GUIDELINES FOR

TAILINGS BASIN WATER BALANCE

MODELLING AND

WATBAL MANUAL

WATBAL © copyright 1989 - 1997 Golder Associates Ltd. Release 4.0

Prepared by

Donald E. Welch, P.Eng. Golder Associates Ltd.

2180 Meadowvale Blvd. Mississauga, Ontario, Canada

L5N 5S3

Tel: (905) 567-4444 Fax: (905) 567-6561

April 1997

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PREFACE

Golder Associates started to consider water balance modelling in tailings basins in 1981 when the firm became involved with the design and operation of tailings facilities in Eastern Canada. The first numerical model was a stand alone computer program but later versions were developed on commercial spreadsheets by Mike Ankenmann, Leon Botham and John Gilby in the Mississauga (Toronto) office of Golder Associates. Rick Firlotte, Juris Balins, Ken Bocking, Roy Lopes and Shiu Nam Kam also made significant contributions to the program logic. Ultimately, the program acquired the code name WATBAL which is used to day. Tailings management is primarily a water management problem. The solids are relatively easy to store but the effluent has to be safely passed through a basin in a never ending stream, either to the environment or recycled to the mill. Over the years WATBAL has proven to be a reliable model for predicting flows in a tailings basin within the accuracy required to cover the extreme ranges in climatic and operating conditions over which a tailings basin has to be designed and operated. In addition to being a design tool, WATBAL can be used by operators and regulators to monitor tailings basins during operation. No special skills are required to run the program but when determining the input parameters sound engineering judgement is needed and the advice of a hydrologist should be sought. Golder Associates disclaims any responsibility for the correctness of the data generated by WATBAL or for the consequences resulting from the use thereof. Any misuse of the WATBAL program is the sole responsibility of the user. WATBAL is merely a mathematical tool which adds and subtracts water inflows and losses to a small watershed operating under simulated and simplified field conditions. The output is only as reliable as the data that are put into it. Golder Associates welcomes suggestions from users of the program and feedback should bugs or glitches be discovered. Further information on the application and operation of WATBAL can be obtained from the author and practitioners in Golder Associates. Donald E. Welch, P.Eng. Mississauga, Ontario, Canada April 1997

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TABLE OF CONTENTS SECTION PAGE

PREFACE......................................................................................................................................... i

1.0 INTRODUCTION .............................................................................................................. 1

2.0 REQUIREMENTS FOR AN EFFECTIVE WATER BALANCE MODEL...................... 3

3.0 PERIOD OF WATER BALANCE ACCOUNTING.......................................................... 3

4.0 MANAGEMENT OF STORM FLOWS ............................................................................ 4

5.0 INTRODUCTION TO WATBAL...................................................................................... 5

6.0 WATBAL LOGIC .............................................................................................................. 6

7.0 SENSITIVITY ANALYSES ............................................................................................ 13

8.0 EXAMPLE - A TYPICAL WATBAL APPLICATION .................................................. 15 8.1 Input Parameters .................................................................................................. 15 8.2 Results of the Base Case Analysis (Precipitation Version) ................................. 17 8.3 Sensitivity Analyses............................................................................................. 18 8.4 Best and Worst Case Scenarios............................................................................ 19 8.5 Conditions Required for Total Containment (Zero Discharge to the Environment)20

9.0 WATBAL OPERATION - USERS MANUAL FOR RELEASE 4.0 .............................. 22

10.0 SOME CONSIDERATIONS FOR WATER BALANCE MODELLING IN EXTREMELY DRY CLIMATES..................................................................................... 28

11.0 DISCUSSION AND CONCLUSIONS ............................................................................ 30

REFERENCES .............................................................................................................................. 32 COMMONLY USED SYMBOLS AND ABBREVIATIONS FIGURES 1. Management of Storm Flows in a Tailings Basin 2. Element of a Tailings Basin Water Balance 3. Inflows, Losses and Net Inflow 4. Decant and Pond Volume

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TABLES 1. WATBAL PRINTOUT - PRECIPITATION VERSION Example Tailings Basin Water Balance (Base Case) 2. WATBAL PRINTOUT - STREAM FLOW VERSION Example Tailings Basin Water Balance (Base Case) 3. Results of Water Balance Sensitivity Analyses 4. Best and Worst Case Water Balance Scenarios

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TAILINGS BASIN WATER BALANCE MODELLING

AND WATBAL MANUAL

1.0 INTRODUCTION

An important objective of effective tailings management is to protect the environment. Because

the main pathway for contaminants is by water, the water balance is a very important element in

any tailings management strategy. It is imperative that the operator of a tailings basin have a

good understanding of the status of the pond at all times and be able to react quickly to unsafe

situations that could adversely impact the environment. Problems arise when:

• there is a prolonged period of greater than average precipitation,

• extreme flood events occur,

• discharge slurry density or recirculation decreases,

• water quality deteriorates,

• waterways and treatment plants are undersized, and

• a basin is not operated as designed.

A tailings basin is a living facility that changes with time as tailings are deposited. It is relatively

easy to effectively contain the tailings solids but the effluent has to be safely passed through the

system in a never ending stream, either to the environment or recycled to the mill. The

management of a tailings facility is primarily a water management problem and it is extremely

important to be able to predict the possible range in flows with a high degree of confidence.

These guidelines, and the attending examples, mainly pertain to water balance modelling in a wet

climate. With respect to a tailings basin, a simple definition of a wet climate is when there is a

net annual accumulation of water (net inflow) which has to be discharged to the environment to

keep a basin in balance on an annual basis. The same principals also pertain to dry climates,

however the goals might be different. For example, in a wet climate, seasonal changes usually

have a large impact on a balance whereas in dry climates water conservation always has a high

priority, hence it is important to minimize evaporation and seepage losses.

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In a wet climate, where water has to be discharged to the environment on an annual basis, costs

and environmental impacts are directly related to the quantity of water (effluent) that has to be

treated and released. For such conditions, inflows can be minimized by:

• keeping the watershed as small as possible to minimize the inflow from runoff,

• providing large ponds to promote evaporation,

• maintaining a high slurry density to minimize the inflow of process water, and

• maximizing the recirculation back to the mill to reduce the discharge to the environment and to minimize the use of fresh make-up water in the mill.

In extremely dry climates total containment (zero discharge) is very easy to attain while

evaporation and seepage losses have to be minimized to encourage recirculation to the mill. In

some in between climatic conditions, total containment may be possible by taking advantage of

relatively high evaporation as compared to precipitation. This is a laudable goal but only

achievable under certain conditions which are discussed, with an example, later in these

guidelines.

Some mine closure plans, in arid climates, rely on evaporation for maintaining zero discharge to

the environment. In other situations permanent flooding is relied upon to inhibit the oxidation of

potentially acid generating tailings. Both these situations require reliable, historical climatic data

on which to make long term predictions.

It is essential that all the parties concerned in the design, approval and operation of a tailings

facility, understand that a tailings basin does not have a single, unique water balance because the

input parameters can vary from day to day, month to month and even yearly. Average, base case

conditions seldom actually occur. Precipitation and evaporation can be significant variables and

often they are not well defined in remote regions because of the scarcity of historical records.

Evaporation varies as the surface area of the ponds and wetted tailings changes. Seepage can

only be approximately estimated unless a detailed hydrogeological investigation and analysis has

been carried out. Recirculation to the mill is frequently a moving target, the discharge slurry

density can vary depending on the operation of the mill, and even the milling rate can change.

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2.0 REQUIREMENTS FOR AN EFFECTIVE WATER BALANCE MODEL

Since a water balance model is a predictive tool, it must be able to estimate flows and pond

volumes over a wide range of operating and hydrological conditions. Care must be taken not to

build sophistication into a model which is not warranted for the degree of accuracy which can be

achieved and to understand which elements have the greatest impact on the outcome.

A tailings basin water balance should be:

• simple to use with data input in easily recognizable terms,

• transparent; that is easy to understand, scrutinize and criticize,

• easy to vary the input parameters to model changes in operating and climatic conditions,

• able to carry out sensitivity analyses, and

• capable of being used by mine personnel and regulators as an operating (monitoring) tool during the life of a tailings facility.

3.0 PERIOD OF WATER BALANCE ACCOUNTING

Tailings basin water balances can be calculated on short or long term bases. With short term

balances there is a concern that a clear picture might not be represented. Also, it is unusual to be

able to find a hydrological data set, which is suitably summarized for a short period analysis.

A year covers one complete hydrological cycle, therefore it is logical to at least summarize a

water balance on an annual basis. However, an annual balance may only be appropriate in an

environment where seasonal changes don’t have a large impact. For example, an annual balance

might be confidentially used in an extremely dry climate where only the increase in basin size has

a significant impact on seepage (infiltration) and evaporation on an annual basis. Changing basin

configuration might also impact a wet climate balance but probably not to the same extent,

especially in a small basin.

