3.7.1 Key findings

3.7.1.1 Coal resource development

There is a long history of coal mining in the Hunter subregion, dating back to the 1790s. As of December 2012, there were 42 open-cut and underground mines commercially producing coal. An additional 22 developments were identified as potentially starting after this time.

A large number of regulatory requirements has been introduced over time to manage the risks associated with coal mining, both on and off site. The planning and approval processes cover a range of areas relating to management of water resources, including mine dewatering, discharges to and extractions from the stream network, treatment and reuse of water and subsidence, which are intended to minimise risks from coal resource development.

3.7.1.2 Hydrological changes

The modelled additional coal resource development in the Hunter subregion results in five discrete drawdown zones. Drawdowns of greater than 0.2 m are very likely (greater than 95% chance) to occur at distances of 5 km from mine sites and very unlikely (less than 5% chance) to occur at distances greater than 20 km. A less extensive area of drawdown is predicted around the proposed Wallarah 2 mine when the regional result set is constrained using local information.

Changes in baseflow due to groundwater drawdown, plus changes in catchment runoff due to disruptions in surface drainage around the mine site, contribute to changes in streamflow that exceed the thresholds defined in Table 6 within and downstream of the drawdown areas. River modelling indicates that these thresholds are also exceeded at the tidal limit of the Hunter River (near Greta) from the cumulative impact due to additional coal resource development throughout the Hunter river basin.

Potentially large changes in flow regime are predicted in the Wyong River, Loders Creek, Saddlers Creek, Wollar Creek and two unnamed creeks near the Mount Pleasant and Mount Thorley–Warkworth coal mines. The unnamed creeks are small, hence impacts are localised.

The Hunter Regulated River, into which these creeks flow, is not very sensitive to changes in inflows from these creeks. Wollar Creek, Saddlers Creek and Loder Creek drain somewhat larger catchments and have a discernible effect on the Goulburn and Hunter rivers into which they flow. However, changes in baseflow to the Goulburn and Hunter rivers due to groundwater drawdown could be more significant than changes in tributary inflows.

Results for the Hunter Regulated River show that decreases in mean annual flow of between 1% and 2% are very likely, and decreases of more than about 2% upstream of the junction with Loders Creek, or 3% to 4% downstream of this point to Greta, are very unlikely. These changes need to be interpreted with caution, since the Australian Water Resources Assessment river model (AWRA-R) has not been constructed to specifically represent operational management of releases from Glenbawn and Glennies Creek storages.

Generally, the modelled changes are small relative to the interannual variability due to climate, especially for annual flow and high-flow days. There is a chance that increases in low-flow days could affect flow regimes in streams near all the mining areas, with smaller intermittent and perennial streams close to additional coal resource developments in the Central Hunter and Lower Hunter particularly at risk.

In the Wyong River, which is part of the water supply system to Wyong, there is a risk of potentially significant changes in flow regime from the proposed Wallarah 2 mine, and also possibly the proposed Mandalong Southern Extension. Based on the regional analysis, changes in low-flow days of more than 200 days per year are possible (5% chance) and are outside the range of previously experienced low-flow days per year. When the regional-scale results are constrained using local hydrogeological information, the predicted changes in flow regime due to additional coal resource development are substantially smaller and unlikely to result in a change in flow regime that is substantially different to that experienced under the baseline.

Seven mining proposals identified as additional coal resource developments (at September 2015) were not included in the surface water and/or groundwater modelling. Austar underground, Chain Valley underground and Mount Arthur open-cut are considered unlikely to result in significant hydrological change. Changes in the flow regimes of Dora, Mannering, Morans, Stockton, Wallarah and Wyee creeks are assessed as likely from the Mandalong Southern Extension Project and Wallarah 2, with potential impacts on forested wetlands and wet and dry sclerophyll communities along these creeks. The new West Muswellbrook mine could expand the groundwater zone of potential hydrological change further into the Wybong Creek catchment, with potentially more significant impacts on Wybong Creek flow, and contribute to greater reductions in baseflow to Dart Brook and the Hunter River. Additional hydrological changes at Wambo underground and Wilipinjong open-cut mines will likely impact areas that have been or will be affected by baseline developments. The magnitude of additional change is uncertain, but based on the size of the proposed mining projects and results from the modelled additional coal resource development, it appears that spatial extent will be relatively contained and not result in impacts on instream habitat and GDEs beyond those likely to be affected by hydrological changes under the baseline.

