3.6.2 Potential impacts of non-modelled additional coal resource developments Mandalong Southern Extension (underground)

The Mandalong Southern Extension (Mandalong SE) Project involves expanding Centennial Coal’s existing underground operations into the Exploration Licence 6317 area (the Southern Extension Area), approximately 4470 ha of land south of existing workings (Figure 78). This represents a 50% increase on the 9000 ha of operations under development consent 97/800. The proposed development would extend the life of Mandalong’s underground operations by 21 years to 2040. Development consent SSD-5144 was granted by the Planning and Assessment Commission on 12 October 2015.

The surface mine footprint will include parts of Morans Creek (which flows into Stockton and then Dora creeks), Wyee Creek and Mannering Creek catchments, which drain into Lake Macquarie; and Buttonderry Creek catchment, which drains into the Wyong River. The streams overlying the site are typically first- and second-order streams, with some third-order streams, and are ephemeral (Umwelt, 2013b). Subsidence modelling suggests there will be some vertical subsidence, but that connective cracking is unlikely (Ditton Geotechnical Services, 2013). The predicted subsidence was not identified as affecting catchment boundaries or stream alignments (Umwelt, 2013b). There may be an increase in scouring along some creeks, but negligible increase in areas affected by ponding (3.6 ha).

Drawdown from mine dewatering and the consequent effect on fluxes of groundwater to streams are likely to have a larger effect on streamflow hydrology than changes in surface runoff from disruption of natural drainage. Mine dewatering is predicted to increase to 2154 ML/year from the 1825 ML/year approved under Mandalong’s current dewatering licence (20BL169424). Drawdowns have been modelled as part of the BA for the Hunter subregion, but model nodes were not specified for any of the draining streams and the change in the groundwater flux to streams was not computed. Results from the Wallarah 2 underground mine in the adjacent Wyong river basin provide some insights into potential effects on streamflow hydrology. A comparison of the groundwater modelling parameters for the two mines, extracted from Table 23 in companion product 2.1-2.2 for the Hunter subregion (Herron et al., 2018a), is provided in Table 46 . In summary, Mandalong SE covers a larger area, extraction is on average at a shallower depth, maximum flow rates are almost 2.5 times higher, and the period of extraction (as modelled) is slightly shorter. Thus, maximum drawdown of the regional watertable from mining of Mandalong SE is predicted to be greater than from mining at Wallarah 2 (see companion product 2.6.2 for the Hunter subregion (Herron et al., 2018b); and Figure 18 of this product). The implications are that streamflow in perennial and intermittent streams that drain the Mandalong SE area will experience potentially significant changes in hydrology, similar to the changes modelled in the Wyong River (see Section 3.3.3). Potentially affected streams include Dora, Mannering, Morans, Stockton, Wallarah and Wyee creeks (Figure 78). Monitoring of these streams will contribute to understanding the hydrological effects and impacts on instream habitat of the additional coal resource development in this area. Quantification of changes in the number of zero-flow days per year and duration of zero-flow day spells would enable a preliminary assessment of the risk to instream habitat using the Hunter subregion receptor impact models for perennial streams and intermittent streams (see companion product 2.7 for the Hunter subregion (Hosack et al., 2018)), with local information used to better resolve the magnitude of risk.

Figure 78

Figure 78 The Mandalong Southern Extension Project area

The extent of the coal resource developments in the coal resource development pathway (CRDP) is the union of the extents in the baseline and in the additional coal resource development (ACRD).

