South Galilee model review

RPS Aquaterra (2012) and RPS Aquaterra (2013) provide details on the South Galilee groundwater model. The model presented in RPS Aquaterra (2013) is a revision of the original model as outlined in RPS Aquaterra (2012), thus model details presented here are derived from RPS Aquaterra (2013). However, RPS Aquaterra (2012) contains much of the background information for the model.

Some basic parameters for the steady-state model are outlined in Table 5. The South Galilee model area includes the China First Coal Project area. It has a variable cell size of 50 m in the mining tenement, and 1 km at the model boundaries.

The model contains seven layers. The uppermost model layer represents the Quaternary and Cenozoic material. Model layer 2 represents the Moolayember Formation, Clematis Group and the Rewan Group. Model layer 3 represents the primary coal resource. Model layers 4 to 6 represent the other coal seams, interburden and underburden. Model layer 7 represents the Joe Joe Group. Including the Moolayember Formation in model layer 2, which otherwise represents aquifers, partial aquifers, and coal resources, is likely to affect the ability of the model to predict impacts on shallow groundwater levels in the Quaternary and Cenozoic material resulting from changes in deeper groundwater levels.

The northern, western and much of the eastern boundaries for the models were designated as general-head boundaries. An exception to this was the western boundary of the Rewan Group (model layer 3) and Joe Joe Group (model layer 8), which were designated as no-flow boundaries as they represent aquitards. Other areas, including the southern boundary were designated no-flow boundaries (RPS Aquaterra, 2013).

Recharge was applied to the model only where alluvium was present at the surface, or where the Clematis Group outcropped. Alluvium was assigned a recharge rate of 2.75 mm/year (or 0.5% rainfall), Cenozoic (called Tertiary in the model) 5.5 mm/year (1% rainfall) and the Clematis Group was assigned recharge of 13.75 mm/year (2.5% rainfall).

Surface water bodies were represented with the MODFLOW River package, allowing flow both from aquifers to streams and from streams to aquifers. River stages were set at 5 m below the river bank elevation. The riverbed of Native Companion Creek was set at 1 m below the river stage. All other surface streams had the riverbed set at 0.1 m below the river stage. This arrangement allows a low level of constant leakage to occur from surface water to groundwater (RPS Aquaterra, 2012), however most of the streams in the model domain are ephemeral.

Mining-induced fracturing was simulated in the transient model by allowing for changes in conductivity over time to occur as mining progresses in model cells that represent mine areas. Some parts of the China First Coal Mine areas were incorporated into the model so as to investigate cumulative impacts from the China First and South Galilee coal projects (RPS Aquaterra, 2013). The model was run for 100 years to simulate groundwater recovery post mining.

Predicted mine inflows range from 2 to 8 GL/year, which equates to 5 to 23 ML/day (RPS Aquaterra, 2013) with a total cumulative inflow volume of 187 GL over life of mine. A sensitivity analysis on the effects of faulting on the Rewan Group was undertaken by increasing the permeability of the Rewan-Dunda layer to simulate the potential interconnectivity effect due to faulting and fracturing. The analysis showed that the model was not sensitive to drastic changes to hydraulic conductivity in the Rewan Group. This implied that the presence of faults in the Rewan Group would not significantly increase inflows to the mine. The cumulative model runs suggest that there will be minimal impacts to the Alpha town water supply. Cumulative drawdown in the Clematis Group aquifer during mining was in the order of 3 to 5 m, which is considered to be overly conservative as a result of the Clematis Group aquifer being lumped with the Rewan Group aquitard in the model.

Last updated:
6 December 2018
Thumbnail of the Galilee subregion

Product Finalisation date