8543-Volume 2-1.4 - Groundwater Numeric Modelling

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Golde r Assoc iates Africa (Pty) LtdReg . No. 2002/007104/07

JOHANNESBURGPO Box 6001 Halfwa y House 1685Sou th AfricaTha nd a na ni Pa rk, Ma tuka C loseHalfway Ga rd ens, MidrandTel + (27) (0)11 254-4800Fa x + (27) (0)11 315-0317http://www.golder.com

Direc tors : P Onley (Au stralia), FR Suthe rlan d, AM va n Niekerk, JA Wate s

OFFICESIN JOHA NNESBURG, DURBAN, A USTRALIA, INDONESIA, NEW ZEALAND , PEOPLESREPUBLIC OF C HINA, PHILIPPINES, SINGAPORE

OFFICESAC ROSSAFRICA, ASIA, AUSTRALASIA, EUROPE, NO RTH AM ERICA , SOUTH AM ERICA

REPORT ON

HYDROGEOLOGICAL INVESTIGATION FOR

THE PROPOSED HEIDELBERG OPENCAST

MINE

NUMERIC MODELLING OF PIT INFLOWSAND GROUNDWATER IMPACTS

Report No : 7475/8297/7/G

Submitted to:

Anglo Coal Project Services

Private Bag X9

Leraatsfontein

DISTRIBUTION:

2 Copies - Anglo Coal Project Services1 Copy - Golder Associates Africa (Pty) Ltd Library

April 2006 7475
http://www.golder.com/http://www.golder.com/http://www.golder.com/


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EXECUTIVE SUMMARY

Anglo Coal Projects Services (ACPS) initiated the feasibility of developing the Heidelberg opencast

mine with an operational life of approximately twelve years for long-term coal supply to Eskoms

Grootvlei power station, which is to be re-commissioned.

Golder Associates Africa (GAA) was appointed to determine the possible impacts that the proposed

opencast mine will have on the groundwater regime in the area. A key aspect of the Heidelberg in-pit

water balance is the determination of groundwater inflows into the opencast workings during

operation and post mine closure.

Golder conducted a groundwater study to develop an understanding of the existing groundwater

environment and to evaluate the changes to this environment, as a result of pit impacts.

The main objectives of the groundwater modelling study are to:

Construct and calibrate a numerical groundwater flow model, to accurately simulate groundwater

flow through the aquifers

Simulate inflow rates into the mining sections using the preliminary mine plans in conjunction

with the calibrated groundwater flow model

Determine the necessity of dedicated dewatering well fields around mining sections

Simulate the potential influence of the Suikerbosrand River on mining

Simulate the extent of the influence of mining operations on the hydrogeological regime in the

study area.

Determine the influence of mining on community boreholes.

Simulate the effect of mine closure on groundwater levels and recommend an operational pit

groundwater/surface water level elevation to prevent decant of groundwater / surface water intothe Suikerbosrand River.

Identify the need for any additional hydrogeological field investigations in order to reduce

uncertainties identified from the hydrogeological flow modelling investigation conducted.

The results of the investigation are presented in this report and incorporate results of the

hydrogeological and geochemical study into the groundwater model.


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A numerical groundwater model using the FeFlow code was constructed for the proposed Heidelberg

opencast pit. The conceptual model, model assumptions, construction, and results are presented in

Sections 12 - 14 of the report.

The key findings of the study can be summarised under:

Alternative A: Worst Case Scenario

The average influx of groundwater during operation will approximate 1 820 m3 /d, reduced to

1 160 m3/d (36 % reduction on average) with active dewatering of nine (9) dewatering wells and

825 m3/d (55 % reduction on average) with active dewatering of seventeen (17) dewatering wells

(see Figure S1 below).

The natural filling of the pit, to an elevation of 1476 mamsl (1 m below the base of the

Suikerbosrand River) will take approximately 1 980 days or more than 5 years after closure. The

average flow rate will be at 841 m3/d, reducing over time as driven by the rising head conditions

within the pit, to average flows of 452 m3 /d. If the opencast pit is allowed to fill above the

recommended waterlevel of 1476 mamsl, outflow of contaminated water to the Suikerbosrand

River could approximate > 280 m3/d.

There will be an increase in flows through the alluvium to the pit, as the pit development

approaches the Suikerbosrand River. Average flows will approximate 171 m3 /d without

dewatering and roughly contribute 9 % of groundwater inflow to the pit. Active dewatering will

increase the flow through the alluvium to an average of 214 m3/d and 265 m

3/d, for the 9 and 17

dewatering well scenarios respectively (Scenario 2).

Operation of the opencast pit will lead to the development of a dewatered cone of depression

extending to the Suikerbosrand River in the East, 1 400 m North, 615 m to the South and 410 m to

the West, with no active dewatering (Scenario 1). The footprint area will be influenced by the

nine (9) dewatering well field, operating six (6) months prior to pit development for Scenario 2.

This will result in a more extensive area of influence that could approximate 1 470m to the North,

720 m to the South and 570 m to the West.

The extent of dewatering by increasing the number of dewatering wells to seventeen (17), indicate

a cone of depression developing to 1 500 m North, 800 m South, 700 m West and to theSuikerbosrand River on the East. Operation of a 17 dewatering well field will ensure a more

effective barrier to groundwater inflow into the opencast pit.


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Figure S1: Alternative A Pit Inflows

Alternative B: Best Case Scenario

The average influx of groundwater during operation of the Heidelberg opencast pit will

approximate 1 060 m3/d, reduced to 272 m

3/d (74 % reduction on average) with active dewatering

of nine (9) dewatering wells and 37 m3/d (97 % reduction on average) with active dewatering of

seventeen (17) dewatering wells (see Figure S2 below).

The natural filling of the pit, to an elevation of 1476 mamsl will take approximately 3 960 days,

almost 11 years after closure. The average flow rate will be at 617 m3 /d, reducing over time to

average flows of 442 m3 /d. If the opencast pit is allowed to fill above the recommended water

level of 1476 mamsl, outflow of contaminated water to the Suikerbosrand River could

approximate > 280 m3/d.

The average alluvial flow will approximate 76 m3 /d without dewatering and roughly contribute

7 % of groundwater inflow to the pit. Active dewatering will increase the flow through the

alluvium to an average of 98 m3 /d and 121 m

3 /d, for the 9 and 17 dewatering well scenarios

respectively (Scenario 2).

0

500

1000

1500

2000

2500

3000

3500

0 365 730 1095 1460 1825 2190 2555 2920 3285 3650 4015 4380

Time (d)

Inflow

(m3/d) Scenario 1

Scenario 2_9 Wells

Scenario 2_17 Wells

Pump_9 Well

Pump_17 Well


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Operation of the opencast pit will lead to a similar development of a dewatered cone as for

Alternative A, but extending to 710 m South and 860 m to the East of the opencast footprint.

Scenario 2 indicates an approximate area of influence extending to 860 m South and 960 m east.

By characterising the Suikerbosrand River as a gaining stream only, the developed dewateringcone will extend up to 980 m South and 1 125 m East, with the increase of dewatering wells to

seventeen (17).

