situ vitrification and washing/flushing. It is particularly important, as regards the final monitoring where gas stripping has been employed, to distinguish between ultimate success of the operation and mere perturbation of the equilibrium. As regards many supposed aquifer remediations by this technique, the latter would appear to have been the case (Travis and Doty, 1990).

4 CONCLUSION

In conclusion, it must be said that unlike other areas of environmental science, where an input from other disciplines (e.g. physics, civil engineering etc) must form part of an integrated approach, the investigation of contaminated sites hinges almost exclusively upon analytical chemistry. Analytical chemistry governs not only the investigation but also provides the raison d' etre for determining what constitutes an acceptable background concentration for the determinand. Indeed, the need to investigate many contaminated sites remains unclear because there is a general lack of information as regards background levels of contaminants in soil. In terms of substances which can only contaminate sites as a result of anthropogenic input, this value would obviously be zero. With other contaminants (e.g. heavy metals), the situation is not as clear cut. Background levels must be clearly distinguished from levels of concern. Frequently the former are well below the latter, but the reverse may also be true. A region heavily mineralised with uranium for example would have that element present in soil at a concentration far above the level of

265

concern. If analytical chemistry is to provide the cornerstone it should for investigation of contaminated sites, then a definition of background levels for substances such as heavy metals in soils is an absolute priority (Sadler and Shaw, 1991). The Government Chemical Laboratory is currently embarking upon the first stage of such a sUlVey in Queensland.

5 REFERENCES

Australia and New Zealand Environment and Conservation Council and National Health and Medical Research Council (1992). National guidelines for assessment and management of contaminated land, ANZEC, Canberra.

Australian Water Resources Council (1991). A preliminary guide to the standard operatipg procedures for sampling contaminated groundwater, Occasional paper WRMC No 2, Water resources management committee, AWRC.

Braun, D.H., De Wilde, P.G.M., Rood, G.A. and Aalbers, T.H.G (1991). A standard leaching test, including solid phase extraction, for the determination of P AH ·leachibility from waste materials. Chemosphere 22:713-722.

Chern Unit (1992). A guide to the contaminated land act. (GoprintBrisbane.)

Chern Unit (1992). Guidelines for the assessment of contaminated land in Queensland. (Goprint:Brisbane.)

El Saadi, O. and Langley, A. (1991). The health risk and assessment of contaminated sites: Proceedings of a national workshop in the health risk assessment of contaminated sites. (South Australian Health Commission:Adelaide.)

Freeman, H. (1985). Innovative thermal hazardous org~ waste treatment processes. (Noyes Publications, Park Ridge.)

Hawthorne, S.B. (1990). Analytical-scale supercritical fluid extraction. Analytical . ChemiStry 62:633A-642A.

Hofstraat, J.W., Tleltooij, J.A., Compaan, H. and Mulder, W.H. (1991). Fluidized-bed solid-ppase extraction: A novel approach to time-integrated sampling of trace metals:m surface water. Environmental Science and Technology 25: 1722-1727.

Hopper, D.R. (1989). Cleaning up contaminated waste ,tes; Chemical Engineering 96:94-110.

Jackson, D.R.; Garrett, B.C. and Bishop, T.A. (1984). Comparison of batch and column methods for assessing leachability of hazardous waste. Environmental Science and Technowgy 18:668-673 .

. ~

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Junk, G.A. (1987). Synthetic polymers for accumulating organic compounds from water. In 'Organic Pollutants in Water' (Eds I H Suffet and M Malaiyandi.) pp 201-246. (American ChemicaISociety:Washington, USA.)

Longwell, C.R. and Flint, R:F. (1962). Introduction to physical geology. (John Wiley and Sons:New York.)

Lowenbach, W., Ellerbusch, F., King, J.A. and Cheremisinoff, P.N. (1977). Leachate . testing techniques surVeyed. Water and Sewerage Works 124:36-46:

:,

Meuller, J.G., Chapman, P.1,. and Pritchard, P.N. (1989). Creosote contaminated sites. Their potential for bioremediation. Environmental Science and Technology 23: 1197-1201.

Olszowy, H. (1992). Inorganics in soils. In 'Joint seminar/workshop on contaminated sites'. (Australian Water and Wastewater Association and Environmental Chemistry Division of Royal Australian Chemical Institute:Brisbane, Australia.)

Parkhurst, D.F. (1984). Optimal sampling geometry for hazardous waste sites. Environmental Science and Technology 18:521-523.

Paustenbach, D.J. (1989). The risk assessment of environmental hazards: A textbook of case studies. (Wiley Interscience:New York.)

Queensland contaminated land act (1991). (GoprintBrisbane.)

Queensland contaminated land regulations (1991). (GoprintBrisbane.)

Queensland local government (planning and environment) act (1990). (Goprint:Brisbane.)

Sadler, R. (1992). Sampling of contaminated soils. In 'Joint seminar/workshop on contaminated sites'. (Australian Water and Wastewater Association and Environmental Chemistry Division of Royal Australian Chemical Institute:Brisbane, Australia.)

Sadler, R and Shaw, G. (1991). Contaminated sites: A chemical perspective. Chemistry in Australia 58:452-455.

Sadler, R, Olszowy, H., Shaw, G. and Neville, G. (1990). Report on contaminated sites at Mt Isa. GCL Report Series No 5, Government Chemical Laboratory, Brisbane.

Sims, R, Sorensen, D., Sims, J., McLean, J., Mahmood, R., Jurinak, J. and Wagner, K. (1986). Contaminated surface soils in-place treatment techniques. (Noyes Publications:New Jersey.)

267

Smith, M.A. and Bell, R.M. (1986). Upward movement of metals in to soil covering metalliferous waste. In 'Contaminated Soil' (Eds J WAssink and W J van den Brink). pp 133-135. (Martinus Nijhoff:Dordrecht, Holland.)

Australian Standard. Selection of containers and preservation of water samples for chemical and microbiological analysis. Part I Chemical, AS 203 1. 1-1986. Standards Association of Australia, Sydney.

Australian Standard. Preparation of disturbed soil samples for testing. AS1289.1-1991. Standards Australia, Sydney.

Australian Standard. Chemical analysis - interlaboratory test programs - For determining precision of analytical methodes) - Guide to the planning and conduct. AS2850-1986. Standards Australia, Sydney.

Thornton, I. (1986). Metal contamination of soils in the UK Urban gardens:Implications to· health. In 'Contaminated Soil' (Eds J WAssink and W J van den Brink). pp 203-209. (Martinus Nijhoff:Dordrecht, Holland.)

Travis, c.c. and Doty, C.B. (1990). Can contaminated aquifers at superfund sites be remediated? Environmental Science and Technology 24:1464-1466.

U.S Environmental Protection Agency (1987). Test methods for evaluating solid wastes. EPA SW846, Method 3050. (U S Government Printing Office:Washington.)

U.S Environmental Protection Agency (1987). Sample preparation procedure for spectrochemical analysis of total recoverable elements. Methos 200.2. (U S Government Printing Office: Washington.)

U.S Environmental Protection Agency (1986). Sonication extraction. Method 3550. (U S Government Printing Office: Washington.)

U.S Federal Register (1988). Part 268 - Land disposal restrictions, U S Federal Register 53: 18792-19797.

Yu, X., Wang, X., Bartha, R. and Rosen, J.D. (1990). Supercritical fluid extraction of coal tar contaminated soil. Environmental Science and Technology 24:1732-1738.

268

HYDROLOGY AND CHEMISTRY OF LANDFILLS

W. E. Razzell Scientific Services Branch

Department of Water Supply and Sewerage Brisbane City Council

ABSTRACT

A description is given of the major considerations in the design and operation of conventional, modem landfills suitable for domestic and commercial refuse. The coverage is not adequate for engineering design, but does illustrate the principles involved. Emphasis is placed on the microbiological nature of the waste stabilisation process; on gas formation, composition and recovery; leachate control and recycling; protection from the ingress of rainwater; and monitoring requirements. Comments are added on the next generation of landfills: secure landfills requiring multiple barrier design, with surrounding environmental monitoring.

1 INTRODUCTION

The substance of this topic can be covered under the following headings:

Premises

Purposes and Principles Hydrology Leachate Recovery and Reinjection Microbiology - aerobic and anaerobic processes Organic and Inorganic Chemistry Gas Production/Composition Monitoring Codisposal Concepts Sanitary vs Secure Landfills

It helps to keep the topic manageable if one starts simply and adds complications later. Therefore let us conceive of a simple situation in which a piece of land is available on which to build a landfill for municipal/domestic refuse. And let's assume the geology provides a generous clay pan under the proposed site. And let's assume average Australian weather conditions: no worries about snow and pipes freezing; no translation from regular, predictable rainfalls to real Australian chaos.

The situations created by adding complications to this simple scenario can then be dealt with individually - and endlessly.

Finally, this is not an engineering manual; to construct and operate a landfill properly, a qualified civil engineer must be employed. This article is an outline of the considerations applicable to landfills in generaL

269

2 PURPOSES AND PRINCIPLES

There are two fundamental considerations which apply to planning a landfill intended to receive putrescible (domestic, municipal) wastes: it can either take the maximum amount of waste per unit land area, or it can take a shorter time to achieve a stable condition, allowing the reuse of the land for a specific range of purposes. It is not possible to achieve both objectives in one landfill.

Thus, heavily compacted landfills having some 500kg/m3 and 80-90 thousand· tonnes/ha emplaced will become anaerobic in about 8 years, and require some 15-20 years to become stabilised to the point that methane generation and leachate concentrations are acceptably low; even then, there will be considerable variation between landfills - some taking a decade longer to stabilise.

Less-compacted landfills, having less than 300kg/m3 and 12 thousand tonnes/ha emplaced, and employing leachate reinjection will remain aerobic for 2-3 years, producing strong leachate from day one, and then go through a brief, 3-year anaerobic period quickly followed by stabilisation by the end of year 5-6.

Thus, the requirements of the community need taken into account if land is cheap and a long period of methane generation is desired, the big, dense landfill is the right one. However, it is evident that a low-density landfill will occupy more space for a short period; it is impossible to have a compact landfill stabilise quickly

3 HYDROLOGY

3.1 The Basic Case

Under ideal conditions, a landfill would shed water like a duck, both during periods of waste deposition and afterwards. There is enough water in waste, both as a constituent of the waste and as a contaminant of it, to satisfy the needs of the degradation processes.

Added water is merely a problem to be addressed. A properly-designed landfill does not allow ingress of rainwater, nor does it allow egress of leachate laterally; any water entering the landfill must arrive as rain or as backup in the leachate control system. Since the latter is both undesirable and controllable, we are left with the events following rainfalls. In Australia particularly, that is not trivial: it is possible to have 20cm of rain in a day, which both soaks into the landfill and causes scouring to increase the rate of penetration: the surface of a soil-covered landfill is moderately permeable unless drainage is provided to allow rain to escape (Figure 1).

270

Vegetative Layer

Drainage Layer

Low Permeability' Layer

FML-' __ _

(~ 20 mils in thickness)

Compacted Soil Layer

~ 24"

1 ~~~~"~~~~~~~~~~7=~~

Figure 1 Drainage Provision for Surface Water ",~;·,tg~~. '

[ Functions I Vegetation or Other

Erosion Control Material at and Above Surface

Top Soil for Root Growth

Remove Infiltrating Water

\

Increases Efficiency of Drainage Layer and

Minimizes Infiltration into Unit

The course of water within the landfill will not be simple, either physically nor in timescale. , '

Physically,the water would ideally take the shortest route to the leachate coll~ction system; practically it doesn't (Figure 2). In timescale, the rate of migration of water would be predictable; practically, it isn't.

Dyed water

-----------~ .. ----.. --.------.. ~-----------------------~~----~ ...... --------

-------.. _ ... _ .... -------

-------_ ............ ---..

Figure 2 Erratic Pathway of Water Penetration

271

The reasons are apparent from the process of landfilling: layers of heterogeneous waste are separated by layers of cover material, all of which is tilted to. the contour of the face of the working tip, and must also slope at the edges (Figure 3). Therefore, especially if the construction shown in Figure 1 is not used to exclude rain, there will be sharp discontinuities in the permeability of the mass, water travelling quickly to permeate the waste, slowly to penetrate the cover material. As a result, for prolonged periods, there will be "lenses" of waterlogged waste, and probably free water, within the landfill mass. It would be helpful if the "perched water" of these lenses were more common towards the top of the mass, but the lack of uniformity in waste and cover material properties over·time, and of the water content of the cover material because of past rain events when it was. exposed (or prolonged drying weather, for that matter), means that the water may "perch" lower in the mass than simple common sense would suggest.

3.2 The COPlplications

The question is, "so what?". If there is good design and construction, the water will eventually travel to the bottom of the mass via the collection drains (Figure 4). If however, there are any weaknesses in the mass - particularly at the sides where the hydraulic pressure gradient will be greatest, the water may exit laterally, run down and scour the side, release more perched leachate, and create a leachate escape of startling proportions - far in excess of what might seem at the time to be a reasonable expectation from a small added water loading event.

The consequence of creatltig' a lens of perched water is the practically complete exclusion of air from the waterlogged waste, leading to anaerobic reactions, with the creation of "perched leachate" of decidedly unpleasant composition. The impact of the rapid escape of this leachate would be far worse than the escape of the original rainwater. The composition of the leachate will vary depending on the age of the waste in the location where the water had perched: fresher waste would create strong leachate during fermentation, whereas older waste . - nearer the bottom of the pile - would presumably be more stabilised and therefore contribute less in a fermentation to the strength of the leachate.

As suggested,' the heterogeneity of the density through the mass means that no easy assumptions should be made about the level at which· water will perch.

272

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~ Q

~ ~ ~ Q

~ ~

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273

Leachate Collection Pipe

To Leachate Collection Sump

Figure 4 Water Course and Leachate Collection

Another complication is the impact of planning on waste behaviour: planning carefully is crucial. In particular, popular assumptions should be avoided: facts are more useful (Table I).

Table 1 (Modern Archaeology) - Volume Composition of Compacted Refuse in Landfills

Constituent Folklore Reality (USA)

Fast-food Packaging 20 - 30% 0.25% Polystyrene Foam 30 - 40% 0.90% Disposable Diapers 25 - 45% 0.80%

Sub Total 75 2 Plastics 20 - 30% 10% * Paper 15 - 20% 50% ** Newspaper 8 - 10% 10 - 15% Industrial Waste (demolitions) 10% 15%

*1972 = 1986 **Lower Figure Includes effects of recycling

274

Data in Australia are unavailable; but it would be surprising if the "Actual" figures given in Table I were not found for this country too.

Subsequent sections will deal with the processes of waste degradation in landfills; here we need merely note that aerobic processes, occuning early in the creation of the landfill, produce highly oxidised products (keto acids and small fatty acids) whereas later anaerobic reactions produce mostly methane.

The former are rapid, the latter are slow; the first phase is a matter of months, the latter phase lasts decades. Therefore, the leachate arising for most of the life of a landfill will be generated under anaerobic conditions; it will contain alcohol and fatty acid precursors of the methane-formation process.

The methane will itself be contaminated with any volatile compounds produced from, for example, biodegradation of petroleum hydrocarbons (waste oil) - which should not be in the landfill in the first place, if use of methane is an objective.

4 LEACHATE RECOVERY AND REINJECTION

It is reasonable to expect that any landfill constructed after 1990 will have leachate recovery drains installed during the buildup of the landfill, and will probably have methane recovery pipework also - if the fill is large enough. The former is to protect the environment; the latter is to gain some future income as well as protect the earth from.Q1e Greenhouse Effect.

Although the concept of collecting and returning the leachate to the landfill is widely­recognised, the construction required to achieve it is not. One can apply leachate to the landfill surface; getting it to proceed into the landfill is less assured. I am trying to find some examples of design of return systems, so far without success.

It would seem pointless to have a complete system for leachate recovery without the facility for its reinjection: why waste a good thing? If, in the case where the anticipated leachate will be dilute and/or of low volume, then its discharge to sewer (sewer being available - which is not necessarily so) is a simple answer which should represent a low cost. But there will be a cost, because the BOD at least will not be negligible.

Figure 5 "Dry" Configuration

275

In short, in any landfill operation there will be leachate and methane: the controls should be in the initial landfill design. A design along the lines of that shown in Figure 5 appears "nice" but would not do: how is leachate supposed to be applied to such a structure? The only acceptable construction along the lines illustrated would be one impervious to rain, so that there was no leachate to recycle (see Figure 1).

If there is appreciable entry of water to the landfill from rain, there may well be times when the volume of leachate exceeds the reasonable volume which could be reapplied to the landfill: new leachate + recycled leachate> leachate control capacity! There thus needs to be a sewer access and an access agreement with the sewerage authority to provide for both the physical input of leachate to the sewer system, and the legal agreement on volume, strength (BOD, suspended solids, etc) and, of course, the charges to be levied on the owner of the landfill.

Alternatively, if it can be shown that excess leachate in rainy.periods would be confined to the dilute portion, discharge to a waterway may be negotiated with the relevant authority; unlikely, perhaps.

What leachate reinjection accomplishes is both an acceleration of the stabilisation process; so that the landfill becomes "habitable" sooner, and a better conversion of carbon compounds to methane, so the gas yield is higher and the leachate correspondingly leaner.

5 MICROBIOLOGY-, AEROBIC AND ANAEROBIC

With the exception of the rusting (oxidation) of the steel cans and debris, as well as the formation of metallic sulfides by reaction of soluble metals with sulfide ion, all the changes observed in a landfill are the result of microbiological action. That is, the "chemical" reactions observed are actually biochemical reactions consequent upon the reproduction of bacteria. The three major types of bacteria are aerobic, facultative, and anaerobic.

Aerobic bacteria can only gain energy by connecting the oxidation of (electron removal from) a chemical species to the reduction of gaseous (dissolved) oxygen. Anaerobic bacteria not only cannot do that, but positively suffer from the presence of oxygen, which acts as an inhibitor of ceLl biochemistry; therefore anaerobic bacteria must transfer the electrons from a chemical being oxidised, to a chemical which becomes reduced:

alcohol(A) minus 2e ------> aldehyde(A) + useable energy aldehyde(B) plus 2e ------> alcohol(B) etc.

A facultative bacterium can use oxygen as electron acceptot, and also a very few other chemicals, suc~ as nitrate (----> nitrite), carbonate (---> formate); note than facultative bacteria cannot reduce sulfate: that is an anaerobic process.

Methane-producing bacteria are a special class of very oxygen-sensitive bacteria, which exist in association with each other, and form methane as the final product of electron transfer. The reactions :leading to methane generally proceed through fatty alcohols and acids (R.CH2.CH2.CH20H and R.CH2.CH2.COOH), creating and. utilising hydrogen gas. For example, valeric acid ----> propionic acid + CH4; propionic acid ----> acetic acid + CH4; acetic acid ----> C02 + CH4. In a diverse mixture of compourids, formic acid, methanol and C02 are also formed, some being converted to CH4 also.'

276

The energy gained by aerobic bacteria per electron pair transferred to oxygen is 10 times greater than for an electron pair transferred by anaerobic bacteria to form methane. Or, the aerobic conversion of massive amounts of carbohydrate, fat, protein, cellulose, hydrocarbons, etc., in a landfill by bacteria would create huge masses of bacteria and some byproducts (ideally C02 and water, but often fatty- and keto-acids) ; but under anaerobic conditions, leads to massive volumes of methane and not much residual bacterial mass.

Intermediate in the former reactions are many partially oxidised compounds which are relatively innocuous; intermediate in the latter are some truly obnoxious compounds - which is why landfill leachate (usually formed under strictly anaerobic conditions) is such an environmental menace.

Facultative bacteria act to confuse the picture, because they convert some of the intermediate chemicals to products not produced by the other types. They do not, however, compete well with either the aerobes or the anaerobes, and in a free situation - such as a landfill - the facultative bacteria as well as the aerobes (when they have used up all the oxygen initially trapped in the mass) disappear.

The establishment of good anaerobic conditions for methane production takes time: oxygen must be zero, nitrate, sulfate, mu.st be zero, fatty alcohols and acids must be formed - only then can the methanogenic bacteria slowly begin to take over. And that can take 10 years!

If leachate is sampled and analysed over time, a loosely-compacted landfill will show a picture as in Figure 6A; a tightly-compacted, readily-anaerobic' nuidfillwill be as in Figure 6B.

Figure 6 -A: Upper Plots (loose) 0

0_

BOOs - Fe .. ... .. I!' -- 0_ -..

0 ..

0 - ... ... ... .. ! I" • IL H

• . , .. ,. .. ·r .. .. . . M .... , .. . ... ,,. . ..

B: Lower Plots (dense)

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• IL

M

.II\.L \\ \ • i a. M ... 'r« .,. 'M M ... .,. 'M ... , .. ",."tlt.

277

6 ORGANIC AND INORGANIC CHEMISTRY

As previously indicated, there is very little chemistry in a landfill; lots of biochemistry, but a trivial amount of chemistry.

7 GAS PRODUCTION AND COMPOSmON

As shown in Figure 6, high-BOD leachate is formed after months or years of completion of the landfill (or of several lifts, if the landfill is large). Until the leachate is high in BOD, there cannot be any methane production, nor hydrogen. There will be production of C02 as the air entrapped in the landfill is used up by the aerobic and facultative bacteria, and as the available iron is oxidised. However, once the oxygen is all converted to water, reducing conditions are created by the further metabolism of the anaerobes, strong leachate becomes funned, ferric ions (in the fraction ,not trapped as sulfides) become reduced to ferrous, and thereby "iron" becomes soluble in the moderately acid (pH 6.4+) conditions - as shown in Figure 6.

As "gas", only C02 and CH4 are formed, with some hydrogen; however, volatiles including methanol, formic acid and acetic acid are formed from non-aromatic compounds generally. Aromatic compounds, such as are found in petroleum wastes, are converted also, but with appreciable residues of lower MW acids as well as aromatic solvents (dimethyl- and methylbenzene), which show up in the gas phase and upon oxidation tend to be corrosive to equipment such asenginesj< ,'"~I ,,'

About 0.35m3 of CH4 is produced for every kilogram of COD consumed (although I would rather have been told how many kg of BOD were consumed; it is possible that the number would be higher, ie., more CH4/kg BOD than 0.35m3).

The CH4 : C02 ratio is about 65:35, with some hydrogen (we have measured 56.5 + 41.3 + 0.04 in mole%). Thus, attempts to use the CH4 for fuel, etc., usually require removal of the C02 in order to save space in the CH4 containers, and some care to establish that the H2 is not too high - otherwise spectacular results can be obtained.

8 MONITORING

The first rule of monitoring for any waste stream is to decide why. There may be statutory requirements, but there can also be additional logical objectives which may assist in understanding iilorder to manage the site or process - or in understanding why the regulatory data are what they are when they're not within regulated limitS.

If the landfill is large, and constructed properly, there will be runoff which is not leachate, but merely "site" drainage; nevertheless, the innocuous nature of that must be demonstrated frequently, so its disposal may be kept simple.

Figure 6 presents cases of monitoring leachate which reveal much about the behaviour of landfills. The . problem is, once having found unsatisfactory results, it is awkward to reconstruct the landfill to rectify the situation!

278

Usually, regulatory agencies are concerned about water contaminants: BOD, suspended solids (maybe), pH, nutrients (particularly ammonia), heavy metals, salinity (or its surrogate, conductivity), and of course, the flow rate of leachate. Any responsible analytical services laboratory supervisor can prepare a monitoring plan to suit whatever objectives are identified -and also assist in identifying objectives.

During the early phase of landfill construction, there will be suspended. solids in runoff which may be beyond acceptable limits. A settling pond, in which the bulk of such solids will remain and the water will become clear enough to meet discharge limits, is an inexpensive control device. A sand seepage filter downstream of a settling pond will give even better results. For leachate, however, whether early in the development or later when the high­strength leachate of an actively fermenting mass has become the norm, a purposeful liquid . waste treatment installation will be necessary.

Monitoring a leachate treatment plant is analogous to monitoring any other: flow rates" concentrations of scheduled constituents, and other data required for optimising performance must be collected often enough to be both useful and convincing. Often, the negotiated discharge permit will nominate the minimum; it is always wiser to obtain more than the minimum data.

9 €ODISPOSAL CONCEPTS

The problem of codisposal is primarily one of credibility and consistency: out of sight must not become out of mind; success so far must not become an excuse for future laxity. What am I talking about?

