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WATER RESEARCH COMMISSION

ECOLOGICAL AND ENVIRONMENTAL IMPACTS OF LARGE VOLUME GROUNDWATER ABSTRACTION IN

THE TABLE MOUNTAIN GROUP (TMG) AQUIFER SYSTEM

DISCUSSION DOCUMENT FOR SCOPING PHASE

July 2003

WRC project K5/1327

WATER RESEARCH COMMISSION

ECOLOGICAL AND ENVIRONMENTAL IMPACTS OF LARGE VOLUME GROUNDWATER ABSTRACTION IN

THE TABLE MOUNTAIN GROUP (TMG) AQUIFER SYSTEM

DISCUSSION DOCUMENT FOR SCOPING PHASEJuly 2003

WRC Project K5/1327

Authors of this report:Cate Brown – Southern Waters

Christine Colvin – CSIRChris Hartnady – Umvoto AfricaRowena Hay – Umvoto Africa

David Le Maitre – CSIRKornelius Riemann – Umvoto

Africa

Project Manager: Paul Lochner - CSIR

Report published by:CSIR-Environmentek

P O Box 320Stellenbosch 7599

South AfricaTel: (021) 888 2400Fax: (021) 888 2693

Southern WatersP O Box 13280Mowbray 7705

South AfricaTel: (021) 685 4166Fax: (021) 685 4630

Umvoto Africa (Pty) LtdP O Box 61

Muizenberg 7950South Africa

Tel: (021) 788 8031Fax: (021) 788 6742

This report is to be cited as: Water Research Commission (WRC), 2003. Ecological and environmental impacts of large volume groundwater abstraction in the Table Mountain Group (TMG) aquifer system: Discussion Document for Scoping Phase. WRC Project K5/1327. Published by CSIR-Environmentek, Southern Waters and Umvoto Africa. CSIR Report Number: ENV-S-C 2003-076, Stellenbosch.

CSIR Report Number: ENV-S-C 2003-076

Contents

1. PURPOSE OF THIS DOCUMENT____________________________________11.1 The project________________________________________________________________11.2 The team__________________________________________________________________21.3 The plan__________________________________________________________________31.4 A one-day Workshop_________________________________________________________3

2. AN INTRODUCTION TO GROUNDWATER_____________________________42.1 What is groundwater_________________________________________________________42.2 Types of aquifers____________________________________________________________52.3 Properties of fractured-rock aquifers_____________________________________________7

3. GROUNDWATER INTERACTIONS IN THE ENVIRONMENT_______________83.1 Soil and Rock Interactions____________________________________________________103.2 Interactions with non-marine aquatic ecosystems__________________________________11

3.2.1 Surface water bodies__________________________________________________113.2.2 Subsurface water bodies________________________________________________13

3.3 Interactions with terrestrial ecosystems_________________________________________133.4 Interactions with marine ecosystems___________________________________________153.5 The nature of groundwater dependency_________________________________________15

4. TMG AQUIFERS_________________________________________________164.1 Table Mountain Group: Geological pattern and process____________________________184.2 Structure and functioning of the deep fractured-rock TMG aquifer_____________________194.3 Chemistry of TMG Aquifers___________________________________________________214.4 Why the confined TMG aquifer is being explored for bulk water supply.________________214.5 Types of TMG aquifer settings_______________________________________________224.6 Interactions TMG aquifer - ecosystems__________________________________________25

5. ASSESSING GROUNDWATER DEPENDENCE________________________265.1 Possible evaluation methods__________________________________________________275.2 Assessing impact of long-term changes in hydrologic cycle__________________________29

6. INPUT REQUIRED FROM SPECIALISTS_____________________________30

7. REFERENCES__________________________________________________31

Appendix A Glossary of hydrogeological terms

1. PURPOSE OF THIS DOCUMENT

This document aims to assist specialists from a range of earth and life science disciplines in developing an understanding of how abstraction of groundwater from the Table Mountain Group (TMG) aquifer system, in the Western Cape, may affect water availability in the surface environment. It does this by:

giving a brief introduction to this project, its objectives and the team involved; providing an introduction to groundwater (see Section 2); providing an overview of the role of groundwater in the environment and its interactions

with ecosystems, in general (see Section 3); providing an overview of the character and functioning of the TMG aquifer in the Western

Cape (see Section 4); and summarising pertinent groundwater terminology to facilitate discussion (see Appendix A).

Isolating the potential impacts on the environment as a result of abstraction of water from the TMG aquifer is likely to be a complex process, which cannot be addressed by one scientist or discipline alone. Specialists will require a basic understanding of the above-mentioned issues, and of each other’s disciplines, in order to promote cross-disciplinary interactions and facilitate informed evaluation of the possible implications of abstraction on the ecosystems in which they specialise.

The document represents the first in a series of reports and other structured activities aimed at promoting the required interaction, including various one-on-one meetings between specialists and core team members, and an inter-disciplinary workshop in March 2003.

1.1 The project

The project, “Ecological and Environmental Impacts of Large-scale Groundwater Development in the Table Mountain Group (TMG) Aquifer Systems”, forms part of a programme of research funded by the Water Research Commission (WRC) in partnership with Department of Water Affairs and Forestry (DWAF) on “The hydrogeology of the TMG aquifers”. The programme was created in the light of possible future large-scale groundwater development of the TMG aquifer.

The full WRC programme has the following specific objectives:1. Development of an understanding of the occurrence, attributes and dynamics of the TMG

aquifer systems.2. Development of an understanding of the environmental impacts of exploitation from TMG

aquifer systems.3. Integration of groundwater into the broader water management framework.

This project aims to initiate an assessment of the ecological role of groundwater in the TMG aquifer systems as a first contribution towards the Programme Objective 2 as stated above.

Furthermore, the project aims to: use existing knowledge and develop existing study sites; promote capacity building to strengthen the human resources available to monitor and

manage possible water-resource developments of the TMG aquifer.

The critical areas that will be addressed are:

WRC Project K5/1327page 1

The development of predictive tools, innovative techniques and indicators to assess the impact of groundwater abstraction on the environment.

The improvement of our understanding of groundwater dependent ecosystems in the TMG and sensitivity to groundwater level fluctuations.

The development of an improved, conceptual understanding of the impact of changing low flows on freshwater ecosystems.

The project is being run in close conjunction with the City of Cape Town’s (CCT) Pilot Study on the feasibility of extracting groundwater from the TMG for municipal water supplies. The CCT project also is in its public participation and scoping phase at present. The WRC project will provide information that will help the CCT project in selecting the site for the wellfield and in designing the monitoring component.

The proposed key deliverables of this project are: A spatial database of information on groundwater dependent ecosystems, related to the

TMG. A prototype design for the assessment of the groundwater dependent ecosystems and

monitoring results at field sites over one annual cycle. Establishment of an infrastructure for monitoring groundwater dependent ecosystems in

selected study areas. Analysis of field results to assess the extent of dependency of the identified ecosystems

on patterns of groundwater discharge at various spatial and temporal scales, and inferred sensitivity to abstraction impacts.

Recommendations for application of current knowledge and critical areas for future research.

1.2 The team

The project team comprises a consortium of Western Cape scientists, which includes hydrogeologists, terrestrial and aquatic ecologists, hydrologists and data base specialists. Organisations involved in the study include:

Southern Waters Ecological Research and Consulting cc Umvoto Africa Environmentek – CSIR Ninham Shand University of Cape Town University of Western Cape Stellenbosch University Geoss Western Cape Nature Conservation Board Department of Water Affairs and Forestry.


1.3 The plan

The project has been organised in four phases:

Phase 1 Scoping: Detailed inception study and field study planning.Phase 2 Field data collection.Phase 3 Analysis of results.Phase 4 Recommendations for the application of current knowledge and

identification of critical areas for future research.

The study commenced in June 2002, and is currently in Phase 1. The objective of Phase 1 is to capture available knowledge that may be of use in assessing the dependency of ecosystems on groundwater in the TMG. Phase 1 comprises three main activities:

Collation of readily available date and information. Compilation of this Discussion Document and related aids to inform key specialists. One-on-one discussions and a workshop with key specialists.

1.4 A one-day Workshop

During the workshop, invited specialists will be asked to contribute:

Background to their area of expertise; Their initial ideas on potential impact on groundwater dependent ecosystems; Suggestions for impact and baseline monitoring.