There is no indication that the development of the dewatering cone, during operation of the

Heidelberg opencast pit, will influence current water supply boreholes, under hydraulic continuity

or no hydraulic continuity conditions (Alternative A or B).

Water quality from the deeper semi-confined aquifer is of a good quality. Most of the pumped water

for mine dewatering purposes during the operational phase will be derived from this aquifer. It can

therefore be safely assumed that the pumped water can be discharged into the Suikerbosrand Riverwithout any detrimental effect to the environment. In-pit management practices during the operational

phase should be such that no potential contaminated surface water be allowed the time to infiltrate

into the groundwater system.

Figure S2: Alternative B - Pit Inflows

0

500

1000

1500

2000

2500

3000

3500

0 365 730 1095 1460 1825 2190 2555 2920 3285 3650 4015 4380

Time (d)

Inflow(

m3/d) Scenario 1

Scenario 2_9 Wells

Scenario 2_17 Wells

Pump_9 Wells

Pump_17 Wells


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Key uncertainties in the study are:

The extent of influence from the peat area (south-west of the proposed Heidelberg opencast pit) to

flows in the groundwater environment.

The hydrogeological properties of the Suikerbosrand riverbed.

The study aimed at developing a first order understanding of the groundwater impacts associated with

the Heidelberg opencast pit operation. To narrow down the envelope of possibility, the numeric model

results should be re-evaluated, once additional information is available from construction of the

dewatering well field and extension of the hydrocensus.


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TABLE OF CONTENTS

SECTION PAGE

1 INTRODUCTION.................................................................................... 12 OBJECTIVES......................................................................................... 13 METHODOLOGY................................................................................... 24 LOCATION............................................................................................. 25 TOPOGRAPHY...................................................................................... 26 DRAINAGE ............................................................................................ 47 RAINFALL ACROSS STUDY AREA....................................................... 48 EVAPORATION ..................................................................................... 49 GEOLOGY............................................................................................. 5

9.1 Regional Geology.............................................................................. 59.2 Site Specific Geology ........................................................................ 5

10 HYDROGEOLOGY ................................................................................ 710.1 Aquifers............................................................................................. 710.2 Groundwater Flow Directions and Gradients ..................................... 810.3 Water level depth below surface and blow yields............................... 810.4 Aquifer test analyses and Hydraulic Parameters................................ 8

11 GROUNDWATER FLOW MODEL DEVELOPMENT.............................. 812 CONCEPTUAL MODEL....................................................................... 10

12.1 Modelling assumptions .................................................................... 1012.2 Software Selection (Modelling Code)............................................... 11

13 CONSTRUCTION OF THE NUMERIC FLOW MODEL ........................ 1113.1 Finite Element Grid.......................................................................... 11

13.2 Model Boundaries............................................................................ 1313.2.1 Model Boundary Conditions............................................... 14

13.3 Calibration ....................................................................................... 1414 UTILIZING THE GROUNDWATER FLOW MODEL TO ADDRESS

THE STUDY OBJECTIVES.................................................................. 1814.1 Introduction...................................................................................... 1814.2 Operational Mining Phase ............................................................... 18

14.2.1 Mining method ...................................................................1814.3 Modelled Scenarios......................................................................... 2014.4 Scenario Discussion ........................................................................ 21

14.4.1 Operational Phase............................................................. 2114.4.2 Developing Cone of Depression......................................... 24

14.4.3 Closure Phase...................................................................3114.4.4 Contribution from Alluvium................................................. 34

14.5 Groundwater Quality........................................................................ 3414.5.1 Local Background Groundwater Quality

Concentrations................................................................... 3415 CONCLUSIONS AND RECOMMENDATIONS..................................... 3916 REFERENCES..................................................................................... 44


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LIST OF FIGURES

Figure 1: General Geographical Reference Map .................................................................................3

Figure 2: Monthly Rainfall Record for Heidelberg (1901 2000)....................................................... 4

Figure 3: Monthly Evaporation Record for Heidelberg (1901 2000) ................................................ 5

Figure 4: General Geological Reference Map.....................................................................................6

Figure 5: Schematic Representation of the Heidelberg Environment................................................... 7

Figure 6: Regional Piezometric Water levels and Flow Directions...................................................... 9

Figure 7: Model Matrix (Finite Element Grid).................................................................................. 12

Figure 8: Three Dimensional Model Domain ................................................................................... 13

Figure 9: Calibrated Water levels.....................................................................................................15

Figure 10: Geological Representation of Modelled Area and Water Levels ...................................... 15

Figure 11: Frequency Distribution of Conductivities for Shallow Aquifer system............................. 16

Figure 12: Frequency Distribution of Conductivities for Deep Aquifer system ................................. 17

Figure 13: Transient Calibration of Borehole D................................................................................18

Figure 14: Proposed Heidelberg Opencast Pit Development & Dewatering Well Field ..................... 19

Figure 15: Pit Inflows and Pump Rates for Alternative A .................................................................22

Figure 16: Pit Inflows and Pump Rates for Alternative B .................................................................23

Figure 17: Cone of Depression Scenario 1 (Alternative A) ............................................................25

Figure 18: Cone of Depression Scenario 1 (Alternative B) ............................................................26

Figure 19: Cone of Depression Scenario 2 (Alternative A) [9 Well Dewatering]............................ 27

Figure 20: Cone of Depression Scenario 2 (Alternative B) [9 Well Dewatering] ............................ 28

Figure 21: Cone of Depression Scenario 2 (Alternative A) [17 Well Dewatering].......................... 29


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Figure 22: Cone of Depression Scenario 2 (Alternative B) [17 Well Dewatering] .......................... 30

Figure 23: Head in Pit vs. Fill Rate for Alternative A....................................................................... 32

Figure 24: Head in Pit vs. Fill Rate for Alternative B ....................................................................... 33

Figure 25: Alluvial Inflows for Alternative A...................................................................................35

Figure 26: Alluvial Inflows for Alternative B ................................................................................... 36

Figure 27: Shallow Aquifer Chemistry.............................................................................................38

Figure 28: Deep Aquifer Chemistry .................................................................................................38

Figure 29: Groundwater Flow Lines at 1476 mamsl ......................................................................... 39

LIST OF TABLES

Table 1: Calibrated Hydrogeological Parameters..............................................................................16

Table 2: Modelled Operational Timeframe.......................................................................................20

Table 3: Pit Water Elevation & Inflow Rates at Closure................................................................... 31

Table 4: Background Groundwater Qualities.................................................................................... 37

Table 5: Comparative Scenario Results for Pit Operation.................................................................43

LIST OF APPENDICES

Appendix A Groundwater Figures


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1 INTRODUCTION

Anglo Coal Projects Services (ACPS) initiated the feasibility of developing the Heidelberg opencast

mine with an operational life of approximately twelve years for long-term coal supply to Eskoms

Grootvlei power station, which is to be re-commissioned.

Golder Associates Africa (GAA) was appointed to determine the possible impacts that the proposed

opencast mine will have on the groundwater regime in the area. A key aspect of the Heidelberg in-pit

water balance is the determination of groundwater inflows into the opencast workings during

operation and post mine closure.

