Water and Environment - EPA WA · and Marra Mamba rocks forming the east-west trending ridge, the Davidson Creek site is covered with aeolian sand, recent colluvium and alluvium

Water and Environment

DAVIDSON CREEK PRELIMINARY MINE DEWATERING ANALYSIS

Prepared for FerrAus

Date of Issue 17 November 2009

Our Reference 1009B/032 As part of Aquaterra’s commitment to the environment this PDF has been designed for double sided printing and includes blank pages as part of the document.


Prepared for FerrAus

Date of Issue 17 November 2009

Our Reference 1009B/032


EXECUTIVE SUMMARY


EXECUTIVE SUMMARY

PROJECT BACKGROUND

In October 2008, Aquaterra were requested by FerrAus Limited to prepare a proposal to conduct a prefeasibility level dewatering investigation of their Davidson Creek Deposit. Aquaterra carried out a desktop study in late 2008, which included the collection of existing mine and mineral exploration drilling information. The fieldwork only commenced in September 2009 and included the airlift testing of fifteen RC exploration bores at the Davidson Creek site. This report presents the work carried out and provides a preliminary estimate of the dewatering requirements of the proposed mine over 10 years of operation, as well as recommendations of further work required to upgrade these current assessment to a feasibility Study level of certainty.

HYDROGEOLOGY

Airlift testing of mineral explorations bores, targeting the Davidson Creek ore-body (hosted in the Marra Mamba Iron Formation), indicated that the ore body forms a relatively permeable aquifer, with an average hydraulic conductivity of 3 m/day. The average yield of the 10 holes airlift-tested during this study is ~11 L/s. The depth to the groundwater-level below the mine site varies between 18 and 24 metres below surface. The groundwater in the Marra Mamba aquifer is relatively fresh, with a total dissolved solids (TDS) content of <1000 mg/L.

The groundwater yield potential of the adjoining geological units, namely the overlying West Angela (Wittenoom Formation) shale and underlying shale of the Jerrinah Formation, is comparatively low. Five holes tapping these geological units gave an average airlift yield of ~1 L/s. The salinity of the water in these aquifers is higher (TDS of >1400 mg/L) than that in the Marra Mamba aquifer. The nature and degree of hydraulic connection between surrounding shallow, alluvial aquifers and the above hard-rock aquifers is currently unknown, but expected to be low.

DEWATERING REQUIREMENTS AND IMPACTS

Simple analytical methods were used to provide first-order estimates for the proposed Davidson Creek mine dewatering requirements over 10 years of operation, as well as to assess the associated potential impacts on the regional groundwater levels. The findings are summarised below:

Estimated Dewatering Requirement Year of Mine Operation

(L/s) (m3/day)

Estimated Radius of Cone of Waterlevel Drawdown (m) in Marra Mamba Aquifer

1 38 - 45 3,280 – 3,890 1,000

2 22 - 26 1,900- 2,250 1,600

3 22 - 26 1,900- 2,250 2,500

4 35 - 40 3,020 – 3,460 3,200

5 38 - 44 3,280 – 3,800 4,200

6 44 - 50 3,800 – 4,320 4,500

7 24 - 30 2,070 – 2,600 4,900

8 38 - 44 3,280 – 3,800 5,100

9 44 - 50 3,800 – 4,320 5,300

10 46 - 52 4,000 – 4,500 5,500


EXECUTIVE SUMMARY


RECOMMENDATIONS

The following key recommendations are made:

▼ There is a need to undertake a bore survey and field hydrological data collection that includes mineral exploration holes and privately-owned bores located in an area, extending up to 5km from the mine site.

▼ Drilling and pump-testing of 4 to 6 production test holes at selected sites near to the proposed mine pit to obtain a better understanding of hydraulic properties of and hydraulic connectivity between, the various aquifer systems.

▼ Numerical groundwater flow modelling to firm up estimates of mine inflows and dewatering requirements to a BFS level of certainty.

▼ Assessment of mine water balance and disposal options for any excess dewatering water.

▼ Evaluation of future groundwater monitoring and regulatory reporting requirements.


CONTENTS

Our Reference 1009B/032 i

CONTENTS

1 INTRODUCTION ...................................................................................................1 1.1 BACKGROUND ............................................................................................. 1 1.2 LOCATION................................................................................................... 1 1.3 TOPOGRAPHY .............................................................................................. 1 1.4 CLIMATE ..................................................................................................... 1 1.5 GEOLOGY.................................................................................................... 1

1.5.1 REGIONAL GEOLOGY ............................................................................ 1 1.5.2 LOCAL GEOLOGY.................................................................................. 1

1.6 HYDROGEOLOGY.......................................................................................... 2

2 DAVIDSON CREEK AIRLIFT TESTING ...................................................................7 2.1 AIRLIFT TESTING PROGRAMME ...................................................................... 7 2.2 DATA ANALYSIS AND RESULTS ...................................................................... 7 2.3 GROUNDWATER QUALITY .............................................................................. 8

