Breadcrumb

2.3 Conceptual modelling for the Gloucester subregion

Executive summary

View of the Gloucester valley NSW with the Barrington River and associated riparian vegetation in the foreground and the township Gloucester in the distance looking south from the Kia Ora Lookout, 2013 Credit: Heinz Buettikofer, CSIRO

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 (BAs), conceptual models are developed to describe the causal pathways, the logical chain of events ‒ either planned or unplanned ‒ that link coal resource developments to water-dependent assets.

Methods

This product details the conceptual model of causal pathways of the Gloucester subregion, closely following the methods described in companion submethodology M05 for developing a conceptual model of causal pathways. For the subregion it identifies:

  • the key system components, processes and interactions, which essentially define pathways over and through which water can move (Section 2.3.2)
  • the ecosystems in terms of landscape classes and their dependence on water (Section 2.3.3)
  • the potential hydrological changes that may occur due to coal resource development by describing and documenting the baseline coal resource development (baseline) and coal resource development pathway (CRDP) (Section 2.3.4), including a summary of water management for coal resource development (Section 2.3.4.2)
  • hazards from coal resource development using an Impact Modes and Effects Analysis (IMEA) hazard analysis approach (Section 2.3.4.3)
  • causal pathways from coal resource development through to hydrological changes, both for the baseline and the coal resource development pathway (CRDP) (Section 2.3.5).

Summary of key system components, processes and interactions

The Gloucester subregion is a small sedimentary geological basin. The groundwater flow processes and interactions in the Gloucester subregion are controlled by the layering, faulting and fracturing of the coal measures and shallow weathered and fractured rock layer. Groundwater recharge mainly occurs at the margins and areas of outcropping lower layers, and discharges in the central valley floor and associated alluvial deposits. Under most natural conditions the streams and rivers in the Gloucester subregion are gaining and connected to local alluvium groundwater systems. Surface water in the subregion is divided into two distinct catchments, with the Avon River flowing to the north and the Karuah River to the south. These rivers draining the subregion are relatively small parts of a larger river system draining surrounding landscapes. According to climatological equilibrium water balance analysis, changes in the surface water volume draining north from the Gloucester River (considering the contribution from the Avon River) will not be reasonably detectable at the confluence with the Manning River, where it supplies only about 3% of the streamflow at that point. Similarly for the Karuah River flowing south, contributions to the total streamflow at Port Stephens are less than 5% from the Gloucester subregion.

Ecosystems

The ecosystems in the Gloucester subregion are classified in terms of landscape classes and their dependence on water. The classifications are based on key landscape properties related to patterns in geology, geomorphology, hydrology, ecology and human-modified land use. The landscape classes were grouped into five broad landscape groups, defined to reflect different connections to surface water and groundwater systems:

  • ‘Riverine’
  • ‘Groundwater-dependent ecosystem (GDE)’
  • ‘Estuarine’
  • ‘Non-GDE’
  • ‘Economic land use’.

These landscape groups are expressed as a percentage of the geographic area associated with a subregion or bioregion in which the potential water-related impact of coal resource development on assets is assessed. This geographic area is called the preliminary assessment extent (PAE). Less than 3000 ha (6.4%) of the PAE (46,820 ha in total) was classified in the riverine, estuarine and GDE landscape groups. The majority of the subregion is cleared of native vegetation and supports agricultural uses.

Coal resource development

The CRDP for the Gloucester subregion includes the Duralie, Stratford and Rocky Hill mines. AGL’s proposed coal seam gas (CSG) development in the Gloucester Gas Project, stage 1 gas field development area is also included. For numerical modelling purposes the CRDP was finalised in October 2015. Although there may be further stages (beyond Stage 1) of AGL’s proposed CSG development in the Gloucester Gas Project, there is no publicly available documentation of these as of October 2015. In December 2015 AGL withdrew from their proposed Gloucester Gas Project and, according to the companion submethodology M04 for developing a coal resource development pathway, once the CRDP is finalised (October 2015) it is not revisited. In the Gloucester subregion, water management plans for existing and proposed coal and CSG resource developments are designed to achieve no overflow from on-site water storages to the neighbouring water bodies. Excess water is disposed through on-site irrigation, on-site reuse for dust suppression, and is evaporated back to the atmosphere from holding dams.

Water management

Information on water management is available for the two existing coal mines with expansion plans (Duralie Coal Mine and Stratford Mining Complex), one proposed coal mine (Rocky Hill, currently on hold as of 15 November 2015) and a CSG project (AGL’s proposed CSG development in the Gloucester Gas Project, under development). Each of these developments has a water management plan.

Common elements of these water management plans are: (i) mine areas are isolated from the larger surface water catchment area by diversion drains early in the development process; (ii) surface water, where possible, is diverted around the mine areas; (iii) any water in the mine area is utilised for mining purposes, such as dust suppression and fracture stimulation, including pumped groundwater and (iv) progressive rehabilitation of mined-out areas as mining advances. The surface area that is disconnected from a catchment due to mining may vary during the life of the mine. There may be some provision for each of the mines to discharge off site during surface water high-flow periods.

