Hazards associated with CSG operations and open-cut coal mines that are considered to be in scope for BA in the Maranoa-Balonne-Condamine subregion are aggregated into four main causal pathway groups. These four causal pathway groups represent a conceptual model of the chain of events that begins with an activity and ends with a potential impact on a groundwater- or surface water-dependent asset. Four main causal pathway groups are identified in the Maranoa-Balonne-Condamine subregion:
- ‘Subsurface depressurisation and dewatering’
- ‘Subsurface physical flow paths’
- ‘Surface water drainage’
- ‘Operational water management’.
Groundwater extraction leads to hydrostatic depressurisation of the target coal seam and connected aquifer layers. Hydrological effects arising from this group of hazards depend on the local environment of the individual well (groundwater supply bore, CSG exploration or production bore or mine dewatering bore). If there are no faults or fractures nearby then the head changes caused by depressurisation must be passed to other layers diffusively according to hydrological conductivity, layer structure and the presence of aquitards. These affect the magnitude of head change, the spatial extent of head change and the time it takes for maximum head change to occur. Therefore, there is no general rule for how depressurisation of the target aquifer will affect connected aquifers. Where a fault or fracture does exist then pressure change can be transmitted more readily. However, this depends on the geometry of the geological compartments defined by the faults or fractures. Furthermore, it may be possible that prolonged depressurisation will reactivate a fault pathway, and thus create a pathway that was not active prior to the aquifer depressurisation.
Water is removed from most mines to allow the safe extraction of coal, and this decrease in local groundwater level creates a gradient toward the pit, and induces flow into it. The primary sources of this water are the geological layers in which the mine is sited, down to the layer being mined. The spatial extent of the influence area of the pit dewatering is a function of the depth of mining, the local hydraulic properties of conductivity and storativity, and the time elapsed. For example, a particular asset may be so distant from an open-cut mine that, within the life of the mine, drawdown will not affect it. However, in the years following mine closure, the spread of the drawdown cone may affect it. This can only be quantified with targeted monitoring and numerical groundwater modelling. Groundwater level monitoring can directly measure drawdown changes over time, whereas numerical groundwater modelling provides predictive estimates of drawdown changes to inform future planning. Mine pit dewatering can also affect alluvial aquifers, which can also affect the volume and timing of groundwater that is discharged as baseflow to connected watercourses. If the dewatering of an open-cut mine allows a drawdown cone to intersect with an alluvial aquifer supporting a stream, then potentially the water that would naturally discharge to the stream is instead drawn away from the alluvium toward the open-cut mining pit. Changes to surface water – groundwater interactions and depth to the water table can affect GDEs, including terrestrial vegetation and aquatic ecosystems.
The spatial context, hazards (impact modes and activities) and hydrological effects associated with the ‘Subsurface depressurisation and dewatering’ causal pathway group following water and gas extraction are shown in Figure 33. The cumulative effects of aquifer depressurisation associated with baseline CSG operations and dewatering associated with the five baseline and two additional coal mines is likely to be widespread, affecting target and non-target aquifers within the tenements and potentially affecting connected aquifers, surface water systems and GDEs outside the tenements. The groundwater numerical modelling (companion product 2.6.2 for the Maranoa-Balonne-Condamine subregion ) describes the cumulative effects of aquifer depressurisation caused by the difference between results for the CRDP and the baseline (due to the additional coal resource development (ACRD)) for only those developments that can be modelled in the Maranoa-Balonne-Condamine subregion.
‘Groundwater pumping…’ refers to ‘groundwater pumping enabling coal seam gas extraction’ and ‘groundwater pumping enabling open-cut coal mining’ causal pathways
Subsurface physical flow paths
Preferential flow paths can be affected by hydraulic fracturing (including deviated drilling and changes to non-target aquifers), well integrity (including incomplete and/or compromised cementing/casing, miss perforation of target aquifers) and 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). The spatial context, hazards (impact modes and activities) and hydrological effects associated with the ‘Subsurface physical flow paths’ causal pathway group are shown in Figure 34. Potential effects are likely to be localised, that is, they are restricted within less than 1 km of the preferential flow path, but will continue until remedial actions are taken. Potential effects include the escape of gas from the coal seams to the overlying geological layers and ultimately to the atmosphere. Inter-aquifer mixing can potentially compromise aquifer water quality. Effects on surface water systems are thought to be minimal due to the limited extent of impacts associated with changes to preferential flow paths and will be restricted to aquifer outcrop areas within tenement areas.