3 DEWATERING.....................................................................................................17 3.1 CONCEPTUAL MINE PLAN............................................................................. 17 3.2 ASSUMPTIONS AND DEWATERING ANALYSIS ................................................. 25 3.3 DISCUSSION OF RESULTS........................................................................... 26

4 CONCLUSION AND RECOMMENDATIONS............................................................33

5 REFERENCES ......................................................................................................34

TABLES

Table 2.1: Airlift Drill Hole Information ........................................................................... 11 Table 2.2: Observation Drill Hole Information .................................................................. 12 Table 2.3: Aquifer Parameters ....................................................................................... 13 Table 2.4: Groundwater Quality Determinations made during Airlift-Testing ......................... 16 Table 3.1: Conceptualised Pit Geometry over 10-year Mining Plan ...................................... 17 Table 3.2: Bulk Aquifer Properties used in the Davidson Creek Mine Dewatering Analysis ....... 25 Table 3.3: Estimated Yearly Dewatering Requirements...................................................... 27

FIGURES

Figure 1.1: Davidson Creek Location ................................................................................ 3 Figure 1.2: Geology ....................................................................................................... 5 Figure 2.1: Groundwater Chemistry – Piper Diagram .......................................................... 9


CONTENTS

ii Our Reference 1009B/032

Figure 3.1: Conceptual Model of Mine Dewatering ............................................................ 19 Figure 3.2: Davidson Creek Mine Pit Plan ........................................................................ 21 Figure 3.3: Mining Schedule - East-West Pit Profiles ......................................................... 23 Figure 3.4: Predicted Annual Dewatering Requirements of the Davidson Creek Mine.............. 29 Figure 3.5: Dewatering Zone of Influence ....................................................................... 31

APPENDICES

APPENDIX A: LABORATORY WATER ANALYSIS ANALYTICAL REPORT


INTRODUCTION

Our Reference 1009B/032 Page 1

1 INTRODUCTION

1.1 BACKGROUND

FerrAus Limited requested Aquaterra to carry out a Prefeasibility Level Dewatering Investigation for their Davidson Creek Iron Ore Project. This report details the results of the field data collection programme and the subsequent analysis, to estimate pit dewatering requirements at the Davidson Creek mine site.

1.2 LOCATION

The Davidson Creek Iron Ore Project is located in the East Pilbara region approximately 75km east of the township of Newman, on the northern border of the Jigalong Aboriginal Reserve. Access is via the Jimblebar access road. The Site is shown on Location Plan Figure 1.1.

1.3 TOPOGRAPHY

The area around Davidson Creek is flat to gently sloping. A low east west ridge runs immediately south of the ore body with the land directly above the deposit sloping gently to the north. Drainage is predominantly to the north with major drainage features running through the western end (Thirteen Creek) of the proposed pit and 1km to the east (Davidson Creek) of the pit.

1.4 CLIMATE

The Pilbara Region is characterised by an arid climate, receiving summer rainfall. Cyclones occur during this period, bringing heavy rain and causing potential destruction to inland and coastal towns.

The region has an extreme temperature range, potentially rising to 50°C during the summer, and dropping to around 0°C in winter. At Newman, mean monthly maximum temperatures range from 39°C in January to 22°C in July (with corresponding monthly minimum temperatures range 25°C and 7°C). High summer temperatures and humidity seldom occur together, giving the Pilbara its very dry climate.

The region has a highly variable rainfall, which is dominated by the occurrence of tropical cyclones, mainly during the period from January to March. The moist tropical storms from the north bring sporadic and drenching thunderstorms. With the exception of these large events, rainfall can be erratic and localised, due to thunderstorm activity. Therefore, rainfall from a single site may not be representative of the spatial variability of rainfall over the entire catchment during an event. The driest months are September to November.

The annual average rainfall for Newman is 300mm pa. Variability is high, with annual rainfall varying between about 150mm and 500mm. The mean annual pan evaporation rate is about 3200-3600mm, which exceeds annual rainfall by around 3000mm. Average monthly pan evaporation rates vary between a minimum 144mm in June and a maximum 384mm in December.

1.5 GEOLOGY

1.5.1 REGIONAL GEOLOGY

The Davidson Creek Iron Ore Project area is situated in the eastern margin of the Hamersley Iron Province, Western Australia. Figure 1.2 shows the geology at Davidson Creek

The ore body at Davidson Creek is contained within the Archaean Marra Mamba Formation. This formation is part of The Hamersley Group which is commonly characterised by banded iron formation (BIF). Structurally, folding and faulting within The Hamersley Group is a common feature. Alluvial deposits of Cainozoic age often overlie the sequence.

