- Bioregional Assessment Program
- Hunter subregion
- 3-4 Impact and risk analysis for the Hunter subregion
- 3.3 Potential hydrological changes
- 3.3.4 Potential impacts on water quality
Regional changes in surface water and groundwater flows due to additional coal resource development could potentially lead to changes in surface water and groundwater quality. While water quality was not modelled as part of this BA, the implications for water quality in the Hunter subregion are considered here in light of the modelled hydrological changes due to additional coal resource development.
Relevant factors for assessing the potential for changes in regional groundwater and surface water quality from the additional coal resource developments in the Hunter subregion are:
- High levels of connate salt in the Greta Coal Measures and Wittingham Coal Measures, which were formed during marine transgressions (Kellett et al., 1989). Estimated salt yields of 30 to 40 T/km2/year from these measures, compared to 4 to 5 T/km2/year for Carboniferous and Triassic units have been reported (Creelman (1994) reported in NSW EPA (2013)).
- Areas of high background salinity, including the Jerry Plains area and the Wollombi Brook valley between Broke and Singleton, where Permian rock units occur at the surface (Figure 28). Saline discharges occur from springs associated with geological and geomorphological features, such as the Hunter-Mooki Fault Thrust System and break-of-slope areas.
- Stream salinity is a significant management issue in the Hunter river basin. Sources of salt include rainfall and weathering products, which enter the stream via surface runoff pathways, and groundwater sources, particularly from Permian coal measures. Streams with identified groundwater interactions often have high salinities. Of the surface water salinity observations from across the Hunter subregion, median electrical conductivities exceed 5500 μS/cm in Loders Creek in the Singleton water source, Saddlers Creek and Saltwater Creek in the Jerrys water source, minor creeks around Mount Arthur coal mine in Muswellbrook water source and Big Flat Creek in the Wybong water source. In the Upper Goulburn River and Wollar Creek, median electrical conductivities exceed 2300 μS/cm (Figure 29; NSW EPA, 2013). Coal mining is thought to contribute to stream salinity, although this is difficult to confirm due to lack of long-term monitoring data and a highly variable climate.
- The Hunter River Salinity Trading Scheme (HRSTS) was introduced to manage the discharge of saline water from coal mining and power generation sites along the Hunter Regulated River.
- Some coal resource developments in the subregion are required to manage discharges according to volumes, quality and discharge windows specified in environment protection licences (EPL), which are a condition of their approval to operate.
- There is no CSG development in the CRDP and potential water quality issues from use of fracking chemicals is not relevant; nor can well failure leading to leakage between aquifers be considered a real risk because construction of large numbers of wells is not a feature of open-cut and longwall coal extraction methods.
- None of the additional coal resource developments proposes to re-inject co-produced water into depressurised aquifers.
Figure 28 Areas of dryland salinity and salinity risk in the Hunter river basin
Source: NSW EPA (2013)
Figure 29 Surface water electrical conductivity (μS/CM) levels in the Hunter river basin
Source: NSW EPA (2013)
In the following sections the groundwater and surface water causal pathways that could potentially lead to regional impacts are identified and the risk of impact is assessed. The extent of influence and existing regulation and management practices are used to inform the assessment of risk.
22.214.171.124 Groundwater quality
Changes in groundwater quality from coal resource development can occur as an indirect result of depressurisation and dewatering of aquifers and changes to subsurface physical pathways between aquifers, which enhance leakage between aquifers of different quality water. Changes in groundwater quality can also occur as a direct result of coal resource development and operational water management, such as when water is deliberately injected into an aquifer or coal seam to manage surplus water or counter the effects of groundwater depressurisation. Unless hydrologically isolated from their surroundings, the creation of coal stockpiles, rock dumps and tailings dams on coal mine sites can result in leaching of contaminants to groundwater. In all these cases, a hazard arises when the quality of the receiving water is changed such that it reduces its beneficial use value. BAs are concerned with the risk from non-accidental changes to water quality off site, which may be cumulative where mining operations are in proximity.
Table 15 lists potential causes of changes in groundwater quality from coal resource development in the Hunter subregion and identifies the potential for off-site impacts. Regional changes in groundwater quality from bore leakage is considered unlikely, as bore construction and maintenance must be undertaken in accordance with state regulation to minimise leakage. In NSW, a water supply work approval is needed under NSW’s Water Management Act 2000 for a new bore. Construction of a bore must be undertaken by a licensed driller and drillers are expected to meet minimum requirements set out in guidelines developed by the National Uniform Drillers Licensing Committee (NUDLC, 2012). These guidelines detail mandatory requirements and good industry practice for all aspects of the bore life cycle from bore design, bore siting, drilling fluids, casing, maximising bore efficiency, sealing and bore completion. While some leakage from older bores is considered likely, these bores are not part of the potential impact due to additional coal resource development and not within the scope of this BA. Three of the four causal pathways in Table 15 could potentially have off-site impacts. In the remainder of this section, the likelihood of impacts is considered in the context of existing regulatory controls.
