Breadcrumb

2.3 Conceptual modelling for the Galilee subregion

Executive summary

Artesian Spring Wetland at  Doongmabulla Nature Refuge, QLD, 2013 Credit: Jeremy Drimer, University of Queensland

Conceptual models are abstractions or simplifications of reality. During development of conceptual models, the essence of how the key system components operate and interact is distilled. In bioregional assessments (BA), conceptual models are developed to describe the causal pathways, the logical chain of events – either planned or unplanned – that link coal resource development and potential impacts on water resources and water-dependent assets.

Methods

This product details the conceptual model of causal pathways of the Galilee subregion, following the methods described in the companion submethodology M05 on the development of conceptual models. It identifies:

Summary of key system components, processes and interactions

The Galilee subregion has a protracted geological history of deposition, deformation, uplift and erosion. This history has implications for the degree of connectivity that may exist between various geological units, coal resource development and water-dependent assets. Geological aspects that may influence connectivity include variations in the thickness and extent of the Moolayember and Rewan formations; thickness of sediments that overlay the upper Permian coal measures; and location and extent of faulting, in particular around the margins of the geological Galilee Basin.

The hydrogeological conceptualisation outlines three major groundwater flow systems: aquifers in Cenozoic sediments, Great Artesian Basin (GAB) aquifers (Eromanga Basin), and Galilee Basin aquifers in the Clematis Group, upper Permian coal measures and Joe Joe Group.

In the regional artesian aquifer systems of the GAB (i.e. Hutton Aquifer to Cadna-owie – Hooray Aquifer) groundwater flow moves westerly away from the recharge areas that occur where the aquifers outcrop along margin of the Eromanga Basin. Groundwater flow in the Galilee Basin (Clematis Group, upper Permian coal measures and Joe Joe Group) is more complex than what is apparent in the GAB aquifers. Hydrochemical information suggests that Galilee Basin aquifers may be discharging into the Hutton Sandstone aquifer along parts of the western margin of the Galilee subregion. Also, there are several features that are unique to the hydrodynamics of the Galilee Basin aquifers:

  • a north trending groundwater divide. This feature has been found to be common to all Galilee Basin groundwater systems for which groundwater mapping has been undertaken as part of the BA for the Galilee subregion
  • trends in coal seam gas (CSG) content, its relationship to groundwater systems and structure.

North of Barcaldine Ridge, a groundwater divide segregates groundwater system into easterly and westerly flow components, with potential for discharge to occur towards the margins of the Galilee Basin. It is hypothesised that regional-scale landscape features and processes may be influencing the hydrodynamics of the Galilee Basin aquifers and the location and formation of this regional groundwater divide.

An apparent north-east trend in the CSG gas content is parallel to direction of groundwater flow that are inferred from potentiometric surface mapping for upper Permian coal measures. The trend in CSG gas content is also parallel to some significant faults that have been mapped at the top of the upper Permian coal measures. One hypothesis that may explain the CSG trend is that groundwater hydrodynamics and geological structure are exerting control on the gas distribution and gas content of the coal seams.

The Galilee subregion encompasses the headwaters of seven major river basins with almost all proposed coal resource developments situated in the headwaters of the Burdekin river basin. In the majority of rivers, water flow is strongly seasonal and, from year to year, flows can vary greatly from almost no flow to significant floods. Surface water – groundwater interactions in the Galilee subregion take the following forms: baseflow from shallow groundwater systems to rivers, losing streams (surface water recharging shallow aquifers), spring discharge to spring outflow pools, discharge to lakes (e.g. Lake Galilee) and shallow groundwater being utilised by deep-rooted plants.

Due to the highly variable nature of surface water flow volumes in any given year, there is a strong dependence on groundwater supplies. Most groundwater in the Galilee subregion is extracted from GAB aquifer systems, in particular the Hutton Sandstone and Cadna-owie – Hooray Aquifer. The most utilised aquifer system in the Galilee Basin is the Clematis Group aquifer. The main uses for groundwater are either for agricultural purposes or town water supplies. Groundwater is also extracted from Cenozoic sediments and is the source of some town water supplies (e.g. Alpha township).

Ecosystems

In the Galilee subregion potential changes to water resources and water-dependent assets from coal resource development may have an impact on ecosystems at the land surface. Dividing the Galilee subregion into landscape classes enables a structured approach for assessing these potential impacts. These landscape classes are expressed as a percentage of the preliminary assessment extent (PAE), identified as the geographic area where potential water-related impacts of coal resource development are assessed.

Landscape classification for the Galilee subregion is based on five elements derived from the Australian National Aquatic Ecosystem (ANAE) classification framework involving topography, landform, groundwater source, water type and water availability. In addition, each area was identified as either remnant or non-remnant vegetation based on Queensland remnant Regional Ecosystem (RE) mapping. This classification produced a typology consisting of 41 landscape classes that were further collapsed into 12 broad landscape groups.