1.5.2 LOCAL GEOLOGY

At Davidson Creek the rocks of the Hamersley Basin dip at 35 to 45 degrees towards the north. The Davidson Creek ore body is hosted in the Hamersley Group Marra Mamba Iron Formation


INTRODUCTION

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and, more specifically, in the Mount Newman Member which is the uppermost unit of the Marra Mamba Formation. This formation is characterised by massive to finely laminated chert, shale and BIF, as well as abundant vughs and brecciation. At the Davidson Creek site, the Marra Mamba Iron Formation is about 200m thick, some 50m of which consists of Mount Newman rocks (Darvall et al., 2009).

The Marra Mamba Formation is underlain by the Jerrinah Formation of the Archaean Fortescue Group. This formation is characterised by a volcanogenic sequence including massive and vesicular basalts, intermediate flows and tuffs. Sediments, minor chert and pebble conglomerate are intercalated throughout the entire sequence. Overlying the Marra Mamba Iron Formation is the Wittenoom Formation, with the basal West Angela Member consisting of shale, manganiferous shale with minor BIF and chert. With the exception of the outcropping Jerrinah and Marra Mamba rocks forming the east-west trending ridge, the Davidson Creek site is covered with aeolian sand, recent colluvium and alluvium. These sediments are up to 60m thick at Davidson Creek (Darvall et al, 2009).

The Fortescue, Hamersley and younger rocks envelop a core of Sylvania Inlier basement rocks, which outcrop just south of the propose mine site, and consists of meta-granite and meta–diorite. Darvall, et al (2009) observed NE and SW trending, sub-vertical strike-slip faults that cut across all the units at the Davidson Creek site. They describe dextral fault displacements of up to 1200m along the major NE trending faults located on the eastern edge of the area. The fault zones are characterised by silicified gouge and breccia of up to 1m thick.

1.6 HYDROGEOLOGY

During mineral exploration drilling at Robertson Range groundwater was encountered in variable quantities and a range of groundwater in-flows to bores have been noted anecdotally. At some locations, inflow rates have been high enough to impede the progress of drilling. At ‘Davidson’ (some 17 to 18km to the west of the site) a bore has provided reliable supply of good quality stock water for over 50 years. Details of the bore construction are not available however the groundwater level at Davidson is believed to be within 30m of surface. It is likely that the bulk of the aquifer recharge takes places during periodic flooding associated with significant rainfall events.

The groundwater level at the Davidson Creek mine site lies between 18 and 24 metres below ground level or m.bgl (Table 2.1), whilst the watertable lies 10 to 15 m below the alluvial floodplain to the north of the mine site. The groundwater gradient in the vicinity of the Davidson Creek site estimated at 0.002. The airlift-testing of 10 holes drilled into the ore body (Mt Newman Member) indicate the high yield capacity of this aquifer (i.e. average immediate yield of 10.6 L/s, Table 2.1), whilst those tapping the West Angela shales (Wittenoom Formation) to are low yielding (average 1.1 L/s).

FIGURE 1.1AUTHOR:

REPORT NO:JOB NO:

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LEGENDLocation

Davidson Creek

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FIGURE 1.2AUTHOR:

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Davidson Creek

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Airlift Test Site


DAVIDSON CREEK AIRLIFT TESTING


2 DAVIDSON CREEK AIRLIFT TESTING

2.1 AIRLIFT TESTING PROGRAMME

The airlift testing programme utilised existing mineral exploration bores that were ‘cleared-out’ using a RC drilling-rig, prior to lowering airlifting apparatus down the hole. The apparatus consisted of 6.6m lengths of 50mm steel pipe with camlock joints. The basal pipe housed a pressure transducer for recording waterlevels during drawdown and recovery. The pressure transducer was set to take readings every 30 seconds. Compressed air pumped into the pipe escaped through holes drilled several metres above the pressure transducer, to lift water from the drill hole. Bores were airlifted for approximately 1 hour, with groundwater flows from the hole measured by v-notch weir. Waterlevels were also monitored in nearby drill holes during airlifting and for 1 hour after completion of airlifting.

Ten locations within the Davidson Creek Ore Body were selected for testing, largely based on the occurrence and volume of water encountered during mineral exploration drilling. Many of these holes had since undergone rehabilitation including cutting-off of the collar pipe below ground level, capping and covering with soil. These sites were located and uncovered by FerrAus staff prior to the airlift testing programme. An additional five sites were selected to provide information on the hydraulic properties of the bedrock outside of the ore body. Figure 1.2 shows the airlift-test locations.

Fourteen of the 15 airlift tests produced data suitable for analysis. One test was re-run as airlifted water was found to have run back down the hole during the initial test. The failed test on bore 13 produced no water during airlifting and resulted in loss of the airlift apparatus down the hole. Table 2.1 and Table 2.2 show details of the airlift and observation holes used during the programme and the airlift yields achieved during airlift testing. In general, data recorded by the pressure transducer in the airlift hole produced usable data during both airlifting and recovery. On some occasions waterlevel recorded during airlifting were erratic and not suitable for analysis. It is likely that drill hole instability and collapse during testing caused these anomalous readings. In each of these cases, however, the data collected during waterlevel recovery monitoring was suitable for analysis.