We intend to: Scope the full range and types of potential ecological impacts; Identify geographical areas considered likely to be dependent on groundwater; Prioritise areas for future monitoring and research.

The criteria for prioritising future research and monitoring sites will be discussed during the workshop. It is expected that they will include:

Relevance of the impacted ecosystem in the wider environment; Degree of ecosystem dependency on groundwater; Potential ecosystem responses to changes in the hydrogeological regime; Sensitivity of groundwater discharge zones to long and short term abstraction and climatic

variations; Capacity to monitor and evaluate anticipated changes in the ecosystem.


Note: This workshop was held on 28 March 2003 at CSIR Stellenbosch. The findings of the workshop are contained in a separate report entitled Scoping

Workshop Report prepared by CSIR, Umvoto and Southern Waters.

2. AN INTRODUCTION TO GROUNDWATER

Water is a scarce and unevenly distributed national resource which occurs in many different forms, all of which are part of a unitary, interdependent cycle (National Water Act, 1998). Groundwater is one part in the hydrologic cycle. In order to understand interactions between the components of the hydrologic cycle it is required to develop an understanding of the processes and the driving forces.

Figure 2.1: Groundwater in the hydrologic cycle (from Domenico and Schwarz, 1990)

2.1 What is groundwater

Groundwater is water that occurs underground and (‘daylights’) as springs, wetlands, seeps or as baseflow into rivers or the sea. Hydrogeologists think of groundwater in terms of water that is found in the saturated zone, as opposed to soil moisture, which is the water in the unsaturated or vadose zone (see Figure 2.2). Potentially usable or abstractable groundwater is found in aquifers.

An aquifer is formed from permeable earth material, such as porous sediments or fractured hard rock, which is saturated with groundwater. An aquifer is considered to be a rock type (including ‘soft’ unconsolidated material), which has sufficient permeability to allow flow to a borehole or a well. This implies that only utilisable groundwater constitutes an aquifer. Low permeability and impermeable rock types are called aquitards and aquicludes, respectively (see Figure 2.4).

In addition to the permeability of an aquifer, the amount of groundwater stored is a key parameter used in the assessment of aquifers. The water stored is normally described in terms of water released per unit drop in water level. This is determined by the porosity of the aquifer, and the compressibility of the aquifer material and water.


Figure 2.2: Unsaturated and saturated zone, capillary fringe and water table (Hölting 1984)

2.2 Types of aquifers

There are essentially two main categories of aquifers (see Figure 2.3):

Primary aquifers: Aquifers in which the water moves through the spaces that were formed at the same time as the geological formation was formed, for instance intergranular porosity in sand (e.g. alluvial deposits).

Secondary aquifers: Aquifers in which the water moves through spaces that were formed after the geological formation was formed, such as fractures in hard rock.

a) b) c)

Figure 2.3: Schematic block-diagramms of different aquifer types; a) primary aquifer, b) secondary aquifer (red block not porous), c) double porosity aquifer

(yellow block porous) (van Tonder, 2001)


Within this broad characterisation, aquifers can be described as either unconfined or confined:

Unconfined aquifer: An aquifer where the water pressure is equal to atmospheric pressure. The ‘top’ of an unconfined aquifer, where the aquifer is fully saturated, is termed the ‘water table’. If the water table reaches the surface, water flows from the ground as springs or seepages, thus generating or augmenting the baseflow of rivers or wetlands. Where water is drawn from an unconfined aquifer, the water table drops in a cone of depression. There is a direct relationship between the volume of water in the aquifer and the height of the water table.

Confined aquifer: An aquifer that is overlain by a geological layer of low permeability, under which the water is confined at a pressure greater than atmospheric pressure. The interaction between confined aquifers and the surface environment tends to be restricted over most of its area by the low permeability layer. The water in these systems remains trapped below the aquitard/aquiclude, and flows from the ground only in areas where the impermeable layer is broken or weathered away at the surface. Drilling into a confined aquifer will result in a release of the pressure and an expansion of the water into the additional space. Thus, the water level in the borehole at rest after drilling (rest water level) is higher than that intercepted during drilling (strike water level). If water is abstracted from a confined aquifer, the pressure-head in the aquifer, represented by the piezometric surface, falls in the vicinity of the borehole.

Confined and unconfined aquifers are the opposite end members of a continuum from pressurised aquifers (confined) to ones that are in equilibrium with the atmosphere (unconfined) (see Figure 2.4). Semi-confined (leaky) and semi-unconfined (delayed yield) aquifers are found in between.

Figure 2.4: Differences between confined and unconfined aquifers (van Tonder, 2001)

Flow within aquifers, and therefore discharge to springs and baseflow, is controlled by the permeability of the aquifer and the gradient of the water table or piezometetric surface.

Groundwater flows through aquifers from areas of recharge to areas of discharge (see Figure 2.5). Recharge occurs where water from rainfall, surface water, neighbouring aquifers or even from water supply systems, is added to the aquifer. Rainfall, which is not intercepted, transpired, evaporated or does not run-off the land surface, percolates through the unsaturated zone. In the


unsaturated zone, water infiltrates downwards under gravity flow through pores and macro pores such as cracks and fractures, until it reaches the water table. Once in the aquifer the flow is controlled by hydraulic permeability and the gradient of the water level. Flow paths may be fairly short in time and length (several 100m, weeks/months) or long (100s kms and thousands of years).

Figure 2.5: Groundwater flow in unconfined and confined aquifers (after Driscoll, 1986)

2.3 Properties of fractured-rock aquifers

In fractured (secondary) aquifers, such as the TMG, most of the water flows along fractures. The macro fractures are usually embedded in a porous matrix formed by blocks (e.g. sandstone) or micro fissured blocks (e.g. quartzite). This matrix has lower permeability than the fractures, but is capable of storing water in the innumerable pores or micro fractures (see Figure 2.3). In extreme cases, the blocks between the fractures are of such low permeability (e.g. granite) that very little water can be exchanged between the fracture network and the matrix, which is then called ‘inert’.

The most striking hydrogeologic feature of a fractured rock is the variability of aquifer parameters. A parameter such as the hydraulic conductivity or permeability normally varies by several orders of magnitude within the same rock unit, often within short distances, and over different scales. Similarly, the storativtiy of an aquifer may vary at different scales. The flow of groundwater (in both primary and secondary aquifers) may be anisotropic. In fractured aquifers, preferential flow along fractures and fracture zones results in strongly anisotropic flow.

If fractures are densely interconnected, they conform to a ‘fracture network continuum’ characterised by a large storage capacity that contributes substantially to the volume extracted by a pumped well. A fracture network may be considered as continuum depending on the following three properties:

Representative elementary volume; Fracture connectivity; Conductivity contrast between fracture and matrix.


These properties of the fracture network control its hydraulic behaviour. Importantly anisotropic flow occurs in restricted zones of high permeability where the water flows from recharge to discharge areas.

According to the standard equations derived for primary aquifers, the radius – or distance – of influence increases with increased permeability and decreased storativity. Since water bearing fractures can be seen as conduits with very high permeability, the radius of influence can easily reach several kilometers in the direction of the fracture set, while the radius perpendicular to the fractures reaches only several meters. An example of the possible cone of depression during a hydraulic test is shown in Figure 2.6.

Figure 2.6: Cone of depression (blue line) during hydraulic test at one borehole (green dot), showing the anisotropy along the water bearing fault

zones (red lines) (Umvoto, 2002)

In most of the fractured-rock formations in RSA the flow of groundwater is controlled by two kinds of fractures, namely: vertical and sub-vertical fractures (i.e. vertical faults and fault zones due to tectonic stresses; contacts along dykes) and horizontal or bedding-plane fractures (i.e. fractures formed by tension release, uplifting and weathered zones).

3. GROUNDWATER INTERACTIONS IN THE ENVIRONMENT

Groundwater and surface water interact at many places throughout the landscape. These interactions can be highly dynamic as they respond to the variations and changes in the hydraulic gradients which drive the flows between them. The potential functions of aquifers in the broad environment can be summarised as provision of :

sources or sinks for water; sources or sinks for nutrients or trace elements; habitat for aquatic and semi-aquatic biota.


Figure 3.1. Types of aquatic and terrrestrial ecosystems, using groundwater as water

resource (Le Maitre, in press).