In wet or cold climates, where seasonal changes do impact a water balance, the flows in a tailings

basin can be reasonably modelled on a monthly basis for at least one complete, annual cycle.

There are two reasons why this is a suitable period. Firstly, the impact of the seasons can be

modelled and secondly, the climatological data (precipitation or runoff and evaporation) are

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usually published in a monthly format. These statistical data can be used directly in a water

balance model without further manipulation. In remote regions, precipitation and particularly

evaporation data are often scarce or unreliable. This requires an awareness and responsibility of

mine owners to collect meterological and runoff data to support the environmental management

strategy for a mine.

4.0 MANAGEMENT OF STORM FLOWS

A tailings basin has to be designed for a given set of annual hydrological conditions or more

correctly a range of conditions in which the storm events are statiscally included. Decants,

pumping systems, piping and treatment plants are sized for such criteria. However, a basin still

has to be able to cope with individual extreme runoff events as they occur, particularly if one

occurs when the pond is at its maximum operating level. Such an event might come from an

extremely heavy rainfall or a combination of rainfall and rapid snowmelt. Handling storm flows

in a tailings basin is usually easy because the collecting watersheds are relatively small, typically

less than 5 km2 in a small basin.

One approach is to use two design floods; an environmental design flood or design inflow (EDF)

and a dam design flood (DDF). The EDF is typically from a storm with a finite return period of

say 10 to 100 years, or it may be a rainfall:snowmelt event (where applicable). It is the maximum

storm event which still does not result in an unscheduled discharge of water to the environment.

The flood from the EDF is retained, within a freeboard allowance below the invert of the

emergency spillway, and managed within the normal operation of the basin. For this reason, a

maximum operating water level (MOWL) must be specified in conjunction with the EDF. As a

matter of due dilligence, the MOWL should not be exceeded under normal operating conditions.

The MOWL should provide for adequate freeboard to store the EDF without discharge over the

emergency spillway. Floods in excess of the EDF are either allowed to spill unimpeded (very

large storms) or to spill slowly with a reduced retention time through the emergency spillway.

This concept is shown on Figure 1. If spillage cannot be tolerated under any circumstances, then

the EDF and the DDF have to be the same.

The overtopping of a dam could result in a catastrophic failure, extensive erosion, a loss of

tailings to the environment, a very large uncontrolled spill of water and even the complete

emptying of the pond. For these reasons, an emergency spillway is a safety measure, which must

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be included in every tailings facility, to handle unforseen events and to protect the environment.

Under no circumstances should tailings be allowed to escape from a basin or a dam be allowed to

overtop. The DDF must be chosen to ensure that this does not happen. The routed DDF is used

to size the emergency spillway. For a tailings basin, it is frequently based on the largest possible

storm resulting from the probable maximum precipitation (PMP) which might or might not occur

in conjunction with snowmelt. A flood routing analysis is required to size an emergency

spillway.

5.0 INTRODUCTION TO WATBAL

A tailings basin water balance merely adds and subtracts inflows and losses to a system and is

therefore really quite simple. Runoff models, such as might be used for urban areas, are designed

for storm events and, as such, are not applicable for a tailings basin water balance. There are

tailings basin water balance models that do not reflect seasonal changes. Others don’t include

seepage which may be a significant component. In some, an attempt is made to predict pond

elevation (level) which does not fit very well into a generic model. In any event, pond elevation

can be readily determined from a struck level storage capacity curve (elevation plotted against

volume) which should be developed for every stage of a pond’s life.

Considering the uncertainty in predicting climatic and operational conditions, experience has

shown that a simple model, along with good engineering judgement, can adequately predict the

performance of a tailings basin over a range of anticipated conditions. Golder Associates has

developed and refined a computerized, spreadsheet based, tailings basin water balance model

called WATBAL which has been successfully used in many basins to predict flows; to size

ponds, decant facilities and treatment plants; and to determine schedules for dam raising. Earlier

versions of the WATBAL model have been previously discussed in other fora (Welch and

Firlotte, 1989; Welch, Botham and Bronkhorst, 1992; and Welch, Botham and Johnson, 1995).

WATBAL is merely a mathematical tool which adds and subtracts inflows and losses to a

system. The results of the output are only as good as the numbers that are put into it; however,

actual experience has proven it to be an effective program. The user must ensure, however, that

the assumptions and input data are appropriate for the intended application. The advice of a

hydrologist or hydrotechnical engineer should be sought to assist with the interpretation of

hydrological records, the estimation of runoff and the routing of storm flows. The program is a

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simplified model of simulated field conditions operating in a small watershed under specific

hydrological conditions.

6.0 WATBAL LOGIC

All water balance models have to use the same basic elements. These are shown schematically

on the attached Figure 2 and listed below. WATBAL spreadsheets have been developed to

handle precipitation data (Table 1) and also stream flow data or specific runoff (Table 2). For

reference to the column numbers given below, refer to WATBAL's output format which is the

same in both versions of WATBAL.

INFLOWS • Transport water in the tailings slurry (Column 2) • Miscellaneous inflows such as mine water (Column 3) • Runoff from precipitation (Column 4) • Summation of the inflows (Column 5)

LOSSES

• Water retained in the pore spaces of the tailings (Column 6) • Seepage (Column 7) • Evaporation from ponds and wetted tailings surface (Column 8) • Recirculation to the mill (Column 9) • Summation of the losses (Column 10)

In addition to calculating inflows and losses WATBAL includes:

ACCUMULATION • Net inflow - Column 5 minus Column 10 (Column 11) • Water displaced - if the pond volume has to be reduced to make room for tailings (Column

12) • Total to be decanted - sum of Columns 11 and 12 (Column 13) • Decant strategy (Column 14) • Net monthly change (Column 15) • Accumulated pond volume (Column 16)

The net inflow (Column 11) is merely the difference between the inflows and the losses. The net

annual inflow is the volume of water that accumulates in the system on an annual basis and has to

be discharged to the environment each year if the basin is to remain in balance. Otherwise the

pond has to be sized to store that portion of the net inflow which accumulates. If the pond

volume has to be reduced by tailings inflow then this volume has to be added to the net inflow to

give the total to be decanted in Column 13.

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The following describes the logic and equations used in WATBAL. With reference to Tables 1

and 2, the tailings water (Column 2) is computed from the percent solids (by weight) in the

tailings slurry by solving for Vw in the following formula:

SW

W W100 where, V

100 WS

W

rs

w ww

ss

w=

+

=

−

where:

Ws = dry weight of tailings solids in the slurry W w = weight of water in the slurry (ρw x Vw) Vw = volume of water in the slurry ρw = density of water (unity in the metric system) S = slurry density (% solids by weight)

The tailings water is always calculated from the tailings slurry density. The tailings water is

essentially the total mill water requirements except for that which is lost as spillage and

evaporation in the mill and any water that goes out with the product or concentrate (Figure 2).

Miscellaneous inflows, such as mine water (Column 3) are sometimes put into the tailings pond.

It is normally better to handle mine water and other miscellaneous inflows separately because

they usually have different holding and treatment requirements or no treatment requirement at all.

On the other hand, a single point of effluent discharge from a mining property is attractive from a

regulatory point of view.

If water is being discharged from pond to pond, as might be the case from a sedimentation pond

to say a conditioning or polishing pond, and a separate balance is required for each pond, then the

inflow into the receiving pond has to be entered into WATBAL as a miscellaneous inflow.

Surface runoff is computed in Column 4. Runoff is a function of watershed size, the larger the

watershed the greater the runoff and the more important it becomes in a water balance. Runoff

can essentially be assessed from three different sources.

• precipitation records,

• stream flow measurements at or close to the site,

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• regional specific runoff (l/sec/km2).

Specific runoff is a published number which is determined from flow measurements in a large

(regional) watershed. However, it may not be realistic to apply specific runoff, determined in

such a way, to a small watershed. In a large watershed, which contains lakes, swamps, streams

and vegetation, the attenuation of runoff and the evapo-transpiration would be much greater and

the corresponding runoff less than from a small tailings basin watershed where the same elements

may not be present. In many regions, such as the mining areas in Eastern Canada, the specific

runoff is typically about 50% of the total annual precipitation whereas in a small tailings basin

watershed the runoff might be 65% to 75% of the total precipitation. In dry climates, the specific

runoff might be as low as 20% and 10% and possibly even zero in extremely dry desert regions

such as Northern Chile.

Actual stream flow measurements can be used if they are accurately obtained and cover enough

years to determine a realistic range. This, however, is seldom the case, particularly in a remote

region. However, even cursory stream flow measurements are useful to assess the monthly runoff

distribution which, in areas with winter snow, doesn’t mirror precipitation.

Precipitation records are often the only reliable means to assess runoff. Precipitation records,

covering enough years to obtain a realistic average, are usually available from nearby stations in

most regions of the world. In fact most agencies publish average precipitation data in a monthly

format which is suitable for direct input into a tailings basin water balance.

