2.3.5.3 Causal pathways

Causal pathways are considered for CSG operations and coal mines separately, for both the baseline coal resource development (baseline) and CRDP. Many of the causal pathways that exist are common to both baseline and the CRDP because the latter includes extensions of Stratford and Duralie mines beyond December 2012.

Water flow paths are controlled primarily by changes in total head. For surface water flows, the water depth is small relative to land-surface elevation changes, so that surface topography and slope are the main drivers of both direction and velocity. For groundwater flows, the thickness of saturated material (i.e. the aquifer) is often larger than changes in the base elevation of the layer, so absolute groundwater elevation is the main driver of flow direction. Flow velocity is a function of the aquifer material and geological connections, so horizontal and vertical hydraulic conductivity are the main drivers of groundwater flow velocity. With groundwater flows, however, the presence of faults and discontinuities can alter both direction and velocity. These are present in the geological Gloucester Basin.

There are three hydrogeological units in the Gloucester Basin:

The natural cycle for water within the Gloucester Basin is for rainfall to recharge outcropping layers at the margins of the basin causing upward pressure in these layers that discharges to the alluvial aquifer and valley floor. Streams and rivers hosted in the alluvial aquifer are gaining and connected under natural conditions. Diffuse rainfall recharge occurs to the alluvial aquifer and supports baseflow.

The main controls for groundwater interactions with the surface are in the SRL, with its connectivity between the coal layers and the alluvium, and the fracture and fault networks within it. A necessary part of CSG exploration and production is depressurisation of individual gas-bearing layers. Target seams in the Gloucester Basin are between 150 and 600 m depth with dry ash-free gas content of 10 to 25 m³/t, increasing with depth (Hodgkinson et al., 2014, p. 13). While layers at these depths are considered to be water-bearing strata rather than aquifers, the weathered and fractured rock zone extends as far as 150 m depth, so the uppermost target coal seams may be just below this upper aquifer. As water volumes extracted as part of CSG operations are likely to be relatively small, changes would be propagated to the surface via SRL through lowering the hydrostatic pressure in coal seams, rather than direct water transfer.

Coal seams outcrop on the land surface within the Gloucester Basin and are open-cut mined, but these seams have lost their gas content long ago and are not targets for CSG. Localised groundwater pressure drops can be transferred to the SRL by natural diffuse pathways and vertical conductance, and via fractures and structural features where present. If the underlying groundwater level within the SRL decreases enough, then it may induce flow away from the alluvial aquifer, reducing natural stream discharge.

A potential impact of CSG operations occurs if a CSG well leaks. If this occurs then it may create a connection between the targeted coal seam and any other layer(s) above it, including the land surface. The results include localised groundwater level reduction, and the induced transfer of water and any introduced material between geological layers. If such a leak occurred within the alluvial aquifer, then stock and domestic bores that draw water from it may be affected. There is no evidence of any leakage from the four gas test wells drilled and fracture stimulated as part of the Waukivory Pilot Project.

Open-cut coal mines in the Gloucester subregion are focused at the land surface. The pit must be dewatered so that coal can be removed, and this causes a groundwater level gradient toward the pit, which induces more groundwater seepage that needs to be removed. This groundwater level gradient is induced within the geological layers the mine occupies, and the area of drawdown is controlled by the local hydraulic conductivity. If there is connectivity between other outcropping layers and hydraulic properties allow it, then water could be drawn from these layers. If water flows are induced away from the alluvial aquifer, then natural discharge to streams may be reduced.

Open-cut coal mines also affect surface flows. Typically, all rain falling within the active mining operations area must be retained on site. This means that any runoff produced in that area does not contribute to normal surface flows to streams, wetlands and any other surface water features.

In general, negative effects of changes in groundwater level gradient hinge on the presence of a connection between an asset and CSG or coal mining operations via direct or diffuse pathways, or the creation of a new connection due to changes in the rocks and layers as a result of drilling and/or other operations. The scale of change depends on rates of hydraulic conductivity, and the behaviour of faults and discontinuities as either carriers of, or barriers to, groundwater movement.