Aquifers typically provide water to non-marine aquatic ecosystems, such as baseflow to rivers, wetland and spring ecosystems, cave and aquifer ecosystems, and water to terrestrial plant communities (Figure 3.1). They also supply freshwater and nutrients to marine and, in particular, coastal environments such as lagoons, estuaries and the near-zone intertidal zone.

River ecosystems: In perennial and seasonal rivers, the character and composition of the aquatic (in-stream) or riparian (near stream) ecosystem is often dependent on groundwater discharge as base (dry season) flows. In many cases the flows are also critical for meeting human needs both directly and by sustaining human enterprises.

Wetlands and spring systems: In terms of groundwater interaction, there is a continuum from springs, which have definite discharge points, to wetlands, where discharge tends to be diffuse. Groundwater dependent wetlands would include wetlands with a known or likely component of groundwater discharge in their hydrological cycle; at least some endorheic pans and many of the coastal wetlands are examples.

Aquifer and cave ecosystems: These include groundwater contributions to “hypogean life” (subterranean), including those in the aquifer itself (Hatton and Evans 1998), and to associated cave ecosystems. Areas with karst geology, such as dolomitic rock systems or limestones, are examples.

Terrestrial ecosystems: Where terrestrial vegetation is directly dependent on groundwater (as opposed to interflow) it is in areas where the groundwater body is within the rooting depth of the plants and groundwater discharge occurs through the plant root systems. This is also called “cryptic” discharge, and is most noticeable as oasis-type vegetation in arid environments.

Marine ecosystems. Groundwater inflows to the near-shore marine ecosystems contribute nutrients and silica and affect the productivity of these systems. Evidence indicates that groundwater may also play a significant role in estuarine and salt marsh ecosystems, and that freshwater seeps on the ocean floor could contribute to up-welling effects.

Each of these possible groundwater interactions is addressed in more detail in the sub-sections that follow, with general descriptions and graphics based on examples and geological settings of


impermeable layer“Cryptic” seasonal andephemeral rivers, pans

Cave ecosystems

Spring, wetland

Baseflow, bank storage

impermeable layer“Cryptic” seasonal andephemeral rivers, pans

Cave ecosystems

Spring, wetlandSpring, wetland

Baseflow, bank storageBaseflow, bank storage

primary aquifers. This is for illustrative purposes only. It is not directly translatable into the context of the TMG geology and topography. This aspect is dealt with more fully in section 4.

3.1 Soil and Rock Interactions

Groundwater also reacts with the soils, weathered materials and rocks in which it occurs and the longer the residence time, the further these reactions progress. These reactions alter the biological and geochemical characteristics of the water through:(1) acid-base reactions, (2) precipitation and dissolution of minerals,(3) sorption and ion exchange, (4) oxidation-reduction reactions, (5) biodegradation, and (6) dissolution and exsolution of gases.

The net inflow of oxygen-rich surface water into the subsurface environment creates an environment where bacteria and geochemically active sediment coatings are abundant. Aerobic (oxygen-using) micro organisms are particularly active and may use up all the dissolved oxygen so that the “downstream” part of the localised sub-surface flow system may be dominated by anaerobic micro organisms. Anaerobic bacteria can use nitrate, sulfate, or other solutes in place of oxygen for metabolism. The result of these processes is that many solutes are highly reactive in shallow groundwater in the vicinity of streambeds and the return flow to the surface water may be relatively nutrient-rich (see Figure 3.2 for an example of the kinds of changes).

Figure 3.2: How concentrations of various substances changed as the

groundwater was drawn from a river to a borehole (Winker et al 1999)


3.2 Interactions with non-marine aquatic ecosystems

Aquatic systems are broadly defined as ones where water is the primary medium or substrate. Although some saline water bodies are included in this section, the emphasis here is on freshwater systems where groundwater plays a significant role. This includes both surface aquatic systems, such as rivers, springs and wetlands, and sub-surface aquatic systems, such as caves and aquifers. Lakes and vleis and endorheic systems, such as pans, sometimes also have groundwater connections, but this is not always the case.

Estuaries and lagoons are dealt with in the section on the coast (Section 3.4).

3.2.1 Surface water bodies

RiversMany rivers originate as springs, where there is a localised or point discharge of groundwater. These are most common where groundwater flows are confined to conduits, such as faults, or concentrated in geological contact zones in the underlying rocks.

Thereafter the interactions between groundwater and surface water bodies are usually in the form of net fluxes (e.g., groundwater discharge from or recharge to a river, Figures 3.3 and 3.4). Figures 3.3 and 3.4 represent distinct states along a continuum, where the direction and rate of flow between the river and groundwater varies both spatially and temporally. As the water level in a river rises during rainfall events, so water may flow into the groundwater (losing river), and as it drops again following a dry period, so the direction of flow may reverse (gaining river). Some of the major rivers in the Western Cape are gaining rivers in the upper reaches but become losing rivers in their lower reaches, particularly in summer.

Thus, the stability of riverine habitats is enhanced by the interaction with groundwater. By receiving water that sustains water levels during dry seasons and droughts and replenishing this water during periods of high flow, the extremes of high or no-flow are moderated, and the risk of complete loss of aquatic habitat is reduced.


Figure 3.3: A gaining stream, with a net inflow of groundwater. (Winker et

al 1999)

Figure 3.4: A losing stream, with a net outflow of groundwater (Winker

et al 1999)

The temperature beneath the surface is also more constant with time so groundwater discharge can help stabilise the temperature of the surface water body it interacts with. This is particularly important in cold temperate climates but may also be important in the hotter, drier parts of South Africa. For instance, in the Doring River groundwater-fed pools are used by the indigenous fish during the dry season when surface flow ceases. During a recent survey in the summer indigenous fish were only found in pools that had a freshwater source, while alien fish were found in all pools.

A river may also become a losing stream when the transpiration of the riparian vegetation creates a zone of depression, with the water table at a lower elevation than the water level in the river. This can result in distinct diurnal cycles in the water levels in the river, which have been used to provide rough estimates of transpiration by the riparian vegetation.

WetlandsSimilar situations to those described in Figures 3.3 and 3.4 apply in the case of wetlands. Most wetlands are gaining systems, where there is a net discharge of groundwater into the wetland, although there are some well-known examples of losing wetlands, such as the endorheic Okavango Swamps. Many river floodplains in dry areas operate as gaining wetlands during floods, and as losing wetlands, feeding back to the river, during the dry season. Rooted vegetation in wetlands typically has a fibrous root mat which is highly permeable. Water absorption through these roots drives a significant interchange between surface water and pore water in the underlying sediments and can result in substantial nutrient fluxes. Nutrients from groundwater may also play a significant role in the functioning of wetlands.

Hyporheic zonesInteraction between river water and groundwater typically occurs through the hyporheic zone, which is a localised area or strip of permeable substrate beneath the river surface (Figure 3.5). Extensive hyporheic zones are likely to be a feature of most of the streams and river systems in and fed by the TMG. In Western Cape cobble-bed rivers, the hyporheic zone is recognised as an important component of the habitat assemblage in a river, providing, inter alia, nursery areas for larvae and juveniles, refuge areas for adults and supporting processes such as mineralisation and organic matter decomposition.

The size and geometry of hyporheic zones surrounding streams vary greatly in time and space. Mixing of ground and surface water in this zone may cause the chemical and biological character of the hyporhoeic zone to differ markedly from adjacent river or groundwater. The upwelling groundwater creates patches of high productivity in the hyporheic zones and in aquatic systems, which support greater animal densities and diversity compared with non-up welling situations.


Figure 3.5: Illustrations of the location of a hyporhoeic zone beneath a river and two situations where there is flow or interchange between the stream water and the groundwater driven by the hydraulic gradients (Winker et al.

1999)

Riparian vegetationGroundwater may comprise a substantial proportion of the dry season flow that is essential for maintaining the vegetation in riparian areas. Riparian zones, especially in semi-arid to arid areas, are important for maintaining biodiversity, offering refugia and habitat for a variety of organisms that are not able to survive solely in the adjacent terrestrial or aquatic environments. Riparian zones create a buffer between the terrestrial and aquatic ecosystems, protect the river from the effects of activities in the adjacent terrestrial environment, and stablise the river banks. They also provide shade, lower temperatures and natural resources (e.g. reed, wood, fruit).

3.2.2 Subsurface water bodies

Subsurface water bodies are found mainly in cave systems and karst landscapes where the water table reaches the bottom of the cave system. Where the rocks (e.g. limestone, dolomite) are fairly easily dissolved, groundwater interactions can influence the formation of underground caverns and caves, thus affecting the types and distribution of the different habitats, and the organisms that live in them.