As generally understood, codisposal is the addition of specified types and proportions of hazardOUs wastes with domestic/commercial refuse. Hazardous wastes often codisposed include:'

hospital wastes.

waste lubricating oil, and other petroleum products (greases, lubricants);

paints and paint thinners (solvents);

metal hydroxide sludges from the electroplating industry;

The first two groups are either bio-reactive, so that non-hazardous products are formed which can be biodegraded, or they can be biodegraded readily under appropriate conditions. Much has been made recently about hospital wastes; however, they are generally not appreciably different from domestic refuse, once the trace amounts of reactive compounds have indeed reacted - apart from the psychological aspects.

The major difficulty with medical and oily wastes is that they biodegrade much better under aerobic conditions than anaerobically - but didn't I say that about almost everything?

279

Therefore the only problem is ensuring that the leachate problems are properly addressed while the co-disposed materials are degrading.

As for the second group - the petroleum-based solvents require a period of aerobic biodegradation to avoid the deposition in the landfill of a mass analogous to the waste oils.

In contrast, the ultimate stable condition of heavy metal wastes is as metallic sulfides; for that, fermentation reactions by facultative bacteria in the presence of inorganic sulfate are essential. Thus, the mass must become depleted in gaseous oxygen, if not completely anaerobic, before adding metal hydroxide sludges.

It should thus be evident that careful, load-by-load management of a developing landfill must be conducted if codisposal is being pursued; mistakes can take years to manifest themselves, and more years to be rectified - at great cost.

A better option is the use ofa properly-sited and designed secure landfill as part of a managed system· for hazardous wastes generally.

10 SANITARY VS SECURE LANDFILLS

It should be obvious by now that a sanitary landfill is not a garbage dump.

Therefore there is only a degree of distinction between a proper sanitary landfill and a secure landfill: the latter has braces as well as the belt on the former; while a secure landfill requires a greater degree of environmental monitoring.

The standard for a secure landfill by now has become double-barrier construction (Figure 7).

COIrI!lOnents

r Protective Soil or Cover (Optional)

~;;;;;;;;==r;;::. ~ r Drain Pipe (Typ)

Leachate Collection and Removal System

Leachate Detection; Collection. and RemOVal System (LDCRS)

. -:;-;-;-:-'-::r--------------;:-::'7 .. ~;--.' ::. • 1 : Solid Waste • " .,'

.: .' .. ". \f,'" ....... c ... '

Native Soil Foundation

Figure 7 Basic Secure Landfill Design

280

Lower Component (compacted soil)'

Leachate Collection System Sump (MonItoring Compliance Point,'

The rationale for secure landfill designs is to provide a basal impermeable layer above which any leachate will be collected for removal and subsequent treatment. The drainage below the "top liner" - noted in Figure 7 as FML, or Flexible Membrane Liner - is encouraged toward a central sump for sensitive detection and for practical recovery. In practice, a Geotextile Net (much like a plastic netting used on kitchen draincounters) is placed over the FML to facilitate the flow of leachate, and a filter cloth is laid on top of the Net, to keep fines from plugging the Net mesh. Altogether not as simple as selecting a flat piece of land.

While the Figure 7 example notes the Protective Soil as being "optional", that is not considered so nowadays; it is essential to eliminate penetration of rain or floods, and any ponding, over all the exposed surface (as in Figure 1).

Furthermore bores, to a level to intersect the groundwater (well below the base of the secure landfill) are essential to monitor the environment beyond the landfill, ensuring that the fill has no detrimental effect on the adjacent land or water resources. Decades of compromise with such designs have revealed the shortcomings of all but the most stringent. We are now applying the most stringent designs to the secure landfill being built by the Brisbane City Council at Gurulmundi, in Murilla Shire. The concept is very much like that in Figure 7; but as such designs are marketable, I will not be disclosing it here.

11 READINGS

1. Waste Decomposition: mostly relating to sewage -

Principles of Water Quality Management W.W. Eckenfelder, Jr CBI Publishing Co 51 Sleeper St Boston, Mass, USA 02210

2. More detailed microbiology than related by Eckenfelder -

General Microbiology, Third Edition R.Y. Stanier, M Doudoroff, and E A Adelberg The Macmillan Press Ltd London, etc

3. What are landfills really like?

Once and Future Landfills W.L. Rathje National Geographic May, 1991

Modelling long-term dynamic behaviour M.J. Knight and G Beck Proc.lnternat. Conf. Groundwater Systems Under Stress A WRC Conf. Series, 13:475-90 (1986)

281

282

LAND DISPOSAL OF EFFLUENT FROM INTENSIVE RURAL INDUSTRIES

E.A. Gardner, M.A. Gilbert and R.J. Shaw Natural Resource Management, QDPI, Indooroopilly, Qld, 4068.

ABSTRACT

EnvironmentaUy sustainable disposal of effluent from intensive rural industries, agri­industrial processors and sewage treatment plants is one of the major chaUenges facing agriculture today. Land based disposal schemes using pastures are attractive in both a practical and ecological sense but it is important that export of nutrients and salt off site are kept to acceptably smaU values. This paper discusses the disposal of N, P and salt in particular to highlight the processes involved in treating waste water and the likely influence of external factors in long term sustainability. The mass balance approach is used to trace the fate of reactive organics, salts, N and P from the animal manure, through pre treatment lagoons to application to land and subsequent partitioning into soil, plant, atmosphere and water bodies. For P, the major sink is the soil where it is immobilised in slow and fast sorption reactions. However for N, major gaseous loss occurs due to volatilisation and denitrification during effluent pre treatment and land application. Plant biomass is the major sustainable sink for N. Similar mass balance calculations are described for the salt balance of an irrigated effluent disposal area, and for sizing wet weather storage-irrigation area combinations to ensure lagoon overtopping is an infrequent event. We conclude with the observation that a monitoring strategy is a critical part of effluent management program compared with an effluent planning exercise.

1 INTRODUCTION

During the summer of 1991/92 the largest ever recorded outbreak: of blue-green algae occurred on the Darling River, stretching over 1,000 Ian from Wilcannia to Mungindi. In a subsequent analysis of the nutrient balance of the Murray Darling River Basin, Gutteridge, Haskins and Davey (1992) identified sewage treatment plants, discharging treated effluent into the river system, as the major point source of nitrogen and phosphorus (essential requirements for algal outbreaks). In an average flow year, about 25% of the N and P inputs comes from sources such as these.

Gutteridge, Haskins and Davey (1992) also commented on the potential for intensive rural industries such as feedlots and piggeries to discharge large quantities of N and P into the river system equivalent to the production from a human population of 4-5 million. Although there was no documented evidence that these industries caused nutrient discharge (indeed feedlots are licensed as nil farm discharge operations) there is a keen community interest to ensure their land based effluent disposal practices follow the highest possible standards.

In Queensland, changes to the Clean Water Act will ensure that discharge of any effluent to water bodies will be an option of last resort. In addition to these legislative initiatives, charges by water treatment authorities are starting to reflect the true cost of treating high strength organic waste generated by those processing industries which can discharge to sewer. Charges of $2,000 up to $10,000 per megalitre are not uncommon, making relocation to rural areas with land based effluent disposal systems financially attractive.

283

Government agencies with legislative, custodial or traditional responsibilities for protecting the rural environment are concerned that problems of sub optimal effluent disposal practices are not just simply transferred from water bodies to land systems. The objective of land based disposal, as indeed for any farm management practice, is that ecological sustainability is maintained. By this we mean that the quality and integrity of the environment, human health and safety are not degraded in the short or long term. Environmentally sustainable disposal practices should be capable of being continued for many future generations or for the life of the industry or installation.

In this chapter we discuss: nutrients of concern in the environment; intensive rural industries and their waste characteristics; effluent pretreatment before land application; and water, nutrient and salt balances of effluent disposal areas, and estimation of the pasture biomass available as a sink for nutrients.

2 NUTRIENTS AND OTHER CHEMICALS OF CONCERN

Traditionally the description of the quality of effluent came from the sewage treatment industry where the concern was on the effect of effluent discharge on discolouration of the receiving water body and its oxygen consumption by organic material leading, amongst other things, to fish death. These quality aspects were paramatised by the Suspended Solid Concentration and the Biochemical Oxygen Demand, BOD5 which is the quantity of oxygen required for microbial degradation of the oxidisable organic compounds in waste water at 20°C (the standard BOD test is carried out in a laboratory for a five day incubation period (BOD5) and represents about 70-80% of the total BOD). The use of these parameters to describe effluent quality have lead to the 20/30 rule for effluent discharge to water bodies: 20 mg/L BOD and 30 mg/L SS. Another index of the amount of reactive material is the Chemical Oxygen Demand (COD) which is the quantity of oxygen required to oxidise all the organic matter and reducing compounds such as ferrous ions, sulphur compounds and nitrate (Feigin et al 1991).

Nitrogen compounds of concern are the inorganic ions ammonium (NH/) and nitrate (N03·).

Ammonium is biologically transformed by bacteria to N03 (nitrification) causing oxygen consumption in the receiving water body (and hence likely fish death). Nitrate is of major concern in potable water supplies, especially groundwater, where a concentration greater than 10 mg/L N03-N (or 45 mg/L N03) can cause methaemoglobinaemia in infants (blue baby syndrome). Elevated N03 levels in surface water bodies (>3 mg/L), when combined with elevated phosphorus levels, can cause algal blooms which can be either toxic in themselves, or which give off foul odours and consume dissolved oxygen during their subsequent decay (see the companion paper by Connell and Arthington).

Phosphorus is usually the element most limiting to algal growth in surface water bodies. Concentrations greater than 0.05 mg/L are considered to be excessive in northern hemisphere waters (Vollemweiller 1985) but with the more turbid waters in Australia, data suggests that soluble P levels must exceed 0.17 mg/L for algal blooms to occur (Donnelly et aI1992). As with nitrogen, P comes in a variety of organic and inorganic forms, but the forms which are available for biological uptake are the orthophosphate ions, H2P04·, HP042-, PO/.

A well known example of excessive N03- in potable water supplies in Australia is the Mt

284

Gambier aquifer in South Australia where excessive N levels are caused by N03 leaching from legume-grass pastures in the recharge area during winter rainfall events (Dillon 1988). In Western Australia, the Peel-Harvey inlet, located to the south of Perth, is a well known example of algal bloom caused by excessive P levels sourced by surface and groundwater export from heavily fertilised farm lands (Birch 1982). Because orthophosphates are readily sorbed by most soils, the major export pathway of P from fann lands is via its attachment on eroded soil particles. An exception is very sandy soils, typical of the south west corner of Western Australia, which allow P04 movement into the regional water tables. Similar concern for excessive levels of agriculturally sourced P on the coral ecology of the Great BaIrier Reef in north Queensland have been expressed (Baldwin 1990).

Another parameter of concern in effluent is Total Dissolved Salt (IDS) which can cause soil salinisation and hence reduced plant growth on the effluent disposal area itself, or contribute to salinity increases in adjacent streams via leaching and thence groundwater flow. This latter process is of particular relevance in seasonally flowing rivers where a constant salt input can increase river salinity beyond the recommended level for drinking of 1,000 - 1,500 mg/L IDS (ANZECC 1992) during low flow conditions.

IDS is often described by the more easily measured Electrical Conductivity, (EC), which has units of deci Siemens/metre. The conversion between TDS and EC depends on the type of dissolved salts (e.g. Na, Ca, Mg, K, Cl, S04' HC03) but a good working rule is

EC (dS/m) * 640 = TDS (mg/L) (1)

Pathogens are also of concern in effluent disposal to land - apart from the often large number of faecal coliform and streptococcal bacteria, there can be unacceptably large concentrations of enteroviruses, protozoans, nematodes and tapeworms, depending on the source and pre treatment of the effluent. The companion chapter by Beavers discusses in detail pathogen composition from treated sewage.

Heavy metals (such as cadmium, arsenic, chromium) and hazardous and/or toxic organic material such as PCBs, pesticides etc can also occur in effluent and these are discussed in a companion chapter by Rayment and Barry. However it is important to stress that with the exception of treated sewage effluent, and sewage sludge in particular, the composition of effluent from intensive livestock production and associated agri-industrial processing is largely free of these trace contaminants. The land based disposal challenge is to ensure that the water, nutrient and salt balances are maintained and animal and human health aspects are protected.

3 WHO ARE THE EFFLUENT PRODUCERS

The major producers of effluent from intensive livestock production are beef and dairy feedlots, piggeries and poultry sheds. Agri industrial processors cover a wider range and include abattoirs, wool scourings, hide preservation and tanneries, distilleries, yeast and starch manufacturers, food processors, pulp manufacturers and sewage treatment plants (STP). The latter is included in the list because of the essentially biological nature of its liquid waste (if the chemical contaminants are concentrated in the sludge).

285

3.1 Emuen! volumes and compositions

In designing environmentally-sound land-based disposal schemes using the mass· balance principle, it is essential that infonnation be available on the mass of nutrients, salt and solids produced and the associated effluent volumes. Mass balance refers to the conservation of mass principle which states (with some licence) matter can be neither created nor destroyed but can only move from one sink to another (e.g. from a manure pile to plant uptake) or transform from one form to another (e.g. solid organic nitrogen to nitrous oxide gas).

IT the mass of a nutrient excreted by a given livestock mass is known, and this is combined with total livestock mass, the total nutrient source mass is easily calculated and treatment facilities and disposal areas can be appropriately sized. Whilst this approach seems self evident, many effluent disposal designs use previously published data on nutrient concentration in either the waste stream, or more commonly the concentrations in a treatment lagoon prior to land disposal. The difficulty with this latter approach is that such concentrations can vary three or four fold in response to variations in mass inputs, lagoon evaporation, effluent recycling and the frequency of effluent irrigation.

Mass balance can take two basic approaches

either (1) Nutrient mass = Food intake - Animal storage

or (2) Nutrient mass = Nutrient excreted * Totalliveweight 1,000 kg liveweight

The first approach has been used by Barth (1985) to estimate the BOD load to effluent ponds where the composition and digestibility of known food rations (e.g. corn-soybean) was used to estimate the· production of Total and Volatile Solids per unit mass of feed consumed by various animal types (Volatile solids is the proportion of 105°C oven dry total solids which is driven off as volatile combustible gas after heating to 550°C for 1 hour). Casey (1992) extended this approach to nutrient balance. van Horn (1991), on the other hand, did a simple Nand P mass balance for dairy cows and showed that excreted mass equalled N and P in the feed intake minus Nand P contained in milk minus change in.dairy cow mass (due to calt). The analysis also showed that the N andP excreted could be significantly reduced by changing food rations without a reduction in animal performance. This reduction is important where disposal area is limited.

The second m~s balance approach uses the manure (faeces and urine) production per animal mass and its nutrient concentration. Table 1 lists relevant charSfteristics for dairy cows, beef, pigs and poultry. Note the very high N content of poultrym~ure relative to that of other animals (0.84 kg N/l,OOO kg liveweight) and the high fraction: of NH4-N in the pig manure. We will show later this has implications for the N loss in the sequence from excretion to land application.

286

Table 1 Fresh manure production and composition per 1,000 kg live animal mass per day (from ASAE D384.1).

Parameter Units Dairy Beef Swine Poultry Oayer)

Total Manure kg 86 58 84 64 Urine kg 26 18 39 Density kg/m3 990 1000 990 970 Total Solids kg 12 8.5 11 16 Volatile Solids kg 10 7.2 8.5 12 BOD kg 1.6 1.6 3.1 3.3 pH 7.0 7.0 7.5 6.9 Total N kg 0.45 0.34 0.52 0.84 ~-N kg 0.079 0.086 0.29 N/D Total P kg 0.094 0.092 0.18 0.30 Orthophosphate-P kg 0.061 0.030 0.12 0.09

Potassium kg 0.29 0.21 0.29 0.30

Calcium kg 0.16 0.14 0.33 1.30 Magnesium kg 0.07 0.05 0.07 0.14 Sodium kg 0.05 0.03 0.07 0.10

Chloride kg 0.13 N/D 0.26 0.56

Sulphur kg 0.05 0.045 0.076 0.14

Standard Derivation can be up to 100% of the mean values listed All nutrients and metal values are given in elemental form

Animal· age and changing dietary requirements affect manure production (MWPS 1985) and excretion per unit liveweight decreases with increasing liveweight and change in diet (Shulte

. et a11985). Table 2 demonstrates this type of calculation for a range of pig ages using three different estimation methods. Note that the ASAE method can return manure production values which are up to twice that for the DAMP method.

The volume of liquid associated with manure and nutrients is important because of its implication to sizing the treatment lagoon/wet weather storage and the irrigation area, and ensuring that concentrations of salt and nutrients don't become toxic to fermenting bacteria in the lagoon or retarding to pasture growth.

287

Table 2 Manure production characteristics produced for different classes of pigs estimated using different techniques and standards (from Barth 1985).

kg/animallday

Animal Parameter DAMP DAMP + 5% Feed Waste

Gilt - TS 0.261 0.372 replacement (125 kg) FS 0.035 0.041

VS 0.226 0.331

Boar (160 kg) TS 0.204 0.295 FS 0.024 0.028 VS 0.180 0.267

Sow - TS 0.227 0.328 gestating FS 0.027 0.032

VS 0.200 0.296

Sow - TS 0.567 0.820 Lactating (150 kg) FS 0.068 0.080

VS 0.499 0.740

Pig- TS 0.087 0.120 45 to 11.3 kg FS 0.017 0.020 nursing VS 0.070 0.100

Pig- TS 0.104 0.146 11.4 to 20.4 kg FS 0.018 0.021 weanling VS 0.086 0.125

Pig- TS 0.179 0.255 20.5 to 56.7 FS 0.024 0.028 grower VS 0.155 0.227

Pig- TS 0.272 0.393 56.8 to 99.8 kg FS 0.033 0.038 finisher VS 0.239 0.355

VS = Volatile Solids FS = Fixed Solids TS = Total Solids = VS + FS

=

= Digestibility Approximation of Manure Production American Society of Agrie. Eng. Standard D384.1

ASAE MWPS

0.490 0.122 0.368

0.780 0.454 0.195 0.073 0.585 0.381

0.685 0.372 0.172 0.073 0.513 0.299

0.685 0.907 0.172 0.181 0.513 0.726

0.046 0.056 0.009 0.011 0.037 0.045

0.090 0.086 0.018 0.014 0.072 0.072

0.231 0.231 0.046 0.045 0.185 0.186

0.476 0.476 0.095 0.095 0.381 0.381

DAMP ASAE MWPS = Mid West Plan Service: Livestock Waste Facilities Handbook (1985)

For piggeries, water is needed for three major purposes:

Fresh drinking water Fresh pen washdown water Fresh or recycled effluent flushing water -

288

8 L/animal/day 2 L/animal/day 14 L/animal/day

For a 1,000 sow piggery, the total animal water requirement is about 60 Ml/year after allowance has been made for lagoon flushing to control salt build up.

For feedlots, drinking water requirements vary from 35 to 70 L/animal/day with an annual fresh water demand of about 20,000 L/animal/year. The largest volume of effluent is derived from runoff from the feedlot pens and access roads during rainfall events. As a rule of thumb, runoff equals 30% of annual rainfall and the annual runoff from a oqe in ten rainfall year must be contained on farm (QDPI 1989). For a 10,000 head feedlot with 24 m2/animal in a 750 mm annual rainfall zone, average annual runoff is about 55 MI, increasing by a factor of 1.2 to 1.5 for one in ten year events.

Similar information can be obtained for agri-industrial processing such as abattoirs where about 1,800 litres of water ate used per (beef) animal processed. A common throughput is 350 to 400 animals/day.

For dairies, effluent volume is generated from washdown and cleaning water in the holding yards and milking shed. Water use is about 5,000 L/day for an 80-100 head herd. Manure voided in the dairy is about 10% of daily excretion. Over a milking season, about 1.8 MI of effluentds produced.

3.2 Embent Pre treatment before land application

Anaerobic lagoons

Effluenttfrom intensive livestock production is rarely applied directly to land. In feedlots for example~manure is scraped from the pens on ~ regular (monthly) basis and stockpiled for up to a year before it is trucked off site. In some cases, the piles are windrowed and regularly turned as: part of an aerobic composting operation.

More commonly, effluent is in a high strength liquid form and the readily oxidisable organic material~must be broken down.to less reactive substances and gaseous loss. This is usually achieved using an anaerobic lagoon in which bacteria convert reactive organics to carbon dioxide and methane in an oxygenfree (anaerobic) environment (Figure 1).

Anaerobic digestion can be described as a three stage process (Parkin and Owen 1986), which is shown schematically in Figure 1 and Figure 2.

(1) Hydrolysis and liquification of complex organic compounds is achieved by extra cellular enzymes produced by bacteria.

(2) The acid forming phase involves fermentation of the hydrolysed organics into long chain organic acids, amino acids and eventually to smaller Volatile Fatty Acids such as . propionic, butyric and valeric acid. At the same time a different group of bacteria convert part of the organic feed stock into acetic acid.

289

N03- AEROBIC ZONE

MICROAEROBIC ZONE E

""" A .c -ANAEROBIC C-Q)

ZONE "0 "0

Fermentative c: 0

Bacteria a..

Figure 1

Acetogenic Bacteria

Biological environment in an anaerobic pond showing schematically the production of methane, carbon dioxide and ammonia (after Zhang et al 1990).

(3) The third phase, where the completed waste stabilisation step occurs, is the conversion of acetic acid and hydrogen into methane, which is essentially insoluble in water and readily escapes from the lagoon system. Carbon dioxide is also produced and either escapes as gas or is converted to bicarbonate alkalinity. This step is particularly important· to balance the excess hydrogen produced in step two as not only are methanogenic bacteria strict anaerobes, but their optimum metabolic activity occurs over a narrow pH range (6.4 to 7.2). Acidic conditions strongly inhibit their metabolism.

An excessive build up of Volatile Fatty Acids in stage 2 can produce unbelievably objectionable malodours at parts per billion concentration. It is for this reason that particular care is required in designing the appropriate loading of organics to a lagoon.

The acid forming and methanogenic bacteria are mesophilic with an optimum temperature range of 30-45°C, hence the process is responsive to seasonal temperature variation. We will show later how average annual temperature at a given location sets the upper limit for organic loading rate to lagoons.

290

Figure 2 Chemical pathways for methane fermentation of organic effluent in an anaerobic pond (after Parkin and Owen 1986).

Effluent from intensive livestock production is rich in organic nitrogen which in tum leads to the formation of NH/, following the general equation:

Effluent Protein + H20 -7 Methane (CH4) + Carbon Dioxide (C02) + Cell Protein + Ammonium (NH/) + Bicarbonate (HC03-)-

The NH/ and ammonia gas (NH3) co exist in solution and the balance between the two is determined largely by pH and temperature. Figure 3 shows that above pH 8.5, over 25% of ammoniacal N occurs in the gaseous form and hence can be lost by volatilisation. It is for this reason that up to 70 % of the total N entering a lagoon system can be lost as NH3 gas, even at near neutral pH's (Koelliker and Miner 1973, Humenik and Overcash 1976, Vanderho1m 1975).

The magnitude of this loss pathway comes as a surprise to many scientists and engineers who are usually familiar with the traditional anaerobic N loss pathway of nitrate (N03) being reduced by bacteria under anoxic conditions to produce nitrous oxide, (N20), and nitrogen, (N2), gases.

291

Figure 3

c: 100 0 ~ NH3 ::::J 0 en 80 .s c: 0 60 ~ ::::J .0 .c: - 40 en =0 CD

~ 20 NH4+ -c: CD e a CD D.

4 5 6 7 8 9 10

Solution pH

The effect of pH on the relative concentrations of ammonium and ammonia in solution (from Freney et al 1983)

The primary purpose of an anaerobic lagoon is to stabilise reactive organic material into CH4

and CO2 with a minimum of odour generation. Because of the potential imbalance between the acid forming and methane forming reactions and associated odour problems, considerable effort has been spent on examiilingthe optimum loading rate of organic material to ponds and their hydraulic retention times. In the waste-water discipline, organic loading is described by the kg of BOD per hectare of lagoon surface per day - typical values range up to 4,000 kg/ha/day with a retention time of up to 50 days (Eckenfelder 1980).

Since BOD is a time consuming and relatively costly parameter to measure, agricultural engineers use Volatile Solids as an index of the reactive organic fraction (Barth 1985).

Using average seasonal air temperature as a measure of the rate of biological activity, isochrones of recommended loading rates of VS have been developed for America (ASAE 1990) and Australia (Casey 1992). Figure 4 shows iso K values (i.e. biological activity index) for Australia and these are approximately numerically equivalent to VS loading rate in 10-2 * g VS/day/m3 of pond volume (i.e. a K of 0.9 is approximately equivalent to a 90 g VS/day/m3 loading rate). As expected, loading rates for eastern New South Wales and Victoria are substantially lower than for south east Queensland.