2.3.5.3.1 Coal seam gas operations

The hazards associated with CSG operations (identified as part of the IMEA) were considered in relation to the scope (Section 2.3.5.2.3) and were aggregated into four main causal pathway groups:

‘Subsurface depressurisation and dewatering’ causal pathway group

Groundwater extraction for CSG production can lead to hydrostatic depressurisation of aquifer layers. The water pathway for this hazard depends on the local environment of an individual CSG well (Figure 17). If there are no faults or fractures nearby then the head changes caused by depressurisation must be passed to other layers diffusively according to 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 change to occur. Therefore, there is no hard and fast rule for how depressurisation of a particular CSG target layer will affect surrounding layers or the SRL or alluvial aquifer, if at all. Where a fault or fracture does exist then pressure change may potentially be transmitted much quicker and much further. However, this depends on the geometry of the geological compartments defined by the faults or fractures and their properties. 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 extractive activity. Predictions of fault locations by the revised geological model for Gloucester subregion (Figure 13 in companion product 2.1-2.2 for the Gloucester subregion (Frery et al., 2018)) indicate that major faults exist within the Stage 1 gas field development area of AGL’s proposed CSG development in the Gloucester Gas Project.

Figure 17 'Subsurface depressurisation and dewatering' causal pathway group arising from coal seam gas operations

‘TDS’ (total dissolved solids) and ‘TSS’ (total suspended solids) are examples only of a range of metrics that could be assessed for water quality.

‘Subsurface physical flow paths’ causal pathway group

Well construction may lead to enhanced connection between layers. The water pathway for this hazard is a result of drilling the well for CSG operations (Figure 18) or for any well that penetrates between distinct geological layers. A well that is not completely sealed against the surrounding material may provide a direct conduit for water to any other layer it is drilled through, including to the land surface. This may be as a result of well construction methods, degradation of the well sealing materials over time, or changes in the aquifer material or structure over time. Hydraulic fracturing is designed to alter connectivity within target layers but may potentially alter inter-aquifer connectivity and introduce additional preferential pathways. Well construction may increase local connectivity (Stuckey and Mulvey, 2013), and allow the mixing of waters from previously disconnected layers of different quality and chemical properties, or of any fluid introduced down the well.

Figure 18 'Subsurface physical flow paths' causal pathway group arising from coal seam gas operations

‘TDS’ (total dissolved solids) and ‘TSS’ (total suspended solids) are examples only of a range of metrics that could be assessed for water quality.

‘Operational water management’ causal pathway group

This causal pathway group is less about the impact of water removal from the system, and more what happens after it is removed and collected (Figure 19). The water produced from CSG operations will contain the products added to water pumped into the well, along with the salt (and may also contain other naturally occurring dissolved components that occur in the coal seam groundwater system) in the water removed from the coal seam to release gas. If the water is transported off site, then the problem may no longer exist in the subregion. Other options include use for mine site management, such as dust suppression, irrigation of land locally, or release into local stream and rivers during flow events. If the produced water is of poor quality it may require dilution with fresh water, which introduces an issue of gathering fresh water either from the local environment or from outside the area. Any produced water that is disposed of locally via irrigation or released to rivers can affect water quality and quantity. Section 2.3.4 details the specific plans for each of the mines in the Gloucester subregion, and water quality ranges for its various uses. Water quality monitoring by Parsons Brinckerhoff (2012) indicates that salinity of water generally increases with depth, and that water in the coal seams and interburden layers is three to four times more saline than water in the alluvial aquifer and up to 50 times more saline than Avon and Gloucester river water.

Figure 19 'Operational water management' causal pathway group arising from coal seam gas operations

‘TDS’ (total dissolved solids) and ‘TSS’ (total suspended solids) are examples only of a range of metrics that could be assessed for water quality.

