The conceptual models of causal pathways describe the logical chain of events ‒ either planned or unplanned ‒ that link coal resource development and potential impacts on water resources and water-dependent assets. These conceptual models integrate existing knowledge about the main system components, processes and interactions of the Galilee subregion’s geology, groundwater and surface water, as well as the surficial ecology. The most plausible and important potential impacts predicted from the CRDP are specifically considered, in both space and time. The causal pathways provide the logical and transparent foundation for the impact and risk analysis, as well as underpinning the development of the hydrological models.
As a starting point for developing causal pathways, a systematic hazard analysis using the Impact Modes and Effects Analysis method (described in companion submethodology M11 (as listed in Table 1) for hazard analysis (Ford et al., 2016)), was undertaken for the Galilee subregion. This analysis was used to identify the activities that occur as part of CSG and coal mining development that might result in a change in the quality or quantity of surface water or groundwater. Hazards were prioritised according to the likelihood, severity and detectability of potential impacts (Bioregional Assessment Programme, Dataset 1). Importantly, all hazards identified during this process need to be addressed for the impact and risk analysis to meet the necessary quality criteria of the BA. This does not mean that all causal pathways need to be assessed in the same manner or to the same level of detail, only that they are all addressed in some way.
The many individual ‘hazards’ themselves were not represented directly in the hydrological models, but instead they were aggregated into four causal pathway groups. These groups reflect the main hydrological pathways via which the effects of a hazard can propagate from its origin, and such pathways were broadly represented in the BA hydrological models. The causal pathway groups are:
- ‘Subsurface depressurisation and dewatering’
- ‘Subsurface physical flow paths’
- ‘Surface water drainage’
- ‘Operational water management’.
Figure 8 generically illustrates the causal pathway groups associated with both open-cut and underground coal mining, whereas Figure 9 shows the causal pathway groups for generic CSG operations. Table 4 summarises the main causal pathways and hazards within the ‘Subsurface depressurisation and dewatering’ causal pathway group, Table 5 highlights similar information for the ‘Subsurface physical flow paths’ causal pathway group, and Table 6 for the ‘Surface water drainage’ causal pathway group. The main causal pathway in the ‘Operational water management’ causal pathway group is ‘storing extracted water’, and this mainly relates to storing untreated water associated with both mining and CSG operations (e.g. co-produced water extracted from coal seams during CSG production), which has the potential to leak from storage ponds and affect groundwater and surface water quality. Further details about hazards, their identified effects and their link to causal pathway groups are explained in companion product 2.3 for the Galilee subregion (Evans et al., 2018).
The hydrological models represent causal pathways through their conceptualisations and parameterisations. The outputs from the hydrological models do not align with individual causal pathways but rather integrate the effects of the possible causal pathways into the predicted hydrological response, at particular points in space and time.
The effects of some hazards were not modelled in this BA. Some cannot be quantitatively modelled due to scale or complexity, and these were addressed qualitatively using the current conceptual understanding and knowledge base for the Galilee subregion. Changes in water quality due to coal resource development were considered only through potential effects on the salinity of either groundwater or surface water (Section 3.3.4). Some identified hazards were deemed to be local in scale and addressed adequately by existing site-based management procedures (such as leaching of various compounds from on-site coal stockpiles or waste rock dumps), whereas some were considered knowledge gaps (e.g. because the long-term means of disposal for co-produced water extracted during CSG production is currently unknown for CSG projects in the Galilee subregion). Other hazards were considered of such low likelihood and/or consequence for contributing to broader cumulative impacts at the regional scale that they were not included (e.g. littering and minor fuel spills associated with ground support staff at mining operations are potential hazards to groundwater and surface water, but are considered to be of low priority for contributing to regional-scale cumulative impacts, and are generally well managed on site).
While the causal pathway groups are generic, the physical characteristics of a subregion, such as its geological, geophysical and topographic architecture, and related surface water and groundwater networks, will influence the regional hydrological connectivity. The Assessment team’s conceptual understanding of the dominant geological and topographic influences on surface water and groundwater connectivity in the Galilee subregion are described in companion product 2.3 (Evans et al., 2018). Importantly, these conceptualisations have provided the knowledge base that underpins many aspects of the impact and risk analysis presented here.
This is a schematic diagram that is not drawn to scale. This generic diagram does not specifically relate to any proposed coal mines in the Galilee Basin, nor does it represent any specific geographic features, geological units or land uses in the subregion. Rather, the mining operation shown here illustrates examples of the four causal pathway groups defined in bioregional assessments. A summary of causal pathways is in the accompanying text, with a more detailed description in companion product 2.3 (Evans et al., 2018). The arrows shown below ground in this diagram refer to groundwater extraction, whereas the arrows above ground illustrate various aspects of mine water management, which may include transferring extracted groundwater around the mine site.
This schematic diagram is not drawn to scale. The inset schematic shows a zoomed view of hydraulic fracturing of a coal seam, where a mixture predominantly composed of water (blue) and sand (yellow), with minor amounts of chemical additives, is injected at high pressure into the well to produce small cracks in the coal (lighter grey zone). This process (which is an example of causal pathway group B) enhances the permeability of the coal seam, enabling larger volumes of gas and water to be subsequently pumped from the well. The diagram does not specifically relate to any proposed coal seam gas developments in the Galilee subregion, nor does it represent any specific geographic features, geological units or land uses in the subregion.
Table 4 Main causal pathways in the ‘Subsurface depressurisation and dewatering’ causal pathway group in the Galilee subregion
Full descriptions of the causal pathways and causal pathway groups are available in companion submethodology M05 (as listed in Table 1) for developing a conceptual model of causal pathways (Henderson et al., 2016). Hazards that potentially change groundwater or surface water flow may alter characteristics such as the direction, timing and magnitude of flow within or between aquifers, or within streams.
The impact mode of the hazards associated with the coal seam gas causal pathways listed in this table involves reducing the pressure of groundwater in the coal seam to enable gas to desorb from the coal matrix, so that the gas (and associated water) can be extracted by pumping. The reduction in water pressure within the coal seam (i.e. hydrostatic depressurisation) differs from aquifer dewatering associated with mining, as the coal seams are generally not completely pumped dry.
Table 5 Main causal pathways in the ‘Subsurface physical flow paths’ causal pathway group in the Galilee subregion
Full descriptions of the causal pathways and causal pathway groups are available in companion submethodology M05 (as listed in Table 1) for developing a conceptual model of causal pathways (Henderson et al., 2016). Hazards that potentially change groundwater or surface water flow may alter characteristics such as the direction, timing and magnitude of flow within or between aquifers, or within streams.
Table 6 Main causal pathways in the ‘Surface water drainage’ causal pathway group in the Galilee subregion
Full descriptions of the causal pathways and causal pathway groups are available in companion submethodology M05 (as listed in Table 1) for developing a conceptual model of causal pathways (Henderson et al., 2016). Hazards that potentially change groundwater or surface water flow may alter characteristics such as the direction, timing and magnitude of flow within or between aquifers, or within streams. Surface infrastructure for coal seam gas operations can include roadways and easements, gas-gathering pipeline networks, gas and water processing plants, ponds for storage of treated water and brines, pipelines and well head sites.