Hydraulic fracturing is designed to alter connectivity within target layers but may potentially alter inter-aquifer connectivity and introduce additional preferential flow paths. Aquifer 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. The intended impact of changing aquifer properties is expected to be limited to the coal seams with a smaller risk of impacting neighbouring aquifers or aquitards. The lateral extent to which aquifer properties are changed diminishes with distance from the well and is therefore dependent on the number of wells where this process is implemented. The water quality of the fractured aquifer and neighbouring aquifers can be compromised, but is subject to management controls (such as compliance with standards and regulations) and monitoring. At the end of hydraulic fracturing, water is pumped out, which is discussed for the ‘Subsurface depressurisation and dewatering’ causal pathway group. Potential effects of disposal of co-produced water removed after hydraulic fracturing are discussed for the ‘Operational water management’ causal pathway group.
Well construction may lead to enhanced connection between aquifer layers , and allow the mixing of waters from previously disconnected layers of different quality and chemical properties, or of any fluid introduced down the well. CSG wells are drilled vertically from the surface to the coal seam and within the coal seam by directional or deviated drilling. Maintaining well integrity throughout construction, operation and decommissioning phases is crucial to ensuring sustainable gas production and avoiding adverse environmental impacts. Incomplete and/or compromised casing and seals could introduce preferential flow paths. Miss perforation of the target aquifer can create a connection between previously disconnected aquifers. Preferential flow paths have the potential to connect any two or more consecutive or non-consecutive geological layers up to the land surface through failure of well integrity or via faults affected by fracture stimulation.
Open-cut coal mines can have a localised effect on preferential flow paths in surrounding aquifers, affecting surface water – groundwater interactions. This includes changes to hydraulic gradients in the alluvial aquifer and connected aquifers associated with mine pit dewatering and preferential drainage and recharge associated with post-closure water filling the pit. An important component of streamflow is baseflow, where groundwater discharges to the stream from the alluvial aquifer. Changes to hydraulic gradients can change the timing and volume of baseflow contributions to streams, which can affect the stream ecosystem within and downstream of tenements. Changes to baseflow contributions will be restricted to the aquifer outcrop areas, where direct interactions between watercourses and unconfined aquifers are possible. The locations of watercourses in aquifer outcrop areas in the Maranoa-Balonne-Condamine subregion are shown in Figure 35. The locations of catchments and watercourses potentially affected by the CRDP in the subregion are shown in Figure 36.
SW = surface water, GW = groundwater, groundwater composition = mixing groundwaters of different composition (in terms of natural dissolved solids)
The mines in the coal resource development pathway (CRDP) are the sum of those in the baseline and in the ACRD.
ACRD = additional coal resource development, CSG = coal seam gas, GAB outcrop = Great Artesian Basin aquifer outcrop areas
The mines in the coal resource development pathway (CRDP) are the sum of those in the baseline and in the additional coal resource development (ACRD).
CSG = coal seam gas
Surface water drainage
Disruption of the surface drainage network may lead to a loss, or redirection, of runoff that can have long-term cumulative effects on downstream watercourses. The physical infrastructure of CSG operations, including land clearing, land levelling, the construction of hard packed areas such as roads and tracks, pipelines and plant for collection and transport of gas can all disrupt natural surface flows and pathways by redirecting and concentrating flows. Water flow and landscape topography co-evolve in natural systems such that the areas of most concentrated flow tend to be the most resistant to erosion. Changes in flow regime and catastrophic events can alter flows and pathways either temporarily before returning to the previous state, or semi-permanently until the next event. In the same way, anthropogenic structures and earth works associated with CSG exploration and production may divert and concentrate surface flow. This may lead to erosion of the land surface, stream banks or streambeds, and alter water quality in streams if new material is mobilised and washed into them. Geomorphological changes may create chemical or hydraulic barriers to migration of aquatic organisms that can affect upstream watercourses such as changes to the pH or dissolved oxygen concentrations (chemical) and stream velocity or turbulence (hydraulic).
