a9 mesa c h3 hydrogeological level assessment (rio tinto

OKP – Water Resource Evaluation & Services Mesa C H3 Level Assessment

i

Executive Summary

The H3 hydrogeological level assessment describes the hydrogeological conceptualisation and

analytical model developed to predict dewatering requirements of the Mesa C deposit with the

purpose of predicting dewatering requirements and aquifer drawdown across dependent systems,

including the Robe River.

The conceptual hydrogeological model considers the local aquifer as a closed system. While the

uncertainty analysis considered an open hydrogeological system, whereby groundwater

throughflow between the CID, the Ashburton Aquitard and the Robe River Aquifers occurs.

Analytical modelling was completed using AQTESOLV, with predictions indicates that Mesa C

dewatering requirements and estimated drawdown are:

• Total maximum abstraction volume during the Life of Mine (LoM) is approximately 8.5 GL;

• Peak annual groundwater abstraction of up to 3.5 GL is expected;

• Up to 15 m drawdown is predicted in the CID aquifer;

The uncertainty run indicates that:

• Total abstraction volume during the LoM to be up to 21 GL;

• A drawdown extending east of the Mesa C deposit towards the Robe River was predicted up

to 6 m.

The overall risk to other groundwater users, the environment and regional aquifer is considered

low and groundwater abstraction of the Mesa C deposit for dewatering purposes is not envisaged

to have adverse effects. This is as a result of drawdown predicted to be limited to the CID aquifer

area only. Any unlikely reduction of the groundwater level east of the Mesa C, along the Robe

River, is not expected to have adverse effects, due to direct rainfall and stream flow events along

the Robe River catchment several order of magnitude higher than the estimated reduction in the

alluvium storage.

Groundwater and environmental monitoring and management procedures are planned to be in

place to reduce and assess potential impacts. This includes, but is not limited to, groundwater

level and abstraction monitoring, hydro-geochemical testing and regular reporting on the aquifer.

Future work is planned to assess and improve the hydrogeological understanding in the area,

particularly to assess the hydraulic connections between pit areas, the Ashburton Formation and

the Robe River alluvium aquifer. Long term data and increased hydrogeological understanding will

allow modelling to be updated as this information becomes available.


ii

Contents

Section 1 - Introduction .................................................................................................. 1

Section 2 - Climate and Rainfall ...................................................................................... 3

Hydrology and drainage .................................................................................................................... 3

Section 3 - Hydrogeology ................................................................................................ 6

Bore network ...................................................................................................................................... 6

Hydrostratigraphy .............................................................................................................................. 6

Groundwater levels .......................................................................................................................... 14

Groundwater Recharge ..................................................................................................................... 17

Groundwater Discharge .................................................................................................................... 17

Conceptual model .............................................................................................................................. 17

Water Balance .................................................................................................................................. 18

3.7.1 Aquifer Storage ..................................................................................................................... 18

3.7.2 Recharge ................................................................................................................................ 18

3.7.1 Throughflow .......................................................................................................................... 19

Section 4 - Existing Groundwater Use ........................................................................... 20

Mesa H & Mesa J .............................................................................................................................. 20

Mesa A / Warramboo ....................................................................................................................... 20

Station Bores and Exploration Bores .............................................................................................. 23

Section 5 - Groundwater Investigation .......................................................................... 24

Drilling .............................................................................................................................................. 24

Test Pumping.................................................................................................................................... 24

Groundwater chemistry ................................................................................................................... 25

5.3.1 Chloride ................................................................................................................................. 26

5.3.2 Isotopes ................................................................................................................................. 28

5.3.3 Summary of hydro-geochemical observations: ................................................................... 28

Section 6 - Groundwater Model .................................................................................... 30

6.1.1 Analytical Model Set-up ....................................................................................................... 30

6.1.2 Model Calibration ................................................................................................................. 32

6.1.3 Model Predictions ................................................................................................................. 33

6.1.4 Model Summary .................................................................................................................... 37

Assessment of Potential Impacts ..................................................................................................... 37

6.2.1 Groundwater Users ............................................................................................................... 38


iii

6.2.2 The Aquifer ............................................................................................................................ 38

Section 7 - Groundwater Management .......................................................................... 39

Groundwater monitoring ................................................................................................................. 39

Management Approach .................................................................................................................... 40

Figures

Figure 1 Mesa B and C location map and existing bore locations ................................................... 2 Figure 2 Mesa C annual (October- September) rainfall ................................................................... 4 Figure 3 Yarraloola stream gauging station yearly average flow volumes ...................................... 5 Figure 4 Mesa C Monitoring Bore Locations and CID deposit (aerial photo April 2017) ................. 8 Figure 5a Mesa C Geological cross Sections A-A’ ........................................................................ 10 Figure 5b Mesa C Geological cross Sections B-B’ ........................................................................ 11 Figure 5c Mesa C Geological cross Sections C-C’ ........................................................................ 12 Figure 5d Mesa C Geological cross Sections D-D’ ........................................................................ 13 Figure 6 Mesa C Groundwater levels Vs Rainfall (refer to Figure 3 for bore locations) ................ 15 Figure 7 Pre-mining water table contour map ................................................................................ 16 Figure 8 Monitoring Summary – Warramboo Abstraction Vs Water Level .................................... 21 Figure 9 Monitoring Summary – Mesa A and Warramboo Rainfall Vs Water Level. ..................... 22 Figure 10 Piper Diagram for initial chemistry results at Mesa C by Formation .............................. 26 Figure 11 Chloride concentrations in Ashburton Formation and CID aquifer ................................ 27 Figure 12 Isotopes Deuterium and Oxygen 18 .............................................................................. 28 Figure 13 Mesa C Model domain and pit zones ............................................................................ 31 Figure 14 Bench progressions vs Predicted Water levels based on Sy of 0.15 ............................ 34 Figure 15 Bench progressions vs Predicted Water levels based on Specific yield 0.15 ............... 34 Figure 16 Base Case drawdown prediction ................................................................................... 35 Figure 17 Uncertainty Run: drawdown prediction .......................................................................... 36

Tables

Table 1 Current operating, long term daily rainfall stations in the Robe River catchment ............... 4 Table 2 Summary of local key geological and hydrogeological characteristics ............................... 9 Table 3 Adopted Hydraulic Parameters for throughflow estimation............................................... 19 Table 4 Annual Pre-mining Water Balance – Uncertainty Run ...................................................... 19 Table 5 Bench progression by pit zone .......................................................................................... 32 Table 6 Base Case- Abstraction rates by pit area with different specific yield scenarios .............. 33 Table 7 Uncertainty Run- Abstraction rates by pit area with different specific yield scenarios ..... 33 Table 8 Current Monitoring Schedule ........................................................................................... 39

