Regional changes in surface water and groundwater flows due to additional coal resource development could potentially lead to changes in the quality of surface water and groundwater. Although water quality changes due to the additional coal resource development were not modelled explicitly as part of this BA, the implications for water quality in the Gloucester subregion are considered in this section in light of the modelled hydrological changes due to the additional coal resource development.
Relevant factors for assessing the potential for changes in regional groundwater and surface water quality from the four additional coal resource developments in the Gloucester subregion are:
- Under the Gloucester Gas Project Stage 1 proposal, AGL intends to hydraulically fracture target coal seams to facilitate the release of coal seam gas. This fracturing process would involve the injection of a mixture of water, sand and various chemical components (such as biocides) at high pressures.
- Under the Gloucester Gas Project Stage 1 proposal, AGL does not propose to re-inject co-produced water into depressurised aquifers. This water disposal mechanism is not relevant to the coal mines.
- All four developments operate under a ‘no discharge’ rule, which means that they are not licensed to discharge co-produced or other operational water into the stream network.
- All four developments include the use of existing or new water storages, for storage of co-produced water and drainage from waste rock emplacements.
- All four developments propose to reuse co-produced water on site. Uses include dust suppression, coal handling and preparation, and irrigation. The Gloucester Gas Project Stage 1 proposal includes construction of an on-site water treatment plant to treat saline co‑produced water to a quality suitable for use by a local irrigator, with salt and solid by-products trucked off site for disposal (AGL Energy Limited, 2014).
In the following sections the groundwater and surface water causal pathways that could potentially lead to regional impacts on water quality are identified and the risk of impact is qualitatively assessed. The extent of influence and existing regulation and management practices are used to inform the assessment of risk.
18.104.22.168 Groundwater quality
Changes in groundwater quality from coal resource development can occur as an indirect result of subsurface depressurisation and dewatering of aquifers and changes to subsurface physical pathways between aquifers, which may modify groundwater flow paths and flow rates 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, counter the effects of groundwater depressurisation and/or facilitate the process of CSG extraction. 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 different mining operations are in proximity.
Table 12 lists potential causes of changes in groundwater quality from coal resource development in the Gloucester subregion and identifies the potential for off-site impacts. Three of the four causal pathways in Table 12 could potentially have off-site impacts. However, one of these, ‘Hydraulic fracturing’ causal pathway, is not within the scope of the BAs and is identified here simply to acknowledge the hazard. Hydraulic fracturing is often, but not always, used in CSG extraction. It involves injecting water, often containing chemicals and sand, into the coal seams to create pathways to liberate gas and could have the potential for water quality impacts off site. Although AGL put their Gloucester Gas Project on hold in February 2016, this development is included in the CRDP for the Gloucester subregion and must be considered a potential cause for water quality impacts off site. In the remainder of this section, the risk to water quality off site is considered in the context of the scale of the effect and existing regulatory controls.
Table 12 Potential causal pathways for changes in groundwater quality and potential for off-site impacts
BA = bioregional assessment, CSG = coal seam gas
While not specifically identified for each development, wells are necessary parts of CSG extraction, and monitoring bores and production bores are typical of coal mining developments. Well integrity can be an issue, with well failure considered an inevitable consequence of CSG extraction. NSW Department of Industry Resources and Energy (DIRE) have a code of practice for CSG well integrity (DTI, 2012), which includes mandatory standards for well design and construction to protect groundwater resources. As a condition of title to explore, extract or produce under NSW’s Petroleum (Onshore) Act 1991, wells must be designed, constructed, maintained and abandoned to: (i) prevent any interconnection between coal seams and aquifers; (ii) ensure isolation between different aquifers and water-bearing zones; and (iii) not introduce substances that may cause environmental harm. All chemicals to be used must be disclosed during the approvals process. Given this, off-site impacts on water quality from well leakage are considered unlikely.
Bore leakage may result in local changes in groundwater quality, but the propagation of these changes off site, and hence the potential for regional, cumulative impacts, is considered unlikely. 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 through to 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 from additional coal resource development and not within the scope of this BA.
The potential impacts on watertable level, water pressure and groundwater quality from environmentally relevant activities such as CSG operations and coal mines are managed through the NSW Aquifer Interference Policy (DPI, 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 Gloucester subregion, no change in the beneficial-use category of a groundwater source further 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, groundwater-dependent ecosystem, 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, that the proposed development is undertaken in accordance with the policy. Given this, the potential for significant changes in regional groundwater quality is likely to be low.
In relation to leaching of contaminants from mining-related contaminant sources, NSW DIRE, under NSW’s Mining Act 1992, requires mines to have an approved mining operations plan. The mining operations plan provides details of how the mining operation will be carried out, including details of management of stockpiles, rock dumps and tailings dams.
22.214.171.124 Surface water quality
Changes in surface water quality from coal resource development can occur as 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 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. Depressurisation and dewatering of aquifers and changes to subsurface physical pathways between aquifers can lead to a change in baseflow to streams and potentially affect the water quality of the stream. Table 13 lists potential causes of changes in surface water quality from coal resource development in the Gloucester subregion and identifies the potential for off-site impacts, having regard to the relevance of the causal pathway in the subregion and the likely scale of the effect.
Table 13 Potential causes of changes in surface water quality and potential for off-site impacts
TSS = total suspended solids
Altering the surface water system is an inevitable consequence of open-cut mining operations and will therefore directly impact on streamflow. Changes in baseflow on the other hand will depend, in the case of open-cut mines, on the proximity of mining to the stream network, hydraulic conductivity of the rock around the mine and mine pumping rates, which ultimately determine the drawdown zone and changes in direction and magnitude of hydraulic gradients.
The likelihood of off-site water quality impacts from disruptions to surface drainage 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 mining operations plan. Environmental protection licences, 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, 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.
The likelihood of off-site deterioration in stream water quality caused by changes in baseflow following dewatering of mines and/or changes in subsurface physical flow paths will depend on the quality of the groundwater relative to the quality of the water in the stream into which it discharges. Modelling of the hydrological changes due to additional coal resource development predicts a probable reduction in baseflows to streams in the Gloucester subregion. 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 improve stream water quality. This is likely as reported electrical conductivity measurements in Gloucester subregion streams vary between 100 and 600 μS/cm (Section 126.96.36.199 of companion product 1.1 for the Gloucester subregion (McVicar et al., 2014)), while published electrical conductivity readings of groundwater vary from around 400 to 6000 μS/cm in alluvial aquifers and around 2400 to 9400 μS/cm in sandstone‑siltstone and coal seam aquifers (Section 188.8.131.52 in McVicar et al., 2014). Streamflow and groundwater level data indicate that streams in the Avon river basin are generally gaining streams and that groundwater is exchanged between the alluvium and surrounding shallow weathered rock aquifer (Section 184.108.40.206 in McVicar et al., 2014). These data suggest that any reduction in baseflow due to the drawdown from additional coal resource development could lead to a decrease in stream salinities, but there are other factors at play and a broader analysis may be warranted.
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 were not modelled
- 3.7 Conclusion
- Contributors to the Technical Programme
- About this technical product