Data: Bioregional Assessment Programme (Dataset 3, Dataset 4, Dataset 5, Dataset 6)

Table 46 Groundwater model input parameters for Mandalong Southern Extension and Wallarah 2 underground mines


Maximum footprint area


Minimum extraction depth


Maximum flow rate


Period of mine dewatering

Mandalong Southern Extension





Wallarah 2





Similarly, the forested wetlands and wet and dry sclerophyll forest communities that occupy the riparian lands along some of these creeks could be affected by the additional coal resource development. Better understanding of the groundwater dependency of the wet and dry sclerophyll forest communities and streamflow and alluvial groundwater dependencies of the forested wetlands in this area will assist in assessing the risk to their persistence and condition from coal mining induced groundwater drawdowns and streamflow changes. Wambo underground

The Wambo coal mine is located in the Hunter Coalfield 30 km west of Singleton. Nearby mines include Hunter Valley Operations to the north and Mount Thorley–Warkworth and Bulga mine complexes to the south-east. The Wambo additional coal resource development relates to Modification 15 to Development Consent 305-7-2003 to access coal reserves in the Wambo seam underneath the approved South Bates underground mine (Whybrow seam). The proposal is for three additional longwall panels (14–16) under already approved panels (11–13). From a surface water perspective, the new panels will result in negligible additional disturbance to surface drainage and no change to water demand and supply, and were therefore deemed to not affect catchment runoff. This is consistent with conclusions from the surface water impact assessment (Advisian, 2015) prepared as part of the Environmental Assessment (Resource Strategies, 2015) for South Bates underground mine. The environmental assessment concluded that there could be some additional surface cracking above panels 15 and 16 at the North Wambo Creek diversion end, however changes in drainage due to cracking were not part of the surface water modelling for the subregion.

The groundwater modelling for the subregion did not include the Wambo additional coal resource development, thus the modelled predictions do not take account of the effect of regional watertable drawdown from dewatering the Wambo seam, below the Whybrow seam. In the groundwater assessment (Hydrosimulations, 2015a) undertaken as part of the Environmental Assessment, peak inflow rates were estimated to increase to 179 ML/year from 124 ML/year under the approved layout. Since the location of the new panels is the same as that for panels 11 to 13 (except deeper), and since a multiplier was used in the uncertainty analysis to vary the inflow rates between 0.5 and 1.5 of the specified input rates, baseline results based on the 1.5 multiplier can be interpreted as broadly consistent with inclusion of additional coal resource development. Hydrosimulations (2015a) modelling results suggested no significant change to the regional watertable from dewatering of the Wambo Seam and no discernible change to baseflow.

The groundwater modelling of the likely change in drawdown due to additional coal resource development at the nearby Mount Thorley–Warkworth (about 2 km away at closest points), which has a bigger maximum footprint and peak flow rates of 1560 ML/year, suggests that a 5% chance of >0.2 m drawdowns will not extend west of Wollombi Brook. The drawdown associated with an increase in flow rates of 54 ML/year at Wambo is likely to be comparatively small. The Ashton South East Open Cut additional coal resource development has a similar peak inflow rate (51 ML/year) and results indicate that its contribution to drawdown of the regional watertable is negligible. While the Wambo mine is 2 km from the Mount Thorley–Warkworth development, modelling results for Mount Thorley–Warkworth and Ashton South East Open Cut suggest a low likelihood that drawdown of the regional watertable from the Wambo additional coal resource development will intersect the drawdown cone from Mount Thorley–Warkworth. However, it cannot be ruled out. The generalised ‘worst case’ estimate of drawdown extent from the Hunter groundwater modelling (see companion product 2.6.2 for the Hunter subregion (Herron et al., 2018b)) suggested that the drawdown extents from mines within 20 km could potentially intersect. More likely, any additional drawdown will be localised and impact very local intermittent streams such as Stony Creek and North Wambo Creek.

Figure 79 includes the greater than 0.2 m drawdown under the baseline. Woodland GDEs to the north-west and forested wetland GDEs to the north and south are potentially impacted due to baseline developments, which include mining at Wambo. It is possible that the drawdown due to additional coal resource development could compound hydrological changes in these GDEs, but the significance of additional drawdown, on top of what is predicted under the baseline, is unknown.

Figure 79

Figure 79 The Wambo baseline mine area

Drawdown under the baseline is shown to identify the area potentially affected by existing operations. The extent of the coal resource developments in the coal resource development pathway (CRDP) is the union of the extents in the baseline and in the additional coal resource development (ACRD).