Using the manure production data from Table 1, a 45 kg pig produces about 380 g VS/day. For a VS loading of 90 g VS/day/m3, the anaerobic lagoon volume per pig is about 4.2m3

which is in good agreement with current QDPI guidelines of 4 m3/pig (Casey 1984).

The Hydraulic Retention Time (HRT) of a lagoon is defined as the ratio of pond volume to effluent throughflow rate. For most anaerobic lagoons in Queensland a HRT of 40-50 days is required. Hence provided the average rate of effluent volume is known, lagoon volumes can also be designed on HRT considerations. The lagoon design volume will be the larger of the VS loading or the HRT calculations.

292

-15

-20

-25

-30

-35

-40

-45 110

Figure 4

125 130 135 140 145 150 155 -10

-15

-20

-25

-30

-35

-40

~ 115 120 125 130 135

Isochrones of annual average biological activity (iso K values) in anaerobic lagoons in Australia (from Casey 1992)

Sludge accumulation

Not all the effluent entering a lagoon is bio-degradable, some fraction of the livestock feed is classified as Fixed or Non Volatile Solids, whilst part of the organic fraction degrades so slowly that it too accumulates as sludge in the bottom of the lagoon. The rate of sludge accumulation is of particular interest as it determines the length of time before the lagoon must be desludged of usually nutrient rich material. Barth and Kroes (1985) have commented extensively on this problem and Table 3 lists the design sludge accumulation rate for mature anaerobic lagoons subjected to moderate organic loading rates. The units are m3 of sludge per kg of Total Solids Added, which in tum can be obtained from Tables 1 or 2.

Table 3

Dairy Beef Swine

Sludge accumulation for mature anaerobic lagoons subject to moderate loading rates (from ASAE D384.1).

Animal m3 sludgelkg Total Solids Added

0.00455 0.004 (approx.) 0.00303

Poultry (layer) 0.00184

293

It is usual for anaerobic lagoons to require des1udging every 10 years or so, which can pose difficult management problems in terms of the nutrient loading rate to land.

Screening of effluent

In order to decrease both the rate of sludge accumulation and the VS loading rate, effluent can be screened before it enters the lagoon. Screening reduces the solid content and the nutrient concentration in both the solid and the liquid effluents. Table 4 lists the typical performance of a screen and drum type separator for a range of livestock effluents (NZAEI 1984) but performance information for screens alone is very incomplete.

Table 4 Reduction (%) in concentration of solids, nutrient and BOD in fresh livestock effluent after passing through a rundown screen (from NZAEI 1984)

PARAMETER SOLID COMPONENT LIQUID COMPONENT

Nitrogen 30 70 Phosphorus 30 70 Potassium 20 80 Total Solids :::: 50 :::: 50 BOD :::: 55 :::: 40

3.3 Composition of Lagoon Effluents and Sludges

A difficulty in using the mass balance approach to estimate nutrient production, and hence design loading rates to land, is estimating the volatile losses in the system (for N) and in partitioning nutrients into the solid and liquid phases of a treatment lagoon. We will illustrate later how this estimation can be achieved, However as an alternative, Table 5 list typical chemical compositions of anaerobic ponds for effluent from feedlots, piggeries and secondary treated sewage for populations of 10,000 animals/persons. Of particular interest is the high salt content of the cattle and pig lagoons, the dominance of NH/ as the inorganic form of N, and the relative innocuousness of the sewage effluent (although its effluent volume is far higher than the other two sources leading to a much larger annual production of N, P, K).

294

Table 5 Nutrient Concentrations in Emuent Lagoons

FEEDLOT PIGGERY 2° SEWAGE 10,000 head 1,000 sows 10,000 people

pH 8.5 - 9.0 7.9 ",8.0

EC dS/m 5.5 8.7 '" 1.2 SAR (1) 5.5 ",6.0

Total N * 160 640 50

NH4-N * 130 520 40

N03-N * 2 10 10

p* 30 82 10

K* 500 520 25

BOD * 1500 330 30

* Units = mg/l (1) Sodium Adsorption Ratio

Table 6 lists similar information for the solid component of waste, which refer to manure stockpiles in the feedlot and sludge accumulation in the piggery and STP lagoons.

Table 6 Nutrient and organic concentrations, and production characteristics of solid emuent from three sources.

N

P

K

BOD

Production Rate

Type of Solid

FEEDLOT 10,000 head

27,000

4,700

21,500

250,000

13,000 dry tonnes/yr

Manure

* Units = mg/l or mg/kg

295

PIGGERY 1,000 sows

4,400

3,700

1,800

",3,000 m3/yr

Anaerobic Lagoon Sludge

.2° SEWAGE 10,000 people

39,000

19,000

1,400

15,000

310 d.tJyr

Activated Sludge (Aerobic)

4 EFFLUENTS AND SOIL SALINITY

Because the effluent from most treatment lagoons is more saline than 2 dS/m, special care must be taken to ensure salts do not accumulate in the soil profile to levels which limit plant growth. This can be achieved by ensuring that adequate leaching or deep drainage below the root zone occurs, since under steady state conditions, the mass of salt entering and leaving the root zone are equal. This concept can be expressed by the simple mass balance equation:

(2)

where

~ is the electrical conductivity, EC, of the input water CdS/m)

Cd is the salt cOIlcentration of the soil water moving below the root zone.approximated by the EC of the soil matrix at 'field capacity'

Dj is the depth of rainfall (Plus irrigation) in mm

Dd Deep drainage, is defined as the quantity of water draining below the root ZOIle depth of a soil in a defined time period. Here the root zone depth is taken as O.9m and Dd expressed as mm/year.

On rearrangement of Equation 2, the EC at the bottom of the root zone is

D .. C = C. * --.:!i"': i 7

d 'D d

(3)

It follows that for a given amount of irrigation (and rainfall), Di, the smaller Dd, the larger the ratio D/Dd which is the concentration factor for the input water Ci. The inverse, DJDi' is termed the Leaching Fraction, LF.

Hence the smaller the value of LF, the larger is the value of Cd' or the salinity concentration experienced by.plant roots.

Soil salinity values have been tabulated for a range of plant species, which if exceeded, will cause a reduction in plant growth. Following Mass and Hoffrpan (1977) the effect of root zone salinity is. calculated as

Yr = 100 - B (ECse - A) (3)

where

Yr = relative yield A = salinity threshold above which yield is affected B = % yield reduction per dS/m increase above the threshold value Ee.., = average f<?Ot zone salinity CdS/m)

In this case ECse refers to the EC of the soil saturation extract which is about twice as dilute as the field capacity moisture conditions which apply to the Cd values of Equation 2. Hence ECse :::: 0.5 Cd.

296

For Rhodes grass, a highly salt tolerant species, A = 7.0 dS/m and B = 3.2%. Table 7 lists A and B values for a selected range of crops covering the low, medium and high salt tolerant groupmgs.

Table 7 Salt tolerance of a selected range of pasture/fodder species expressed in terms of the soil salinity threshold causing yield reduction, ECse' and the yield decrease per dS/m increase in soil salinity (% Y) (from Shaw et al 1987).

Low Salt Tolerance Medium Salt Tolerance High Salt Tolerance

ECse %y ECse %y ECse

White Clover 1.5 12.1 Lovegrass 2.0 8.5 Wheatgrass 7.5 Forage Com 1.8 7.4 Lucerne 2.0 7.3 Kikuyu 3.0 Setaria 2.4 12.2 Green Panic 3.0 6.9 Barley 6.0 Paspalum 1.8 9.0 Pangola 2.0 4.0 Buffel 6.9 Townsville Stylo 2.4 20.4 Sudangrass 2.8 4.3 Couch 6.9

Oats 5.0 20.0 Rhodes 7.0 Fescue 3.9 5.3 Sorghum 8.3

Apart from the simple but approximate dilution relationship between Cct and ECse, the difficulty of applying Equation 2 is estimating average drainage below theroot zone, Dct.

Shaw and Thorburn (1982) examined over 700 dryland soil profiles in Queensland which covered a wide range of rainfall regimes and soil properties and showed that Dct (calculated from Equation 2 using the appropriate salt concentrations) could be described by the equation

(4)

where DR is the annual depth of rainfall which ranged from 300 - 2,000 mm for the experimental data set they examined; a and b are regression coefficients which vary with clay content and clay mineralogy; and n is a constant coefficient which linearises the relationship between Exchangeable Sodium Percentage (ESP) and Leaching Fraction (i.e. DJDJ

Equation 4 can be applied to irrigation situations on the assumption that the new hydrological combination of rainfall plus irrigation behaves as an equivalent depth of rainfall (:5: 2,000 mm) provided adjustment is made for soil permeability increases due to electrolyte effects (Quirk and Schofield 1955). On the basis of experience with heavy textured soils in the Lockyer Valley using variable salinity irrigation water, a non linear adjustment to the Leaching Fraction was developed (Shaw et al 1987) and this is shown in Figure 5.

Note that ECj refers to the "shandy" of irrigation water and rainfall, and ECR refers to the salinity of the rainfall alone. For an EC ratio of 50, the adjustment calculates a Leaching Fraction which is about 17 times larger than that which would have occurred under rainfall alone.

297

%y

6.9 3.0 7.0 6.8 6.3 3.2

11.2

* EC I

o 1 234 5 6

* for EC, = 0.03 (dS/m)

- ~ :. - ... '- ~-.. - :""----

- Y = 2.65Xo.5 - 1.35

o ~~~~~~~~~~~~--~ o 50 100 150 200

EC ratio = EC j IECr

Figure 5 The electrolyte adjustment factor to predicted Leaching Fraction (LF) as a function of the ratio of input water salinity ECI to rain water salinity, ECr (after Shaw et aI1987).

There is one fulther- correction required for root zone salinity, and this refers t6 more agronomically relevant average root zone salinity which incorporates the pattemofroot water uptake. Remembering that DJDj is the Leaching Fraction at the bottom of the root zone, Equation 3 can be expressed as

c. . C =-'

a LF

Following Rhodes (1983) and Shaw et al (1987), LF can be converted to profile average Leaching Fraction, LF average, by the equation

LF average = [0.976 * LFbottom + 0.022] 0.625

Hence the average root zone salinity ex, is calculated as

Cx = __ C..;..i_

LF average

(5)

(6)

~ is converted to the equivalent saturation extract salinity (ECse)' on which all published plant responses are based, by dividing by 2.

In summary then

298

• The amount of deep drainage D d is calculated from Equation 4 for a given soil profile paramatised by its ESP, clay % and clay mineralogy and for a given depth of rainfall and irrigation Dj •

• Calculated Dd is then expressed as a Leaching Fraction, DJDj, and is scaled up using Figure 4 to account for electrolyte affects on soil permeability.

• The corrected LF at the bottom of the root zone is then adjusted for the root water uptake pattern, using Equation 5 to obtain the profile average Leaching Fraction LFaverage.

• The average root zone salinity ~ is calculated using LF average in Equation 6 noting that Cj is a depth weighted average salinity from rainfall and irrigation sources.

• Mter converting ~ to equivalent ECse values, Equation 3 is used to calculate relative yield for the selected crop or pastures.

Fortunately this procedure has been automated using the computer program SALF (Salt and Leaching Fraction) and Table 8 lists the results for a given soil profile (a typical vertisol) in a 750 mm rainfall zone irrigated with 500 mm per year of effluent of varying salinities. The relative yield for three crops of high, medium and low salinity tolerance are the major results of interest. Note that the salt tolerant Rhodes grass is unaffected in this soil type for effluent salinities as high as 8 dS/m. "

Table 8 Predicted SOil(l) salinities and relative yield of high, medium and low salt tolerant crops irrigated with 500mm(1) of irrigation water of varying salinities.

EC irrigation water (dS/m)

1 2 3 4 5 6 7 8

Leaching Fraction (2)

0.06 0.08 0.10 0.12 0.14 0.15 0.16 0.17

EC.e rooting depth dS/m (3)

5.0 6.0 6.6 7.1 7.5 7.9 8.2 8.5

Low

Forage Corn

100 100 98 95 91 88 ~

86 83

Relative Yield %

Salt Tolerance

Medium High

Kikuyu Grass Rhodes Grass

100 100 100 100 100 100 100 100 100 100 99 100 98 100 97 100

(1) Soil was a vertisol with 45% clay, 8% ESP and CEC/g clay ratio (CCR) of 0.60. Average annual rainfall was 750 mm.

(2) Leaching Fraction corrected for electrolyte effect on soil permeability.

(3) ECse * 2.2 ::::: EC in situ at bottom of root zone.

299

It is important to realise that the preceding analysis is based on long term steady state behaviour. There may be extended periods of below average rainfall in a given year which cause soil salinity values to exceed the calculated steady state value because of reduced leaching.

4.1 Other Water Quality Parameters

Other water quality parameters of interest are specific ion effects such as chloride and boron and sodicity effects due to excessive amount of sodium in the water. If chloride levels in irrigation water exceed 5 meq/L and are applied directly to leaves, foliar injury will occur in sensitive crops such as citrus. For less sensitive crops such as corn and cotton, concentrations up to 20 meq/L chloride are acceptable. For boron, results have been expressed as soil solution concentrations and vary from 1 mg/L for citrus crops to 6 mg/L for cotton and lucerne (Westcot and Ayers 1985).

The sodicity hazard of a water is paramatised by its Sodium Adsorption Ratio, SAR, defined by

(7)

where Na, Ca, Mg are the ion concentrations in meq/L.

Waters high in ',. §AR result in sodium displacing divalent cations such as calcium and magnesium from·the exchange surface on clay particles (i.e. the ESP increases) resulting in dispersion and permeability reduction on subsequent rewetting. However clay dispersion can be inhibited by increasing electrolyte concentration and the acceptable SAR value for an irrigation water increases with increasing effluent salinity. This relationship is shown in Figure 6 where SAR-EC combinations to the left of the line will have unstable soil profile permeability. The stability line is also dependent on clay content and mineralogy, and more quantitative analysis, similar in concept to that of Equation 4, requires a better understanding of ESP changes to various SAR values. Shaw et al (1987) discuss the predicted changes in ESP with varying SAR in more detail.

4.2 Hydraulic loading

The design hydraulic loading of an effluent disposal area is the depth of effluent applied per hectare per year (the actual amount applied at each irrigation is dependent on the antecedent soil water deficit). On an annual basis, the maximum amount of effluent irrigation equals the potential crop .evapotranspiration less rainfall which is discounted for runoff and deep drainage. However there is a strong seasonal trend in irrigation demand because of reduced potential evaporation in the winter months, which can be exacerbated by winter dominant rainfall. When this seasonal trend is combined with a constant monthly effluent inflow into a treatment lagoon, there is a relatively complex hydrological problem to ensure that all effluent is contained on-site in at least the 90 percentile wet year. That is. treatment and storage lagoons did not overtop more frequently than once every ten years.

300

Figure 6 Permeability threshold values for 2 general soil types as a function of contamination of Sodium Adsorption Rates (SAR) and salinity of the input water ECI•

Potential evaporation ET is calculated as the product of a Class A pan evaporation (Ep) and a crop OJ! lagoon factor (t). For lagoons without crusts, the pan factor is 0.7 (Watts and McKay 1986). For crops, the factor varies with crop type and time of the year reflecting vigour and cover of the transpiring crop canopy. Crop factors for seasonally growing pasture, lucerne and eucalypt trees of increasing age are listed in Table 9.

Table 9

Crop

Irrigated Pasture

Lucerne

Eucalypts. 1 year old

2 year old

>4 year old

Monthly Crop Factors for irrigated pasture, lucerne and eucalypt stands of various ages (from Thomas 1992).

Crop Factor

Jan Feb Mar April May June July Aug Sept Oct Nov Dec

0.70 0.70 0.70 0.60 0.50 0045 0040 0045 0.55 0.65 0.70 0.70

0.95 0.90 0.85 0.80 0.70 0.55 0.55 0.65 0.75 0.85 0.95 1.00

0040 0040 0040 0.40 0040 0.40 0040 0.40 0.40 0040 0.40 0040 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

301

Effective rainfall is total rainfall less runoff and deep drainage, and is usually calculated from a simple bucket type, daily-water-balance model where rainfall in excess of the soil water deficit is routed as runoff or drainage. If a daily water balance is not available, then the conservative assumption of equating total to effective rainfall is adopted. In a water balance sense, the partitioning of excess rainfall into either runoff or draining is unimportant since both are loss pathways. However in terms of soil salinity control, drainage from rainfall is essential since it is this source which allows most of the salt leaching to occur.

4.3 Necessary Areas for Irrigation and Pond

Table 10 describes the one in ten year water balance for a hypothetical intensive rural industry in south east Queensland where a monthly effluent production of 30 Ml per month passes through treatment and storage lagoons of 4 ha in surface area before being applied to 100 hectares of irrigated pastures. The 100 ha is relatively arbitrary.

The pond rainfall (Ml) is the product of monthly rainfall and pond area (4 ha).

The pond evaporation (Ml) is 0.7 * Class A pan evaporation * pond area.

The potential evaporation (mm) is monthly crop factor * Class A pan.evaporation.

The Irrigation Demand (mm) is the difference between potential ET and rainfall discounted by runoff and drainage (obtained from a daily water balance). For a 100 ha irrigation area, a mm of Irrigation Demand is numerically equivalent to ML

The change in net pond input (Ml), or .6S, is the sum of pond rain and pond effluent, minus pond evaporation.

The cumulative pond storage in a given month, ESt, is the sum of the storage in the previous month, ESt- l , plus the net pond input for the current month, .6St, less the Irrigation Demand (ID) for the current month i.e.

The final calculation in Table 10 specifies the amount of wet weather storage required, over and above normal operating conditions, to prevent the lagoon system overtopping.

Assuming the wet weather storage volumes, comprising a dedicated storage lagoon and freeboard on the treatment lagoons, are empty at the start of the 1:10 rainfall year, 180 MI of storage is required to prevent overtopping. This is six months production of effluent. The major reason for the large storage volume is the small irrigation demand between February and July inclusive when effective rainfall approximates or exceeds potential evaporation. Consequently increasing the irrigation area from 100 to 150 ha has little effect on wet weather storage volume, reducing it to only 156 MI. Considering that the capital cost of spray irrigation is about $4,OOO/ha compared with $l,OOO/M1 for earthen storages, the economic choice is obvious.

302

Table 10 1 in 10 year Pond Water Balance for a processing factory in south east Queensland

Rain Pan Effluent Pond Pond Crop RIO Drainage Potent. Irrigation Demand Net Pond Cumulative Evap Input Rain Evap Factor ET Inflow Pond Storage

~S kS

100 ha** 150 ha Pond Initially Pond Empty

Initially 100 150 Full ha ha

mm mm Ml Ml Ml mm mm mm mm Ml Ml Ml Ml Ml 180 Ml

Jan 170 170 30 6.8 5.4 0.95 8 27 162 27 27 41 31 4 0 184 (180)*

Feb 173 145 30 6.9 4.6 0.90 8 28 131 0 0 0 32 37 32 212 (180)*

Mar 136 136 30 5.4 4.4 0.85 6 22 116 8 8 12 31 60 51 203 (180)*

April 92 106 30 3.7 3.4 0.80 4 15 85 12 12 18 30 78 64 198 (180)*

May 87 70 30 3.5 2.2 0.70 4 14 49 0 0 0 31 109 95 211 (180)*

June 68 64 30 2.7 2.0 0.55 3 11 35 0 0 0 21 140 126 211 (180)*

July 62 67 30 2.5 2.1 0.55 3 10 37 .. 0 0 0 30 170 156 210 (180)*

Aug 47 88 30 1.9 2.8 0.65 2 7 57 19 19 29 29 180 156 190 (180)*

Sept 49 117 30 2.0 3.7 0.75 2 8 88 49 49 74 28 160 110 159

Oct 102 139 30 4.1 4.4 0.85 5 16 118 37 37 56 30 153 84 152

Nov 108 145 30 4.3 4.6 0.95 5 17 138 52 52 78 30 130 36 130

Dec 157 175 30 6.3 5.6 1.00 7 25 175 50 ··50 75 21 III 0 110 E= 254 mm

Pond Factor = 0.7 ** Irrigation area of 100 or 150 ha Potential ET = Class A pan evaporation '" Crop Factor * 180 Ml is the maximum wet weather storage. Hence in these months, Irrigation Demand = Potential ET - (Rain - RIO - Drainage) lagoon overtops. Net input to pond, ~S = Pond Rain + Pond Effluent - Pond Evaporation Cumulative pond storage, ESt = ESt.! + ~St - Irrigation Demandt for month t

Increasing the size of an irrigation area also introduces a management difficulty in that below average rainfall may severely limit pasture production every few years requiring additional (fresh) irrigation water to be available. There is currently no simple answer for the optimum economic balance between irrigation area/storage volume which produces an acceptable failure risk.

Table 10 also lists the cumulative monthly pond storage volume for the assumption of ponds full at the start of the year, and a maximum wet weather storage of 180 Ml. For the first eight months of the year, the ponds overtop making this initial storage conditions unacceptable. Solutions include: increasing wet weather storage volumes; decreasing effluent production (including factory closure), and ensuring storage ponds are empty at the start of each wet season.

The water balance problem can become more complex when effluent inputs are episodic, such as rainfall runoff from feedlot pens. However the problem can be easily solved if storage lagoons are built which store the 1:10 year annual runoff giving the operator maximum flexibility for when to dispose of the' effluent on a dedicated irrigation area. For a 10,000 head feedlot, this storage is usually between 75 and 100 Ml.

5 SUSTAINABLE DISPOSAL AREAS

We can now use the principles described in the previous sections to estimate the sustainable disposal areas for feedlots andpiggeries.

5.1 Phosphorus, balance of a feedlot

As phosphorus is neither volatile nor particularly mobile in soils, it is possible to develop a quantitative P cycle using mass balance principles. This is shown in Figure 7 for a typical feedlot (Watts et al 1992). P enters the feedlot system in the form of live cattle and feed. P is intentionally removed from the system in the form of finished cattle, biomass (i.e. crops) from the waste utilisation area, and as sale of manure off farm. P may unintentionally leave the system as pond overflow, soil erosion and possibly groundwater. The objective of an ecologically sustainable system is to keep these unintentional losses to an acceptable mlIlltIlum.

304

BOUNDARY OF FEEDLOT SYSTEM I---------~--------------------------~--I

11- - - - - -.. -.-_-..... ' _____________________ pl.CiiiATiiTL.E.OU.T~.~

:: I ~ -------------------I FRESH I

FEED IN I CROPS

• ~ , MANURE , I b ~MANURE' SCRAPED I MANURE.

II· ~ PEN ~ MANURE ~ DISPOSAL RUNOFF/

•.• C.AT.T.LE.IN_ .. ll ... ~ , SURFACE ~ STOCKPILE 7 AREA I SOIL EROSION

I;, :11 L...I _A_P_",--, I ~ II CATTLE, I MANURE SALES

II A . MORTAUTY, FRESH D D .. -----... ,---. -.-~ D RUNOFF I II _ _ _ _ _ __ "II SLUDGE : :

I GROUNDWATER I

CARCASS I I E;I~L~~NTI DISPOSAL SEDIMENT I RETENTION

EFFLUENT DISPOSAL

AREA

Ap AREA I BASIN I POND

Ap : B L __________ _ - - - - Returned Water - - --

CROPS

RUNOFF I naSION

OPTION TAILWATER

DAM

_______________________________________ I POND OVERFLOW

Figure 7 Phosphorus cycle of a cattle feedlot (from Watts et at1992)

5.2 Production of P in the feedlot

The traditional approach to determining the area required for P disposal is to estimate the annual amount of manure taken from the stockpile and its P concentration, However because of variation in dry matter composition during storage, varying manure moisture content and varying P concentration (3-16 g P/kg dry manure) the design mass of P can vary from 16 to 175 tonnes P/year for a 10,000 head feedlot stocked with 450 kg animals (Watts et at, 1992), This method is clearly a poor estimate of the design P load on the waste utilisation area,

As an alternative, Watts et al (1992) calculated the total P production using a mass balance method,

where

Pm = P contained in manure (kg/yr) ~ = Mass of feed (kg/yr) C, = P concentration of feed (%) ~c = Mass of incoming cattle (kg/yr) M..c = Mass of outgoing (finished) cattle Ckg/yr)

(3)

305

~ = Mass of dead animals (kg) Cc = P concentration of whole cattle (%)

The mass of feed intake can be estimated from cattle liveweight whilst the P content of typical barley and sorghum rations vary between 3.5 to 4.6 g P/kg feed dry matter. The liveweight of cattle entering is easily obtained whilst predicted liveweight at leaving is obtained from daily weight gain, feeding period and average feedlot occupancy. The overall P concentration in feedlot cattle is relatively stable at 8g/kg of liveweight.

