2.6.2.2.4 China Stone model review

AGE Pty Ltd (2015) reports on the details of the numerical model for the proposed China Stone Coal Mine. Model dimensions and runtimes are outlined in Table 5. The model consists of 18 layers. Model layers 1 and 2 represent Quaternary and Cenozoic sediments, and if present regolith material. Model layer 3 represents the Ronlow beds, while layers 4 to 8 represent the Galilee Basin sequence from Moolayember Formation down to the base of the Rewan Group. Model layers 9 to 17 represent the upper Permian coal measures while the Joe Joe Group is represented by layer 18.

All model boundaries except the base of the model were set as general-head boundaries. Variable diffuse recharge rates were applied across the model domain and were dependent on the surface geology. They ranged from 0.08 ML/day (outcrop areas of Rewan Group and upper Permian coal measures) up to 1.13 ML/day (Clematis Group outcrop). The model included fracture zones developing as the result of longwall mining in the upper Permian coal measures, which were conservatively assumed to propagate through the entire thickness of overlying strata into the Clematis Group.

Major drainage features incorporated into the model had varying vertical riverbed conductance ranging from 0.0038 m/day (Lake Buchanan) up to 9.6 m/day (Carmichael River) (riverbed conductance is defined as C = K×W×L/M, where K is hydraulic conductivity of the streambed material, W is stream width, L is length of the stream, and M is thickness of stream layer).

The steady-state calibration utilised an observed water level measurement from 127 bores. Many of the water level measurements were taken when the area was experiencing drought conditions, a result being that monitoring did not record any significant changes to water levels. Thus a transient calibration was not considered to be useful with data at hand. Changes to hydraulic conductivity due to cracking above underground mine longwall panels was incorporated into the transient model as were changes to recharge and hydraulic properties due to storing waste rock in areas of open-cuts where mining was complete. An estimate of cumulative impacts from both the China Stone and Carmichael mines was also carried out.

During the 50 years that mining operations were simulated in the model, total groundwater inflows were calculated to be up to 20 ML/day, although the overall mean could be in the order of 4 to 8 ML/day. Up to 33 m drawdown may occur in the Clematis Group (where it is saturated) in the vicinity of the northern underground mine workings. However, drawdown impacts diminish away from mine areas to less than 1 m within 5 km of proposed mine workings. Groundwater levels in Cenozoic sediments are likely to be drawn down immediately east of the southern open-cut mine areas. Cumulative drawdown in Cenozoic sediments post closure of the mine may be up to 10 m near mine areas but decreases to around 1 m, 10 km east of the mine tenement areas.