‘Surface water drainage’ causal pathway group

This impact mode is defined by the physical infrastructure of CSG operations, and the associated surface works. 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 (Figure 20). In many natural systems water flow and landscape topography co-evolve so that in a stable system the areas of most concentrated flow are 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 another such event. In the same way, engineered 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 stream beds, and alter water quality in streams if new material is mobilised and washed into them. As noted in Hodgkinson et al. (2014, pp. 27–29) the CSG development approved in the Gloucester subregion is for 110 wells in the 50 km² stage 1 CSG development area of AGL’s proposed CSG development in the Gloucester Gas Project, and subsequent stages estimated as 200 to 300 wells over the full 210 km² gas field development area. This allows for what the NSW Department of Planning (2011, p. 5) calls:

... construction, operation, decommissioning and rehabilitation of gas wells and associated infrastructure including gas and water gathering lines and temporary construction facilities ....

Figure 20 'Surface water drainage' causal pathway group arising from coal seam gas operations

‘TDS’ (total dissolved solids) and ‘TSS’ (total suspended solids) are examples only of a range of metrics that could be assessed for water quality.

SW = surface water

The influence of the various impact modes is described in Table 9. While all of the effects are generally local to the CSG operation, for example, less than 1 km from the site, both aquifer depressurisation and inter-aquifer connectivity are affected by the local geological environment. The spread of drawdown or water mixing is controlled by the presence and nature of faults and fractures, and local hydraulic properties. Drawdown cones from pumping wells will take decades to reach maximum effect in both groundwater level drop and diameter, while water mixing from a leaky well may be recognised in less than a year. Similarly, disruption to surface flows is generally localised, but the changes in water chemistry or flow input location along a reach may have effects on water or other assets many kilometres downstream. Soil erosion resulting from changes in runoff pathways may cause damage with an individual storm event, and changes to the amount and type of material discharging into the stream over many years.

2.3.5.3.2 Open-cut coal mines

The hazards associated with open-cut coal mines (identified as part of the IMEA) were considered in relation to the scope (Section 2.3.5.2.3) and were aggregated into four main causal pathway groups:

‘Surface water drainage’ causal pathway group

Disruption of surface drainage network may lead to a loss, or redirection, of runoff. The water issue with this hazard is that any rain that falls within the limits of the mining operations area must be retained on site (Figure 20). This on-site water retention minimises the chances of any runoff from the mining operations or infrastructure being contaminated and then exported to the rest of the surface catchment or watercourse. Due to this requirement, however, any runoff that would naturally be generated from 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.

Open-cut mining area may also alter the surface water pathways. While the total amount of runoff in a surface water catchment might be reduced by only a few percent, the locations that water enters within a stream network may be altered. For example, a mining site may alter runoff pathways such that a single upland stream that contributes only a few percent of overall streamflow contributes none of its normal surface water volumes at the confluence with the network at lower elevations. This has implications for the local stream environment and the next downstream reach where the contribution at this point may be much more significant. It can lead to a greater concentration of flow, so that erosion risk is greater, or a lack of contribution to a water dependent asset. This hazard will have a greater impact the closer an open-cut mine is to the first order streams (or headwater streams) of a surface water network. In the Gloucester subregion the maximum extent of the CRDP mining footprints is 16.9 km², or 5.5% of the surface area of the subregion. This should result in a maximum of 5.5% direct reduction in runoff to the entire stream network, assuming uniform runoff production.

‘Subsurface physical flow paths’ and ‘Subsurface depressurisation and dewatering’ causal pathway groups

The dewatering of an open-cut coal mine will lower the watertable, affect inter-aquifer connectivity and may potentially lead to a loss of baseflow (Figure 18). Mines must have water removed to allow the safe removal 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 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. It is the time elapsed that affects the spatial extent of this impact. For example, a particular asset may be so distant from an open-cut coal mine that within the life of mining that drawdown will not affect it, but in the years following the spread of the drawdown area may affect it. This can only be quantified with monitoring and estimated with modelling. Streams exist in an alluvial aquifer and recharge within this aquifer discharges to the stream as baseflow. If the dewatering of an open-cut coal mine allows a drawdown cone to intersect with an alluvial aquifer supporting a stream, then potentially that water that would naturally discharge to the stream is instead drawn away from the alluvium toward an open-cut mining pit.