The potential impacts on watertable level, water pressure and groundwater quality from environmentally relevant activities such as coal mining are managed through the NSW Aquifer Interference Policy (DPI Water, 2012). This policy requires that all water taken from an aquifer is properly accounted for; minimal impact considerations on the watertable, water pressure and water quality are addressed; and remedial measures are planned for in the event that actual impacts are greater than predicted. For aquifers in the Hunter subregion, no change in the beneficial use category of a groundwater source farther than 40 m from the activity is permitted, unless studies can demonstrate that the change in groundwater quality will not affect the long-term viability of any water sharing plan, GDE, culturally significant site or water supply work. An increase of more than 1% per activity of the long-term average salinity is not permitted in a highly connected water source at the nearest point to the activity. As part of their groundwater monitoring and modelling plans, mining companies must demonstrate to the satisfaction of the NSW Department of Primary Industries Water, that the proposed development is undertaken in accordance with the policy. Given this, the potential for significant changes in regional groundwater quality are likely to be low.
Table 15 Potential causes of changes in groundwater quality and potential for off-site impacts in the Hunter subregion
Changes in tensile and compression forces in the overburden above longwall panels following their collapse can lead to fracturing above longwall panels and hydraulic enhancement of the goaf, with the potential for freer movement of water between aquifers of potentially different water quality. Hydraulic enhancement was modelled in the Hunter groundwater model (companion product 2.6.2 for the Hunter subregion (Herron et al., 2018b)) and was shown to affect the extent of drawdown zone and surface water – groundwater exchanges, but implications for groundwater quality were not modelled. Sometimes groundwater assessments undertaken by mines represent changes in hydraulic properties above longwall panels in their modelling, but sometimes these changes are ignored because the scale of influence is deemed too local to affect larger-scale drawdown predictions. If hydraulic enhancement of the goaf is ignored, the hydraulic properties of the interburden may be overestimated to compensate for the lack of groundwater flowing into the mine. Invariably the groundwater models do not represent changes in groundwater quality or surface water quality due to changes in hydraulic properties. The effect of coal resource development on the water quality of nearby aquifers and streams in the Hunter subregion remains largely a knowledge gap.
In relation to leaching of contaminants from mining-related contaminant sources, the Department of Industry Resources and Energy (DIRE), under NSW’s Mining Act 1992, requires mines to have an approved mining operations plan (MOP). The MOP provides details of how the mining operation will be carried out, including details of management of stockpiles, rock dumps and tailings dams. Mining companies, as part of best practice management, are required to design storages that are secure and stable over their life and have a low risk of spills.
126.96.36.199 Surface water quality
Changes in surface water quality from coal resource development can occur as a result of disruptions to surface drainage from the removal of vegetation and disturbance of soil in construction of roads, site facilities, excavation of open-cut pits and landscaping of the site during production and rehabilitation. Bare surfaces increase the risk of erosion with potential to increase total suspended solids (TSS) in waterways. The discharge of mine water into the stream network as part of operational water management is potentially hazardous if the quality of the discharged water lowers the quality of the receiving water below its current beneficial use level. Groundwater pumping and subsurface fracturing above longwall panels can lead to changes in baseflow to streams and potentially affect the water quality of the stream. Table 16 lists potential causes of changes in surface water quality from coal resource development and identifies the potential for off-site impacts in the Hunter subregion, having regard to the likely scale of the effect and existing management. One causal pathway is considered unlikely to lead to significant off-site water quality impacts; three could potentially have off-site impacts on water quality.
Table 16 Potential causes of changes in surface water quality and potential for off-site impacts
The likelihood of off-site soil loss and sedimentation impacts from altering the surface water system on the mine sites is considered unlikely. There is a long history of soil erosion management in NSW, which has its origins in the agricultural sector, but has been extended to minimise the generation and mobilisation of sediments in all developments where disturbance of the soil occurs. NSW DIRE requires mines to provide details of how the mining operation proposes to minimise soil loss at all life stages of the mine and post-mining as part of an approved MOP. EPLs, issued by DIRE under NSW’s Protection of the Environment Operations Act 1997, may also specify erosion control conditions. Furthermore, DIRE requires authorised mines to develop, implement and report on environmental monitoring programs. In annual environmental management reports (AEMR), the coal mining companies must publish their monitoring data in order to demonstrate that they are meeting their environmental objectives under their licence to operate.