For a 10,000 head feedlot producing large cattle for the Japanese market over 200 day periods with a 90% occupancy, total P production was estimated as 177 t/year. Encouragingly, similar figures were obtained using estimated fresh manure1 production (5.8% of liveweight), its P concentration (about 16g/kg of dry matter) and its moisture content (15% d.b).

Some of this P will enter the storage lagoon during runoff events. For a 24 ha feedlot in a 700 mm rainfall zone with 30% average annual runoff, the total P stored in lagoon effluent with a concentration of 50 mg/L P is about 2.5 t P/year which is a trivial amount compared with the P remaining in the manure stockpile.

5.3 Phosphorus sinks for sustainable disposal

The objective of disposing on P on cropping areas is to ensure that adverse biological and chemical effects on the dispO,sal area, and adjacent off-site areas and water bodies, are minimal. There are three potentially significant P sinks:

(i) Exporting manure off farm to be used by neighbouring farms. However without regulation, there is no surety that sustainable application rates won't be exceeded.

(ii) Incorporation into above ground biomass.

(iii) Storage of inorganic P in the soil profile.

5.3.1 Biomass sink

For biomass to be a permanent sink, the plant material must be harvested and removed from the disposal area. The amount of P which can be recovered from fodder and grain production is determined by climate, soils, species and application rate.

Most Queensland feedlots receive 700-900 mm of annual rainfall which can be expected to produce 10-15 t of pasture DM per hectare per year with a P concentration of 3 g/kg. This represents a potential sink of 30·45 kg of P per hectare per year. For irrigated pasture in southern Queensland, P removal can reach up to 100 kg/ha. The pasture growing season is restricted by temperature to the months October to April in most feedlot locations. This has implications for the erosion hazard of applying manure to pasture.

1 Manure includes both faeces and urine

306

In the major wheat, barley and sorghum growing shires in Queensland, average grain yields range up to 2.1, 2.5 and 4.3 t/ha respectively. Assuming a P concentration of 4 mg/kg, these yields represent potential P sinks of 8-17 kg P/ha/yr which could be approximately doubled if all the above ground biomass is removed.

5.3.2 Soil Sink

P exists in inorganic and organic compounds. Inorganic compounds are mainly insoluble aluminium, iron and calcium compounds, and it is only the orthophosphates (H2P04-, HPO/", POl) in the soil solution which are available for uptake by plants. Organic P is contained in organic matter (humus and plant residues, manure) and is unavailable to plants in this form.

The inorganic orthophosphate compounds can be sorbed by soil minerals (a fast reaction) and is largely reversible. The capacity of a soil to sorb P depends on the concentration of hydrous oxides of Fe and AI and their positive charge characteristics. They can also be fixed in the crystal lattice of soil minerals or precipitated as insoluble inorganic compounds in calcareous soils and this slow reaction is considered to be essentially irreversible (Barrow 1989).

The capacity of a soil to sorb P varies widely from low levels in sandy soils to high levels in strongly weathered clay soils e.g. krasnozems. Table 11 shows a typical range of P sOIption capacities for a range of Australian soils (Probert 1983). The range is due in part to the initial P content of the soil. Assuming a bulk density of 1,500 kg/m3, the P storage capacity will range from 300-3,600 kg/ha/m soil depth fo.rtopsoil and 900-14,400 kg/ha/m soil depth for the subsoil. ib·

Table 11 P sorption (mg/kg) of a range of Australian soils· equilibrated for 24 hours with a solution P concentration of 0.2 mg/L (Probert 1983).

Soil

Krasnozems Xanthozens Euchrozens Red Earths Yellow Earths Red Podsolic Solodic

P sorbed (mg/kg)

Top soil

133-240 55 -135 25 - 80 10 -100 10 -70 25 -155

18

Sub soil

445-965 120-290 360-250

60-485 90-165

110-315 N.D

It is important to appreciate that the P sorption or fixation is a strong function of the inorganic P concentration in the soil solution. The relationship between P sorbed and the solution concentration is called a sorption isotherm and an example of this for a range of Queensland soils is shown in Figure 8. Using the conservative acceptable leachate P concentration of 0.05 mg/L, the amount of P sorbed by the sesquioxide rich krasnozem in Figure 7 is 110 mg of P/kg of soil and for the vertisol, it is only 10 mg/kg. The krasnozem

307

loam can have a P storage of 1,700 kg/ha per metre depth of soil whilst for the vertisol, it is only 125 kg/ha/m soil depth. To calculate the total P storage per hectare, the relevant design soil depth should be the depth to bedrock or groundwater (less an appropriate leaching buffer soil storage), but allowances for changes in the P sorption characteristics with depth may ne~d to be made.

800 -- Krasnozem A-A Podzolic

600 .-. Vertisol

Oi ~ Ol E .......... "0 Q)

400 .0 .... 0 CJ)

a.

200

o~~~~~~~~~~~~

o 5 1 0 15 20 25 30

Solution P concentration (mg/L)

Figure 8 Psorption isotherms of 3 Australian soils of varying P fixing ability

An alternative estimate of soil P storage is to use the mass of P sorbed corresponding to the P concentration in the effluent (Ryden and Pratt 1980). Using a weighted effluent storage pond P concentration of 17 mg/L (4 Ml/ha irrigation at 50 mg/P/L + 8 Ml/ha rainfall), P storage capacity increases to 8,000 kg/ha/m soil depth for the krasnozem and 1,600 kg/ha/m soil depth for the vertiso!. Obviously, such large increases have a substantial effect on the sustainable life of the soil for effluent disposal.

For example, assuming an effluent P source of 2,500 kg P Iyr and a disposal area of 12 hectares growing irrigated pasture with a biomass sink 100 kg P/ha/year, the P loading on the soil will be 108 kg P/ha/year i.e.

[2,500 - i~OO * 12) = 108]

For a 2m depth of krasnozem the sustainable life before excessive P leaches through the soil profile will be 148 years. For the 2m deep vertisol, this reduces to 30 years. Neither time could reasonably be termed environmentally sustainable, and this will require either an increase in the disposal area, thereby reducing the 4Ml/ha/year effluent loading, or an alternative chemical method to immobilise the P (e.g. precipitation with red mud from the alumina industry).

308

5.3.3 Solid Effluent

Because of uncertainties in the concentration of inorganic P in the soil solution of soils used for manure disposal, calculation of the size of the soil sorption sink is not as: straightforward as that for effluent. Howeve.r it is reasonable to adopt a soil solution concentration of 1 mg PjL which is of the order occurring in top soils, heavily fertilised with superphosphate.

With this assumption, the P; storage capacities for 2m deep profile are 780p kg/ha for the krasnozem and 1,750 kg/ha for the vertisol. Remembering the total P production is 177 t P/year less 3 t/year for pond ~ffluent, and adopting the current industry practice of 250 ha of disposal area per 10,000 heaFi, the loading rate to the land is 696 kg P/ha/year. Assuming a combination of grain cropping and pasture can provide a biomass sink of 50 kg P/ha/year, the P loading to the soil storage sink is f.. 650 kg P/ha/year giving a sustainable life of 12 years for the krasnozem reducing to 2.7 years for the vertisol. Clearly the$e loading rates can't be sustained and the ohly practical alternative is to export the manure off farm. To achieve a 200 year sustaindble life would limit P application to 90 kg P/ha/yr for the krasnozem (i.e. 7,800/200 + ;50 kg biomass sink) and 60 kg P/ha/yr for the vertisol. This latter loading rate would reql[lire that about 11,700 tonnes of the estimated 12,700 tonnes of stockpiled manure produced per year must be sold off farm. '

5.3.4 P export via erosion pathways

Unintentional losses of P can occur when it is transport~ into surface streams attached to eroded soil material. Table 12 provides some estimates of likely losses.

"''''''(:cr_:

A vertisol on the eastern Darling Downs with a range of slopes and stubble management practices has been chosen for purposes of illustrative calculation. Using the cropping systems model PERFECT (Littleboy et al 1992), the average annual soil erosion losses have been calculated using long term climate records (100 years). These losses ranged from 1 t/ha/yr (2% slope, zero till) to 85 t/ha/yr (8% slope; stubble burnt). The biologically active P concentration of the top soil after extensive manure application is assumed to be 200 mg P/kg soil. Because manure application to cropping soil usually requires discing in, stubble incorporated is the most likely management treatment. From Table 12, the average P export is therefore likely to range from 0.7 to 4.1 kglha/yr for this soil-climate-management combination.

Table 12

Land Slope

Estimated average annual export of P adsorbed to soil sediments eroded from a vertisol on the eastern Darling Downs for a range of slopes and surface management P concentration is assumed to be 200mglkg of soil.

2% 4% 6% 8%

Stubble Management Export of P (kg P/ha/yr)

Burnt 3 6.5 11.3 17.2 Incorporated 0.7 1.6 2.7 4.1 Reduced till 0.2 0.5 0.8 1.2 Zero till 0.1 0.2 0.4 0.6

309

Losses of P can also occur as leaching flux below the design soil depth. Deep drainage loss from well structured vertisols on the eastern Darling Downs are about 20 mm per year. Assuming a soil leachate concentration of 1 mg P/L, the likely P export is therefore 0.2 kg Plha/yr.

These calculations suggest that P exports into streams are dominated by soil erosion pathways. Therefore guidelines need to be framed to limit the soil slope used and tillage strategy adopted when disposing of feedlot manure onto land.

To place these loss rates into perspective, the P export from catchments draining into the Peel-Harvey inlet of Western Australia (with notorious algal bloom problems) range from 0.1 to 1.3 kg P/ha/yr (Birch 1982).

The concept of sustainability. for intensive rural industries raises the interesting philosophical problem of defining for just how long sustainability should be maintained. For example, it is difficult to imagine that animal effluent production from feedlots and piggeries will occur for generations at the same location. Perhaps a more plausible definition of sustainability for planning purposes should explicitly consider a realistic life span for these operations. An exception is effluent from sewage treatment plants where the high capital cost ensures little change in "land use".

6 NITROGEN BALANCE OF AN INTENSIVE PIGGERY

It was appropriate to discuss the disposal of P from a beef feedlot, because with P the system is relatively simple; there are no volatile losses of P and leaching is minimal except on extremely sandy -Soils. The N cycle, however, is both complex and highly variable (Figure 9).

Nitrogen entering the disposal as waste comprises organic forms of N, or the inorganic NH4-N and NH3• Generally much of the inorganic N in manure is NH4-N or NH3• When waste is applied to soil, N undergoes the following biological transformations: (1) immobilisation of inorganic forms of N by plants and micro organisms to form organic N compounds; (2) ammonification, the decomposition of organic N to NH/; (3) mineralisation or nitrification, the oxidation of NH/ to N02- and then onto N03-; (4) nitrate reduction and denitrification of N03- to N20 and N2 (from Keeney 1981).

The amount of N mineralised or immobilised is a function of the form of organic matter, temperature, and moisture status of the soil. Under suitable cOllditions, rapid increases in the microbial population occur, providing a large sink for N for use in cell synthesis. If plant residues have . a low carbon/nitrogen ratio (often a C/N ratio less than 22, with N concentrations of about 2%), rapid net N mineralisation occurs., For a C/N ratio greater than 22, net immobilisation of N occurs.

Nitrification is an aerobic process in which the relatively immobile NH/ form is transformed into N03-, which can be readily leached from the soil. Under aerobic, warm conditions there is almost complete conversion of NH/ to N03- in the surface soil of the disposal area within a few days of' effluent application. Temperature and oxygen supply govern the rate of nitrification.

310

Denitrification occurs under anaerobic conditions when bacteria use N03- or N02- as an electron acceptor when soil oxygen is in short supply to produce both N20 and N2 which are lost from the soil as volatile gases.

EFFLUENT I Organic N NH4-N N03-N I

I INPU TS I

~ Nitrification I Mineralization , 'l N02- J NH4+ I Soil Immobilization I I

OrganicN

tl CI) , ZI-

~ 00::

~ \ -0 NH4+ 1-11..

\!t <t:CI)

~ ::2 z

- N03- O::<t: 00:: LLI-

~o Leaching Denitrification <t:z

o::<t:

I I-Residues

I I N20, N2 I tl Plant I Biomass

I :;

NH3

t \ ,

I

-Atmosphere Harvest Groundwater Atmosphere OUTPUTS

Figure 9 Nitrogen transformations and pathways of effiuent in a land based disposal system (from Keeney 1981).

To simplify this discussion of N effluent disposal, a piggery case study has been chosen because there is considerable data on the N flows within the piggery system, and the waste disposal system is relatively well controlled (i.e. all manure is disposed of in an effluent pond) in comparison with the feedlot system. Within the piggery, N enters as feed and exits as finished pigs for slaughter, and as volatile gases to the atmosphere and manure to the disposal pond. Here we consider the fate of N as manure (Figure 10) from a 1,000 sow piggery which is a breeding and finishing operation. Casey (1992) estimated the manure output from the piggery to contain 119 kg N/day, or 43.4 tN/year, though estimates are as high as 89.4 tN/year (Table 13). The manure is flushed into an effluent pond where N separates into two fractions, the sludge (20% of manure N output), and the pond solution (80%) of which 50% is lost as NH3 through volatilisation and 30% remains in solution.

The major losses of nitrogen (up to 70%) occurs as NH3 volatilisation from the pond surface. Surface area of the pond is the chief determinant of NH3 volatilisation, though other factors such as increasing temperature, pH and wind speed also increase volatilisation. Volatilisation is proportional to the difference in NH3 concentrations in the solution and the air, and since the NH3 concentration in the air is low under field conditions, volatilisation is proportional

311

to the solution NH3 concentration.

To minimise NH3 losses to the atmosphere, the surface area/volume ratio of the pond can be reduced; alternatively, this ratio can be increased to maximise NH3 loss (Casey 1992). The latter system appears to be commonly employed to minimise the need for N disposal.

Table 13 Comparison of nutrient production in manure by category of pig for a 100 sow piggery using various estimation techniques (from Casey 1992)

Pig Class Number Nutrient Nutrient Production

(kg/head/day) DAMP (1) ASAE (2)

Gilt (110 kg) 10 N 0.0176 0.0572 P 0.0061 0.0198 K 0.0098 0.0319

Boar (160 kg) 5 N 0.0140 0.0832 P 0.0048 0.0288 K 0.0078 0.0464

Dry Sow (125 kg) 77 N 0.0155 0.0650 P 0.0054 0.0225 K 0.0087 0.0363

N 0.0388 0.0780 Lactating Sow (150 kg) ,23 (

P 0.0134 0.0270 K 0.0216 0.0435

Sucke:r (4.5 - 11.3 kg) 210 N 0.0057 0.0041 P 0.0020 0.0014 K 0.0032 0.0023

Weaner (11.4 - 20.4 kg) 175 N 0.0069 0.0083 P 0.0024 0.0029 K 0.0039 0.0046

Grower (20.5 - 56.7 kg) 210 N 0.0121 0.0201 P 0.0042 0.0069 K 0.0067 0.0112

Finisher (56.8 - 99.8 kg) 250 N 0.0186 0.0407 P 0.0064 0.0141 K 0.0104 0.0227

(1) Digestibility Approximation of Manure Production (Barth 1985).

(2) ASAE Standard D384.1 (1985).

(3) Mid West Plan Service (1985).

312

MWPS(3)

0.0410 0.0140 0.0270

0.0350 0.0120 0.0230

0.0280 0.0100 0.0180

0.1000 0.0350 0.0680

0.0073 0.0024 0.0045

0.0173 0.0057 0.0117

0.0355 0.0118 0.0232

>6.34} 1,1..38 ........ .

Figure 10

Pond Sludge

8.7t

~ 10 Years

87t

Mineral ization

Year 1 =35t (45%) Year 2= 7.8t (10%) Year 3= 3.9t ( 5%)

NITROGEN PRODUCTION IN MANURE 43.4t1Year

Pond Solution

13.Ot

Pond Volatilisation

21.7t

~O% 80% --

Soil Pasture 10.4t Volatilisation

2.6t

tion I ,10% +15% + 45%

Leaching Denitrification Biomass

1.ot 1.6t Uptake 4.2t

20% 10%

Soil Organic Volatilisation Matter 1.0t

1.6t

Flow diagram for Nitrogen disposal to land from a 1,000 sow piggery

Of the N remaining in the disposal pond, the solution N is available for land disposal on a regular basis; sludge N accumulates for longer periods (e.g. 10 years) before disposal onto land. As with P, nitrogen may be lost from the pond from the disposal system as pond overflow during wet periods and soil erosion, but unlike P, nitrogen can be readily leached through the soil to the groundwater if loading rates are excessive and rainfall is high (Barraclough et al 1992), particularly during cool periods when there is little active pasture growth.

When the waste pond effluent is applied to land, an estimated 80% enters the soil and 20% is lost as volatile NH3. The loss of NH3 is reduced if the solution is applied to standing pasture, due to reduced temperature and wind effects, and if the effluent is flood irrigated rather than spray irrigated. Volatilisation occurs within a number of days when the NH3 concentration in the effluent is high (Cameron and Rate 1992).

Of the effluent N reaching the soil, pasture biomass can account for up to 45%, the soil organic pool 20%, leaching 10%, denitrification 15% and volatilisation 10%. Approximately 10% has been estimated for N03 leaching, though leaching depends on loading rates, biomass uptake and the time series nature of rainfall events.

Given an ample supply of effluent water, biomass production will be governed by the growth

313

response of tropical pasture species to low temperatures during the autumn, winter and spring periods. At average daily temperatures (max + min) from 12°C to 25°C, pasture growth rate increases linearly from 0 to 100 kg dry matter/ha/day for a range of tropical pasture grasses. Therefore biomass production from irrigated tropical pastures has a marked seasonal distribution (Figure 11) such that the N sink: will be non-existent during the coldest months. Growth data for Kikuyu pasture indicate that N recovery in the biomass may be as high as 50% of the N applied to the soil. In order to minimise N leaching, annual application rate should not be greater than 300 kg N/ha. Therefore to dispose of the estimated 10.4 tN/year to land (Figure 10), a disposal area of 35 ha would be required.

Turning now to the pond sludge, assuming accumulation over 10 years and negligible break­down and loss of N in storage, there is some 87 t N for land disposal. If this sludge is immediately incorporated into the soil, there is virtually no loss of NH3 through volatilisation. However, we have allowed for a 10% loss of NH3, leaving 78.3 t N in the soil. Once again using an annual application rate of 300 kg N/ha, 260 ha of disposal land would be required once in ten years. However, not all of this sludge N is immediately available to the plant but becomes available at the following rate: approximately 45% of the Nin sludge will be mineralised in year one, 10% in year two and 5% in year three (Pratt etal1976). Therefore, sludge N application rates could be doubled to provide 300 kg N/ha, and the disposal area halved to 130 ha.

-.l: -c::: o E "iii .l: -~

1/1

= E o :is

i J

Z

Figure 11

2:- 2:- .r::. iii' QI ~ - ~ ~ ~ ~

~ .~ 1/1 QI QI QI QI nf res c::: J J .0 .0 .0 .0 J J res « ~ J ...., Cl E ~ E E c::: ...

~ ....,

J res .0 « ! QI QI ...., QI 0 ~ u

lL. QI QI

(J) Z 0

Seasonal biomass uptake of N by Kikuyu grass at Kingaroy using a plant N concentration of 2.5%.

314

CONCLUSIONS

Planning land based disposal schemes for agricultural effluent to achieve environmental sustainability in either the long term, or for the life of project, requires first and foremost good estimates of the volumes and quantities of nutrients, salt and reactive organics in the effluent. We suggest this can best be done using mass balance techniques rather than scaling published lagoon concentrations by estimated effluent volumes.

For P, calculation of loading rate to land is relatively straightforward because no volatile losses occur and the major disposal sink is the soil sorption capacity. However care must be taken to ensure unintentional off site losses do not occur due to soil erosion.

Acceptable N loading rates are considerably more difficult to calculate because of substantial volatilisation losses, mineralisation and leaching processes and the fact that seasonal growing biomass is the major storage sink. None the less, we demonstrate that reasonable estimates of N available for land disposal can be obtained because volatilisation of NH3 from treatment lagoons is a major loss mechanism (up to 70% of N input) whilst up to 26% of input N remains in the semi solid pond sludge.

Anaerobic treatment lagoons are an essential component of effluent disposal and it is important they overtop only infrequently (e.g. less than 1 in 10 years). This requires calculation of an appropriate wet weather storage volume - irrigation area combination which can be readily obtained using a monthly water balance. More sophisticated analysis of optimum irrigation area to account for dry periods requires detailed computer modelling.

Effluent from intensive livestock treatment lagoons is usually very saline requiring careful management to ensure the irrigation disposal area does not become salinised. The problem is similar to those for irrigation water quality and has been well researched in Queensland and overseas.

Finally plans are just that - plans! Accurate estimate of future behaviour are difficult to obtain at cost effective prices and a monitoring system is essential to manage and fine tune effluent disposal practice during the operational· life of the· production facility.

ACKNOWLEDGEMENT

We thank Mr Paul Jenkins for producing the figures.

REFERENCES

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ANZECC (1992). Australian Water Quality Guidelines for Fresh and Marine Waters. Aust. and New Zealand Environment and Conservation Council.

Ayres, R.S. and Westcot, D.W. (1985). Water Quality for Agriculture. Irrigation and Drainage Paper No. 29, F.A.O. Rome.

315

Baldwin, C. (1990). Impact of elevated nutrients in the Great Barrier Reef: Great Barrier Reef Marine Park Authority. Research Publication No. 20.

Barraclough, D., Jarvis, S.c., Davis, G.P. and Williams, 1. (1992). The relation between fertiliser nitrogen applications and nitrate leaching from grazed grassland. Soil Use and Management. 8. p.51-56.

Barrow, J. "Surface Reactions of Phosphate in Soil". Agric. Science 2, 1989. pp.23-37.

Barth, c.L. (1985). Livestock Waste Characterisation - A New Approach. in Agricultural Waste Utilisation and Management, Proceedings of the Fifth International Symposium on Livestock Wastes, pp. 286-294. St. Joseph, Michigan: American Society of Agricultural Engineers.

Barth, C.L.and Kroes, J. (1985). Livestock Waste Lagoon Sludge Characterisation. in Agricultural Waste Utilisation and Management, Proceedings of the Fifth International Symposium on Livestock Wastes, 660-671. St. Joseph, Michigan: American Society of Agricultural Engineers.

Birch, P.B. "Phosphorus Export from Coastal Plain Drainage into the Peel Harvey Estuary System of Western Australia". Aust. J. Mar. Freshwater Res., Vol. 33, 1982, pp. 23-32.

Cameron, K.c. and Rate, A.W. (1992). The fate of nitrogen in piggery waste applied to a shallow stony pasture soil. In "The use of wastes and by products as fertiliser and soil amendments for pastures and crops". Eds. P.E.H. Gregg and L.D. Currie. Proc. Workshop Fertiliser and Lime Research Centre. Massey Univ. NZ. Feb. 1992. Occasional Report No.6. pp. 314-326.

Casey, K.D. (1992). A computer model to evaluate the design of anaerobic lagoons for pig wastes. Unpublished Masters Thesis pp. 202. Clemson University, North Carolina, USA.

Casey, K.D. (1984). Effluent Management - anaerobic ponds. QDPI Farm Note. AGDEX 440/048.

Dillon, P.J. (1988). An evaluation of the source of nitrate in groundwater near Mt Gambier, South Australia. CSIRO Water Resources Series No. 1.

Donnelly, T.H., Caitcheon, G.G. and Wasson, R.J. (1992). Algal blooms in inland Australian water systems: sourcing nutrients and turbidity. In 'Research Areas Petinent to Intensive Rural Industries Waste Management'. Eds. F.H. Bowmer and P. Laut. CSIRO Div. Water Resources Report 92/4. pp.74-81.

Eckenfelder, W; (1980). Principles of Water Quality Management. CBI Publishing. Boston, USA. 717 pp.

Feigin, A., Ravina, 1. and Shalhevei, J. (1991). Irrigation with treated sewage effluent. 217pp. (Advanced Series in Agricultural Science) - Springer-Verlag.

316

•

Freney, J.R., Simpson, J.R. and Denmead, O.F. (1983). Volatilisation of ammonia in 'Gaseous losses of Nitrogen from plant soil systems'. Eds. J.R. Freney and J.R. Simpson. pp. 1-32. Martinus Nijhoff - The Hague.

Gutteridge, Haskins and Davey Pty Ltd. "An Investigation of Nutrient Pollution in the Murray-Darling River System". Report prepared for the Murray-Darling Basin Commission. Canberra, Jan. 1992. Ref No: 311/1048/0504.