Open-cut mines may also alter the surface water pathways, through diverting site drain lines and on-site water retention. While the total amount of runoff in a surface water catchment might be reduced by only a few percent, creek line diversions change where water enters the stream network. For example, a mine may alter runoff pathways such that a single upland stream that contributes only a few percent of overall catchment streamflow is diverted around the mine to a watercourse further downstream. This has implications for the local stream environment and downstream reaches where the contribution at this point may be more significant. It can lead to flow being more concentrated, so that erosion risk is greater, or reduce the flow contributions to a water-dependent asset. It may also change surface water – groundwater interactions in the alluvium and watercourses in aquifer outcrop areas. The relative effect will be greater the closer an open-cut mine is to the stream. On-site water retention minimises the chances of any runoff from the mining operations or infrastructure being contaminated and released to the surface water catchment or watercourse. Therefore, any runoff that is naturally generated within the mining operations area is lost to streamflow and the environment. After mining ceases mine-site rehabilitation occurs and, at some stage following this activity, some proportion of the rehabilitated land area will again become connected to the wider surface water catchment.
The spatial context, hazards (impact modes and activities) and hydrological effects associated with the ‘Surface water drainage’ causal pathway group are shown in Figure 37. The cumulative effects on natural surface drainage in the subregion are likely to have medium to long term cumulative effects on watercourses within, upstream and downstream of tenements.
GW = groundwater, SW = surface water
Operational water management
CSG operations remove water from the coal seam to release gas during the production stage. If the produced water is of poor quality it may require dilution with fresh water, or treatment to remove salts, gas and other contaminants sourced from the coal seam. Open-cut coal mines produce water of varying quantity and quality at different stages in the life of a mine. Operation water management includes activities related to sourcing water for on-site operations, storing extracted water, discharging extracted water into the surface water system, processing and using extracted water and reinjecting co-produced water into the aquifer.
Priorities for co-produced water disposal in the subregion are purposes that are beneficial to the environment, new water users, existing or new water-dependent industries. ‘…after feasible beneficial use options have been considered, treating and disposing CSG water in a way that firstly avoids, and then minimises and mitigates impacts on environmental values’ . In addition, Healthy Waters Management Plans that address water quality requirements of both the Queensland’s Environmental Protection Act 1994 legislation and the Commonwealth’s Basin Plan 2012 are currently being developed. The plans assess risks to water quality and identify water quality targets based on local data (including electrical conductivity, nutrients, turbidity, pH). The water quality targets are designed to assist in informing regulatory conditions on environmentally relevant activities such as CSG operations and coal mines.
Beneficial uses include water for site management such as dust suppression or washing, discharge to rivers for conveyance to beneficial uses and reinjection to depleted aquifers. Co-produced water that is disposed of locally via irrigation or release to rivers can affect water quality and quantity. Discharge of co-produced water to rivers and for irrigation can affect watertable levels and soil salt mobilisation along watercourses and near irrigation areas. Aquifer reinjection can increase groundwater pressures and change the volume and timing of groundwater discharge to springs and watercourses in aquifer outcrop areas. Reinjection can also change aquifer composition, with the extent of water quality changes being limited by local hydraulic properties (conductivity and storativity) and time.
Water management structures (dams, levee bunds and diversions) associated with open-cut coal mines can change natural surface drainage or cause excessive runoff during closure that has a cumulative effect on surface water and groundwater systems in the subregion. Upstream impacts that change watercourse geomorphology or create barriers to the migration of aquatic organisms are also possible. The location of watercourses potentially affected by the CRDP in the subregion are shown in Figure 36.
The spatial context, hazards (impact modes and activities) and hydrological effects associated with the ‘Operational water management’ causal pathway group are shown in Figure 37. Water management structures are likely to have medium- to long-term effects on surface water quality, direction and flow. ‘Discharge to river’ (impact mode) is likely to have episodic- to short-term effects on surface water flow and quality in the alluvium and watercourses in aquifer outcrop areas in the subregion. Changes to groundwater level and quality and surface water quality associated with ‘discharge to river’ and irrigation that raises the watertable and mobilises salt is likely to be episodic to short term and localised. Effects are likely to be in the medium to long term and include the alluvium and watercourses in aquifer outcrop areas that are within, upstream and downstream of tenements.
Product Finalisation date
- 2.3.1 Methods
- 2.3.2 Summary of key system components, processes and interactions
- 2.3.3 Ecosystems
- 2.3.4 Baseline and coal resource development pathway
- 220.127.116.11 Developing the coal resource development pathway
- 18.104.22.168 Water management for the coal resource developments
- 22.214.171.124 Gaps
- 2.3.5 Conceptual model of causal pathways
- 126.96.36.199 Methodology
- 188.8.131.52 Hazard analysis
- 184.108.40.206 Causal pathways
- 220.127.116.11 Gaps
- Contributors to the Technical Programme
- About this technical product