Regional-scale modelling and predictive uncertainty analysis of cumulative groundwater impacts from coal seam gas and coal mining developments

The geological basins in Australia across which the hydrogeological Great Artesian Basin (GAB) exists, hold significant coal and coal seam gas (CSG) resources. Resource development from deep sedimentary basins often involves the risk of impacting groundwater resources. Predictive analysis of potential impacts on water resources are important for the risk analysis of resource development projects. A regional-scale numerical groundwater model was developed to probabilistically assess potential groundwater impacts due to additional coal resource development from a deep sedimentary basin underlying the GAB. The probabilistic simulation considered the plausible variability of the model parameters and accounted for uncertainties. Predictive uncertainty analysis was undertaken using a rejection sampling method after screening the model runs using predefined objective functions to evaluate the performance of the model runs with respect to historical observations. The predictive simulations were undertaken using 2,618 model runs to obtain maximum head drawdown caused by CSG and coal mining developments. The results showed that the water-table drawdown from an individual coal mining development becomes insignificant (maximum difference in drawdown <0.2 m) beyond 5 km. In general, less water-table drawdown was produced near the CSG development site but small amounts of drawdown spread further from it. Separation of surficial aquifers from deeper coal formations, as generally found in the GAB, limits the propagation of CSG-induced drawdown into the aquifers closer to the surface. While this study was specifically done for the Namoi region, similar outcomes could be expected in the broader GAB and other basins where equivalent hydrogeological conditions exist.