The WATBAL program has been designed to use both precipitation records (Table 1) and stream

flow or specific runoff (Table 2).

In the precipitation version, WATBAL accounts for runoff from two different parts of a

watershed; the virgin land which surrounds the tailings and drains into the pond and also the

tailings and ponds. The runoff from the virgin land is computed as a percentage of precipitation,

which could be in the order of 65% to 75% for a rocky slope surrounding a tailings basin. As is

discussed above, this percentage is typically greater than the runoff factor for a large watershed.

In this document the percentage runoff has been defined as the runoff factor. All the precipitation

that falls on the tailings surface and ponds is assumed to enter the ponds directly and therefore the

runoff factor for this part of the watershed is 100%. If the tailings are not completely saturated

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then some of this precipitation could infiltrate into the tailings before reaching the ponds, but

eventually most of it will get to the pond at least in a wet climate. Evaporation from the ponds

and wetted tailings surface is then accounted for separately in WATBAL.

As is mentioned above, runoff doesn’t always mirror precipitation, at least not in areas with

winter snow. During the winter months, the runoff can be reduced because the snow is held until

a thaw. WATBAL can account for this by inputing a runoff distribution each month as a

percentage of the total accumulated to date. For example, if it snow accumulates for 1 or more

consecutive months and 100% is input in the month that snowmelt occurs, then the total

accumulation will enter the inflow side of the water balance in the month of snowmelt.

The precipitation and evaporation data may be available from the hydrological services in a

monthly format, which is suitable for direct use in WATBAL. Regardless, the advice of a

hydrologist should be sought in estimating runoff factors.

The stream flow version of the WATBAL spreadsheet (Table 2) works exactly the same as the

precipitation version except that the data used to compute runoff is input either as “stream flow”

or “specific runoff ”. The latter is usually given as an average annual rate in L/s/km2. If average

annual specific runoff is used, then the area of the collecting watershed also has to be entered,

along with a monthly distribution which is a percentage of the annual total. The sum of the

monthly distributions must equal 100%. For both inputs, the impact of changes can be easily and

quickly evaluated by inputting a plus or minus “% change” in the appropriate row.

Precipitation data usually have to be used for tailings basin water balances simply because stream

flow data are not available and specific runoff values are not normally applicable to small

watersheds.

Column 5 is the summation of the inflows.

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Water retained in the pore spaces of the tailings (Column 6) is entered into the program by

“water content” in the traditional soil mechanics sense, which is the ratio of the weight of water

over the dry weight of tailings. If the tailings are saturated then the water content can be simply

calculated as follows:

w wGs

or

we

Gs

d= −

=

ρρ

1100

100

where: w = saturated water content (%) ρd = dry density of tailings Gs = specific gravity of tailings particles ρw = density of water (unity in the metric system) e = void ratio (vol. of voids / vol. of solids)

The dry density (or corresponding void ratio) that is chosen to calculate the saturated water content

can have a significant impact on a water balance. Void ratio and density are a function of specific

gravity. It is not always a simple matter to choose a realistic density. Tailings dry density depends

on many factors such as specific gravity of solid particles, gradation, content of clay, gypsum and

precipitates, ice inclusions in cold climates, degree of consolidation (due to self loading) and the

distance and depth from the discharge point. Some average value has to be used for design. A few

general guidelines are given below:

• If the tailings are relatively coarse grained and do not contain clay minerals then consolidation,

after initial liquid/solids separation, will be relatively minor and occur quickly. It can be argued

that, in this case, most of the water, that will be squeezed out of the tailings by consolidation,

due to self loading, will end up in the pond in a relatively short period of time and therefore the

ultimate assumed dry density can be used to calculate the water retained without introducing a

significant inaccuracy in the water balance. Typical dry densities for this case might range

between 1.3 and 1.4 t/m3 for tailings with a specific gravity in the 2.7 range.

• If the tailings are finely ground and/or contain clay, gypsum or other precipitates, then

consolidation will take place more slowly and may not even be completed until long after the

basin has been decommissioned. In this case a density lower than the ultimate dry density

should be used for the water balance computations. This can be estimated by laboratory testing

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to determine the initial density and the rate at which water will be liberated by the consolidation

process.

• In extremely dry climates there can be high evaporative losses on exposed beaches which can’t

be recovered. In this case, a low dry density should be used which typically might be in the 1.0

t/m3 range because it doesn’t matter to the balance whether the water is lost as evaporation or

permanently kept in the pore spaces.

• If a portion of a tailings mass is partially drained then the average water content will be less

than the saturated water content. The water content, in this case, has to be arbitrarily reduced

by estimation. This reduction is not a function of density.

Laboratory testing is required to determine tailings properties and to assist in estimating deposited

density.

Unrecoverable seepage (Column 7) may not be accurately known unless a full scale geological

and hydrogeological assessment has been made of the basin. In valley basins, where the

groundwater is mounded, above the tailings deposit, seepage from the basin does not usually

represent a significant volume with respect to the balance although it can be an environmental

problem. In fact, in such a case, seepage into the basin may contribute to the net inflow and

possibly should be assessed, but WATBAL does not account for this. However, in very dry

climates seepage can be a significant component of a tailings basin water balance. This is

discussed further in Section 10.0.

Evapo-transpiration is implicity taken into account in the runoff factor applied to the

precipitation falling on the virgin land surrounding the tailings. However, once the runoff reaches

the ponds, water can be lost through evaporation. Lake evaporation (as opposed to pan

evaporation) is therefore applied to the ponds and wetted surface of the tailings (Column 8). It is

a function of the surface area of the ponds and wetted tailings surface. Lake evaporation is

generally about 70% of measured pan evaporation. Evaporation can be increased by spraying

techniques. In a dry climate, on the other hand, it may be important to inhibit the evaporation by

keeping the pond area small. Evaporation data are normally obtained from the same source as

precipitation and also in a monthly format. The impact of changes in evaporation can be easily

and quickly evaluated by inputting a plus or minus “% change” in the appropriate row.

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Water that is recirculated from the tailings pond for reuse in the mill (Column 9) is entered into

the model as a percentage of the tailings discharge water. For practical purposes, all the water

that enters a mill has to come out of the mill in the tailings stream except for that which goes out

with the product or is lost as spillage and evaporation in the mill (Figure 2). The rate of

recirculation is readily available from designers and mill operators. Depending on the

circumstances, a recirculation rate as high as 80% to 90% may be realistic but 100% is seldom

achievable because a mill requires some clean make-up water for reagent mixing, compressors,

gland water, showers, etc.

In WATBAL there has to be tailings discharge to calculate recirculation. However, a balance can

be done for a pond, such as a polishing pond which is not receiving tailings but from which water

is being recirculated. For such a case, the recirculation can be accounted for by subtracting it

from the input flow (from the pond above) which has to be inputted into the balance as

miscellaneous inflow.

Column 10 is the summation of the losses.

The "Net Inflow" (Column 11) is the excess water that accumulates in the pond. It is the

difference between Columns 5 and 10. It is the volume that has to be decanted, treated and

discharged to the environment if the pond volume is to remain constant on an annual basis.

Water displaced by tailings is calculated in Column 12. If an existing body of water has to be

displaced to make room for tailings then the displaced water has to be added to the net annual

inflow which has to be decanted to the environment each year if the basin is to remain in balance.

For example, such a situation could exist if a lake had to be displaced over the life of the mine to

make room for tailings. In this case, a portion of the lake’s water would have to be decanted each

year. It would, of course, be preferable to pump out the lake before depositing the tailings but

assuming that this isn’t possible, for whatever reason, then the water would have to be displaced.

Now almost in every case tailings migrate into a sedimentation pond but this does not mean that

water is displaced to make room for tailings if the pond volume remains constant on an annual

basis. The pond may merely move or the surface may rise but it does not necessarily change in

volume on an annual basis. Only if water is displaced to make room for tailings does the

displaced water have to be added to the net inflow. In WATBAL this is accounted for as follows:

The water retained in the pore spaces of the tailings, has already been accounted for in the

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balance, therefore the volume displaced must equal the total space occupied by the deposited

tailings slurry (solids plus water). This can be calculated by:

Water displaceddry weight of submerged tailings ( sW )dry density of deposited tailings ( )

= dρ

1

The dry weight of tailings is that portion of the tailings that actually ends up displacing water and

not necessarily the total production. It is stressed again that, because the pore water has

already been accounted for separately in WATBAL, the volume of water displaced equals

the total deposited volume (tailings solids plus pore space) and not just the water displaced

by the tailings solids.

Another way to handle displaced water in WATBAL is to merely add an arbitrary volume as a

miscellaneous inflow.

Column 13 is the total that has to be decanted to the environment. It includes the net inflow

(Column 11) plus any water that has to be displaced to make room for tailings.