Little is known about these systems or their interaction with groundwater. However, work done elsewhere, particularly in Eastern Europe and Australia, has indicated that they can exhibit surprisingly diverse assemblages of biota, and that they should not be excluded from an assessment of groundwater/surface water interactions.

3.3 Interactions with terrestrial ecosystems

Terrestrial ecosystems, using groundwater as the main water resource, are a significant feature of semi-arid and arid environments but are also found in higher rainfall areas. They are commonly found on seasonal and ephemeral river systems where there is no visible surface water and are indicated by the presence of plant species not represented in the adjacent dryland areas, for example, gallery forest or woodland. These types of terrestrial vegetation may also occur in association with dykes, geological contact zones, faults and fractures and are often used to site boreholes where there is no other evidence of groundwater near the surface.


Figure 3.6: Schematic illustrating some typical interactions between

vegetation and groundwater (Scott & Le Maitre, 1998; Le Maitre et al., 1999).

Groundwater usage of vegetation is highly likely in areas where the water table is <10 m deep but may also occur where the water table is deeper, as many tree root systems can reach depths of 20 - 50 m (Scott and Le Maitre 1998; Le Maitre et al. 1999, Figure 3.7).

Figure 3.7: Mean (+/- standard deviation) and maximum root depths for different plant growth forms (After Canadell et al. 1996).

3.4 Interactions with marine ecosystems


-70

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0Trees Shrubs

Grasses &herbaceous plants

Dep

th (m

) MeanMaximum

By volume, groundwater discharge into the marine environment is fairly small, but may play a significant role in the ecological functioning of coastal, estuarine and salt marsh. Where groundwater discharges into the marine environment, it not only moderates the salinity of the receiving water, but also keeps saline water out of terrestrial ecosystems. Discharge can occur either as broad seepage, often along the coastline, or as a point-discharge from a subsurface spring at the shoreline or offshore (Figure 3.8). The discharge rate varies with time according to, inter alia, variations in the sea level, increasing as sea levels decline.

In areas of groundwater seepage the groundwater flux through the seafloor influences the fauna living in the sediment, providing the organisms with freshwater, dissolved silica and nutrients. For instance, blooms of Anaulu, (a diatom) in the wave zone at the Alexandria Dunefield, Eastern Cape have been linked to nutrient-rich groundwater entering the ocean just off the beach. A similar phenomenon occurs on the False Bay coast off Zeekoeivlei, associated with silica-rich groundwater (Engelbrecht, pers. comm.). The Anaulus is food for inter-tidal mussels and snails that are eaten by shore-birds.

In brackish systems, such as salt marshes or estuaries, the fresh groundwater can provide a refuge habitat for freshwater organisms by maintaining relatively low salinities.

Figure 3.8: Groundwater seepage into the surface water and groundwater discharge through a sub-aqueous spring (Winker et al. 1999)

3.5 The nature of groundwater dependency

A groundwater dependent ecosystem, or component of an ecosystem, can be defined as: ‘An ecosystem, or component of an ecosystem, that would be significantly altered by a change in the volume and/or temporal distribution of its groundwater supply’.

A fundamental tenet of ecology is that ecosystems generally use a resource in proportion to its availability, whether it is water, light, nitrogen or some other resource, and that the availability of different resources will be a significant determinant of the structure, composition and dynamics of an ecosystem (Tilman 1988). Thus, where groundwater is accessible, ecosystems will develop some degree of dependence on it, and that dependence is likely to increase with increasing aridity of the associated environment. The following definitions of dependency may assist in this evaluation:

Entirely dependent – Were groundwater fluxes to diminish or be modified only slightly the ecosystem will collapse.


Highly dependent – Moderate changes to groundwater discharge or water tables would lead to substantial decreases in either the extent or the condition of the ecosystem. One form of response could be where the ecosystem has a critical threshold in its tolerance of change. Its response would be abrupt but only evident once the threshold has been exceeded.

Proportionally dependent – For a number of systems it is likely that a unit change in the amounts of groundwater will result in a proportional change in the condition of the ecosystem. For example, a 50% change in discharge would lead to the same change in the ecosystem.

Facultative dependency – Changes in groundwater would have a minor effect on the condition of the ecosystem.

No dependence – ecosystem independent of groundwater. It may be very difficult to restore these ecosystems once they have been affected or altered because of changes in the groundwater situation.

This section provided an overview of the types of interactions that generally can occur between groundwater and other ecosystems. In reality there are few studies and even less data on the nature of the interactions, or on the importance of those for maintaining the ecosystems. Often the exact nature of a groundwater dependency may only be realised once an ecosystem has been stressed beyond a critical threshold. Furthermore, it is worth remembering that groundwater dependence is not limited to quantity considerations and could also include dependence on the physical and chemical characteristics of the groundwater.

4. TMG AQUIFERS

TMG formations underlie much of the Western Cape and extend into both the Eastern and Northern Cape (Figure 4.1). The focus of this project is the confined secondary aquifer, where water moves at depth through cracks and fractures in the Table Mountain Group quartzites.


Figure 4.1a: Outcrop areas of TMG formations in the Western Cape, Eastern Cape and Northern Cape with associated caves (Mlisa, 2003). Box indicates

area of interest for large scale abstraction for the City of Cape Town.

Figure 4.1b: Outcrops and subsurface extent of TMG formations with associated hot springs and major fault zones (Hartnady and Hay, 2000).

Yellow box indicates ‘Cage’ study area.


4.1 Table Mountain Group: Geological pattern and process

The occurrence of groundwater in the deep, fractured rocks of the Table Mountain Group Aquifer has an ancient geological history that can be traced to the protracted deposition of sediments of the Cape Supergroup more than 400 million years ago.

The sediments, comprising quartz sands, clays and silts, were deposited in a shallow marine environment. An estimated 5 – 8 km depth of sediment was deposited in the Cape Basin. Over time the layers became buried, eventually forming rock under the increasing pressures and temperatures. The Table Mountain Group, which is the focus of this study, is the lowest component of the Cape Supergroup and forms the backbone of the Cape Fold Belt Mountains. Continental movement caused the layers to be squeezed into folds. The pressure on these layers of rock was immense, causing buckling and fracturing in the layers.

Two major events led to the current deformed state of the Table Mountain Group. They include a mountain building period called the Cape Orogeny during which uplift and thickening occurred, and the break up of Gondwana, a super-continent that existed before the formation of the Atlantic Ocean. The Cape Fold Belt was formed around 250 million years ago. The results of these events are fractures and faults in the brittle or competent layers e.g. the quartz sandstones, and extensive folding in the more pliable shale layers.

Table 4.1 Lithostratigraphy1 (after De Beer 2002) and hydrostratigraphy2 (after Hartnady and Hay 2002) of the TMG (thickness values mostly apply to southwestern outcrops)

Subgroup Formation Lithology (Rock type) Max. Thickness (m)

Hydro-stratigraphy

Nardouw Rietvlei/Baviaanskloof Feldspathic sandstone 280 Aquifer (limited)

Verlorenvalley Shale Mini-aquitardSkurweberg Quartz sandstone 290 AquiferGoudini Silty sandstone,

siltstone230 Meso-

aquitardPeninsula Cedarberg

PakhuisShale, siltstone Diamictite shale

12040

Meso-aquitard

Peninsula Quartz sandstone 1800 AquiferGraafwater Impure sandstone,

shale420 Meso-

aquitardPiekenierskloof Quartz sandstone,

conglomerate, shale900 Aquifer

(limited)

1 The sequence in which rock types are layered2 The sequence in which hydrological units (aquifer, aquitard) are layered


4.2 Structure and functioning of the deep fractured-rock TMG aquifer

The fractured rock groundwater systems of the tectonically folded Table Mountain Group (TMG) constitute a vast aquifer system extending from just north of Nieuwoudtville southwards to Cape Agulhas and eastwards to Port Elizabeth. The full volume of the aquifer rocks in this whole region comprises a staggering 100 000 km3.

The geological structure and geohydrological settings of the aquifers in the TMG primarily considered for medium to large scale abstraction (i.e. the Skurweberg Aquifer in the Nardouw Subgroup and the Peninsula Aquifer in the Peninsula Subgroup) differ in several respects. The Piekenierskloof Aquifer has no regional relevance and is not considered further. The Rietvlei aquifer, while having average thickness, is fine grained, has high feldspathic content and in general a low storativity. It is therefore not considered suitable for large scale abstraction schemes.