2.2 DATA ANALYSIS AND RESULTS

Several methods were used to obtain estimates of aquifer transmissivity (T) and where possible storativity (S) from the airlift test data, including the Theis and Cooper-Jacob time-drawdown methods, as well as the Theis recovery method (Kruseman & de Ridder, 1994). Specific capacity (i.e. waterlevel drawdown divided by airlift yield) information was also used to derive T-values using Logan’s (1964) method and the Bradbury-Rothschild (1985) computer code.

Aquifer thickness (h) was estimated from the bore geological logs and the hydraulic conductivity (k) of the aquifer was calculated using k = T/h. As the holes were logged for mineral exploration purposes, information relating to groundwater occurrence is scant and the aquifer thickness was not always apparent. In most cases, within the Marra Mamba Iron Formation, the ore body thickness was assumed to be the aquifer thickness.

Table 2.3 displays the aquifer parameters and method used for each airlift test.

The average airlift yield of 10 holes in the Marra Mamba Iron Formation is 20.7 L/s (standard deviation of 1.7). The average transmissivity and hydraulic conductivity of the Marra Mamba aquifer is 128 m2/day (standard deviation of 85.1) and 3.1 m/day (standard deviation of 2.6), respectively. The geometric mean of the aquifer transmissivity and hydraulic conductivity is 106 m2/day and 2.3 m/day, respectively. The average storativity of the aquifer is 0.001 (i.e. specific storage of 2.7×10-5). Hydrogeological investigations carried out by Aquaterra elsewhere in the Pilbara region, have shown that the mineralised Marra Mamba Iron Formation (i.e. ore body) have K and specific yield (Sy) values that range between 2.8 to 8.0 m/day and 0.01 to 0.10, respectively (Aquaterra, 2009).

The average airlift yield of the 5 bores tapping the Wittenoom and Jerrinah Formations is 1.0 L/s (standard deviation 1.0). The arithmetic and geometric mean of the T values obtained for these rocks are 10 m2/day (standard deviation 10) and 5 m2/day, respectively. Similarly, the




arithmetic and geometric mean of the hydraulic conductivity is 0.2 and 0.1 m/day. Aquaterra’s estimates of K values for the West Angela Member (Wittenoom Formation) and Jerrinah Formation, obtained elsewhere in the Pilbara region, are 0.01 – 0.50 and 0.03 m/day, respectively. Similarly, the average specific yield of the West Angela shale and Jerrinah Formations is 0.001 and 0.03, respectively. The storativity of the Jerrinah Formation is ~2.0×10-4, whilst that of the West Angela shale ranges between 2.0×10-4 and 1.0×10-5 (Aquaterra, 2009).

2.3 GROUNDWATER QUALITY

Field water quality testing was undertaken during each airlift test, generally just prior to the cessation of airlifting. Figure 2.1 shows the results of these tests. The TDS of the groundwater in the Marra Mamba Iron Formation is highly variable (average 686 mg/L and ranging between 360 and 1210 mg/L), whilst the pH is neutral. The groundwater salinity increases away from the ridge and onto the alluvial covered floodplain, i.e. the TDS of the water in bores 11, 12 and 13 varies between 1400 and 1600 mg/L, and the pH exceeds 8.0.

In addition to the field water quality testing, groundwater samples were collected for laboratory analysis from five airlift test holes, namely 2, 6, 11, 12 and 15. The laboratory analytical report is included as Appendix A. The chemical character of the sampled groundwater is graphically presented on a Piper diagram (Figure 2.1), where it is evident that the water in the Marra Mamba Iron, Wittenoom and Jerrinah Formations are chemically distinct from one another. The groundwater in the Marra Mamba Iron and Jerrinah Formations are of a sodium-chloride (Na-Cl) type, whilst that in the Wittenoom is more a sodium-magnesium-bicarbonate water (bore 12). The groundwater from bore 12 also has a high fluoride content of 1.6 mg/L. The calcium concentration of the water sampled from bore 15 is anomalously low (4.8 mg/L).

GROUNDWATER CHEMISTRY – PIPER DIAGRAM FIGURE 2.1

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Table 2.1: Airlift Drill Hole Information

Airlift Test # FerrAus Drill Hole #

Easting MGA94, z51

Northing MGA94, 51

Elevation (mAHD)

SWL (m.bgl) Airlift Yield (L/s) Target Formation

1 DCRC0271 242800 7407901 524 19.46 11.2 Marra Mamba Iron










11 DCRC0536 239006 7408300 526 10.7 0.6 Jerrinah

12 DCRC0544 240201 7411095 513 15.79 2.7 Wittenoom

13 DCRC0571 248601 7408301 527 22.64 No Flow Wittenoom

14 DCRC0558 249541 7409695 529 20.31 1.2 Wittenoom

15 DCRC0546 250701 7406492 544 37.64 0.8 Jerrinah




Table 2.2: Observation Drill Hole Information

Airlift Test # FerrAus Drill Hole # Easting MGA94, z51 Northing MGA94, 51 Elevation Radial Distance Pump Hole (m)