To characterise the prevailing hydrogeological conditions in the area of the proposed opencast mine,

Golder launched a hydrogeological field investigation to characterise the aquifer(s) within which the

proposed opencast mine will function. A conceptual hydrogeological model was subsequently

developed for the study area based on results forthcoming from this field investigation (GAA Report

No: 7475/8235/4/G, March 2006).

GAA was appointed to construct and calibrate a numerical flow model based on the developed

conceptual model (GAA Report No: 7475/8235/4/G, March 2006), to determine the impacts that the

proposed mine will have on the groundwater regime of the area. The model is used to simulate the

groundwater flow through the groundwater system.

This report describes the modelling process and modelled impacts of the opencast mine on the

groundwater regime in the area investigated.

2 OBJECTIVES

The main objectives of the groundwater modelling study are to:

Construct and calibrate a numerical groundwater flow model, to accurately simulate groundwater

flow through the aquifers

Simulate inflow rates into the mining sections using the preliminary mine plans in conjunction

with the calibrated groundwater flow model

Determine the necessity of dedicated dewatering wellfields around mining sections

Simulate the potential influence of the Suikerbosrand River on mining

Simulate the extent of the influence of mining operations on the hydrogeological regime in the

study area.

Determine the influence of mining on community boreholes.

Simulate the effect of mine closure on groundwater levels and recommend an operational pit

groundwater/surface water level elevation to prevent decant of groundwater / surface water into

the Suikerbosrand River.


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Identify the need for any additional hydrogeological field investigations in order to reduce

uncertainties identified from the hydrogeological flow modelling investigation conducted.

3 METHODOLOGY

The approach followed in this groundwater modelling study was to focus only on the proposed

Heidelberg opencast pit and its immediate environment, based on available data.

The study was therefore aimed at creating a first-order understanding of groundwater impacts through

numeric modelling and to develop knowledge gaps and recommendations for refinement of the model

in the future.

In order to achieve the objectives of the investigation the following methodology was adopted:

A desk study was conducted on information made available by Anglo Coal and the previous work

undertaken (WMB Report No: 3758/1713/1/W, March 1999)

Development of a conceptual model for the Heidelberg opencast pit, based on the desk study and

information available from Golder (GAA Report No: 7475/8235/4/G, March 2006)

Selection of the most appropriate modelling code

Model construction and set-up

Calibration of the model for steady state groundwater flow conditions, based on average water

levels measured during the hydrocensus conducted in 2005 across the study area

Calibration of the model for transient state groundwater flow conditions based on historic aquifer

test data collected from deep boreholes (WMB Report No: 3758/1713/1/W, March 1999)

Conduct model simulations in order to evaluate the groundwater impact associated with the

proposed development of the Heidelberg opencast pit over time

4 LOCATION

The proposed opencast is to be located on portion 6 of the farm Elandsfontein 412IR approximately

12 km south-east of Heidelberg Gauteng province in South Africa. The mine will be bound to the east

by the Suikerbosrand River and to the south by the R549, Heidelberg to Deneysville road (Figure 1).

Also indicated on Figure 1, is the location of the boreholes identified during the hydrocensus

conducted by Golder (2005), as a general geographical reference.

5 TOPOGRAPHY

The studied area falls within the South Rand Coal field comprising Karoo sediments. The area is

surrounded by hills of the Witwatersrand Supergroup and the Ventersdorp Group.


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Figure 1: General Geographical Reference Map


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The Suikerbosrand River rises in the eastern portion of the Vaal River catchment near Devon and

Leandra, from where it flows in a south-westerly direction, before turning to the north-west upstream

of its confluence with the Blesbokspruit (GAA Report No: 7475/8290/6/W, April 2006).

6 DRAINAGE

The natural groundwater level is mostly controlled by the local topography, with groundwater

draining towards surface stream channels. Below its confluence with the Blesbokspruit, the

Suikerbosrand River flows in a south-westerly direction for about half its length before turning to a

westerly course until its confluence with the Vaal River at Vereeniging. This portion of the catchment

is also largely undeveloped and dominated by grassland and dry land agriculture (GAA Report No:

7475/8290/6/W, April 2006).

7 RAINFALL ACROSS STUDY AREA

The rainfall record for Heidelberg (1901 2000) is depicted on a monthly basis in Figure 2. The

average mean annual precipitation (MAP) for Heidelberg is 687 mm/a. Rainfall is strongly seasonal

with most rain occurring in the summer period (October to April) The peak rainfall months are

December and January (GAA Report No: 7475/8290/6/W, April 2006).

Figure 2: Monthly Rainfall Record for Heidelberg (1901 2000)

8 EVAPORATION

The mean annual evaporation (MAE) of Heidelberg equals 1625 mm/a. The evaporation record is

depicted on a monthly basis in Figure 3. The highest Class A-pan monthly evaporation is in January

(range 180 mm to 260 mm) and the lowest evaporation is in June (80 mm to 110 mm) (GAA ReportNo: 7475/8290/6/W, April 2006).

0

50

100

150

200

250

300

350

400

450

500

1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Date (year)

Rain(mm/m)

MMP


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Figure 3: Monthly Evaporation Record for Heidelberg (1901 2000)

9 GEOLOGY

9.1 Regional Geology

The studied area falls within the South Rand Coal field comprising Karoo sediments. The area is

surrounded by hills of the Witwatersrand Supergroup and the Ventersdorp Group. Figure 4 shows the

regional geology of the study area as obtained from the 1:250 000 map of Heidelberg

(2628 East Rand). Generally the geology comprises sandstone, mudstone, shale and coal of the

Vryheid formation in the Karoo Supergroup. Geological structures include the Suikerbos and

Malanskraal fault to the south and east of the study area.

Regionally the coal deposit at Heidelberg comprises an infilled glacial valley on the edge of the South

Rand Karoo basin. The deposit itself is therefore relatively localised and elongated NW SE. It

underlies the flood plain of the Suikerbosrand River and also the area to the west of the River where

the topography rises.

The relatively straight North - South drainage of the Suikerbosrand River, indicates the river channel

to be structurally controlled.

9.2 Site Specific Geology

Locally the flood plain in the study area is characterised by a shallow soil profile, located on top of

the Karoo strata. The flood plain is the product of hillwash from the surrounding hills and the

alluvium associated with the Suikerbosrand River. The shallow soil profile comprises dark, soft tofirm, sandy clay and clay rich silty sands.

0

20

40

60

80

100

120

140

160

180

200

1 2 3 4 5 6 7 8 9 10 11 12

Month

Evaporation(mm/m)

MME


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Figure 4: General Geological Reference Map


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The modelled area is underlain by a layer of alluvium and sandy clays ( 6 m thick), overlaying shales

and sandstones ( 6 m thick). The Karoo rocks are 27 m thick and contain the No. 3 coal seam with

the largest economic potential.

A schematic cross section of the geological environment is presented in Figure 5.

Figure 5: Schematic Representation of the Heidelberg Environment

10 HYDROGEOLOGY

10.1 Aquifers

A shallow aquifer system is present in the floodplain of the Suikerbosrand River and surrounding

hills, comprised of alluvial material and hill-wash. These sandy clays are saturated and limited in

extend, which makes it an inefficient semi-confined aquifer, as it still allows seepage through to the

underlying geology, developing a perched water level at shallow depth (at the base of residual or

transported soils) above the unweathered rock layers.