For further discussion of groundwater flow in the TMG and interactions with other ecosystems it is necessary to distinguish between these two important aquifers. The main characteristics and differences are listed in Table 4.2 below. For the occurrence and outcrops of these aquifers refer to Figure 4.1a.

Table 4.2 Main characteristics and differences of the Skurweberg and Peninsula Aquifer

Characteristic Skurweberg Aquifer Peninsula Aquifer

Elevation of outcrops Middle and lower ranges High mountain ranges

Vegetation cover Karroid shrublands, Renosterveld, Fynbos

Fynbos, often bare rock

Weathering Highly weathered, fractured Resistant, highly fractured

Confined / unconfined Often confined, overlaid by Bokkeveld Shale

Mostly confined, overlaid by Cedarberg Shale

Material Sandstone, siltstone, shale layers Sandstone, Quartzite

Bedding & jointing Thin bedding, cross-bedding, small vertical fractures; medium fracture network

Thick bedding, dense vertical fractures; good fracture network

Storage capacity Medium storativity due to less thickness and fracturing

High storativity due to thickness and dense fracture network

Recharge Less rainfall and less recharge percentage

High rainfall and higher recharge percentage

Discharge Seep zones, seasonal low flow springs

High elevation wetlands, perennial springs

The groundwater intersections of pathways in the TMG aquifer are commonly at depths of greater than 100 m below ground. The depth and capacity of the system is evidenced by the several powerful hot-springs in the region with outflow temperatures of up to 64° C (e.g. Brandvlei, see Figure 4.4). These temperatures are caused by the water being circulated to depths of at least 2000 m below ground level. All of the hot springs are situated in the Peninsula Aquifer and linked to either the contact with the over- or underlying aquitard or an impermeable fault zone, or where the Peninsula is hydraulically connected with the Nardouw due to faulting (see Figures 4.3 – 4.8).


Most springs and other natural groundwater discharge areas in the TMG can be linked to highly permeable fractures, which can have a large extent (called hydrotect). Usually the water flows in the fracture network towards zones of high permeability. These fractures or fault zones act like conduits, transporting the water over a long distance in a very narrow zone (see Figure 4.2).

Figure 4.2: Hydrotect near Voelvlei, connecting recharge and discharge area across a synclinal structure (Umvoto 2001)

Due to the harder structure and texture the higher lying outcrop areas of the TMG consist mainly of the quartzites from the Peninsula Formation, while the Nardouw forms the middle and lower ranges in the mountainous regions. The valley infills consist of Bokkeveld rocks which overlie the Nardouw except in upper regions and Alluvium cover. The river alluvium is generally derived from the TMG and thus varies from coarse sand and boulders in upper regions to sand with increasing silt content derived from Bokkeveld downvalley.

The recharge to the Table Mountain Group aquifer system is believed to be in the range of 7% to 23% of Mean Annual Precipitation (MAP). On the higher mountain peaks of the TMG in the Western Cape the MAP exceeds 1000 mm and over the remaining TMG catchment it usually exceeds 600 mm. High snowfalls in the vicinity of the Hex and Ceres Valley add considerably to the total recharge. Recharge is dominant in the winter months. The average winter temperature is significantly less than the average summer temperature. The EVT applicable to recharge estimation is based on the winter month’s temperature average. The surfaces of the TMG aquifers are also conducive to infiltration, comprising a spongelike jointed and blocky rock with minimal soil cover. Taking the orographic rainfall pattern, the elevation of the outcrop areas of the Peninsula Aquifer and its weathered and highly fractured surface into account, it is evident that the Peninsula Aquifer receives the higher amount of recharge.

In the study area for the concurrent feasibility study for CMC, the potential groundwater recharge for the Peninsula Aquifer only is of the order 100 million cubic metres or more a year (Umvoto 2001).

4.3 Chemistry of TMG Aquifers


Table 4.3 provides a breakdown of the general water chemistry characteristics expected for water abstracted from the Peninsula formation of the TMG aquifer. The water tends to be oligotrophic (low in nutrients), acidic and low in salinity, which is characteristic of water flowing through or over TMG formations.

Table 4.3 Typical water chemistry characteristics expected for water abstracted from the confined TMG aquifer. All concentrations in mg/L unless otherwise indicated (Smith et al. 2002).

EC (mS/m)

pH Na Mg Ca Cl SO4 Alkali-nity as CaCO3

Si K Fe D (‰)

18O(‰)

Boreholes from Nardouw SubgroupMean 30.0 6.0 30.8 5.8 10.2 56.2 27.6 42.8 6.4 5.1 3.3 -45.2 -7.3Minimum 9.2 3.1 7.2 1.5 1.3 6.1 3.2 1.0 2.1 0.4 <0.1 -53.2 -7.9Maximum 155.0 8.3 232.8 43.1 73.4 395.2 220.5 147.3 18.1 16.2 15.4 -27.7 -6.3Boreholes from Peninsula FormationMean 10.4 6.2 11.1 1.8 3.3 18.0 5.2 14.5 3.8 0.8 0.2 -42.8 -7.3Minimum 2.6 4.3 2.0 0.9 0.4 4.5 1.0 3.5 1.4 0.2 0.1 -51.1 -7.7Maximum 26.3 7.6 21.2 3.2 30.4 34.1 14.0 77.9 9.4 2.3 0.2 -35.5 -7.06

Thus, unlike the Nardouw Aquifer in the Western Cape, which tends to be high in iron and salts, the water from the Peninsula formation can be expected to be relatively pure. In contradiction, results from a study in the Hermanus area show high iron content and higher EC values for water samples from the Peninsula Aquifer due to the position of the water strikes close to the Cedarberg / Pakhuis contact (Umvoto 2002).

4.4 Why the confined TMG aquifer is being explored for bulk water supply.

As with a surface water resource being considered for use as supply, the main concerns when evaluating a possible groundwater source are the quality, abstractability, economic (i.e. the higher the yield the lower the cost in R/m3), storage capacity of the aquifer and environmental and social considerations.

The great thickness of the TMG, its high permeability in faulted areas, the generally good water quality (especially in the Peninsula Formation) and its exposure in high rainfall areas means that this aquifer may be able to provide significant volumes of useable water on a sustainable basis. In several areas, farmers are already abstracting significant volumes of groundwater from the Nardouw Aquifer, associated alluvium and alluvial fans, and Bokkeveld sandstones, e.g. Hex River (20 mill m3/yr Rosewarne 2002), Olifants – Doring (~12 mill. m3/yr, Hartnady and Hay 2000). There is currently only little abstraction from the Peninsula Aquifer for bulk water supply, e.g. Kammanassie area (1 mill. m3/yr), Hermanus (planned 1 mill. m3/yr). The CCT has started a project to gain a better understanding of the potential yield and impact of the TMG within 200 km of Cape Town. This project aims to establish a pilot well field delivering 5 mill. m3/yr within three years.


4.5 Types of TMG aquifer settings

The geological setting and geometry of the TMG aquifer requires a three-dimensional conceptual understanding of groundwater flow when evaluating flow paths and groundwater discharge into surface water bodies. The types of interaction with aquatic and terrestrial ecosystems can differ from the examples given in section 3 above. Groundwater discharge is mostly locally restricted and directly linked to lineaments such as fractures and faults.

In this context it is also important to distinguish between the two main TMG aquifers, namely the Skurweberg (in the Nardouw sub-group) and the Peninsula (Table 4.1). For instance, the Skurweberg aquifer contributes more directly to river baseflow both via the river bottom and via springs at the Nardouw – Cedarberg contact, while the Peninsula contributes to river flow mainly as surface run-off. Springs related to the Skurweberg/ Nardouw are often low volume springs and seasonal (depending on the short term rainfall patterns), while Peninsula springs mostly flow all year around and respond to loner term climatic patterns.

The following diagrams illustrate some of the main types of situations – type-settings – in the TMG aquifer showing the groundwater flow paths and the kinds of springs that are associated with these systems. Note that these diagrams are two-dimensional cross-sections. In some cases additional flow at an angle to the cross-section can occur in both the Peninsula in the overlying confined Skurweberg/ Nardouw aquifers.