SWL (mbgl) Target Formation

1 DCRC0270 242801 7407848 526 53 19.62 Marra Mamba Iron

1 DCRC0272 242800 7407949 517 48 Blocked Marra Mamba Iron
















13 DCRC0570 248611 7408250 522 52 22.73 Wittenoom




Table 2.3: Aquifer Parameters

Target Formation Airlift Test # Analysis Method Transmissivity (T in m2/day)

Storativity (S) Aquifer Thickness (m)

Hydraulic Conductivity (K in m/d)

Marra Mamba 1 Logan's Approximation 88.61 - 82 1.08

Marra Mamba 1 Theis (1935) 98.7 - 82 1.20

Marra Mamba 1 AT35 Theis 32.5 1.720E-03 82 0.40

Marra Mamba 1 Theis Recovery 108 - 82 1.32

Marra Mamba 1 CJ Time DD 97.6 - 82 1.19





Marra Mamba 2 CJ Time DD 446 - 46 9.70







Marra Mamba 4 AT35 Theis 236 3.800E-05 28 8.43


Marra Mamba 4 132 - 28 4.71










Marra Mamba 5 Theis Recovery 47.5 - 36 1.32

Marra Mamba 5 CJ Time DD 131 2.230E-09 36 3.64




























Jerrinah 11 Logan's Approximation 2.4 - 60 0.04

Jerrinah 11 Theis (1935) 2.1 - 60 0.04

Jerrinah 11 AT35 Theis 1.18 - 60 0.02

Jerrinah 11 Theis Recovery 3.2 - 60 0.05

Jerrinah 15 Logan's Approximation 1.66 - 63 0.03

Jerrinah 15 Theis (1935) 1.4 - 63 0.02

Jerrinah 15 Theis Recovery 3.49 - 63 0.06

Wittenoom 12 Logan's Approximation 24.8 - 50 0.50

Wittenoom 12 Theis (1935) 25.7 - 50 0.51

Wittenoom 12 Theis Recovery 22.4 - 50 0.45

Wittenoom 14 Logan's Approximation 17.08 - 60 0.28

Wittenoom 14 Theis (1935) 17.1 - 60 0.29

Wittenoom 14 Theis Recovery 3.2 - 60 0.05




Table 2.4: Groundwater Quality Determinations made during Airlift-Testing

Airlift Test Bore #

pH EC mS/m

TDS ppm

Temp 0C

1 7.23 104 520 29.1

2 8.1 96 480 27.6

3 7.85 127 630 27.8

4 7.78 72 360 28.2

5 7.43 107 530 29.2

6 7.93 242 1210 31.2

7 7.86 119 590 26.7

8 7.77 133 660 26.1

9 8.1 179 890 26.6

10 7.8 198 990 29.8

11 8.61 315 1580 25.3

12 8.08 296 1470 29.5

14 7.84 142 710 31.1

15 8.13 272 1360 29.2


DEWATERING


3 DEWATERING

3.1 CONCEPTUAL MINE PLAN

The Davidson Creek final mine pit plan shows a long, east-west orientated, open cut void. The conceptual model for estimating the mine’s dewatering requirements is that of a ‘strip mine’ in which groundwater inflow into the mine pit is calculated assuming parallel inflow from the pit walls, as well as a radial inflow component accounting for the four pit wall corners (Figure 3.1). The bounding material on the north face of the pit is considered to be West Angela shale (Wittenoom Formation), whilst the southern face is composed of Roy Hill shale (Jerrinah Formation). The shorter faces at the east and west ends of the pit are considered to be Marra Mamba Iron Formation with similar hydrogeological properties to that of the ore body. The model assumes that no inflow occurs through the base of the pit. Stored water from the excavated material is also taken into account.

The mine scheduling and progression of the pit excavation is adopted from the Davidson Creek Iron Ore Project Mining Scoping Study of December 2008. The geographic extent and contours of the mine pit plan is illustrated in Figure 3.2. This information was used to estimate the basic geometry of the mine pit on an annual basis for the 10-year life span of the mine. Figure 3.3 shows annual east-west sections of the pit profile from year 1 to year 10. The ‘average’ waterlevel in the vicinity of the Davidson Creek mine is estimated at 507 mAHD. For calculation purposes the pit profiles for each year have been approximated by one or two rectangles or sections (Table 3.1) from which groundwater inflows were calculated. Only the western portion of the ore body (Section 1) will be mined during the first 4 years, whereafter the mine pit will gradually be extended towards the west (Section 2). The Section 1 mine pit will attain a maximum depth of 147 below the ‘average’ watertable or 360 mAHD, after 9 years of mining.

