Model development


The entire Maranoa-Balonne-Condamine subregion falls within the boundary of the geological Surat Basin, which forms part of the wider Great Artesian Basin (GAB). The regional aquifers are important groundwater supplies for agriculture, industries and towns in the subregion. Coal mining and coal seam gas (CSG) development in the Maranoa-Balonne-Condamine subregion targets the Walloon Coal Measures of the Surat geological basin. Groundwater recharge occurs via infiltration and leakage from streams or overlying aquifers in the aquifer outcrop areas in the north, north-west, north-east and east along the Great Dividing Range. Groundwater flow is predominantly from the recharge areas to the south, south-west, and west. Natural discharge from the Surat Basin occurs via vertical leakage through aquitards, springs, rivers and subsurface flow into adjoining areas.

The Office of Groundwater Impact Assessment (OGIA) model covers an area of 550 km (east–west) × 660 km (north–south), comprising more than three million 1.5 km × 1.5 km grid cells, stacked into 19 layers based on available hydrostratigraphic data. The Walloon Coal Measures are represented by three model layers: an upper aquitard, a composite middle layer representing all coal seams and a lower aquitard. The OGIA model was calibrated in steady state to replicate pre-CSG extraction conditions prior to 1995. Groundwater modelling by OGIA for the Bioregional Assessment Programme estimates hydrological changes arising from coal resource development by comparing the difference in predicted water levels between two possible futures – the baseline and the coal resource development pathway (CRDP). The baseline includes all CSG development included in the 2014 P&G Production Run (OGIA 2014) plus five baseline coal mines. The CRDP includes the baseline plus the additional coal resource development (ACRD – which consists of two coal mines) in the subregion. The difference in predicted water levels between the baseline and CRDP provides an estimate of water level impacts that are attributable to the ACRD.

The OGIA model is a finite difference numerical model based on the MODFLOW–2005 code. The PCG-N solver (preconditioned conjugate gradient solver with improved nonlinear control) is used to solve the finite difference equations in models that are characterised by highly anisotropic hydraulic conductivity values, which is particularly useful for large sedimentary basins where significant contrasts between horizontal and vertical conductivity occur. Model calibration and uncertainty analysis was performed using the parameter estimation (PEST) suite of software.

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

Product Finalisation date