report revision 1 - epa wa

Report Revision 1Wallaby Project Scoping Study, Hypersaline Groundwater Management 30 August 1999for Placer (Granny Smith) Pty Ltd Page i

Ref: JMD:sor/17667-100-071/DK:515-F1443.1/DOC/PER DAMES & MOORE

TABLE OF CONTENTS

Page No.

1. INTRODUCTION..........................................................................................................................................1

2. BACKGROUND TO THE SCOPING STUDY ..........................................................................................2

2.1 CLIMATE................................................................................................................2

2.2 LOCAL SETTING ...................................................................................................3

2.3 CONCEPTUAL MINE DEWATERING REQUIREMENTS....................................3

2.4 SALT LOADINGS...................................................................................................4

2.5 PRELIMINARY MANAGEMENT OPTIONS .........................................................5

3. SCOPING STUDY DESIGN CRITERIA....................................................................................................5

4. SITE VISIT.....................................................................................................................................................6

5. HYPERSALINE GROUNDWATER MANAGEMENT OPTIONS ........................................................7

5.1 INTRODUCTION....................................................................................................7

5.2 OPTION 1 – EVAPORATION PONDS ...................................................................8

5.2.1 General ........................................................................................................8

5.2.2 Land-Based Facility (Options 1a and 1b) .................................................... 10

5.2.3 Lake Carey Based Facility (Option 1c)........................................................ 14

5.2.4 Discussion.................................................................................................. 16

5.3 OPTION 2 – DISCHARGE TO NORTHWEST SALINALAND............................ 17

5.4 OPTION 3 – DISCHARGE INTO ABANDONED PITS........................................ 18

5.4.1 General ...................................................................................................... 18

5.4.2 Discharge into Jupiter Pit (Option 3a) ......................................................... 18

5.4.3 Discharge into Goanna, Granny and Windich Pits (Options 3b and 3c) ........ 20

5.4.4 Discussion.................................................................................................. 21

5.5 OPTION 4 – DISCHARGE ONTO LAKE CAREY ............................................... 22

5.5.1 Bunded Areas on Lake Carey (Option 4a) ................................................... 22

5.5.2 Direct Discharge onto Lake Carey (Option 4b)............................................ 25

5.5.3 Discharge into Intra-Island Bunded Areas on Lake Carey (Option 4c).......... 26

5.5.4 Discussion.................................................................................................. 26

5.6 OPTION 5 – DISPOSAL BY REINJECTION........................................................ 27

5.6.1 General ...................................................................................................... 27

5.6.2 Deep Well Injection (Option 5a).................................................................. 28

5.6.3 Shallow Well Injection (Option 5b) ............................................................. 32

5.7 OPTION 6 – CONSTRUCT PALAEOCHANNEL BARRIER ............................... 34

5.7.1 General ...................................................................................................... 34

5.7.2 Grout Curtain (Options 6a and 6b).............................................................. 35

Report Revision 1Wallaby Project Scoping Study, Hypersaline Groundwater Management 30 August 1999for Placer (Granny Smith) Pty Ltd Page ii


TABLE OF CONTENTS (cont'd)

5.7.3 Slurry Cut-off Wall (Option 6c).................................................................. 37

5.8 OPTION 7 – DESALINATION OF HYPERSALINE GROUNDWATER .............. 39

5.9 OPTION 8 – SALT HARVESTING....................................................................... 41

5.10 COMBINED OPTIONS ......................................................................................... 44

5.11 SUMMARY OF COSTS ........................................................................................ 44

5.12 RANKING OF OPTIONS ...................................................................................... 44

6. DISCUSSION AND CONCLUSIONS .......................................................................................................48

7. RECOMMENDATIONS.............................................................................................................................51

8. REFERENCES.............................................................................................................................................51

LIMITATIONS OF REPORT ..................................................................................................... 53

LIST OF TABLES

1 Conceptual Mine Dewatering Requirements .................................................. 4

2 Scoping Study Design Criteria ...................................................................... 5

3 Cost Estimate for Land-Based Evaporation Pond ........................................ 13

4 Cost Estimate for Lake Carey based Evaporation Pond ............................... 15

5 Viable Discharge in Abandoned Pits ........................................................... 19

6 Cost Estimate for Discharge into Abandoned Pits........................................ 21

7 Cost Estimate for Discharge onto Lake Carey ............................................. 25

8 Modelled Deep Well Injection ..................................................................... 30

9 Cost Estimate for Deep Well and Shallow Well Injection............................. 31

10 Minimum Area of Dunal Terrain Required for Shallow Injection ................. 33

11 Cost Estimate for Desalination Plant ........................................................... 41

Report Revision 1Wallaby Project Scoping Study, Hypersaline Groundwater Management 13 August 1999for Placer (Granny Smith) Pty Ltd Page ii


LIST OF TABLES (cont'd)

12 Plant and Equipment Required for a Salt Field ............................................ 42

13 Cost Estimate for Salt Harvesting ............................................................... 43

14 Summary of Costs for all Management Options .......................................... 45

15 Technical Ranking of Options..................................................................... 47

LIST OF FIGURES

1 Project Area Locality Plan

2 Conceptual Hydrogeological Model(a) Section View(b) Model Domain

3 Evaporation Pond Facility Siting

4 Evaporation Pond Embankment Cross-sections

5 Discharge to Northwest Salinaland (Option 2)

6 Discharge into Abandoned Pits (Option 3)

7 (a) Potential Bunded Discharge Sites on Lake Carey, Selected Lake Embayments (Option 4a)

(b) Direct Discharge into Lake Carey (Option 4b)(c) Discharge into Intra-island Bunded Areas on Lake Carey (Option 4c)

8 Bunded Area Perimeter Embankment Cross-Section

9 Known and Inferred Distribution of Injection Domains

10 Modelled Distribution of Deep Injection Wells

11 Conceptual Distribution of Deep Injection Wells

12 Deep Well Injection Modelling Results(a) Variable Injection Rates(b) Injection Strategies

13 Shallow Injection Domain

14 Schematic Outline of Grout Curtain and Palaeochannel Aquifer

15 Schematic Cross-Section of Slurry Cut-Off Wall

Report Revision 1Wallaby Project Scoping Study, Hypersaline Groundwater Management 13 August 1999for Placer (Granny Smith) Pty Ltd Page iii


LIST OF APPENDICES

A Detailed Breakdown of Discharge Option Cost Estimates

B Method Statement for Slurry Cut-off Wall Construction

Report Revision 1Wallaby Project Scoping Study Into Hypersaline Groundwater Management Options 30 August 1999for Placer (Granny Smith) Pty Ltd Page 1


REPORTWALLABY PROJECT SCOPING STUDY INTO

HYPERSALINE GROUNDWATER MANAGEMENT OPTIONSfor

Placer (Granny Smith) Pty Ltd

1. INTRODUCTION

Placer (Granny Smith) Pty Ltd (PGS) manages the Granny Smith Project, centred at the Granny Smith

Mine, about 25 km south of Laverton, Western Australia.

The Granny Smith Project proposes to develop the Wallaby Deposit, located on the shore of Lake

Carey, approximately 10 km from the Granny Smith Mine (Figure 1). Mining feasibility studies are in

progress at present.

Hydrogeological studies of the Wallaby Deposit, including site investigations of the local groundwater

resources and assessment of the mine dewatering requirements, form an integral part of the feasibility

studies. The hydrogeological studies are incomplete, but the site investigations have identified three

discrete aquifer systems within the proposed pit area. They being:

• a shallow superficial formations aquifer system that occurs within dunal terrain and underlying

alluvial and colluvial deposits, to depths of 20 to 30 m;

• a significant palaeochannel aquifer, formed by the Carey Palaeodrainage, that occurs at or near

the base of the transported formations at depths from 60 to 100 m; and

• a bedrock aquifer formed by a weathered and fractured regolith within conglomeratic rocks in

the transition zone above fresh bedrock.

Each aquifer system will need to be locally dewatered and depressurised to accommodate mine

development. All sampled groundwater resources are hypersaline, with Total Dissolved Solids (TDS)

concentrations of the order of 250,000 mg/L.

The poor quality of the local groundwater resources raises several environmental and management

issues associated with the management of mine dewatering discharges. In order to assess these issues

in the short-term, semi-quantitative estimates of the mine dewatering requirements have been assessed

using a simple conceptual hydrogeological and groundwater flow model of the project area. Results

from the conceptual model indicate:

• initial rates of dewatering abstraction of about 1,300 L/s (112,320 kL/day), sustainable for one

year; and

• steadily reducing rates of dewatering abstraction, from about 740 to 460 L/s (63,936 to

39,744 kL/day) during the ensuing years as the dewatering impacts approach steady-state.



Notwithstanding the semi-quantitative and conceptual nature of the completed mine dewatering

simulations, the results clearly indicate the large scale of the project – both in terms of groundwater

abstraction and groundwater management.

Options for groundwater management have been previously reported by DC Blandford & Associates,

July 1999 (Reference PGS99-029, Saline Water Disposal) and discussed in several dedicated project

meetings. The identified management options were outlined in conceptual detail only and consequently

were difficult to compare and rank based on physical aspects, engineering design, capital and operating

costs, technical feasibility and security. Accordingly, Dames & Moore has been engaged to complete a

scoping study of the management options. The results of the scoping study are reported herein, with

emphasis on:

• developing conceptual designs for each management option based on water balance parameters,

salt containment, and specified rates of discharge;

• broadly defining the design specifications for each management option, inclusive of

construction, operating and closure aspects;

• broadly defining the design specifications for water transfer systems;

• identifying alternative management options;

• estimating capital and operating costs; and

• ranking of the management options on the basis of:

- cost;

- engineering aspects;

- practical issues associated with construction, operation, management and closure;

- areas of environmental disturbance;

- salt management;

- technical risk;

- operating risk; and

- closure.

2. BACKGROUND TO THE SCOPING STUDY

2.1 CLIMATE

The region is arid, with an average annual rainfall of 222 mm and average annual evaporation of

2,960 mm. Evaporation exceeds rainfall for every month of the year.

Significant rainfall events occur due to cyclones and thunderstorms during the summer and autumn

months. There may be considerable variation in the areal distribution of rainfall during these events.



2.2 LOCAL SETTING

The Wallaby Deposit is located on the shore of Lake Carey (Figure 1). Surface areas are characterised

by low relief undulating aeolian sand dunes, with gentle footslopes to the lake surface. The water table

occurs within 0.3 to 4.0 m of the surface, with relief provided by the dunal terrain.

Lake Carey is a very large salt lake salinaland, that forms a regional sink for surface water and

groundwater flows. The lake surface is characterised by alluvial beds of silts and fine sands. Beneath

the surface, the superficial soils are gypsiferous, with common occurrence of large crystals of gypsum.

In general, the surface of the lake is not covered by a salt crust; though temporal crusts may occur

locally and selected areas may be more prone to the accumulation of salt due to isolation and/or limited

drainage/throughflow.

The surface of the lake is also host to innumerable islands in the form of low relief aeolian dunes that

vary significantly in shape and size. Some of the dunes are bedded onto outcrops or near-surface

subcrops of bedrock.

Available evidence suggests that there is a distinct fauna and flora community living within the Lake

Carey salinaland habitat. On the surface of the lake itself, it is understood that the life cycles of the

indigenous fauna (particularly brine shrimp) are linked to the local climate and the temporal inundation

of the lake with runoff from storm events.

All sampled groundwaters, from the project area are hypersaline. Included in the sampling were

groundwaters from the water table beneath the lake and the superficial formations on the lake shore.

After significant rainfall events, areas on the lake and those onshore that have high infiltration

capacities may be characterised by an ephemeral fresh to low salinity water table. It is expected that

these resources would be very limited and quickly degrade due to evaporative effects.

Due to their hypersaline nature, the shallow groundwater resources are thought not to be closely linked

to the habitat and life cycles of the fauna and flora of the lake. Consequently, the discharge of

hypersaline groundwater onto the lake may result in adverse environmental impacts to the lake surface

habitats and resident fauna communities.

2.3 CONCEPTUAL MINE DEWATERING REQUIREMENTS

A conceptual hydrogeological model of the aquifer systems identified within the Wallaby Deposit has

been developed to provide preliminary estimates of the mine dewatering requirements. Outputs from

the conceptual model provide semi-quantitative estimates of the volumes of hypersaline groundwater to

be disposed.

The form of the conceptual model is shown on Figure 2. Estimates of the mine dewatering

requirements are outlined in Table 1. These estimates are broadly based on the following assumptions:



• the palaeochannel formed by the Carey Palaeodrainage forms a continuous, uniform aquifer

throughout the model domain;

• the palaeochannel extends to a depth of about 100 m within the proposed Wallaby Pit;

• areas of the palaeochannel that occur within the proposed Wallaby Pit would be

dewatered/depressurised within about one year of the commencement of pumping; and

• the operating life-of-the-pit would be ten years and the dewatering would be continuous for this

duration.

Table 1

Conceptual Mine Dewatering Requirements

Dewatering Abstraction

Period of Pit Operation(L/s) (m3/day) Annual Volumes

(GL)

CumulativeGroundwater Volume

(GL)

Year 1 1,300 112,320 41.0 41.0

Year 2 740 63,936 23.2 64.2

Year 3 620 53,568 19.6 83.8

Year 4 570 49,248 18.0 101.8

Year 5 550 47,520 17.3 119.1

Year 6 520 44,928 16.4 135.5

Year 7 490 42,336 15.5 151.0

Year 8 470 40,608 14.8 165.8

Year 9 460 39,744 14.5 180.3

Year 10 460 39,744 14.5 194.8

We reiterate that the above dewatering requirements are preliminary and will change during the course

of formal feasibility study evaluations of the aquifer systems and dewatering designs.

2.4 SALT LOADINGS

Based on the available groundwater quality data, it is anticipated that the TDS concentration of

groundwater discharged by mine dewatering would average about 250,000 mg/L. Therefore, during the

first year of the dewatering programme approximately 10.3 million tonnes (10.3 Mt) of salt would be

contained in the discharged groundwater.

Over the proposed ten-year pit life, about 48.7 Mt (194,800,000,000 L x 250,000 mg/L x 10–15 =

48.7 Mt) of salt would be removed from storage within the local aquifer systems. Assuming an average

dry density of 1.36 t/m3 for the contained salt (Gorenc et al., 1984), this equates to a salt storage

volume requirement of approximately 35.8 x 106 m3.