Table 3.1: Conceptualised Pit Geometry over 10-year Mining Plan

Section 1 Section 2 Year

Length (m) Width (m) Depth Pit (m)*

Length (m) Width (m) Depth Pit (m)*

1 660 380 37 0 0 0

2 980 400 57 0 0 0

3 960 400 87 0 0 0

4 940 400 107 690 400 37

5 930 400 137 690 400 67

6 1620 400 137 840 400 7

7 1620 400 137 840 400 17

8 1620 400 147 840 400 47

9 1620 400 147 840 400 87

10 2460 400 147 0 0 0

Note: * - Depth of Pit below ‘average’ waterlevel (507 mAHD)

CONCEPTUAL MODEL OF MINE DEWATERING FIGURE 3.1

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FIGURE 3.2AUTHOR:

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Davidson Creek

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Airlift Test Site

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MINE SCHEDULE EAST-WEST PIT PROFILE FIGURE 3.3

350

370

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430

450

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530

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Yr 1 Yr 2 Yr 3 Yr 4 Yr 5 Yr 6 Yr 7 Yr 8 Yr 9 Yr 10 SWL


DEWATERING


3.2 ASSUMPTIONS AND DEWATERING ANALYSIS

In order to obtain preliminary estimates of potential groundwater inflows into the Davidson Creek mine pit and therefore their dewatering requirements, assumptions regarding the hydrogeological properties of the ore body and surrounding country rock have been made and are summarised in Table 3.2.

Table 3.2: Bulk Aquifer Properties used in the Davidson Creek Mine Dewatering Analysis

Parameter Unit Assumed Value Justification

Marra Mamba Iron Formation (MMIF)

2.0 Corresponds to the geometric mean of values derived from analysis of the airlift tests

Wittenoom Formation 0.1 Corresponds with the values derived from analysis of airlift-tests

Hydraulic Conductivity (K in m/day)

Jerrinah Formation 0.04 Corresponds with the values obtained elsewhere by Aquaterra (Section 2.22.2)

Marra Mamba Iron Formation

0.04

Wittenoom Formation 0.005

Specific Yield (Sy)

Jerrinah Formation 0.005

Corresponds with lower-end of values derived in other areas with similar geology (Section 2.2)

Statistical analysis of hydraulic conductivity values derived from pumping tests carried out elsewhere in the Marra Mamba Iron Formation provided an arithmetic and geometric mean of 2.2 and 1 m/d, respectively (Aquaterra, 2009).

It has been assumed that the Marra Mamba Iron Formation (MMIF), and more specifically the Davidson Creek ore body, is not in direct hydraulic connection with the relatively thick alluvial aquifer that extends to the north and south of the mine site. It is assumed that the relatively low permeability West Angela and Jerrinah shale rich units form a relatively low permeability envelope around the MMIF, which will restrict the rates lateral recharge from adjoining aquifer systems, including the alluvial aquifers.

The dewatering analysis was conducted for a number of scenarios in order to take into account uncertainties related to the limited knowledge of the aquifer parameters (Table 3.2) and makes use of Aquaterra’s experience gained from similar projects elsewhere in Western Australia:

▼ Scenario 1 – The aquifer hydraulic parameters are as per Table 3.2, using information derived from the pumping-tests (Section 2).

▼ Scenario 2 – as per Scenario 1, except the hydraulic conductivity of the ore body in the MMIF is decreased to 1 m/day.

▼ Scenario 3 - as per Scenario 1, except the hydraulic conductivity of the ore body in the MMIF is doubled to 4 m/day.

▼ Scenario 4 – as per Scenario 1, except the specific yield of the MMIF ore-body decreased to 0.01.

▼ Scenario 5 – as per Scenario 1, except the specific yield of the MMIF ore-body increased to 0.08.

▼ Scenario 6 – as per Scenario 1, except the hydraulic parameters of the Jerrinah and Wittenoom host rock are doubled (i.e. the K values of the Wittenoom and Jerrinah Formations are increased to 0.2 and 0.08 m/day, respectively; and the specific yield of these rocks is increased to 0.01).


DEWATERING


It must be noted that the analytical equations used in this dewatering assessment are based on a number of general theoretical assumptions regarding the flow of water in aquifers that are not always met in a natural system and hence represent a first-order approximation rather than an exact result.

3.3 DISCUSSION OF RESULTS

A standard analytical solution for estimating groundwater inflow into a strip mine excavation under unconfined conditions was used to estimate the dewatering requirements of the Davidson Creek Mine over the proposed 10 years of operation. Anticipated dewatering pumping rates are presented in Table 3.3 and graphically summarised in Figure 3.4.

Based on the current mining plan and the current hydrogeological understanding of the Davidson Creek area, the following observations can be made:

▼ Dewatering Scenarios 1 to 3 produce similar results, with the exception of groundwater inflows during year 1. This indicates that if the estimated hydraulic conductivity of the Marra Mamba (ore body) aquifer is doubled or halved, it does not significantly alter the potential groundwater inflow rates into the mine pit.