188.8.131.52.1 Stream salinity
Reductions in catchment runoff can increase the salinity of a receiving stream, where the runoff is less saline than the receiving stream. Catchment runoff occurs during and shortly after significant rainfall events and is typically the main contributor of fresher water to peaks in the streamflow hydrograph. Where the mine footprint (which for surface water includes areas where runoff is intercepted by mine pits and storages, and is retained on site) is small relative to the contributing area of the stream into which it drains, the risk of large increases in stream salinity from reducing catchment runoff is likely to be very low; where the opposite is true, the salinity of peak flows could become increasingly biased towards the salinity of baseflow. If, at the same time, groundwater drawdown has contributed to a reduction in baseflow or a disconnection between the stream channel and groundwater, then stream salinity will reflect the changes in the relative contributions from catchment runoff, baseflow and streamflow from up catchment.
184.108.40.206.1.1 Discharges to regulated river
There are many competing demands on water resources in the rivers of the Hunter subregion and water needs to be of a quality to support a diverse range of agricultural uses, town water supply and environmental needs. Background salt levels are naturally high in some parts of the subregion where geological layers, such as the Permian coal measures, which formed under marine transgressions, outcrop at the surface (Figure 28). In addition, discharge of saline water from coal mining and power generation operations has been identified as a significant source of salt, and concerns about increasing stream salinities in the Hunter River led to the introduction of the HRSTS in the late 1990s. The scheme introduced a capped system of tradeable salinity credits to limit annual discharges of salt to the Hunter River, and established rules to govern the timing of discharges from participants in the scheme to ensure water quality is maintained at an acceptable level for other users. Dartbrook, Mount Arthur, Bengalla, Hunter Valley Operations, Liddell, Ravensworth, Wambo and Mount Thorley–Warkworth coal mines are all participants of the scheme. Discharges are permitted during high-flow and flood-flow windows when the natural salinity of the river decreases in response to the influx of relatively fresh surface runoff and the river can accommodate extra salt from industrial discharges without exceeding salinity thresholds. Discharges are monitored and reported in an annual statement by the NSW EPA, which summarises the flow and salinity of the river at three locations over the year and details the mine discharges that occurred (see NSW EPA (n.d.)). These monitoring reports indicate that HRSTS is operating as intended and mine discharges are not leading to periods of unacceptable salinity.
Results from the hydrological modelling of additional coal resource development can be used to assess whether the modelled changes in Hunter River flows are likely to impact upon the opportunities the mines and power generators have to discharge saline water to the river under the scheme. Flow rate thresholds are defined at model nodes 6, 20 and 51 for each of the three river reaches making up the HRSTS. Table 17 summarises the discharge thresholds at each node. Industry discharges to the river are permitted when flow rates are above these thresholds. Table 18 summarises the mean annual change in discharge opportunities in the Hunter Regulated River due to additional coal resource development for each 30-year period. The modelled number of discharge days under the baseline are provided to show the interannual variability in discharge days due predominantly to climate. In the most downstream reach of the HRSTS (model node 6), there is a greater than 95% chance of greater than 26 discharge days and a 5% chance of greater than 99 discharge days; in the upstream reach, represented by model node 51, there is a 95% chance of greater than 34 discharge days and a 5% chance of greater than 151 discharge days. Discharge days tend to be fewer in the middle reach. The modelling results suggest that there is very unlikely to be an impact upon discharge days under the HRSTS due to additional coal resource development. At the two upstream model nodes (20 and 51), additional coal resource development has no impact on discharge opportunities, with average reductions of less than 1 day per year in all three periods. At the Singleton gauging station (model node 6), additional coal resource development could potentially cause an average reduction of 1 to 2 days in discharge days.
Table 17 Flow rate threshold (ML/day) for mine discharges to the Hunter River under the Hunter River Salinity Trading Scheme
Flow rate threshold
Hunter R at Singleton
Hunter R upstream Glennies Ck
Hunter R at Denman
Table 18 Number of days per year when Hunter River streamflow rates are above the discharge threshold under the baseline and reduction in discharge days due to additional coal resource development (ACRD)
Discharge day = a day when flow rates at the model node exceed the flow rate threshold for discharge under the Hunter River Salinity Trading Scheme
Data: Bioregional Assessment Programme (Dataset 12)
220.127.116.11.1.2 Discharges to unregulated river
Some of the Hunter subregion mines are not part of the HRSTS and occur along unregulated rivers in the Hunter river basin and Macquarie-Tuggerah lakes basin. These coal mines are required to hold an EPL, which specifies conditions attaching to the mine’s licence to operate, including those relating to the management of mine water. Some examples for the Hunter subregion additional coal resource developments are:
- At the proposed Bylong Mine, the condition of operation is that the mine is not permitted to discharge mine water off site.