Humenik, F.J. and Overcash, M.R. (1976). Design Criteria for Swine Waste Treatment Systems. EPA-600/2-76-233. Ada, Oklahoma: U.S. Environmental Protection Agency.

Keeney, D.R. (1981). Soil nitrogen chemistry and biochemistry. In 'Modelling Wastewater Renovation - land treatment'. Ed. I.K. Iskandar. pp. 259-277. John Wiley.

Koelliker, J.K. and Miner, lR. (1973). Desorption of ammonia from anaerobic lagoons. Transaction ASAE 16 p.148-151.

Littleboy, M., Silburn, D.M., Freebairn, D.M., Woodruff, D.R., Hammer, G.L. and Leslie, J.K. (1992). Impact of soil erosion on production in cropping systems. Parts I and ll. Aust. J. Soil Res. 30. 757-788.

Maas, E.V. and Hoffman, G.l (1977). Crop salt tolerance - current assessment, Journal of the Irrigation and Drainage Division, (ASCE) 103: 115-130.

MWPS. (1985). Livestock Waste Facilities Handbook. MWPS-18. Ames, Iowa: Midwest Plan Service.

NZAEI (1984). Agricultural Waste Manual. New Zealand Agricultural Engineering Institute, Lincoln College, Canterbury, New Zealand.

Parkin, G.F. and Owen, W.F. (1986). Fundamentals of anaerobic digestion of wastewater sludges. J. Env. Eng. (ASCE) 112. p.867-920.

Pratt, P.F., Davis, S. and Sharpless, R.G. (1976). A four year field trial with animal manures. I. Nitrogen balances and yields. Hilgardia. 44. 99-125.

Probert, M.E. "The Sorption of Phosphate by Soils", In "Soils - an Australian Viewpoint". CSIRO Division of Soils. CSIRO/Academic Press. 1983, pp. 427-435.

QDPI (1989). Queensland Government Guidelines for establishment and operation of cattle feedlots. QDPI. September 1989. pp.15.

Quirk, J.P. and Schofield (1955). The effect of electrolyte concentration on soil permeability. J. Soil Science. 6. 163-177.

Rhoades, J.D. (1983). Using saline waters for irrigation, International Workshop on Salt­affected Soils of Latin America, October, 1983, Venezuela.

Ryden, J.C. and Pratt, P.F. "Phosphorus Removal from Waste Water Applied to Land", Hilgardia, Vol. 48, 1980, pp. 1-36.

317

Schulte, D.D., Kottwitz, D.A. and Gilbertson, C.B. (1985). Nitrogen content of scraped swine manure. In' Agricultural Waste Utilisation and Management'. Proc. 5th Int. Symp. on Agric. Wastes. Chicago, ASAE. p.321-328.

Shaw, RJ., Hughes, K.K., Thorburn, PJ. and Dowling, A.J. (1987). Principles of landscape, soil and water salinity - process and management options. Part A. in "Landscape, Soil and Water Salinity. Proceedings of the Brisbane Regional Salinity Workshop, Brisbane, May 1987". Queensland Department of Primary Industries Conference and Workshop Series QC 87003.

Shaw, RJ. and Thorburn, PJ. (1985). Prediction of leaching fraction from soil properties, irrigation water and rainfall. Irrigation Science. 6, 73-83.

Thomas, J.W.M. (1992). Guidelines for Waste Water Irrigation. Publication No. 168. E.P.A. Victoria 103 pp.

Van Hom, H.H. (1990). Achieving environmental balance o{nutrient flow through animal production systems. Presented at 'American Feed Industry Association - Nutrition Symposium'. November 1990. St. Louis, MO.

Vanderholm,D.H. (1975). Nutrient Losses from Livestock Waste During Storage, Treatment and Handling. In Managing Livestock Wastes, Proceedings of the Third International Symposium on Livestock W~~s, 282-285. St. Joseph, Michigan: American Society of Agricultural Engineers.

Vollemweiller, RA. "Phosphorus, the Key Element in Eutrophication Control", Proc. Int. Conf. on Management Strategies for Phosphorus in the Environment, Lisbon, July 1985, pp. 1-60.

Watts, P.J., Gardner, E.A., Tucker, R.W., Moody, P.W. and Gilbert, M. (1992). Phosphorus balance for cattle feedlots. In 'Conference on Engineering in Agric. Albury, NSW, Australia' .. October 1992. pp. 153-158.

Watts, P.J. and McKay, M.E. (1986). Simulation Modelling :of Cattle Feedlot Hydrology. in Proceedings of the Conference on Agricultural Engineering, Bundaberg. 1986, 393-398. Canberra, Australia: Institution of Engineers, Australia.

Zhang, R.H., Day, D.L. and Ishibashi, K. (1990). Generation!lIld transport of gases in and out of liquid swine manure in under floor pits. In' Agricultural and Food Processing Wastes'. Proc. 6th Int. Symp., ASAE, Chicago, 1970. pp.486-493.

318

ABSTRACT

CHEMISTRY, TREATMENT AND DISPOSAL OF MUNICIPAL SEWAGE EFFLUENT AND SLUDGE

P.D. Beavers

Water Resources Commission, QDPI, Qld, 4000.

The issue of wastewater treatment and disposal is one of the major problems currently facing water authorities. Regulatory authorities are placing a greater emphasis on stringent water quality requirements for discharge to receiving waters. Alternative means of disposal such as land application are now being given higher priority by the water authorities. However, it is essential that all disciplines involved with land application have a sound understanding of the wastewater sources, volumes, biological and chemical composition. The chemical constituents of the wastewater must be known before land application proceeds in order that there is no long term contamination of the soil, crops and groundwater. A knowledge of the biological treatment processes that the wastewater receives before" disposal is essential. Wastewater contains pathogenic organisms which can be transmitted through the environment back to man. Sound management practices are to be implemented to ensure that the public health of the community is not compromised in any way.

1 INTRODUCTION

The treatment and disposal of faecal wastes and other waste material from human activity has been a constant problem wherever communities have developed. The problem can be easily solved - "dig a hole and bury it," - and the consequences of failure to do so are confined to those who are responsible. The pattern in fact is laid down in the Book of Deuteronomy (Chapter 23, V.12-13).

"Designate a place outside the camp where you can to relieve yourself. As part of your equipment have something to dig with, and when you relieve yourself, dig a hole and cover up your excrement."

Where others are involved, however, the risks are greatly increased, and responsibility for adoption of sanitary disposal methods is greater.

We now live in a society where cities and towns generate such large quanTITIes of wastewater and sludge that the infrastructure for the safe disposal of these wastes is stretched beyond its capacity. No longer can effluent be discharged to the nearest receiving water and then be ignored. Also with the increasing emphasis on higher quality sewage effluents, the spectre of huge stockpiles of sewage sludge looms over the community.

319

The question that is being asked is "How can we safely dispose of our wastes to the environment?" Before this question can be satisfactorily answered an understanding of the source, volumes and composition of the wastewater is necessary. Therefore, the first part of this paper looks at the volume of wastewater produced and the chemistry of the wastewater that is produced from the treatment processes.

A brief introduction to the treatment of wastewater is presented in the second part of the paper. The treatment processes produce two products; effluent and sludge. The disposal and public health aspects of effluent and sludge are also included.

2 GENERATION OF EFFLUENT AND SLUDGE

Municipal wastewater in developed countries is generated from two principal sources;

domestic dwellings in urban areas; and industry.

Domestic wastewater is produced by households that have an in-house multiple-tap water supply service and flush toilets connected to a sewer system. All the household wastewater, with the exception of rainfall run-off, is discharged into the sewer. Industry sources are numerous, but include wastewater after pre-treatment, from food processing, breweries, manufacturing, etc.

The domestic and industrial wastewaters are collected in sewers where they are transported to a treatment plant (Figure 1). Treatment of the wastewater results in two main products:

the secondary or tertiary treated effluent, and the raw, secondary or tertiary sludge.

WASTEWATER TREATMENT

SLUDGE TYPE

SLUDGE TREATMENT

......... _ .. r::l ...... ~ ...... Odor Control and "L:..J ..,....~...,.... Pathogen Reduction t . Stabilization

EJ Water Removal. ...... Raw ...... Volume Reduction,

Secondary ...,.... Secondary ...,.... and Possibly Mass Reduction

• Thickening

• Conditioning

• Dewatering

SLUDGE USE/DISPOSAL

• Land Application • Distribu ticn and

.. Marketing • Landfilting • Incineration • Ocean Disposal

Tertiary .. Raw .. cg -Crying

L..I_Ad_v_an_ce_dl....J Tertiary '--____ -'

= Wastewater

.. = Sludge

Figure 1 Generation, Treatment and Disposal of Municipal Wastewater

320

3 QUANTITIES OF WASTEWATER

Wastewater is the general term applied to the liquid waste collected in sanitary sewers and treated in a municipal wastewater treatment plant. The volume of wastewater generated in a community on a per capita basis varies from 180 to 600 L/cap/day. A typical average per capita flow for Queensland is 240 L/cap/day which includes domestic wastewater plus infiltration inflow but would exclude industrial wastewaters.

The wide range of per capita flows reflects differences in water consumption among commumtIes. For Brisbane and surrounding South-East Queensland urban areas the average water consumption is 600 L/cap/day. However, some 400 L/cap/day is external use; ie garden watering, car washing etc. The balance is internal use comprising toilet flushing, laundry, bathroom and kitchen.

Based on the above, a domestic dwelling of four persons generates 350 kL of wastewater per annum.

Wastewater is composed of 99.6% water and 0.4% other material (suspended, colloidal and dissolved solids). Sludge, which is a product of the treatment process, is 98% water and 2% solids. In Queensland, approximately 20,000 tonnes per annum of sludge are generated on a dry weight basis.

Brisbane generates some 48 dewatered tonnes per day (225 cubic meters per day) of sewage sludge (Lever, 1990) .. To put this in some perspective, if this sludge was placed on Lang Park, the stockpile would be 15 metres deep in twelve months.

Urbanisation and more intensive and efficient wastewater treatment systems are resulting in the production of increasing quantities of sludge for disposal. Sludge handling is responsible for about 30-40% of the capital cost of a treatment plant, and about 50% of the operating cost.

4 WASTEWATER CONSTITUENTS AND COMPOSITION

The physical, chemical and biological constituents of wastewater are important parameters in the design and operation of treatment and disposal facilities. These parameters are also of importance in the engineering management of environmental quality. The constituents of concern in wastewater treatment and disposal are listed in Table 1.

Composition refers to actual amounts of physical, chemical and biological constituents present in the wastewater. The composition of the raw wastewater and the treated effluent depend upon the following:

· composition of the municipal water supply, · number and type of commercial and industrial establishments, · nature of the residential community.

321

Table 1 Constituents of Concern in Wastewater Treatment and Irrigation with Reclaimed Wastewater

Constituent

Physical

Suspended Solids

Biological

Biodegradable organics

Pathogens

Chemical

Nutrients

Stable (refractory) organics

Hydrogen ion activity

Heavy Metals

Dissolved inorganics

Residual chlorine

Measured Parameters

Suspended solids including volatile and fixed solids

Biochemical oxygen demand, Chemical oxygen demand

Indicator organisms, total and faecal coliform bacteria

Nitrogen Phosphorus Potassium

Specific compounds (e.g., phenol, pesticides, chlorinated hydrocarbons)

pH

Specific elements (e.g., Cd, Zn, Ni, Hg)

Total dissolved solids, electrical conductivity, specific elements (e.g., Na, Ca, Mg, C7, B)

Free combined chlorine

After Pettygrove, G & Asano, T., 1984

Reason for Concern

Suspended solids can lead to the development of sludge deposits and anaerobic conditions when untreated wastewater is discharged in the aquatic environment. Excessive amounts of suspended solids cause plugging in irrigation systems.

Composed principally of proteins, carbohydrates, and fats. If discharged to the environment, their biological decomposition can lead to the depletion of dissolved oxygen in receiving waters and to the development of septic conditions.

Communicable diseases can be transmitted by the pathogens in wastewater: bacteria, virus, parasites (See Section 7.0).

Nitrogen, phosphorus, and potassium are essential nutrients for plant growth. and their presence normally enhances the value of the water for irrigation. When discharged to the aquatic environment, nitrogen and phosphorus can lead to the growth of undesirable aquatic life. When discharged in excessive amounts on land, nitrogen can also lead to the pollution of groundwater.

These organics tend to resist conventional methods of wastewater treatment. Some organic compounds are toxic in the environment, and their presence may limit the suitability of the wastewater for irrigation.

The pH of wastewater affects metal solubility as well as alkalinity of solids. Normal range in municipal wastewater is pH= 6.5-8.5, but industrial waste can alter pH significantly.

Some heavy metals accumulate in the environment and are toxic to plants and animals. Their presence may limit the suitability of the wastewater for irrigation.

Excessive salinity may damage some crops. Specific ions such as chloride, sodium, boron are toxic to some crops. Sodium may pose soil permeability problems.

Excessive amount of free available chlorine (>0.05 mg/L Clz) may cause leaf-tip burn and damage some sensitive crops. However, most chlorine in reclaimed wastewater is in a combined form, which does not cause crop damage. Some concerns are expressed as to the toxic effects of chlorinated organics in regard to groundwater contamination.

Consequently, the composition of wastewater varies widely among different communities. Typical data on the composition of raw wastewater in Queensland is presented in Table 2.

322

TABLE 2 Typical Composition of Domestic Wastewater in Queensland

Weak

Constituent Unit Strong

Suspended Solids mg/L 350 140

BODs mg/L 450 150

Total Organic Carbon mg/L 250 160

Ammonia Nitrogen mg/L 60 20

Total Phosphorous mg/L 30 6

pH 6.5 - 8.0 6.5 - 8.0

Conductivity ).ls/cm 900 700

Total Dissolved Salts mg/L 650 500

Total Sulphide mg/L 15 5

Alkalinity mg/L 300 50

Total Oil and Grease mg/L 100 10

5 EFFLUENT CHARACTERISTICS

Wastewater quality data routinely measured and reported are mostly in gross pollutional parameters (eg. biochemical oxygen demand, suspended solids). These are the parameters of most interest when discharging the effluent to a surface water.

In contrast, for land application of effluent, the characteristics of importance are the specific chemical elements and compounds that affect plant growth or soil permeability. These parameters are not usually measured by Local Authorities as part of their routine water quality monitoring program. Therefore, when a land application scheme is being planned, it will be necessary to sample and analyse the effluent for the constituents that define the suitability of the water for agricultural or landscape irrigation.

5.1 Chemical Parameters

The parameters which largely determine the suitability of an effluent for land application are listed in Table 3 (Pettygrove & Asano, 1984). They relate to the presence of dissolved salts and other chemicals which may impact on plant growth directly through toxicity effects or indirectly through adverse affects on soil characteristics.

323

Table 3 Laboratory determinations needed to evaluate irrigation water suitability

Parameter

Salinity

Electrical Conductivity Total Dissolved solids

Individual Cations and Anions

Calcium Magnesium Sodium Carbonate Bicarbonate Chloride Sulphate

Miscellaneous

Boron pH Sodium Adsorption Ratio

Symbol

EC TDS

Ca++ Mg++ Na+

C03

HC03-

Cf S04-

B pH SARa

Unit

JlS/cm mg/L

mg/L mg/L mg/L mg/L mg/L mg/L mg/L

mg/L

Usual range in irrigation water

0-3000 0-2000

o - 400 (0 - 20.0 meq/L) 0-60 (0 - 4.9 meq/L) o -900 (0 - 39.1 meq/L) 0-3 (0 - 0.1 meq/L) o -600 (0 - 9.8 meq/L) 0- 1100 0- 1000

0-2 6.5 - 8.5 0- 15

a. SAR is calculated from Equation l.Source Pettygrove and Asano, 1984.

5.1.1 Salinity

Salinity is the single most important parameter used in determining the suitability of treated effluent for irrigation. It is measured by electrical conductivity (EC expressed as micro Siemens/cm) which is a measure of the ability of water to carry electric charge by virtue of the ions in solution. Therefore as salinity, which is due to the concentration of anions and cations in water, increases so does electrical conductivity. The test provides a rapid indication of salinity.

Total dissolved salts (IDS, expressed in mg/L) in irrigation water is normally calculated from EC. For any water there is a direct relationship between TDS and EC in the range of 0.5 to 0.7 depending on type and quantity of dissolved salts. For irrigation assessment a factor of 0.64 is commonly used, that is:

TDS mg/L = EC, lls/cm x 0.64

Salt is continually added to the soil with the effluent, and a problem occurs if the added salts accumulate to a concentration that is harmful to the crop or landscape. The rate of accumulation depends upon the quantity of salt applied in the effluent and the rate at which salt is removed by leaching. That is, over an extended period of time.

Salts in = Salts out.

324

Fortunately, most salts are soluble and easily transported by water added to the soil. Applying more effluent than can be used by the crop assures that salt removal takes place (leaching). Establishing a net downward flux of water and salt through the root zone is the only practical way to manage a salinity problem. Under such conditions, good drainage is essential in order to allow a continuous movement of water and salt below the root zone.

Water can be classified into salinity groups based on EC as shown in Table 4. Such classifications are based on average conditions of soil, drainage, climate and salt tolerance. Interpretation of data for effluent use requires a knowledge of the salt tolerance of the plant species and the properties of the soil type to be used.

5.1.2 Sodium and Permeability Hazard

High concentrations of sodium in the irrigation water may adversely affect the soil structure and reduce the soil hydraulic conductivity in fine textured soils.

Table 4 Salinity Classes for irrigation water

Classification EC, microSiemens/cm

Low salinity water - safe to use on practically all grasses and soils

Medium salinity - generally safe for grasses with moderate salt tolerance on relatively permeable soils

High salinity - should not be used on soils with inadequate subsurface drainage. Salt tolerant grasses should be used. Appropriate soil/water management practices will be required.

Very high salinity - suitable for high salt tolerant species and use appropriate soil/water management techniques.

Too salty for irrigation

USDA"

<250

250 - 750

720 -2250

a: USDA Handbook 60, 1954

-<280

280 - 800

800 - 2300

> 5500

b: ANZEEC, Draft Australian Water Quality Guidelines, 1992 c: Queensland DPI d: Figures are for Corrected Electrical Conductivity

< 650

650 - l300

l300 - 3000

5000 - 8000

>8000

The sodium hazard of irrigation water is expressed In terms of the sodium adsorption ration (SAR) and is calculated using the formula:

325

SAR Na+

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. (1)

Where the ion concentrations are expressed in meq/L as shown in Table 3.

It is a measure of the rate at which sodium is adsorbed by the soiL Clay minerals in soils have cation exchange sites (measured by the Cation Exchange Capacity) for which the cations calcium, magnesium, sodium and potassium compete. When calcium is the dominant cation adsorbed, the soil will have a granular structure which is readily workable and permeable. When sodium is the dominant cation absorbed (> 10 to 15% of the total exchange capacity) the soil, when wet, becomes dispersed thus lowering the permeability and, when dry, it forms a hard impermeable crust. Most researchers agree that problems with soil permeability increase when the SAR approaches 6 - 10 depending on soil type. Gypsum application is the normal ameliorative soil treatment in such situations. In some situations an adjusted SAR is calculated to allow for the calcium that may precipitate out in the soil resulting in an increased SAR in the soil environment.

Sodium sensitive plants can be injured by sodium accumulation in the plant tissue at levels in the water much lower than those that are associated with soil structural problems. Woody plants, in particular fruit trees tend to be most sodium sensitive. Figure 2 can be used to classify the potentialsQdium hazard to the soil structure.

Potassium can have similar effects to that of sodium but in most waters its concentration is quite low, posing little hazard (Beehag, 1992).

~ !. -c: .. ";i > '5 C'

~ ] ~ ::J 0 51

Figure 2

32

28

24

20

16

12

8

4

0 0

, . , I

S4 Very High-Sodium

Water

5 10

S2 Medium-Sodium

Water

IS 20 25 30 35

CALCIUM PLUS MAGNESIUM (milli1Quivalenu per litre I

Water quality criteria for sodium

326

Low Sodium Water (SI) can be used for inigation on almost all soils with little danger of the development of a sodium problem. However, sodium sensitive crops such as stone fruit trees and avocados, may accumulate injurious amounts of sodium in the leaves.

Medium Sodium Water (S2) may present a moderate sodium problem in fine textured (clay) soils unless there is gypsum in the soil. This water can be used on coarse textured (sandy) or organic soils that take water well.

High Sodium Water (S3) may produce troublesome problems in most soils and will require special management, good drainage, high leaching and additions of organic matter. If there is plenty of gypsum in the soil, a serious problem may not develop for some time. If gypsum is not present, it, or some similar material may have to be added.

Very High Sodium Water (S4) is generally unsatisfactory for inigation except at low or medium salinity levels, where the use of gypsum or some other additive makes it possible to use such water.

5.1.3 Chloride

High chloride levels in the effluent will affect the growth of many plants. No problems are apparent if the chloride concentration is less than 140 mg/L, however problems will increase with concentrations between the 140 - 350 mg/L range and severe problems when it is above 350 mg/L (Bouwer and Idelovitch, 1987). These concentrations apply to surface or other inigation systems where water is absorbed by roots only.

Direct foliar application of effluent by spray inigation can cause leaf damage to chloride senslttve species. Bouwer and Idelovitch, (1987) recommend that the chloride concentration should be below 100 mg/L to avoid problems during spray inigation.

5.1.4 Trace Elements

Sewage effluent which is predominantly domestic in ongm is not likely to have high concentrations of heavy metals. The recommended maximum concentrations in inigation water of trace elements for long term inigation use are summarised in Table 5. These levels take account of both the toxicity to plants and long term soil accumulation from water application.

Typical concentrations of trace elements found in effluent from Brisbane City Council Treatment Plants are given in Table 6. The concentrations show little potential problem from trace element accumulation. Small to medium size communities which are not highly industrialised would be expected to have lower concentrations of trace elements.

327

Table 5

Element

Al (Aluminium)

As (Arsenic)

Be (Beryllium)

Cd (Cadmium)

Co (Cobalt)

Cr (Chromium)

Cu (Copper)

F (Fluoride)

Fe (Iron)

Li (Lithium)

Hg (Mercury)

Mn (Manganese)

Mo (Molybdenum)

Ni (Nickel)

Pb (Lead)

Se (Selenium)

V (Vanadium)

Zn (Zinc)

Recommended Maximum Concentrations of Trace Elements in Irrigation Waters

Recommended maximum concentration. mg/L

5.0

0.10

0.10

0.01

0.05

0.1

0.2

1.0

1.0

2.5

0.002

0.2

0.01

0.2

0.2

0.02

0.1

2.0

Remarks

Can cause reduced productivity in acid soils (PH < 5.5) but more aI.kaIine soils at pH > 5.5 will precipitate the ion and eliminate toxicity.

Toxicity to plants varies widely. ranging from 12 mg/L for Sudan grass to < 0.05 mg/L for rice.

Toxicity to plant varies widely. ranging from 5 mg/L for kale to 0.5 mg/L for bush beans.

Limit based on potential for accumulation in plants and soils to concentrations harmful to humans. Cd in nutrient solutions in range 0.1 to 1.0 mg/l toxic to variety of plants.

Tends to be inactivated by D8utral and alkaline soils. Toxic to variety plants in range 0.1 to 5.0 mg/L in nutrient solution.

No evidence that is essential to plant growth. Conservative limit recommended because of lack of knowledge on plaJit toxicity.

Essential plant nutrient but toxic to a number of plants at 0.1 to 1.0 mg/L in nutrient solutions. Plant uptake greater in acid soil

Inactivated by D8utral and alkaline soils. Main concern is for animal and human consumers of plants high in fluoride.

Staining of plants. equipment, buildings etc. Deposits can interfere with plant photosynthesis and respiration and clog irrigation equipment.

Tolerated by most crops up to 5 mg/L [except citrus· toxic at low levels (> 0.075 mg/L)]. Mobile in soil

Strongly retained by soils especially those high in organic matter. Not readily taken up by most plants due to low availability in soil solution.

Essential for plant growth but toxic to a number of crops at levels as low as· 0.5 mg/L. usually only in acid soil. As for iron may cause staining and clogging problems.

Not toxic to plants at normal concentrations in soil and water. Can be toxic to livestock if accumulated in forage.

Toxic to a number of plants at 0.5 to 1.0 mg/L. Reduced toxicity at neutral and alkaline pH.

Can inhibit plant cell growth at very high concentrations. Uptake favoured by acid soil pH « 4.5). .

Toxic to some plants at concentrations as low as 0.025 mg/L. Essential to livestock in very low concentrations but toxic if forage grown on soils with high levels of available selenium.