The water that has to be decanted to the environment each month can be controlled by design and

can vary from month and even be zero in the winter months. Many mines are only permitted to

discharge in certain seasons of the year, typically when receiver flows are high, or when natural

processes have provided optimal polishing. However, if the system is to remain in balance, on an

annual basis, then the total volume of water decanted must be equal to the net annual inflow. Few

tailings basins have enough excess storage capacity which would allow them to safely accumulate

water from year to year without threatening to overtop dams. The amount of water decanted

(discharged to the environment) each month is entered into the program as a percentage of the

total net inflow (Column 14). It is controlled by design and can vary from month to month, and

even be zero. Column 15 gives the net monthly change and Column 16 the accumulated

volume in the pond at the end of each month.

7.0 SENSITIVITY ANALYSES

It is extremely important that the critical factors affecting a water balance be thoroughly

understood. A little effort in addressing the parameters affecting the operation of a tailings basin

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can often have a beneficial impact on development and operating costs. WATBAL can be used

to quickly investigate the effect that variations in the input parameters can have on the volume of

water that has to be treated and discharged to the environment. The parameters that normally

impact a balance to varying degrees depending on the situation, include:

• discharge slurry density,

• diversion or reduction in watershed area,

• precipitation and evaporation,

• recirculation to the mill,

• miscellaneous inflows such as mine water,

• water retained in the tailings (pore water),

• seepage, and

• pumping out existing water in a basin prior to start-up.

For example, if the slurry density is increased from 30% to 50% solids, then the tailings transport

water is reduced by more than half. Runoff is of course a function of watershed size and it is easy

to evaluate the impact of diverting part of a watershed which may be an economical trade-off

against managing a larger volume of water. Evaporation is a function of pond area and wetted

tailings surface; large shallow ponds result in greater evaporative losses. A high recirculation rate

reduces the need to introduce clean make-up water to the system as well as reducing the

accumulation of water in the tailings pond.

Based on the results of a sensitivity analysis, parameters can then be chosen to simulate best

(optimistic) and worst (pessimistic) case scenarios to establish the range of operating and

hydrological conditions that a tailings facility might have to contend with. The choice of

parameters for the worst case scenario depends on the level of risk that the operators and

regulatory authorities are willing to take. A probabilistic risk assessment (program) can be

applied to determine the probability that a certain net inflow will occur for a given set of

conditions.

Another variable which must be considered during the operating life of a mine is that a tailings

facility normally changes shape as tailings are deposited. The surface area of the tailings usually

increases with time. The pond configuration can change even though the volumetric capacity

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remains constant; for example, it may become shallower with a larger surface area which

increases evaporative losses. Such changes, which may have to be considered for design, impact

runoff, evaporation and seepage. Balances should be prepared for the various stages of tailings

facility’s life and in a very dry climate they may have to be prepared for each year to obtain a

realstic assessment of what is actually happening.

8.0 EXAMPLE - A TYPICAL WATBAL APPLICATION

8.1 Input Parameters

As is discussed above, a tailings basin must be designed to operate within a range of operating

and hydrological conditions. All the significant inflows and losses must be identified and

accounted for in the water balance. The first step is to established a realistic set of “base case”

design parameters which represents normal (average) operating and hydrological conditions.

Then, based on a sensitivity analysis, the “best case” and “worst case” scenarios can be

established to set the boundary conditions on which to base the design. The best and worst case

scenarios should be developed to reflect the uncertainty and natural variability in the parameters.

As is discussed above, a probabilistic based risk assessment program could be used to assist with

this task.

For an example, using precipitation, the input data for a hypothetical gold mill operating in the

Canadian Northwest Territories with a severe winter climate are assumed. A reasonable set of

base case parameters for such a mill and tailings basin could be as follows:

Tailings production

• design tonnage 2100 t/day (metric)

• mill availability 95%

• nominal tonnage (planned average annual over 365 days)

1995 t/day

Operating period 12 months/year

Discharge slurry density 45% solids by weight

Miscellaneous inflows zero m3/month

Precipitation (average annual) 300 mm/yr. (monthly distribution required)

Total watershed area 5.2 km2

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Area of tailings and ponds 1.7 km2

Runoff factors (% of precipitation) - virgin land surrounding the tailings 70% of precipitation - tailings and ponds 100% of precipitation

Runoff period 7 months (May to November)

Specific gravity of tailings solids 2.70

Assumed void ratio of deposited tailings 1.0 (volume of voids / volume of solids)

Dry density of tailings (calculated using assumed void ratio) 1.35 t/m3 (no ice inclusions)

Saturated water content of tails (calculated as wt. water/dry wt. tails)

37% (assume tailings remain saturated)

Estimated basin seepage 2000 m3/month

Lake evaporation from ponds and wetted tailings surface 470 mm/yr. (monthly distribution required)

Area of pond and wetting tailings beach 0.5 km2

Recirculation to the mill 90% of discharge water

% of tails displacing water None

Decant strategy 6 months (June to November)

Starting month (beginning of an annual cycle) December

Initial pond volume (end of Nov.) 400,000 m3 (dead storage)

For our example, WATBAL is run for both the precipitation and runoff versions. For the runoff

version on Table 2, the total annual runoff (determined in the precipitation version) is distributed

over the 7 summer months from may to November inclusive. The results of the two versions are

therefore identical.

The input data can be entered into WATBAL in simple, easily recognizable terms which

facilitates changes and enables sensitivity analyses to be carried out. The input data, as they are

entered into the program for the base case, are shown on the upper part of Tables 1 and 2. For

most items, the input data can either be entered as an average in the “VALUE” column or

distributed with a variable distribution in the monthly columns. For example, the water content

of the tailings might increase in the winter months to account for process water that gets

permanently entrapped as ice. The precipitation and evaporation has to be inputed on a monthly

basis. If the “VALUE” column is blank or contains a zero then the program automatically uses

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the monthly distributions to carry out its computations. If both the “VALUE” column and the

monthly columns contain data then the “VALUE ” column will dominate.

8.2 Results of the Base Case Analysis (Precipitation Version)

The output from WATBAL is given on the lower part of Table 1 (and Table 2) and is summarized

below on an annual basis. The annual totals are merely from the total line on the table.

INFLOWS M m3/year

Tailings water 0.89 Miscellaneous inflows 0 Runoff 1.25 ____ TOTAL 2.14 LOSSES Retained in tailings 0.27 Seepage 0.02 Evaporation 0.24 Recirculation 0.80 ____ TOTAL 1.33 NET ANNUAL INFLOW 0.81 Water displaced by tails 0 TOTAL TO BE DECANTED 0.81 MAX. POND VOLUME 0.72 M m3 (May)

The net annual inflow is 0.81 M m3/yr. This is the quantity of water that has to be discharged to

the environment each year if the pond is to remain in balance.

As can be seen, the runoff component is larger than the tailings discharge water. So even with a

high recirculation rate to the mill (90% of discharge water) and full saturation of the tailings there

is still a considerable amount of water that has to be discharged to the environment on an annual

basis to keep the pond in balance. The losses to the system are simply not great enough to

balance the inflows. The above example is quite typical for a tailings basin in the northern

regions of Canada where the conditions required for a total containment (zero discharge) are

difficult to attain. This is discussed further in Section 8.5 below.

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In our example, the maximum accumulated water volume for the average base case conditions

(Column 16) is 0.72 M m3 in May after the spring runoff. In the example, decanting stops at the

end of November when the pond is drawn down. However, the pond continues to decrease in

volume over the winter months, because the recirculation to the mill is large and there is limited

runoff.

Note that a provision has not been made for water losses due to ice inclusions which could

conceivably happen in a cold climate. As is discussed above, ice can be accounted for by

arbitrarily by increasing the water content of the deposited tailings in the winter months.

On Figure 3 the Inflows, Losses and Net Inflow are plotted against time and on Figure 4 Decant

and Pond Volume are also plotted against time.

8.3 Sensitivity Analyses

The sensitivity of the balance can be easily investigated by varying the significant input

parameters which, in our example, includes precipitation, tailings discharge slurry density,

recirculation to the mill and a scenario where the water, that is pumped from the mine, is put into

the basin as a miscellaneous inflow.

The results of the sensitivity analyses on these parameters, in our example, are given on Table 3.

The analyses are carried out by varying the input parameter being studied and keeping all of the

other parameters constant at the base case.

All the parameters investigated on Table 3 have a significant impact on the balance. If the annual

precipitation increases by 25% then the net annual inflow increases by 38%. Such an increase in

precipitation is likely to have a relatively short return period.

If the slurry density were allowed to decrease from 45% solids to say 20% solids then the net

annual inflow would increase by 25% even if the recirculation rate remains high at 90%. If it is

100% there would, of course, be no change. It is therefore necessary to maintain a high discharge

slurry density.