Appendix A

Mesa C and Warramboo Hydrogeological Drilling and Test Pumping 2016 Report


1

Section 1 - Introduction

Mesa B and Mesa C Channel Iron Deposit (CID) are located east and adjacent to the Mesa A and

Warramboo mine sites, approximately 50 km west of the Pannawonica Township and 140 km

southwest of Karratha in the Pilbara region of Western Australia. Located adjacent to the Robe

River, both deposits form part of the chain of Robe River palaeochannels and tributaries in the

Robe Valley. The mesas are flat topped geomorphological structures, elevated roughly 50m above

the valley plain. Adjacent to Mesa B and C are a number of ephemeral pools and one semi-

permanent pool near the railway. The nearest Permanent pool to Mesa C is more than 7km away,

located downstream to the northwest of the deposit at the North West Coast Highway and Robe

River intersection (Figure 1).

Approximately 5% of the Mesa C orebody occurs below the water table (BWT) and the pre mining

groundwater level of the mesa stands at between 64 to 70 mAHD. Groundwater level in the vicinity

of Mesa B is approximately 55 mAHD. The entire inferred orebody at Mesa B occurs above the

water table (AWT); subsequently no dewatering will be required at this deposit. Groundwater

abstraction would be limited to the Mesa C deposit for dewatering purposes.

Dewatering at the Mesa C deposit is not expected to be extensive and all groundwater abstracted

is in support of mine dewatering. Dewatering volumes are expected to be used for dust

suppression. Between 10 and 15 m of drawdown is required to achieve dry mining conditions at

Mesa C. Based on drilling, hydrogeological analysis and modelling, the predicted groundwater

abstraction volume from Mesa C deposit is estimated to be 6 GL for the Life of Mine (LoM).

As part of the Mesa BC proposal, extensive drilling, test pumping and hydrogeological

investigations at Warramboo have been conducted for the Mesa BC wet plant water supply. The

wet plant water supply is proposed to be sourced from the Yarraloola Aquifer located west of the

Warramboo Mine. The estimated water supply for Mesa BC wet ore processing and other mining

requirements is 6-11 GL/a over an eleven year period. Warramboo Mesa BC wet plant water supply

investigations is presented in the accompanying Warramboo H3 level assessment (RTIO 2017).

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LEGEND

Geospatial Information and Mapping

I r o n O r e ( W A )

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Department of Water Bore

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Plan No:PDE0162155v1Proj:MGA94 Zone50

Drawn:T.M.Date: June, 2018

Figure 1:Mesa B and CLocation mapand Existing

Bore Locations

This document has been prepared to the highest level of accuracy possible, for the purposesof Rio Tinto’s iron ore business. Reproduction of this document in whole or in part by anymeans is strictly prohibited without the express approval of Rio Tinto. Further, this documentmay not be referred to, quoted or relied upon for any purpose whatsoever without the writtenapproval of Rio Tinto. Rio Tinto will not be liable to a third party for any loss, damage, liabilityor claim arising out of or incidental to a third party using or relying on the content containedin this document. Rio Tinto disclaims all risk and the third party assumes all risk and releasesand indemnifies and agrees to keep indemnified Rio Tinto from any loss, damage, claim orliability arising directly or indirectly from the use or reliance on this document.

Disclaimer

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1:75 000 @ A4

SCALE

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YandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicooginaYandicoogina

Mesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa J

Mesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa A

Port Hedland

Cape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape Lambert

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Western Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner SynclineWestern Turner Syncline

NammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiNammuldiBrockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2Brockman 2

Mount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom PriceMount Tom Price

MarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooMarandooBrockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4Brockman 4

ChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarChannarEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern RangeEastern Range

ParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdooParaburdoo West AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasWest AngelasHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeHopeDowns 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4Downs 4

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Mesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa JMesa J

Mesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa A

Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1Hope Downs 1

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Port Hedland

Cape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape LambertCape Lambert

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Location Map


3

Section 2 - Climate and Rainfall

The climatic conditions experienced in the Pilbara region are classified as semi-arid to arid with hot

and dry conditions prevailing for the majority of the year. Average maximum temperatures are high

and range from ~41C in January to ~26.7 C in July. The high temperatures and solar radiation

levels contribute to high evapotranspiration rates in the order of 3400 mm/a (BoM, 2008). Annual

rainfall for the area is taken from four long-term rainfall stations operational in the Robe Valley

catchment with key characteristics summarised in Table 1.

Long-term mean annual rainfall for the Mesa C deposit is 342 mm, and exhibits large inter-annual

variability with annual rainfall ranging between 54 mm (1923/24) and 736 mm (1933/34). This

variability is due to the episodic nature of tropical cyclones and thunderstorms which occur

generally between November and March accounting for about 70% of total annual rainfall during

these seasonal periods. The annual gridded rainfall series as taken from the BoM’s (website) long

term daily data is shown in Figure 2, with the annual rainfall trend showing a long term rise of

0.9 mm/a. On average, the Mesa BC area experiences 36 rain days per year, but daily rainfall

totals larger than 50 mm are uncommon.

Hydrology and drainage

The Robe River catchment has an area of 7,104 km2. The Mesa C deposit is a divide between

Yarraloola sub-catchments of the Robe Valley Catchment. Rainfall and runoff in upper sub-

catchments contribute to the recharge of lower sub-catchments and the Robe River Alluvial Aquifer.

Mesa C sub-catchment is approximately 40 km2 and confined to the south and west by other CID

deposits and in the east by the Robe River. Records indicate over the period from 1974 – 2016,

that the average annual flow through Robe River Catchment was 119 GL, as recorded by the

Yarraloola Stream Gauging station located 20 km upstream of the Mesa C area at the North West

Coast Highway. Figure 3 shows the yearly average recorded flows.

The Robe River runs directly east of Mesa C and is the major watercourse in the region. Robe

River is ephemeral due to sporadic rainfall events with a few ephemeral and semi-permanent pools

observed southeast and north of Mesa C (Figure 1). Rainfall run-off is directed over the edges of

the mesa on all sides with surface water flow and drainage lines eventuating north of the mesa

toward the Robe River.