Figure 4.3: Cross-section of TMG-flow –fold & lithology controlled [‘CAGE’-Type]

Recharge to exposed Peninsula either side of a folded syncline. Cold spring discharge from short flow paths. Hot springs discharge from long, deep flow paths. Flow follows the folded formation and is

focussed by faulting (parallel to cross-section).


Figure 4.4: Cross-section of TMG-flow – ‘Brandvlei’-Type, fault ledFault itself is ‘sealed’ (impermeable) but fault zone has high permeability. Recharge in high elevation area of the Peninsula and flow controlled by lithology. Long, deep flow paths as shown by hot springs

controlled faul tzone creating hydraulic contact between Peninsula and Nardouw.

Figure 4.5: Cross-section of TMG-flow – ‘Wemmershoek’-Type, fault boundedFaulting throws up impermeable basement. Short local flow paths. Discharge to springs against

impermeable basement and aquitard.


Figure 4.6: Cross-section of TMG-flow – Synclinal aquifer & fault ledComplex interaction of faults and folds. Short and long pathways to hot and cold springs, including

coastal and sub-marine springs.

Figure 4.7: Cross-section of TMG-flow – ‘Voelvlei’-Type, synclinal aquifer & alluvium

Precipitation and topographic gradients drive longer flow paths in the Peninsula. Discharge to springs and alluvium. Flow paths in Nardouw are perpendicular to the cross-section


4.6 Interactions TMG aquifer - ecosystems

Using the same grouping of ecosystems as in section 3, the TMG specific interactions between groundwater and ecosystems are described below, as far as they are known:

River ecosystemsThe groundwater fed river low flow or baseflow is mainly contributed by springs in the upper parts of the river system. Streams, rivers and estuaries are not directly connected to the TMG aquifers. However, the alluvium aquifer which might feed into the river or estuarine system might receive some contribution from higher lying areas in the TMG aquifer via springs and seep zones.

A WRC funded study has been carried out in the Kammanassie Mountains on TMG discharge and the impacts of abstraction. Groundwater in the area is thought to play a key role in providing baseflow to rivers and riparian vegetation. The environmental importance of groundwater in the area in and around the Kammanassie Nature Reserve is high.

Wetlands and spring ecosystemsIn the above-mentioned study area (Kammanassie Nature Reserve), three main types of springs were identified (see Figure 4.8):

Type 1: Shallow seasonal springs and seeps emanating at perched water tables, which can be interpreted as interflow or rejected recharge

Type 2: Lithologically controlled springs, due to the presence of inter-bedded aquitards, located mainly at the Peninsula - Cedarberg, Goudini – Skurweberg and Nardouw – Bokkeveld contacts

Type 3: Fault controlled Springs (FCS)

Figure 4.8: Typical spring localities in the Kammanassie area (Cleaver et al., in prep)


Type 1 springs occur across the Peninsula and Nardouw aquifers, and are not connected to the greater groundwater flow system (Kotze, 2001). The springs seep from a network of fractures within the TMG aquifers directly above localised aquitards and are highly seasonal. According to Kotze (2001) groundwater abstraction from any part of the TMG aquifers will not impact Type 1 springs.

Types 2 & 3 are the most significant with regard to the regional water balance (Kotze, 2001). Type 2 refers to those emanating from contacts between Cedarberg shale aquitard and the Peninsula Aquifer, the different strata in the Nardouw group and the Nardouw and Bokkeveld shales, as well as unconformities. The Type 2 springs provide an important portion of the stream run-off in the form of groundwater fed baseflow. Type 3 is represented by the hot springs present in the area. Both types can be impacted by groundwater abstraction provided there is interconnection and the spring occurs within the radius of influence of abstraction.

In the high elevation areas of the TMG outcrops a number of high lying wetlands occur, which depend on either the regional water table or perched water.

Aquifer and cave ecosystemsSubsurface water bodies and caves occur in certain formations within the Cape Supergroup (Mlisa 2003, see Figure 4.1a). These caves are typical of those more usually found in limestone formations and thus the Table Mountain Group is said to have a pseudokarstic character. They are likely to be important in TMG aquifer recharge and discharge patterns and in particular in the distribution of highland seeps and localised occurrence of apparent groundwater dependent ecosystems.

Terrestrial ecosystemsThe typical vegetation in the high elevation areas, underlain by TMG formations, utilise rain, air moisture and soil moisture as their main water sources. Vegetation depending on groundwater is fairly localised and can be linked to springs, seep zones and wetlands.

Marine ecosystemsDue to the physical character of the fractured TMG aquifers discharge of groundwater direct into the sea occurs mostly at distinct points, which could provide small but important ecosystems, as described in section 3.4. There is no direct groundwater discharge from the TMG into lagoons and estuarines.

5. ASSESSING GROUNDWATER DEPENDENCE

Evaluation of the potential impacts of water abstraction from TMG aquifer systems requires an understanding of the nature and extent of dependency of the ecosystems which use groundwater from this system.

As described in section 3.5 a groundwater dependent ecosystem, or component of an ecosystem, can be defined as: ‘An ecosystem, or component of an ecosystem, that would be significantly altered by a change in the volume and/or temporal distribution of its groundwater supply’.

From this definition, and using the information provided in Sections 3 and 4, it follows that: demonstration of groundwater use does not necessarily equate to groundwater

dependence;


demonstration of groundwater use in general does not necessarily equate to use of TMG groundwater in particular;

abstraction of water from the TMG aquifer will not necessarily affect the supply of TMG groundwater to a TMG-dependent ecosystem.

Section 3.5 provided an overview of the types of interactions that can occur between groundwater and other ecosystems. In reality there are few studies and even fewer data on the nature of the interactions, or on the importance of those for maintaining ecosystems. Often the exact nature of a groundwater dependency may only be realised once an ecosystem has been stressed beyond its range of tolerance of change. Groundwater dependence is not limited to the quantity or flows it can also include dependence on the physical and chemical characteristics of the groundwater. Thus, different parameters may be important in assessing dependency in different ecosystems. For instance, the depth to the water table is likely to be an important hydrogeological parameter controlling the availability of groundwater to the terrestrial plant communities, but salinity gradients and distributions are an important parameter in estuaries.

5.1 Possible evaluation methods

There are generally two ways of evaluating the groundwater dependency of a specific ecosystem, namely (1) assessing the relative groundwater contribution to the ecosystem as a whole and (2) assessing the extent to which key species are dependent on groundwater and their ability to tolerate water stress. While the possible methods for (1) vary widely from field measurements to remote sensing approaches, the methods for (2) depend upon field and laboratory experiments.

The methods to identify and assess the groundwater contribution to the ecosystem include:

Identifying groundwater discharge regimes and related water availability to terrestrial ecosystems, using either direct measurements in the field or remote sensing approaches;

Identifying the spatial relationship of ecosystems to groundwater flow regimes, particularly with respect to the recharge-discharge patterns of TMG aquifers.

Detailed geohydrological mapping and interpretation of the flow systems on, and affecting, the ecosystem;

Quantifying the groundwater contribution to baseflow in TMG rivers and streams, using hydrograph separation methods.

Methods to assess groundwater dependency of terrestrial ecosystems in southern Africa have been summarised in Colvin et al., 2002. The figure below summarises some of the tools available for use at different scales.


Figure 5.1: Tools to assess groundwater use by plants according to spatial scale (Colvin et al., 2002)

One technique using remote sensing is the change vector analysis technique (Hay, 2002). It identifies vegetation change anomalies that arise from seasonal or circumstantial growth patterns. Relationships between groundwater, surface water, terrestrial and aquatic ecology are a function of rainfall, topography, stratigraphy and geological structure. In order to facilitate the objective assessment of the probable combination of causes for vegetation change, a geospatial model is required. Combined with regional groundwater flow path modeling, the tool has potential to assess the impact of groundwater abstraction on the terrestrial and aquatic ecology. Geospatial modeling techniques allow for semi quantitative evaluation of precedent or event response causes.The methods to assess the groundwater dependency of specific plant species involve at least the following:

Botanical and ecological surveys of wetlands to develop a classification of groundwater dependence and to identify indicators and key species.

Measurement of evaporation and plant water use in representative wetlands. Sampling of selected plant species to assess their groundwater use and moisture stress

tolerances. Measurement of impacts of water abstraction on surface water flows and of the responses

of the stream(s) and /or springs to rainfall inputs. Measurement of ground and surface water quality.