▼ In Scenarios 4 and 5 the specific yield of the Marra Mamba aquifer is decreased to 0.01 and increased to 0.08, respectively. This has a dramatic effect on the predicted pit inflow rates and shows the sensitivity of the results to uncertainties in the specific yield of the ore-body. The results of the Scenario 4 and 5 analyses are thought to represent the two unlikely extremes that form an envelope which encompasses the more likely pit inflow rates.

▼ Scenario 6 takes into account the possibility that hydraulic properties of the adjoining shale units in the Wittenoom and Jerrinah Formations have been substantially under-estimated, i.e. 100%.

▼ During the first year of mining it is estimated that groundwater inflows into the mining area will range between 38 to 45 L/s (3280 to 3880 m3/day).

▼ Lower pumping rates of between 22 to 26 L/s (1900 to 2250 m3/day) will be required to dewater the mine excavations during years 2 and 3.

▼ Groundwater inflows into the mine will continuously increase over years 4 to 6, as the mining area and pit depth is increased. During year 6 peak inflows of 44 to 50 L/s (3800 to 4320 m3/day) are predicted.

▼ In year 7 the groundwater inflows into the mining area will drop-off dramatically to between 24 and 30 L/s (2070 to 2600 m3/day), where after inflows will gradually increase from year 8 onwards up until the end of the operational life of the mine, i.e. 10 years. Maximum pumping rates of between 46 and 52 L/s (3970 to 4500 m3/day) are likely to be required to adequately dewater the mine pits during year 10.

It must be noted that these estimates of the dewatering requirements assume average climatological and aquifer recharge conditions and do not take into account possible short–term periods of surface water flooding following cyclonic events and associated high rates of aquifer recharge.

The analytical methods used, also provide first order estimates of potential effects of dewatering on the regional waterlevels in the ore body and adjoining aquifer systems. The development of the ‘zone of maximum waterlevel drawdown’ in the Marra Mamba (Ore Body), Wittenoom and Jerrinah aquifers over the 10 year operational life of the Davidson Creek Mine are graphically summarised in Figure 3.5, where it can be observed that:

▼ Dewatering of the Marra Mamba Iron Formation (ore body) will result in the gradual development of an elongated cone of waterlevel drawdown within the ore body, which could extend up to ~6 km away from the mine after 10 years.

▼ Dewatering of the ore body in the Marra Mamba Iron Formation will also result in the gradual development of a cone of waterlevel drawdown in the adjoining Wittenoom and Jerrinah Formations that could extend up to 1km north and 3km south of the mine, respectively.


DEWATERING


Table 3.3: Estimated Yearly Dewatering Requirements

Scenario 1 Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10

Q (L/s) 34 21 21 31 37 44 20 38 33 45

Q (m3/day) 2941 1793 1828 2649 3187 3774 1696 3245 2850 3911

Scenario 2

Q (L/s) 28 21 21 30 37 43 19 37 33 45

Q (m3/day) 2446 1785 1816 2634 3167 3756 1680 3228 2834 3896

Scenario 3

Q (L/s) 42 21 21 31 37 44 20 38 33 46

Q (m3/day) 3640 1805 1846 2669 3214 3798 1719 3269 2872 3932

Scenario 4

Q (L/s) 15 8 10 14 18 23 16 21 20 26

Q (m3/day) 1334 705 897 1223 1591 1963 1392 1855 1718 2228

Scenario 5

Q (L/s) 54 37 35 53 61 71 24 59 50 71

Q (m3/day) 4657 3236 3058 4536 5297 6173 2087 5083 4345 6142

Scenario 6

Q (L/s) 37 25 28 39 49 59 34 53 48 64

Q (m3/day) 3176 2127 2401 3379 4224 5113 2969 4617 4173 5561

PREDICTED ANNUAL DEWATERING REQUIREMENTS OF THE DAVIDSON CREEK MINE FIGURE 3.4

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DEWATERING ZONE OF INFLUENCE FIGURE 3.5

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CONCLUSION AND RECOMMENDATIONS


4 CONCLUSION AND RECOMMENDATIONS

The Davidson Creek Mine will require dewatering throughout the 10-year operational life span of the mine. Groundwater levels lie at a depth of 18 to 24 metres below ground surface at the mine site and according to the current mine plan the pit will penetrate ~37m of saturated ore-body during its first year of operation, with an expected groundwater inflow rate of 38 to 45 L/s. The mine dewatering requirements will increase to a maximum of 46 to 52 L/s after 6 years of operation, which will then decline and gradually increase to similar inflow rates by year 10 when the base of the pit will lie ~147m below the pre-mining waterlevel. The groundwater in the Marra Mamba Iron Formation is relatively fresh (i.e. TDS < 1000 mg/L). Mine dewatering will result in the gradual development of an ellipsoidal-shaped cone of waterlevel drawdown, orientated parallel to the strike Marra Mamba Iron Formation, that is likely extend up to 6km away on either side of the mining area.