4

Table 1 Current operating, long term daily rainfall stations in the Robe River catchment

Number Station Name Location Record Period Data Recorded

005032 Yarraloola 383,897 mE

7,614,316 mN

1899 - present 94%

005022 Red Hill 403,499 mE

7,569,646 mN

1898 - present 92%

005029 Yalleen 437,182 mE

7,602,834 mN

1930 - present 53%

005069 Pannawonica 430,750 mE

7,606,959 mN

1971 - present 80%

Figure 2 Mesa C annual (October- September) rainfall


5

Figure 3 Yarraloola stream gauging station yearly average flow volumes


6

Section 3 - Hydrogeology

The preliminary hydrogeological conceptualisation of the Mesa C deposit was developed based on

data collected from Resource Evaluation Drilling in 2015 and a hydrogeological investigation

conducted in 2016 that included drilling, test pumping and hydro-geochemical analysis of selected

locations at the Mesa C deposit and the east valley between the Robe River and Mesa C.

Bore network

An initial groundwater monitoring network was first established in 2015. It included installation of

one monitoring bore on Mesa B and four monitoring bores at Mesa C through the conversion of

resource geology drill holes. A hydrogeological drilling program carried out in 2016 expanded this

groundwater network to support the Mesa BC wet Plant Pre-Feasibility Study (PFS). In total, the

Mesa C groundwater network includes 2 test production bores and 13 monitoring bores. Bore logs

and constructions are in Appendix A. Majority of the bores were installed to target the CID Aquifer,

however bores installed in the valley adjacent to the Mesa and Robe River were installed outside

of the CID resource (Figure 4).

Hydrostratigraphy

The following section describes the geology and groundwater characteristics encountered during

hydrogeological and resource evaluation drilling and testing activities to-date. In the vicinity of Mesa

C, four key hydro-stratigraphic units have been identified. They are summarised below and in Table

2. Hydrostratigraphic cross sections of Mesa C are presented in Figure 5 (Sections A – D).

• Robe River Alluvial Aquifer: Robe River Alluvials have an estimated 15-20m saturated

thickness with an average width of 400m. This aquifer is considered to be highly

conductive with recharge occurring through rainfall and stream flow events from the

upper catchment, however there is minimal data relating to the Robe River aquifer in the

immediate vicinity of Mesa C.

• CID Aquifer: This consists of channel iron deposit pisolite sediments. This aquifer has

a variable saturated thickness across the deposit ranging on average 10 and 20 m. It is

up to 500m wide and is limited to the east length of the Mesa C deposit (Figure 4). The

water table predominantly lies at the top of the basal Robe Pisolite unit. The basal Robe

Pisolite is considered to have impermeable characteristics and typically 5 to 10m thick,

but up to 30m thick in some areas. The basal unit is not considered to be part of the iron

ore resource.

• Ashburton Aquitard: Comprises of interbedded volcanoclastics with tuff, mudstone,

shale and sandstone with varying encounters and thickness across the east area below

the CID. The thickness and hydraulic properties of the Ashburton volcanoclastics and

sediments is not well defined, but is considered to have low permeability and as such is


7

referred to as the Ashburton Aquitard. Hydraulic connection and permeability however

may be associated discontinuous intersections of sandstone and fractured

volcanoclastics. The Ashburton Formation volcanoclastics and sandstone lithology has

only been intersected on the east strike of mesa and adjacent to the Robe River.

• Ashburton Aquiclude: The Ashburton Formation to the west strike of the mesa

underlying the CID predominantly consists of shale and mudstone and is hence

considered to be impermeable. The local thickness and extent of the Ashburton shale is

not well defined, however the Ashburton Formation basement is extensive across the

area.

7,602,000 mN

7,600,000 mN

394,000 mE392,000 mE

392,000 mE 394,000 mE

7,60

0,00

0 m

N7,

602,

000

mN

LEGEND



Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

Robe

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Robe








River



Figure 4: Mesa C Monitoring

Bore Locations and CID deposit


Disclaimer


1:25 000 @ A4

SCALE


Water bore

Monitoring

VWP

Hydrology

Railway

CID Aquifer

Pit

0.25 0 0.25 0.5 0.75


9

Table 2 Summary of local key geological and hydrogeological characteristics

Unit Formation

Name Aquifer Type General Lithology

Hydraulic Characteristics

Robe River Alluvials

Quaternary Alluvials

Unconfined, sedimentary,

regional

Well rounded, pebbles, gravels to fine unconsolidated

conglomerate of varying rock types

High permeability, high storage, hosts major water

course

CID Robe Pisolite Unconfined, sedimentary,

local

Hard capped pisolite, undifferentiated pisolite (clayey, iron rich, and mixed), clay/silica-rich

basal pisolite

heterogeneous, high and low permeability

associated with varying clay content, vertically and

horizontally limited.

Basement Ashburton Formation

Aquitard, local

Volcaniclastics, mudstone, shale, and

sandstones. Highly variable, discontinuous

lithologies

Heterogeneous, low permeability, zones of

permeability associated with fracturing and

immature sandstone lithology

Aquiclude, local

Shale and mudstone not permeable, creates local groundwater divide


10

Figure 5a Mesa C Geological cross Sections A-A’


11

Figure 5b Mesa C Geological cross Sections B-B’


12

Figure 5c Mesa C Geological cross Sections C-C’


13

Figure 5d Mesa C Geological cross Sections D-D’


14

Groundwater levels

The pre-mining groundwater elevation in the CID Aquifer stands between the 64 and 70 mAHD

and depth to water is approximately 55 mbgl from the mesa surface (Figure 6 and Figure 7). The

hydraulic gradient of the CID aquifer is 0.002. Groundwater and surface water elevation in the

Robe River Alluvial Aquifer is highly dynamic with seasonal variation, rainfall events and recharge

in the upper catchment.

Over the review period between January 2016 and April 2017, during periods of stream flow along

the Robe River from February 2017, varying degrees of recharge has been observed across

monitoring points in the Mesa C area. During drier periods, and when the rainfall event has not

lead to stream flow and surface runoff, groundwater levels have remained fairly stable.

Groundwater levels in the CID aquifer have been relatively stable since monitoring began in early

2016 with groundwater level variance of up to 0.5 m that appears to be seasonal compared to

monitoring bores in the valley locations screened in Alluvials and Ashburton volcanoclastics where

groundwater levels have been observed to vary by up to 2 m (MB16MEC0008 and

MB16MEC0009). Figure 6 presents groundwater levels and rainfall data observations since Mesa

C groundwater monitoring began.

The hydrographs suggests that recharge of the CID aquifer occurs during streamflow events in the

Robe River due to the raise of the surface water level and consequential leakage to the aquifer.