Data: Bioregional Assessment Programme (Dataset 3, Dataset 4, Dataset 5, Dataset 6) West Muswellbrook open-cut

The West Muswellbrook Project is a new development proposal in the Hunter Coalfield for two open-cut pits and associated infrastructure in Assessment Lease 19, approximately 12 km north-west of Muswellbrook (Figure 80). The development would operate over 30 years and disturb an area of approximately 5620 ha, including diversion of a creek (IESC, 2015). At the time the CRDP was being defined, this proposal had not progressed beyond a Gateway application and there were insufficient details available to enable modelling as part of the additional coal resource development. As of December 2016, an environmental assessment had not been lodged.

The proposed development is located to the west and north-west of, but in proximity to, Dartbrook, Mount Pleasant and Bengalla mines and to the north-east of Mangoola mine. At its closest points, the mine footprint is about 10 km east of Wybong Creek itself, but less than 3 km from some of its tributaries; and 5 km from Dart Brook. The effects on catchment runoff, groundwater drawdown and changes in groundwater fluxes to streamflow are expected to compound any changes from the additional coal resource developments at Mount Pleasant and Bengalla, and also from the baseline developments at Mangoola mine, which is approved to 2026, and Dartbrook, which has been in care and maintenance since 2007, but looks set to re-open following its acquisition by Australian Pacific Coal Ltd in 2016.

Based on predictions of drawdown from the modelled additional coal resource development, drawdowns of at least 0.2 m are very likely within a 5 km radius of the mine footprints with a possibility of extending to about 20 km away (companion product 2.6.2 for the Hunter subregion (Herron et al., 2018b)). Thus it is likely that the West Muswellbrook Project would expand the groundwater zone of potential hydrological change further into the Wybong Creek catchment to the west of the development, particularly if the drawdowns from Bengalla, Mount Pleasant and West Muswellbrook compound each other. Under the modelled additional coal resource development, which included Bengalla and Mount Pleasant developments, the hydrological modelling indicated a possibility of increases in the number of low-flow spells in Wybong Creek. With the addition of the West Muswellbrook open-cut mines, it is likely that Wybong Creek will experience changes in flow regime across more of the hydrological response variables than just the low-flow spells. Monitoring of groundwater levels and streamflow in the Wybong Creek catchment would enable the impacts of the West Muswellbrook and neighbouring mines to be determined.

Similarly, to the north and east, it is likely that groundwater drawdowns will extend further into the Dart Brook catchment and potentially enhance drawdowns from the Mount Pleasant and Bengalla mines near the Hunter River. Greater reductions in baseflow in the lower Dart Brook and along parts of the Hunter River cannot be ruled out. Both Sandy Creek North and Sandy Creek South, which flow out of the proposed mine area, are in the zone of potential hydrological change due to the Mount Pleasant and Bengalla developments. It is likely that these streams would experience larger hydrological changes if the West Muswellbrook mine was included in the modelling. Some ephemeral streams in these catchments will be directly impacted by the excavation of the mine pits.

Figure 80

Figure 80 The proposed West Muswellbrook Project area

The extent of the coal resource developments in the coal resource development pathway (CRDP) is the union of the extents in the baseline and in the additional coal resource development (ACRD).

Data: Bioregional Assessment Programme (Dataset 3, Dataset 4, Dataset 5, Dataset 6, Dataset 7)

Since the Hunter River has minimum environmental flow requirements specified in the regulated river, any reductions in tributary inflows could mean additional releases of water from Glenbawn Dam would be needed to ensure the minimum flow condition is met. However, the analysis of changes in environmental water releases presented in Section for the modelled additional coal resource development suggests that additional releases would be relatively small.

