3.3.4 Potential water quality changes


Regional hydrological changes due to additional coal resource development could potentially impact surface water and groundwater quality. Although water quality was not modelled as part of this BA, the implications for water quality in the Galilee subregion are briefly considered in this section, in light of the previously discussed modelled changes due to additional coal resource development.

Relevant factors for assessing the potential for changes in regional groundwater and surface water quality from the seven additional coal resource developments in the Galilee subregion are:

  • Natural streamflow in the Galilee subregion varies considerably, with long periods of little to no streamflow during times of low rainfall. This is likely to affect surface water quality (e.g. variations in salinity and turbidity) between wet and dry seasons.
  • Approval conditions for off-site discharge of mine-related water are yet to be finalised (as of December 2017).
  • No groundwater discharge off site is envisaged (e.g. it is not proposed as part of environmental impact statement (EIS) documentation), with most groundwater extracted as part of routine mine dewatering operations expected to be utilised on site for various mining and processing applications (see Section 2.5.2 in companion product 2.5 for the Galilee subregion (Karim et al., 2018a) for further information). Therefore, it is possible that salts derived from this groundwater will need to remain contained on site.
  • Source of external water supplies for mines is yet to be finalised. Transferrals of large volumes of water will include dissolved solutes and may influence distribution of salt within the catchment, or possibly in other (nearby) catchments.
  • Surface water salinity data are patchy, with large gaps (both spatial and temporal) between measurements in most of the streams in and near to the zone.
  • Other than salinity information, there is very little available analytical data for minor, trace and organic analytes from groundwater and surface water in the zone of potential hydrological change.
  • None of the additional coal resource developments propose to re-inject co-produced water into depressurised aquifers.
  • Water quality impacts from non-modelled developments (see Section 3.6) will be additional to any potential water quality changes derived from modelled development areas, but are not further assessed here.

In the following sections the groundwater and surface water causal pathways that could potentially lead to regional water quality impacts are identified and the risk of impact is qualitatively assessed. The extent of influence and existing regulation and management practices are used to inform this assessment of risk.

3.3.4.1 Groundwater quality

Groundwater quality in the Galilee subregion is covered by the Environmental Protection (Water) Policy 2009 (EPP (Water)), which achieves the objectives of Queensland’s Environmental Protection Act 1994. The groundwater modelling results presented in companion product 2.6.2 (Peeters et al., 2018) indicate that any potential groundwater impacts will be confined to hydrostratigraphic units assigned to the Galilee Basin (primarily the upper Permian coal measures, Rewan Group, and to a lesser extent the Clematis Group aquifer) and in areas where alluvium and Cenozoic sediments overlie the Galilee Basin aquifers. These conditions largely occur around the central-eastern margin of the Galilee Basin in the vicinity of the modelled additional coal resource development.

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 may enhance leakage between aquifers that contain groundwater of different qualities. Changes in groundwater quality can also occur as a direct result of coal resource development and its associated operational water management practices, such as when water is deliberately injected into an aquifer or coal seam to manage surplus water, counter the effects of groundwater depressurisation or to 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 also result in leaching of contaminants to shallow groundwater systems. 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 occur in proximity.

Table 17 lists potential causes of changes in groundwater quality from coal resource development in the Galilee subregion and identifies the potential for off-site impacts. Groundwater quality (including aquifer properties and groundwater composition) is potentially affected by up to eight causal pathways in the Galilee 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 will mainly be addressed by future mine water management and monitoring plans within tenements. In some areas of Queensland (e.g. around the Surat Basin), Healthy Waters Management Plans may also exist for potentially affected 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.

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 developed for the Galilee subregion. Preferential flow paths can also be affected by changes to surface water – groundwater interactions (including changes to aquifer interconnectivity, mine expansion close to a river or lake, preferential drainage and recharge associated with post-closure water filling the pit). Mine developments that link aquifers and lead to preferential drainage can affect groundwater quality, but may be limited to the vicinity of open-cut 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, monitoring bores and dewatering bores are required for coal resource developments. 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.

Fracturing above underground longwall panels due to removal of coal and hydraulic enhancement (of aquifers and aquitards) above the goaf potentially affects the rate, volume and timing of groundwater flow between aquifers. Enhanced aquifer connectivity has the potential to lead to mixing between different quality groundwater sources, or to locally enhance recharge of surface water to shallow aquifers. However, the degree of change in connectivity is dependent on site-specific conditions at each underground operation, and requires substantially more local-scale studies than what has been possible to undertake for this BA.

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.

Table 17 Potential causes of changes in groundwater quality and potential for off-site impacts in the Galilee subregion


Causal pathway

Water quality concern

Scale

Off-site impacts in Galilee subregion

Subsurface depressurisation and dewatering for coal mines

Leakage between aquifers that diminishes the beneficial use value of a productive aquifer due to changes in water quality

Local to regional

Potential for off-site impacts from changes in the hydraulic gradients between connected aquifers of differing water quality

Failure of bore integrity

Leakage between aquifers that diminishes the beneficial use value of a productive aquifer due to changes in water quality

Local

Off-site impacts are unlikely. State regulation and best practice guidelines are in place to minimise potential adverse impacts from bore construction, use and abandonment practises

Subsurface fracturing above longwall panels

Leakage between aquifers and/or surface that diminishes the beneficial use value of a productive aquifer due to changes in water quality

Local

Potential for off-site impacts from possible enhanced connectivity between aquifers of differing water quality or increased recharge