The fact that some bores in the mesa are showing a raise in the water level and others are not,

corroborates the assumption of limited connection potentially caused by localised faults and other

geological structures in the basement.


15

Figure 6 Mesa C Groundwater levels Vs Rainfall (refer to Figure 3 for bore locations)

7,600,000 m

N7,602,000 m

N

394,000 mE392,000 mE

392,000 mE 394,000 mE

7,60

0,00

0 m

N7,

602,

000

mN

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Figure 7: Pre-mining water level contour map


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Hydrology

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SCALE

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VWP


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Groundwater Recharge

Recharge to the CID aquifer is limited to throughflow or seepage. Depth to water at the top of the

mesa is greater than 50 m, subsequently recharge from direct rainfall recharge is considered

negligible. Any rainfall runoff is directed off the edges of the mesa to the north and east towards

the Robe River. In steady state and drier conditions or during seasons with inconsequential rainfall

events, the CID Aquifer is likely to be isolated from throughflow recharge. However, seasonal

throughflow or seepage to the CID aquifer may occur during periods of streamflow in the Robe

River due to the short-timed raise of the water table, as indicated on 3.3.

Groundwater Discharge

Discharge from the CID Aquifer is presumed to be occurring as throughflow only, as

evapotranspiration is negligible from the Mesa as water levels are >50 meters below ground level

(mbgl) and there is currently no groundwater abstraction occurring from the CID aquifer. Discharge

throughflow potentially occurs at the northern intersect between the CID and Robe River Alluvial

Aquifer. Throughflow potential is increased during periods of streamflow and recharge within the

upper catchment. The Robe River Alluvial Aquifer discharge occurs via evapotranspiration and

throughflow.

Conceptual model

The conceptual model is guided by the key themes that prescribe the current understanding of

groundwater occurrence and flow in the vicinity of Mesa C. These themes are summarised below:

- CID Aquifer has low storage and is spatially limited to the east strike of Mesa C. The clayey

nature of the basal pisolite unit of the CID likely act as a partial barrier to vertical flow,

limiting potential hydraulic connection between aquifers

- Robe River Alluvial Aquifer has high storage and high throughflow volumes. It is

periodically recharged by direct rainfall and surface water runoff in the immediate vicinity

and within the upper catchment.

- Ashburton Aquitard hydrogeological characteristics are largely undefined, spatially and

geologically, however, limited hydraulic connection between the Robe River Alluvial

Aquifer and CID Aquifer may occur through isolated intersects of local sandstone and

fractures.

- The Ashburton Aquiclude is not permeable due to shale and mudstone content and creates

a groundwater divide through the basement at Mesa C and features consistently along the

west strike of the Mesa C area.

The conceptual base case the CID aquifer is isolated in steady state conditions, it was considered

appropriate to evaluate an uncertainty model run to assess the outcome sensitivity regarding the


18

Robe River due to the proximity of the Robe River and potential throughflow component of

sandstone intersects in the Ashburton Aquitard. Therefore, the base case and the uncertainty run

are referred to as the following:

• Base Case: A closed system, whereby the CID aquifer is considered to be isolated from

the surrounding regional aquifers

• Uncertainty Run: An open system, whereby the CID aquifer is considered to be in

hydraulic connection with the Ashburton Aquitard.

Water Balance

3.7.1 Aquifer Storage

Due to insufficient data to define the hydraulic connection along the boundary extents between the

CID, the Ashburton Aquitard and the Robe River Alluvial Aquifer, an Uncertainty Run was

developed to incorporate the hydraulic connection between the CID and the Alluvial Aquifer.

In the Base Case there are no inflows (recharge or throughflow) or outflows to be considered in

the water balance when dewatering has not yet commenced. Thus this closed conceptualisation

assumes groundwater is sourced from storage only. CID Aquifer storage was based on an aquifer

area of 2 km2, aquifer thickness of 20 m and a specific yield of 0.15. Therefore the aquifer storage

of the Mesa C CID Aquifer has been estimated to be ~6GL.

The Uncertainty is based on an “Open” aquifer system, a water balance was devised based on

Darcy’s law and chloride mass balance. The water balance considers the extents of the CID

aquifer, average rainfall, chloride concentrations and results from test pumping completed in 2016.

This run was developed to assess the maximum possible hydraulic connection between the CID

and the Robe River.

3.7.2 Recharge

To calculate catchment recharge by rainfall the mean chloride concentration of Mesa C

hydrogeochemical analysis was used with the precipitation chloride concentration and the chloride

mass balance equation below:

Recharge = P x Cl(p)/ Cl(gw)

P – precipitation

Cl(p) - concentration of chloride in the precipitation

Cl (gw) - Concentration of chloride in the groundwater

Based on a mean groundwater chloride at the Mesa C deposit of 1110 mg/L, precipitation chloride

of 3.4 mg/L (Dogramaci et al, 2012) and average rainfall of 342mm/a, the recharge by rainfall was


19

estimated to be 1.04 mm/a, similar to that reported in the Warramboo Modelling Report (RTIO-

PDE-0116844). This value is 0.2% of the annual average rainfall. Over the sub-catchment area of

40 km2 this becomes 40 ML/a.

As a result, seepage throughflow is assumed to be the only meaningful groundwater inflow into the

Mesa C CID Aquifer via the Ashburton Formation in the Uncertainty Run.

3.7.1 Throughflow

Discharge from the CID Aquifer is presumed to be occurring as throughflow only, as

evapotranspiration is considered to be negligible in this water balance, due to the depth to the

water table from the top of the mesa being greater than 50m, and the CID not occurring in the lower

areas where surface runoff and recharge does happen. Discharge throughflow occurs via the

northern intersect between the CID Aquifer and Robe River Alluvial Aquifer (Figure 5 Section D).

Throughflow was estimated using the hydraulic parameter results from test pumping analysis of

the CID Aquifer only. Values used for throughflow estimation is given in Table 3 and was calculated

using Darcy’s Law: Q= KA(dH/dL). Table 4 indicates the calculated water balance for the

Uncertainty Run.

Hence the CID throughflow is estimated to be 175 ML/a.