It is recommended that further hydrogeological fieldwork be undertaken at the Davidson Creek mine site in order to firm up the current estimates of the dewatering requirements and potential impacts thereof over the 10 year period of operation to a PFS or DFS level of assessment. In order to complete such a study it is recommended that the following work be undertaken:

▼ Bore survey and field hydrological data collection that includes mineral exploration holes and other privately-owned bores located in an area extending up to distance of 5km from the mine site.

▼ Drilling and pump-testing of between 4 and 6 production test holes at selected locations within the proposed mining area. Of importance to this study, will be an assessment of the possible hydraulic connection between the Marra Mamba (ore body) and the extensive alluvial aquifers located to the north and south of the mine.

▼ Re-evaluation of the mine inflow and dewatering requirements, as well as potential impacts thereof on the regional waterlevels, to a BFS level of certainty using a numerical groundwater flow model.

▼ Assessment of the mine water balance and disposal options for excess dewatering water.

▼ Evaluation of future groundwater monitoring and regulatory reporting requirements.


REFERENCES


5 REFERENCES

Aquaterra, (2009): Hydrogeological Assessment for Jimblebar Iron Ore Project, Technical Report 1008/600/057B, Report prepared for BHP Billiton Iron Ore Pty Ltd., August 2009.

Bradbury, K.R. & Rothschild, E.R., (1985): A computerized technique for estimating the hydraulic conductivity of aquifers from specific capacity data, Ground Water, Vol. 32, No. 2, pp 240-246.

Darvall, P., McCarthy, R. & Hawke,P., (2009): Journey to the Edge of the Basin – Stratigraphic Setting and Iron Mineralisation at the Davidson Creek and Robertson Range Projects, Hamersley Province, Western Australia, Iron Ore Conference, 27-29 July 2009, Perth, pp 67-712.

Environmental Simulations (1996): Guide to using WinFlow – Two-Dimensional Groundwater Flow Model, Developed by J. & D. Rumbaugh, Herdon, Virginia, pp 85.

Kruseman, G.P. & de Ridder, N.A., (1994): Analysis and evaluation of Pumping Test Data, International Institute for Land Reclamation and Improvement Publication No. 47, The Netherlands, 377p.

Logan, J. (1964): Estimating transmissibility from routine production tests of water wells, Ground Water, Vol. 2, No. 1, pp 35-37.

APPENDIX A: LABORATORY WATER ANALYSIS ANALYTICAL REPORT

Detection 2 6 11 12 15

Limits

Date Sampled 01-Oct-2009 28-Sep-2009 02-Oct-2009 03-Oct-2009 29-Sep-2009

pH pH Units <0.1 7 8.1 8.5 8.1 8.3Conductivity @25oC µS/cm <2 860 2100 2800 2700 2400

TDS (Calculated) mg/L <5 520 1300 1700 1600 1400

Sodium, Na mg/L <0.5 92 240 520 300 630

Potassium, K mg/L <0.1 9.1 24 20 16 25

Calcium, Ca mg/L <0.2 44 120 88 140 4.8

Magnesium, Mg mg/L <0.1 30 92 57 120 20

Hardness (as CaCO3 mg/L <5 230 670 460 830 96

Soluble Iron, Fe mg/L <0.02 0.03 <0.02 <0.02 <0.02 0.14

Chloride, Cl mg/L <1 160 460 490 370 510

Bicarbonate, HCO3 mg/L <5 130 330 370 1000 240

Sulphate, SO4 mg/L <1 83 250 530 410 420

Nitrate, NO3 mg/L <0.2 47 <0.2 100 <0.2 59

Fluoride, F mg/L <0.1 0.8 0.9 0.9 1.6 1

Free Cyanide mg/L <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

Soluble Aluminium, Amg/L <0.02 <0.02 <0.02 <0.02 0.03 0.03

Soluble Arsenic, As mg/L <0.002 <0.002 0.002 0.002 <0.002 <0.002

Soluble Manganese, mg/L <0.005 <0.005 0.044 0.017 <0.005 0.12

Soluble Lead, Pb mg/L <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

Soluble Cadmium, C mg/L <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Soluble Copper, Cu mg/L <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

Soluble Zinc, Zn mg/L <0.01 0.024 0.11 <0.01 <0.01 0.029

Sediment n/a Moderate Moderate Moderate Heavy Moderate

Odour n/a None None None None None

Colour n/a Light Light Light Light Light

Turbidity n/a Light Light Moderate Light Light

Cation/Anion balance% -0.6 1.81 -0.35 -8.83 3.16

Sum of Ions (calc.) mg/L 591 1512 2185 2366 1901

Airlift Test Bore Number

Determinand

Documents

Water and Environment - EPA WA · and Marra Mamba rocks forming the east-west trending ridge, the Davidson Creek site is covered with aeolian sand, recent colluvium and alluvium