2.5 PRELIMINARY MANAGEMENT OPTIONS

A total of twelve saline water management options have been identified by DC Blandford & Associates

Pty Ltd (July 1999). These include:

• discharge to the northwest Salinaland;

• joint discharge to the northwest Salinaland and Jupiter Pit;

• full containment on Lake Carey;

• full containment on the land;

• partial-temporal containment on Lake Carey;

• deep well injection within the Carey Palaeodrainage;

• discharge to nearby abandoned pits (Jupiter, Goanna, Granny and Windich);

• containment for harvesting of salt;

• discharge to Lake Carey;

• discharge to depressions on the surface of Lake Carey;

• direct discharge to Lake Carey with passive recharge; and

• desalination.

The general areas for discharge are identified on Figure 1.

Each of these options has been further investigated and developed as part of this study. Additional

options have also been investigated, including:

• shallow well injection;

• grout curtains within the palaeochannel aquifer, to limit throughflow of groundwater into the

pit; and

• combinations of selected options.

3. SCOPING STUDY DESIGN CRITERIA

The design criteria assumed for this scoping study are listed in Table 2.

Table 2

Scoping Study Design Criteria

Item Assumed Design Criteria

Wallaby Deposit Life-of-Mine 10 years

Groundwater TDS 250,000 mg/L

Dry Density of Contained Salt 1.36 t/m3

Average Annual Pan Evaporation (Laverton) 2,960 mm

Average Annual Rainfall (Laverton) 222 mm



Table 2 (cont’d)

Item Assumed Design Criteria

Wallaby Pit Annual Average Dewatering Rates Year 1 - 1,300 L/s

Year 2 - 740 L/s

Year 3 - 620 L/s

Year 4 - 570 L/s

Year 5 - 550 L/s

Year 6 - 520 L/s

Year 7 - 490 L/s

Year 8 - 470 L/s

Year 9 - 460 L/s

Year 10 - 460 L/s

Evaporation Factor (fraction of Epan) 0.4 for ponds, 0.7 for in-pit lakes

Evaporation Pond Design Flow 600 L/s

Jupiter Pit Storage Capacity 6.02 x 106 m3

Goanna Pit Storage Capacity 8.71 x 106 m3

Granny Pit Storage Capacity 17.91 x 106 m3

Windich Pit Storage Capacity 14.97 x 106 m3

4. SITE VISIT

A site visit of the proposed Wallaby Deposit and surrounding areas was conducted on Tuesday 20 July

1999. In attendance were Mr Ian Kerr from PGS and Messrs Joe Dwyer and Jason Fong from Dames

& Moore. The intent of the site visit was to ascertain the physical aspects of the Wallaby Deposit and

surrounds, thus gaining a perspective of the geotechnical and hydrological issues linked to the

respective management options.

The following areas were inspected during the site visit:

• the site of the proposed Wallaby Pit;

• clay pans adjacent to Wallaby Deposit;

• preferential drainage paths of catchments proximal to the Wallaby Deposit;

• available areas for a land-based storage facility and potential borrow areas;

• northeastern shoreline of Lake Carey;

• northwest Salinaland;

• TiTree Dam and catchment;

• Jupiter Pit; and

• Windich Pit.

Observations made during the site visit were incorporated into this scoping study document in

circumstances where factual data were unavailable.



5. HYPERSALINE GROUNDWATER MANAGEMENT OPTIONS

5.1 INTRODUCTION

The groundwater management options reviewed and specified in detail include:

• Option 1 – evaporation ponds.

This option has been subdivided into:

- Option 1a – land-based pond with minimum ground surface treatment;

- Option 1b – land-based pond with a compacted clay basal liner;

- Option 1c – Lake Carey based pond with minimum ground surface treatment; and

• Option 2 – northwest salinaland.

• Option 3 – discharge into abandoned pits, including:

- Option 3a – discharge into Jupiter Pit;

- Option 3b – discharge into Goanna, Granny and Windich pits at 200 L/s; and

- Option 3c – discharge into Goanna, Granny and Windich pits at 600 L/s.

• Option 4 – discharge onto Lake Carey, including:

- Option 4a – bunded areas on Lake Carey;

- Option 4b – direct discharge onto Lake Carey; and

- Option 4c – discharge into intra-islanded bunded areas.

• Option 5 – management by reinjection:

- Option 5a – deep well injection; and

- Option 5b – shallow well injection.

• Option 6 – construct palaeochannel barrier, including:

- Options 6a and 6b – grout curtains; and

- Option 6c – slurry cutoff wall.

• Option 7 – desalination.

• Option 8 – salt harvesting.

Each of these options is discussed in detail below.



5.2 OPTION 1 – EVAPORATION PONDS

5.2.1 General

Evaporation ponds typically comprise shallow cells to dispose of water in net evaporation

environments. For technical reasons, and to minimise cost, the following features are desirable for an

appropriate evaporation based facility site.

• reasonable proximity to the pit-perimeter dewatering infrastructure;

• underlain by clayey soils to minimise seepage;

• suitable topography to minimise the impact on surface and subsurface hydrology;

• suitable local fill material for bulk earthworks;

• outside the zone of influence of the dewatering bores to minimise recharge to the dewatering

zone; and

• access for light and heavy vehicles.

In addition, selection of the optimum site and pond design will take into consideration:

• seepage control;

• freeboard for extreme rainfall events and wave action;

• method of operation and site management; and

• decommissioning, rehabilitation and closure.

Hypersaline water discharge to a facility either based on land or on Lake Carey was investigated.

Evaporation pond systems considered included:

• Land based evaporation pond.

(i) Option 1a - minimum ground surface treatment.

(ii) Option 1b - compacted clay liner on pond base.

• Evaporation facility on Lake Carey:

(i) evaporation pond sited on Lake Carey:

Option 1c - minimum ground surface treatment.

(ii) discharge to an area on Lake Carey using natural high points (islands or shoreline) and

bunding for containment (Option 4 – Section 5.5).

A water balance developed for the evaporation pond system was used as the basis to size the facility.

The inputs into the system include the annual average design discharge from the mine dewatering and

average monthly rainfall. The outputs from the system include evaporation and subgrade seepage.



The following assumptions were made in the water balance calculations:

• average groundwater discharge rate of 600 L/s stored in pond system;

• a pan factor of 0.4 has been adopted as the design case for hypersaline waters and brines in

evaporation ponds;

• a pan factor of 0.7 has been adopted for hypersaline waters in lakes in abandoned pits;

• subgrade seepage is negligible;

• evaporation rate data from Laverton (Yamarna) station 012219 is representative of the

Wallaby site (due to lack of on-site information); and

• rainfall data from Laverton station 012045 is representative of the Wallaby site (due to lack of

on-site information).

Sensitivity analyses should be conducted during the detailed design phase as variations of the above

may markedly influence the final sizing of the pond.

An evaporation pan factor of 0.4 for evaporation ponds was selected following discussions with site

staff from WMC St Ives Gold. This figure is based on experience with evaporation studies conducted

by WMC on Lake Lefroy Salt Lake near Kambalda, Western Australia.

Evaporation is treated as the only loss from the system and therefore the evaporation pond sizing is

sensitive to the assumed pan factor. For example, a pan factor of 0.7 (as typically quoted for “fresh”

water) results in a 40% reduction in the evaporation pond area. It is recommended that hypersaline pan

factors be further investigated during detailed design in order to optimise any evaporation pond system

design.

At this stage there is insufficient data (relating to soil types and siting uncertainties) to undertake a

seepage analysis for the facility. To be conservative in sizing the facility, subgrade seepage was

assumed to be negligible in the water balance, however the conceptual design of the facility includes a

perimeter seepage interception trench drain.

The effects of greater than average rainfall on the facility were not fully assessed. However, a 1m

freeboard (the vertical distance between the top water level and the embankment crest under normal

operating conditions) has been included in the conceptual pond design to allow safe storage of extreme

rainfall events and prevent overtopping.

The water balance identified an area requirement of approximately 1,440 Ha (3.8 km x 3.8 km) for

discharge of the hypersaline water, either on land or on Lake Carey. The design pond is divided into

four equal sized cells to enhance evaporation effects. Figure 3 presents the conceptual size and

indicative locations of both evaporation pond systems. Typical embankment cross-sections for the

pond systems are shown on Figure 4.



The conceptual size and locations for the discharge of hypersaline water to a bunded area on Lake

Carey (Option 4) is presented on Figure 7. These areas are either already substantially more salt

affected than other areas of the lake (around Windich Creek), require a minimum fill volume for

bunding (islands to the south of Wallaby) or have small surface runoff contributing catchments.

5.2.2 Land-Based Facility (Options 1a and 1b)

(a) General Description

Two ground preparation cases were investigated. The first option (Option 1a) involved minimal ground

preparation (clear and grub, stripping topsoil and grade floor) and the second option (Option 1b)

involved the reworking of in situ clayey material to form a basal liner.

Option 1a will include the following:

• clear and grub storage area;

• strip topsoil from storage area and stockpile;

• grade pond-floor level;

• construct perimeter earthfill embankments;

• construct internal earthfill embankments;

• construct perimeter seepage interception and recovery works;

• install groundwater delivery system (pumps and pipes); and

• construct perimeter access track.

Option 1b includes all of the above items plus the following:

• rework in situ clayey material to form a basal liner.

Typical embankment for cross-sections for Options 1a and 1b are shown on Figure 4. A

comprehensive site selection assessment will be required during the detailed design phase to determine

the optimum location for the pond.

The groundwater delivery system will be the same for both options and will include:

• a duty and standby end-suction pump;

• φ 800 mm HDPE pipework from the pump to the centre of the pond system;

• a valve station within the pipework to control the discharge position; and

• telemetry controlled pipeline leak detection.



(b) Specifications

The pond system would be constructed by experienced contractors in accordance with standard

earthworks, piping and mechanical specifications, and the relevant regulatory requirements applicable

to mining facilities. This has been allowed for in the cost estimate.

(c) Operating Description

It is assumed that the mine dewatering pumps will discharge the groundwater to a manifold adjacent to

the Wallaby Pit. The groundwater will then be pumped from this manifold arrangement to the

evaporation pond via a single HDPE pipeline. The pipeline will be installed along the shoulder of the

access track and along the crest of one wall of the pond system to a multi-point discharge arrangement

in the pond centre.

Discharge into the separate ponds will be rotated on a regular cycle. A valve station at the multi-point

discharge will enable easy switching of discharge points. The discharge location will be rotated with

the objective to deposit thin layers of hypersaline groundwater at any one time, thus enhancing

evaporation.

Seepage from the pond system will be intercepted by a downstream trench collection system and

pumped back into the evaporation pond.

It is anticipated that the pond system will be able to be operated and monitored remotely. Control logic

will be programmed for the mutual operation of the pond and pit dewatering systems. The system will

operate 24 hours a day, all year round. It is anticipated that a full-time operator will only be required

during day-shift and pit operations staff will regularly check the pond system during night-shift.

Typical maintenance of the pond system would comprise normal pump, pipework and earthworks

maintenance.

(d) Decommissioning, Rehabilitation and Closure

For the purpose of this scoping study to determine closure cost estimates, it has been assumed that the

rehabilitation and closure requirements for tailing storage facilities (TSFs) are applicable to the

evaporation pond system. The Department of Minerals and Energy (DME) has prepared guidelines on

TSF rehabilitation and closure (1999). The DME require that a decommissioned TSF be safe, stable,

and aesthetically acceptable. To achieve this the DME recommends that:

• the outer walls be 20° or less and covered with a waste rock layer and drainage control to

minimise long-term erosion;

• the top surface be covered with a minimum 500 mm of suitable waste where saline process

water has been used, followed by spreading of topsoil and seeding;



• a self-regenerating cover be established; and

• measures to control dust, water erosion and contamination of surface and subsurface waters be

implemented

Decommissioning of the facility will involve removal of the delivery pipework and pumps.

The evaporation pond system outer embankment walls will be designed to be less than 20° and covered

with a 0.5 m thick layer of rockfill to reduce the requirement for earthworks as part of closure.

The evaporation ponds will contain at least 2 m of salt and it is considered a thicker capping layer (up

to 2m thick) may be required. The thicker capping would promote long-term protection of the facility

and limit capilliary rise of saline water into the root zone of the vegetation cover. A 2 m capping layer

(1.5 m clay and 0.5 m rock) has been allowed for in the cost estimate. Excess material removed and

stockpiled during grade levelling of the site will be utilised to cap the facility. Similarly, stripped

topsoil and cleared vegetation will be placed on top of the cap to assist in establishing a vegetation

community. Further studies, including liaison with the DME, into this area will be required to

determine the full extent of rehabilitation works required.

Stormwater diversion works constructed as part of the pond system will be designed to meet the criteria

storm flows considered acceptable for closure standards.

Seepage from the interception trenches is expected to reduce markedly shortly after decommissioning of

the evaporation pond. Flow from the system would be monitored following decommissioning to

determine the timeframe to close the facility. Closure of the seepage collection system would involve

plugging the collection pipes, removing the recovery pumps and reinstating the holding ponds.

(e) Cost Estimate

A summary of the costs associated with developing and constructing both land-based facility options is

included in Table 3. A detailed breakdown of the cost estimates is included in Appendix A.



Table 3

Cost Estimate for Land-Based Evaporation Pond

Cost EstimateItem

Option 1a Option 1b (clay liner)

Capital Construction 1 $27,000,000 $43,000,000

Design, Tender and Contract Administration2 $1,900,000 $3,000,000

Engineering Procurement and ConstructionManagement (EPCM)3 $2,700,000 $4,300,000

Operating 1 $6,000,000 $6,000,000

Decommissioning, Rehabilitation and Closure 1 $68,000,000 $68,000,000

TOTAL LIFE-OF-MINE ESTIMATE 1 $105,600,000($106M)

$124,300,,000($124M)

Notes: 1 Cost is + 25% accuracy.2 Allow 7% of capital cost of works, based on the Association of Consulting Engineering of Australia (ACEA) guidelines.3 Allow 10% of capital cost of works.

The cost estimate is considered to be +25% due to insufficient data on ground conditions, uncertainty

on the availability of suitable embankment construction materials, and uncertainty on facility siting.

The major cost associated with a land-based facility is the cost of bulk earthworks to prepare the base

area required for the evaporation pond. In order to reduce these earthworks costs, the facility could be

developed with an irregular shape to match existing contours and the cells constructed at different base

elevations. It was assumed in the cost estimates that an average 300 mm depth of cut across the

evaporation pond footprint would be required to provide a suitably level base. This material was

assumed to be suitable for use in the construction of internal and external embankments. However, the

volume of material required for embankment construction is less than the cut volume, therefore the

excess would be stockpiled for use in the closure works.

The seepage interception system was sized to intercept a maximum 25% of flow from one cell at any

given time. A detailed seepage analysis will be required to confirm this figure.