Toxic to many plants at relatively low concentrations.

Essential for plant growth but excess may be toxic to plants at widely varying concentrations. Reduced toxicity at pH >6.0 and in fine textured or organic soils.

(Adapted from Hart, 1992 and Pettygrove and Asano, 1985)

328

Table 6 rrace Element Concentrations in Treated Emuent from Selected Brisbane City Council Plants (mg/L)

BRISBANE CITY COUNCIL GOLD COAST

ELEMENT LUGGAGE POINT COOMBABAH

Aluminium 0.12 Arsenic ND Cadmium < 0.01 0.001 Chromium < 0.01 0.007 Copper 0.012 0.15 Iron 0.300 0.16 Lead < 0.05 0.001 Mercury < 0.0003 ND Manganese 0.07 Nickel < 0.015 0.003 Selenium ND Silver ND Zinc 0.055 0.07

5.1.5 Nitrogen

Nitrogen is an intriguing constituent of treated effluent because of its importance as a fertiliser in irrigated agriculture, its adverse effects when discharging the effluent to a receiving water and the various forms in which it can be found in the effluent.

The nitrogen content of a secondary treated effluent is generally within the range of 15 - 40 mg/L, but the nitrogen concentration, as well as the form of nitrogen (NH4-N, N03-N, organic-N), depend on the degree and type of treatment that is given. Table 7 shows the variation in total nitrogen and the forms of nitrogen that occur in effluent from two plants operated by the Gold Coast City Council.

Table 7 Nitrogen Concentration in Gold Coast Treated Emuents

NUTRIENT

Nitrate nitrogen (N03-N), mg/L as N Ammonia nitrogen (~-N), mg/L as N Organic nitrogen (Org-N), mg/L as N Total nitrogen (Total N), mg/L as N

na: not available

COOMBABAH

2.6 0.6 2.0 5.2

ELANORA

20.7 1.1

na > 21.8

With each mg/L of nitrogen contained in the wastewater, about 10 kg/ha N are applied with each metre of effluent. Considering that the nitrogen concentration of secondary effluent is generally within the range of 15 to 40 mg/L, the nitrogen application with effluent will vary between 150 - 400 kg/ha for every metre of effluent per annum.

329

Treatment processes, discussed in Section 7, show that it is possible to reduce the nitrogen in the effluent down to 5.0 mg/L. But, reducing nitrogen down to these levels may negate the value of effluent as a fertiliser.

Another problem related to the use of nitrogen in effluent as fertiliser is that the water demand and the nitrogen demand are not parallel. For most crops, nitrogen demand is highest during the period of active growth and lowest during initial growth stages and when harvest time approaches.

5.1.6 Phosphorous

Phosphorus is another major nutrient found in sewage effluent. The phosphorus in the effluent from secondary treatment systems varies from 6 to 15 mg/L unless removal has been accomplished during treatment.

The available information on the effect of irrigation with phosphorus-rich effluents is limited. Many soils, however, are successfully irrigated with sewage effluent having normal P-concentrations of about 5 - 10 mg/L.

5.1.7 Potassium

Potassium is not a constituent measured in effluents by Local Authorities. Petty grove and Asano, (1984), give the range of potassium in secondary effluents as 10 - 30 mg/L. Because of the value of potassium in agriculture, Local Authorities will be advised to include it as a measw~J:>le constituent.

5.2 Physical Parameters

5.2.1 Suspended Solids

The level of suspended solids in the effluent . depends on the degree of treatment. Approximately 60 - 70% of the solids are taken out during primary treatment. However, a primary treated effluent with a high suspended solids has the potential to actually kill crops.

Secondary treated effluents generally have suspended solids ranging from 5 to 50 mg/L. A good quality effluent would have a suspended solids concentration of < 10 mg/L.

Suspended solids, which are mostly organic in nature in sewage effluents, adversely affect the efficiency of disinfection since bacteria and viruses can be protected by organic particles. Suspended solids also detract from the aesthetics of using sewage effluent for irrigation, through the deposition of colloidal particles on leaves.

Clogging problems with sprinkler and drip inigation systems have been reported due to heavy concentrations of algae and suspended solids. Sand filtration of the effluent can be used to remove suspended solids.

330

5.3 Biological Parameters

5.3.1 Biodegradable Organics

The biodegradable organics, which enter the sewage water with human and kitchen wastes, are readily decomposed in the soil. The presence of biodegradable organics is not a major concern for inigation waters. However, the presence of organics in the effluent discharged to a surface water can deplete the available oxygen in the surface water.

All aerobic organisms require oxygen for respiration. Therefore, it is essential to determine the five day biochemical oxygen demand (BODs) of the effluent. The BODs test measures the oxygen required by micro-organisms during degradation of a water sample.

An effluent which has a high BODs will cause oxygen to be used up by the micro-organisms in the effluent and receiving water. Eventually, the oxygen is depleted in the receiving water, thus causing the plants and aquatic animals to die.

6 MUNICIPAL WASTEWATER SLUDGE

Municipal wastewater sludge is a by-product of wastewater treatment as shown in Figure 1. Typically, a liquid sludge has a solids content of 1 - 10%, and a dewatered sludge 15 - 40% solids, which includes chemical additions used during. treatment.

The characteristics of the sludge depend on both the iHitial composition and subsequent wastewater and sludge treatment processes used. DIfferent treatment processes will generate radically different types and volumes of sludge.

6.1 Classification of Wastewater Sludges

The simplest classification of wastewater sludges is according to the process from which they are produced.

Raw or Primary Sludge is drawn from the primary sedimentation tanks. It contains all of the readily settleable matter from the wastewater, has a high organic content, mainly faecal matter and food scraps, and is thus highly putrescible in nature. In its fresh state, raw sludge is grey in colour with a heavy faecal odour. Storage under anoxic conditions will rapidly lead to the onset of putrefaction and extremely unpleasant odours.

Humus Sludge comprises the underflow from the humus tanks which follow trickling filters. It consists of the biological solids sloughed or scoured from the surfaces of the filter media. Being largely organic matter in content, it exhibits similar problems to primary sludges under anoxic or anaerobic conditions.

Waste Activated Sludge is drawn off the return sludge system from the underflow of the final sedimentation tanks in the activated sludge process. It consists of light flocculant biological solids with a significant demand for oxygen, largely due to the respiration of

331

the sludge micro-organisms. Activated sludges have a much higher water content than other organic wastewater sludges and exhibit greater dewatering problems.

Chemical Sludge is produced by processes involving chemical coagulation or chemical precipitation. Such processes are seldom used in conventional biological wastewater treatment. The sludge comprises mainly the reaction products of the applied chemical and the impurity to be removed.

Digested Sludge is the product of either aerobic or anaerobic digestion. It is a well stabilised material able to be dewatered by mechanical means or on open drying beds. Well stabilised anaerobically digested sludge has a black appearance with a tarry odour, is non-putrescible and no longer attractive to flies and other vectors.

6.2 Sludge Characteristics

The characteristics of the sludge to be treated require definition for the rational selection of the unit processes for sludge treatment and disposal. Major sludge treatment processes and their benefits are described in Section 8. The following discussion is concerned with defining some important parameters used in measuring sludge characteristics.

Total Solids (TS) is the usual method of measuring the gross solids content. It IS

expressed as a percentage of the original wet sludge mass.

Volatile Solids (VS) is a measure of the organic content of the sludge. It gives an indication of the degree of stabilisation which could be achieved by biological processes. Volatile solids content is usually quoted as a percentage of the total solids, but can also be quoted as a percent of the original wet sludge mass.

Fixed Residue (FR) is the mass of inorganic matter in the sludge (FR = TS - VS). It is also the minimum mass of solids which would remain for ultimate disposal after incineration.

Some typical ranges of total and volatile solids encountered in practice are presented in Table 8.

6.2.1 Moisture Content

Since sludges generally contain between 1 - 10% solids by weight, their major component is water. Furthermore, since sludge solids are of similar density to water, the water content accounts for most of the volume of wet sludges. Sludge moisture is therefore the parameter which has the greatest effect on the volume of sludge to be processed.

Doubling the solids content of a sludge (e.g. raising it from 5 to 10 percent TS) halves the volume of sludge that must be used or disposed (See Figure 3).

332

Table 8 Total and Volatile Solids Content of Sludges

SLUDGE TYPE

Primary sludge

Activated sludge

Digested sludge

Humus sludge

High rate plastic media sludge

Source: Bliss 1988

w (!) o :::> ~ (/)

40

10

o 10

TOTAL SOLIDS CONTENT (%)

20

3 - 7

0.5 - 2.0

1 - 3

1 - 4

3 - 8

This example is for a sludge with a mass of 910 kg (2,000 Ibl of dry sludge solids. This mass remains constant at every point on the curve. Other sludge masses would show similar reductions in volume with increasing solids concentration.

30 40 50 60 SLUDGE SOLIDS CONCENTRATION (%J

VOLATILE SOLIDS

(% DRY WEIGHT)

60 - 80

67 - 75

45 - 60

64 - 86

Figure 3 Change in Sludge Volume with increase in Sludge Solids Concentration

6.2.2 pH

The acidity of a sludge affects the availability of heavy metals, the pathogen content of the sludge and the corrosivity of the sludge. High pH (greater than 11) sludges destroy many bacteria and, in conjunction with soils of neutral or high pH, can inhibit movement of heavy metals through soils and uptake of heavy metals by plants. Conversely, low pH (less than about 6.5) sludges promote leaching of heavy metals and greater crop uptake of metals. Leaching of heavy metals can occur at landfills because acid conditions often prevail. Thus, pH affects the suitability of sludge for land application, distribution and

333

marketing and landfilling. Low pH sludges are also corrOSIve and must be treated to prevent equipment damage.

6.2.3 Degree of Stabilisation

Stabilisation refers to a number of processes that reduce the potential for odours, reduce pathogen levels and volatile solids content. Major methods of stabilisation include anaerobic digestion, aerobic digestion, lime stabilisation and composting. Because much of the organic material has already been eliminated, stabilised sludges tend not to have odour problems.

6.3 Sludge Constituents

The composition of sludge can limit the choice of use/disposal options or make some options more appealing. The five constituents that are usually the most important in decision-making are:

• organic content; • nutrients; • pathogens; • metals; • toxic organic chemicals.

Figure 4 demonstrates the importance of these constituents for the vanous sludge use/disposal options.

6.3.1 Organic Content

The organic content of a sludge is usually expressed as a percent volatile solids. Unstabilised sludges usually contain 75 - 85% volatile solids on a dry weight basis. Stabilisation of the sludge reduces the organic content to 45 - 65% volatile solids. Organic content is an important determinant of the potential for odour problems (in storage and land application), value as a soil conditioner (in land application), thermal value (in incineration) and gas production during digestion.

6.3.2 Nutrients

Sludge contains three essential nutrients for plant growth: nitrogen, phosphorous and potassium. Typical nutrient levels in sewage sludge reported by the Brisbane City Council are given in Table 9.

The exact ratio for these nutrients is not that of a well balanced formulated fertiliser; nevertheless most agronomic crops respond favourably to the nutrients in sludge.

334

(:l z z-

z ~

Ow 0 -",

;: ~a: :l< <0(

u ~~ 0:::; Z"- ~o

<0('" '!!z .... < 0<

Volatile Solids ~ ~

Nutrients • ~

Path09"ns • • Metall • • Toxic: Orl1anic ~ ~ Chemicala:

• Based on current EPA infounation: however. assessment of pot.,nt;"1 effects continues.

(:l Z 0

z ;: .j ;:j < <0( .... ex: Z'" ~ w

0 z <0 Z U ...... <0(

~ u'!!

.... 00

~ • 0

Q 0 Q

. o· 0 0

Q ~ ~

~ Q ~

• V.-:y Il'T1i>Ortant

~ Mode~tely Important

o N~ Important

{

Figure 4 Importance of Sludge Constituents to Sludge Use/Disposal Options (USEP A, 1984)

Table 9 Nutrient Content of Sludges from Oxley Creek Sewage Treatment Plant (1/4/90 - 22/6/90)

All results reported on dry weight sludge basis mg/kg

PARAMETER MEAN STD. DEV. RANGE

Calcium 20,300 7,440 9,300 32,500 Potassium 1,570 309 700 2,120 Magnesium 3,760 690 1,640 4,650 Sodium 958 221 601 1,390 Total Nitrogen 35,800 2,000 33,600 38,900 Total Phosphorus 16,500 200 16,000 16,800 Ammonia/Nitrogen 3,890 303 3,330 4,560

Nitrogen may occur in sludge as organic nitrogen (org.N), and the inorganic forms of ammonium (NH/) and nitrate ions (N03). The concentrations of org.N, NH/ and N03 in sludge are affected by the treatment and handling processes used. Unless aerobic conditions have prevailed during sludge treatment, over 90% of the inorganic N will be as NH/.

335

Most of the org.N in sludge is associated with the sludge solids and thus org.N levels are not appreciably altered by sludge dewatering or drying processes. In contrast, NH/ and N03 are water soluble and their concentrations decrease rapidly during dewatering. Heat or air drying reduce the NH4+ through ammonia volatilisation but not the N03 level.

Phosphorus is present in sludge in organic and inorganic forms. The soluble phosphate is directly utilised by plant roots or absorbed by mineral clay and humus particles. Phosphate in excess of crop needs is strongly held to the soil particles and thus will remain as a valuable soils resource.

6.3.3 Pathogens

Sewage sludge contains a wide spectrum of bacteria, viruses, protozoa and eggs of parasitic worms. A detailed discussion on pathogens and the public health aspects is given in Section 7.

6.3.4 Metals

The "Heavy" metals which occur in sludge can be defined as those with densities greater than 5.0' g/cm3•

The metals of most interest which occur in sludge due to domestic contributions and the presence of industrial wastes include zinc, copper, nickel, cadmium, mercury, lead, chromium, and to a lessei'e~nt arsenic, molybdenum and mercury. At low concentrations in soil, some of these metals are essential micro-nutrients re,quired by plants and animals. However, at high concentrations, they may be toxic to humans, animals and plants. Table In lists ranges of metal concentration in sludge.

Metals may be present in sludge as carbonates, sulphides, organically bound, absorbed or exchangeable forms. The dominance of any fraction varies with the metal and with the treatment which the sludge has undergone. Generally, the cationic species are less mobile than the non-cationic forms of chelates and complexes and the organic ligands are humic substances arising from cellular matter. The nature of the metals present will influence the proportionment of metals between the solid and liquid phases. They could also influence initial availability to plants, when applied to agricultural land (Dean and Suess, 1985).

This paper will not cover specific details of the various metals in sludges. Some of the metals can cause health problems to humans and animals. However, for excessive exposure to a toxic substance to occur, the sludge must contain the substance at higher levels than those in the soil, or the substance applied in the sludge must be more bioadvailable than the soil contents to plants or animals. Table 11 from Dean and Suess (1985), groups the trace elements according to the potential effects of sludge application.

336

Table 10 Heavy Metals in Sewage Sludge (mg/kg dry solids)

BRISBANE CITY USEPA SLUDGE SURVEY* COUNCIL -

GOLD COAST CITY COUNCIL -COOMBABAH

LUGGAGE POINT METAL

No. OF MEAN STANDARD MEAN MEAN RANGE SAMPLES DEVIATION JUNE '91 - MAY '92

Arsenic 199 9.9 18.8 9.0 Cadmium 198 6.9 11.8 26.0 < 3.0 1 <6 Chromium 199 118.6 339.2 313.0 33.0 <6 50 Copper 199 741.2 961.8 719.0 526.0 506 560 Lead 199 134.4 197.8 255.0 70 45 83 Mercury 199 5.2 15.5 5.0 < 8.0 <6 10 Molybdenum 199 9.2 16.6 Nickel 199 42.7 94.8 82.0 13 <6 20 Selenium 199 5.2 7.3 2.5 Zinc 199 1,202.0 1,554.4 1,170 319 69 438

*USEPA, National Sewage Sludge Survey, 1990

The concentrations of metals are primarily a function of the type and amount of industrial waste that is discharged into the wastewater system. Trade waste controls are now being implemented by the major Local Authorities to reduce the metal levels in the sewage sludges. Good management practices in land application, landfilling and incineration will minimise or eliminate the potential for adverse effects.

Table 11 Response of Plants, Animals and Humans to Elements in Municipal Sewage Sludge Applied to Agricultural Land

GROUP ELEMENT

(1) Potentially injurious to humans, increased in plants or animals {Cadmium { Lead { Mercury f Nickel

(2) Increases in livestock eating sludge and phytotoxicity { Cobalt* { Copper* { Fluorines * { Iron { Molybdenum*

(3) Increased in plants, some phytotoxicity { Boron {Manganese { Zinc

(4) No effects on plants or animals { Antimony { Arsenic

* Some plant uptake

337

{ Beryllium { Chromium { Selenium* { Silver { Thallium { Tin { Tungsten

6.3.5 Toxic Organic Chemicals

Sludges can contain synthetic organic chemicals from industrial wastes, household chemicals and pesticides. The sources of organic substances vary and in many cases will be specific to a particular location.

Sludge treatment processes have relatively little effect on hazardous organic compounds. There is little biodegradation of polychlorinated biphenyls (PCBs), organochlorine pesticides during sludge treatment in contrast to degradation by soil bacteria (Dean and Suess, 1985).

With the exception of a small number of substances of special concern, very little is known about the concentrations and fate of organic chemicals in treated wastewater and sludge. Removal of polynuclear aromatic hydrocarbons (PARs), PCBs and pesticides from the wastewater occurs in conventional treatment plants. Much of the reported removal from the effluents undoubtedly takes place by transfer to the sludge.

The principal organic chemicals expected to occur in sludge are presented in Table 12. Also shown in Table 12 are usual ranges of these organic contaminants. The data for the "usual range" are based on large surveys of plants in Europe, North America and Canada. Local data is also shown.

Table 12

CONTAMINANT

PAR PCG Dieldrin Lindane Aldrin DDT DDE Chlorpyrifos

Summary of Concentrations of Some Organic Contaminants found in Sewage Sludge

CONCENTRATION (MG/KG DRY SOLIDS)

BRISBANE CITY COUNCIL

USUAL RANGE* GOLD COAST LUGGAGE POINT OXLEY

0.01 50.00 0.16 9.11 0.018 3.900 1.052 0.306 < 0.10 0.025 0.410 < 0.10 0.02 0.25 0.02 0.80 < 0.10

0.02 0.093 0.056 < 0.10 0.305 0.120

* Adopted from Dean and Suess, 1985.

7 PUBLIC HEALTH ASPECTS

Wastewater contains pathogenic organisms which can be transmitted from the wastewater back to man. The cycle of pathogen transmission through the environment is demonstrated in Figure 5.

338

Fiigure 5 Cycles of Pathogen Transmission through the EnVironment

Municipal wastewater generally contains four major types of human pathogenic (disease causing) organisms:

• Bacterial (Salmonella, Clostridium perfungens); • Protozoan parasites (Giardia, Entamboeba, Cystosporiduim); • Viruses (Enteroviruses, Adenoviruses, Gastroenteritis); • Helminth parasites (Ascaris lumbricoides, Taenia saginata, Taenia soluim).

The actual species and density of pathogens present in wastewater from a particular municipality depend on the health status of the local community and may vary substantially at different times. The level of pathogens present in sludge depends on the reductions achieved by the wastewater and sludge treatment processes.

The pathogens in wastewater are primarily associated with insoluble solids. Primary wastewater treatment processes concentrate these solids into sludge, so untreated or raw primary sludges have higher densities of pathogens than the incoming wastewater. Biological wastewater treatment processes such as lagoons, trickling filters and activated sludge treatment may reduce a number of pathogens in wastewater. Table 13 lists some principal pathogens of concern that may be present in wastewater and/or effluent and sludge.

339

For land application of effluent or sludge to pose an actual risk to health requires all of the following to occur (Mara and Cairncross, 1989):

(a) either an infective dose of an excreted pathogen reaches the field or pond, or the pathogen multiplies in the field or pond to form an infective dose;

(b) the infective dose reaches a human host;

(c) the host becomes infected; and

(d) the infection causes disease or further transmission.

The risk remains a potential risk if only (a),- or (a) and (b), or (a), (b) and (c) occur, but not (d).

There is ample evidence (Feacham et al, 1983) that wastewater and sludge may contain high concentrations of excreted pathogens. Many of these pathogens can survive in the soil or on crops for sometime and can also withstand most conventional treatment processes. Therefore,they can arrive at the disposal site in large enough numbers for human infection to be theoretically possible. The only way that this can be prevented from happening is to remove or kill the pathogens before they reach the disposal site.

Infection will only occur if an infective dose is received by a susceptible host. For some pathogens, such as Taenia Saginata, an intermediate host is required before infection can occur. Other pathogens are infective immediately on· excretion. Bacteria, such as salmonella spp., can survive longer transmission routes therefore they can pose real risks in effluent and sludge reuse schemes.

The extensive literature on the survival times of excreted pathogens in soil and on crop surfaces has been reviewed by Feacham et al 1983. There are wide variations in reported survival times, which reflect both strain variation and differing climatic factors. A summary of current knowledge on pathogen survival in soil and crops in warm climates (20 - 300~) is shown in Table 14.

The available evidence indicates that almost all excreted pathogens can survive in soils for a sufficient length of time to pose potential risks to agricultural workers and consumers of root vegetables. Pathogen survival on crop surfaces is much shorter than in soil. Survival times can last long enough to pose potential risks to crop handlers and consumers.

The pathogens in secondary treated effluent and sewage sludge can be reduced to below detectable levels by adequately treating the effluent or sludge prior to land application. Tertiary treatment processes such as rapid. sand filtration, slow sand filtration, land treatment and maturation ponds have varying degrees of pathogen removal.

Chlorination of the effluent is the most commonly used method of disinfection in Australia. However, effluent chlorination does have a number of serious limitations. Chlorine has to be applied in heavy doses (10 - 30 mg/L) to achieve coliform concentrations of less than 100 per 100 millilitres of effluent. If the effluent is discharged into a river or lake, the chlorine residual may adversely affect the ecology of the receiving water.

340

Table 13 Principal Pathogens of Concern in Municipal Wastewater and Sludge

Bacteria

Viruses

ORGANISM DISEASE/SYMPTOMS

Salmonella spp. Salmonellosis (food poisoning), typhoid fever

Shigella spp. Bacillary dysentery

Yersinia spp. Acute gastroenteritis (incl. diarrhoea, abdominal pain)

Vibrio cholerae Cholera

Campylobacter jejuni Gastroenteritis

Escherichia coli (pathogenic strains) Gastroenteritis

Poliovirus

Coxsackievirus

Echovirus

Hepatitis A virus

Rotavirus

Norwalk agents

Reovirus

Poliomyelitis

Meningitis, pneumonia, hepatitis, fever, common colds, etc.

Meningitis, pneumonia, hepatitis, fevet, common colds,

diarrhoea, etc.

Infectious hepatitis

Acute gastroenteritis with severe diarrhoea

Epidemic gastroenteritis with severe diarrhoea

Respiratory infecti0Ils, ,gastroenteritis R'!;~ ,''.-, ~

Protozoa

Cryptosporidium

Entamoeba histolytica

Giardia lamblia

Balantidium coli

Toxoplasma gondii

Helminth Worms

Ascaris lumbricoides

Ascaris suum

Trichuris trichiura

Toxocara canis

Taenia saginata

Taenia solium

Necator americanus

Hymenolepis nana

Source: USEPA 1989

Gastroenteritis

Acute enteritis

Giardiasis (including diarrhoea, abdominal cramps, weight

loss)

Diarrhoea and dysentery

Toxoplasmosis

Digestive and nutritional disturbances, abdominal pain,

vomiting, restlessness

May produce symptoms such as coughing, chest pain and

fever

Abdominal pain, diarrhoea, anaemia, weight loss

Fever, abdominal discomfort, muscle aches,neurological

symptoms

Nervousness, insomnia, anorexia, abdominal pain, digestive

disturbances

Nervousness, insomnia, anorexia, abdominal pain, digestive

disturbances

Hookworm disease

Taeniasis

341

Table 14 Survival Times of Selected Excreted Pathogens in Soil and on Crop Surfaces at 20 - 30°C

Viruses

Bacteria

Protozoa

PATHOGEN

Enterovirusesa

Faecal coliforms Salmonella spp. Vibrio cholerae

Entamoeba histolytica cysts

Helminths Ascaris lumbricoides eggs Hookworm larvae Taenia saginata eggs Trichuris trichiura eggs

From Feachem et al1983.

