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2.6.2 Groundwater numerical modelling for the Hunter subregion

Executive summary

Google Earth image of the Hunter River west of Muswellbrook

Coal resource development can potentially affect water-dependent assets through impacts on groundwater hydrology. The bioregional assessment (BA) groundwater numerical modelling provides probabilities of groundwater drawdown and changes in the surface water – groundwater exchange flux from coal resource development in the Hunter subregion.

This product describes the model development and presents the modelled hydrological changes in response to coal resource development in the Hunter subregion. Results are reported for the difference between the two potential futures considered in a BA:

  • 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 due to the additional coal resource development – all coal mines and CSG fields, including expansions of baseline operations, that are expected to begin commercial production after December 2012.

The Hunter subregion has a long history of coal mining. In the CRDP for the Hunter subregion there are 42 baseline mines, 1 of which was not included in the modelling and 22 additional coal resource developments, 5 of which were not included in the modelling. Reasons for not modelling these 6 mines were insufficient data or that the additional coal resource development produced negligible changes to the flow rates of groundwater into the mine. Therefore, the groundwater numerical modelling in the Hunter subregion includes 58 mines comprising 41 baseline mines and 17 additional coal resource developments. In the Hunter subregion there are no CSG fields in the CRDP.

The modelling approach in the Hunter subregion is consistent with that detailed in submethodology M07 for groundwater modelling, and in submethodology M09 for propagating uncertainty through models. The modelling approach is implemented in the Hunter subregion as follows:

  • The model chain comprises a subregion-wide groundwater model, built specifically for the Hunter subregion in the Multiphysics Object Oriented Simulation Environment (MOOSE).
  • Model emulators, trained on a large number of groundwater model simulations, characterise the prediction uncertainty for two hydrological response variables: dmax, the maximum difference in drawdown between CRDP and baseline due to the additional coal resource development, and tmax, the year of maximum change.
  • The model is built upon the nine geological horizons in the Hunter subregion geological model outlined in companion product 2.1-2.2 (observations analysis, statistical analysis and interpolation) for the Hunter subregion. Additional layers are added to represent near-surface groundwater processes and the presence of coal seams.
  • The modelling domain spans an area of about 34,000 km2, with a best spatial resolution in plan view of 500 m, and with depths below surface ranging from about 300 m to 3000 m.
  • Representation of the modelling domain in plan view is by a variable triangulated mesh with greater resolution around mines and streams.
  • All major rivers, along with some minor reaches, are represented in the model and define the locations where the groundwater model interacts with the Hunter river model. Changes in these surface water – groundwater fluxes are incorporated into the streamflow changes in the river model and are reported in companion product 2.6.1 (surface water numerical modelling) for the Hunter subregion.
  • The model has 22 parameters, which need to be specified. Ten of these are varied in the uncertainty analysis.
  • Regional-wide observational data to constrain model parameters are poor or lacking. Groundwater level observations from 64 groundwater monitoring sites and observed streamflow data are used to impose upper and lower bounds on the modelled drawdowns.
  • The surface water – groundwater flux simulations are constrained as part of the surface water modelling.

The predictions from the groundwater numerical modelling for the Hunter subregion show that the extraction of water by coal mines generally causes baseflow (i.e. the groundwater contribution to stream) to decrease.

More than three-quarters of the model nodes have drawdowns due to additional coal resource development of less than 2 m; two-thirds have drawdowns less than 0.2 m. The drawdowns resulting from the mining operations of the additional coal resource development are localised around the mines. At a distance of about 20 km from these mine sites, there is approximately a 5% probability of the drawdown exceeding 0.2 m. In general, the year of maximum change occurs relatively quickly in the immediate vicinity of the mines, but progressively later with increasing distance from the mines.

As in any modelling exercise, a number of assumptions have been made, some of which are not included in the formal uncertainty analysis. Many are necessitated through insufficient data, but lack of resources to undertake more rigorous analyses and technical issues have also necessitated some assumptions. In general, these assumptions lead to overestimation of the groundwater changes from mining. Results of the sensitivity analysis indicate that the depth of the river bed and parameterisation of hydraulic conductivities have the biggest effect on historical model predictions; whereas drawdown due to the additional coal resource development (additional drawdown) is sensitive to some of the same parameters but also sensitive to the baseline porosity parameter and the parameters that define the changes arising from coal resource development.

The use of regional parameters can result in overestimations of the range of potential drawdown at any particular point in the landscape. Constraining the regional model results in the Wyong River based on local geological and hydrogeological data indicated that the area affected by drawdowns exceeding 0.2 m due to the additional coal resource development is likely to be far less extensive than that predicted using the full set of regional parameterisations.

The results of groundwater numerical modelling for the Hunter subregion inform product 3-4 (impact and risk analysis). Estimates of the probability of hydrological changes due to coal resource development will be used to assess direct impacts on water-dependent assets, such as groundwater-dependent ecosystems and economic bores.

Last updated:
5 November 2018
Thumbnail of the Hunter subregion
PRODUCT FINALISATION DATE
2018
PRODUCT CONTENTS
ASSESSMENT