Area

Leaf transpirationPlant transpirationCommunity transpiration

Xylem analysis

Evaporation modelling

Isotopes/chemistryTracers

Plant moisture stress

mm2 m2 100m2 1ha 1km2 100km2 10000km2

SatelliteSPOT VegLandsat MSSLandsat TMSPOT HR

AirborneScannerVideoPhotography

Ikonos

Resistivity

Experimental abstraction

Guidance on identifying groundwater dependent terrestrial ecosystems in order to set Resource Quality Objectives (RQOs) as a protective measure is given in Colvin et al..

5.2 Assessing impact of long-term changes in hydrologic cycle

The next step after evaluating groundwater dependency of an ecosystem is to assess the impact of long-term changes of the water flow pattern and water quality can have on the ecosystems. The first step for the assessment is to quantify the changes related to different causes.

Long-term changes of the flow pattern are here defined as relevant changes, which are observed over a cycle of several years and trends against the mean annual behaviour. They can be caused by several factors:

change in climatic conditions, such as global warming, and therefore increase or decrease of available water in the local system of the hydrologic cycle;

change in preferential flow paths due to reduced permeability of fracture zones, e.g. caused by earthquakes or chemical precipitation;

change in flow paths due to constructions, which intersect the natural flow path, such as tunnels, roads, dams;

decrease of actual recharge due to change of landuse in recharge area;

decrease of water table due to over abstraction.

It is often difficult, or even impossible, to distinguish between different potential causes, as the impact cannot be related to one cause separately. Furthermore, the significance of the impact of any one variable depends on the contribution this variable may have to the total change.

It is therefore of utmost importance for the assessment to establish a properly designed monitoring system, in order to quantify the changes and related impacts. Without proper baseline measurements and ongoing monitoring, it is impossible to quantify changes or predict the associated impacts.

Data from the monitoring system and the assessment of impacts can be used to defined and quantify the related risks.. Based on this approach a risk management plan can be derived for an adaptive risk management and mitigation strategy.


6. INPUT REQUIRED FROM SPECIALISTS

In the Scoping Phase of the project, the intention is to obtain an indication from a range of specialists of the type, nature and degree of interaction between their area of ecological experience and groundwater. It is planned to use this information as a basis from which to:

Evaluate the potential links between ecosystems and the TMG aquifer, as opposed to those from other aquifer systems in the Western Cape;

Identify key areas of information deficiency.

It is planned that the information, meta data, hard data, description, and opinion required from specialists, where available, will include an assessment of:

The temporal degree of utilisation of groundwater, i.e., seasonal or year-round.

The significance of groundwater for the ecosystem and the likely consequences of changes in groundwater supply, e.g., if groundwater was no longer available would this result in the demise of the ecosystem? The definitions of dependency provided in section 3.5 may assist in this evaluation.

The spatial extent of the dependence, e.g. widespread or localised. This includes consideration of both the nature and conservation importance of the ecosystem. It should be noted that even a small aquifer may support a keystone ecosystem, which has an importance greater than its geographical extent.

The nature of the dependency. This is possibly the most difficult to define given current knowledge on groundwater dependence and, as already mentioned, may only be realised once an ecosystem has been stressed beyond a critical threshold. Section 4 provided some clarification using examples of groundwater interactions in the Western Cape, where possible, and specialists may find these useful in their assessments.


Note: This section was provided in the February 2003 version of this Discussion Document in order to assist specialists in preparing for the scoping workshop

held on 28 March 2003.

7. REFERENCES

Bear, J. 1972. Dynamics in fluids of porous media. American Elsevier, 764 pp.

Canadell, J, Jackson, RB, Ehleringer, JR, Mooney, HA, Sala, OE & Schulze, E-D 1996. Maximum rooting depth of vegetation types at the global scale. Oecologia 108: 583-595.

Cleaver, G, Brown, LR, Bredenkamp, GJ and Smart, M in prep. Assessment of environmental impacts of groundwater abstraction from Table Mountain Group (TMG) aquifers on ecosystems in the Kammanassie Nature Reserve and environs. Draft report to the Water Research Commission (WRC)

Colvin, CA, Le Maitre, DC, Hughes, S. 2002. Assessing terrestrial ecosystem dependence on groundwater. Water Research Commission.

De Beer, CH 2002. The stratigraphy, lithology and structure of the Table Mountain Group. In: A synthesis of the hydrogeology of the Table Mountain Group – formation of a research strategy (eds K. Pietersen and R Parsons), pp 8-18. Report No. TT 158/01, Water Research Commission, Pretoria.

Driscoll, F.G. 1986. Groundwater and wells. Second edition. Published by Johnson Filtration Systems Inc., St Paul, Minnesota.1089 p.

Domenico, PA, and Schwartz, FW. 1990. Physical and chemical hydrogeology. John Wiley and sons, New York.

Hartnady, CJH and Hay, ER 2000. Reconnaissance Investigation into the Development and Utilisation of Table Mountain Group artesian Groundwater using the E10 Catchment as a Pilot Study Area [CAGE Study]. Report by Umvoto Africa cc to the Department of Water Affairs and Forestry

Hartnady, CJH and Hay, ER 2002. Fracture system and attribute studies in Table Mountain Group groundwater target generation. In: A synthesis of the hydrogeology of the Table Mountain Group – formation of a research strategy (eds K. Pietersen and R Parsons), pp 89-96. Report No. TT 158/01, Water Research Commission, Pretoria.

Hatton, T and Evans, R 1998. Dependence of ecosystems on groundwater and its significance to Australia. Occasional Paper No 12/98, Land and Water Resources Research and Development Corporation, CSIRO Australia.

Hay, ER, Tyson, C and Mlisa, A 2002. Vegetation Change Vector Analysis for evaluating the impact of groundwater abstraction on terrestrial and riparian vegetation. AAWG

Hölting, B 1984. Hydrogeologie - Einführung in die Allgemeine und Angewandte Hydrogeologie (Geohydrology – Introduction into general and applied geohydrology). Ferdinand Enke Verlag Stuttgart, Germany

Kelbe, B 2001. The importance of groundwater in the hydrological cycle and the relationship to surface water bodies. Progress Report on the WRC-Research Project K5/1168 to the WRC


Khan, R, Hay, ER, Hartnady, CJH, Shand, M, Görgens, A, Brown, C, Boucher, C and MacDevette, M 2000. Groundwater – Enhancing the water supply to the Western Cape. Presentation to the Minister of Water Affairs 4 September 2000

Kotze, JC 2001. Modelling of groundwater flow in the Table Mountain fractured sandstone aquifer. PhD Thesis, University of the Free State.

Le Maitre, DC, Scott, DF and Colvin C 1999. A review of information on interactions between vegetation and groundwater. Water SA 25: 137-152.

Le Maitre, DC, Colvin C and Scott, 2002. Groundwater dependent ecosystems in the fynbos biome and their vulnerability to groundwater abstraction. In ed Peitersen, K, and Parsons, R, A synthesis of the hydrogeology of the Table Mountain Group – Formulation of a Research Strategy. Report TT 158/01 Water Research Commission.

Luger, M and Hay, ER 2002. Are Environmental Impact Assessments meeting the challenges of groundwater utilisation? Motivation for an alternative risk management approach. In Proceedings Groundwater – The hidden treasure, Conference of the Groundwater Division Branch Western Cape September 2002

Mlisa, A 2003. GIS and Remote Sensing Application for Determining the Relationship between Geological Structures and Cave Distribution. B.Sc. (Hons.) Thesis at University of Fort Hare, Department of GIS

Pietersen, K. and Parsons, R. 2002. A synthesis of the hydrogeology of the Table Mountain Group – formulation of a research strategy. Report No. TT 158/01, Water Research Commission, Pretoria.

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Rosewarne, P 2002. Case study: Hex River valley. In: A synthesis of the hydrogeology of the Table Mountain Group – formation of a research strategy (eds K. Pietersen and R Parsons), pp 178-182. Report No. TT 158/01, Water Research Commission, Pretoria.

Scott, DF and Le Maitre, DC 1998. The interaction between vegetation and groundwater: research priorities for South Africa. Report No. 730/1/98, Water Research Commission, Pretoria.

Smart, M in press. Kammanassie

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APPENDIX A - GLOSSARY OF HYDROGEOLOGICAL TERMS

Acidic – water with a low pH.