A deeper aquifer system is associated with the fractured Karoo sandstone and coal seams and will be

the main system contributing to groundwater flow into the proposed Heidelberg opencast pit. This

deeper aquifer is confined to semi-confined, non-continuous and multi-layered, causing elevated

piezometric heads and in some instances artesian conditions.


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10.2 Groundwater Flow Directions and Gradients

The natural groundwater level is mostly controlled by the local topography, with groundwater

draining towards surface stream channels.

The average water level distribution was estimated by the hydrocensus conducted and has been used

to calibrate the model for steady state conditions as represented in Figure 6. In general groundwater in

the study area flows towards the Suikerbosrand River (from the East) and the Blesbokspruit (from the

North).

The measured groundwater elevations, indicate that groundwater levels mirror the topography

(see Figure 6) and confirms that the groundwater flow is to the Suikerbosrand River and

Blesbokspruit at a gradient of approximately 1:300. Locally no preferential pathways for the

movement of groundwater away from the site, along fracture zones or deep weathered zones, have

been identified.

Artesian conditions are present in a peat area to the south-west of the proposed opencast pit, where

groundwater flows from subsurface to the ground surface, due to confining to semi-confining

conditions within the deeper aquifer system.

10.3 Water level depth below surface and blow yields

The newly drilled shallow boreholes (GAA Report No: 7475/8235/4/G, March 2006) in the study

area, indicating water levels of 7 to 9 meters below ground level (mbgl) with blow yields ranging

from


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Figure 6: Regional Piezometric Water levels and Flow Directions


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WMB report (now Golder Associates Africa (Pty)Ltd), 3758/1713/1/W (March 1999): Heidelberg

Opencast Coal Mine Specialist Studies for Surface and Groundwater

Hydrocensus, drilling and aquifer testing data (GAA Report No: 7475/8235/4/G, March 2006)

Current Heidelberg pit layout data in digital format, as provided by Anglo Coal

Lithological boundaries as from the geological database, distinguishing between soft (alluvium &

sandy clays), hard (sandstone & shale) and base of the No. 3 coal seam (coal seams &

sandstone / shale layers)

This data was used in the development of a conceptual model and finite element numerical model,

simulating the impact on groundwater from the operation of the proposed Heidelberg opencast coal

mine, starting in 2007 to 2018.

12 CONCEPTUAL MODEL

Groundwater flow modelling depends on the physical properties of the site. For a numerical model to

be useful as an assessment tool, it is necessary to integrate the physical geometry and properties of the

site into the model. Controlling factors are:

The geology, as well as the hydrogeological properties of the aquifer system

The topography and relief

Surface hydrology and precipitation

A conceptual model reduces the actual problem and domain, to an acceptable simplification, based on

a set of assumptions.

12.1 Modelling assumptions

The modelling assumptions used in the development of the conceptual model for the Heidelberg

opencast pit groundwater system, included:

The model boundary is represented by topographical highs (along water sheds) based on a digitalterrain model constructed from the 1:50 000 topographical maps of the study area (2627 & 2628).

Thickness of the modelling layers were based on an average thickness of the soft, hard and floor

of the No. 3 coal seam data, provided by Anglo. These layers are underlain by pre-Karoo

basement rock

Alluvial zones were spatially assigned according to the 1:250 000 geological map (2628 East

Rand). Exploration drilling across the footprint of the proposed opencast pit area confirmed that

no paleo channel is present

Operational life of the Heidelberg opencast pit is from 2007 to 2018 (12 years). After operation,the void within the pit will be allowed to naturally fill with water


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12.2 Software Selection (Modelling Code)

FEFLOW is the software package selected for this modelling study. It provides a sophisticated three-

dimensional finite element modelling environment and allows interactive simulation of groundwater

flow systems within the subsurface. The software is also capable of simulating cross-sectional, fluiddensity-coupled, thermohaline or uncoupled systems. It also handles variably saturated, transient or

steady state flow, mass and heat transport systems in 3D. These systems could be simulated with or

without one or more free surfaces.

The main advantage of using the finite element approach (above the finite difference approach) is that

the boundaries of various features can be accurately represented.

13 CONSTRUCTION OF THE NUMERIC FLOW MODEL

A groundwater flow model was developed for the Heidelberg opencast pit in order to simulate

operational and post closure groundwater flow conditions. The model was calibrated for pre-mining

steady state conditions; these served as starting heads for the transient simulations, in which the effect

of the opencast pit on the groundwater environment was considered.

13.1 Finite Element Grid

The FEFLOW pre-processing software was used to generate a 6-noded triangular prism element

network across the area investigated (Figure 7).

The grid mesh consists of 300 448 elements and 190 005 nodes. Refinement of the grid mesh (finer

density, closer nodal spacing) was specified along the footprint area of the proposed Heidelberg

opencast pit, where a more accurate solution of groundwater flow is required.

The model consists of four (4) layers as schematically shown in Figure 5:

The first layer with a thickness of 6 m represents alluvial and sandy clay horizons, where these are

present. Elsewhere sandstone and shale formations were assigned to this layer where they outcrop

according the geology map of the area

The second layer represents sandstone and shale, approximately 6 m thick

The third layer represents sandstone and the bottom of the No. 3 Coal Seam, approximately 5 m

thick

The fourth layer represents fractured sandstone , approximately 22 m thick

A three dimensional view of the modelling grid is provided in Figure 8.


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Figure 7: Model Matrix (Finite Element Grid)


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Figure 8: Three Dimensional Model Domain

13.2 Model Boundaries

A surface catchment approach was adopted to reduce or eliminate the influence of incorrectly

specified boundary conditions in the numerical model (model boundaries are selected on localwatersheds, i.e., the model covers a larger area and thus the extent of potential dewatering impacts

will not be constrained by the size of the modelling domain). The natural catchment boundaries were

determined from an investigation of the local topography maps and subsequently assigned to the

model.

The model boundary includes the catchments of the Suikerbosrand River and Blesbokspruit

respectively to the east and north of the proposed mine site (Figure 7).

The modelled area covers 1 150 800 000 m2, with the proposed Heidelberg opencast pit covering

1 191 350 m2.


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Figure 9: Calibrated Water levels

Figure 10: Geological Representation of Modelled Area and Water Levels

R = 0.95

1460

1480

1500

1520

1540

1560

1580

1460 1480 1500 1520 1540 1560 1580

Measured (mamsl)

Calculated(mamsl)

Observed

Modelled

R2=0.9043

-80000 -75000 -70000 -65000 -60000 -55000 -50000 -45000

-2965000

-2960000

-2955000

-2950000

-2945000

-2940000

-2935000

-2930000

-2925000

-2920000

Suikerbosrand River

Blesbokspruit

Pit

A

B

C

Zone:


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A sensitivity analysis was conducted on the material underlying the alluvium, as a measure to address

the uncertainty in the properties of the material underlying the Suikerbosrand River. This analysis did

not have a significant impact on the calibration result of the model with regards to the water level

distribution.