Table 3 Adopted Hydraulic Parameters for throughflow estimation

Outflow calculated using Darcy's Law

K (m/day) 24

Thickness m 20

Width m 500

Hydraulic gradient 0.002

Table 4 Annual Pre-mining Water Balance – Uncertainty Run

Inflow Outflow

Rainfall 0.048 ML/a* Evapotranspiration 0

Throughflow 175 Ml/a Throughflow 175 ML/a

Total 175 ML year 175 ML year

* Catchment rainfall recharge


20

Section 4 - Existing Groundwater Use

Groundwater use in the area is driven by mining operations, pastoral station use and groundwater

dependent ecosystems (GDE). The Robe Valley has a number of operating and proposed mine

sites including Mesa A and Warramboo near to the Mesa C deposit as well as Mesa J and Mesa

H situated approximately 30 km east of Mesa C at the confluence the Robe River and

Jimmawurrada Creek. Within the Mesa H and J area a number of GDEs and pools have been

identified along the Robe River, including significant ethnographic sites.

Mesa H & Mesa J

Groundwater use and abstraction is dominated by mining activities in the area. Mesa J located 30

km upstream of the Mesa C deposit has been in operation for more than 20 years. The annual

abstraction rate is approximately 12 GL/a. Discharge to the Jimmawurrada Creek and Robe River

Tributary occur at an approximate rate of 3 GL/a in total. Much of the abstracted groundwater is

recycled through the use of Waste Fines Storage Facilities. Groundwater abstracted from sumps

and mine borefield is used for ore processing and other mining activities. The Mesa H deposit

directly west of the Mesa J operation is proposed to begin operation in 2020. The water supply

demand for both the Mesa H and J operation is predicted to increase by 2 GL/a. Extensive studies

have been conducted in support of the Mesa H study with hydrogeological assessments focused

on the potential impact to Robe River and significant sites adjacent to Mesa H.

Mesa A / Warramboo

Mesa A and Warramboo mine site borefield, located 15 km west of the Mesa C deposit, has been

abstracting groundwater since 2010. The current annual abstraction rate is approximately 1.3 GL/a,

with a current groundwater license expiring 16th October 2023 (GWL162500(7)) allowing up to 3

GL/a to be abstracted. Future water demand, in combination with Mesa B and C is estimated to be

up to 11 GL per annum.

Figure 8 shows the water level versus abstraction (kL/ month) over time for bores installed across

Mesa A and Warramboo. Figure 9 shows rainfall (mm) versus water levels from the Mesa A

weather station, as reported on the 2016 Mesa A Annual Aquifer Review.


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Figure 8 Monitoring Summary – Warramboo Abstraction Vs Water Level


22

Figure 9 Monitoring Summary – Mesa A and Warramboo Rainfall Vs Water Level.


23

Station Bores and Exploration Bores

Six pastoral station bores and one Department of Main Roads supply bore have been identified in

the vicinity of the Mesa B and C deposits (Figure 1). Including a number of exploration bores, the

status and use of these bores are unknown. Impact to these bores is unlikely as the groundwater

drawdown due to dewatering activities is not expected to extend to these areas.


24

Section 5 - Groundwater Investigation

Groundwater monitoring of the Mesa C deposit began in 2015 with resource evaluation drillhole

conversions to monitoring bores in several locations across the deposit. Prior to this groundwater

water investigation in the area were limited to the Mesa A and Warramboo deposits. With initial

resource evaluation drilling, approximately 5% of the Mesa C resource was characterised BWT

alluding to further hydrogeological investigations. Groundwater investigations to date have

included, but not limited to hydrogeological drilling, test pumping and analytical modelling. All

relevant groundwater work carried out to date is presented below in chronological order.

• Rio Tinto 2015. Out of scope Resource Evaluation drillhole conversion to monitoring bores

• Rio Tinto 2016 – PFS Hydrogeological Drilling Bore Completion Report: Installation of 2

production bores, 9 monitoring bores (includes two nested piezometers), pumping test

program and water sampling campaign to support PFS.

These drilling programs and investigations were used to support this report. Summaries of drilling,

test pumping and hydrogeochemical analysis used to support groundwater analytical modelling is

presented in the following sections. Relevant bore completion and hydrogeological assessment

reports can be found in Appendix A.

Drilling

Initial drill hole conversions in 2015 included four monitoring bores at Mesa C and one monitoring

bore at Mesa B for baseline groundwater levels and hydraulic gradient. Drilling in 2016 was

conducted to develop hydrogeological conceptualisation, support groundwater modelling for

dewatering strategies and to establish a groundwater monitoring network for baseline groundwater

levels and hydro-geochemistry. The full 2016 PFS Hydrogeological Drilling and Test Pumping

report is in Appendix A.

Test Pumping

In 2016, preliminary test pumping of 2 production bores was conducted to assess the hydraulic

conductivity and specific yield / storage parameters of the CID Aquifer and Ashburton Aquitard.

The following summarises results of the two tests.

Summary

• CID is presumed to be an unconfined aquifer;

• Ashburton Aquitard has hydraulic connection with the CID in isolated areas, however the

spatial extent of the Ashburton Aquitard is undefined;

• Hydraulic conductivity of the CID aquifer ranged between 23 and 41 m/day with a storage

coefficient of approximately 2.5E-03 ;


25

• A conclusive result of transmissivity value for CID Aquifer of 480 m2/day based on

WB16MEC0002 CRT and monitoring bore observations;

• Inconclusive transmissivity of the Ashburton Aquitard was estimated as 70 m2/day, and a

storage coefficient of 2.30E-03. Results are inconclusive as test pumping of production and

observation bore construction did not specifically target the Ashburton Formation;

• The bulk average hydraulic conductivity of the CID and Ashburton Aquitard was estimated

at between 1.5 and 50 m/day with a storage coefficient of approximately 5.1E-03.

Test pumping results also found that boundary conditions and/or hydraulic connection between the

CID and Ashburton Aquitard could not be conclusively identified. Groundwater level responses

associated with the two pump tests, with observation bores screened in CID versus the Ashburton

Aquitard, indicated hydraulic connection between units in one test but not the other.

Groundwater chemistry

Baseline hydro-geochemical information was collected in 2016 for Mesa C. Full details of sampling

and hydro-chemical laboratory results can be found in the Appendix A. Major cations and anion

distribution is shown in the piper plot in Figure 10.

Field parameters collected upon the completion of each bore drilling indicated that pH and electrical

conductivities (EC) reached levels in the range of 6.98 to 7.95 and 2,570 to 5,560 µS/cm

respectively.