3.7.1.3 Ecological impacts

Results from receptor impact modelling of perennial streams, as indicated by changes in the probability of presence of riffle-breeding frogs and density of Hydropsychidae larvae, suggest that instream habitats of Wyong River could be impacted. Elsewhere in the subregion, it is very unlikely that instream habitats of perennial streams will be impacted, except possibly Dora Creek, which was not modelled but likely to experience similar drawdowns to those in the Wyong River catchment. However, local information from the Wyong River catchment suggests that the hydrological changes predicted using the regional parameter sets are grossly over estimated and that the risk to instream and riparian habitats is probably much lower.

Results from receptor impact modelling on intermittent streams, as indicated by changes in the probability of presence of riffle-breeding frogs and richness of hyporheic taxa, suggest that the instream habitats of Saddlers and Loders Creeks in the Hunter river basin could be impacted. Instream habitats of other intermittent streams around all additional coal resource developments are also potentially impacted, but the hydrological changes in these streams were not modelled. To improve the predictions of risk to instream habitat in these streams, more consideration needs to be given to local factors. This was not within the scope of this regional assessment, although geological and hydrogeological information from the Wyong River catchment, and stream salinity and stream condition information for Loders and Saddlers creeks, were used to illustrate this step.

The median result from receptor impact modelling suggests little likelihood of wet and dry sclerophyll forest being impacted, however, there is at least a 5% chance, as indicated by changes in projected foliage cover, that 10 to 15 km² of these landscape classes will be impacted, predominantly in the Macquarie-Tuggerah lakes basin. It is very unlikely that more than 8.6 km² will be subjected to groundwater drawdown of more than 2 m.

The median result from receptor impact modelling suggests little likelihood of riverine forested wetlands being impacted, however, there is at least a 5% chance, as indicated by changes in projected foliage cover, that 10 to 15 km² of riverine forests along the Goulburn River will be impacted. The ecological impact on the coastal swamp community within the ‘Forested wetland’ landscape class was not represented in the receptor impact model.

Most of the rainforest communities in the ‘Rainforest’ landscape class (in the ‘GDE’ landscape group) are unlikely to be impacted because, if they are dependent on groundwater at all, it is from local groundwater sources. The exceptions are rainforests along the perennial Wyong River, the water-dependency of which represents a gap in the BA of the Hunter subregion.

Of the 1652 water-dependent ecological assets in the Hunter subregion, 921 are very unlikely to be impacted by additional coal resource development because they are outside the zone of potential hydrological change. Of the remaining 731 assets, 210 are 'more at risk of hydrological changes' owing to their association with potentially impacted landscape classes.

3.7.1.4 Economic impacts

Five groundwater sources and 19 surface water sources are potentially impacted by hydrological changes due to additional coal resource development.

Changes in water availability, as indicated by the modelled change in mean annual flow, are very likely to exceed 5 GL/year in the Hunter Regulated River at Greta, but very unlikely to exceed 12 GL/year over the period from 2013 to 2042. Reductions of 3 to 6 GL/year in the Singleton, Muswellbrook, Jerrys and Wyong River water sources are possible (at least 5% chance). In the Wyong River, the median decrease in cease-to-pump days over the three 30-year periods is predicted to be between 6 and 8 days per year, with a less than 5% chance of 145 days per year (2043 to 2072).

The reliability of supply, as indicated by change in the number of cease-to-pump days, is likely to be affected in Singleton, Muswellbrook, Jerrys and Wyong River water sources.

‘Make good’ provisions under the NSW Aquifer Interference Policy (DPI Water, 2012) might be necessary (greater than 5% chance) for 11 bores, located in the Sydney Basin – North Coast groundwater source (7) and the Jilliby Jilliby Creek (2), Tuggerah Lakes and South Lake Macquarie water sources. Another 159 bores were identified as likely to experience drawdowns of at least 2 m due to additional coal resource development, but since they are on mining and exploration leases, the requirement to ‘make good’ on these impacts was considered less likely.