Alluvial – recent unconsolidated sediments, resulting from the operations of modern rivers, thus including the sediments laid down in the river beds, flood plains, lakes, fans at the foot of mountain slopes, and estuaries.

Anisotropic – having physical properties that vary in different directions.

Aquiclude – An impermeable geological unit that cannot transmit water at all. (Very few natural geological materials are considered aquicludes.)

Aquifer - A saturated permeable geological unit that can transmit significant (economically useful) quantities of water under ordinary hydraulic gradients. Specific geologic materials are not innately defined as aquifers and aquitards, but within the context of the stratigraphic sequence in the subsurface area of interest.

Aquitard - A saturated geological unit of relatively lower permeability within a stratigraphic sequence relative to the aquifer of interest. Its permeability is not sufficient to justify production wells being placed in it. (This terminology is used much more frequently in practice than aquiclude, in recognition of the rarity of natural aquicludes.)

Capillary fringe - (a.k.a. tension-saturated zone) - The subsurface zone directly above the water table in which pores within the geologic matrix are saturated with water, but the fluid pressure is less than atmospheric. Water is held within pores of the geologic matrix by capillary forces.

Cone of depression - The cone-shaped area around a well where the groundwater level is lowered by pumping. The shape of the cone is influenced by both the permeability and storativity of the aquifer, and the abstraction rate and time. In fractured-rock aquifers the geometry of the fracture network and the contrast between fractures and matrix have the main impact on the shape.

Confined aquifer - An aquifer that is physically located between two aquicludes, where the piezometric water level is above the upper boundary of the aquifer. The water level in a well tapping a confined aquifer usually rises above the level of the aquifer. If the water rises above ground level, the aquifer is called artesian.

Contact zone – the join or interface between different geological units. Where a permeability contrast occurs, springs may form.

Discharge area - The area or zone where ground water emerges from the aquifer naturally or artificially. Natural outflow may be into a stream, lake, spring, wetland, etc. Artificial outflow may occur via pump wells.

Down gradient - Direction toward lesser hydraulic head than point of origin, or point of interest

Endorheic – a chatchement area with a closed drainage system i.e. one with no outlets. Often with a perennial or seasonal water body (pan) at its lowest point.

Fracture – breaks in rocks such as joints, due to intense folding or faulting

Fracture connectivity – a measure of how well the individual fractures or fracture systems, are connected to each other and thus of the potential flows.


Groundwater - Water in the subsurface, which is beneath the water table, and thus present within the saturated zone. In contrast, to water present in the unsaturated or vadose zone which is referred to as soil moisture.

Heterogeneous - A characteristic of the geologic matrix of interest in which hydraulic conductivity is dependent upon position and or direction.

Homogeneous - A characteristic of the geologic matrix of interest in which hydraulic conductivity is independent of position and or direction

Hydraulic conductivity - The constant of proportionality in Darcy’s law. It is defined as the volume of water that will move through a porous medium in unit time under a unit hydraulic gradient through a unit area measured at right angles to the direction of flow. Hydraulic conductivity is a function of both the porous medium and the fluid flowing through the porous medium.

Hydraulic gradient - The difference in hydraulic head between two measuring points within an aquifer located to each other in the direction of flow, divided by the distance between the two points.

Hydraulic head - The fluid potential for flow through porous media largely comprised of pressure head and elevation head. This satisfies the definition of potential in that it is a physical quantity capable of measurement (such as with manometers, piezometers, or wells tapping the porous medium), where flow always occurs from regions of higher values to regions of lower values.

Hydrotect – high permeable fracture or fault zone that extends over a long distance

Hyporheic zone - The saturated and biologically active zone in the unconsolidated material underlying and next to a water-course. The hyporhoeic zone is important in river system nutrient budgets as it acts as a nutrient storage system. It also provides a habitat and refuge for aquatic organisms, thus also serving a buffering function which promotes rapid recovery of aquatic ecosystems after floods or droughts.

Interflow – the lateral movement of water through upper soil horizons, normally during or following significant precipitation events.

Lithology – the description of the macroscale features of a rock, eg texture, grain type etc.

Oligotrophic – refers to lakes with considerable oxygen in the bottom waters and with limited nutrient matter

Orographic – related to elevation; orographic rainfall pattern means a strong relationship between amount of precipitation and elevation of the area

Perched water - Unconfined groundwater held above the water table by a layer of impermeable rock or sediment

Percolate - The downward flow of water through the pores or spaces of unsaturated rock or soil

Permeability - The capacity of rock or soil to transmit water. It is the portion of the hydraulic conductivity, which is a function of the porous medium alone. In primary aquifers permeability is an intrinsic property, which is a function of mean grain diameter, grain size distribution, sphericity and roundness of grains and the nature of grain packing.

Porosity - The degree to which the total volume of soil or rock is permeated with spaces or cavities through which water or air can move.

Potable water - Water, which is free from impurities that may cause disease or harmful physiological effects, such that the water is safe for human consumption


Potentiometric or piezometric surface - An imaginary surface formed by measuring the level to which water will rise in wells of a particular aquifer. For an unconfined aquifer the potentiometric surface is the water table; for a confined aquifer it is the static level of water in the wells. (Also known as the piezometric surface.)

Primary aquifer - Aquifers in which the water moves through the spaces that were formed at the same time as the geological formation was formed, for instance intergranular porosity in sand (e.g., alluvial deposits).

Recharge areas - Areas of land that allow groundwater to be replenished through infiltration or seepage from precipitation or surface runoff.

Representative elementary volume – a volume of sufficient size where there are no longer any significant statistical variations in the value of a particular property with the increasing size of the element (Bear, 1972).

Rejected recharge – groundwater discharge occurring relatively close to the recharge area of a regional aquifer system. The groundwater discharge has followed a flow path that is short in relation to the rest of the flow system.

Salinity - The concentration of dissolved salts in water. The most desirable drinking water contains 500 ppm or less of dissolved minerals.

Saturated zone - The subsurface zone below the water table where pores within the geologic matrix are filled with water and fluid pressure is greater than atmospheric. Aquifers are located in this zone.

Secondary aquifer – (a.k.a. as fractured-rock aquifer) - Aquifers in which the water moves through spaces that were formed after the geological formation was formed, such as fractures in hard rock.

Semi-confined aquifer – (a.k.a leaky aquifer) - An aquifer that is physically located between two aquitardes, and where the piezometric water level is above the upper boundary of the aquifer.

Storativity – capacity of the aquifer to store water in its pores, voids, fissures and fractures. It is given as the volume of water released from storage per unit surface area of the aquifer per unit decline in the hydraulic head (typically m3/m2/m, ie dimensionless).

Surface water - Bodies of water, snow, or ice on the surface of the earth (such as lakes, streams, ponds, wetlands, etc.).

Syncline – a fold in rocks in which the strata dip inward from both sides toward the axis, often forming a valley

Tectonic – designating the rock structure and external forms resulting from the deformation of the earth’s crust

Unconsolidated – where the matrix of the aquifer is formed from uncemented materials such as sand, gravel pebbles or mixtures of these.

Unconfined aquifer - (a.k.a. water table aquifer) - An aquifer which is not restricted by any confining layer above it. Its upper boundary is the water table, which is free to rise and fall. The water level in a well tapping an unconfined aquifer is at atmospheric pressure and does not rise above the level of the water table within the aquifer. An unconfined aquifer is often near to the earth's surface and not protected by low permeable layers, causing it to be easily recharged as well as contaminated.

Unsaturated zone - An area, usually between the land surface and the water table, where the openings or pores in the soil contain both air and water. (See also “vadose zone”)

Up gradient - Direction toward greater hydraulic head than point of origin, or point of interest.


Vadose zone - (a.k.a. unsaturated zone) - The subsurface zone above the water table and the capillary fringe in which pores within the geologic matrix are partially filled with air and partially filled with water, and fluid pressure is less than atmospheric.

Water table - The top of an unconfined aquifer where water pressure is equal to atmospheric pressure. The water table depth fluctuates with climate conditions on the land surface above and is usually gently curved and follows a subdued version of the land surface topography.

Watershed - All land and water within a drainage area, defined by topographic high points.

Well - An opening in the surface of the earth for the purpose of removing fresh water.


Documents

Conceptual understandingfred.csir.co.za/project/tmg/documents/scoping discussion... · Web viewprecipitation and dissolution of minerals, sorption and ion exchange, oxidation-reduction