26

Figure 10 Piper Diagram for initial chemistry results at Mesa C by Formation

5.3.1 Chloride

Chloride levels are elevated in the Mesa C deposit area with up to 1540 mg/L. The lowest range of

concentration were from the valley locations at up 757 mg/L, which suggests, as expected, that the

valley area is more exposed to direct rainfall recharge and/or groundwater throughflow. Figure 11

shows the distribution of Chloride results from sample locations indicating the respective slotted

formation: CID (orange) or Ashburton Formation (green). Elevated chloride levels are indicative of

low recharge rates compared to other areas. In conjunction with elevated sodium levels, it suggests

the deposit area is an endpoint for groundwater throughflow.

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Figure 11: Chloride concentrations in Ashburton Formation

and CID aquifer


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Ashburton aquiclude

CID aquifer


28

5.3.2 Isotopes

Isotope samples were collected only from bores on Mesa C. These signatures are generally

enriched in Oxygen 18 compared with other areas of the Pilbara region. These signatures suggest

either evaporation prior to recharge or alternatively that the groundwater source has an enriched

isotope composition due to the source of water being older and from a drier climatic past compared

to the current rainfall regime, corroborating with the current conceptualisation. Figure 12 shows the

Mesa C Isotopes for Oxygen and Deuterium plotted with the Local Meteoric Water Line (LMWL)

for the Pilbara (Dogramaci et al, 2012). For comparison, the Mesa C isotopes have been plotted

with the data collected to support the Mesa H study and include isotope samples from the nearby

pools and from the Mesa H deposit (RTIO-PDE-0153233). The comparison is that the Mesa C CID

and Ashburton Formation signatures are more enriched than Mesa H area however they are less

enriched than Robe River pools near Mesa H as a result of evaporation.

Figure 12 Isotopes Deuterium and Oxygen 18

5.3.3 Summary of hydro-geochemical observations:

The CID aquifer, in general, shows high sodium and chloride signatures and enriched Isotopes.

The high chloride levels may suggest the water is fairly stagnant, corroborating with the current

conceptualisation and therefore signatures may reflect aged groundwater rather than high degrees

of evaporation of groundwater throughflow.

The Ashburton Formation has more variation in the hydro-geochemical results across the Mesa C

area however it is also enriched in chloride and other major anion/cations relative to other areas.

Hydro-geochemical variations from the Ashburton may be attributed to the different mineralogy in

this formation (volcanoclastics, sandstone, mudstone, tuff and shale). Since Ashburton Formation

lithologies are highly variable across the formation and difficult to log as continuous units, the


29

results may be representative of isolated hydro-stratigraphic areas within the formation. For

example, nested bore samples collected from the CID and Ashburton are very similar in one

location (HM16MEC0002), but for the other nested bore location the hydro-geochemistry is

different between the slotted zones. This suggests the CID Aquifer and Ashburton Aquitard may

be hydraulically connected in some places of the Mesa C area and isolated in others.


30

Section 6 - Groundwater Model

The key hydrogeological concepts established from drilling, test pumping and hydro-geochemistry

was gathered to develop the Mesa C groundwater analytical model. The purpose of the model was

to devise a dewatering strategy for Mesa C BWT and to assess the potential impacts of dewatering

activities on the Robe River. The analytical model was considered suitable for the small scale

dewatering requirements predicted in conjunction with hydrogeological understanding of the area.

6.1.1 Analytical Model Set-up

The conceptual model was represented in an analytical superposition solution constructed using

the AQTESOLV interface (Duffield, 2007). Superposition treats the variable rate as a sequence of

steps in which the discharge rate is constant in each step. Although AQTESOLV has limited

modelling capacity, the interface is considered adequate for the Mesa C dewatering prediction.

The model was based on hydrogeological understanding of the Mesa C CID aquifer, whereby the

main uncertainty in the conceptual model is whether the Ashburton Aquitard is in hydraulic

connection with the CID aquifer. Hence two predictive models have been developed as a result of

uncertain boundary conditions. The set boundaries for each the base case and uncertainty run are:

• Base Case: two parallel north south trending no-flow boundary conditions and 2 parallel

east west trending no-flow boundary conditions located along the length of the extents of

the CID deposit and edges of the mesa (Figure 13).

• Uncertainty Run: has two no flow boundaries, 1 along the west edge of the deposit and 1

at the south edge of the deposit; the eastern boundary was removed from the model to

allow the throughflow between the alluvial aquifer and the CID aquifer.

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LEGEND



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Figure 13: Mesa C Model domain and

pit zones


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SCALE

Hydrology

Railway

Pit

0.25 0 0.25 0.5 0.75

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Model boundary

Tracking point

Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1Pit zone 1




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The model comprises a single layer and hydraulic parameters in an unconfined aquifer. To simulate

the dewatering predictions however Theis confined solution was used. The assumptions for this

model are as follows:

• No-flow boundaries are vertical and orthogonal to the flow;

• No recharge is considered in the model during the pumping period;

• The model domain is homogeneous with a uniform thickness of 20m (the average

saturated thickness of the CID);

• Based on test pumping and historical knowledge of CID, the Transmissivity (T) was

allocated as 480 m2/day;

• AQTESOLV assumes the “sump” is a pumping well with horizontal flow when pumping and

is full penetrating the aquifer;

• Abstraction assumes 100% utilisation and availability of pumps;

• Initial head conditions from 2016 monitoring were used to calculate simulated drawdowns

and the drawdown required for dry mining conditions;

• Modelled mine plan schedule for dewatering targets assumes the lowest bench level

progressions of the year is reached the first day of that year;

• “Sump” locations are based on deepest parts of each pit zone (1, 2 and 3), indicated by

tracking points Sump 1, Sump 2 and Sump 3 respectively (Figure 13).

Based on other operations in the Robe Valley a sump pumping strategy is assumed as suitable for

the Mesa C dewatering. Three pit zones have been defined for sump location simulations (Figure

13). Bench progression and BWT levels in each pit zone is in Table 5. Modelled dewatering in each

pit zone is cumulative and hence pumping in one pit affects the other. Therefore dewatering

performance and prediction is reliant on concurrent and successive pumping in each pit zone. Due

to the cumulative effects of pumping in one pit area on another, changes to the pit design and

schedule may affect the dewatering volumes and abstraction periods required that are modelled

here.

Table 5 Bench progression by pit zone

Year 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033

Pit 1 100 84 80 80 68 56 56

Pit 2 120 112 104 92 88 64 64 64 56

Pit 3 112 108 100 96 80 80 60 60 *benches in mRL (AHD) / below water table pits are highlighted in blue

6.1.2 Model Calibration

There is no long term transient data available as no groundwater abstraction, other than test

pumping has occurred. Therefore the analytical model could not be calibrated against transient

data and hence initial test pumping results have been used for hydrogeological parameters and

prior knowledge of channel pisolitic aquifer characteristics. The base case steady state conditions


33

are such that there is no recharge and or discharge influences to add to the model. The steady

state conditions for the Uncertainty run has recharge and discharge occurring via throughflow at a

rate of 175 ML/a. Rainfall and evapotranspiration is considered negligible in both scenarios.