In Figure 80, it can be seen that some additional areas of forested wetlands on Wybong Creek to the west and on Dart Brook and Kingdon Ponds to the north-east of the proposed mine could potentially be affected by the West Muswellbrook development. Wilpinjong open-cut

Wilpinjong Mine is located in the Western Coalfield and comprises seven open-cut pits. The additional coal resource development is a proposal to increase the area of existing open-cut pits by incremental additions totalling 500 ha, and develop an eighth pit of approximately 300 ha (Figure 81), which together would necessitate some infrastructure changes and extend the life of the operation by 7 years. The proposal requires no changes to key sources of water supply (i.e. runoff collected on site, dewatering of open-cut pits and groundwater), nor to water disposal as per the existing Environment Protection Licence 12425. The proposed changes on catchment runoff have been modelled as part of this Assessment, but the river modelling does not include changes in baseflow from this development because it was not included in the groundwater modelling (noting that the information summarised below was released in 2015 after the BA groundwater model was set up). Pit inflows have been predicted as part of the groundwater assessment (Table 6-6, Hydrosimulations, 2015b) for the Wilpinjong Extension Project Environmental Assessment. (Peabody Energy, 2015). They are predicted to reach a maximum of 1269 ML/year in 2018 to 2019 across the eight pits, decreasing to 10 ML/year by 2034 when mining ceases. The inflow predictions do not differentiate the baseline and additional coal resource development components, except for the new pit (pit 8), which represents about 30% of the inflow rate in 2018 to 2020, but significantly lower proportions in other years. The extra 7 years of operations are associated with predicted inflow rates of between 263 and 753 ML/year.

The Hydrosimulations (2015b) groundwater assessment found considerable drawdown of the watertable at the pit margins, but overall watertable drawdowns were laterally restricted with only limited areas of drawdown of 1 to 2 m further away from the mine pits. Drawdowns were predicted to induce a flux of water from the Wollar Creek alluvium (up to 170 ML/year during the period of mining) with peak baseflow reductions in Wilpinjong Creek and Cumbo Creek of 0.47 ML/day (cf. 0.48 ML/day from prior modelling of baseline development).

Considering the groundwater modelling of the baseline and additional coal resource development mines, and Hydrosimulations’ (2015b) predicted mine inflow rates and results from site-scale modelling, it is probable that the drawdown cone associated with additional coal resource development could extend towards the Goulburn River in the north and potentially affect baseflows in the Goulburn River. At the 95th percentile under the baseline, drawdowns exceeding 0.2 m are predicted under a stretch of the Goulburn River (Figure 20 in Section 3.3). However, it is unclear the extent to which the operations at Wilpinjong are contributing to this, since the Ulan and Moolarben mines are closer to this area and the effects of the mines were not modelled individually.

Wollar Creek, 2 km to the east of the eastern edge of the Wilpinjong operation, could also be affected given its proximity and that it is potentially affected under the baseline (Figure 81). The potential for some impact on nearby streams, at the very least a prolonging of baseline impacts due to the 7-year extension to operations under the additional coal resource development proposal, cannot be ruled out.

Figure 81

Figure 81 The Wilpinjong mine and associated drawdown extent under the baseline and due to additional coal resource development

The extent of the coal resource developments in the coal resource development pathway (CRDP) is the union of the extents in the baseline and in the additional coal resource development (ACRD).

Data: Bioregional Assessment Programme (Dataset 3, Dataset 4, Dataset 5, Dataset 6)

The GDEs in the vicinity of Wilpinjong mine include forested wetlands, dry sclerophyll forests and woodlands. All of these are in the zone of potential hydrological change for the modelled additional coal resource development. The additional drawdown from representing the Wilpinjong mine is unlikely to impact on additional GDEs, but could enhance the hydrological changes at the sites that have already been identified as potentially impacted. The area of greater than 0.2 m of drawdown under the baseline, shown in Figure 81, does not intersect any additional GDEs to those already identified as being in the zone. Additional drawdown from the new pit and nearby workings (on the eastern side of the Wilpinjong mine site) could increase the impact on a small area of dry sclerophyll forest on Wilpinjong Creek and potentially some forested wetlands on the Goulburn River (downstream of the junction with Wollar Creek), but most of the streams in this area are mapped as ephemeral and would be unlikely to be affected if the drawdown area were larger.

Last updated:
15 March 2019
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