The capital cost estimate assumes no salt harvesting from the facility and that the salt will remain

stored in the ponds . There is scope to reduce the volume of embankment earthworks if salt can be

periodically removed from the evaporation pond cells.

The cost estimates include allowance for 5 km of pipework and the associated pumping and control

equipment to deliver the hypersaline water to the evaporation pond facility.

Capping the pond at closure is a major cost item. Should the cap thickness be reduced to 1 m (0.5 m

clay and 0.5 m rock) a saving of approximately $15 M would result.



5.2.3 Lake Carey Based Facility (Option 1c)


Construction of an evaporation pond on Lake Carey was investigated. Reworking of in situ clayey

material to form a basal liner was not considered feasible as the natural water table is close to the lake

surface.

Option 1c will include the following:

• construct perimeter earthfill embankment;

• construct internal earthfill embankments;

• install delivery system (pumps and pipes); and

• construct perimeter access track.

(b) Specifications

A lake-based facility will be constructed to the same specification as would apply for a land-based

facility (refer Section 5.2.2(b)).

(c) Operating Description

A lake-based facility would generally be operated in the same manner as a land-based facility (refer

Section 5.2.2(c)). However, a lake-based facility will not have a seepage interception and collection

system considering the high natural water table.

Access to the pond will be via an earthfill causeway constructed from Wallaby Pit across Lake Carey.


Decommissioning, rehabilitation and closure of a lake-based facility would be in accordance with

current regulatory requirements (refer Section 5.2.2(d)). It is however assumed that the pond surface

will not require capping and revegetation as part of the closure works. Closure of the system would

involve placing erosion protection on the pond downstream walls, removing the pumps and pipes and

removing the access causeway.

(e) Cost Estimate

A summary of the costs associated with developing and constructing the Lake Carey based facility is

included in Table 4. A detailed breakdown of the cost estimate is included in Appendix A.



Table 4

Cost Estimate for Lake Carey based Evaporation Pond

ItemCost Estimate

Option 1c

Capital Construction 1 $19,600,000

Design, Tender and Contract Administration2 $1,400,000

Engineering Procurement and Construction Management (EPCM)3 $2,000,000

Operating 1 $6,000,000

Decommissioning, Rehabilitation and Closure 1 $2,900,000

TOTAL ESTIMATE 1 $31,900,000($31.9M)

Notes: 1 Cost is +25% accuracy.2 Allow 7% of capital cost of works, based on the Association of Consulting Engineering of Australia (ACEA) guidelines.3 Allow 10% of capital cost of works.

The cost estimate is considered to be +25% due to insufficient data on ground conditions, uncertainty

on the availability of suitable embankment construction materials and uncertainty on facility siting.

The comparatively level areas available on Lake Carey relative to any land-based areas lead to a

minimisation of earthworks costs. A lake-based facility would be developed with a regular shape and

the cells constructed at similar base elevations. It was assumed that material for embankment

construction would be sourced from the surrounding Lake Carey area from a balanced cut-to-fill

activity.

It is anticipated that the water table beneath Lake Carey is close to the surface. Therefore, a seepage

cutoff trench and interception drain may not be the most appropriate option for seepage control.

Further geotechnical site investigations of the Lake Carey subsurface will be required to determine the

most appropriate seepage control method. Therefore, a cost component for seepage control has not

been included in this cost estimate. It is recommended that a detailed seepage analysis be undertaken

during the detailed design phase in order to clearly define this requirement.

The capital cost estimate assumes no salt harvesting from the facility and that the salt will remain

stored in the ponds. There is scope to reduce the volume of embankment earthworks if salt is

periodically removed from the evaporation pond cells.

The cost estimates include allowance for 5 km of pipework and the associated pumping and control

equipment to deliver the hypersaline water to the evaporation pond facility.



5.2.4 Discussion

Consideration of costs alone favour discharge of the hypersaline groundwater to a facility on Lake

Carey. The difference in the costs of land-based discharge compared to discharge to Lake Carey arises

from the substantial cost associated with clearing and grubbing activities and bulk earthworks for the

former option. Due to the greater relief and presence of scrub and other vegetation, any land-based

facility would require more surface preparation and earthworks than a facility sited on Lake Carey,

which is relatively flat. In order to minimise the volume of earthworks associated with a land-based

facility, the individual cells of the evaporation pond may have irregular shapes and different base

elevations. This may have some management implications, and would involve increased operator

involvement.

At this stage, the cost of the Lake Carey based evaporation pond facility is not fully representative of

likely capital costs as seepage management has not been included. Further work to assess the quantity

of seepage is required before seepage control options can be developed and costed. However, it is

anticipated that seepage control for an evaporation pond on Lake Carey is unlikely to raise the cost to

that of the land-based evaporation pond.

Management of the salt load remaining in the evaporation ponds presents both short-term and long-term

issues. In the short-term increasing salt loads may reduce the evaporation efficiency of the system.

Rehabilitation and closure of a salt stockpile presents long-term implications. These will require

additional studies. Broad options that may be applicable for long-term management of the salt include:

• salt harvesting;

• encapsulating by containment cap; and

• leave salt surface exposed.

Issues associated with salt harvesting for commercial sale are discussed in detail in Section 5.8 and

include:

• the salt would have to be sold overseas because of insufficient demand from the local market;

• transportation costs are high;

• transport infrastructure is currently inadequate;

• high up-front capital costs to establish the processing system; and

• the harvesting operation would have to be a long-term activity to be economically viable.

An alternative is to contain the salt in the evaporation ponds and cap the salt column at the end of the

mine life to shed rainfall runoff and minimise seepage through the salt. The construction cost to cap the

ponds presented in Table 3 includes hauling, end-dumping and spreading of the capping material. This

cost could be in the order of up to 60% greater if a more rigorously designed and constructed lowpermeability cap is required by the regulators to minimise seepage through the salt column.



The lowest cost option would be to leave the salt surface exposed to the environment. This raises a

number of environmental issues and would require detailed studies and liaison with the regulators.

In order to refine the costings associated with the evaporation pond facility options, additional work is

required on:

• investigation into appropriate evaporation pan factors and their fluctuation with changing TDS

concentrations;

• geotechnical and hydrogeological site investigations (with laboratory testing) of proposed

facility locations to identify suitable soils for construction, parameters for embankment design

and groundwater conditions;

• determining the level of closure works required (as acceptable to the regulatory agencies);

• detailed seepage analyses;

• investigation of appropriate capping designs; and

• detailed topographic survey of the area proposed for the chosen facility.

The above work does not take into account siting considerations based on environmental or

ethnographic issues.

5.3 OPTION 2 – DISCHARGE TO NORTHWEST SALINALAND

DC Blandford & Associates (1999) proposed this disposal option where groundwater is discharged to

the series of saline playas making up the northwest salinaland, allowed to move through the system,

eventually entering Lake Carey (Figure 5). The groundwater would enter the system through the

biggest of the depressions and then be controlled by low-key engineering works, eventually entering

Lake Carey via the existing inlet.

The total surface area of the saline playas is approximately 75.6 Ha and, at an estimated average

storage depth of 1.5 m, the available storage in the northwest salinaland is estimated to be

approximately 1.1 x 106 m3. This storage capacity is substantially less than the total salt storage

capacity required for a long-term discharge option (35.8 x 106m3); and would only provide

approximately 20 days storage based on the averaged mine dewatering discharge requirements. The

system would require an overflow through an outfall onto Lake Carey. The outfall would operate

continuously, discharging the hypersaline water directly onto Lake Carey. This option is therefore very

similar to the Lake Carey direct discharge option (Option 4b) presented in Section 5.4.2.

Minimum earthworks construction is required for this option, however their is uncertainty on the extent

of rehabilitation works required to the affected Lake Carey area.

The life-of-mine cost estimate, assuming no rehabilitation works are required on Lake Carey, is

approximately $10 M. A detailed breakdown of this cost estimate is included in Appendix A.



As this system offers limited storage capacity it is not considered economically attractive as a

management option. However, it may be possible to utilise this option for long-term discharge of low

flows in conjunction with other management options, such as an evaporation pond.

5.4 OPTION 3 – DISCHARGE INTO ABANDONED PITS

5.4.1 General

Abandoned pits nearby to the Wallaby Deposit provide potential ready-made containment facilities for

discharge of mine dewatering discharge. The local abandoned pits investigated to quantify their storage

potential include:

• Jupiter Pit (of the Mt Morgans Mine); and

• Goanna, Granny and Windich pits of the Granny Smith Mine.

The location of these pits is shown on Figure 1.

In order to define the available storage in each pit, data have been collated on:

• final void volumes based on survey records that define surface areas at various elevations;

• estimated overflow elevations based on surface topography;

• estimates of the elevations to which each pit is presently inundated; and

• groundwater abstraction records during pit development, to estimate the potential of the local

aquifers to transmit the disposed groundwater laterally from the pit.

The collated data have subsequently been manipulated to quantify incremental storage volumes within

each pit and to provide estimates of:

• rates of groundwater discharge that are sustainable for ten years;

• surge capacities of the pits for disposal of an average discharge of 600 L/s (51,840 kL/day);

• rates of groundwater discharge that are sustainable for a one-year period; and

• rates of groundwater discharge that are sustainable for a two-year period.

An evaporative rate of 2.1 m/annum (Laverton Epan x 0.7) has been applied to these assessments to

account for water losses from the surfaces of the in-pit lakes. Results of this work are outlined in

Table 5.

5.4.2 Discharge into Jupiter Pit (Option 3a)

The Jupiter Pit operated as a satellite of the Mt Morgans Gold Operations. The decommissioned pit

has approximately 6 x 106 m3 storage capacity.

The alternative discharge flow rates investigated for this option are detailed in Table 5.



Table 5

Viable Discharge in Abandoned Pits

Short-term Sustainable Discharge 1

Abandoned Pit Final Void Volume(m3)

Current LakeLevel

(m AHD)

Current LakeVolume

(m3)

Long-term SustainableDischarge

(no seepage to localaquifers, L/s)

Long-term SustainableDischarge1

(with estimated seepage tolocal aquifers, L/s)

One-Year Period(L/s)

Two-Year Period(L/s)

Capability to dispose of600 L/s Flows

(days)

Jupiter 6.02 x 106 291 0.01 x 106 24 24 200 104 116

Goanna 8.71 x 106 364 0.85 x 106 36 48 266 139 168

Granny 17.91 x 106 335 2.73 x 106 63 98 509 263 346

Windich 14.97 x 106 335 1.30 x 106 57 67 451 231 289

Notes: 1 Estimates of seepage to the local aquifers have been derived based on dewatering abstraction records and include:• Jupiter Pit – nil;• Goanna Pit – 11.6 L/s (about 1,000 kL/day);• Granny Pit – 35 L/s (about 3,025 kL/day); and• Windich Pit – 10 L/s (about 865 kL/day).



It is considered that in order to maximise the benefit of disposing groundwater into Jupiter Pit, the

storage life of the pit should be maximised. Therefore, the options of filling Jupiter Pit within the first

two years of operation are not considered further.

The remaining option (sustain flow over life-of-mine) is further split into the following two alternatives:

• Option 3a/1 - upgrade existing access tracks and install pipework along these tracks.

• Option 3a/2 - construct new access tracks and install pipework along these tracks.

The above options are shown schematically on Figure 6.

The length of pipework, and access track required to be upgraded, for Option 3a/1 is approximately

16 km. Tracks for Option 3a/2 will be constructed directly to Jupiter Pit for a total length of

approximately 10 km. An allowance for access track construction over clay pans has been included for

Option 3a/2.

A HDPE welded pipe of φ 355 mm and φ 280 mm has been sized to deliver the sustained flow of 24 L/s

for each option.

As part of decommissioning it has been assumed that the pump and pipeworks will be removed and the

access track regraded and repaired for use by locals. No closure works associated with the Jupiter Pit

have been allowed for.

A summary of the total costs for Options 3a/1 and 3a/2 is included in Table 6. A detailed cost

breakdown for both options is included in Appendix A. It must be borne in mind that an additional

groundwater storage facility is required to operate in conjunction with either of these options.

5.4.3 Discharge into Goanna, Granny and Windich Pits (Options 3b and 3c)

(a) Option 3b – 200 L/s Long-Term Sustainable Discharge

The combined long-term sustainable discharge to the Goanna, Granny and Windich Pits is

approximately 200 L/s.

This management option is shown schematically on Figure 6. The length of pipework, and access track

required to be upgraded, for this option is approximately 15 km. To deliver this flow a φ 450 mm

HDPE welded pipe has been sized.

Decommissioning, rehabilitation and closure requirements would be the same as assumed for discharge

options into Jupiter Pit (refer Section 5.4.2).



A summary of the total cost for Option 3b is included in Table 6. A detailed breakdown of this

estimate is attached in Appendix A.

Table 6

Cost Estimate for Discharge into Abandoned Pits

Cost Estimate

Jupiter Pit Goanna, Granny and Windich PitsItem

Option 3a/1 4 Option 3a/2 4 Option 3b 5 Option 3c 6

Capital Construction 1 $1,800,000 $950,000 $2,600,000 $5,200,000

Design, Tender and Contract Administration 2 $130,000 $70,000 $180,000 $400,000

Engineering Procurement and ConstructionManagement (EPCM) 3

$180,000 $100,000 $260,000 $500,000

Operating 1 $1,450,000 $1,450,000 $2,300,000 $620,000

Decommissioning, Rehabilitation and Closure 1 $240,000 $150,000 $225,000 $225,000

TOTAL ESTIMATE 1 $3,800,000($3.8 M)

$2,720,000($2.7 M)

$5,565,000($5.6M)

$6,945,000(6.9M)

Notes: 1 Capital cost is + 25% accuracy.2 Allow 7% of capital cost of works, based on the Association of Consulting Engineering of Australia (ACEA) guidelines.3 Allow 10% of capital cost of works.4 Sustained discharge of 24 L/s into the Jupiter Pit.5 Sustained discharge of 200 L/s into the Goanna, Granny and Windich pits.6 Discharge of 600 L/s into the Goanna, Granny and Windich pits.

(b) Option 3c – 600 L/s Discharge Rate

At 600 L/s discharge rate, the Goanna, Granny and Windich pits will provide approximately 2.2 years

storage capacity (refer Table 5).

A φ 800 mm HDPE pipe will be required to deliver this flow. The pipework route, access track

requirements and closure works would be the same as per Option 3b.

A summary of the total cost for Option 3c is included in Table 6 and a detailed breakdown is attached

in Appendix A.

5.4.4 Discussion

Results from the water balance studies involving the abandoned pits indicate:

• The Jupiter Pit is of limited value as a management option and is the least preferred of the four

pits (Option 3a).

• The combined long-term sustainable discharge to the Goanna, Granny and Windich pits is

about 200 L/s and consequently would provide for about 32% of the total requirement (Option

3b).