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A drop in the recirculation rate to the mill has a major impact. A drop to zero increases the net

inflow by 99%. An increase from 90% to 100% decreases the net inflow by 11%. This points

out the importance of recirculation to the mill. It has to be maintained at a high level if the basin

is to be operated efficiently.

Putting the mine water into the tailings pond can have a large adverse impact on the balance. At a

modest rate of 30,000 m3/month (~ 183 US gpm) the net annual inflow would increase by 44%

from the base case and at 90,000 m3/month the increase is 135% in our example. The

consequences of putting miscellaneous flows into a tailings pond must be carefully considered at

the design stage and not after the mill has started up. As is pointed out earlier in this document, it

is normally better to handle miscellaneous inflows separately because they usually have different

holding and treatment requirements.

8.4 Best and Worst Case Scenarios

As can be seen from the discussion above, a combination of adverse conditions, occurring

simultaneously, could have a disastrous impact on a tailings basin water balance. Using the

results above, reasonable best and worst case scenarios can be developed to determine the range

of operating and hydrological conditions that our hypothetical tailings basin has to contend with.

For our example, a reasonable set of best and worst case parameters could arguably be:

PARAMETER BEST CASE BASE CASE WORST CASE

Tailings Production 1995 1995 1995

Slurry density (%) 60 45 25

Miscellaneous inflows - Mine water (m3/month)

0 0 30,000

Precipitation -25% average +25%

Recirculation (%) 100 90 30

It is not unreasonable to assume that the annual precipitation could vary by at least plus or minus

25% during the operating life of a mine. Recirculation to the mill and the discharge slurry density

can drop for many reasons; usually associated with the operation of the mill. Also, it is not

inconceivable that unplanned miscellaneous inflows such as mine water could be discharged to

the tailings pond. This frequently happens.

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The results of the best case and worst case water balance scenarios are tabulated on Table 4. For

the worst case scenario the net annual inflow increases by 260% (a factor of 3.6) over the base

case and for the best case scenario it is approximately half. This clearly demonstrates the wide

range over which our hypothetical tailings facility would have to operate in order to prevent an

unscheduled discharge to the environment and for the system to stay in balance on an annual

basis. The net annual inflow, in our example, ranges from 0.45 M m3/yr. to 2.91 M m3/yr. When

it is agreed by the various stakeholders what the worst and best case scenarios should be, then the

system must be designed to accommodate the range plus an allowance for routed storm flows.

Also, the pond size is an important consideration. For the worst case scenario, the maximum

pond volume (in May) has to be about 3 times larger than the maximum pond volume for the base

case. It should be noted that there cannot be negative numbers in the accumulated pond volume

column (Column 16). To ensure that this does not happen the initial pond volume (dead storage),

that is inputted into WATBAL, has to be chosen accordingly.

8.5 Conditions Required for Total Containment (Zero Discharge to the Environment)

In extremely dry climates, with high evaporative and seepage losses, zero discharge from a

tailings basin to the environment can be easily achieved. In fact, in such cases, the collection and

reuse of every drop of water is usually a high priority.

In environmental settings where the precipitation is low and the evaporation relatively high, zero

discharge may be attainable under certain unique conditions when:

• the runoff component is small relative to the tailings production (either a dry climate or a small watershed),

• the recirculation to the mill is high (approaching 100%), and

• the volume of tailings is large enough to store the runoff component in the pore spaces.

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For our hypothetical tailings basin, the conditions for zero discharge to the environment are

explored. For the sake of argument let us assume that the watershed can’t be decreased in size

and that the recirculation to the mill is increased to 100% (difficult to attain). Under these

condition, the only way to achieve zero discharge is if the tailings production can be increased to

about 7,300 t/day and then the balance would look like this:

INFLOWS M m3/year Tailings Water 3.26 Miscellaneous inflows 0 Runoff 1.24 ____ TOTAL 4.50 LOSSES Retaining in tailings 0.98 Seepage 0.02 Evaporation 0.24 Recirculation 3.26 ____ TOTAL 4.50 NET ANNUAL INFLOW 0 MAX. POND VOLUME 0.42 (M m3) (Nov.)

It is interesting to note that the runoff component has to be virtually all stored in the pore spaces

of the tailings because the other losses due to seepage and evaporation are relatively small. This

is the reason why the tonnage has to be increased to meet the requirement for zero discharge.

Also, the pond has to be relatively large in order to be able to recycle water during the winter

when there is no runoff. It is highly unlikely that, in practice, the production of a 2,000 t/day

operation could be increased to 7,300 t/day which would make the goal of zero discharge,

unattainable in our example. The only other variably that can possibly be considered is the runoff

which might be reduced if the watershed area can be decreased in size. Assuming that this is not

possible then zero discharge is not a feasible option for the example in question.

While total containment is a desirable environmental goal, in practice it is extremely difficult to

attain in wet environments. In very dry climates it is, of course, easily achievable.

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9.0 WATBAL OPERATION - USERS MANUAL FOR RELEASE 4.0

WATBAL, Release 4.0 is programmed for the Windows based spreadsheet, EXCEL Release 5.0.

The program will run efficiently with the minimum computer memory (RAM) specified for the

software.

Two versions of WATBAL 4.0 have been developed; WATBAL 4.0A which uses precipitation

data to compute runoff and WATBAL 4.0B which uses stream flow or specific runoff in the

runoff calculations. Both versions are included on the attached WATBAL diskette.

The spreadsheet file is a read only file and when it is closed, it will ask to be saved with a

different file name. It cannot be saved as WATBAL.

To start, open the spreadsheet and load the file. The welcome screen will appear. Click on OK to

continue. The input sheet will appear. Data can only be entered into the shaded cells. The

unshaded cells cannot be changed. Print that is highlighted in blue can also be changed.

Computations are locked in and cannot be changed unless the input data are changed.

Every change that is made on the input sheet will automatically calculate the appropriate change

on the output form. A special command is not required to run the program.

The following notes explain the entry of the input data. Section 6.0 above “WATBAL LOGIC”

contains an explanation of the logic used. The upper part of the spreadsheet contains all the

information that WATBAL requires to compute the output on the lower part of the spreadsheet.

The balance is completely in metric units. Entries can either be inputted into the “ VALUE ”

column which will give an even monthly distribution or in most rows, variable distributions can

be inputted into the monthly columns. If the “ VALUE ” column is blank (no value) or contains a

zero, then the program automatically uses monthly distributions to carry out its computations. If

both the “VALUE ” column and the monthly columns contain data (other than zero) then the

“VALUE ” column will dominate. The input data are entered into Table 1A as follows:

Starting month:

Enter the month number, in the “ VALUE ” column, for the month that corresponds to

the start of the program which is usually the month at the beginning of the annual

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operating cycle such as when decanting is stopped for the season or when the pond

volume is the smallest. For example at a gold mill in the southern hemisphere, May is

typically the end of the annual decant period therefore June is the beginning of the annual

cycle. Water is normally held in a gold tailings pond during the winter when there is

minimal natural cyanide degradation.

Tailings Production:

A simple procedure is to divide the planned annual tailings production (nominal tonnage)

by 365 days to get an average tonnage for each day. This will include planned shutdowns

and the operational availability of the mill. The tonnage value can either be inputted as an

average value in the “ VALUE ” column or as a variable distribution in the monthly

columns.

Planned shutdowns can be modelled but then the design tonnage has to be used for the

months in which there is full production and prorated (reduced) acordingly for those

months in which there is partial production. For example if a mill, with a design

production rate of 2100 t/day has 95% availability (shut down for 18 ¼ days all in one

month) then the value that is inputed into that monthly column would be 822.5 t/day

which is calculated by [2100 x (30-18.25) ÷ 30] and the full design production rate would

be inputted into the other monthly columns. Alternatively, if an unplanned shutdown

occurs then the nominal tonnage for the month in which the shutdown occurs merely has

to be reduced accordingly if the balance is being used as a monitoring tool.

Note that the design tonnage and the nominal tonnage are different. The design tonnage

is the maximum tonnage (hourly or daily) for which a mill is designed to handle, whereas

the nominal tonnage is the annual planned or actual tonnage that is put through a mill

taking into account planned shutdowns. The design tonnage, which is always the larger,

is used to design the mill, pumps and pipelines whereas the nominal tonnage is used to

size the tailings basin and for the water balance.

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Solids in Discharge (Slurry Density):

The slurry density is used to compute the quantity of water discharged with the tailings.

It is the percentage (%) solids by weight in the total slurry (solids plus water). It can be

inputted in the “ VALUE ” column or as a variable distribution in the monthly columns.

Miscellaneous Inflows:

Enter a value in m3/month for any miscellaneous water entering the pond (such as mine

water), independent of the tailings water and direct runoff from precipitation. The value

can either be inputted as an average value into the “VALUE” column or as a variable

distribution in the monthly columns. If water is being discharged from pond to pond then

the inflow into the receiving pond, from the pond above, has to be entered into

“WATBAL” as a miscellaneous inflow.

