Observations and predictions


The regional steady-state model, which is intended to replicate long-term average conditions pre 1995 (i.e. pre coal seam gas (CSG) extraction), was calibrated to average groundwater levels from 1541 boreholes. Pre-1995 groundwater levels represent a period before significant CSG and open-cut coal mine development in the Surat cumulative management area (CMA). For this reason, the Office of Groundwater Impact Assessment (OGIA) model was not recalibrated when the open-cut coal mine boundary conditions were added for the bioregional assessment (BA) groundwater modelling in the Maranoa-Balonne-Condamine subregion. Calibration of the regional steady-state model and transient sub-model using the parameter estimation (PEST) suite of software considered model sensitivity to different combinations of parameters, representation of the Walloon Coal Measures, recharge estimates and model robustness.

For each bore, the OGIA model produces a time series of drawdown caused by the historical and baseline coal resource development (baseline) conditions and a time series of additional drawdown caused by the historical and coal resource development pathway (CRDP) conditions. Two measures are used to summarise these time series: dmax, the maximum difference between the baseline and CRDP drawdown; and tmax, the time when maximum drawdown occurs.

The water balance assessment presents a quantitative water balance for the Maranoa-Balonne-Condamine subregion for both the baseline and CRDP futures in order to quantify the effects of depressurisation of the coal seams for CSG production and mine pit dewatering. The water balance is reported for a defined control volume that includes all model grid cells where additional drawdown is greater than the minimum detectable difference (0.02 m) for the groundwater model.

Modelled CSG water production in The Range water balance area falls from 17% to 0% of modelled losses following modelled cessation of CSG production in 2065. Coal mine dewatering accounts for 2% of modelled losses in the first 30-year period and zero after modelled mine closure in 2041. In the New Acland Coal Mine water balance area, CSG water production falls from 3% to 0% of modelled losses over successive 30-year periods following modelled cessation of CSG production in 2065. Modelled coal mine dewatering during mine operation (2012 to 2029, 0.85 GL/year) is consistent with values reported in the New Acland Coal Mine Stage 3 environmental impact assessment documents (maximum of 1.4 GL/year).

Last updated:
17 October 2018
Thumbnail of the Maranoa-Baloone-Condamine subregion

Product Finalisation date