Table 1: Calibrated Hydrogeological Parameters

Hydraulic Conductivity (m/d)Layer

Thickness

(m)Zone

AZone

BZone

C

Storage

Compressibility

(Operation)

1 6 0.24 0.11 0.03 1.28 E-4

2 6 0.24 0.11 0.03 1.28 E-4

3 5 0.19 0.11 0.03 1.28 E-4

4 22 0.24 0.11 0.03 1.28 E-4

Aquifer Test - 0.001-0.8 0.02-1.3 - -

Recharge

(% MAP)- >3


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The calibrated groundwater flow model was further validated using transient abstraction and water

level data obtained from the aquifer tests conducted by Wates Meiring & Barnard (1997) [now Golder

Associates Africa (Pty)Ltd]. This was conducted in order to ensure that the model is sufficiently

calibrated on a small scale around the proposed footprint area of the Heidelberg opencast pit.

Figure 12: Frequency Distribution of Conductivities for Deep Aquifer system

A well-calibrated model will ensure that reliable estimates of flow rates and impacts on the

groundwater regime are approximated during operation of the Heidelberg opencast pit. A storage

coefficient value of 0.005 was assumed for the whole system modelled. This is reconcilable with

values for storativity typically encountered for Karoo aquifers.

Figure 13 shows the simulated and observed water level response for borehole D during a constant

discharge aquifer test. It is clear from the results obtained, and taking into account borehole losses,

that the flow model is capable of simulating the real aquifer test data sufficiently well to allow

realistic simulation predictions.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3

Hydraulic Conductivity (m/d)

0

1

2

3

4

F

equency

Calculated values from aquifer tests

Value used in model for shale

Value used in model for sandstone




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Figure 13: Transient Calibration of Borehole D

14 UTILIZING THE GROUNDWATER FLOW MODEL TO ADDRESS THE

STUDY OBJECTIVES

14.1 Introduction

The first objective of this study (see Section 2) is to develop and calibrate a numerical flow model

(discussed in Section 12) that accurately simulates groundwater flow through the aquifer systems to

and from the proposed Heidelberg opencast pit. The construction and calibration of the flow model

were discussed in detail in Section 13. This section describes the simulated inflows and impact

assessment on the groundwater environment for the operational and post closure phase of the

Heidelberg opencast pit with and without a dedicated pit dewatering system.

14.2 Operational Mining Phase

14.2.1 Mining method

The British Box-cut method of strip-mining using truck and shovel equipment will be used during the

operational phase with a planned production life of the mine at 12 years, indicated by Figure 14.

For each strip the topsoil is removed, followed by the overburden, and finally the coal. Spoils will be

loaded on trucks and selectively dumped in the void left by mining, leveled and the topsoil will be

replaced, leveled and re-vegetated.

1476

1476

1477

1477

1478

1478

1479

1479

1480

1480

0 0.2 0.4 0.6 0.8 1 1.2 1.4Time (d)

Elevation(mamsl)

Test

Model


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Figure 14: Proposed Heidelberg Opencast Pit Development & Dewatering Well Field

With the British Box-cut method, rehabilitation is undertaken much sooner after the coal is extracted

and the area of disturbance at any one given time is much smaller. It also allows for more flexibility

during operation for materials handling.

The area that will comprise the active mining area and ramps is relatively small, due to the zigzag

nature of the mining operation.

This proposed mining method was introduced to the model on a monthly basis, with the use of

constant head conditions, as a function of the lowest block elevation and constrained as to only allow

water to be removed by the heads once active mining commence in a particular block.

Legend

2007\Jan - 2007\Dec

2008\Jan - 2008\Dec2009\Jan - 2009\Dec

2010\Jan - 2010\Dec

2011\Jan - 2011\Dec

2012\Jan - 2012\Dec

2013\Jan - 2013\Dec

2014\Jan - 2014\Dec

2015\Jan - 2015\Dec

2016\Jan - 2016\Dec

2017\Jan - 2017\Dec

2018\Jan - 2018\Dec

-2946000 -2946000

-71000

-71000

-71395mE

-71395mE

-70059mE

-70059mE

-2945000mN

-2946617mN -2946617mN

-2945000mN

Dewatering Borehole Locations


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Operational timeframes is provided in Table 2, as proposed by Anglo Coal.

Table 2: Modelled Operational Timeframe

Rehabilitation Start of operation date Completion date

Block 1 2007/01/01 2007/12/31

Block 2 2008/01/01 2008/12/31

Block 3 2009/01/01 2009/12/31

Block 4 2010/01/01 2010/12/31

Block 5 2011/01/01 2011/12/31

Block 6 2012/01/01 2012/12/31

Block 7 2013/01/01 2013/12/31

Block 8 2014/01/01 2014/12/31

Block 9 2015/01/01 2015/12/31

Block 10 2016/01/01 2016/12/31

Block 11 2017/01/01 2017/12/31

Block 12 2018/01/01 2018/12/31

14.3 Modelled Scenarios

Two alternatives were modelled to simulate the groundwater inflows to the proposed Heidelberg

opencast pit and the subsequent impact on the surrounding groundwater environment. These two

alternatives are:

Alternative A It is assumed that the constant heads specified on the Suikerbosrand River and

Blesbokspruit can supply water into the modelling domain or remove water from the modelling

domain depending on the hydraulic gradient present. This situation would represent a worst case

scenario in terms of modelled inflow into the pit (for the set of calibrated hydraulic properties) as

additional water is derived into the model through the constant head boundary conditions specified on

the rivers.

Alternative B It is assumed that the constant heads specified on the Suikerbosrand River and

Blesbokspruit can only remove water from the modelling domain depending on the hydraulic gradient

present. This situation would represent a best case scenario in terms of modelled inflow into the pit

(for the set of calibrated hydraulic properties) as no additional water is derived from outside the model

domain through river inflow.

These two different approaches (scenarios) were adopted to address the uncertainty as to whether the

Suikerbosrand River is in hydraulic continuity with the underlying strata or not.

For each of the above described alternatives simulations were conducted for a Base Case scenario

(Scenario 1) and a Base Case plus dewatering from dedicated dewatering boreholes scenario

(Scenario 2):


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Scenario 1 Base Case: Development of the Heidelberg opencast pit by truck and shovel

method, from 2007 till 2018, excluding a dedicated dewatering well field.

Scenario 2 Base Case plus Dewatering: Develop the Heidelberg opencast pit by truck and

shovel method, from 2007 till 2018, including a dedicated dewatering well field consisting of 9and 17 dewatering boreholes.

Assumptions made for Scenario 1 (Operational Phase): Operation of the Heidelberg opencast pit

will initiate in 2007 and function by truck and shovel method on a monthly time step over a period of

twelve years. Completion of the Heidelberg opencast pit is in 2018.

Assumptions made for Scenario 2 (Operational Phase): The same assumptions have been made as

for Scenario 1, with the following addition: Operation of the Heidelberg opencast pit will include a

dewatering well field, represented by nine (9) and seventeen (17) fully penetrating extraction

boreholes on the perimeter of the opencast footprint area, actively dewatering the system from

6 months prior to operation in 2007 at a pump rate of approximately two litres per second (2 l/s) per

borehole.