Precipitation (precipitation version):

For each month, enter the base case precipitation in mm/month in the “Average

Precipitation” row. Alternatively, a value can be entered in the “VALUE ” column which

will then apply that value evenly to each month.

When investigating the sensitivity of runoff on the balance, the precipitation can be

increased or decreased by merely inputting the appropriate plus or minus percentage in

the “VALUE ” column in the “Change in Precipitation” row. This will then change the

computed runoff accordingly using the same distribution as the base case. If no change is

required then enter 0%. Alternatively, individual percentages can be inputted in the

monthly columns in the “Change in Precipitation” row. In this way, the distribution can

be changed from the base case.

Area of Virgin Land in the Basin:

Enter the total area, in hectares, of the watershed minus the area of the tailings and ponds

in the “VALUE ” column. The area cannot be inputed as a variable distribution in the

monthly columns because it is unlikely that the watershed area will have a significant

short term (monthly) variable distribution.

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Runoff Factor from Virgin Land:

Enter the runoff factor in per cent (%) for the area of virgin land in the basin (normally

ranges between 65% and 75% annually for rocky slopes surrounding a tailings basin in

the Canadian environment). In dry areas it could be much lower. Either an average value

can be entered in the “ VALUE ” column or a variable monthly distribution can be used.

Area of Tailings and Ponds:

Enter the total area in hectares of the tailings and ponds in the “VALUE ” column. As

with the area of virgin land, a variable distribution in the monthly columns cannot be

used. However, it is possible that the area of tailings and ponds can change over the long

term from year to year. To model this, sequential annual balances have to be run.

Runoff Factor for Tailings and Ponds:

Enter the runoff factor in per cent (%) for the area of tailings and ponds. This is usually

100% if it is assumed that all the precipitation falling on the tailings and ponds enters the

ponds and there is no transpiration. However, there are situations where this won’t be the

case such as in dry areas where precipitation merely re-wets a desiccated tailings beach

and evaporates before running into the pond. Either an average value or a variable

monthly distribution can be used. Evaporation from the wetted tailing surface and ponds

is accounted for separately in WATBAL.

Monthly Runoff:

In WATBAL runoff can be accumulated from month to month if all the precipitation that

falls in a given month dosen’t runoff in that month. For example, runoff doesn’t always

mirror precipitation, at least not in areas with winter snow. In WATBAL, the amount of

runoff can be controlled by inputting a percentage (%) into the monthly columns which

calculates the amount of runoff from the accumulated precipitation up to the end of the

month in question. For example, if zero percentage is inputted for January, February and

March and 100% is inputted in April then the total accumulation for the four months will

enter WATBAL as an inflow in April. Note that there is a difference between “Monthly

Runoff ” and “Runoff Factor ”. The “Monthly Runoff ” controls when runoff will occur

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but the “Runoff Factor ” controls the quantity. If all the precipitation runs off in the

month that it occurs for each month of the year, then 100% can be inputted into the

“Value” column.

Using Stream Flow or Specific Runoff Instead of Precipitation (stream flow version):

For the “Stream Flow Version” (Table 2), WATBAL operates exactly the same way

except that flow data are entered as m3/month in the “stream flow” row either in the

“VALUE ” column or as a distribution in the monthly columns. Changes in stream flows

can be evaluated by inputting a plus or minus % in the “VALUE ” column.

If specific runoff is used then nothing should be inputted into the “stream flow” row.

Specific runoff can be inputted into the “VALUE ” column or with a monthly distribution

in the monthly columns. Because specific runoff is normally quoted as an average annual

value, when it is inputted into the “VALUE” column the monthly variations in flow can

be evaluated by inputting a % of total flow in the monthly columns in the “monthly

distribution” row. Be careful that the percentages add up to 100%. The area of the

tailings basin watershed also has to be inputted in hectares when using specific runoff.

If data are entered into both the stream flow and specific runoff rows, the specific runoff

data will take precedent.

Water Displaced:

If pond water has to be displaced to make room for tailings then the volume of water

displaced is calculated by dividing the dry density of the deposited tailings into the dry

tonnage submerged. The dry tonnage submerged is inputted in the “Tailings Submerged”

row as a percentage (%) of the total tailings production which may be 100% or something

less. It can be inputted as an average value in the “VALUE” column or as variable

distributions in the monthly columns. The “Deposited Dry Density”, in tons/m3, can only

be inputted as a average value in the “ VALUE ” column.

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Water retained in Tailings:

Enter the percentage (%) of water retained in the tailings in the “VALUE” column. This

is the saturated water content of the tailings on a dry weight basis (wt. water/dry wt. of

tailings). If the tailings are partially drained this has to be reduced by estimation and if

slurry water is permanently trapped as ice in the tailings then the water content has to be

increased, which in effect reduces the dry unit weight of the tailings. Once again, either

an average value or a variable monthly distribution can be used.

Estimated Seepage Losses:

Enter the volume of water, in m3/month estimated to be lost from the tailings

impoundment as unrecoverable seepage in the “estimated seepage losses ” row. Either an

average value or a variable monthly distribution can be used.

Evaporation:

For each month, enter the estimated base case evaporation rates in mm/month in the

“Average Evaporation” row. Alternatively, an average value can be entered into the

“VALUE” column which will then apply that value evenly to each month.

When investigating the sensitivy of evaporation on the balance, the evaporation can be

increased or decreased by merely inputting the appropriate plus or minus percentage (%) in

the “VALUE” column opposite “Change in Evaporation”. This will then change the

computed evaporation using the same distribution as the base case. If no change is required

then enter 0%. Alternatively, individual percentages can be inputted into the monthly

columns in the “Change in Evaporation” row. In this way the distribution can be changed

from the base case.

Area of Ponds and Wetted Tailings Surface:

This is the area, in hectares, that the evaporation rates will be applied to. Enter the area in

hectares of the ponds and wetted tailings surface in the “VALUE” column. Alternatively,

this area can be inputted as a variable distribution in the monthly columns however, for

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most balances, it is unlikely that the area could vary enough to appreciably impact the

balance.

Recirculation to the Mill:

Enter the percentage (%) recirculation to the mill which is expressed as a percentage of the

tailings discharge water. Either an average value or a variable monthly distribution can be

used. In WATBAL there has to be tailings discharge to calculate recirculation. If a pond

does not receive tailings, and water is being recirculated from it, then the recirculation has

to be accounted for by subtracting the recirculation from the inflows which have to be

inputted from the pond above as a miscellaneous inflow.

Decant Strategy:

The decant strategy controls the pond’s discharge to the environment or to a treatment

plant. For each month, when there is discharge, enter a percentage (%) of the total annual

net inflow. This represents the volume of water which will be discharged to the

environment each month. If the total net annual inflow has to be discharged to keep the

pond in balance then the monthly percentages must add up to 100%.

Initial Water Volume in Ponds:

The initial volume of water in the pond is the volume preceding the starting month.The

pond can never have a negative value therefore the initial volume must be chosen to

ensure that this does not happen.

10.0 SOME CONSIDERATIONS FOR WATER BALANCE MODELLING IN EXTREMELY DRY CLIMATES

In extremely dry climates, such as the Coastal Plain of Peru, water losses in a tailings basin may

be very high. In such cases, every possible drop of water has to be conserved, collected and

reused in the mill unless there is an inexhaustible supply of groundwater.

In dry climates, seepage and evaporation usually have a significant impact on a water balance;

however in wet environments, they are relatively small as compared to the inflows. Also in dry

climates, seasonal variatons may have little impact on a water balance therefore modelling on a

monthly basis may not be necessary. On the other hand, the changing shape of a basin can have

a large impact on a balance because as a basin increases in size, larger wetted areas become

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available for evaporation and infiltration. These conditions can usually be adequately modelled

on an annual basis and in fact annual balances are normally developed for each year over the

entire life of a tailings basin in a dry climate. In dry climates, where losses are extremely large,

flows can be conveniently expressed in cubic metres of water lost per ton of tailings produced or

ton of ore milled (m3/ton of tailings).

Frequenty in dry climates the tailings have a high clay content. If this is the case, then the density

at initial liquid/solids separation may be quite low and a long time may be required to complete

consolidation under the weight of self loading. The consolidation water released in reaching the

ultimate density can be relatively large as illustrated in the following example.

Discharge slurry density (solids by wt.) 48% Specific gravity of tailings solids 2.7 Volume of slurry water (1/0.48 - 1) 1.08 m3/t of tails (100%) Initial liquid/solids separation Dry density (assume) 1.05 t/m3 Pore water retained (saturated) (1/1.05 - 1/2.70) 0.58 m3/t of tails (54%) After consolidation Dry density (assume) 1.35 t/m3 Pore water retained (saturated) (1/1.35 - 1/2.70) 0.37 m3/t of tails (34%) Water released by consolidation (0.58 - 0.37) 0.21 m3/t of tails (19%)

In the above simple example, the water released by consolidation is 19% of the total discharge

water. This is a significant quantity. It may be possible to collect, a small portion of this water

but this is difficult to predict with any degree of confidence. Consolidation water that rises to the

surface on an inactive beach may be lost to evaporation and that which goes vertically

downwards may be lost as seepage. Consolidation water that migrates to dams and dykes can

possibly be collected. Given the uncertainty, the water retained at the initial deposited density is

probably a reasonable estimate of the water that will be lost.

