The bioregional assessment (BA) of the Hunter subregion has identified a risk of potentially significant hydrological changes and ecosystem impacts due to additional coal resource development in a small number of catchments within the subregion. These include the perennial Wyong River and potentially Dora Creek in the Macquarie-Tuggerah lakes basin; relatively small intermittent streams – Loders Creek, Saddlers Creek, Dry Creek and a couple of unnamed streams – that drain additional coal resource developments in the Hunter Coalfield; and the intermittent Wollar Creek in the Western Coalfield.
Larger changes are associated with the low-flow part of the flow regime in these systems, including the number of zero-flow days per year.
Results from receptor impact modelling suggest a risk to the instream habitat of Wyong River due to modelled changes in zero-flow days. However, when the predictions of groundwater drawdown are constrained using local hydrogeological information, the impacts on Wyong River flow are substantially smaller and less likely to lead to adverse ecological impacts.
Potentially large impacts on instream habitat of intermittent streams may occur in Loders Creek and Saddlers Creek. Forested wetlands along the Goulburn River could also experience changes in projected foliage cover. Decreases of up to 0.1 in projected foliage cover are possible, although median results indicate the likelihood of negligible change due to additional coal resource development.
The BA of the Hunter subregion has been conducted using the best available information within the operational constraints and timing of the Bioregional Assessment Programme. There are opportunities to build on this Assessment and address science gaps to reduce predictive uncertainty and improve the utility of the results.
The Assessment is regional and cumulative, and provides an important framework for local-scale environmental impact assessments of new coal resource developments, and the local geological, hydrogeological and hydrological modelling that support them. The results do not replace the need for detailed site-specific studies, nor should they be used to supplant the results of detailed studies that may be required under state legislation. There are opportunities to tailor the BA modelling results for more local analyses (e.g. combining detailed local geological information with the groundwater emulators developed through BA, where appropriate).
There are also opportunities to consider alternative futures through different combinations of these developments and the inclusion of new developments, or to assess the potential impact of individual or small groups of developments.
Incorporating additional data that were not readily available at the time the geological model was constructed might reduce some of the uncertainties carried forward into the hydrological modelling. These additional data would include well and bore logs and seismic data. The greatest potential to reduce predictive uncertainty lies in better characterisation of hydraulic properties of the sedimentary rocks, especially the porosity and storage parameters.
Improved mapping of depth to groundwater, including its spatial and temporal variation, has potential to constrain drawdown and baseflow predictions, and provide better context for interpreting the ecological impacts due to hydrological change. Interactions between changes in groundwater availability and the health and persistence of terrestrial groundwater-dependent vegetation remain uncertain due, in part, to sparse mapping of groundwater depths outside of alluvial layers.
More closely coupled surface water and groundwater models could incorporate feedback mechanisms not included in the current Assessment. This could reduce predictive uncertainty and help to ensure that any dependencies between hydrological response variables, particularly between groundwater and surface water hydrological response variables, is carried through to receptor impact modelling.
A more extensive set of surface water model nodes would improve the interpolation of surface water hydrological response variables, resulting in more reaches with quantifiable changes and fewer stream reaches assessed as ‘potentially impacted’.
Additional high-resolution vegetation mapping and ongoing research to identify groundwater-dependent ecosystems (GDEs) in the subregion would improve the assessment of impacts on water-dependent assets. Receptor impact models and qualitative models for the ‘Forested wetland’, ‘Freshwater wetland’ and ‘Rainforest’ landscape classes contain knowledge gaps or are not considered appropriate for subsets of those landscape classes.
Identifying water-dependent assets valued by the local Indigenous communities would provide a more comprehensive account of sociocultural assets, even if many of those assets are already in the water-dependent asset register through other sources, for example, a wetland may have both ecological and Indigenous value.
Future monitoring of groundwater levels in the five discrete drawdown zones due to additional coal resource development is recommended. Suggested priorities, based on potentially impacted bores, are the Sydney Basin – North Coast groundwater source and Jilliby Jilliby, Tuggerah Lake and South Macquarie Lake water sources, and the area west of the proposed West Muswellbrook mine, where it appears likely that drawdown from this development will extend into that from other nearby mines.
It is recommended that the streams modelled to experience large changes in flow regime, particularly the Wyong River, but also possibly Loders Creek, Saddlers Creek and Wollar Creek, be monitored. Local information is needed to better determine the priorities. Streamflow monitoring could be of value for Dora, Mannering, Morans, Stockton, Wallarah and Wyee creeks given potential changes in flow regime that may arise from the proposed Mandalong Southern Extension and Wallarah 2. Monitoring of the Goulburn and Hunter rivers should continue, given potential changes in baseflow. Additional streamflow monitoring in the Wybong Creek catchment would help to assess potential impacts from the proposed West Muswellbrook mine.
Product Finalisation date
- 3.1 Overview
- 3.2 Methods
- 3.3 Potential hydrological changes
- 3.4 Impacts on and risks to landscape classes
- 3.5 Impacts on and risks to water-dependent assets
- 3.6 Commentary for coal resource developments that are not modelled
- 3.7 Conclusion
- Contributors to the Technical Programme
- About this technical product