The non-water-dependent landscape classes, ‘Dryland’ and ‘Dryland, remnant vegetation’, dominate the area of the PAE (68.54%). Of the water-dependent landscape classes, 26.52% of the area of the PAE consists of floodplain landscape classes with the remaining 4.87% of the area occupied by non-floodplain, water-dependent landscape classes. For both the floodplain and non-floodplain water-dependent landscape classes, most of the area consists of terrestrial groundwater-dependent ecosystems (GDEs).

Coal resource development

Changes in water resources and water-dependent assets due to coal resource development are quantified in the Galilee subregion by considering two potential futures:

  • baseline coal resource development (baseline): a future that includes all coal mines and CSG fields that are commercially producing as of December 2012
  • coal resource development pathway (CRDP): a future that includes all coal mines and CSG fields that are in the baseline as well as those that are expected to begin commercial production after December 2012.

The difference in results between CRDP and baseline is the change that is primarily reported in a BA. This change is due to the additional coal resource development– all coal mines and CSG fields in the Galilee subregion, including expansions of baseline operations that are expected to begin commercial production after December 2012.

In the Galilee subregion, the absence of commercially producing coal mines and CSG fields as of December 2012 means that there are no coal resource developments being quantitatively modelled in the baseline for the purposes of BA. There are 17 proposed new developments in the Galilee subregion in the CRDP, which is the combination of proposed coal mine and CSG developments that the Assessment team has evaluated as most likely to progress to commercial production at some time in the future (post the baseline date of December 2012). There is enough publically available information (e.g. detailed mine development plans and scheduling, results from groundwater and surface water modelling) to include seven of these developments in the numerical modelling being undertaken for the Galilee subregion BA.

The seven coal resource developments to be included in surface water and groundwater models for the Galilee subregion BA are the open-cut coal mines Alpha and Hyde Park, and the combined open-cut and underground coal mines Carmichael, China First, China Stone, Kevin’s Corner and South Galilee. Other coal mines (Alpha West, Blackall, Clyde Park, Hughenden, Milray, Pentland and West Pentland) and CSG developments (Galilee Gas, Gunn and Blue Energy) in the CRDP will not be assessed by hydrological modelling in this iteration of the BA. However, qualitative analysis of potential coal resource development-related impacts to water resources and water-dependent assets for the non-modelled CRDP will be reported in companion product 3-4 (impact and risk analysis) for the Galilee subregion.

Hazard analysis

Coal resource development hazards with the potential to impact hydrology are identified using IMEA as outlined in companion submethodology M11 for hazard analysis. A large number of hazards are identified, many of which are beyond the scope of a BA or are assumed to be adequately addressed by site-based risk management processes and regulation. Hazards that are beyond the scope of a BA are those that are not causing impacts via water-related pathways (e.g. hazards related to fire), as are hazards that result in surface water – groundwater effects beyond changes in water quantity or salinity. Hazards that have potential for cumulative impacts for coal mine development include: dewatering and depressurisation around coal mine developments, fracturing and subsidence that can occur above underground coal mine longwall panels, and effects of mine infrastructure on surface water systems. For CSG operations hazards include: depressurisation of coal seams, well integrity and co-produced water management-related issues. The hazards are grouped according to four causal pathway groups (refer to Appendix B in companion submethodology M05 for developing a conceptual model of causal pathways).

Causal pathways for coal seam gas

The most advanced CSG project in the Galilee subregion is the Glenaras CSG Project. As of December 2015, Galilee Energy Limited had refined the CSG well designs for the Glenaras pilot wellfield and was conducting an extended pump test to assess CSG flow rates to surface. It is possible that the potential for cumulative impacts from CSG development may only become significant once a project (or several projects) ramp up towards a production phase. Four causal pathway groups have been identified for CSG projects.

‘Subsurface depressurisation and dewatering’ causal pathway group includes the depressurisation of coal seams by removal of groundwater. This is required to produce CSG to the surface from the coal seams. In the Galilee subregion, the main target seams for CSG development are in the upper Permian coal measures. Factors that can affect the extent to which depressurisation can occur and propagate away from target coal seams include:

  • local geological complexity (e.g. lithological variation, structures)
  • configuration of groundwater flow systems (e.g. aquifers, aquitards, flow direction), hydraulic properties, connectivity of aquifer systems and CSG reservoirs
  • rate and duration of pumping (extent of depressurisation is time dependent)
  • gas desorption pressure and depth of coal seams
  • overburden thickness.

Due to these various complexities, depressurisation at depth in the coal measures does not necessarily equate to the same magnitude of depressurisation near surface.