Competing against the consolidation process is water permanently lost due to evaporation and

desiccation on inactive beaches. Desiccation has the same effect as consolidation except that

water is removed by evaporation instead of being squeezed out by pressure. This can be

modelled in a water balance by simply using a lower density to calculate the water retained in the

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pore spaces. The model understands that this is water that is permanently lost; whether it be due

to evaporation or permanently retained in the pore spaces doesn’t really matter to the balance.

Seepage can also be difficult to handle in a water balance in a dry climate because it is usually

large and unpredictable. It can consist of several components:

• Saturation of unsaturated soils beneath a basin.

• Seepage from ponds in contact with natural soils, the area of which can change drammatically as a basin develops.

• Seepage from the ponds on top of the tailings. Depending on the hydraulic conductivity of the tailings this could be quite small.

• Pore water squeezed vertically downwards by consolidation may be lost as seepage.

• Water may be permanently lost in saturating desiccated inactive beaches when they are rewetted.

Depending on the subsurface conditions, seepage might be collected in wells, underdrains and

from pervious dykes and dams.

In summary, it is more difficult to realistically model a tailings basin water balance in an

extremely dry climate than in a wet climate. A monthly balance is not normally required but

annual balances should be carried out for each year of the entire life of the basin to evaluate the

impact of changing basin configuration with time. Normally it is necessary to develop a stand

alone water balance format for each case because a generic format doesn’t cover site specific

situations. Depending on the complexity of the balance, the use of several interconnected

spreadsheets may be appropriate. However, the principles used in WATBAL are the same.

11.0 DISCUSSION AND CONCLUSIONS

WATBAL is merely a mathematical tool which adds and subtracts inflows and losses to a system.

No special skills are required to operate the program, but sound engineering judgement is needed

to ensure that the input data are appropriate for the intended application. Also, the advice of a

qualified hydrologist or hydrotechnical engineer should be sought to advise on precipitation,

evaporation, runoff factors and the routing of storm flows. The results are only as good as the

numbers that are put into it. WATBAL has proven to be a reliable tool for predicting pond

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Golder Associates

volumes and effluent flows from tailings basins (Welch, et al 1995). It is easy to use and more

importantly it requires very little computer capacity. The input data are entered into the program

in easily recognizable terms which facilitates changes and enables sensitivity analyses to be

carried out easily and quickly.

An important conclusion, which can be reached from the hypothetical example analyzed in this

document, is that a tailings basin must be able to operate over a wide range of operating and

hydrological conditions. There is no such thing as an unique water balance for a tailings basin.

As is demonstrated in this document, the net annual inflow for a reasonable set of worst case

parameters can be easily three times greater than the average base case flow.

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Golder Associates

REFERENCES

Welch, D.E. and Firlotte, F.W. 1989. “Tailings Management in the Gold Mining Industry”. Proceedings Vol. 14, International Symposium on Tailings Effluent Management. Halifax, Aug. 1989. The Metallurgical Society of the Canadian Institute of Mining and Metallurgy.

Welch, D.E., Botham, L.C., and Bronkhorst, D. 1992. “Tailings Basin Water Management, A Simple Effective Water Balance”. Environmental Management for Mining, 1992 Saskatchewan Conference, Saskatchewan Mining Association.

Welch, D.E., Botham, L.C., Johnson, J.M. 1995. “Prediction of Tailings Effluent Flows” Tailings and Mine Waste 95 and Summitville Forum Conference. Colorado State University, Fort Collins, Colorado, USA. January 17-20, 1995.

Canadian Climate Normals, 1961-1990. Environment Canada, Atmospheric Environment Service.

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Golder Associates

COMMONLY USED SYMBOLS AND ABBREVIATIONS FACTORS TIME

G giga (billion - 109 ) sec or s second (basic unit)

M mega (million - 106 ) min minute

k kilo (thousand - 103 ) hr or h hour

c centi (hundredth - 10-2 ) yr year

m milli (thousandth - 10-3 )

µ micro (millionth - 10-6 ) AREA ha hectare (10,000 m2 ) LENGTH km2 square kilometre (1,000,000 m2 )

m metre (basic unit) m2 square metre

km kilometre (1,000 m) cm2 square centimetre

cm centimetre (1/100 m) SOIL (TAILINGS) PROPERTIES

mm millimetre (1/1,000 m) e void ratio (vol. voids/vol.solids)

µm or µ micro (1/1,000,000 m) n porosity (vol. voids/total vol.) w water content by mass (mass water/mass solids)(N.2) VOLUME

wt water content by mass (mass water/total mass) (N. 2)

V volume (Vv - voids, Vs - solids, Vw - water, wv water content by volume (vol. water/total vol.)

and Vt - total) S or Cw slurry density (mass solids/total mass) (Note 3) L litre (1000 cm3 ) Cv solids content by vol. (vol.solids/total vol.) (Note 3)

m3 cubic metre s degree of saturation (vol.water/vol.voids)

cm3 cubic centimetre ρ density (mass per unit volume) (Note 4)

gal gallon (US or imperial as stated) ρs density of solid particles (mass solids/vol.solids)

MASS (Note 1) ρd dry density (mass solids/total vol.)

W mass (Ws - solid particles, Ww - water and ρw density of water (mass water/vol.water) (Note 5) Wt - total) ρt total or bulk density (total mass/total vol.)

g or gm kg

gram (“g” also used for acceleration due to gravity) kilogram (1000 g) (basic unit)

ρ ′

Gs

bouyant density (ρt (sat.) - ρw )

specific gravity ( ρs / ρw )

t ton (1000 kg) (metric unless otherwise stated) σ pressure or stress

NOTES: 1. “Mass” and “Weight” are often incorrectly interchanged. Mass (or inertia) is a constant of an object irregardless

of where it is in the universe. It is a measure of the amount of matter that an object contains and it controls the response of an object to an applied force. Weight is the gravitational force that causes a downward acceleration. This is Newton’s second law (F=Ma) in which W(weight) = M(mass) x g(acceleration due to gravity).

2. In soil mechanics ‘w’ is % of dry solids mass and in process engineering ‘wt’ is the % of total mass. 3. In pumping terminology the symbol for slurry density is ‘Cw’ and solids content by volume is ‘Cv’. 4. “Unit Weight” is often incorrectly used instead of “density”. An older symbol for density (in imperial units) was

‘γ’ which is now reserved for unit weight. 5. The density of water (ρw) in the metric system is unity, therefore ‘Gs’ and ‘ρs’ have the same value.

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Dam crest

Freeboard

Storage capacity for the environmental design flood (EDF)

Emergency spillway

Maximum operating pond level

Figure 1 Management of Storm Flows

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2

January, 2002 011-1820

WATBAL PRINTOUT - STREAM FLOW VERSIONExample Tailings Basin Water Balance

Figure 3INFLOWS, LOSSES AND NET INFLOW

Figure 4DECANT AND POND VOLUME

Golder Associates

-100000

0

100000

200000

300000

400000

500000

600000

Dic Ene Feb Mar Abr May Jun Jul Ago Sep Oct Nov

Month

Volu

me

(m3)

Inflows Losses Net Inflow

0

100000

200000

300000

400000

500000

600000

700000

800000

Dic Ene Feb Mar Abr May Jun Jul Ago Sep Oct Nov

Month

Volu

me

(m3 /m

onth

)

Accumulated Pond Volume Decant

Watbal Manual-Table 2 & Fig 3-4.xls-Plots

April 2000 000-0000TABLE 1

WATBAL PRINTOUT - PRECIPITATION VERSIONExample - Dry Northern Climate

TABLE 1AINPUT DATA

Precipitation VersionUNITS VALUE Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

Starting month no. 1

Tailings production t/day 1935 706275

Solids (by weight) in discharge % 45

Miscellaneous inflows m3/mo. 0 0

Average precipitation mm/mo. 17 15 17 12 17 21 42 49 29 32 25 24 300.0

Change in precipitation % 0

Total precipitation mm/mo. 17 15 17 12 17 21 42 49 29 32 25 24 300.0

Area of natural land in basin ha 350

Runoff factor % 70

Area of tailings and ponds ha 170

Runoff factor % 100

Monthly runoff (% of accumulation) % 0 0 0 0 100 100 100 100 100 100 100 0

IF TAILS DISPLACE POND

Tailings submerged (% of total) %

Deposited dry density t/m3 1.35

Water retained in tailings (dry wt basis) % 37

Estimated seepage losses m3/mo. 2000 24000

Average Evaporation mm/mo. 0 0 0 0 0 160 154 107 49 0 0 0 470

Change in evaporation % 0

Total evaporation mm/mo. 0 0 0 0 0 160 154 107 49 0 0 0 470

Concentrate production t/day 60 21900

Concentrate slurry density (by wt.) % 40

Area of ponds and wetted tailings ha 50

Recirculation to mill (% of process water) % 90

Decant strategy (% of net inflow) % / mo. 0 0 0 0 0 16.67 16.67 16.67 16.67 16.67 16.65 100