• With co-disposal, the Goanna, Granny and Windich pits could provide:

- disposal of all year-one mine dewatering discharge;

- surge capacity, for emergency discharge for at least several months; and

- surge capacity during long-term sustainable discharge over the 10-year project life.

• At 600 L/s discharge rate the Goanna, Granny and Windich pits would provide storage

capacity for about 2.2 years (Option 3c).

Notwithstanding the above results, the feasibility of using the Goanna, Granny and Windich pits for

discharge of hypersaline groundwater needs to be reviewed in connection with an existing tripartite

Water Agreement between Ashton Mining Limited, Wesfarmers CSBP Limited and PGS. The Water

Agreement is linked to groundwater abstraction from the Mt Weld carbonatite, for process water

supplies, by the Granny Smith Project. Aspects of this Agreement that need review relate to

commitments to divert Windich Creek into the Windich Pit to capture and store good quality (low

salinity) streamflow. The inundation of Windich Pit, and due to their proximity the Granny and

Goanna pits with hypersaline groundwater, may contravene this part of the Agreement.

5.5 OPTION 4 – DISCHARGE ONTO LAKE CAREY

5.5.1 Bunded Areas on Lake Carey (Option 4a)


We are unaware of any guidelines or criteria that define preferred areas for discharge onto Lake Carey.

Subsequently, in order to develop this option, the topography of Lake Carey and areas of catchments in

the hinterland of selected potential discharge sites have been investigated. The intent of these

investigations has been to select potential discharge sites based on:

• areas already influenced and/or degraded by discharge of minewater;

• areas or embayments on the lake surface with small runoff catchments; and

• areas or embayments on the lake surface with comparatively large runoff catchments.

The areas with small runoff catchments could potentially be isolated from the lake hydrology and be

fully contained (except for seepage under the bund walls).

Areas with comparatively large runoff catchments could host discharge facilities that promote flushing

by runoff from significant storm events.

Both options require bunded areas of approximately 3,500 to 4,000 Ha to accommodate containment

of salt residues for a ten-year project life. The facility would most likely be developed with an

irregular shape to take advantage of natural features that would be included as a part of the



containment structure. Detailed survey of the local lake areas would be required to determine

containment volumes and the extent of flooding for the shore based sites.

Potential discharge areas in reasonable proximity to the Wallaby Deposit that have been identified are

shown on Figure 7a and include:

• Mt Margaret lake-shore, a low-runoff domain;

• Pike Hill lake-shore, a low-runoff domain;

• Windich Creek outfall, a high-runoff domain already influenced by historical discharge from

the Granny Smith Mine; and

• Cement Creek outfall, a high-runoff domain and embayment on the northwest shore of Lake

Carey.

This option will include the following:

• perimeter earthfill bunds;

• minor floor regrading;

• delivery system (pumps and pipes);

• access causeway from Wallaby deposit; and

• crest access track.

Ground preparation has been assumed to be limited to the areas forming the foundations to the

embankments. It is assumed that no surface treatment of the discharge area is required. The delivery

pipe will be installed along the lake surface to the discharge area.

The Pike Hill lake-shore and Windich Creek outfall discharge sites also may be supported by shallow

well injection in onshore dunal terrain that fringes the lake.

(b) Specifications

The mine dewatering pumps will deliver the groundwater to a manifold adjacent to the Wallaby Pit.

The groundwater will be pumped to the bunded area via a single HDPE pipeline from the manifold.

Where possible, the delivery pipeline will be aligned along the shoreline of the lake to minimise bedding

works. At the bunded area, the pipe will be run along the shoulder of the access track and along the

perimeter wall crest to a discharge point at the edge of the facility.

Water levels will be limited to less than 1m (due to restrictions in the elevation of dune crests/islands

above the lake base levels). The system will operate 24 hours a day, all year round. It is anticipated

that a full-time operator will only be required during day-shift and pit operations staff will regularly

check the pond system during night-shift.

Maintenance of the facility is anticipated to be generally limited to normal pump, pipework and

earthworks maintenance.




At this stage, options for rehabilitation range from a “do nothing” option to closure in accordance with

DME guidelines for tailings storage facilities (1999). Flood events, seepage and wind action could

prove to be significant factors in reducing the quantity of retained salt in the facility over the life of the

mine. The remainder of the salt could be left to dissipate naturally due to exposure to the environment.

Regardless of the rate of release of salt, the total quantity released could have significant environmental

impacts. It is recommended that the potential environmental impacts be investigated further before

pursuing this closure option.

(e) Cost Estimate

The major cost associated with a bunded area on Lake Carey is bund wall construction. It was

assumed that material for bund wall construction would be sourced from the surrounding Lake Carey

area from a balanced cut-to-fill activity.

No seepage control has been included in this conceptual design due to the anticipated high water table

beneath Lake Carey.

The capital cost estimate for this option assumes that there will be no salt harvesting from the facility

and that the salt will generally remain at the base of the ponds. Closure works have been assumed to be

limited to removing pipework and removing the bund walls. It was assumed that neither capping nor

revegetation would be required.

The cost estimates have allowed for 10 km of pipework and the associated pumping and control

equipment to deliver the hypersaline water to the facility.

A summary of the costs associated with developing and constructing a bunded discharge facility on

Lake Carey is included in Table 7.



Table 7

Cost Estimate for Discharge onto Lake Carey

Cost EstimateItem

Option 4a Option 4b Option 4c

Capital Construction 1 $4,800,000 $2,200,000 $5,400,000

Design, Tender and Contract Administration2 $330,000 $154,000 $400,000

Engineering Procurement and ConstructionManagement (EPCM)3 $480,000 $220,000 $500,000

Operating 1 $6,000,000 $6,000,000 $6,000,000

Decommissioning, Rehabilitation and Closure 1 $1,300,000 $200,000 $900,000

TOTAL ESTIMATE 1$12,910,000

($12.9 M)$8,774,000

($8.8 M)$13,200,000

($13.2 M)

Notes: 1 Cost is +25% accuracy.2 Allow 7% of capital cost of works, based on the Association of Consulting Engineering of Australia (ACEA) guidelines.3 Allow 10% of capital cost of works.

5.5.2 Direct Discharge onto Lake Carey (Option 4b)

The option of direct discharge into Lake Carey was investigated. This discharge option includes a duty

and standby pump and 5 km of pipeline installed along the Lake Carey shoreline to an outfall position.

No embankments are required and the only earthworks is the construction of an access track adjacent to

the delivery pipe. This option is shown schematically on Figure 7b.

The discharged groundwater pool position will not be controlled and its behaviour is uncertain.

Extensive bathymetric studies of the lake terrain will need to be undertaken in order to determine the

likely movement of discharge water on the lake surface.

Similarly, extensive environmental studies will be required for this option to determine likely impacts of

the groundwater discharge on the lake ecosystem.

It was assumed that the only closure works required for this option will be minor earthworks to

reinstate the outfall, regrading the access track and removal of the delivery pipe. It was further

assumed that rehabilitation works associated with the area affected on Lake Carey by the groundwater

discharge will not be required. This assumption will need to be reviewed as part of the detailed

environmental and bathymetric studies of the lake.

The total cost for this option is summarised in Table 7, and a detailed breakdown is included in

Appendix A.



5.5.3 Discharge into Intra-Island Bunded Areas on Lake Carey (Option 4c)

A similar system to option 4a would be to utilise islands within Lake Carey bridged together with bund

walls to form a groundwater discharge area. An area that has sufficient storage capacity has been

identified close to the Wallaby deposit and within mining leases held by PGS. This option is shown

schematically on Figure 7c.

A 5 km causeway constructed across Lake Carey from the Wallaby Pit will be required for access to

the discharge area and to carry the delivery pipeline. Approximately 17 km of bund wall will be

required to bridge the islands and form a closed cell. Borrow material for the construction works have

been assumed to be locally available from the islands. No seepage control works have been included in

the conceptual design. A typical cross-section of the perimeter bund wall is included on Figure 8.

The groundwater will be delivered via a single HDPE pipeline with a single discharge outfall into the

disposal cell.

As per options 4a and 4b, capping and revegetation of the salt surface stored within the cell is assumed

to be not required. This will need to be confirmed with the regulators following further environmental

studies. Closure works have been limited to removal of the delivery pipe, removal of the access

causeway and placement of erosion protection on the downstream bund wall.

The total cost for option 4c is summarised in Table 7, and a detailed breakdown is included in

Appendix A.

5.5.4 Discussion

Discharge of hypersaline water onto Lake Carey is a lower cost option than discharge to a similar

evaporation pond facility on Lake Carey. In addition, the shallow storage depth (of salt and water)

associated with these options may allow salt to be dissipated by the effects of flooding and wind

associated with extreme rainfall events. This may result in lower operating and rehabilitation costs but

the tradeoff is the increased area required for this option compared to an evaporation pond option.

Site observation suggests that the difference between the base of the bunded area and the top elevation

of the natural containment features is generally small, say 1 to 2 m (as determined by the level of

existing dune or island levels). Therefore, the area needed for full containment of the hypersaline water

within a lake bunded area is estimated to be up to three times the area required for the evaporation

ponds. The required area could be reduced if higher bunds are developed. According to information

from PGS, the islands on Lake Carey provide between 2 to 25 m relief. The ‘useable’ relief is

considered to be practically between 2 and 5 m for storage purposes. This option allows overflow onto

Lake Carey following extreme rainfall events.



Further environmental and bathymetric studies are required on these options to determine the extent of

any impacts and the rehabilitation and closure requirements acceptable to the regulators. Following

these studies a more accurate estimate on the total cost for these options could be made.

5.6 OPTION 5 – DISPOSAL BY REINJECTION

5.6.1 General

Two re-injection concepts, that explore the feasibility of disposal systems, have been investigated.

These concepts include reinjection into:

• the palaeochannel aquifer formed by the Carey Palaeodrainage; and

• dunal superficial formations on the shore of Lake Carey.

The known and inferred distribution of these domains are shown on Figure 9.

The palaeochannel aquifer has been targetted because of its significant regional extent and known local

high transmissivity, albeit the aquifer profile is fully saturated and confined. Discharge to the

superficial dunal terrain is considered because the unsaturated profile above the current water table is

expected to be transmissive and able to provide some storage capacity. The shallow depths of the water

table in the vicinity of Lake Carey is a linking factor with both re-injection options.

Other areas that are further from the lake and higher in the catchment, thus with a deeper water table

setting, may exist. However, local experience shows that most of the superficial formations and

weathered bedrock profiles above the water table have high clay contents and poor transmissivity.

Formations of this nature would not be conducive to re-injection process.

The concepts for re-injection are not new and have been investigated at numerous sites around the

world, principally as a means of artificially recharging depleted aquifer zones and oil reservoirs. It is

broadly recognised that re-injection systems are constrained by operating factors that limit rates of

recharge in individual bores. One or more physical, chemical or biological factors leads to clogging of

screened aquifer zones to restrict flow. These factors vary from site to site depending on local

conditions, groundwater quality and aquifer characteristics. They might include:

• filtration of suspended solids and particulate matter;

• microbial growth;

• chemical precipitation;

• air entrapment and gaseous binding; and

• mobilisation of aquifer fines.

In order to develop an understanding of the behaviour of the local aquifers under re-injection conditions

a suite of field tests were conducted. The tests involved three bores:



• one screened only within the palaeochannel;

• one screened throughout the superficial formations; and

• one screened throughout the superficial formations, palaeochannel and bedrock regolith aquifer

zones.

Results from the tests have been applied to determine:

• rates of injection that are sustainable in the short-term;

• static pressures that are required to achieve the nominal rates of sustainable injection; and

• bore designs that would need to be implemented to direct the re-injected groundwaters into

appropriate aquifers.

To evaluate the local feasibility of the two re-injection options, the conceptual hydrogeological model of

the Wallaby Deposit has been expanded and modified. To accommodate the deep-well re-injection this

model has been extended to 80 km south of the Wallaby Deposit. For the shallow re-injection option,

the surface layers of the model have been redefined to include:

• the topography of the dunal terrain on the northern and northeastern shore of Lake Carey;

• estimates of the regional water table elevations and gradient; and

• unsaturated dunal sands.

Results of the predictive modelling are outlined below. In evaluating the results it needs to be

understood that:

• evaporative effects are not included in the modelled water balances, largely because rates of

evaporation from the lake surface and dunal terrain are unknown;

• the local aquifer systems are in a steady-state and fully saturated, the consequence of which

being there is very limited storage available for additional groundwater proposed to be

introduced by re-injection; and

• the local water table occurs at very shallow depths, significantly limiting the storage that might

be available within the unsaturated profile.

5.6.2 Deep Well Injection (Option 5a)

The deep well injection option is based on the re-injection of mine dewatering discharge into the

palaeochannel formed by the Carey Palaeodrainage.

Key design aspects for this option include:

• The re-injection of groundwater into the palaeochannel aquifer only. Flow into other (shallow)

aquifer zones would need to be prevented by the construction of injection bores with the

annulus around the casing grouted from the top of the palaeochannel aquifer to the surface.



• Location of the injection bores at least 5 km from the Wallaby Deposit so that the dewatering

system is not compromised and the flow of injected groundwater towards the Wallaby Deposit

is minimised.

The recirculation of some groundwater would appear to be unavoidable; the designs therefore

attempt to minimise recirculation volumes during the first year and promote re-injection into

the more distant injection bores during subsequent periods.

• Location of the injection bores within the palaeochannel aquifer and downstream of the

Wallaby Deposit. The conceptual hydrogeological model and palaeochannel aquifer profiles

are assumed to be continuous and consistent to 80 km south of the Wallaby Deposit. Known

intersections of the palaeochannel aquifer provide data to a distance of about 20 km south of

the Wallaby Deposit. These data show the palaeochannel aquifer occurs beneath Lake Carey.

• Limiting of the mounding of groundwater levels and rise of the water table. Due to the shallow

water table beneath the lake and in near-shore dunal terrain, a groundwater mound of 2 m has

been assumed the maximum acceptable.

The number of injection wells required to dispose of the mine dewatering discharge is

controlled by the permissible increase in water table elevations and not by the rate of injection.

Results of groundwater flow modelling based on the outlined design aspects indicate that in order to

adequately distribute the averaged mine dewatering discharge within the palaeochannel domain, a large

number of injection wells with relatively small injection rates would be required. The most significant

aspects of the design borefield include:

• A total of 175 injection bores disposed over distances from 5 to 54 km south of the Wallaby

Deposit.