6.1.3 Model Predictions

The analytical model predicts cumulative drawdown impacts for the LoM across the CID deposit

and also assesses the extent of drawdown at the Robe River intersect adjacent to the Mesa C. Not

included in the model however is the high throughflow volume and storage of the Robe River

Alluvial Aquifer, whereby any dewatering impacts to the river are likely to be mitigated by periodic

recharge and streamflow events. Dewatering drawdown impact at the Robe River intersect is hence

over estimated with the AQTESOLV simulations. As a result an increase in the CID aquifer storage

of Sy 0.2 was included in the Uncertainty prediction, to account for additional hydraulic connectivity

and throughflow from the Robe River.

The model dewatering volume predictions is presented in the Table 6 and Table 7 below. The

predicted hydrographs of dewatering by pit area is presented in Figure 14 and Figure 15 for the

Base Case and Uncertainty Run respectively, Figure 16 and Figure 17 shows the predicted

drawdown contour map based for both scenarios respectively

Table 6 Base Case- Abstraction rates by pit area with different specific yield scenarios

Specific

Yield

Pit 1

(kL/ Day)

Pit 2

(kL/Day)

Pit 3

(kL/Day)

Approximate Total

Abstraction during

LoM (GL)

Sy= 0.05 2000 500 0 2

Sy= 0.1 3000 1500 0 3

Sy=0.15 4000 2200 0 4.5

Table 7 Uncertainty Run- Abstraction rates by pit area with different specific yield

scenarios

Specific

Yield

Pit 1

(kL/ Day)

Pit 2

(kL/Day)

Pit 3

(kL/Day)

Approximate Total

Abstraction

during LoM (GL)

S= 0.05 6000 5000 2200 15

S= 0.1 7000 6000 2200 17

S=0.15 8000 7000 2200 18

S=0.2 8000 8000 2200 21


34

(Colour coded by Pit zone, Orange = Pit 1, Red = Pit 2, Green = Pit 3, dash lines = Predicted water levels). Figure 14 Bench progressions vs Predicted Water levels based on Sy of 0.15

(Colour coded by Pit zone, Orange = Pit 1, Red = Pit 2, Green = Pit 3, dash lines = Predicted water levels). Figure 15 Bench progressions vs Predicted Water levels based on Specific yield 0.15

45

50

55

60

65

70

75

80

85

90

1/01/2021 28/09/2023 24/06/2026 20/03/2029 15/12/2031

m R

L

Year

Mine Plan and Water level predictions (base case)

Pit 1

Pit 2

Pit 3

MB15MEC004

MB15MEC003

MB15MEC001

45

50

55

60

65

70

75

80

85

90

1/01/2021 28/09/2023 24/06/2026 20/03/2029 15/12/2031

m R

L

Year

Mine Plan and Water level predictions (uncertainity run)

Pit 1

Pit 2

Pit 3

MB15MEC004

MB15MEC003

MB15MEC001

7,6

00

,00

0 m

N7

,60

2,0

00

mN

394,000 mE392,000 mE

392,000 mE 394,000 mE

7,6

00

,00

0 m

N7

,60

2,0

00

mN

15151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515

14.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.5

14.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.514.5

14141414141414141414141414141414141414141414141414141414141414141414141414141414141414141414141414

LEGEND



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River



Figure 16: Base Case

drawdown predictionJanuary 2033


Disclaimer


1:25 000 @ A4

SCALE

Hydrology

Railway

Pit

0.25 0 0.25 0.5 0.75

Pit design

Drawdown contours

Tracking point

Mesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa AMesa A

LocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocationLocation

394,000 mE 396,000 mE

7,6

02

,00

0 m

N7

,60

4,0

00

mN

394,000 mE 396,000 mE

7,6

00

,00

0 m

N

392,000 mE

7,6

02

,00

0 m

N7

,60

4,0

00

mN

7,6

00

,00

0 m

N

392,000 mE

1111111111111111111111111111111111111111111111111

2222222222222222222222222222222222222222222222222

4444444444444444444444444444444444444444444444444

3333333333333333333333333333333333333333333333333

5555555555555555555555555555555555555555555555555

6666666666666666666666666666666666666666666666666

7777777777777777777777777777777777777777777777777

8888888888888888888888888888888888888888888888888

LEGEND



Robe

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River



Figure 17: Uncertainty Run:

drawdown predictionJanuary 2033


Disclaimer


1:30 000 @ A4

SCALE

Hydrology

Railway

Pit

Pit design

Drawdown contoursUncertainty Run

Tracking point

0.25 0 0.25 0.5 0.75km


37

The removal of above water table material will expose the local aquifer to an increased recharge

from rainfall due to the removal of the vadose zone. The Mesa C has a total area of approximately

1.5 Km2, assuming a mean annual rainfall of 342 mm, during the planned 8 years of BWT

operations, an estimated extra 4 GL will need to be abstracted due to local aquifer recharge.

6.1.4 Model Summary

Summary of the predicted dewatering requirements and groundwater drawdown is below:

- Overall drawdown of the CID is predicted to be a maximum of 6 m below the lowest mined

bench level (56mAHD).

- Drawdown is uniform across the deposit and predicted to be 15 m by the year 2033.

- The sumps are predicted to operate at a maximum rate of 8 ML/ day to achieve dry mining

conditions.

- Groundwater abstraction of up to 4.5 GL over the LoM that results in drawdown of the CID

aquifer only. Amounting to approximately 75 % of the local aquifer storage.

- Uncertainty Run: A groundwater abstraction of up to 21 GL over the LoM was predicted

but, based on Mesa J’s dewatering experience such volume is very unlikely. The open east

boundary results in drawdown of the CID Aquifer and up to an estimated 6 m drawdown in

the Alluvials immediately east of the Mesa C deposit. However this drawdown is

considered to be overestimated due high throughflow volume of the Robe River catchment

and periodic streamflow events that could not be, due to technical limitations, incorporated

in the analytical model.