The initial estimation of the 9 borehole dewatering field was based on the average inflows into the pit

(approximating 1 800 m3/d), estimated from Scenario 1 and also taking into account the six months of

active dewatering prior to operation. This related to an approximate extraction rate of 1 555 m3/d,

from the nine wells, pumping at 2 l/s. Boreholes were then added between the initial 9 dewatering

wells, to more effectively overlay the dewatering interference from the well field, around the

perimeter of the proposed opencast footprint. This established the 17 dewatering wells, pumping at2 l/s (approximately 2 938 m

3/d).

Assumptions made for the Post closure phase: Changes in recharge conditions (15 % of MAP) and

hydraulic conductivities (8.64 m/d) are assumed after 2018 over the opencast footprint, from which

date the opencast pit was allowed to naturally fill with water. Natural filling of the opencast pit was

without the operation of the dewatering well field, considering both alternatives over a 12 year period.

14.4 Scenario Discussion

14.4.1 Operational Phase

Groundwater flows into the pit, over the operational period are represented in Figure 15 and Figure 16

for Alternative A and Alternative B respectively. A comparative evaluation of the base case scenario

(Scenario 1) and the dewatering well field (Scenario 2), by interpretation of the graphical results from

these figures indicate that the average influx of water into the pit will be approximately 1 820 m3/d,

reduced to 1 160 m3 /d (36 % reduction on average) with active dewatering of nine (9) dewatering

wells and 825 m3/d (55 % reduction on average) with active dewatering of seventeen (17) dewatering

wells, for Alternative A (see Table 5).


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Figure 15: Pit Inflows and Pump Rates for Alternative A

0

500

1000

1500

2000

2500

3000

3500

0 365 730 1095 1460 1825 2190 2555 2920 3285 3650 4015 4380

Time (d)

Inflow(

m3/d) Scenario 1

Scenario 2_9 Wells

Scenario 2_17 Wells

Pump_9 Well

Pump_17 Well


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Figure 16: Pit Inflows and Pump Rates for Alternative B

0

500

1000

1500

2000

2500

3000

3500

0 365 730 1095 1460 1825 2190 2555 2920 3285 3650 4015 4380

Time (d)

Inflow(

m3/d) Scenario 1

Scenario 2_9 Wells

Scenario 2_17 Wells

Pump_9 Wells

Pump_17 Wells


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For Alternative B the average influx of water into the pit will be approximately 1 060 m3/d, reduced to

272 m3 /d (74 % reduction on average) with active dewatering of nine (9) dewatering wells and

37 m3 /d (97 % reduction on average) with active dewatering of seventeen (17) dewatering wells

(see Table 5).

The average pump rate will approximate 1 136 m3/d to 1 120 m

3/d with nine (9) dewatering wells and

1 829 m3/d to 1 726 m

3/d with seventeen (17) dewatering wells for Alternative A & B respectively.

From the simulations conducted for the operational phase the following is evident:

The difference in the inflows to the pit is significant between Alternative A (river is in hydraulic

continuity to allow inflow or outflow of water from the system) and Alternative B (river is not in

hydraulic continuity and only allow the outflow of water from the system). The model is therefore

highly sensitive as to the uncertainty that exists whether the river is in hydraulic continuity with

the underlying aquifer.

Dewatering using dedicated abstraction boreholes in advance of mining will significantly reduce

inflow of groundwater into the mine workings.

A trend of incremental groundwater inflow increases is observed over the 12 year operational life

of the pit, as a direct result of generally increases in floor elevations over time as the pit develops.

14.4.2 Developing Cone of Depression

The development of the cone of depression (impact on the surrounding groundwater environment)from 2007, with the start of the Heidelberg opencast pit, to the year 2018, are presented in

Appendix A [indicated on the sequential figures is the groundwater drawdown in meter(s) from the

calibrated steady state water level = Cone of Dewatering].

This development of the cone of depression around the footprint of the Heidelberg opencast pit over

time is indicating that the development will follow the progression of the mine, starting in the South-

West in 2007 and extending to the North until 2018.

The cone of depression will extend to the Suikerbosrand River in the East and could have a zone of

influence approximating 1 400 m to the North, 615 m to the South and 410 m to the West, with no

active dewatering for Alternative A, as indicated in Figure 17 (Scenario 1). Alternative B indicates

that the zone of influence from dewatering could extend up to 710 m in the South and 860 m to the

East of the opencast footprint (Figure 18).


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14.4.3 Closure Phase

Natural refilling of the opencast pit from the year 2018, to an elevation of 1476 mamsl, one metre

below the base of the Suikerbosrand River (1477 mamsl), is proposed to ensure that any possible

contaminated water in the opencast pit will not flow into the Suikerbosrand River and will takeapproximately 1 980 days or more than 5 years, for Alternative A and approximately 3 960 days,

almost 11 years for Alternative B (Appendix A).

The average natural filling of the pit will be relatively high at approximately 840 m3/d and 617 m

3/d,

reducing over time, as driven by the reduction in head differential between outside aquifer and inside

pit head conditions to average flows of 452 m3 /d and 442 m

3 /d, as represented by Figure 23 and

Figure 24 for Alternatives A & B respectively.

Table 3 summarises the natural refilling of the Heidelberg opencast pit under constant head and steady

state conditions. It should be noted that where the flows are expressed as negative values, the flows

are not into the pit, but out of the pit and into the Suikerbosrand River.

Table 3: Pit Water Elevation & Inflow Rates at Closure

Alternative A

(River/Aquifer Hydraulic

Continuity)

Alternative B

(No River/Aquifer

Hydraulic Continuity)Pit Water

Elevation

(mamsl)AverageInflow

(m3/d)

Duration(Days)

AverageInflow

(m3/d)

Duration(Days)

1473 841 1 260 617 2 880

1474 712 1 440 580 3 2401475 582 1 680 523 3 600

1476 452 1 980 442 3 960

1477 323 2 340 317 4 380

1478 -67 (out) 2 790 -70 (out) -

1479 -171 (out) 3 510 -175 (out) -

1480 -280 (out) - -284 (out) -


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Figure 23: Head in Pit vs. Fill Rate for Alternative A

1455

1460

1465

1470

1475

1480

1485

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Time (d)

Elevation(mamsl)

-1000

0

1000

2000

3000

4000

5000

Flux(m3/d)

Head

Inflow


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Figure 24: Head in Pit vs. Fill Rate for Alternative B

1455

1460

1465

1470

1475

1480

1485

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Time (d)

Elevation(mamsl)

0

500

1000

1500

2000

2500

3000

3500

Flux(m3/d)

Head

Inflow


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14.4.4 Contribution from Alluvium

Figure 25 and Figure 26 show the simulated contribution from modelling Layer 1 (shallow aquifer)

into the Heidelberg opencast pit for Alternatives A & B respectively. It is clear that the inflows

increase as the pit development approaches the Suikerbosrand River, under river/aquifer hydrauliccontinuity conditions (Figure 25), as well as an increased flow, due to increased pumping activity

from a 9 and 17 well dewatering field. Average flows from the alluvium, for Alternative A,

approximates 171 m3/d without dewatering and roughly contributes 9 % of groundwater inflows to the

pit. Active dewatering will increase the flow through the alluvium to an average of 214 m3 /d and

265 m3/d, for the 9 and 17 well scenarios respectively.