• Spacing of the injection bores on a regular gridded pattern that incorporates:

- seven injection bores per linear 2 km of the palaeochannel aquifer;

- bore spacings of 250 m across the channel;

- bore spacings of 1,000 m along the channel;

- bores grouped in threes and fours on alternate grids; and

- bores laterally off-set from one grid to another.

• Injection bores screened from the top of the palaeochannel aquifer to fresh bedrock.

• A maximum injection rate of 3.5 L/s (300 kL/day) for individual injection bores. This rate of

injection is conservatively low-based on the results of short-term field trials. The low design



rate is intended to provide some security to the design to accommodate operating factors that

may limit the longer-term discharge to individual bores.

The design injection bore specifications are summarised in Table 8, the borefield layout is shown on

Figures 10 and 11. Results of the predictive simulations showing head distribution due to the

groundwater discharge by re-injection into the palaeochannel aquifer are shown on Figure 12.

Table 8

Modelled Deep Well Injection

RunTime

(years)Number of

Injection Wells

Rates ofInjection(m3/day)

Injection Scenarios

B 0 0 0 Pit abstraction only.

RRB1 0 – 10 175 300 Stead-state injection.

RB2 0 – 1

1 – 2

1 – 1.5

1.5 – 6

6 – 7

7 – 10

175

175

112

85

71

70

300

300

300

300

300

300

Total injection matches averaged abstraction, with decreasingnumber of injection wells during the project duration.

RB3 0 – 1

1 – 2

2 – 3

3 – 6

6 – 7

7 – 10

175

175

175

175

175

175

300

150

140

125

112.5

105

Total injection matches averaged abstraction, with decreasinginjection rates during the project duration.

Analyses of the deep well injection simulations show:

• Most (50 to 60%) of the disposed groundwater is transmitted from the palaeochannel aquifer,

where it is injected, to the unsaturated profile above the water table. Mounding of the water

table over a very large area is the outcome of this.

In the model, the transmission of groundwater from the palaeochannel aquifer to the superficial

formations predominantly occurs within the weathered bedrock domains and areas not overlain

by thick profiles of transported clays.

• Significant volumes are likely to be transmitted, within the palaeochannel aquifer, back to the

dewatering borefield and consequently recycled through the discharge system. These volumes

have not been quantified, but during the later years of the dewatering programme would be

expected to contribute significantly to the aggregate abstraction.



• Increased heads in the confined aquifer zones formed by the palaeochannel and bedrock

domains contribute to the storage of the disposed groundwater.

Based on the model simulations, the following conclusions have been drawn regarding the feasibility of

deep well injections within the palaeochannel aquifer:

• This option is not viable. Intrinsically (because the aquifer systems are fully saturated) the

groundwater disposed by this means would:

- be transmitted to the water table below the lake surface; and

- be transmitted to and recycled by the dewatering system.

• In practice, it is anticipated that groundwater flow would be preferentially transmitted from the

palaeochannel aquifer to the water table by permeable structures within the bedrock and

saprolite profiles. This would provide excessive, localised mounding of the water table and

discharge of hypersaline groundwater onto the surface of Lake Carey.

• The design injection system would compromise significant areas of the Lake Carey salinaland

due to construction of roadways for pipelines and access to the injection bores.

Roadways above would occupy about 1,000 Ha and the total system would significantly alter

the lake bathymetry and hydrology characteristics over an area of about 60,000 Ha. Included

in this area are isolated cells for containment of spillages.

The total cost of the deep well injection option is summarised in Table 9, and a detailed breakdown of

the cost is included in Appendix A.

Table 9

Cost Estimate for Deep Well and Shallow Well Injection

Cost EstimateItem

Option 5a Option 5b

Capital Construction 1 $45,000,000 $32,500,000

Design, Tender and Contract Administration 2 $3,200,000 $2,300,000

Engineering Procurement and Construction Management(EPCM) 3

$4,500,000 $3,300,000

Operating 1 $6,000,000 $6,000,000

Decommissioning, Rehabilitation and Closure 1 $10,800,000 $4,800,000

TOTAL LIFE-OF-MINE ESTIMATE 1 $69,500,000($69.5M)

$48,900,000($48.9M)




5.6.3 Shallow Well Injection (Option 5b)

This option seeks to utilise the available storage within the unsaturated superficial formations (above

the water table) for hypersaline groundwater discharge. The water table is generally within 1 m of the

ground surface in low-lying areas close to the lakeshore. However, in local dunal terrain further from

the shoreline, the depth to groundwater increases to 3 to 5 m. The shallow injection domain is shown

on Figure 13.

Groundwater would be disposed into the nearshore dunal terrain through a network of shallow injection

bores or a subsurface irrigation pipeline. Re-injection would need to be balanced and limited by:

• water table rise to within 1 m of the ground surface within the dunal terrain; and

• lateral flow and discharge from the dunal terrain near or on the lake shore.

Once the available storage has been utilised, subsequent re-injection rates would be balanced by

evaporative losses and seepage to deeper aquifer zones.

In order to evaluate the storage potential of the shallow well injection option, several aspects of the

dunal terrain were investigated. These investigations included and provided insight to:

• Review of the areal distribution of the dunal terrain based on mapping of the surface geology

(Laverton 1:250,000 Geology Sheet) and air-photo interpretation.

• Modelling of a typical section of the dunal terrain, with a small network of injection bores re-

injecting nominal groundwater volumes. The modelling showed the progressive depletion of the

unsaturated storage areas as the water table mounded in the vicinity of the injection bores.

Model parameters for the dunal sediments were estimated as no data are available.

Results of the modelling reaffirm the storage concepts and the need to balance injection rates

with evaporation and throughflow to avoid discharge of hypersaline groundwater onto the

surface.

• Calculation (Table 10) of minimum dunal terrain land areas required for the shallow well

injection system to accommodate the mine dewatering discharge. A number of assumptions

have been made in these calculations, including:

- the topography and water table in a 5,720 m length by 830 m average width strip of

dunal terrain (as depicted on Figure 12) are broadly representative of all nearshore

dunal terrains;

- evaporation from the water table at 1 m below ground surface is 1.5 m per year;

- the specific yield of the dunal terrain is 0.30;



- the water table would uniformly rise to within 1 m of ground surface, without affecting

near-lakeshore vegetation due to initial re-injection; and

- the injection bore network or subsurface irrigation pipeline can be managed so that

there is no discharge of groundwater at the surface near the lake shore or within

interdunal depressions.

Table 10

Minimum Area of Dunal Terrain Required for Shallow Injection

Calculation Parameters

Specific yield of dunal terrain 0.30

Net annual evaporation 1.5 m

Length of dunal strip 5,720 m

Average width of dunal strip 830 m

Volume of dunal strip 11,201,600 m3

Total surface area of dunal strip 4,747,600

Year One Storage Capacity

AVAILABLE GROUNDWATER STORAGE

(Volume of dunal terrain x specific yield) 3,360,480 m3

ANNUAL EVAPORATION LOSS

(Net annual evaporation x Total surface area of dunal strip x Specific yield) 2,131,920 m3

TOTAL FIRST YEAR INJECTION

(Available groundwater storage + Annual evaporation loss) 15,048 m3/day) (174L/s)

Storage Capacity in Subsequent Years

AVAILABLE STORAGE PROVIDED BY ANNUAL EVAPORATION

(Net annual evaporation x Total surface area of dunal strip x Specific yield) 2,131,020 m3

INJECTION RATE

(Available groundwater storage + Annual evaporation loss) 5,840 m3/day(67 L/s)

Minimum Areas Required

Area of dunal terrain required to dispose of 600 L/s averaged abstraction 16,370,000 m2

Volume for discharge in subsequent years in a 4.1 km x 4 km dunal area20,182 m3/day

(234 L/s)

It should be emphasised that the calculations refer to a minimum area required because:

• areal variations in the physical and hydrogeological characteristics of the dunal terrain are

unknown;

• the net water balance of the dunal terrain is unknown;

• 1.5 m of annual evaporation from the soil profile at 1 m depth may be unrealistically high;

• the transmissivity of the dunal terrain is unknown, leading to uncertainty regarding the shape of

the artificial recharge mounds and lateral flow rates; and



• the feasibility of managing the injections so that groundwater mounds are maintained at 1 m

below ground surface, without lateral discharge of groundwater into interdunal depressions on

the lake shore, remains poorly quantified.

The known areas of dunal terrain (Figure 9) totals about 108,000 Ha. Accordingly, it may be feasible

to dispose of the mine dewatering discharge using this option. To minimise adverse environmental

impacts, it would be advisable to disperse the disposed groundwater over the maximum permissible

area and limit rates of injection at individual points.

Due to the uncertainty regarding the local water balance parameters (rates of evaporation within the

dunes, transmissivity of the dunal sands) it may be appropriate to provide for containment bunds on

Lake Carey, along the perimeter of the dunal terrain. The bunds would limit areas influenced by

discharge of hypersaline groundwater.

It is unknown how this discharge system would accommodate increasing salt loads within the dunal

terrain.

A feasibility study is required to further evaluate this discharge option. Key aspects of the study would

include defining the local water balances, developing an understanding of the shallow groundwater flow

systems, and ascertaining the mechanisms and processes that influence the local salt-balance.

The total cost of the shallow well injection option is summarised in Table 9, and a detailed breakdown

of the cost is included in Appendix A.

5.7 OPTION 6 – CONSTRUCT PALAEOCHANNEL BARRIER

5.7.1 General

The volumetric flow of groundwater into the Wallaby Pit could be reduced by constructing a barrier

through the palaeochannel. Such a barrier would form a low permeability structure within the pervious

stratum. The barrier would not halt seepage entirely, but by maximising the loss of hydrostatic

pressure during seepage through or beneath the barrier, flow volumes downstream of the barrier would

be substantially reduced. Barriers are fully effective only when pervious foundation material are

underlain by a continuous impervious stratum of natural material that prevents vertical flow.

The following barrier alternatives were identified for consideration:

• Options 6a and 6b - Grout Curtain

• Option 6c - Slurry Cutoff Wall

These options are discussed further below.



5.7.2 Grout Curtain (Options 6a and 6b)

Grouting of alluvial deposits has been satisfactorily performed using ‘chemical grouts’. This operation

involves the injection at low pressure of low viscosity chemical grout into the voids and/or fractures in

the soils where it gels to seal the groundwater flow paths.

For satisfactory chemical grouting, the aquifer generally requires a permeability of 10-5 m/s or greater,

and not more than 10% of the soil finer than a coarse silt (fraction passing the 20 micron <10%).

Thus, although the material in the palaeochannel aquifer may be suitable for grouting, the permeability

distribution within the aquifer is expected to be irregular and anisotropic and grout holes would

therefore need to be closely spaced in order to provide an effective seal or barrier. Furthermore,

because of the unpredictability of grout flows, a continuous grout curtain cannot be guaranteed.

Grouting seldom reduces the permeability of the grouted material to less than about 10-7 m/s (Vick,

1981).

Prior to excavation of the Wallaby Pit, a grout curtain may be constructed across the width of the

palaeochannel intersected by the pit to cut-off groundwater inflow into the pit. In order to be effective,

the grout curtain would need to “circle” the palaeochannel where it intersects with the pit. The

perimeter length of the grout curtain would be of the order of 2,500m as schematically shown on Figure

14.

The ability of the barrier to significantly limit groundwater inflow from the palaeochannel aquifer into

the pit will be dependent on:

• the final permeability through the barrier;

• the integrity of the barrier, both laterally and vertically within the palaeochannel aquifer; and

• linking of the barrier to low permeability, resistant strata below the palaeochannel aquifer.

Two options involving the design and construction of a grout curtain across the palaeochannel were

investigated. Both options would need to be implemented in conjunction with mine dewatering and

other groundwater discharge options discussed earlier. A further issue is the identification and

treatment of the full thickness of the palaeochannel, which is common to each of the options presented.

Option 6a - Install a grout curtain extending from ground level to the underside of thepalaeochannel.

A grout curtain from the ground level to the underside of the palaeochannel would be installed using

specialist drilling equipment. The grout curtain would extend through the superficial formations, and

the palaeochannel aquifer. Installation of the grout curtain would be required prior to mining

commencing.



To cost this option, the following assumptions were made:

• the water table is 3m below the existing surface level;

• the inclination of drill holes can be controlled to a maximum depth of 100m to allow uniform

grout coverage between drill holes;

• a drill hole spacing of 2m centre-to-centre with three lines of drillholes is required; and

• a grout-curtain thickness of 1.2m is required to resist the hydrostatic load.

The advantages of this option include:

• the grout curtain is constructed from the ground level;

• there is no co-ordination with mine excavation or dewatering activities required to facilitate

wall construction; and

• the technology is available in Australia and multiple crews can be mobilised to complete the

construction.

There are, however, a number of disadvantages, including:

• a conservative construction cost estimate of the order of $180 million;

• full construction of the grout-curtain is required prior to commencement of pit excavation;

• an impermeable barrier cannot be guaranteed, given the potential for inclination control

problems and variability in in situ materials;

• the effectiveness of the barrier in reducing groundwater inflow to the pit is difficult to quantify;

• additional discharge schemes will be required to operate in conjunction with the grout curtain;

• the minimum time for construction is estimated to be between four and six months; and

• the advice from specialist contractor indicates that high risks can be expected.

Option 6b - Install a grout curtain from ground level only to extend over the height of thepalaeochannel.

An alternative to Option 6a is to install the grout-curtain only through the palaeochannel aquifer

profile. The following points are applicable to this option:

• difficulty is expected with dewatering behind the grout-curtain during installation;

• residual seepage through the grout curtain can be contained and removed by internal

dewatering prior to entering the pit; and

• similar wall dimensions and drillhole spacing as per Option 6a are required.



Compared to Option 6a, the advantages for Option 6b include:

• reduced height of the grout curtain; and

• reduced time for construction.

The disadvantages include:

• a conservative construction cost estimate of the order of $85 million;

• construction of the grout-curtain is required prior to commencement of pit excavation;

• additional dewatering points may be required in front of the grout curtain;



• no ability to contain seepage above the palaeochannel if problems occur with pit dewatering;

and

• high risks expected similar to Option 6a.

5.7.3 Slurry Cut-off Wall (Option 6c)

A further alternative involves construction of a slurry cut-off wall from an intermediate platform in the

pit, above the level of the palaeochannel. Construction of the cut-off wall would need to be

incorporated and scheduled within the mining plans.

Deep seepage barriers using the slurry wall excavation method are one of the most effective forms of

cut-off in permeable strata. Permeabilities of 10-8 m/s can be achieved in the slurry wall.

Advantages of this method include:

• trenches can be excavated to substantial depths below the water table;

• the trenches provide a positive cut-off in stratified formations; and

• the barrier is flexible, not subject to cracking with ground movements and able to withstand

high hydraulic gradients.

