2.1.6.6 Tarrawonga Mine (baseline) and Tarrawonga Coal Expansion Project (ACRD)


The Tarrawonga Mine is an open-cut mining operation south of, and adjacent to, the Boggabri Coal Mine. The Tarrawonga Mine commenced operations in 2006, and produces up to approximately 2 Mt/year ROM coal from the Maules Creek Formation using conventional open-cut mining methods. The mine originally had approval to operate until 2017. The Tarrawonga Coal Expansion Project was approved in 2013, allowing for an extension of operations to the east and north and the continued development of the mining operations to facilitate a ROM coal production rate of up to 3 Mt/year until 2030. The information presented here relates to the operations of the Tarrawonga Coal Expansion Project. Unless otherwise stated, the information in this section has been derived from Tarrawonga Coal Pty Ltd (2012).

ROM coal is crushed and screened at an on-site facility and then taken by truck to an off-site coal handling and preparation plant (CHPP).

2.1.6.6.1 Mine water use

The sources of water used at Tarrawonga Coal Expansion Project include the following, in order of priority:

  1. groundwater inflows to the open-cut and associated mine dewatering
  2. water storages containing runoff from active areas
  3. water storages containing runoff from up-catchment areas
  4. licensed groundwater extractions.

Water balance modelling of the performance of the mine water management systems was performed using 122 years of historical climate data, to give 122 possible mine life (17 years) ‘realisations’ (i.e. each model run started on a different year in the 122 year sequence, and the climate record was restarted at year 1, after the end of year 122).

The key water use requirements (demands) and water sources (inflows) in the modelling are summarised in Table 20.

Table 20 Summary of modelled inflows and outflows in Tarrawonga Coal Expansion Project water balance


Water balance elements

Simulated results

(ML/y)

25th percentile

Mean

75th percentile

Inflows

Rainfall-runoff

325

402

480

Groundwater production bore

0

0

0

Groundwater inflow to mine pit

255a

255a

255a

Outflows

Pond evaporation

118

130

141

Mine water spill to environment

0

0

0

Supplied to crusher

8

8

8

Supplied to truckfill

389

394

399

Supplied to irrigation

64

125

193

aThe groundwater inflow rate in the water balance model is not linked to climate, and so does not vary between wet and dry scenarios.

Data: Gilbert and Associates Pty Ltd (2011)

Table 21 shows the volumes that will need to be licensed in the future to account for aquifer interference of the mine void.

Table 21 Predicted licence requirements to address aquifer interference of Tarrawonga Coal Expansion Project


Water source (type)

Predicted mean annual inflow volumes requiring licensing

(ML/y)

Years 1 to 11

Year 12

Years 13 to 17

Post mining

Gunnedah-Oxley Basin

Mean 209

Max. 252

209

209

167

Upper Namoi zone 4

negligible

198

Mean 142

Max. 169

negligible

Data: Merrick and Alkhatib (2012)

2.1.6.6.2 Surface water management

The mine site is situated on the floodplains of the ephemeral Nagero, Goonbri and Bollol creeks, within the Namoi river basin.

The water management infrastructure comprises a mine water dam and a series of sediment dams and basins and drains used for controlling sediment-laden runoff from the mine area. Drainage works divert ‘clean’ water around the site or away from mine disturbed areas. Runoff from the disturbed areas is collected in sediment basins where suspended sediments are allowed to settle out. Mine water is stored in a mine water dam. Water from these storages is used for dust suppression or coal crushing and screening. During extended wet periods excess water is released from a number of sediment control structures as controlled discharge from licensed discharge points.

The mine plan involves mining through the current alignment of Goonbri Creek. Realignment of a 3 km section of the creek, further to the east, has been approved, and will be undertaken by year 12 of the project. The creek realignment will occur in conjunction with the installation of a low permeability barrier in the alluvium to the east and south-east of the open-cut extent (discussed in the following section), and the construction of flood bunds.

A temporary flood bund is required from year 12 of operations for the western bank of Goonbri Creek, to protect the advancing pit against the risk of inundation. The bund has been designed to protect the open-cut for the peak flow resulting from a 1 in 100 year average recurrence interval rainfall event. The bund will be constructed to a nominal height of 1.5 m above the natural surface level.

A flood protection bund will also be required on the eastern bank of Goonbri Creek to protect the construction works of the Goonbri Creek realignment and the low permeability barrier.

Permanent flood bunds will be constructed on both the eastern and western side of the final void. The permanent flood bunds will generally coincide with the alignment of the low permeability barrier and will be designed to a height to provide protection against the peak modelled flood height from a probable maximum precipitation event. At its maximum the bund would be 6 m high; it will also serve as a noise mitigation measure (Gilbert and Associates Pty Ltd, 2011).