Average flows from the alluvium, for Alternative B (Figure 26), approximates 76 m3 /d without

dewatering and roughly contributes 7 % of groundwater inflows to the pit. Active dewatering will


3/d, for the 9 and 17 well

scenarios respectively.

A sensitivity analysis was conducted whereby the hydraulic conductivity of the zone below the

alluvial material (Zone A in Figure 10) was reduced from 0,24 m/d to 0,11 m/d. This simulation was

conducted for Alternative A where river/aquifer hydraulic continuity is assumed. The simulation

indicates a reduction in pit inflow from 1 820 m3/d to 1 250 m3/d a reduction of approximately

31%. This result clearly emphasizes the effect that uncertainty in hydraulic parameters might have on

inflow calculations (and general groundwater impact). The reader is referred again to the range in

field measurements of hydraulic parameters depicted in Figure 11 and Figure 12 vs. the actual values

used in the modelling exercise.

14.5 Groundwater Quality

14.5.1 Local Background Groundwater Quality Concentrations

The pumped water from active dewatering during operation of the Heidelberg opencast pit is to be

discharged into the Suikerbosrand River. It is therefore important to estimate the quality of the water

to be discharged.

Local background groundwater qualities were determined from analysis of current available data for

both the shallow and deep aquifer system.


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Figure 25: Alluvial Inflows for Alternative A

0

50

100

150

200

250

300

350

400

0 365 730 1095 1460 1825 2190 2555 2920 3285 3650 4015 4380

Time (d)

Inflow(

m3/d)

Scenario 1

Scenario 2_9 Wells

Scenario 2_17 Wells


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Figure 26: Alluvial Inflows for Alternative B

0

50

100

150

200

250

300

350

400

0 365 730 1095 1460 1825 2190 2555 2920 3285 3650 4015 4380

Time (d)

Inflow(

m3/d)

Scenario 1

Scenario 2_9 Wells

Scenario 2_17 Wells


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It is clear from Table 4 that unacceptably high concentrations of aluminium and iron are present in the

shallow aquifer system, as evaluated according to the South African Water Quality Guidelines for

Domestic Use. It should also be noted that sodium and sulphate concentrations have been measured at

an unacceptable level. However, the deep aquifer system indicates groundwater of a good quality.

As most of the dewatering water will be derived from the deeper aquifer system during operation, it is

expected that the quality of the water to be discharged should be of an acceptable quality.

Statistical graphs (Box and Whisker plots) showing the chemical distribution of the shallow and deep

aquifer systems is represented in Figure 27 and Figure 28 respectively.

The final in pit void of the Heidelberg opencast pit will be allowed to refill during closure, to a level

one (1) meter below the base of the Suikerbosrand River (1477 mamsl) at 1476 mamsl. This

management strategy will reduce the risk of contaminated water flowing from the pit into the adjacent

Suikerbosrand River, but also allows the pit to function as a sink to groundwater flow.

Figure 29 shows the groundwater flow lines (obtained from particle tracking) for the situation where

the post closure pit water level elevation is maintained at an elevation of 1476 mamsl. It is evident

from this figure that all flow is contained within the pit area.

Any possible contaminated in-pit water will therefore be theoretically contained within the footprint

of the opencast pit, as a result of this proposed in-pit water management strategy.

Table 4: Background Groundwater Qualities

Shallow Aquifer System Deep Aquifer SystemElement Unit

Min Ave Max Min Ave Max

DomesticGuideline

Value

pH 6.40 7.31 8.30 6.38 7.39 8.82 9

Calcium - Ca mg/l 11.80 28.54 73.00 2.60 7.64 11.80 80

Magnesium - Mg mg/l 2.00 17.33 44.00 2.00 4.70 7.10 70

Sodium - Na mg/l 11.80 190.39 503.00 3.60 30.68 65.00 400

Sulphate - SO4 mg/l 4.20 73.72 646.00 1.00 2.13 4.00 400

Aluminium - Al mg/l 0.02 5.84 55.00 0.10 0.10 0.10 0.5

Nickel - Ni mg/l 0.00 0.07 0.63 0.00 0.00 0.00 1

Manganese - Mn mg/l 0.00 0.16 1.00 0.01 0.01 0.01 1Iron - Fe mg/l 0.01 3.02 22.00 0.01 0.06 0.39 1

Zinc - Zn mg/l 0.01 0.03 0.26 0.00 0.00 0.00 10


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Figure 27: Shallow Aquifer Chemistry

Figure 28: Deep Aquifer Chemistry

0.001

0.01

0.1

1

10

100

1000

Concentration(mg/l)

Ca Mg Na SO4 Al Ni Mn Fe Zn

0.1

1

10

100

Conce

ntration(mg/l)

Ca Mg Na SO4 Fe


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Figure 29: Groundwater Flow Lines at 1476 mamsl

15 CONCLUSIONS AND RECOMMENDATIONS

It was prudent to use a conservative approach to the numeric modelling, in view of the sensitivity of

the hydrogeological properties of the underlying riverbed material and location of the proposed

Heidelberg opencast pit to the Suikerbosrand River.

The modelled results were based on the proposed alternatives for each scenario evaluated, to

determine the groundwater inflows to the proposed Heidelberg opencast pit and the impact on the

surrounding groundwater environment, during operation and post closure.

The proposed alternatives to the scenarios address the uncertainty in the hydraulic parameters of thematerial underlying the alluvium. This uncertainty relates to the hydraulic continuity (Alternative A)

or no hydraulic continuity (Alternative B) of the Suikerbosrand River and Blesbokspruit to the

shallow aquifer system. The proposed alternatives were based on the presence of clayey soils that

could restrain water flowing from the river to the opencast pit.

Boundary conditions representing the Suikerbosrand River and Blesbokspruit were assigned to either

allow water to discharge or recharge the system from the constant head nodes, depending on

surrounding head conditions for Alternative A (simulating a worst case scenario) or to be discharged

from the system should a positive hydraulic gradient exist towards the constant head nodes and where

the gradient is zero or negative no water would be added into the system for Alternative B (simulates

a best case scenario).


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The following key conclusions can be drawn from the numerical groundwater modelling presented in

this report.

Alternative A: Worst Case Scenario

The average influx of groundwater during operation will approximate 1 820 m3 /d, reduced to

1 160 m3/d (36 % reduction on average) with active dewatering of nine (9) dewatering wells and

825 m3/d (55 % reduction on average) with active dewatering of seventeen (17) dewatering wells.

The natural filling of the pit, to an elevation of 1476 mamsl (1 m below the base of the

Suikerbosrand River) will take approximately 1 980 days or more than 5 years after closure. The

average flow rate will be at 841 m3/d, reducing over time as driven by the rising head conditions

within the pit, to average flows of 452 m3 /d. If the opencast pit is allowed to fill above the

recommended waterlevel of 1476 mamsl, outflow of contaminated water to the Suikerbosrand

River could approximate > 280 m3/d.