Assessment of Potential Impacts

The risk of potential impact to other groundwater users in the area is considered very low. This is

considering that the model base case limits drawdown to the CID aquifer area (2 km2) and any

drawdown predicted to the east (Uncertainty Run) would be temporary and mitigated by Robe River

Catchment throughflow and direct rainfall recharge due to climatic seasonality. Conceptually due

to the isolation of the CID aquifer, groundwater abstraction is likely only sourced from the CID

aquifer storage; therefore no negative impact is expected on the water balance of the regional

Robe River Alluvial Aquifer.

Groundwater hydro-geochemistry of the CID aquifer and regional aquifer is not likely to be impacted

from dewatering activities at Mesa C. Groundwater abstracted from the Mesa C is planned to be

used entirely for dust suppression purposes at Mesa C and hence water is to be used in the area

of same/similar hydro-geochemistry. Discharge of surplus water to the environment is not expected

due to the low abstraction rates required for dewatering; however hydro-geochemistry of the

dewatering volumes are similar to the surrounds and therefore is not envisaged to have an adverse

effect to the surrounding environment.


38

Although there is a risk of the base conceptualisation being wrong (uncertainty run) the volume of

water to be abstracted from Mesa C (8.5 GL during LoM) is very small in comparison to the overall

available volume along that section of the Robe River, and the recorded average stream flow

events that regularly recharge the Robe River aquifer.

6.2.1 Groundwater Users

Groundwater supply for pastoral bores and other groundwater users identified in the area are not

predicted to be impacted by the localised drawdown of the CID aquifer. It is important to note that

recharge via throughflow and direct rainfall to the Robe River was not included in the predictions

and therefore drawdown is likely to be mitigated by periodic recharge events.

6.2.2 The Aquifer

The approximated area extent of the saturated CID aquifer is 2 km2, which extends the outline of

Mesa C. The saturated thickness of the CID aquifer is between 10 and 30 m and is thickest at the

south of the deposit thinning toward the north. The total drawdown is predicted to reach up to 15m.

The expected drawdown modelled is predicted to extend to up to 6 m below the lowest bench level,

this may include up to 6 m of the Ashburton Aquitard.

Regional groundwater levels are not expected to be impacted. However the local CID Aquifer

groundwater level, is unlikely to recover to pre-mining levels in steady state conditions. However,

mining and back filing to above the pre-mining water table will change the local Mesa C catchment

such that increased surface runoff into the mesa may occur. This change would assist water level

recovery in the CID aquifer. However, in the case of Uncertainty Run, throughflow and rainfall

recharge is predicted to allow recovery to pre-mining water levels.


39

Section 7 - Groundwater Management

Despite there not being any envisaged negative impacts to other groundwater users, the

environment and aquifers in the area, groundwater monitoring and management procedures are in

place to reduce and assess potential impacts. This includes groundwater level and abstraction

monitoring, hydro-geochemical testing, monitoring of GDEs near Mesa C and regular reporting on

the aquifer.

Groundwater monitoring

The current groundwater monitoring schedule of the Mesa B and C deposits includes as a minimum

annual laboratory sampling and biannual water levels. This is to capture baseline data and

seasonal variation. In addition to manual dipping of water levels, many monitoring bore locations

have been installed with water level aqua trolls that record water levels on a daily basis.

An overview of the monitoring schedule for Mesa A and Warramboo, extracted from the approved

site Groundwater Operating Strategy (GWOS) is detailed in Table 8. It is envisaged this monitoring

schedule will be used for Mesa C also and maintained during LoM.

Monitoring data are reviewed and evaluated for validity prior to entry into the licensee’s central

database (EnviroSys) in readiness for reporting requirements. All data collected through the

monitoring programme are included within the Annual and Triennial Aquifer Reviews.

Table 8 Current Monitoring Schedule

Bore Type Frequency Parameter

Production Bores

Monthly Abstraction volume

Monthly EC, pH, Temperature

Annually

Field Chemistry (EC, pH, temp, TSS, Colour, Turbidity, Total

hardness)

Major Ions (CO3, HCO3, Ca, Na, K, Mg, SO4, Si, F, Fe, Al, Cl)

Dissolved metals (Ag, As, B, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo,

Ni, Pb, Sb, Se, Sn, U, Zn);

Nutrients (Total P, Total N, NO2, NO3, NH4)

Microbiological (Thermotolerant Coliforms, E.coli, Naegleria)

Monitoring Bores*

Quarterly Water level (*loggers installed record water level daily)


40

Management Approach

The Proponent has a defined Water Strategy that guides water management within the group. This

strategy requires an integrated approach to water management that promotes, maintains or

improves water quality, minimises fresh water use and maximises reuse and recycling.

The water abstracted from the Mesa C borefield is planned to be used for mining operations,

predominantly dust suppression with the aim to minimise surplus water discharge as much as

feasible.

A maintenance schedule will be implemented to ensure water infrastructure is operating optimally.

Maintenance personnel will visually check the condition of the pipelines and infrastructure

associated with transporting water from the borefield for potential leaks and ruptures.

Monitoring bores will be commissioned with data loggers for continuous monitoring of the water

level. Selected bores will have monitoring devices that will also read conductivity and salinity

changes over time.

All the production bores / sumps will be commissioned with automated telemetry systems to

optimise water abstraction and manage drawdown throughout the borefield. The groundwater

analytical model will be revised yearly based on transient data viability during LoM.


41

References

Commander, 1994: Hydrogeology of the Robe River alluvium, Ashburton Plain, Carnarvon Basin,

October 2012. Department of Water / WA

Dogramaci et. Al, 2015: Estimation of evaporative loss based on the stable isotope composition of

water using Hydrocalculator. Journal of Hydrology, volume 523, p781-789.

DoW, 2010. Lower Robe River ecological values and issues. Department of Water, Environmental

water report series, Report number 14

Geological Survey of Western Australia, 1990: Memoir 3, Geology and Mineral Resources of

Western Australia.

Geological Survey of Western Australia, 1987: Bulletin 133, Geology of the Carnarvon Basin

Western Australia, R.M. Hocking, H.T. Moors and W.J.E Van De Graaff

RTIO-PDE-0149328 - Warramboo Modelling report. Rio Tinto, Australia / WA, 2017.

RTIO-PDE-0152361 - Warramboo H3 Level Assessment. Rio Tinto, Australia / WA, 2017.

RTIO-PDE-0153233 - Mesa H H3 Level Assessment. Rio Tinto, Australia / WA, 2017.


38

Appendix A: Mesa C and

Warramboo Hydrogeological

Drilling and Test Pumping

2016

a9 mesa c h3 hydrogeological level assessment (rio tinto

Documents