SURVIVAL TIME (DAYS)

IN SOIL

< 100 but usually < 20

< 70 but usually < 20 < 70 but usually < 20 < 20 but usually < 10

< 20 but usually < 10

Many months < 90 but usually < 30

Many months Many months

ON CROPS

< 60 but usually < 15

< 30 but usually < 15 < 30 but usually < 15 < 5 but usually < 2

< 10 but usually < 2

< 60 but usually < 30 < 30 but usually < 10 < 60 but usually < 30 < 60 but usually < 30

Complete viral removal may not be achieved with chlorination. Chlorination of the effluent is not effective irf eliminating protozoal cysts, because they are more resistant then either excreted viruses or bacteria.

Ultra-violet disinfection is being considered as an alternative to chlorination. For UV disinfection to be effective, the effluent should pass through a rapid sand filter to lower the suspended solids. However, because of the limitations of chlorination, UV disinfection will gain more prominence.

Various treatment processes are effective in controlling pathogens in sewage sludge. These processes use a variety of approaches to reduce pathogens and alter the sludge so that it becomes a less effective medium for microbial growth. Tables 15 and 16 list processes that have been shown to be effective in controlling pathogens.

The effectiveness of a particular process is best demonstrated by reference to the death curves derived for some pathogens, Figure 6 (Feachem et al 1983). Time-temperature points above the curve for each pathogen represent certain, total destruction.

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Table 15 Regulatory Definition of Processes to Significantly Reduce Pathogens (PSRPst

Aerobic Digestion: The process is conducted by agitating sludge with air or oxygen to maintain aerobic conditions at residence times ranging from 60 days at 15°C to 40 days at 20°C, with a volatile solids reduction of at least 38%.

Air Drying: Liquid sludge is allowed to drain and/or dry on underdrained sand beds, or on paved or unpaved basins in which the sludge depth is a maximum of 230 mm. A minimum of 3 months is needed, for 2 months of which temperatures average on a daily basis above O°C.

Anaerobic Digestion: The process is conducted in the absence of air at residence times ranging from 60 days at 20°C to 15 days at 35°C to 55°C, with a volatile solids reduction of at least 38%.

Composting: Using the within-vessel, static aerated pile, or windrow composting methods, the solid waste is maintained at minimum operating conditions of 40°C for 5 days. For 4 hours during this period, the temperature exceeds 55°C.

Lime Stabilisation: Sufficient lime is added to produce a pH of 12 after 2 hours of contact.

Other Methods: Other methods or operating conditions may be acceptable if pathogens and vector attraction of the waste (volatile solids) are reduced to an extent equivalent to the reduction achieved by any of the above methods.

Source: 40 CFR 257, Appendix II 815°C = 59°F, 20°C = 68°F, O°C = 32°F, 35°C = 95°F, 55°C = 131°F, 40°C = 104°F.

Table 16 Regulatory Definition of Processes to Further Reduce Pathogens (PFRPs)a

Composting: Using the within-veSsel composting method, the solid waste is maintained at operating conditions of 55°C or greater for 3 days. Using the static aerated pile composting method, the solid-waste is maintained at operating conditions of 55°C or greater for 3 days. Using the windrow composting method, the solid waste attains a temperature of 55°C or greater for at least 15 days during the compo sting period. Also, during the high temperature period, there will be a minimum of five turnings of the windrow.

Heat Drying: Dewatered sludge cake is dried by direct or indirect contact with hot gases and moisture content is reduced to 10% or lower. Sludge particles reach temperatures well in excess of 80°C, or the wet bulb temperature of the gas stream in contact with the sludge at the point where it leaves the dryer is in excess of 80°C.

Heat Treatment: Liquid sludge is heated to temperatures of 180°C for 30 minutes.

Thermophilic Aerobic Digestion: Liquid sludge is agitated with air or oxygen to maintain aerobic conditions at residence times of 10 days. at 55°C to 60°C, with a volatile solids reduction of at least 38%.

Other Methods: Other methods or operating conditions may be acceptable if pathogens and vector attraction of the waste (volatile solids) are reduced to an extent equivalent to the reduction achieved by any of the above methods.

Any of the processes listed below, if added to a PSRP, further reduce pathogens.

Beta Ray Irradiation: Sludge is irradiated with gamma rays from certain isotopes, such as 6OCobalt and 131Cesium, at dosages of at least 1.9 megarad at room temperature (ca. 20°C).

Pasteurisation: Sludge is maintained for at least 30 minutes at a minimum temperature of 70°C.

Other Methods: Other methods or operating conditions may be acceptable if pathogens are reduced to an extent equivalent to the reduction achieved by any of the above add-on methods.

Source: 40 CFR 257, Appendix ll. 855°C = 131, 80°C = 176°F, 180°C = 356°F, 60°C = 143°F, 70°C = 158°F.

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9 ~ ::l

E .., Co E " f-

Figure 6

70

65

60

55

50

45

40

35

30

25

. . ~ Entamoeba " hisloiyt;ca

· · ·

40

35

30

25

. 20 L-________ L-________ L--. ____ ~_,._----~~------__r20

10.000 0.1

1 day 1 week I month 1 year

Time (hours)

Influence of Time and Temperature on Selected Pathogens in Night Soil and Sludge

The lines represent conservative upper boundaries for pathogen death - that is, estimates of the time-temperature combinations required for pathogen inactivation. A treatment process with time-temperature effects falling within the "safety zone" should be lethal to all excreted pathogens (with the possible exception of hepatitis A virus at short retention times). Indicated time temperature requirements are at least: 1 hour at ~ 62°C, 1 day at ~ 50°C, and 1 week at ~ 46°C.

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8 WASTEWATER TREATMENT

In previous sections, reference has been made to conventional sewage treatment processes. Sewage treatment is a major topic in its own right, so it is only possible to give at present a very brief summary. The word "treatment" is probably not the best terminology, because each stage is essentially a separation process.

The broad objective of wastewater treatment is to remove impurities so that the effluent is able to be returned to the environment without causing unacceptable degradation of land, air or water and to similarly stabilise and dispose of the solid residues. To achieve this objective, the impurities are separated from the liquid stream.

The main problems in application of this objective are to define:

(a) the required uses of the receiving environment; (b) what constitutes acceptable degradation in the light of those uses; and (c) hence define the· appropriate treatment standard.

General wastewater and effluent quality characteristics to be considered in treatment plant design are:

BOD5 and SS Plant nutrients - nitrogen and phosphorous Pathogenic and indicator micro-organisms Toxic substances Oil and Grease

The stages of treatment, typical processes, their products and means of their disposal are presented in Figure 7. The relationship of these processes to the control of the quality characteristics listed above is discussed below.

8.1 Preliminary and Primary Treatment

Screening and grit removal serve protective functions for removal of rags and coarse matter and fine heavy grit to prevent interference to the main treatment processes.

Primary sedimentation is the main primary process. Its main purpose is to remove settleable suspended solids. About 65% of the raw sewage suspended solids are removed in primary sedimentation. Because this material is mainly faecal, it also removes about 35% of the BOD. Additionally, readily floatable oil and grease rises to the top of the tank: where it is skimmed off as scum. Other materials such as nutrients, micro-organisms and toxins are only removed to the extent that they are associated with the sludge or scum, hence to a very minor extent.

Primary effluent is suitable for disposal only where either very large dilutions are available (as in deep water submarine ocean outfalls), or the effluents may be irrigated over very large areas of land.

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8.2 Secondary Treatment

8.2.1 Objectives

Secondary treatment, by some form of aerobic biological oxidation process, is required to remove the high BOD and SS of primary effluent. As well as BOD/SS removal, it is possible to manipulate some secondary treatment processes so that ammonia in the wastewater is oxidised to nitrate (nitrification). This avoids problems of ammonia toxicity and nitrification oxygen demand in the receiving stream.

The high nutrient content in primary and even conventional secondary effluent often causes a secondary pollution problem by stimulating excessive growth of aquatic weeds and algae (eutrophication). Regulatory authorities are now requiring removal of plant nutrients, nitrogen and especially phosphorous, as part of the licence conditions for many treatment plants.

Nitrogen is best removed in some biological treatment processes· by denitrification under anoxic conditions. Developments in recent years have given rise to other process modifications which provide phosphorous removal to a level comparable to those presently achieved by chemical precipitation.

All aerobic biological treatment systems have the same aim and basic mode of operation -to provide a suitable environment in which a diverse population of micro-organisms (mainly bacteria - referred to as "biomass") may be brought into contact with the wastewater long enough for it to remove the organic content, and use it for growth of new biomass, so reducing the effluent BOD to a low concentration.

8.2.2 Types of Biological Treatment Processes

The processes are separated into two groups, according to support medium for the biomass:

(1) fixed film processes, in which the biomass is grown on a fixed surface - land treatment, trickling filters and rotating biological contactors; and

(2) suspended growth processes in which the biomass is held in suspension in the wastewater - waste stabilisation ponds, aerated lagoons and activated sludge.

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TREATMENT PROCESS TREATMENT UNIT PRODUCT PRODUCT DISPOSAL

R.wwatnllbr .aurial ..

~ .. t • ...... ion .. } Screen' and return f MecNnic:a1 ·RKks -_'!!I!.. --+ to flow .. separation • Screens

t- ~ • Comminutors • Inciner.ion z • t w c ~ :~ t- Ii Mechanical < ~ • Grit _ <i!:.!L_--+ Burial Of w separation cNmben landfill a:: t- (Small inorganic .,Iids) > , a:: < ~ -a::

Mechanical A. • Primary Raw separation lIdimentalion sludge 1-1 (Light. putrescible. links

largely organic ... 1.-tolidsl lti" . "!1 , o· ~

" .. f t- £ u

Z 8iological

= t -. w l~atment - :iE ~ (UsuallVaerobic r 1 1 ... ZI_

< biologIcal oxidation) ri; .. .. • .1 w .!i --.! a: .. -t-

~ - u i .i.f= t- o < > a:: ;: a: • " ~.-

~ 3! 11-< ~ ii! 0 • Z 0

8 Mechanical i~ .... !' Ii! i~ .. -teper.lion ~-w li :fA U:i c en (Biological.,I_1 , - \ , , liolotical1ludge -

> ... Filtration lac:kwath_ R~urn to ---~ plant influent a::~ Physical Disinfection

<~ Chemical T"'~ponds -... t-< BioIogic:l1 a:: UI land filtration V.-ation -powth Anim111fIling wa: Gr..a filt ·'Ition -- . Of mow'ng t- ...

~ ~ eNutrient removal

SIudges _ --+ To sludge disposal Physical or

~ chemical 01 e Removil of dissolved Aaavated.carilroft .. To IW-UIt or biological organics --< ditpou. eDesalting WaN.".. • - -----+ To dispoul

Treated ""~ . I ..J effluent 1 . I Dispoul or re-uII

Figure 7 Wastewater Treatment Process Flow Sheet

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8.2.3 Principles of Biological Treatment Processes

A simplified representation of the action of aerobic bacteria using organic matter as their course of flood and energy for growth is given by the following equation:

Organic Matter (C, 0, H, N) »

Bacteria -------=')'7

/

-----~ New Bacteria Cells

This equation shows how bacteria,}lSe some of the organic matter to grow new cells, breaking the remainder .of it to carbon dioxide, water and ammonium in order to provide the energy needed for growth of new bacterial cells. Since the process is aerobic, a source of available oxygen is essential.

The objective of the bacteria in the process is growth, so the result is the production of new bacterial cells. In treatment processes, this new growth must be removed and disposed as excess or waste biological sludge. So one aim of the treatment process design and operation is to minimise this excess sludge production, through maximising the proportion of organic matter broken down.

Once the flow leaves the main process unit (or reactor), the biomass present in the effluent must be separated to produce a clarified effluent. This is done in a secondary sedimentation tank. One of the most frequent causes of poor effluent quality from a biological treatment process, especially activated sludge, is failure of the biomass to settle and separate effectively in the sedimentation tank.

8.2.4 Trickling FiUers

Trickling filters were the favoured method throughout Queensland until the 196Os. Settled and screened wastewater is intermittently distributed over the top of the media and the organic matter is adsorbed as it flows down through the media.

Effluent is collected beneath the suspended floor of the filter bed. Air circulating naturally through the bed provides the oxygen needed to sustain aerobic biological action.

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The biomass thickness builds up on the media due to growth of new cells. "Patches of the biomass are sloughed off the media through hydraulic shear and shortage of oxygen in the depths of the biomass. The biomass passes out in the filter effluent and is settled out in a humus tank.

Trickling filters are stable in operation, economical to run and produce a good but variable effluent quality. Their mam disadvantages are their high capital cost and the limited degree of control which can be exercised over their performance. They are also not adaptable to the achievement of nutrient removal.

8.3 Activated Sludge

The activated sludge process is the most complex of the available systems. In this case, the biomass is added to the incoming wastewater, the mixture being called "mixed liquor". The biomass concentration of the mixed liquor is characterised by measurement of the suspended solids concentration...; referred to as mixed liquor suspended solids (MLSS).

The high rate of oxygen demand exerted by the aerobic bacterial action must be matched by the rate of air supply by the aeration system. Mter passage through the reactor (aeration tank), the flow passes to the final sedimentation tank for separation of the biomass (activated sludge). Settled activated sludge is returned to the front of the aeration tank to continue the process (return activated sludge). On each transit through the aeration tank, growth of new cells leads to a gradual build-up of MLSS. Left unchecked, the build-up of solids would become too great for effective final separation. To control MLSS at the required concentration, an amount equal to each day's net growth must be drawn-off as waste activated sludge. This waste activated sludge beconi~s part of the total sludge quantity which must be treated and disposed.

One great advantage of the activated sludge process is that it is adaptable for nitrification, denitrification and even biological phosphorous removal.

Most large treatment plants built since the mid 1960s have been conventional activated sludge plants. Small plants are designed in the extended aeration range.

8.4 Tertiary and Advanced Treatment

Some plants are required to provide effective reductions in BOD and SS of the secondary effluent. Such processes are often referred to as effluent polishing. Some simple"" processes include:

Tertiary ponds (sized for 10 - 30 days detention); Grass filtration (overland flow over gently graded grass plots); and Land filtration (irrigation over porous soils with infiltration into the soil).

A more expensive process, but requiring much less land, is rapid filtration through granular media filters.

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Advanced wastewater treatment is a term generally applied to the rather difficult and costly removal of dissolved solids (by desalination) or residual dissolved organics (activated carbon). These are special purpose processes associated with water reclamation for fairly high level re-use.

8.5 Disinfection

In many situations, it is necessary to destroy pathogens because primary and secondary treatment only reduces pathogen numbers by one order of magnitude. The conventional method of disinfection is chlorination, but significant disinfection is also achieved in tertiary ponds.

The 10 - 30 day detention quoted earlier is more related to disinfection than to removal of SS/BOD~

8.6 Treatment of Sludge

The major operation in processing of the primary and secondary sludges is usually anaerobic digestion. This process involves the co-operative action of a sequence of several groups of bacteria all acting in an environment free of dissolved oxygen. The resulting product is humus-like and stable enough to be dried in the air without putrescent odours.

Other treatment processes include thickening, dewatering, heat drying etc. Table 17 summarises the various treatment processes and the effect the process has on the sludge and the disposal option.

8.7 Composting

With the increasing difficulty of finding sludge disposal sites more attention is being paid to the beneficial use of sludge. Composting is one of several approaches for achieving the outcome of beneficial use. Figure 8 shows the basic steps or process flow that apply to all types of compost systems.

Figure 8

Sludge

-Bulkin-g Agent

Mixing

-

Forced Aeration

-~EJ

... Curing r.. Drying r..

(optionall

~

L _____________ _ ------ ----

Composting process flow

350

Bulking Agent - Storage

Recovery (optional)

J Markel

The two components of composting are the sludge and the bulking agent which can consist of many different types of materials, including recycled compost, sawdust, wood chips and shredded tyres. The bulking agent (except tyres) serves as a carbon source.

Composting systems are generally divided into three categories

• windrow • static pile, and • in-vessel.

In the window system, a sludge/bulking agent mixture is composted in long rows (windrows) that are aerated by convective air movement and diffusion. The sludge/bulking agent mixture is aerated mechanically turning over the piles using a front­end loader or specially designed equipment. The piles are turned daily at first when the system has a high oxygen demand. Thereafter, the piles are turned about three times per week. The composted sludge is usually stockpiled for curing before distribution.

Figure 9 is a schematic illustration of a static pile system. The aeration system consists of a series of perforated pipes running underneath each pile and connected to a pump that draws or blows air through the piles. The compost/bulking agent mixture is placed on top of a protective pad to form a pile. The piles are then covered with screened or unscreened finished compost for insulation.

Figure 9

------------------------------------

COMPOSTING EXTENDED PILES WITH FORCED AERATION

Schematic Diagram of Static Pile System

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Table 17 Effects of Pre-Treatment and Sludge Treatment Processes on Sludge and Sludge Use/Disposal Options

TREATMENT PROCESS AND

DEFINITION

Pre-Treatment: Reduction in contaminant levels in industrial wastewater discharge.

Thickening: Low-force separation of water and solids by gravity or flotation.

Digestion (Aerobic and Anaerobic):Biological stabilisation of sludge through conversion of some of the organic matter to water, carbon dioxide and methane.

Lime Stabilisation: Stabilisation of sludge through the addition of lime.

Conditioning: . Alteration of sludge properties to facilitate the sepa:ql.tion of water from sludge. Conditioning can be performed in many ways, e.g., adding inorganic chemicals such as lime and ferric chloride; adding organic chemicals such as polymers; or briefly raising sludge temperature and pressure. Thermal conditioning also causes disinfection.

Dewatering: High-force separation of water and solids.

Composting: Aerobic process involving the biological stabilisation of sludge in a windrow, in an aerated static pile, or in a vessel.

Heat Drying: Application of heat to kill pathogens and eliminate most of the water content.

EFFECT ON SLUDGE

Reduces levels of heavy metals and organics in industrial wastewater discharge, thereby lowering the concentration of these constituents in the sludge.

Increases solids concentration of sludge by removing water, thereby lowering sludge volume.

Reduces the volatile and biodegradable organic content of sludge by converting it to soluble material and gas. Reduces pathogen levels and controls putrescibility.

Raises sludge pH. Temporarily decreases biological activity. Reduces pathogen levels and controls putrescibility. Increases the dry solids mass of the sludge.

ImproV"l3li sludge dewatering characteristics. Conditioning may increase the mass of dry solids to be handled and disposed of without increasing the organic content of the sludge.

Increases solids concentration of sludge by removing much of the entrained water, thereby lowering sludge volume. Some nitrogen and other soluble materials are removed with the water.

Lowers biological activity. Can destroy all pathogens. Degrades sludge to a humus-like material. Increases sludge mass due to addition of bulking agent.

Disinfects sludge. Slightly lowers potential for odours and biological activity.

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EFFECT ON USE/DISPOSAL OPTIONS

Increases the viability of land application, distribution and marketing, and ocean disposal. Reduces need for pollution control devices during incineration, and prevents problems with incinerator ash disposal.

Lowers sludge transportation costs for all options.

Reduces sludge quantity. Preferred stabilisation method prior to landfilling and land application. Reduces heat value for incineration, but anaerobic digestion produces recoverable methane.

May be used prior to land application and landfilling. High pH of lime-stabilised sludge tends to immobilise heavy metals in sludge as long as the high pH levels are maintained.

Increases the amount of auxiliary fuel required in incineration if the amount of inert material in the sludge is increased

Reduces fuel costs for incineration. Reduces land requirements and bulking soil requirements for landfilling. Lowers sludge transportation costs for all options. Dewatering may be undesirable during land application in regions where the water itself is a valuable agricultural resource. Reduction of nitrogen levels mayor may not be an advantage.

Useful prior to land application and distribution and marketing. Often not appropriate for other use or disposal options due to cost.

Generally used only prior to distribution and marketing.

In-vessel composting takes place in partially or completely enclosed containers in which environmental conditions can be controlled. It is essentially just a different way of packaging the window and static pile systems. At present there are no in-vessel systems operating in Australia.

The advantages and disadvantages of the three composting systems are set out below.

Advantages

Window Systems

Static Pile Systems

In-vessel Systems

Disadvantages

Window Systems

Static Pile System

In-vessel System

Rapid drying of the compost because moisture IS

released as piles are turned over. Drier compost material, allows easier separation of bulking material. The capacity to handle high volume of material. Good product establisation Relatively low capital investment. Low capital cost High degree of pathogen reduction Better odour control than window composting Good product stabilisation. Space efficiency Better process control than outdoor operations Protection from adverse climatic conditions Good odour control is possible Potential heat recovery depending on system design.

Not space efficient Although capital investment is relatively low, the equipment maintenance costs are significant Requires more monitoring than static pile Odour release when piles turned Poor operation during adverse climatic conditions. Greater land requirements than in-vessed system Operations affected by climatic variability.

Potentially higher capital costs Reliance on specialised mechanical systems Potential for incomplete stabilisation Compaction of compost mix does not allow adequate air flow Less flexibility in operating modes than with other systems.

For any land application scheme it is important that the site should have favourable characteristics for entry of water into the soil profile and subsequent water movement within the soil. Application rate is also governed by the slope of the land, as surface runoff will increase with slope.

353

The basic disposal field area required is detennined by balancing the volume of effluent with the irrigation requirement .of the crop or pasture being grown, after taking into account any allowable losses. The disposal area is calculated on the basis of acceptable loading rates for each different loading parameter (liquid, N, P or organic) and then selecting the largest area. The loading parameter that corresponds to the largest area requirement then becomes the critical loading parameter.

In addition to the factors previously mentioned other site assessment factors including, climate, hydrology, topography, depth to groundwater, and separation distances must be taken into consideration.

9 SUMMARY OF DISPOSAL/REUSE METHODS

There are a number of disposal/reuse options available for consideration. In the past the approach has been to discharge the secondary treated, disinfected effluent to a surface water and to stockpile the sludge. There has, however, been increasing concern over the effect sewage effluent has on the environmental quality of the receiving water. Regulatory authorities are now shifting the emphasis to land application of effluent and the beneficial use of sludge.

Directing effluent and/or sludge to land may be undertaken via two operational methods:

• Resource utilisation which is the productive use of effluent and/or sludge as a secondary resource rather than just as waste disposal. Examples include irrigating parks, sports fields with effluent, injecting sludge to agricultural land.

• Land disposal to maximise effluent/sludge disposal. It is not concerned with cropping restraints. Crops are selected such that high application rates of effluent will not damage the species or degrade the soil.

Application rates for resource utilisation are contained by cropping requirements. Crop optimisation is the fundamental aim of this procedure, not maximising effluent disposal. Because of this constraint larger areas of land are required for land disposal.

Recognising that areas may vary with local factors indicative figures range from about 100 m2fEP in favourable sites to 500 m2fEP in marginal areas (Water Resources, 1992). This corresponds to an application rate of 870 mm/annum to 175 mm/annum respectively based on a flow rate of 240 LIEP/day.

A buffer area must be allowed outside the disposal area and land also for storage requirements. Storage is required to cater for:

• • •

wet weather conditions, when land disposal is restricted emergency back-up; and peak effluent flows.

Storage calculations must also take rainfall contribution into consideration.

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Land application of sludge is the spreading of sludge on or just below the surface of the land. Three of the four types of land application - agricultural application, forest application and land reclamation - use sludge as valuable resource to improve the characteristics of the land. Site characteristics and parameters discussed in Section 6 must be considered in all land application schemes.

Both liquid and dewatered sludge can be applied beneficially to land. Sludge may also be applied to land in composted, air-dried or heat dried forms. The choice of application method may be limited by local specific factors eg topography.

In a 1987/88 survey of Queensland practices it was found that

• 94% of sewage sludge produced is applied to land in various mode • 70% of sewage treatment plants make no beneficial use of sludge - all the sludge is

dumped to dedicated areas, stockpiled on site or landfilled. • 12% of plants use part of their sludge for agricultural purposes • 18% of plants use part of their sludge for landscaping purposes • approximately 6% of plants in rural areas burn stockpiled sludge.

In Queensland neither ocean dumping nor high temperature incineration have been practised.

Agricultural use of sludge is gallllng wider acceptance in Australia. Application rates generally range from 2 to 70 dry tonnes/ha/year. A typical rate is 15 t/ha/yr. Application rates are usually limited by either nitrogen needs of the crop or by annual or cumulative metals addition to the soil.

It has been reported by USEPA, 1989, that sludge application can greatly improve forest productivity. One major advantage of forest application is that forest products are an insignificant part of the human food chain. The primary environmental and public health concern associated with forest application is pollution of water supplies. Contamination of water supplies by nitrates can be prevented by limiting sludge application rates according to the nitrogen needs of the trees. A typical application rate would be 40 dry tcnnes/ha every 5 years.