The disadvantages are that slurry walls are relatively expensive, cannot easily penetrate fractured

bedrock and are best suited to relatively flat sites.

Generally, slurry backfills employ either soil-bentonite or cement-bentonite mixtures. Soil-bentonite

mixtures are typically well graded gravelly sands, with approximately 20% fines, that are readily

mixed with bentonite and can be placed to sufficient density to prevent excessive settlement. Where

sandy clays and clayey sands are used, mechanical mixing may be required. Cement-bentonite

mixtures offer an alternative to soil-bentonite. These provide similar permeabilities with increased



compressive strength, hence less settlement. However, increased strength is achieved at the expense of

flexibility.

To cost this option, the following assumptions were made:

• the slurry cut-off wall will be constructed using traditional diaphragm wall methods;

• cement-bentonite slurry mix adopted for wall construction;

• the groundwater level would be reduced to the top of the palaeochannel prior to construction

commencing;

• construction of an intermediate platform would be incorporated into the mine plan; and

• 800 mm cut-off wall thickness.

A schematic cross-section of this option is included on Figure 15.

The advantages of Option 6c include:

• potential for low permeability construction;

• no impact on commencement of pit excavation;

• conservative construction cost estimated at $20 million (lowest price estimate of all barrier

options);

• low permeability wall construction and lower risk of groundwater entering the pit; and

• alternative slurry cut-off wall installation methods are available.

The disadvantages of this option include:

• dewatering of the shallow aquifer is required prior to installation;

• discharge of hypersaline water from the shallow aquifer is required;



• co-ordination required with mining program to allow construction of intermediate working

platforms;

• potential delays to pit excavation;

• platform levels are likely to vary in grade around the perimeter of the pit;

• slurry wall construction unlikely to keep up with schedules for mining within the pit;

• grout curtain option does not achieve cost savings given the large number of drill holes

required. Cost estimate for grout curtain construction is of a similar order to that of Option 6b

and similar technical problems are expected;

• up to six grab rigs would be required in order to complete the slurry wall between four and six

months, assuming no construction difficulties; and

• significant volumes of fresh water required to prepare cement/bentonite slurry mix (assumed

readily available).



Specialist technical advice has been obtained from Bachy Soletanche which has an international

reputation in the construction of a wide range of foundation systems including slurry cut-off wall and

grout curtains. A brief method statement for slurry cut-off wall construction of the Harris River Dam

in Collie is contained in Appendix B.

The slurry wall construction involves mobilising to site plant and equipment including cranes which

support clamshell grabs during trench excavation, silos for containment of cement and bentonite,

mixers for bentonite and cement product and agitators and pumps required for grout injection and

placement. Typically, the project requires an engineer and supervisor for each rig and operators for

each crane and the cement/bentonite batching process. The working platform above the palaeochannel

would need to be at least 15 m wide in order to manoeuver plant and equipment during wall

construction.

Preliminary costings are based on a conservative estimate for the duration of the project, the number of

personnel involved and costs for plant, equipment and materials and estimates for the slurry wall

installation are presented in Appendix A.

In addition to the above option, where various cement/bentonite mixes would be adopted for the cut-off

wall, it may be possible to utilise other equivalent slurry type solutions for the purpose of reducing the

permeability of the palaeochannel zone. The principal difficulty with injecting materials, such as

compounds which swell to fill voids within the soil and ground mass, is the ability to control the extent

and uniformity of treatment. Without adequate controls over this process, the risks are high that the

required reduction in permeability may not be uniformly achieved.

Each of the options presented are exclusive of other direct and indirect costs such as the cost of fresh

water supply and impact on the mining schedules. In addition, there is risk associated with each option

in terms of containment of inflow through the cut-off wall and costs for contingency items such as in-pit

pumping need to be considered. Further investigation by field sampling and laboratory testing, and

completion of field trials, would be appropriate to better quantify the actual costs and risk associated

with the implementation of the palaeochannel cut-off wall option.

5.8 OPTION 7 – DESALINATION OF HYPERSALINE GROUNDWATER

The option of circulating groundwater from the mine dewatering discharge through a desalination

process was investigated as a method to manage groundwater discharge from the Wallaby Pit. The

objective for desalination would be to utilise the fresh water product within the plant and sell the excess

product to nearby towns or minesites. The following criteria was identified to further investigate the

operation of a desalination facility:

• plant inflow required between 20,000m3/day (230 L/s) and 52,000m3/day (600 L/s); and

• plant to be capable of handling hypersaline groundwater to 250,000 mg/L TDS.



Enquiries were made with firms specialising in the design and operation of the scale of desalination

facility that would be required for the Wallaby Deposit. The following comments and technical issues

were identified in order to implement this option:

• There are two options presently available for desalination of hypersaline groundwater on a

large scale, namely thermal methods and Reverse Osmosis. Reverse osmosis is the

conventional method for treatment of seawater (30,000 mg/L TDS), however thermal methods

are preferred.

• The treatment of hypersaline groundwater on the scale required for the Wallaby project is

unprecedented worldwide.

• Significant technical problems have been identified with these processes, including water

chemistry and build-up of solids in the treatment process.

• Significant energy resources are required to operate a desalination plant of this size.

• The plant size required to treat hypersaline groundwater from this project are of a similar

magnitude to the largest desalination plants constructed worldwide (the largest desalination

plants in the Middle East do not meet the technical specification requirements for this project).

• Given the hypersaline nature of the groundwater, the materials required for plant construction

are very expensive.

• The efficiency of the desalination process are of the order of 25% to 30%. Therefore,

discharge sites would still need to be identified for the remaining brine and bittern wastes which

would have higher salt concentrations.

• Capital costs are very high for conventional seawater desalination plants.

Capital costs for the provision of a desalination plant meeting the project specification are estimated to

be of the order of $500 M. It is estimated that costs of power consumption would be of the order of

$20 M per annum, supplied from extra generators estimated to cost of the order of $200 M to design

and construct. In addition, discharge sites would need to be identified for the waste fluid from the

desalination process.

A summary of the total cost for this option is presented in Table 11. For completeness, an allowance

for power supply and a waste discharge system were included in the capital cost.



Table 11

Cost Estimate for Desalination Plant

Item Cost Estimate


Design, Tender and Contract Administration 2 $44,000,000


Operating 1 $213,000,000

Decommissioning, Rehabilitation and Closure 1 $60,000,000

TOTAL LIFE-OF-MINE ESTIMATE 1 $999,000,000($999M)


Assuming the plant treats all groundwater pumped from the Wallaby Pit and 30% efficiency then the

fresh water product would need to be sold at a price of around $17/kL to break even.

On the basis of the high capital costs to establish the desalination process at the site and the

unprecedented nature of the treatment of hypersaline groundwater, this option is not considered feasible

for the project.

5.9 OPTION 8 – SALT HARVESTING

Actis Environmental Services (1999) undertook a review of commercial salt harvesting as applicable to

the groundwater discharge for the Wallaby Project. A summary of their findings is given below.

Australia currently supplies approximately 4.5% (8.7 Mt) of the total world salt production. Western

Australia is a major producer of solar salt with the majority of the salt produced along the northwest

coast shipped to Asian markets for use in chemical and manufacturing industries.

Solar salt production generally involves the following steps:

• saline water discharged into concentration ponds;

• evaporation of solution;

• when ‘salt crop’ reaches 100 to 250 mm thickness, the salt is harvested (usually once per year)

using specialist mobile equipment;

• harvested salt is washed in saturated brine to remove impurities;

• washed salt is stockpiled to drain;

• solar salt may then be crushed, screened and dried;

• disposal of residual salts (‘bitterns’); and

• transport of solar salt to export market.



Salt harvesting of the disposed groundwater is technically feasible as local evaporation rates are high

and the area may be more meteorologically stable than the Pilbara where most salt production takes

place.

The specialist plant and equipment required for a commercial salt field (assumed 2 Mtpa salt (annual

production) is of the order $15 M and is listed in Table 12. The plant and equipment has an anticipated

maximum life of 10 years and would require replacement at the end of this period, if not sooner. The

cost of evaporation ponds is additional to these costs.

Table 12

Plant and Equipment Required for a Salt Field

Item Number Cost per Unit Total Cost Comment

Harvestor 2 $800,000 $1,600,000

Dozer 1 $480,000 $480,000 To drain crystalliser

Grader 1 $440,000 $440,000 To drain crystalliser

Road Train 6 $740,000 $4,440,000 Cart wet salt, bottom dump

Dump area 1 $250,000 $250,000 300 tonne storage plus

Conveyor to wash plant 50 $200 $10,000

Wash water holding pond 1 $100,000 $100,000

Pump Station 1 $750,000 $750,000

Classifier 1 $500,000 $500,000

Centrifuge 1 $1,000,000 $1,000,000

Conveyor to stacker 1,500 $200 $300,000

Stacker 1 $750,000 $750,000 15 metre mobile dual side-stacking

Stack area 15,000 $20 $3,000,000360 tonne linear metre dual stack milliontonne stack 1.5m long

Front end loader 1 $1,300,000 $1,300,000 15 m3 bucket

Sundry fittings etc. $500,000

TOTAL $15,420,000

The total cost of the salt harvesting option is summarised in Table 13. A detailed breakdown of the

cost of this option is included in Appendix A. The estimates exclude the cost of transport and the

income from any sales. Closure costs have also been excluded from the life-of-mine estimated. It is

assumed that at the end of the Wallaby Deposit life, PGS will need to replace the plant and equipment

in order to make acquisition of the harvesting operations attractive to a new operator. the new operator

could operate for another 15 years using salt stockpiled in the evaporation ponds.



Table 13

Cost Estimate for Salt Harvesting

Item Cost Estimate


Design, Tender and Contract Administration 2 $3,900,000


Operating 1 $16,000,000

Replace Plant and Equipment 1 $15,000,000

TOTAL LIFE-OF-MINE ESTIMATE 1 $95,400,000($95.4M)


The marginal cost of producing salt would be in the region of $10 per tonne including washing. The

cost of transporting the salt from the Wallaby salt field to a suitable port is not known but anecdotal

estimates have placed this cost as being around $15 per tonne (2 Mtpa haul by truck). There is the

potential that the transport costs may reduce significantly should a rail head be constructed locally (as

is apparently being considered by others). The cost of loading the ships has been estimated as being in

the region of $7 per tonne.

All up, the operational cost of selling the solar salt (FOB) to the export market is around $32 per tonne.

This does not include the capital and marketing cost of selling the salt (royalties). The current price for

salt on the export market is less than AUD $40 per tonne.

Assuming a net profit of $5/t, annual profits for a 2 Mtpa salt harvesting production would be $10 M

and total profit for the 10 years Wallaby project life would be $100 M.

Actis considered the Australian domestic salt market too small to be considered an option for sales.

Actis concluded that solar salt production from Lake Carey will be marginally cost positive. Should a

local salt producer require a supply of salt for marketing reasons the difference in cost may be covered.

However, this is extremely unlikely considering the price difference unless the transport price can be

reduced drastically. This option may become attractive should a local customer be identified.

However, we are unaware of any potential user within the area, particularly considering the large

tonnages anticipated to be produced from the Wallaby Deposit dewatering activities (approximately

48.7 Mt over the life-of-mine, equivalent to six years of Australia’s current world market).

The feasibility of this option is dependent upon identifying a suitable customer and refining the

transport costs. Detailed financial analysis of this option will be required.



5.10 COMBINED OPTIONS

As discussed earlier, the maximum design flow adopted for a number of options within this scoping

study was 600 L/s. Therefore, for these options there is an excess of hypersaline groundwater over the

initial three years that requires discharge. In order to effectively manage discharge of the entire

groundwater flow from the Wallaby pit dewatering it may be necessary to implement a system that

combines either two or a number of alternative discharge options.

The conceptual arrangements combining options may include:

• evaporation pond and abandoned pit discharge;

• evaporation pond and discharge to Lake Carey;

• palaeochannel barrier and evaporation pond;

• palaeochannel barrier and discharge to Lake Carey; and

• palaeochannel barrier and abandoned pit discharge.

The feasibility of combining options into a operating and cost-efficient system will need to be further

investigated.

5.11 SUMMARY OF COSTS

A summary of the design, construction, operating and closure costs for each option investigated is

included in Table 14.

5.12 RANKING OF OPTIONS

All options were evaluated and ranked on the basis of the following criteria considered essential for the

efficient and safe operation of the discharge system.

• Practicality - technically feasible (technical risk) and easily constructible.

• Safety - short and long term stability under static and dynamic loading

and major storm events.

• Robustness - capable of smooth operation with minimal supervision.

• Environmental - minimal disturbance and long-term performance.

• Cost - low $/unit stored or disposed.

• Construction Period - anticipated construction period and coordination with other

mining activities.



Table 14

Summary of Costs for all Management Options

Discharge Option Cost EstimateCost per Unit Stored 4 ($/ML)

No. Description Construction Design andEPCM Operating Closure Total

Total Closure CostExcluded

1a Land-based evaporation pond (no liner) $27 $4.6 $6 M $68 $106 $630 $230

1b Land-based evaporation pond (compacted clay liner) $43 M $7.3 M $6 M $68 M $124 M $735 $335

1c Lake Carey based evaporation pond 3 (no liner) $19.6 M $3.4 M $6 M $3 M $32 M $192 $175

3a/2 Jupiter Pit (24 L/s) – new access tracks $0.9 M $0.16 M $1.5 M $0.15 M $2.7 M $3560 $3370

3b Discharge into Goanna, Granny and Windich and Goanna Pits (200 L/s) $2.6 M $0.44 M $2.3 M $0.22 M $5.6 M $90 $85

3c Discharge into Goanna, Granny and Windich pits (600 L/s $5.2 M $0.9 M $0.6 M $0.2 M $6.9 M $165 $160

4a Direct discharge to bunded area within Lake Carey 3 $4.8 M $0.8 M $6 M $1.3 M $12.9 M $80 $70

4b Direct discharge to Lake Carey (pipeline only) 3 $2.2 M $0.4 M $6 M $0.2 M $8.8 M $52 $51

4c Discharge into bunded area within islands on Lake Carey 3 $5.4 M $0.9 M $6 M $0.9 M $13.2 M $70 $65

5a Deep well injection $45 M $7.5 M $6 M $11 M $70 M $420 $350

5b Shallow well injection $32.5 M $5.5 M $6 M $5 M $49 M $292 $262

6a Grout curtain from ground level to under side of palaeochannel 1 $180 M $31 M $2 M $0.5 M $213.5 M $1550 $1520

6b Grout curtain over height of the palaeochannel 1 $84 M $14 M $2 M $0.5 M $100.5 M $550 $530

6c Slurry Cut-off Wall across palaeochannel 1 $23 M $4 M $2 M $0.5 M $29.5 M $250 $200

7 Desalination $620 M $106M $213 M $60 M $999 M $8500 $6700

8 Salt harvesting 2 $55 M $9.4 M $16 M $15 M $95.4 M $500 $420

Note: 1 Assumed 70% of groundwater effectively treated or blocked off. Excess groundwater to be disposed of using alternative method.2 Does not include cost to transport harvested salt to port or income from sales. No closure costs, just replace all plant and equipment.3 Assumed impacted surface area at closure does not require capping.4 Unit storage cost is calculated using the total volume of hypersaline water stored for each individual option.