- The Ulan, Moolarben and Wilpinjong mines in the upper Goulburn River catchment are permitted under their respective EPLs to discharge 30, 10 and 5 ML/day, respectively, but discharges must not exceed electrical conductivities of 810 to 1000 μS/cm at Ulan (depending on discharge site), 800 to 900 μS/cm at Moolarben and 500 μS/cm at Wilpinjong. In addition, these three mines have entered into a water sharing arrangement, which allows a surplus of water at one site to meet the deficit of water at another, thereby reducing the need for the mines to take more water from the environment and reducing the volume of mine water make discharge from the sites.
- At Mandalong mine, the salinity of water discharged from the mine site is picked up in a general clause of EPL 365, whereby any pollutant not specified in the table/s in the EPL is not allowed to be discharged if it will pollute the waters. The mine is licensed to discharge up to 5 ML/day.
- Wallarah 2 is a new mine and does not yet have an EPL. Modelled estimates of Wallarah 2 surplus water requirements, and hence mine discharges to Wallarah Creek, range from 52 ML/year in year one to median discharge of 250 ML/year (90th percentile of 370 ML/year; maximum discharge of around 500 ML/year). Under their proposal, mine water will be treated to background water quality levels prior to discharge, with the salt in the brine to be disposed underground.
In conclusion, due to a high level of regulation and monitoring of discharges of mine water to surface drainage network in the Hunter subregion, the risk to stream water quality from this causal pathway is considered to be minimal.
18.104.22.168.1.3 Depressurisation, dewatering and hydraulic enhancement
The risk to regional stream water quality caused by changes in baseflow following depressurisation and dewatering of mines and/or changes in subsurface physical flow paths (e.g. from hydraulic enhancement of the goaf) will depend on the magnitude of the hydrological changes and the salinity of the groundwater relative to the salinity of the water in the stream into which it discharges. Modelling of the hydrological changes due to additional coal resource development in the Hunter subregion predicts a probable reduction in baseflows to Hunter subregion streams. If, as is usually the case, the salinity of the groundwater is higher than that of the stream into which it discharges, a reduction in baseflow would be expected to lead to a reduction in stream salinity.
Companion product 1.1 for the Hunter subregion (McVicar et al., 2015) provides details on groundwater and surface water quality. The saline water associated with the Permian coal measures and the intervening marine sequence is thought to have a controlling influence on the overall water quality of the Hunter River (Kellett et al., 1989). Groundwater quality is generally brackish to saline (Mackie Environmental Research, 2006), with electrical conductivity (EC) records in the range 4,000 to 12,000 μS/cm in the hard rock aquifers associated with the Hunter coal seams. In the alluvial aquifers of the Hunter River, the mean total dissolved salts varies from 650 mg/L (~1000 μS/cm) upstream of the Hunter-Goulburn rivers confluence to 840 mg/L (~1300 μS/cm) in the aquifers downstream of the confluence. It is reported that groundwater extractions from alluvial aquifers can lead to upward fluxes from more saline water in the underlying Permian units such as occurred during the 2001 to 2004 drought years (NSW Department of Planning, 2005). Mining operations in some locations have led to reversal of groundwater gradients and decreases in groundwater salinity in alluvial aquifers (Australasian Groundwater and Environmental Consultants Pty Ltd, 2013). Groundwater modelling results from this BA suggest that baseflow reductions are the likely outcome from coal resource development, which could result in reductions in salinity in connected streams. For example, the potentially large reductions in streamflow modelled for Loders Creek and Saddlers Creek, which both have high stream salinities (>5500 μS/cm), are likely to mean less salt is exported from these catchments to the Hunter River due to additional coal resource development.
In all the streams identified from the regional-scale modelling as at risk of potentially large changes in flow regime, the impact on local stream salinity will depend on the relative reductions in catchment runoff and baseflow over time. Reductions in catchment runoff are more likely to affect runoff peaks, while baseflow reductions have a more noticeable effect on low flows. The implications for stream salinity at any given time will depend on how the relative contributions from the quick and slower flow pathways change over time. In streams, such as Loders Creek, Saddlers Creek and the unnamed creeks near the Mount Pleasant and Mount Thorley-Warkworth mines, where modelling results suggest increasing numbers of zero-flow days, it is likely that channel pools will be subject to longer periods of salt concentration by evaporation and less efficient flushing, conditions that favour increasing the salinity of these water bodies.
Increases in baseflow, potentially leading to increases in alluvial aquifer and stream salinity, cannot be ruled out, but this is not an outcome that has been reported in the literature and remains an area for further investigation. The magnitude and extent of water quality changes cannot be determined without specifically representing water quality parameters in the modelling. This remains a knowledge gap.
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