The review of existing groundwater models concluded that the first generation OGIA model built in 2012 meets the fit-for-purpose criteria for groundwater modelling in BA, with the exception of criteria related to the representation of water fluxes in surficial aquifers. The primary purpose of the OGIA model is to probabilistically predict the regional water pressure and water level changes in aquifers within the Surat CMA in response to the depressurisation of the coal seams for CSG production. The OGIA model is re-run annually based on the latest available industry development plans and has been revised for the BA to also simulate water-related impacts of coal mine developments in the Maranoa-Balonne-Condamine subregion.
Hydrological changes arising from coal resource development for two possible futures – the baseline and the CRDP – are assessed using a probabilistic approach. Probabilistic estimates of the hydrological changes associated with coal resource development in the Maranoa-Balonne-Condamine subregion can inform the assessment of direct impacts on groundwater-dependent assets, such as groundwater-dependent ecosystems (ecological assets), or groundwater bores used for stock, irrigation and domestic purposes (economic assets). The focus on the deep regional aquifers targeted by CSG development, means that integration of the OGIA model with a surface water numerical model and receptor impact modelling is not possible at this time in the Maranoa-Balonne-Condamine subregion. Instead, the conceptual model of causal pathways that describes the logical chain of events ‒ either planned or unplanned ‒ that link coal resource development and potential impacts on water and water-dependent assets will inform the assessment of indirect impacts.
The OGIA model uses calibration-constrained uncertainty analysis, which is also known as Null-space Monte Carlo Analysis. This approach provides an efficient method to explore the non-uniqueness of model parameters and resulting model prediction uncertainty. The formal uncertainty analysis considered hydraulic conductivity, recharge and storage values, but did not consider model conceptualisation or the parameters used to specify drain and river boundary conditions. The 200 calibration-constrained parameter sets are defined spatially using pilot points in each model layer, which gives spatial coherence to the model parameter values that is consistent with the model conceptualisation and uses regularisation to solve the problem mathematically.
Model predictions of baseline groundwater drawdown associated with coal resource development are presented as maps of the 95th percentile of baseline groundwater drawdown. The baseline future includes five baseline open-cut coal mines and the five baseline CSG projects that are reported in the 2014 Annual Report for the Surat CMA (OGIA, 2014). Maximum baseline groundwater drawdown associated with CSG production (in excess of 700 m drawdown in the productive Walloon Coal Measures model layer) is predicted near the towns of Chinchilla and Roma. Hydrological changes in excess of 0.2 m baseline groundwater drawdown in the vicinity of the five baseline coal mines are generally within 5 to 10 km (maximum 15 to 20 km) of the modelled pits. Baseline groundwater drawdown associated with CSG production in the vicinity of the coal mines is generally less than 10 m (maximum 20–40 m in the Walloon Coal Measures model layer).
Model predictions of additional groundwater drawdown under the CRDP future are presented as maps of the probability of exceeding additional groundwater drawdown thresholds for each model layer and histograms of maximum additional drawdown (dmax) and time to maximum drawdown (tmax) at the economic bores within the two water balance areas. The CRDP future includes the baseline coal resource developments plus the two proposed open-cut coal mines. Additional groundwater drawdown in the vicinity of the proposed coal mines in excess of 0.2 m (probability, p=0.05) is generally within 20 to 40 km (maximum 50 to 60 km in the Walloon Coal Measures and Hutton / Marburg Sandstone model layers) of the proposed pits. Additional groundwater drawdown in the vicinity of proposed coal mines in excess of 5 m (p=0.05) is generally less than 10 km (maximum 10 to 15 km in the Walloon Coal Measures model layer) of the proposed pits. Overall, 86 of approximately 19,000 bores are predicted to experience hydrological changes in excess of a 95th percentile value of 5 m additional groundwater drawdown in the Maranoa-Balonne-Condamine subregion outside of the proposed pit extents.
The quantitative uncertainty analysis considered hydraulic conductivity, recharge and storage values, but not model conceptualisation or the parameters used to specify drain and river boundary conditions. The representation of hydrological changes in surficial aquifers that affect surface water – groundwater interactions and groundwater-dependent ecosystems are influenced by model assumptions and the availability of water quality and quantity data. Assumptions and model choices related to mine pit dewatering are related to the use of regional-scale hydrostratigraphic interpretation and pit geometry. The revised OGIA model developed for the Surat Underground Water Impact Report (UWIR) and released for public comment in early 2016 (OGIA, 2016) has addressed many of the model data and resource availability and technical issues discussed in the qualitative uncertainty analysis. The patterns of long-term drawdown impacts are broadly consistent between the OGIA 2012 model used for BA and the revised OGIA 2016 model, lending confidence to the BA model predictions and indicating that these improvements to the regional model have a moderate effect on model predictions. Changes to the representation of hydrological changes in surficial aquifers that affect surface water – groundwater interactions and groundwater-dependent ecosystems provide the greatest opportunities to reduce predictive uncertainty in the regional groundwater model used for this Assessment.
Product Finalisation date
- 220.127.116.11 Methods
- 18.104.22.168 Review of existing models
- 22.214.171.124 Model development
- 126.96.36.199 Boundary and initial conditions
- 188.8.131.52 Implementation of coal resource development pathway
- 184.108.40.206 Parameterisation
- 220.127.116.11 Observations and predictions
- 18.104.22.168 Uncertainty analysis
- 22.214.171.124 Limitations
- Contributors to the Technical Programme
- About this technical product