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2.6.2 Groundwater numerical modelling for the Maranoa-Balonne-Condamine subregion

Executive summary

Condamine river weir on Darling Downs in Queensland, 2005 Credit: Arthur Mostead © Commonwealth of Australia (Murray–Darling Basin Authority)

Coal and coal seam gas (CSG) development can potentially affect water-dependent assets (either negatively or positively) through a direct impact on groundwater hydrology. This product presents the modelled hydrological changes in response to likely coal resource development in the Maranoa-Balonne-Condamine subregion after December 2012. First, the methods are summarised and existing models are reviewed, followed by details regarding the development and calibration of the model. The product concludes with probabilistic predictions of hydrological change, including uncertainty analysis and a discussion of model limitations, opportunities and conclusions.

Groundwater modelling for the Maranoa-Balonne-Condamine subregion follows the companion submethodology M07 (as listed in Table 1) for groundwater modelling. Specifically, the groundwater model produces spatially explicit model outputs that are used as inputs to other bioregional assessment (BA) models, including surface water models, uncertainty analysis and receptor impact modelling.

The Queensland Office of Groundwater Impact Assessment (OGIA) regional groundwater model is used to estimate hydrological changes arising from coal resource development for two possible futures – the baseline and the coal resource development pathway (CRDP). The OGIA model was developed with the single purpose ‘to provide a suitable tool for assessing the impacts of CSG development on water levels in the aquifers present within the Surat cumulative management area’. It is ‘therefore on its own not necessarily suitable for predicting responses to arbitrary changes in hydrological conditions, developing sustainable water resource management policies, assessing impacts on groundwater-dependent ecosystems or quantifying surface water – groundwater interactions’. However, it has the best available representation of CSG development in the Surat cumulative management area and is considered fit for purpose for groundwater modelling in BA, with the exception of criteria related to the representation of water fluxes in surficial aquifers.

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 CSG development in the subregion targets the Walloon Coal Measures of the Surat 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 geological Surat Basin occurs via vertical leakage through aquitards, springs, rivers and subsurface flow into adjoining areas.

Groundwater modelling results from the OGIA model estimates hydrological changes arising from coal resource development by comparing the difference in predicted water levels between two possible futures, which provides an estimate of water level impacts that are attributable to the additional coal resource development (ACRD). Five baseline open-cut coal mines, Cameby Downs, Commodore, Kogan Creek, New Acland Stage 2 and Wilkie Creek, are modelled in the Maranoa-Balonne-Condamine subregion. Two proposed open-cut coal mines, New Acland Stage 3 and The Range, are considered sufficiently advanced that under current knowledge and conditions they will most likely proceed. Five baseline CSG projects are modelled in the Maranoa-Balonne-Condamine subregion. This includes three large-scale gas field developments supporting the three liquefied natural gas (LNG) projects on Curtis Island near Gladstone, Australia Pacific LNG Project, the Queensland Curtis LNG Project, and the Santos Gladstone LNG and GLNG Gas Field Development projects. The staged expansion of production for the Surat Gas Project, and the smaller scale Ironbark Project, are considered to be part of the baseline for this Assessment to ensure consistency with OGIA reporting.

Model predictions of baseline groundwater drawdown associated with coal resource development are presented as maps of the 95th percentile of baseline groundwater drawdown. 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 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 five 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. Hydrological changes in excess of 0.2 m additional groundwater drawdown in the vicinity of the proposed coal mines is generally within 20 to 40 km (maximum 50 to 60 km) of the proposed pits. Additional groundwater drawdown in excess of 5 m (p=0.05) is generally within 10 km (maximum 10 to 15 km) of the proposed pits. There is a greater than 5 percent probability of exceeding 5 m additional groundwater drawdown for 86 of the approximately 19,000 bores in the Maranoa-Balonne-Condamine subregion.

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 compared with estimates of fluxes described in the regional-scale conceptual model and localised groundwater models to provide confidence in model predictions. Modelled CSG water production in The Range coal mine 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).

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.

The representations of surface water – groundwater interactions, mine pit dewatering, CSG activities and horizontal and vertical discretisation in the regional model are identified as having the greatest potential effect on model predictions in the qualitative uncertainty analysis. The revised OGIA model developed for the Surat Underground Water Impact Report (UWIR) and released for public comment in early 2016 has addressed many of the model data and resource availability and technical issues. The consistency between OGIA 2012 and revised OGIA 2016 model predictions of hydrological change lends confidence to the BA model predictions. The main opportunities to reduce predictive uncertainty in the regional model are related to the representation of hydrological changes in surficial aquifers that affect surface water – groundwater interactions and groundwater-dependent ecosystems.

The results of this groundwater numerical modelling will be used to inform the impact and risk analysis (product 3-4). Probabilistic estimates of hydrological changes arising from coal resource development will be used to assess direct impacts on groundwater-dependent assets, such as groundwater-dependent ecosystems and economic bores. The focus of the regional model on the deep regional aquifers targeted by CSG development, means that 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 in the impact and risk analysis (product 3-4).

Last updated:
11 July 2017
Thumbnail of the Maranoa-Baloone-Condamine subregion
PRODUCT FINALISATION DATE
30 May 2016
PRODUCT CONTENTS