Hazard analysis

Identification of potential hazards followed the Impact Modes and Effects Analysis (IMEA) method. It is used to systematically identify activities that may initiate hazards, defined as events, or chains of events that might result in an effect (change in the quality or quantity of surface water or groundwater). A large number of hazards are identified; some of these are beyond the scope of an Assessment and others are adequately addressed by site-based risk management processes and regulation.

CSG operations have their immediate impact deep below ground. For CSG operations the highest ranked hazards are: (i) aquifer depressurisation in the coal seams where extraction occurs, (ii) enhanced inter-aquifer connectivity and (iii) the storage and disposal of co-produced water. Open-cut coal mines most directly affect surface water flows and shallow groundwater aquifers; accordingly the highest ranked hazards are: (i) disruption of natural surface drainage, (ii) enhanced inter-aquifer connectivity of shallow aquifers and (iii) the storage and disposal of precipitation.

Causal pathways for coal seam gas

The hazards associated with CSG operations (identified as part of the IMEA) were considered in relation to the scope and were aggregated into four causal pathway groups (refer to Appendix B in companion submethodology M05 for developing a conceptual model of causal pathways).

The ‘Subsurface depressurisation and dewatering’ causal pathway group includes CSG operations that intentionally dewater and depressurise subsurface hydrostratigraphic units (such as coal seams and aquifers) to permit coal resource extraction. The water pathway for this group of hazards depends on the local geological environment of each individual CSG well. Predictions of fault locations by the newly developed three-dimensional geological model for the Gloucester subregion (Figure 13 in companion product 2.1-2.2 for the Gloucester subregion) reconfirm that major faults exist within the Stage 1 gas field development area of AGL’s proposed CSG development in the Gloucester Gas Project.

The ‘Subsurface physical flow paths’ causal pathway group involves physical modification of the rock mass or geological architecture by creating new physical paths that water may potentially infiltrate and flow along. Flow paths may be altered by well construction due to enhanced connection between layers. The water pathway for this group of hazards is a result of drilling the well for CSG operations or for any well that penetrates between distinct geological layers.

The ‘Operational water management’ causal pathway group involves the modification of water management systems and is required for CSG operations due to the use of water during several operational stages. This water may be sourced from either surface water or groundwater systems. 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) indicate 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 shallow alluvial aquifer and up to 50 times more saline than Avon and Gloucester river water.

The ‘Surface water drainage’ causal pathway group 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. The CSG development approved in the Gloucester subregion is for a maximum of 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, not yet approved, are estimated as 200 to 300 wells over the full 210 km2 gas field development area. This may see localised disruption to surface flows, with the changes in water chemistry or flow input location along a reach potentially having 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, as well as changes to the amount and type of material discharging into the stream over many years.

Causal pathways for open-cut coal mines

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

In the ‘Surface water drainage’ causal pathway group there may be a loss, or redirection, of runoff. The water issue with this group of hazards is that any rain that falls within the limits of the mine operations area must be retained on site. This group of hazards 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 baseline plus CRDP mine footprints is 16.9 km2, or approximately 5% of the surface area of the subregion. This should result in a maximum of 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 are combined for open-cut coal mines. Flow paths will be altered by the dewatering of an open-cut coal mine by lowering the local watertable, potentially affecting inter-aquifer connectivity to some degree and thus may potentially lead to a loss of baseflow. Mines must have water removed 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; this is called ‘seepage’. 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 of the geological volume proximal to the mine, and the time elapsed. It is the time elapsed that affects the spatial extent of this impact. For example, a particular water-dependent asset may be so distant from an open-cut mine that within the life of the mine, that drawdown will not affect it, but in the years following, the spread of the drawdown area may have an impact. This can only be quantified with monitoring and modelling.

The ‘Operational water management’ causal pathway group for open-cut mines and CSG operations has similar potential impacts but the volumes of water are likely to be larger as dewatering an open-cut mine, including seepage, usually involves much more water than dewatering a deep coal seam. 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 once mining has ceased.

Gaps

In the Gloucester subregion, the greatest knowledge gap for the flow pathways due to coal mines and CSG operations is knowledge of the locations and characteristics of the subterranean faults and fractures of the geological layers. For example, there is not a clear idea of the location of all the largest faults in the geological Gloucester Basin and the nature, location and extent of smaller potential pathways between adjacent layers is only known theoretically. This makes any definitive statement on the spatial extent of a groundwater level decline due to CSG operations difficult, although uncertainty analysis does allow a probabilistic estimate of maximum groundwater level decline. At the regional scale it was shown that drawdown propagation is minimal for a wide range of randomised faults and well locations, however no modelling was done for local effects at specific locations.

In relation to landscape classes, the underlying data are of such a large scale that it only coarsely covers the small Gloucester PAE. To this end, the final classification was greatly generalised to five landscape classes in the ‘Groundwater-dependent ecosystem (GDE)’ landscape group and seven landscape classes in the ‘Riverine’ landscape group; at the regional scale of analysis, reach lengths of 1 to 3 km were considered too detailed.

Further work

The causal pathways for the baseline and CRDP in this product guide how the modelling (product 2.6.1 (surface water numerical modelling), product 2.6.2 (groundwater numerical modelling) and product 2.7 (receptor impact modelling)) is conducted and how product 3-4 (impact and risk analysis) is framed in the Gloucester subregion.

Last updated:
14 June 2018