‘Subsurface physical flow paths’ causal pathway group includes the causal pathways of ‘Failure of well integrity’ and ‘Hydraulic fracturing’. The impacts associated with compromised well integrity are likely be of a local scale; that is, they are restricted within the close vicinity of the compromised well. However, such impacts may continue until remedial action is taken. It is uncertain whether hydraulic fracturing will become a technique that is readily applied in the Galilee Basin as CSG development is in an early phase. For instance, at the Glenaras pilot wellfield, the drilling of horizontal CSG wells along target coal seams is considered by the proponent to be a more applicable technology for increasing gas flow to CSG wells rather than hydraulic fracturing.

‘Surface water drainage’ causal pathway group encompasses causal pathways relating to changes to the surface drainage network. Disruption or large-scale changes to the surface drainage network may potentially lead to a loss or redirection of runoff.

‘Operational water management’ causal pathway group is required for CSG operations due to the use of water during different stages of the development life cycle. Co-produced water from groundwater pumping may be disposed of via various methods, which are unknown due to the very early development stages of CSG projects in the Galilee subregion. Any treatment and disposal would be subject to regulatory oversight and approval by Queensland state agencies.

Causal pathways for open-cut and underground coal mines

‘Subsurface depressurisation and dewatering’ causal pathway group around coal mine workings has the potential to directly affect the regional groundwater system, and indirectly affect surface water – groundwater interactions in aquifer outcrop areas. There will be a resultant drop in groundwater levels and pressures (drawdown) around mine areas as all proposed coal mine developments in the Galilee subregion will extend deeper than the watertable.

In an open-cut coal mine groundwater drawdown can cause a drop in pressure around the mine area and create the potential for groundwater in the vicinity to flow towards the mine area, depending on the local geological configuration. Drawdown around a mine could potentially lower the watertable, which may decrease water availability to nearby groundwater-dependent ecosystems (GDEs), riparian environments, deep-rooted tree species (e.g. river red gum Eucalyptus camaldulensis), or induce changes to baseflow in river systems. Changes to baseflow may result in local changes to river flow regime such as decreased river flow and duration of flow during low-flow periods.

In an underground coal mine depressurisation will occur to a varying degree around the underground mine workings. This hydrologic depressurisation could be impeded vertically by aquitards in the surrounding geological sequence. Deeper mine workings may also have the potential to increase the lateral extent of the effects of depressurisation and drawdown. Any depressurisation from underground mine workings would be additive to any drawdown associated with dewatering around nearby open pits.

Fracturing and subsidence above underground mine longwall panels (involving causal pathway groups ‘ Subsurface physical flow paths’ and ‘Surface water drainage’) may occur to varying degrees above underground longwall mines in the Galilee subregion. Generally, areas affected by subsidence and fracturing are greatest above or immediately adjacent to areas where longwall coal mining has taken place. Although it is a relatively localised effect that occurs within mining lease areas, cumulatively the combined areas of the proposed underground coal mines could be of significance for the Belyando river basin, a significant tributary to the Burdekin River.

All underground mining is proposed to occur in coal seams in the upper Permian coal measures. The extent to which fracturing and subsidence may change hydrology depends on increased aquifer connectivity due to preferential flow along fractures, or increased flow through an aquitard compromised by fracturing, increased hydraulic conductivity and lower groundwater levels. At surface, the potential changes may include local changes in topography, ponding of water, redirection of surface flows, some changes to surface water flow regime, and if fracturing reaches the surface there is potential for increased recharge to groundwater. The degree of change is strongly dependent on site-specific geological conditions.

‘Surface water drainage’ causal pathway group focuses on potential changes to the surface water regime. Early in the development of a mine site, where the installation of diversion drains is required, bund walls and other measures will divert surface water flows, including overland flow, around the mine site to continue down slope of the coal mine development. Changing surface water drainage may result in changes to some components of the water balance. Redirection of flow to other parts of the catchment may also increase surface water flows locally in areas where it previously did not occur. Whether these changes are significant or not will vary from site to site. The cumulative area that is excised from the Belyando river basin by operational mines identified in the CRDP for the Galilee subregion may be significant.

Gaps

Developing the conceptual models of causal pathways has been based on available plans and discussions with the various proponent companies as projects are at various stages of development and regulatory approval. Knowledge gaps in the conceptual model of causal pathways for the Galilee subregion include: gaps in geological and hydrological knowledge, refinement of mapping and classification of the ecosystems of the PAE, detail on scheduling and operations for coal resource development and finalisation of amount of external water required from off-site sources for coal mine projects.

Further work

The causal pathways described in this product guide how the modelling product 2.6.2 (groundwater numerical modelling) is conducted and how product 3-4 (impact and risk analysis) is framed in the Galilee subregion.

[1] As explained in companion product 1.2 for the Galilee subregion (Lewis et al., 2014), the coal resources at Blackall differ from those at other sites in the CRDP, as they are significantly younger and of lower coal rank. This is because the Blackall coal deposit is hosted in the Upper Cretaceous Winton Formation, a unit of the geological Eromanga Basin which stratigraphically overlies the Permian Galilee Basin.

Last updated:
6 December 2018