The options were each evaluated qualitatively in accordance with the following:

5 - very good (very low)

4 - good (low)

3 - moderate

2 - poor (high)

1 - very poor (very high)

where: very good = best condition or quality

very poor = worst condition or quality

very low = lowest impact

very high = highest impact

Based on the ram weighted rankings alone, the five highest scope options would include:

• Option 4b – Direct Discharge to Lake Carey: Score 74.

• Option 4c – Discharge into Intra-island Areas on Lake Carey: Score 59.

• Option 4a – Discharge to Bunded Areas on Lake Carey: Score 54.

• Option 1c – Lake Carey Evaporation Pond: Score 51.

• Option 1a – Land-based Evaporation Pond: Score 48.

However, several of these options (and other lower scoring options) pose excessive risks in terms of

technical feasibility, environmental aspects and sustainability. Options that fall into this category are

where a ranking of 1 (very poor, highest impact) was evaluated for any of the ranking criteria. These

options are not considered viable and should not be considered further. Based on this understanding,

the five viable options with the highest weighted ranking are:

• Option 4c – Discharge into Intra-Island Areas on Lake Carey: Score 59.

• Option 4a – Discharge to Bunded Areas on Lake Carey: Score 54.

• Option 1c – Lake Carey Evaporation Pond: Score 51.

• Option 6c – Slurry Cut-Off Wall: Score 40.

• Option 8 – Salt Harvesting: Score 36.

Of the remaining options, only those for discharge to the Goanna, Granny and Windich pits (Options 3b

and 3c) should be further considered. Both of these options score highly in the criteria rankings and

offer the project a feasible, low risk and comparatively secure option for co-disposal of all (at times) or

a significant portion of the discharged groundwater.



Table 15

Technical Ranking of Options(assumes all criteria are of equal ranking)

Discharge Option Ranking Criteria Ranking Score

No. Description Practicality Safety Robustness Environmental Cost Construction Period Total %

DischargeCapability

(%)

WeightedRanking 1

(%)1a Land-based evaporation pond (no liner) 4 4 3 1 2 3 17 57 100 48

1bLand-based evaporation pond (compactedclay liner)

4 4 3 1 1 2 15 50 100 42

1cLake Carey based evaporation pond (noliner)

3 3 3 3 3 3 18 60 100 51

3a/2 Jupiter Pit (24 L/s)– new access tracks 5 5 4 4 1 5 24 80 4 3

3bDischarge into Goanna, Granny andWindich and Goanna Pits (200 L/s)

5 5 4 4 4 5 27 90 32 23

3cDischarge into Goanna, Granny andWindich Pits (600 L/s)

5 5 4 4 3 5 26 87 32 22

4aDirect discharge to bunded area withinLake Carey

3 3 3 2 4 4 19 63 100 54

4bDirect discharge to Lake Carey (pipelineonly)

5 5 5 1 5 5 26 87 100 74

4cDischarge into intra-island areas withinislands on Lake Carey

3 4 3 3 4 4 21 70 100 59

5a Deep well injection 1 2 1 1 3 2 10 33 100 285b Shallow well injection 2 2 1 2 3 2 12 40 100 34

6aGrout curtain from ground level tounderside of palaeochannel

2 2 3 5 2 2 16 53 70 32

6bGrout curtain over height of thepalaeochannel

2 2 3 5 2 2 16 53 70 32

6c Slurry cut-off wall across palaeochannel 3 3 4 5 3 2 20 67 70 407 Desalination 1 4 1 2 1 1 10 33 30 88 Salt harvesting 4 4 2 2 3 3 18 60 70 36

Notes: 1 Weighted Ranking – based on discharge capability as a percentage of the total discharge volume. The weighting has been applied based on the relationship:

8x + (4)(0.7)x + (3)(0.3)x + 0.04x = 10

where: 8 options provide a 100% discharge capability;4 options provide a 70% discharge capability;3 options provide a 30% discharge capability; and1 option provides a 4% discharge capability.The factor x equates to 0.85.



6. DISCUSSION AND CONCLUSIONS

The Wallaby Deposit intersects three significant aquifer systems formed by:

• alluvial and laterised deposits within the shallow superficial formations;

• palaeochannel sands within the Carey Palaeodrainage; and

• weathered and fractured rocks in the bedrock regolith.

Each aquifer system is fully saturated and contains hypersaline groundwater of TDS concentrations

about 250,000 mg/L.

Development of the Wallaby Deposit will require a substantial dewatering effort, particularly

associated with the palaeochannel aquifer system. Preliminary, conceptual dewatering rates are

1,300 L/s for the first year of the mine development, progressively reducing to 460 L/s after 10 years.

Discharge of the groundwater is a significant issue, particularly considering the salinity of the

groundwater to be disposed.

Over the 10-year pit life, about 48.7 Mt of salt would be removed from storage within the local aquifer

systems.

Several management options have been investigated including:

• evaporation ponds;

• discharge to northwest salinaland;

• discharge to the Jupiter, Goanna, Granny and Windich abandoned pits;

• discharge onto Lake Carey;

• injection into deep and shallow aquifer systems;

• reducing dewatering and discharge quantities through construction of grout curtains or

alternative low permeability barriers within the palaeochannel aquifer;

• desalination; and

• salt harvesting.

Several of these options may also be combined.

Key finding regarding the specifications and design for, feasibility, practicality and costs of each of

these options is outlined below.

Evaluation of the options (Table 15) has identified a number of discharge systems that are not

recommended to be considered further due to technical risk, cost or environmental sensitivity (namely

options 2, 3a/2, 5a, 5b, 6a, 6b and 7).



Discharge to an evaporation pond system (Options 1a, 1b and 1c), either on land or on Lake Carey, is

considered technically robust and has been utilised for similar installations within Australia. The unit

storage cost for the evaporation pond options is moderate, however the major portion of this cost

(between 40 – 80%) is associated with rehabilitation and closure of the system. With further studies

the potential exists to substantially reduce the total life-of-mine cost for these options. Regulatory

requirements may result in the options being preferred. Therefore, it is recommended that these options

be further developed.

Discharge to a bunded area on Lake Carey or directly onto the lake is economically attractive.

However, a level of environmental risk exists with this option. It is recommended that additional

studies on this option be undertaken, in particular studies related to assessing the potential impacts to

the hydrological regime, and flora and fauna communities within Lake Carey.

Discharge of excess groundwater to Jupiter Pit (Options 3a/1 and 3a/2) is not considered feasible due

to the limited volume that can be discharged over the life-of-mine. However, discharge to the Goanna,

Granny and Windich abandoned pits (Options 3b and 3c) is attractive, particularly during the first three

years of mining when excess groundwater is anticipated.

Disposal by injection into the palaeochannel aquifer system and unsaturated dunal terrain has been

investigated. Both options are significantly constrained by a lack of available storage for the injected

groundwaters.

The deep well injection into the palaeochannel aquifer system would require up to 175 bores, the

furthermost located 54 km south of the Wallaby Deposit. System costs for installation are estimated at

$45 M. Results of simulated injection systems show most of the discharge groundwater is transmitted

to the water table beneath the lake, causing a small scale but regional mounding. Considerable volumes

would also migrate towards the Wallaby Deposit and be recycled by the dewatering programme.

It has been concluded that the deep well injection system is not a viable option. It has been inferred

from modelling results that transmission of disposed groundwater from the palaeochannel aquifer to the

water table would preferentially occur along permeable structures in the bedrock profile. A result of

this would be excessive localised mounding of the water table and discharge of hypersaline groundwater

onto the surface of Lake Carey.

Disposal by shallow well injection would rely on the storage available above the water table in dunal

terrain near the shore of Lake Carey. Dunal terrain covers an area of about 108,000 Ha in proximity to

the Wallaby Deposit, on the western and northeastern shoreline of Lake Carey. System costs for

installation are estimated at $30 M.

The feasibility of the shallow well injection is poorly quantified, largely because key water balance and

hydrogeological parameters are not known; particularly rates of evaporation within the dunal sands and

transmissivity of the sand.



For this option to be feasible, rates of injection would need to be balanced with evaporation losses,

otherwise discharge onto the surface would occur.

This option may be combined with and/or supported by a bunded containment area on Lake Carey.

The bunds would flank the dunal terrain.

The construction of impermeable barrier through the palaeochannel either by grout curtain (Options 6a

and 6b) or slurry cut-off wall (Option 6c) is considered technically ‘risky’ as we are unaware of any

similar installations of the scale required. However, the concept is sound and provides potential to

substantially reduce the volume of groundwater to be discharged for mine dewatering. The barrier will

not intercept all groundwater flow into the pit and therefore additional groundwater discharge systems,

albeit on a reduced scale, will be required in conjunction with construction of the barrier.

Desalination of the groundwater for sale (Option 7) is not considered feasible for the project due to the

large capital required for infrastructure and the unique size of the plant required.

The feasibility of salt harvesting (Option 8) is dependent upon sourcing a customer for the large

tonnage expected to be released on the market (2 Mtpa, which is 25% of Australia’s current export

market). Should they be identified then this option will require further detailed investigation.

In order to evaluate and compare the technical merits, feasibility, environmental impacts and costs for

each discharge option, a ranking system based on specific criteria has been established. The ranking

criteria include:

• practicality;

• safety;

• robustness;

• environmental;

• cost;

• construction period; and

• discharge capability for each discharge option.

The ranking system has been applied to define these options that (i) pose excessive risk in terms of

technical feasibility, environmental aspects and sustainability, and (ii) provide the most practical,

secure, minimal impact and cost-effective management of the groundwater discharge. The results of

the comparative rankings indicate the preferred viable options (listed in order of merit) are:

• Option 4c – Discharge into Intra-Island Areas on Lake Carey.

• Option 4a – Discharge into Bunded Areas on Lake Carey.

• Option 1c – Lake Carey Evaporation Pond.

• Option 6c – Slurry Cut-Off Wall.

• Option 8 – Salt Harvesting.



Of the remaining options, only the discharge to the Goanna, Granny and Windich pits (Options 3b and

3c) should be further considered. These options provide a feasible, low-risk (high rank) discharge

means if co-disposal is considered.

7. RECOMMENDATIONS

It is recommended that the following groundwater discharge options both independently and as

co-disposal systems, be further developed during the Wallaby Deposit feasibility study.

• discharge to intra-island bunded area on Lake Carey;

• discharge to bunded areas on Lake Carey;

• Lake Carey evaporation pond;

• slurry cut-off wall across palaeochannel;

• salt harvesting; and

• discharge to Goanna, Granny and Windich pits (co-disposal only).

8. REFERENCES

Actis Environmental Services, 1999; Review of the Commercial Harvesting of Salt Precipitated from

Mining Groundwater Discharged from the Wallaby Project, Draft Report for Dames & Moore

Pty Ltd, August 1999.

DC Blandford & Associates Pty Ltd, 1999; Saline Water Disposal – a Review of Disposal Options for

the Dewatering Programme at Wallaby, Report for Placer (Granny Smith) Pty Ltd, June 1999.

Department of Minerals and Energy (DME) Western Australia, 1999; Guidelines on the Safe Design

and Operating Standards for Tailings Storage, May 1999.

Gorenc, B.E. & Tinyou, R., 1984; Steel Designers’ Handbook; New South Wales University Press Ltd.

Mackie Martin & Associates Pty Ltd, 1990; Jupiter Project – Minesite Test Pumping Results, Letter

Report for Austmin Gold Mines Pty Ltd, 18 June 1990.



LIMITATIONS OF REPORT

We have prepared this report for the use of Placer (Granny Smith) Pty Ltd in accordance with generally

accepted consulting practice. No other warranty, expressed or implied, is made as to the professional

advice included in this report. This report has not been prepared for the use by parties other than the

client, the owner and their respective consulting advisors. It may not contain sufficient information for

purposes of other parties or for other uses.

It is recommended that any plans and specifications prepared by others and relating to the content of

this report or amendments to the original plans and specifications be reviewed by Dames & Moore to

verify that the intent of our recommendations is properly reflected in the design.

Whilst to the best of our knowledge information contained in this report is accurate at the date of issue,

subsurface conditions, including groundwater levels and contaminant concentrations, can change in a

limited time. This should be borne in mind if the report is used after a protracted delay.

There are always some variations in subsurface conditions across a site that cannot be fully defined by

investigation. Hence it is unlikely that the measurements and values obtained from sampling and

testing during the investigation will represent the extremes of conditions which exist within the site.

Appendix A

Detailed Breakdown of

Management Option Cost Estimates

Option 1a - Disposal on Land to Evaporation Pond (no clay liner)

Item Description Unit Quantity Unit Rate Total

1.0 PreliminaryMobilise/demob. Contractor (inc. establishment) allow $80,000

2.0 ClearingClear and grub m2 14,440,000 $0.04 $577,600Strip topsoil (nom. 150mm depth) and stockpile m2 14,440,000 $0.26 $3,754,400

3.0 ExcavationBulk excavation (allow 0.3m av. across base) m3 4,000,000 $2.50 $10,000,000Grade base - level m2 14,440,000 $0.07 $1,010,800

4.0 EmbankmentsExternal (3m crest, 6m high, 1:3 side slopes) m3 1,915,200 $2.00 $3,830,400Internal (2m crest, 5.5m high, 1:3 side slopes) m3 773,300 $2.00 $1,546,600Topsoil downstream slope m2 288,400 $0.30 $86,520External access track (8m wide) m 15,600 $15.00 $234,000

5.0 Liner (clay) Not Applicable

6.0 Seepage ControlExcavate trench (not exceeding 2m) m 13,600 $10.00 $136,000Supply and install 150mm slotted draincoil m 13,600 $15.00 $204,000Backfill with clean sand drainage material m3 40,800 $25.00 $1,020,000Excavate holding pond m3 3,267 $2.50 $8,168Supply and install HDPE liner m2 1,720 $9.00 $15,480Return Pump & Pipework item allow $25,000

7.0 Saline Water Delivery SystemPipework m 5,000 $300.00 $1,500,000Pumps and control (2 pumps, duty & standby) item allow $250,000

8.0 Stormwater Diversion Works item allow $100,000

Subtotal $24,378,9689.0 Contingency 10% $2,437,897

Total Construction Cost $26,816,864

10.0 Detailed Design and Tender 7% $1,877,180

11.0Engineering Procurement & Construction Management 10% $2,681,686

12.0 Operating CostsSupervision (2 staff, on roster, day shift only) year 10 $200,000 $2,000,000Monitoring & Auditing year 10 $50,000 $500,000Power for pumping year 10 $150,000 $1,500,000Maintenance (pumps, pipes, ponds) year 10 $200,000 $2,000,000

$6,000,000

13.0 Rehabilitation & ClosureCapping Earthworks (1.5m clay) m3 21,660,000 $2.00 $43,320,000Capping Earthworks (0.5m rock) m3 7,220,000 $3.00 $21,660,000Topsoiling Spreading (0.15m thick) m2 2,166,000 $0.30 $649,800Revegetation m2 14,440,000 $0.10 $1,444,000Maintenance item $1,000,000

$68,073,800

TOTAL LIFE-OF-MINE COST ESTIMATE $105,449,531

Note: Cost estimate is based on a 600L/s flow in Year 1 decreasing to 460L/s in Year 10.