There will be an increase in flows through the alluvium to the pit, as the pit development

approaches the Suikerbosrand River. Average flows will approximate 171 m3 /d without

dewatering and roughly contribute 9 % of groundwater inflow to the pit. Active dewatering will


3/d, for the 9 and 17

dewatering well scenarios respectively (Scenario 2).

Operation of the opencast pit will lead to the development of a dewatered cone of depression

extending to the Suikerbosrand River in the East, 1 400 m North, 615 m to the South and 410 m to

the West, with no active dewatering (Scenario 1). The footprint area will be influenced by the

nine (9) dewatering well field, operating six (6) months prior to pit development for Scenario 2.This will result in a more extensive area of influence that could approximate 1 470m to the North,

720 m to the South and 570 m to the West.

The extent of dewatering by increasing the number of dewatering wells to seventeen (17), indicate

a cone of depression developing to 1 500 m North, 800 m South, 700 m West and to the

Suikerbosrand River on the East. Operation of a 17 dewatering well field will ensure a more

effective barrier to groundwater inflow into the opencast pit.


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Alternative B: Best Case Scenario

The average influx of groundwater during operation of the Heidelberg opencast pit will

approximate 1 060 m3/d, reduced to 272 m

3/d (74 % reduction on average) with active dewatering

of nine (9) dewatering wells and 37 m

3

/d (97 % reduction on average) with active dewatering ofseventeen (17) dewatering wells.

The natural filling of the pit, to an elevation of 1476 mamsl will take approximately 3 960 days,

almost 11 years after closure. The average flow rate will be at 617 m3 /d, reducing over time to

average flows of 442 m3 /d. If the opencast pit is allowed to fill above the recommended

waterlevel of 1476 mamsl, outflow of contaminated water to the Suikerbosrand River could

approximate > 280 m3/d.

The average alluvial flow will approximate 76 m3 /d without dewatering and roughly contribute

7 % of groundwater inflow to the pit. Active dewatering will increase the flow through the

alluvium to an average of 98 m3 /d and 121 m3 /d, for the 9 and 17 dewatering well scenarios

respectively (Scenario 2).

Operation of the opencast pit will lead to a similar development of a dewatered cone as for

Alternative A, but extending to 710 m South and 860 m to the East of the opencast footprint.

Scenario 2 indicates an approximate area of influence extending to 860 m South and 960 m East.

By characterising the Suikerbosrand River as a gaining stream only, the developed dewatering

cone will extend up to 980 m South and 1 125 m East, with the increase of dewatering wells to

seventeen (17).

There is no indication that the development of the dewatering cone, during operation of the

Heidelberg opencast pit, will influence current water supply boreholes, under hydraulic continuity

or no hydraulic continuity conditions (Alternative A or B).

Water quality from the deeper semi-confined aquifer is of a good quality. Most of the pumped water

for mine dewatering purposes during the operational phase will be derived from this aquifer. It can

therefore be safely assumed that the pumped water can be discharged into the Suikerbosrand River

without any detrimental effect to the environment. In-pit management practices during the operational

phase should be such that no potential contaminated surface water be allowed the time to infiltrate

into the groundwater system.

Key uncertainties in the study are:

The extent of influence from the peat area (south-west of the proposed Heidelberg opencast pit) to

flows in the groundwater environment.

The hydrogeological properties of the Suikerbosrand riverbed.


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The study was aimed at developing a first order understanding of the groundwater impacts associated

with the Heidelberg opencast pit operation. In order to develop more confidence in the impacts as well

as evaluate suitable dewatering measures, the numeric model results should be re-evaluated, once

additional information is available to recalibrate the current model.

The recommended action to address the key uncertainty is:

To conduct pumping tests at the identified peat area, to estimate a more accurate groundwater

contribution from the area, to the Heidelberg opencast pit. The development of the dewatering

well field will provide this additional information that should be used for refinement of the model.

To estimate the hydrogeological properties of the Suikerbosrand riverbed by conducting a river

profile investigation.

Additional dewatering boreholes (8) should be allocated for and possibly positioned between the

proposed 9 dewatering wells around the footprint of the proposed Heidelberg opencast pit.

A summary of the numeric modelling results is presented in Table 5 below.


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Table 5: Comparative Scenario Results for Pit Operation

Alternative A (Worst Case) Alternative B (Best Case)

Scenario 1(Base Case)

Scenario 2(9 Well

Dewatering)

Scenario 2(17 Well

Dewatering)

Scenario 1(Base Case)

Scenario 2(9 Well

Dewatering)

Scenario 2(17 Well

Dewatering)Annual Block Units

Average

Inflow

Average

Inflow

Average

Inflow

% InflowReduction

from using

9-17

DewateringWells

Average

Inflow

Average

Inflow

Average

Inflow

% InflowReduction

from using

9-17

DewateringWells

Block 1 (2007) m3 /d 827.64 452.94 299.01 45-64 722.16 309.34 100.80 57-86

Block 2 (2008) m3 /d 994.72 500.39 307.52 50-69 803.82 266.37 46.95 67-94

Block 3 (2009) m3 /d 1181.72 587.29 351.58 50-70 957.13 301.37 77.23 69-92

Block 4 (2010) m3 /d 1182.62 637.63 416.27 46-65 930.93 290.45 90.53 69-90

Block 5 (2011) m3 /d 1302.52 770.19 548.03 41-58 1017.42 333.40 85.80 67-92

Block 6 (2012) m3 /d 1440.36 847.42 600.22 41-58 1016.60 247.66 34.28 76-97

Block 7 (2013) m3 /d 1965.90 1342.57 1030.16 32-48 1044.88 249.87 9.42 76-99

Block 8 (2014) m3 /d 2193.07 1501.52 1144.21 32-48 1142.68 267.66 5.15 77-100

Block 9 (2015) m3 /d 2299.18 1479.17 1100.29 36-52 1180.92 202.44 0.01 83-100

Block 10 (2016) m3 /d 2799.96 1808.75 1201.40 35-57 1591.69 391.26 0.00 75-100

Block 11 (2017) m3 /d 2687.91 1843.17 1333.68 31-50 1257.54 262.81 0.00 79-100

Block 12 (2018) m3 /d 2875.98 2037.41 1512.56 29-47 1134.01 166.38 0.00 85-100


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16 REFERENCES

WMB report (now Golder Associates Africa (Pty)Ltd), (March 1999): Heidelberg Opencast Coal

Mine Specialist Studies for Surface and Groundwater, Report No: 3758/1713/1/W

Golder Associates Africa (Pty)Ltd Report, (March 2006): Hydrogeological Investigation for the

Proposed Heidelberg Opencast Mine, Report No: 7475/8235/4/G

Golder Associates Africa (Pty)Ltd Report, (April 2006):Heidelberg Surface Water Specialist Report,

Report No: 7475/8290/6/W

GOLDER ASSOCIATES AFRICA (PTY) LTD

H Marais H van Rensburg G L HubertG:\PROJECTS\7475 - HEIDELBERG EIA\REPORTS AND PRESENTATIONS\7475-8297-7-G NUMERIC GROUNDWATER MODELLING\7475-8297-7-G GROUNDWATER

NUMERIC MODELLING RPT.DOC


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APPENDIX A

Groundwater Figures


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Documents

8543-Volume 2-1.4 - Groundwater Numeric Modelling