Leaching from stockpiles, rock dumps, tailings dams, storage dams

Leaching of contaminants into aquifers that reduces the beneficial use value of a productive aquifer

Local

Potential for off-site impacts, but regulatory controls in place to minimise risk

3.3.4.2 Surface water quality

Surface water quality in the Galilee subregion is covered by the Environmental Protection (Water) Policy 2009 (EPP (Water)), which achieves the objectives of Queensland’s Environmental Protection Act 1994. The surface water zone of potential hydrological change is situated in the headwaters of the Burdekin river basin. Draft environmental values and water quality guidelines for the Burdekin river basin were released for consultation in March 2017 (Newham et al., 2017). These built on work undertaken as part of the Burdekin Region Water Quality Improvement Plan 2016 (NQ Dry Tropics, 2016a). NQ Dry Tropics (2016b) also provides background information on water quality issues in the Burdekin river basin.

Changes in surface water quality from coal resource development can occur following 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 loads of total suspended solids in waterways. Consequently, adequate controls are an important component of managing enhanced erosion risks due to coal mining developments. In addition, the discharge of mine water into the stream network as part of operational water management is a potentially hazardous activity, especially if the quality of the discharged water lowers the quality of the receiving water below its current beneficial-use level. Similarly, any unintentional releases of mine water off site could also be considered as a potential hazard to surface water quality. However, such unintended release events are typically rare (e.g. potentially caused during major floods or in the event of unexpected dam failure), with approved design and containment strategies required to be developed in order to adequately address such aspects of mine site water management.

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 18 lists potential causes of changes in surface water quality from coal resource development in the Galilee 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 18 Potential causes of changes in surface water quality and potential for off-site impacts


Causal pathway

Water quality concern

Scale

Off-site impacts in Galilee subregion

Altering surface water system

Reduced surface water flows due to isolation of part of catchment resulting in reduced runoff to streams

Local to regional

Potential impacts are addressed by site-specific mine water management and monitoring plans within tenements. Managed through regulatory requirements attaching to mining operations plans.

Groundwater pumping enabling coal extraction

Change in baseflow to streams may diminish the beneficial use value of the surface water resource due to changes in water quality

Local to regional

There are potentially substantial off-site impacts and baseflow impacts.

Subsurface fracturing above longwall panels

Change in stream baseflow conditions (e.g. enhanced recharge to aquifers through fracturing) may diminish the beneficial use value of the surface water resource due to changes in water quality from reduced flows

Local

Potential impacts are addressed by site-specific mine water management and monitoring plans within tenements. Broader regional impacts may be assessed as part of specific water quality improvement plans, such as the one developed for the Burdekin Region in 2016 (NQ Dry Tropics, 2016a).

The likelihood of off-site water quality impacts to broader surface water systems is reduced through the capture of surface water on mine sites, which is then utilised for various on-site activities and processes. As of December 2017, conditions for off-site discharge of any excess water are yet to be finalised for the additional coal resource development. Site-specific discharge conditions will form part of future mine approval conditions.

All of the modelled coal mining projects in the Galilee subregion are situated in the headwaters of the Belyando river basin. From the perspective of the greater Burdekin river basin, the mean annual stream discharge from the Belyando River accounts for about 10% of the total discharge volume of the Burdekin River (NQ Dry Tropics, 2016a). During the period 2005 to 2010, Bainbridge et al. (2014) estimated that the mean annual discharge contribution from the Belyando River was about 780 GL/year. In comparison, for the other main rivers above the Burdekin Falls Dam, the mean annual discharge was estimated as 740 GL/year for the Cape River, 820 GL/year for the Suttor River and 4500 GL/year for the Burdekin River (Bainbridge et al., 2014).

Sediment loads in the Belyando and Burdekin river basins vary significantly from year to year and are in part dependent on streamflow volumes (Bainbridge et al., 2014). Sediment loads increase in wetter years or during large cyclone events. For the 2005 to 2010 period, Bainbridge et al. (2014) estimated that the average sediment load contribution from the Belyando River to Lake Dalrymple (Burdekin Falls Dam) was 0.16 Mt/year. In comparison, the sediment contributions from the other main rivers were estimated to be: Cape River (0.27 Mt/year), Suttor River (0.25 Mt/year) and the Upper Burdekin River (5.23 Mt/yr). Thus, the Upper Burdekin River is clearly the main contributor to the overall sediment budget in the Burdekin river basin (above Lake Dalrymple).

It is estimated that the Burdekin Falls Dam (Lake Dalrymple) traps around 65% of the sediment and nutrient load from the combined input of the Upper Burdekin, Belyando, Suttor and Cape rivers (NQ Dry Tropics, 2016a). The trapping efficiency and effectiveness of the Burdekin Falls Dam has implications for the downstream dispersion of any extra sediment load that may occur due to future coal mining activity in the Belyando river basin.

The risk to regional surface 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 above the goaf in longwall mines) 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 Galilee subregion suggests some reduction in baseflow is likely to occur within the zone of potential hydrological change, thereby potentially leading to changes in water quality.

The magnitude and extent of water quality changes cannot be determined without specifically representing the key water quality parameters in the modelling. This remains an important knowledge gap for the Galilee subregion, particularly in the context of multiple large-scale mining developments that may potentially result in cumulative impacts to water quality (see Section 3.7.4 for further discussion about gaps and limitations relevant to this BA).

Last updated:
6 December 2018
Thumbnail of the Galilee subregion

Product Finalisation date

2018
PRODUCT CONTENTS

ASSESSMENT