On a much more local scale exploration and monitoring bores may alter inter-aquifer connectivity through well integrity issues creating preferential pathways. This may be as a result of well construction methods, degradation of the well casing or sealing materials over time, or changes in the aquifer material or structure over time due to operations or natural events.

‘Operational water management’ causal pathway group

The pathways for open-cut coal mines are the same as those for co-produced water from CSG operations (Figure 19) but the volumes of water are likely to be larger, as dewatering an open-cut coal mine including seepage involves much more water than dewatering a deep coal seam. The hazard identification workshop also indicated leaching of water within the mine site from waste rock dumps, coal stockpiles and storage dams of produced water. The pathway here is direct contamination of the aquifer the mine is sited on, or if water escapes over the surface then contamination of local streams.

The spatial and temporal extents of impact modes associated with open-cut mines are shown in Table 10, while the water volumes and water management plans for specific mines are discussed in Section 2.3.4. The inter-aquifer connectivity impact is related to mine pit dewatering, and this must occur over the full life of the mine plus some time into the future. The future impacts are controlled by the management of site rehabilitation (e.g. refilling the mine void with much looser material will allow seepage to continue toward the old mine void and may interrupt local groundwater flow pathways). Similarly, for the disruption of surface drainage, without suitable rehabilitation the mining lease area may have very different properties in runoff production, vegetation health, infiltration characteristics and local groundwater level long into the future.

2.3.5.3.3 Causal pathways for baseline and coal resource development pathway

Baseline coal resource development

There is no coal seam gas development in the baseline for the Gloucester subregion and therefore no associated potential causal pathways.

The causal pathways from open-cut coal mines in the baseline in the Gloucester subregion (see Table 7 in Section 2.3.4) are all those associated with the current Stratford and Duralie mines:

Activities at the Stratford site may affect the ‘Intermittent – gravel/cobble streams’ landscape class in the ‘Riverine’ landscape group of the upper Avon River and the ‘Forested wetlands’ landscape class in the ‘Groundwater-dependent ecosystem (GDE)’ landscape group (see Figure 6 in Section 2.3.3). The mine is otherwise within an area classified as cleared for dryland agriculture with some proximity to native vegetation. The ‘Forested wetlands’ landscape class is associated with the alluvium containing the Avon River locally, and will respond to changes in groundwater quality and level, stream baseflow quantity and variability (see Figure 6 and Table 3 in Section 2.3.3). At the Duralie site the ‘Perennial – gravel/cobble streams’ landscape class (in the ‘Riverine’ landscape group) of Mammy Johnsons River is affected, while the mine is located in land cleared for dryland agriculture.

Coal resource development pathway

AGL’s proposed Gloucester Gas Project introduces the following causal pathway groups related to CSG operations for the CRDP:

The Gloucester Gas Project site will be located within land cleared for dryland agriculture with scattered native vegetation, and these causal pathways may affect reaches of the Avon River in the ‘Intermittent – gravel/cobble streams’ landscape class in the ‘Riverine’ landscape group.

The causal pathways for the CRDP include those for Yancoal Australia Ltd’s (Yancoal) existing Stratford and Duralie mines and are supplemented by expansions at both Stratford and Duralie, and new open-cut mining at Rocky Hill. At the Stratford and Duralie sites no additional causal pathways due to open-cut mines are introduced, and it is expected the same landscape and GDE classes will continue to be affected in the same manner as for the baseline. The Rocky Hill open-cut mining sites are located within land cleared for dryland agriculture with scattered native vegetation, and may affect reaches of the Avon River in the ‘Intermittent – gravel/cobble streams’ landscape class in the riverine landscape group

Summary

Table 11 summarises the causal pathways linking coal resource development to potentially impacted landscape classes in the Gloucester subregion.

Table 11 Causal pathways arising from open-cut mines and coal seam gas operations