Executive summary

This synthesis presents key findings from the bioregional assessment of the Gloucester subregion, part of the Northern Sydney Basin bioregion.

The subregion, north of the Hunter Valley in NSW, is defined by the geological Gloucester Basin and is home to about 5000 people, mainly in the towns of Gloucester and Stroud. Town water is extracted from local rivers and there are no major dams or wetlands. The majority of the subregion is cleared of native vegetation and supports agricultural uses. The groundwater system is confined within the subregion, so potential impacts on groundwater cannot propagate beyond its boundaries.

A small east-west ridge divides the Avon River catchment in the north from the Karuah river basin in the south. The subregion covers 348 km²; however, the total area investigated in this assessment, the assessment extent, is 481 km² as it extends along the Karuah and Gloucester rivers beyond the subregion boundaries (Figure 2).

Hydrological modelling identified potential changes in groundwater and streamflow due to coal resource development for two futures, the baseline and coal resource development pathway (Box 1). The baseline includes two open-cut mines: Duralie Coal Mine in the south, and Stratford Mining Complex in the north. The coal resource development pathway includes the baseline mines plus four additional coal resource developments: mine expansions for the two baseline mines, one new open-cut coal mine at Rocky Hill, north of Stratford, and one coal seam gas (CSG) development, Gloucester Gas Project Stage 1. Note that the Gloucester Gas Project Stage 1 has been withdrawn and in December 2017 the NSW Planning Assessment Commission refused consent for the Rocky Hill Coal Project to proceed.

To rule out impacts on water-dependent ecosystems and assets, a zone of potential hydrological change (Box 4) identified areas where modelling predicted potential changes in surface water and groundwater due to additional coal resource development. The zone comprises 52% of the assessment extent, covers 250 km² and includes 242 km of streams.

Cumulative hydrological impacts due to additional coal resource development are predicted to be minor. No impacts on ecological assets are predicted in the south, given the limited additional coal resource development. In the north, potential impacts on ecological assets are expected to be minor and localised due to the relatively small magnitude of predicted hydrological changes.

All 38.4 km² of the subregion’s estuarine ecosystems are very unlikely to be impacted, along with 7.1 km² of the total 10.3 km² of groundwater-dependent ecosystems, and 67 km of the total 344 km of riverine ecosystems.

In the Avon River and Upper Gloucester River, there is a 95% chance that reductions in water availability are less than 1.6 GL per year due to additional coal resource development, which is a 1% to 2% change relative to the baseline.

Modelling suggests that reliability of water supply is very unlikely to be impacted, with no change in cease-to-pump days in the Upper Gloucester River and Karuah River (upper management zone). It is very likely that there will be fewer than 3 additional low-flow days per year in the Avon River, with the impact on cease-to-pump days expected to be smaller.

The greatest confidence in hydrological modelling results is in areas that are very unlikely to be impacted. Where potential impacts are identified, further local-scale modelling may be required to determine the presence and magnitude of ecosystem impacts.