3.3.4.1 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 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 6 lists potential causes of changes in groundwater quality from coal resource development in the Maranoa-Balonne-Condamine subregion and identifies the potential for off-site impacts. Groundwater quality (including aquifer properties and groundwater composition) is potentially affected by eight causal pathways in the Maranoa-Balonne-Condamine subregion. Effects on groundwater quality are localised within tenements, downstream watercourses and irrigated areas or target aquifers used to dispose of co-produced water. Risks are addressed by Mine Water Management Plans within tenements and by Healthy Water Management Plans in downstream watercourses. 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 6 Causal pathways for potential changes in groundwater quality and off-site impacts

CSG = coal seam gas

CSG operations and coal mines have the potential to change surface water – groundwater interactions. These changes are likely to be within tens of metres of a watercourse and so are not represented in the regional groundwater model. Changes to groundwater quality from environmentally relevant activities such as CSG operations and coal mines are addressed by the Healthy Water Management Plans being developed under Queensland’s Environmental Protection Act 1994 legislation. The plans assess risks to water quality, and identify water quality targets based on local data (including electrical conductivity, nutrients, turbidity, pH) to inform regulatory conditions on environmentally relevant activities such as CSG and coal mines. These plans will improve the monitoring and assessment of threats to surface water quality in the subregion.

Preferential flow paths can also be affected by changes to surface water – groundwater interactions (including changes to aquifer interconnectivity, mine expansion too close to a river or lake, preferential drainage and recharge associated with post-closure water filling the pit). Mine expansion that links aquifers and leads to preferential drainage can affect groundwater quality, but is likely to be limited to the extent of the mine tenements due to the hydraulic gradients toward the mine pits. Changes to surface water – groundwater interactions can also change the timing and volume of baseflow contributions to streams, which can affect the stream ecosystem within and downstream of tenements. These changes are likely to be restricted to areas where direct interactions between watercourses and unconfined aquifers are possible.

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. The code of practice for constructing and abandoning coal seam gas wells and associated bores in Queensland (DNRM, 2013) was developed to ensure that all CSG wells and CSG water bores are constructed and abandoned to a minimum acceptable standard resulting in long term well integrity, containment of gas and the protection of groundwater resources.

Potential effects of leaky wells are likely to be localised, with numerical modelling suggesting that changes to hydraulic gradients are restricted to less than 1 km, but will continue until remedial actions are taken. Hydraulic stimulation involves high-pressure injection of water (and other materials including chemical compounds and sand) to induce changes in aquifer properties to aid the release and flow of gas from the coal seams towards the well. This may also lead to unplanned groundwater changes through mis-perforation of the coal seam. The lateral extent to which aquifer properties and groundwater quality are changed diminishes with distance from the well and is likely to be limited to aquifers within tenements. The groundwater composition and quality of the fractured aquifer and neighbouring aquifers can be compromised and is subject to management controls (such as compliance with standards and regulations) and monitoring.

Disposal of co-produced water by aquifer reinjection has the potential to offset impacts of groundwater depressurisation from CSG production in aquifers in the Maranoa-Balonne-Condamine subregion. Potential impacts include changes to the volume and timing of groundwater discharge to springs and watercourses in aquifer outcrop areas and possible changes to aquifer composition. Aquifer reinjection is not modelled numerically as projects are still at the feasibility testing and trial injection stages. Current studies target the Gubberamunda, Precipice and Hutton sandstone aquifers (APLNG, 2012; Arrow Energy, 2013; Santos, 2013).

Dam construction and other water management structures that change natural surface drainage and runoff have the potential to affect groundwater recharge patterns, in turn affecting groundwater quality and quantity/volume. However, this is likely to be limited to watercourses within and downstream of tenements.