The potential cumulative impacts of the mine on surface water flows as a result of runoff capture are summarised in Table 22 for the Tarrawonga Mine. The table shows that the runoff would be initially reduced. However, runoff would then progressively increase as areas are rehabilitated. The final void and its catchment will remain excised from the Namoi river basin post mining, resulting in a 6% loss of contributing flow from Nagero Creek. The realignment of Goonbri Creek is expected to result in a gain of 2.1% post mining (as parts of the Nagero Creek catchment are diverted to the Bollol/Goonbri creek catchments).

Table 22 Progressive and maximum changes to reductions in the contributing catchment of local creeks and Namoi River as a result of the Tarrawonga Coal Expansion Project


Mine

Percentage reduction in contributing catchment

Nagero Creek

Bollol/Goonbri creeks

Namoi River

Tarrawonga Mine prior to extension

2.4%

1.8%

0.01%

Year 2 – extension

6.9%

2.5%

0.02%

Year 4 – extension

6.3%

2.6%

0.02%

Year 6 – extension

4.5%

2.8%

0.02%

Year 12 – extension

2.9%

2.3%

0.01%

Year 16 – extension

3.0%

3.0%

0.02%

Post-mining

6.0%

–2.1%

0.004%

Data: Gilbert and Associates Pty Ltd (2011)

2.1.6.6.3 Groundwater management

A groundwater assessment by Merrick and Alkhatib (2012) evaluated the potential impacts of the Tarrawonga Coal Expansion Project. Mining since 2006 at Tarrawonga Mine and Boggabri Coal Mine provide strong hydrographic evidence of mining effects on the Maules Creek Formation (porous rock) groundwater system with no discernable effect on the alluvial groundwater system. The Tarrawonga Coal Expansion Project involves advancing the open-cut mine pit to excavate a small portion of the alluvial groundwater system.

A low permeability barrier will be constructed in the alluvium to minimise the rate of alluvial groundwater inflows into the open-cut both during operations and post-mining. The low permeability barrier will be constructed using a soil-bentonite mixture. The barrier will be approximately 2 km long and from 2 to 40 m deep. The base of the barrier will extend into the underlying rocks (Maules Creek Formation) by approximately 1 m (Tarrawonga Coal Pty Ltd, 2012).

The expected pit inflows are listed in Table 23. Modelling indicates that these values are likely to be overestimates. The cumulative effects of the nearby Boggabri, Maules Creek and Rocglen coal mines are likely to mean that groundwater levels in the region around all four mines will be lowered more rapidly, however, as the drawdown results from groundwater inflows to four mines, it is likely that total inflows to each mine will be reduced.

Table 23 Predicted pit inflows for the Tarrawonga Coal Expansion Project acting alone


Project year

Pit inflow

(ML/y)

Project year

Pit inflow

(ML/y)

1

146

10

251.85

2

200.75

11

208.05

3

219

12

405.15

4

229.95

13

332.15

5

219

14

310.25

6

182.5

15

354.05

7

167.9

16

324.85

8

219

17

375.95

9

251.85

Data: Merrick and Alkhatib (2012)

The model simulations suggest that the potential cumulative mining impacts of the four mines on groundwater discharge to the creeks will be minor, at 36.5 ML/year (Merrick and Alkhatib, 2012, p. A-37).

The expected maximum pit depth is to the base of the Braymont Coal Member through to Negero Coal Member in the Maules Creek Formation.

Up to the end of mining, there would be a continuous loss of water from the aquifer system to the mining void. The Maules Creek Formation would be the source of groundwater inflows until year 12 of the project, from which point onwards the alluvium will be the primary source until the end of mining. After the end of mining the long-term groundwater inflow will be drawn from both porous rock and waste rock sources, with a negligible contribution from the alluvium due to the construction of the low permeability barrier. The final void is expected to be about 250 mAHD, assuming the void is partially backfilled, which is about 25 m lower than current levels in the alluvium. The final void will be at the eastern edge of the open-cut. Water levels are expected to reach equilibrium approximately 130 years post mining. The equilibrium long-term groundwater inflow into the void post mining is expected to be about 110 ML/year. This water will come from the Gunnedah-Oxley Basin, with negligible inflows from the alluvium expected post-mining.

Last updated:
6 December 2018
Thumbnail of the Namoi subregion

Product Finalisation date

2018
PRODUCT CONTENTS

ASSESSMENT