Option 1b - Disposal on Land to Evaporation Pond (clay liner)



2.0 ClearingClear and grub m2 14,440,000 $0.04 $577,600Strip topsoil (nom. 150mm depth) and stockpile m2 14,440,000 $0.26 $3,754,400

3.0 ExcavationBulk excavation (allow 0.3m av. across base) m3 4,000,000 $2.50 $10,000,000Grade base - level m2 14,440,000 $0.07 $1,010,800


5.0 Liner (clay)Scarify and recompact insitu clay (600 deep) m2 14,440,000 $1.00 $14,440,000

6.0 Seepage ControlExcavate trench (not exceeding 2m) m 13,600 $10.00 $136,000Supply and install 150mm slotted draincoil m 13,600 $15.00 $204,000Backfill with clean sand drainage material m3 40,800 $25.00 $1,020,000Excavate holding pond m3 3,267 $2.50 $8,168Supply and install HDPE liner m2 1,720 $9.00 $15,480Return Pump & Pipework item allow $25,000








$6,000,000

13.0 Rehabilitation & ClosureCapping Earthworks (1.5m clay) m3 21,660,000 $2.00 $43,320,000Capping Earthworks (0.5m rock) m3 7,220,000 $3.00 $21,660,000Topsoiling Spreading (0.15m thick) m2 2,166,000 $0.30 $649,800Revegetation m2 14,440,000 $0.10 $1,444,000Maintenance item $1,000,000

$68,073,800



Option 1c - Disposal to Lake Carey Evaporation Pond (no clay liner)



2.0 Clearing Not Applicable

3.0 ExcavationBulk excavation (from lake borrow pit) m3 2,779,700 $3.50 $9,728,950

4.0 EmbankmentsExternal (4m crest, 6m high, 1:3 side slopes) m3 2,006,400 $2.00 $4,012,800Internal (2m crest, 5.5m high, 1:3 side slopes) m3 773,300 $2.00 $1,546,600Access track on crest (4m wide) m2 60,800 $4.00 $243,200


6.0 Seepage Control Not Applicable

7.0 Saline Water Delivery SystemPipework m 5,000 $300.00 $1,500,000Causeway m 5,000 $100.00 $500,000Pumps and control (2 pumps, duty & standby) item allow $250,000

8.0 Stormwater Diversion Works Not Applicable






$6,000,000

13.0 Rehabilitation & ClosurePlace rockfill on embankments (500mm thick) m2 300,000 $3.00 $900,000Remove causeway item allow $500,000Remove pipework m 5,000 $10.00 $500,000Maintenance item $1,000,000

$2,900,000



Option 2 - Disposal to Northwest Salinaland



2.0 ClearingClear and grub m2 $0.12Strip topsoil (nom. 150mm depth) and stockpile m2 $0.30

3.0 ExcavationBulk excavation (allow 0.3m av. across base) m3 $3.50Grade base - level m2 $0.60

4.0 EmbankmentsBund walls item allow $100,000Upgrade existing access track from Wallaby m 10,000 $10.00 $100,000



7.0 Saline Water Delivery SystemPipework m 6,000 $300.00 $1,800,000Pumps and control (2 pumps, duty & standby) item allow $250,000Outfall earthworks item allow $100,000


Subtotal $2,530,0009.0 Contingency 10% $253,000


10.0 Detailed Design and Tender 7% $194,810

11.0Engineering Procurement & Construction Management 10% $278,300


$6,000,000

13.0 Rehabilitation & ClosureCapping Earthworks on salinaland (1.5m clay) m3 $2.00Capping Earthworks on salinaland (0.5m clay) m3 $3.00Topsoiling spreading on salinaland (0.15m thick) m2 $0.30Revegetation m2 $0.10Maintenance item $1,000,000

$1,000,000



Option 3a/1 - Disposal into Jupiter Pit (along existing track route)



2.0 Access TracksUpgrade existing tracks m 16,000 $10.00 $160,000Construct new tracks m $20.00 $0Construct access track across clay pan m $50.00 $0






7.0 Operating CostsSupervision (equivalent of 1 staff) year 10 $100,000 $1,000,000Monitoring & Auditing year 10 $10,000 $100,000Power for pumping year 10 $10,000 $100,000Maintenance (pumps, pipes, tracks) year 10 $25,000 $250,000

$1,450,000

8.0 Rehabilitation & ClosureRemove pipework m 16,000 $10.00 $160,000Repair tracks m 16,000 $5.00 $80,000

$240,000


Note: Cost estimate is based on a sustained flow of 24L/s over the life-of-mine.

Option 3a/2 - Disposal into Jupiter Pit (along new track route)



2.0 Access TracksUpgrade existing tracks m $10.00 $0Construct new tracks m 9,000 $20.00 $180,000Construct access track across clay pan m 1,000 $50.00 $50,000

3.0 Saline Water Delivery SystemPipework m 10,000 $55.00 $550,000Pumps and control (2 pumps, duty & standby) item allow $20,000

Subtotal $850,0004.0 Contingency 10% $85,000

Total Construction Cost $935,000



7.0 Operating CostsSupervision (equivalent of 1 staff) year 10 $100,000 $1,000,000Monitoring & Auditing year 10 $10,000 $100,000Power for pumping year 10 $10,000 $100,000Maintenance (pumps, pipes, tracks) year 10 $25,000 $250,000

$1,450,000


$150,000



Option 3b - Disposal into Goanna, Granny & Windich Pits (200L/s)



2.0 Access TracksUpgrade existing tracks m 15,000 $10.00 $150,000Construct new tracks m $20.00 $0

3.0 Saline Water Delivery SystemPipework m 15,000 $140.00 $2,100,000Pumps and control (1 duty pump only) item allow $50,000





7.0 Operating CostsSupervision (equivalent of 1 staff) year 10 $100,000 $1,000,000Monitoring & Auditing year 10 $10,000 $100,000Power for pumping year 10 $100,000 $1,000,000Maintenance (pumps, pipes, tracks) year 10 $20,000 $200,000

$2,300,000


$225,000



Option 4a - Discharge into Bunded Area within Lake Carey (no surface treatment)




3.0 ExcavationBulk excavation (from lake borrow pit) m3 115,600 $2.50 $289,000

4.0 Embankments (Bunds)External (4m crest, 1.4m high, 1:3 side slopes) m3 174,496 $2.00 $348,992Internal m3 $2.00 $0Access track on crest (4m wide) m2 60,800 $4.00 $243,200Access track along shore (8m wide) m 8,000 $15.00 $120,000




8.0 Stormwater Diversion Works Not Applicable






$6,000,000

13.0 Rehabilitation & ClosureRemove bunds m3 174,496 $1.60 $280,000Remove pipework m 10,000 $10.00 $100,000Miscellaneous earthworks item $900,000

$1,280,000


Note: Cost estimate is based on a 1,300L/s flow in Year 1 decreasing to 460L/s in Year 10.

Option 4b - Direct discharge into Lake Carey (pipeline only)




3.0 Excavation Not Applicable

4.0 EarthworksConstruct new access track m 5,000 $20.00 $100,000









12.0 Operating CostsSupervision (2 staff, on roster, day shift only) year 10 $200,000 $2,000,000Monitoring & Auditing year 10 $50,000 $500,000Power for pumping year 10 $150,000 $1,500,000Maintenance (pumps, pipes, tracks) year 10 $200,000 $2,000,000

$6,000,000

13.0 Rehabilitation & ClosureOutfall earthworks repair item allow $125,000Remove pipework m 5,000 $10.00 $50,000Repair tracks m 5,000 $5.00 $25,000

$200,000



Option 4c - Discharge into Intra-island Bunded Areas on Lake Carey





4.0 EarthworksEmbankment (4m crest, 2.5m high, 1:3 side slopes) m3 488,750 $2.00 $977,500Access causeway from Wallaby m 5,000 $100.00 $500,000Access track on island m 5,000 $50.00 $250,000









12.0 Operating CostsSupervision (2 staff, on roster, day shift only) year 10 $200,000 $2,000,000Monitoring & Auditing year 10 $50,000 $500,000Power for pumping year 10 $150,000 $1,500,000Maintenance (pumps, pipes, bunds, tracks) year 10 $200,000 $2,000,000

$6,000,000

13.0 Rehabilitation & ClosureErosion protection rockfill on embankments (500mm) m2 134,400 $3.00 $403,200Remove pipework m 9,000 $10.00 $90,000Remove causeway m 5,000 $80.00 $400,000

$893,200



Option 5a - Deep Well Injection





4.0 EmbankmentsCauseways km 139 $100,000 $13,900,000

5.0 Injection BoresDrilling & materials no. 175 $35,000 $6,125,000

6.0 Saline Water Delivery SystemDelivery pipework m 54,000 $370.00 $19,980,000Feeder pipework m 40,000 $15.00 $600,000Pumps and control (2 pumps) item allow $250,000






$6,000,000

11.0 Rehabilitation & ClosureRemove pipework m 54,000 $10.00 $540,000Remove and grout bores no. 175 $50,000 $8,750,000Repair causeways m 54,000 $10.00 $540,000Storwater diversion works item allow $1,000,000

$10,830,000



Option 5b - Shallow Well Injection




3.0 ExcavationTrenching m 109,000 $14.00 $1,526,000

4.0 Embankments Not Applicable

5.0 Saline Water Delivery SystemDelivery pipework m 47,000 $290.00 $13,630,000Slotted distributor pipe m 62,000 $230.00 $14,260,000Pumps and control (2 pumps) item allow $100,000






$6,000,000

10.0 Rehabilitation & ClosureRemove pipework m 109,000 $30.00 $3,270,000Reinstate dunes item allow $1,500,000

$4,770,000



Option 6a - Grout Curtain from Ground Level to Underside of Palaeochannel



2.0 Grout CurtainGrout curtain installation m 2,500 $65,000.00 $162,500,000

3.0 MiscellaneousEarthworks item allow $50,000Water supply item allow $200,000Instrumentation installation item allow $500,000





7.0 Operating CostsOperation (1 staff, day shift) year 10 $100,000 $1,000,000Monitoring & Auditing year 10 $100,000 $1,000,000Power Not Applicable

$2,000,000

8.0 Rehabilitation & ClosureRemove instrumentation item allow $250,000Miscellaneous earthworks reinstatement item allow $250,000

$500,000


Note: Cost estimate is based on a 1,300L/s flow in Year 1 decreasing to 460L/s in Year 10.Additional cost for disposal of groundwater seeping through grout curtain not included.

Option 6b - Grout Curtain over Height of Palaeochannel



2.0 Grout CurtainGrout curtain installation m 2,500 $30,000.00 $75,000,000







$2,000,000


$500,000


Note: Cost estimate is based on a 1,300L/s flow in Year 1 decreasing to 460L/s in Year 10.Additional cost for disposal of groundwater seeping through grout curtain not included.

Option 6c - Slurry Cut-off Wall across Palaeochannel



2.0 Cut-off WallCranes, grabs, cement and bentonite silos (4 units) day 180 $80,000.00 $14,400,000Materials (cement, bentonite and additives) item allow $2,000,000Consumables (eg fuel) item allow $500,000Project Staff and Operators day 180 $5,000.00 $900,000Other costs, accommodation item allow $250,000





6.0 Engineering Procurement & Construction Management 10% $2,298,000


$2,000,000


$500,000


Note: Cost estimate is based on a 1,300L/s flow in Year 1 decreasing to 460L/s in Year 10.Additional cost for disposal of groundwater seeping through cut-off wall not included.

Option 7 - Desalination Plant



2.0 EarthworksEvaporation ponds for waste (on Lake Carey) item allow $15,000,000

3.0 Desalination PlantMechanical plant and equipment item allow $450,000,000Power supply item allow $100,000,000





7.0 Operating CostsOperation (3 staff, on roster, day & night shift) year 10 $600,000 $6,000,000Monitoring & Auditing year 10 $200,000 $2,000,000Power year 10 $20,000,000 $200,000,000Maintenance (pumps, pipes, ponds) year 10 $500,000 $5,000,000

$213,000,000

8.0 Rehabilitation & ClosureRehabilitate evaporation ponds item allow $50,000,000Decommission desalination plant item allow $10,000,000

$60,000,000

TOTAL LIFE-OF-MINE COST ESTIMATE $1,000,798,500


Option 8 - Salt Harvesting



2.0 ClearingClear and grub m2 11,550,000 $0.12 $1,386,000Strip topsoil (nom. 150mm depth) and stockpile m2 11,550,000 $0.30 $3,465,000

3.0 ExcavationBulk excavation (allow 0.3m av. across base) m3 3,500,000 $2.50 $8,750,000Grade base - level m2 11,550,000 $0.07 $808,500


5.0 Seepage ControlTrenching, ponds, pipes and pumps item allow $1,000,000

6.0 Saline Water Delivery SystemDelivery pipework m 47,000 $290.00 $13,630,000Pumps and control (2 pumps) item allow $100,000


8.0 Plant & Equipment item allow $15,000,000





12.0 Operating CostsOperation (3 staff, on roster, day & night shift) year 10 $600,000 $6,000,000Monitoring & Auditing year 10 $100,000 $1,000,000Power year 10 $400,000 $4,000,000Maintenance (pumps, pipes, ponds) year 10 $500,000 $5,000,000

$16,000,000

13.0 Rehabilitation & ClosureReplace all capital after 10 years item allow $15,000,000

$15,000,000



Appendix B

Method Statement for

Slurry Cut-off Wall Construction

report revision 1 - epa wa

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