Sludge can also be used to reclaim lands disturbed by mining and quarrying. Application of sludge can improve low nutrient levels of soil, problems of acid runoff, high erosion rates and toxic levels of trace metals. The amount of sludge applied at anyone time during land reclamation can range from 7 to 450 dry tonne/ha. Usually the sludge is applied and incorporated into the soil, the land is reseeded and no further sludge is applied.

In dedicated land disposal relatively large quantities of sludge are applied to a land area for many years. No attempt is made to productively use the sludge nutrients. Application rates range from 220 to 900 dry tonnes/ha/yr.

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10 REFERENCES

Beehog, G.W., (1992), "Water Quality Assessment," Qld. Water Management Journal, May 1992.

Bouwer, H., and Idelovitch, E., (1987) "Quality Requirements for Irrigation with Sewage Water." Journal of Irrigation and Drainage Engineering, Vol. 113, No.4, pp. 516-535.

Dean, R.B., and Suess, M.J., (1985), "The Risk to Health of Chemicals in Sewerage Sludge Applied to Land." Waste Management & Research, No.3, pp. 251-278.

Feachem, R.G., Bradley, D.J., Garelick H., and Mara, D.D., (1983) "Sanitation and Disease, Health Aspects of Excreta and Wastewater Management" John Wiley & Sons, Chinchester.

Hart, B.T., (1974) "A Compilation of Australian Water Quality Criteria" Aust. Water Resources Council, Technical Paper No.7.

Hart, B.T. et aI, (1992), "Draft Australian Water Quality Guidelines" ANZEEC.

Lever, M., and Zamek, P., (1990) "Overview of Alternative Sludge Disposal Methods for Brisbane City Council - Interim Findings." AWWA Qld. Branch, Weekend Regional Conference, 1990, Cabarita Beach, N.S.W.

Lewis-Jones, R. & Winkler, M.~ (1~91) "Sludge Parasites and Other Pathogens." Ellis Horwood Limited" Chichester.

Mara, D.D., & Cairncross, S., (1989) "Guidelines for the Safe Use of Wastewater and Excreta in Agriculture and Aquaculture." W orId Bank Organisation, Geneva.

Pettyrove, G$., and Asano, T. (Ed), (1984), "Irrigation. with Reclaimed Municipal· Wastewater - "A Guidance Manual," Report No 84-1 wr, California State Water Resources Control Board, Sacramento.

USEPA (1985), "Composting of Municipial Wastewater Sludges," Seminar Publication, EPN625/4-85/014, Centre for Environmental ResearchJnformation, Cincinati.

USEPA., (1990) National Sewerage Sludge Survey, In: Fedeial Register title 40CFR Part 5m. . . .

USEPA., (1989) "Environmental Regulations and Technology: Control of Pathogens in Municipal Wastewater Sludge." EPN625/1O-89/006~ Centre for Environmental Research Information Cincinati ..

Water Resources Commis~ion, 1992, "Guidelines for Planning and Design of Sewerage Schemes, Vol. 2." Water Resources Commission, Department of Primary Industries.

356

THE REVIEW OF EIS DOCUMENTS

R.J. Barley Water Resources Division, QDPI, Brisbane, Qld, 4000.

ABSTRACT

This paper gives a brief overview of Environmental Impact Assessment (EIA), including consideration of objectives, key principles and basic methodology. The roles of the participants, including the proponents of the development, various government agencies, and interested members of the community, are detailed, with special reference to the Queensland context. The Terms of Reference proposed for an Environmental Impact Statement (EIS), which is a vital part of an EIA, must be thoughtfully prepared, so that meaningful reviews of EIS's may be made. Essential factors in achieving this are discussed.

1 INTRODUCTION

Since 1991 Queensland has seen a great rise in the number of Environmental Impact Statements being prepared for a wide variety of proposed developments, from feedlots to factories. This has been driven principally by legislative reform, in particular the 1990-1992 Local Government (Planning and Environment) Act, but it also reflects the community's growing concern about the environmental effects of Queensland's rapid population growth and the resulting spread and intensification of development: Soil scientists may become involved in the preparation or review of Environmental Impact Statements eElS's) as consultants to developers, as government regulators and advisers, or as members of the community. To fulfil any of these roles requires some understanding of the process of Environmental Impact Assessment of which the EIS document forms a part.

This paper gives a brief overview of Environmental Impact Assessment, including its objectives, key principles and the basic methodology. The legislation applying to Environmental Impact Assessment (ElA) in Queensland is described and used to discuss several current issues, including the role of government advisory agencies, the design of Terms of Reference, and the process of review of the EIS.

Several of the key Acts dealing with EIA in Queensland are presently under review and new legislation is likely to be developed in 1993 or 1994. While it may be hoped that the issues mentioned above will be resolved, it is certain that changes will occur in the procedures as described in this paper. The reader should seek advice from the Department of Housing, Local Government and Planning and the Department of Environment and Heritage on the legal requirements applying to particular developments.

2 ENVIRONMENTAL IMPACT ASSESSMENT

Environmental Impact Assessment arose in the USA in the early 1970's as a tool in resolution of environmental disputes. Such disputes arise over differences in opinion over what is the "best use" of land or other resources, and are characterised by a high degree of uncertainty, a risk of irreversible ecological effects, a wide range of participants, and effects on public as

357

well as private interests (CICMUFIGSR 1991). EIA is able to address these problems in two ways. Firstly, EIA involves a scientific process of investigation and analysis that reduces the uncertainty about the effects of a proposed development. Secondly, EIA is a social process which allows all interested parties to see and to comment on the information about the development and its effects.

ANZECC (1991 p2) defined Environmental Impact Assessment as "a process for the orderly and systematic evaluation of a proposal including its alternatives and objectives and its effects on the environment including the mitigation and management of those effects. The process extends from the initial concept of the proposal through implementation to completion and, where appropriate, decommissioning". The objectives of EIA in Australia are identified (ANZECC, 1991, p3) as follows:

• to ensure that decisions are taken following timely and sound environmental advice

• to encourage and provide opportunities for public participation in environmental aspects of proposals before decisions are taken

• to ensure that proponents of proposals take primary responsibility for protection of the environment relating to their proposals

• to facilitate environmentally sound proposals by minimising adverse impacts and maximising benefits to the environment

• to provide a basis for ongoiI}g environmental management including through the results of monitoring

• to pronlOte awareness and education in environmental values.

Two points are worth emphasising in these· objectives. The first is that EIA is intended to assist and improve decision making, but it is not a decision making process in itself. The output of an EIA process is commonly a set of recommendations and information on environmelltal factors presented to decision makers, in or4er for them to weigh these factors with other' considerations such as economic factors. While EIA legislation may require decision makers to consider the information and to account for environmental effects in their decisions, this does not mean that environmental issues need always take precedence over other factors.

The second point is that EIA is applied to particular proposals for developments. Thus it is a reactive, rather than a proactive process, and does not give prior guidance on what developments will be environmentally acceptable. EIA is most effective, therefore, when it is used in conjunction with other tools of environmental management, such as preparation of Strategic Plans, catchment management plans, standards for environmental quality, and regulation of waste management practices.

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3 THE METHODOLOGY OF ENVIRONMENTAL IMPACT ASSESSMENT

'Environmental Impact Assessment can be examined on several levels, from the legal and administrative perspective through the methodological, to the techniques of its component scientific disciplines (eg Clark et al. 1984). The methodology of EIA describes the stages involved in designing and conducting an assessment (UNEP 1988):

1. Screening: the first and simplest level of evaluation which determines whether or not an EIA is required. This may be a simple administrative procedure where projects of a certain type or size automatically require assessment, or it may involve a simple study or a consultative process to determine whether it is likely that the proposal would have environmental impacts.

2. Scoping: identifying all the relevant issues which need to be examined in the EIA, and determining the level of investigation required. Scoping involves setting the boundaries for the study, in terms of space, time, and the range of issues.

3. Prediction: characterising the direct and indIrect impacts of the proposal in terms of predicted changes to environmental condition. Prediction draws on data and techniques from the natural, physical, social and economic sciences. To contain costs, the sophistication of the prediction methods should be in proportion to the scale of the potential impact and the significance of the receiving environment. As this partly depends on the outcome of the following step it may be necessary to go through several iterations of these stages. It should be noted that, as good scientific practice, prediction should include an estimation of the reliability of the findings in terms of probabilities or margins of error.

4. Evaluation: measuring the significance or importance of the predicted changes against some value system. This may be done, for example, by comparison with regulations or accepted standards, consistency with government policy, or by consultation with the local community.

5. Mitigation: consideration is given to means of avoiding, preventing, reducing or compensating for each of the adverse impacts. Often, an environmental management plan is proposed as a basis for approval of the development.

6. Communication: required between the proponent and the public, the proponent and the government authority, and between the public and the authority. Communication includes the formal documentation of the studies in an EIS but may also include methods such as public meetings, displays, advisory groups, written submissions, workshops, surveys, or public inquiries.

7. Monitoring: although this aspect of EIA is underdeveloped in Australia (Buckley 1989), it is potentially a very valuable step. During construction and operation, monitoring is needed to detect unforseen impacts, to determine the accuracy of predictions and to monitor the effectiveness of the environmental management plan, as well as to enable enforcement of licence conditions.

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· The outcomes of the first five steps are reported in the Environmental Impact Statement (or Impact Assessment Report, Environmental Effects Statement or similar names), a document which then forms the basis of the consultation and review processes which comprise step 6. An EIS generally includes the following sections:

• project description - a description of what is proposed in sufficient detail to allow independent assessment of its feasibility, infrastructure requirements, and wastes generated. This section should also outline the approvals procedure and any opportunities for public involvement.

• project justification - the objectives of the project, economic and marketing feasibility, and a description of feasible alternatives to the proposal, in sufficient detail to demonstrate why the preferred option was selected.

• description of the existing environment - including the site and any other areas directly or indirectly affected by the proposal. Sufficient detail is needed to allow independent review of the predictions and evaluations made in the environmental impact assessment.

• assessment of potential impacts - provides a report of the prediction and evaluation studies.

• environmental management - includes the proposed environmental management plan and any proposed monitoring.

• sources of information - all sources must be documented. Often, detailed technical reports are attached"as appendices, so that the main text can be written in a style suitable for· non-specialist readers.

4 ROLES OF THE PARTICIPANTS IN ENVIRONMENTAL IMPACT ASSESSMENT

There are several different participants in EIA, each with their own requirements of the process. These include the proponent of the development, the government agency responsible for approving the proposal, other government agencies and members of the community with an interest in the proposal or its impacts. There is also the consultant who may be employed by any of these parties to conduct some or all of the assessment.

The responsible authority must decide whether the proposal should proceed, and if so under what conditions. In doing so it has a duty to consider the effects on the environment. In some cases, environmental protection may be the primary function of the authority - for example, the Department of Environment and Heritage in Queensland or the Western Australian EPA. In other cases the requirement to consider environmental impacts comes from umbrella legislation or is secondary to the authority's main purpose. In either case, the authority'S main requirement is an EIS which provides it with clearly understandable information on all the relevant issues, so that it may be confident in making a good decision. To be confident in the quality of the information in the EIS, the authority also depends on other agencies assisting it with technical review.

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In each State there is also an agency, or a number of agencies, responsible for the environmental impact assessment process itself. The role of these agencies is to ensure that a fair, efficient and effective process is conducted in accordance with legal requirements. ANZECC (1991) lists 11 principles for such agencies. These emphasise the importance of providing clear guidance to proponents on all steps of the process, the need to ensure public involvement and reporting, and the importance of monitoring of the effects of developments and the effectiveness of the process.

Many other government agencies may contribute to the assessment of a given proposal, by providing advice on matters in which they have an interest (though not necessarily a statutory responsibility) or technical expertise. The Queensland Department of Primary Industries, for example, is consulted on the potential impacts of proposals ranging from piggeries to the sewage disposal systems of tourist resorts, and on erosion risks of construction in any part of the state. Advice is given on screening, scoping and on the EIS. These agencies require the EIS to be provide sufficient technical detail for the conclusions to be independently assessed. To keep the EIS intelligible to the general reader, technical details are often removed from the main report, but this creates difficulties if the technical reports are not distributed as widely as the EIS. The best solution is to provide these reports as Appendices.

The proponent has the responsibility of preparing and presenting the case for the assessment of the proposal. As such, the proponent needs clear advice from government as to what studies are required, and, as far as possible, what level of impacts will be acceptable. Developers continue to challenge the need to conduct EIA studies, as an additional cost imposed at a time when proposals are not approved and therefore a higher financial risk. However, there are advantages to the proponent, as well as to th~ community, if the need to investigate and describe environmental effects is accepted as part of project costs (McDonald and Brown 1989). The description of the existing environment may be used to formulate proposals that maximise the advantages of the site, avoiding inappropriate uses. Once a basic concept is developed, the prediction and evaluation phase may be used to refine the design so as to minimise the potential for impacts. This reduces the potential for conflict and public controversy, and increases the chance of a speedy acceptance of the project by the authorities. If the opportunity is taken to integrate EIA with the project planning process, down-the-track problems are likely to be avoided, along with the often high costs of mitigating impacts after they occur.

The community has the role of providing information, both on matters of fact such as local knowledge and history, and on matters of value including the acceptability of the proposal and the relative importance to the affected community of the different issues. The community needs to be able to obtain information in a format and style which is readily understandable. At a minimum there should be consultation on the EIS before a decision is made on the project. However the community's input will be the most effective, and the least disruptive to the project, if it is commenced earlier in the EIA process (Formby 1987).

An additional role for the community is as a watchdog on the process. Transparency of process is the greatest guarantee that EIA will be conducted in an effective manner (Martyn et al. 1990).

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5 THE QUEENSLAND CONTEXT

Environmental Impact Assessment is dealt with under several pieces of State legislation. The two principal Acts are the Local Government (Planning and Environment) Act which provides for EIA on planning applications considered by local authorities, and the State Development and Public Works Organisation Act which covers EIA for State Government Departments and other authorities. In addition, some Acts such as the Mineral Resources Act contain references to EIA which extend or complement the State Development Act procedures.

The Local Government (Planning and Environment) Act, which applies to the majority of impact assessments being conducted in Queensland, defines "environment" to include ecosystems, all natural and physical resources, and any social and economic conditions which affect, or are affected by them. The legislation requires that a Local Authority, when considering any application, must take into account any environmental effects. Regulations made under the Act include a Schedule which lists "designated" types of development, and a further Schedule which lists "designated" areas in which a development might occur. Any application for a planning approval in respect of these designated developments or designated areas must be accompanied by an EIS.

The first step for the proponent is to apply to the Department of Housing, Local Government and Planning (DHLGP) for Terms of Reference, using a form which requires a brief outline of the proposal. DHLGP prepares the Terms of Reference in consultation with the Local Authority, the Department of Environment and Heritage, and any relevant advisory agencies. To facilitate this process, DHLGP circUlates a generic model Terms of Reference for that type of designated development, each agency indicating the additions or deletions considered appropriate for the particular application.

This step encompasses both screening and scoping. The Schedules are themselves a tool for screening. In addition, on the basis of advice from the advisory agencies, DHLGP can decide that an EIS is not required if there already exists a relevant EIS that is not outdated and covers all the significant issues, or if it is considered that the consequences of approving the application are minor.

DHLGP then forwards the Terms of Reference to the applicant. Many applications do not proceed beyond this point: in the first year of operation of the Act there were over 250 applications for Terms of Reference (T. Scanlan, DHLGP pers. comm.) but less than 50 EIS's were received by the Department of Primary Industries for review. If the proponent wishes to proceed, an EIS must be prepared and submitted as part of the application to the Local Authority. When the Local Authority receives the application, it refers copies of the EIS to the relevant advisory agencies for review. In most cases, each agency sends its comments directly to the Local Authority (DHLG 1991). However, in April 1992, the Premier's Leading State Economic Statement announced administrative changes in which DHLGP may take a role in coordinating government comments into a "whole-of-government" response. To date this option is being exercised only for a few, particularly significant proposals.

Because the EIS forms part of the application, it is subject to the same requirements for public display and advertisement as the normal planning application. This typically involves a notice displayed on the site and published in the local newspaper, and a copy of the

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application available for perusal (but usually not for loan or photocopying) at the Council Chambers.

The Local Government (Planning and Environment) Act specifies most of the above procedures in the legislation or the Regulations. In contrast, the State Development and Public Works Organisation Act requires only two things. Firstly, all government Departments and agencies are responsible for taking likely major environmental effects into account when considering either granting an application, or undertaking their own works. Secondly, in doing so, they must "have due regard" to policies and administrative arrangements approved by the Minister, to the extent that these are compatible with other legislation.

The policies and administrative arrangements, as set out in the colloquially named "Green Guide" (Coordinator-General 1987), outline a process which is generally similar to the process described for Local Authority approvals. The major differences are:

1. There is no list of designated developments. Screening is achieved by proponents submitting an initial advice statement (in practice this may be the normal application form for the approval which is being sought) and the responsible authority deciding, after consultation with other agencies, whether an EIS is required.

2. The responsible authority manages all stages of the process, from setting the Terms of Reference and liaising with the proponent, to distributing the EIS and coordinating the review process. This requires every Department to maintain skills in EIA administration. In complex or controversial projects it may be agreed to transfer responsibility for administering the EIA to another Department such as Premier's Department, DHLGP or the Department of Environment and Heritage.

3. There is no requirement for public involvement, although the release of the final EIS to the public is encouraged.

4. The significant discretion left to individual agencies has led to the evolution of different practices. For example, some agencies such as SEQEB and FNQEB have established policies on what projects will undergo EIA, while others such as the Department of Primary Industries have until now relied on ad hoc decisions. In some cases a Draft EIS is distributed widely for public comment before the studies were finalised (as in the case of the Garulmundi Toxic Waste Disposal Facility EIS managed by State Services Department) while in others (such as the EIS on the expansion of the Port of Brisbane) the decision was announced before release of the EIS.

The Leading State Economic Statement (1992) announced a review of Queensland's planning system, which is likely to include an overhaul of the EIA processes not only for Local Authorities but also for other government agencies (T. Scanlan, DHLGP, pers. comm.). It would appear likely that the degree of variation in procedures between Government agencies may be reduced, while the requirement for public involvement may be increased.

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6 TERMS OF REFERENCE

Preparing the Terms of Reference for an EIS is an art form which balances conflicting needs. On the basis of often scant information about the nature of the proposed development, Terms of Reference must be composed which do not omit any major impacts, and do not include any inapplicable or trivial impacts. While the proponent wants a clear indication about the level of study and the amount of information required by the authority and advisory agencies, it is almost impossible to detail this exactly. In most cases, until some environmental studies are conducted there are inadequate data available to do a detailed experimental design - for example, to determine the required spacing or depth of soil samples. It is better to compose the Terms of Reference as a set of outcomes to be achieved, rather than treating it as a recipe book to be followed slavishly. Checks are needed, however, to avoid the problems of superficial EIS's which hamper effective decision making and may cause delays while the missing information is obtained. In some cases the Terms of Reference can refer proponents to a standard manual or handbook of methods, such as the Australian Rainfall and Runoff used for hydrological modelling. In others it is better to require consultation with the appropriate agency during the course of the assessment and evaluation phases.

7 REVIEW OF EIS's

Comments by advisory agencies and the community on an EIS should address two separate questions. Firstly, the adequacy of the EIS should be considered: whether it addresses all of the Terms of Reference and gives adequate information. Secondly the adequacy of the proposal itself should be addressed.ANZECC (1991) list the items which should be reviewed by the assessing authoritie~:

•

•

•

•

•

•

whether and why environmental impacts are manageable within tolerable limits

whether and why the degree of uncertainty of impacts (ie the risk to the environment) is sufficiently low to be confident about not encountering unforseen problems

. w~ther there are ways to eliminate avoidable imp~ts, minimise adverse impacts and maximise benefits to the environment

whether the impacts are likely to be cumulative

what the implications are of using community assets

whether and why the programme for minimising, ameliorating, managing and monitoring all impacts is sound and is likely to protect the environment.

Review of EIS's is often a secondary or minor role of the advisory agencies, and the resources available for review are inevitably limited. In practice review is likely to consist of a rapid evaluation based on the knowledge and experience of the reviewing officers. If a problem is evident, and if resources are available, then independent investigations or re­analyses of data may be done. In some States, resource constraints have resulted in advisory agencies being obliged to seek funding from the responsible authority before carrying out any detailed review, and this practice has attracted consideration by more than one Queensland Department.

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In most States the proponent is given the opportunity to respond to comments made by agencies and the public. For example, the first document may be termed a DraftEIS, followed by a Supplement or a Final EIS which includes the response. In Queensland this does not occur as a formal step. However, the growing practice of consultation with agencies during preparation of the EIS serves much of the same function, and has the additional advantage that the communication occurs during, rather than following, the project's design. Some consultants also seek comment from key agencies on a draft of the EIS as an informal step.

Elsewhere in Australia the review process is itself reported, and comments from different agencies are brought together in a summary document, the Assessment Report. Representing a more impartial review of the information provided by the proponent, and allowing the views of different government agencies to be reconciled, the Assessment report is at least as influential a document as the EIS itself (Fookes 1987). While not at present a statutory requirement in Queensland, some Departments and Local Authorities do produce a summary report as a guide to the decision makers.

CONCLUSION

Environmental Impact Assessment is a powerful tool for environmental protection. While not preventing developments from having environmental effects, it does require decisions to be fully informed, and encourages environmentally sensitive project design.

All of the participants in EIA have their requirements of the EIS, many of which conflict. The ideal EIS will be comprehensive, yet without any unimportant information and without unnecessary details. It will be simultaneously intell:tgible to the general reader, and capable of expert analysis and review. Fundamentally, the role of.the EIS is to provide the responsible authority and the community with information which was not otherwise available, either because the data did not exist, or because they had not been collated and synthesised to allow conclusions to be drawn. To fulfil this role, the assessment and evaluation studies reported in the EIS must have scientific validity, and the EIS itself must work as a communication tool.

However, communication in EIA should not be restricted to the EIS and to the formal process of making comments on it to the responsible authority. Consultation with the relevant advisory agencies and with the affected community can commence at the outset of the EIA process, and is the best way to ensure not only that the EIS meets the requirements, but also that the project will be acceptable.

EIA legislation in Queensland presently allows significant procedural discretion to the authorities responsible for approving licences or undertaking works. At the same time, most government Departments suffer resource constraints, making it increasingly difficult to meet the growing community expectations for rigorous environmental assessment. EIA will only be effective in this situation if the professionals involved (both within government and in the private sector) keep in mind the objectives and principles of Environmental Impact Assessment, and understand the requirements of each of the participants in the process.

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9 REFERENCES

Australia and New Zealand Environment and Conservation Council (1991) A National Approach to Environmental Impact Assessment in Australia. ANZECC Secretariat, Canberra.

Buckley, R (1989) What's wrong with EIA. Search 20(5): 146 - 147.

Clark, B.D., Gilad, A., Bisset, R., Tomlinson, P. (Eds.) (1984). "Perspectives on Environmental Impact Assessment" (Farnborough: Saxon House).

Commission of Inquiry into the Conservation, Management and Use of Fraser Island and the Great Sandy Region (1991) Public Dispute Resolution. GoPrint, Brisbane.

Coordinator-General (1987) Impact Assessment in Queensland. Premier's Department, Brisbane.

Department of Housing and Local Government (1991) Local Government (Planning and Environment) Act Legislation Guide. GoPrint, Brisbane.

Fookes, T W (1987) A comparison of Environmental Impact Assessment in South Australia and proposed UNEP Goals and Principles. Environment Planning and Law Journal 4: 204 - 212. -

Formby, J (1987) Environmental Assessment: where has it gone wrong? Environment Planning and Law Journal 4: 191-203.

Martyn, A, Morris, M L and Downing, F (1990) Environmental Impact Assessment Process , in Australia. Environment Institute of Australia, Canberra.

McDonald, G T and Brown, L (1989) Planning processes and environmental assessment. . Impact Assessment Bulletin 8(1 & 2): 261 - 274.

Queensland State Services Department (1992) Garulmundi Toxic Waste Disposal Facility Environmental Impact Statement. GoPrint, Brisbane.

Queensland Government. Local Government (Planning and Environment) Act 1990-1992.

Queensland Government (1992) Queensland: Leading State. GoPrint, Brisbane.

Queensland Government. State Development and Public Works Organisation Act 1971-1981.

Queensland Government. Mineral Resources Act 1990.

United Nations Environment Program (1988) Environmental Impact Assessment. Basic Procedures for developing countries. Regional Office for Asia and the Pacific.

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