2.6.2 Groundwater numerical modelling for the Galilee subregion

Executive summary

Artesian Spring Wetland at  Doongmabulla Nature Refuge, QLD, 2013 Credit: Jeremy Drimer, University of Queensland

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 provides the modelled hydrological changes in response to likely coal resource development in the Galilee subregion after December 2012. First, the methods are summarised and existing models reviewed, followed by details regarding the development of the groundwater flow model. The product concludes with probabilistic predictions of hydrological change, including uncertainty analysis and a discussion of model limitations, opportunities and conclusions.

A bioregional assessment (BA) considers two potential futures:

  • baseline coal resource development (baseline): a future that includes all coal mines and coal seam gas (CSG) fields that are commercially producing as of December 2012
  • coal resource development pathway (CRDP): a future that includes all coal mines and CSG fields that are in the baseline as well as those that are expected to begin commercial production after December 2012.

The difference in results between CRDP and baseline is the change that is primarily reported in a BA. This change is due to the additional coal resource development – all coal mines and CSG fields, including expansions of baseline operations, which are expected to begin commercial production after December 2012.

In the Galilee subregion, there are no coal resource developments in the baseline as there were no commercially operating coal mines as at December 2012. The CRDP includes 14 coal developments and 3 CSG projects. Sufficient information for numerical modelling is available for only 7 of the 14 coal developments. The results reported in this product relate to these seven developments which target the upper Permian coal measures near the central eastern margin of the Galilee subregion: Hyde Park, China Stone, Carmichael, Kevin’s Corner, Alpha, China First and South Galilee.

Groundwater modelling for the Galilee subregion follows the companion submethodology M07 (as listed in Table 1) for groundwater modelling. A review of the existing groundwater models identified that only the Galilee Basin hydrogeological (GBH) model (Turvey et al., 2015) was able to simulate the hydrological change of all coal resource developments modelled in the CRDP. This complex numerical model incorporates the whole of the Galilee Basin, and parts of the overlying Eromanga Basin and Cenozoic sediments. It utilises input data from the Galilee subregion BA. While effective at simulating hydrological change, this model requires further refinement to enhance its predictive capacity. The model’s complexity and associated computational demand mean that in its current form, it cannot be integrated in the probabilistic uncertainty analysis framework designed for BA as outlined in companion submethodology M09 (as listed in Table 1).

A groundwater analytic element model (referred to as GW AEM) was designed and used specifically for this bioregional assessment to predict changes in groundwater levels at specific model nodes, resulting from the cumulative impact of pumping to dewater mines modelled in the Galilee CRDP. Estimated groundwater extraction rates from the seven coal projects were included in the numerical modelling to predict changes in groundwater level at 47 model nodes (points in the landscape where water-related impacts on assets are assessed). All mine developments are situated along the eastern margin of the model domain. The GW AEM generated predicted areas of the maximum difference in drawdown between the CRDP and baseline, due to additional coal resource development (dmax) and time to maximum change (tmax) at the model nodes; these are the key metrics used to assess impacts. Median change in surface water – groundwater flux is also reported.

The GW AEM simulates a simplified hydrostratigraphic model representing the upper Galilee Basin sequence and overlying Cenozoic cover, along the eastern margin of the subregion in the vicinity of CRDP mines. The hydrogeology of the upper Galilee Basin sequence is conceptualised as a series of alternating aquifers and aquitards, outcropping on the Galilee Basin’s eastern margin, which gently dip to the west. This sequence comprises the following hydrostratigraphic units, listed in order from youngest to oldest: the Clematis Group aquifer, Rewan Group aquitard, upper Permian coal measures partial aquifer and the Joe Joe Group aquitard. The model excludes the early Jurassic to late Cretaceous Eromanga Basin sequence, which includes the Hutton Sandstone and Hooray Sandstone and Winton-Mackunda formation aquifers. The Cenozoic cover is represented as the top-most layer in the GW AEM.

The only surface water - groundwater interaction included in the model is with the main channel of the Belyando River. In the model, the Belyando River is the only river system identified as a regional discharge area, while its tributaries can be considered maximally losing.

The GW AEM predictions show that model nodes associated with the Clematis Group have a drawdown generally less than 2 m, occurring on or after the end of the simulation period (2102). The impacts are limited to the vicinity of the Carmichael and China Stone proposed developments, which are close to the eastern-most limit of the Clematis Group. Beyond 20 km of the Carmichael and China Stone mine footprints, the probability of exceeding a drawdown of 0.2 m is generally less than 20%.

The predicted dmax in the upper Permian coal measures is generally in excess of 5 m throughout the model domain. At a distance of more than 100 km west of the footprints of all mines in the CRDP, the probability of exceeding 5 m drawdown is still in excess of 20%. However, large drawdowns in the upper Permian coal measures are very unlikely to propagate vertically due to the hydraulic characteristics of overlying units.

The median change in surface water – groundwater flux at the end of the simulation period is close to 0.6% of baseflow (noting at times the stream does not flow) at Belyando Crossing, as estimated in companion product 2.1-2.2, with the 5th percentile 0.1% and the 95th percentile close to 2.4%. This flux is integrated into the surface water model to evaluate the effect on different aspects of the total streamflow.

While a formal comparison of the probabilistic outcomes of the GW AEM with the deterministic results of the GBH model is not possible, the GBH results are consistent with the results of the GW AEM and the estimated drawdowns of the GBH are close to the 95th percentile of dmax predicted by the GW AEM.

The qualitative uncertainty analysis of the GW AEM assumptions highlights that the assumptions with the highest potential to affect predictions are the implementation of the CRDP, the representation of the Cenozoic and alluvial aquifer system and the conceptualisation of the Belyando River. As an example, the effect of some different conceptualisations for the Cenozoic and alluvial aquifer system on results are detailed as part of the uncertainty analysis.

The main opportunities to improve upon the modelling presented in this product lie in expanding the knowledge base of the shallow aquifer system and the connection status of the river system. Further development of the GBH model and integration of this model in a probabilistic uncertainty analysis framework will allow analysis of several of the simplifying assumptions underpinning the GW AEM which will likely result in more robust and less conservative predictions of hydrological change.

The results from this groundwater modelling are used as inputs in the impact and risk analysis (product 3-4).

Last updated:
6 December 2018
Thumbnail of the Galilee subregion