Initial water volume in ponds m3 400000

TABLE 1B

OUTPUT COMPUTATIONS

INFLOWS LOSSES ACCUMULATION

(m3/mo.) (m3/mo.) (m3/mo.) (m3)

Total (includes

concent. slurry)

Concent. Slurry

1 2 3 4 5 6 7 8 9 9a 10 11 12 13 14 15 16

INITIAL 400000

Ene 73315 0 0 73315 22194 2000 0 65984 2790 90178 -16863 0 -16863 0 -16863 383137

Feb 66220 0 0 66220 20047 2000 0 59598 2520 81645 -15425 0 -15425 0 -15425 367712

Mar 73315 0 0 73315 22194 2000 0 65984 2790 90178 -16863 0 -16863 0 -16863 350850

Abr 70950 0 0 70950 21479 2000 0 63855 2700 87334 -16384 0 -16384 0 -16384 334466

May 73315 0 423300 496615 22194 2000 0 65984 2790 90178 406437 0 406437 0 406437 740903

Jun 70950 0 87150 158100 21479 2000 80000 63855 2700 167334 -9234 0 -9234 135194 -144427 596476

Jul 73315 0 174300 247615 22194 2000 77000 65984 2790 167178 80437 0 80437 135194 -54757 541719

Ago 73315 0 203350 276665 22194 2000 53500 65984 2790 143678 132987 0 132987 135194 -2207 539512

Sep 70950 0 120350 191300 21479 2000 24500 63855 2700 111834 79467 0 79467 135194 -55727 483785

Oct 73315 0 132800 206115 22194 2000 0 65984 2790 90178 115937 0 115937 135194 -19257 464528

Nov 70950 0 103750 174700 21479 2000 0 63855 2700 87334 87367 0 87367 135032 -47665 416863

Dic 73315 0 0 73315 22194 2000 0 65984 2790 90178 -16863 0 -16863 0 -16863 400000

TOTAL 863225 0 1245000 2108225 261322 24000 235000 776903 32850 1297224 811001 0 811001 811001 0

Golder Associates

Decant Net Change

Accum. VolumeTotal Net Inflow Water

Displaced ChangeRetained in Tailings Seepage Pond

Evap.

Recycle

Tailings Water

Misc. Inflows

Tailings Basin Runoff

Total

PRO

CES

S W

ATE

RR

UN

OFF

DIS

PLA

CED

LOSS

ESD

ECA

NT

Watbal Manual-Table 1.xls-Watbal

January, 2002 011-1820TABLE 2

WATBAL PRINTOUT - STREAM FLOW VERSIONExample Tailings Basin Water Balance

TABLE 2AINPUT DATA

Stream Flow VersionUNITS VALUE Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

Starting month no. 12

Tailings production t/day 1995 728175

Solids (by weight) in discharge % 45

Miscellaneous inflows m3/mo. 0 0

Stream flow m3/mo 0 0 0 0 423300 87150 174300 203350 120350 132800 103750 0 1245000

Change in stream flow %

Total stream flow m3/mo 0 0 0 0 423300 87150 174300 203350 120350 132800 103750 0 1245000

OR

Specific runoff L/s/km2

Change in specific runoff %

Total specific runoff L/s/km2

Area of basin watershed ha 520

Monthly distribution (% of total) % 100

IF TAILS DISPLACES POND

Tailings submerged (% of total) % 0

Deposited dry density t/m3 1.35

Water retained in tailings (dry wt basis) % 37

Estimated seepage losses m3/mo. 2000

Average Evaporation mm/mo. 0 0 0 0 0 160 154 107 49 0 0 0 470

Change in evaporation % 0

Total evaporation mm/mo. 0 0 0 0 0 160 154 107 49 0 0 0 470

Area of ponds and wetted tailings ha 50

Recirculation to mill (% of process water) % 90

Decant strategy (% of net inflow) % / mo. 0 0 0 0 0 16.7 16.7 16.7 16.7 16.6 16.6 0 100

Initial water volume in ponds m3 400000

TABLE 2B

OUTPUT COMPUTATIONS

INFLOWS LOSSES ACCUMULATION

(m3/mo.) (m3/mo.) (m3/mo.) (m3)

Tailings Water

Misc. Inflows Runoff Total Retained

in Tailings Seepage Pond Evap.

Recirc-ulation Total Net Inflow Water

Displaced Change Decant Net Change

Accum. Volume

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

INITIAL 400000

Dic 75588 0 0 75588 22883 2000 0 68030 92912 -17324 0 -17324 0 -17324 382676

Ene 75588 0 0 75588 22883 2000 0 68030 92912 -17324 0 -17324 0 -17324 365352

Feb 68273 0 0 68273 20668 2000 0 61446 84114 -15841 0 -15841 0 -15841 349512

Mar 75588 0 0 75588 22883 2000 0 68030 92912 -17324 0 -17324 0 -17324 332188

Abr 73150 0 0 73150 22145 2000 0 65835 89980 -16830 0 -16830 0 -16830 315358

May 75588 0 423300 498888 22883 2000 0 68030 92912 405976 0 405976 0 405976 721334

Jun 73150 0 87150 160300 22145 2000 80000 65835 169980 -9680 0 -9680 134531 -144210 577124

Jul 75588 0 174300 249888 22883 2000 77000 68030 169912 79976 0 79976 134531 -54555 522569

Ago 75588 0 203350 278938 22883 2000 53500 68030 146412 132526 0 132526 134531 -2005 520564

Sep 73150 0 120350 193500 22145 2000 24500 65835 114480 79021 0 79021 134531 -55510 465054

Oct 75588 0 132800 208388 22883 2000 0 68030 92912 115476 0 115476 133725 -18249 446805

Nov 73150 0 103750 176900 22145 2000 0 65835 89980 86921 0 86921 133725 -46805 400000

TOTAL 889992 0 1245000 2134992 269425 24000 235000 800993 1329417 805574 0 805574 805574 0

Golder Associates

PRO

CES

S W

ATE

RR

UN

OFF

DIS

PLA

CED

LOSS

ESD

ECA

NT

Watbal Manual-Table 2 & Fig 3-4.xls-Watbal

March, 2002 TABLE 3 RESULTS OF WATER BALANCE SENSITIVITY ANALYSES

VARIATION IN NET ANNUAL INFLOW

(Column 11)

PARAMETER RANGE M m3/yr. % Change from base case

Precipitation

*

+50% +25% +10%

Average -10% -25% -50%

1.43 1.12 0.93 0.81 0.68 0.49 0.18

*

+77 +38 +15

0 -16 -40 -78

Slurry Density (% Solids)

*

20 30 45 60

1.01 0.89 0.81 0.77

*

+25 +10

0 -5

Recirculation (%)

*

0 30 60 90 100

1.61 1.34 1.07 0.81 0.72

*

+99 +65 +32

0 -11

Mine water (m3/month) * 0 15,000 30,000 45,000 75,000 90,000

0.81 0.99 1.17 1.35 1.71 1.89

* 0 +22 +44 +67 +111 +133

Notes: 1. *denotes base case condition 2. The sensitivity analyses were carried out by varying the parameter being investigated and keeping all the other parameters constant at the base case.

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March, 2002 TABLE 4 BEST AND WORST CASE WATER BALANCE SCENARIOS

ANNUAL FLOWS

Reasonable Best Case Scenario

(M m3/yr.)

Average Base Case

(M m3/yr.)

Reasonable Worst Case

Scenario (M m3/yr.)

INFLOWS

Tailings Water 0.49 0.89 2.18

Mine Water 0 0 0.36

Runoff 0.93

1.25

1.56

TOTAL 1.42 2.14 4.10

LOSSES

Retained in tails 0.27 0.27 0.27

Seepage 0.02 0.02 0.02

Evaporation 0.24 0.24 0.24

Recirculation 0.44

0.80

0.66

TOTAL 1.97 1.33 1.19

NET ANNUAL INFLOW 0.45 0.81 2.91

MAX. POND VOLUME (M m3) 0.30 0.72 1.73

INPUT PARAMETERS

Tailings Production (t/day) 1995 1995 1995

Slurry density (% solids) 60 45 25

Mine water (m3/month) 0 0 30,000

Runoff (%) -25% avg. +25%

Recirculation (%) 100 90 30

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