2.3 Conceptual modelling for the Hunter subregion

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 the 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 for the Hunter subregion, following the methods described in the companion submethodology M05 (as listed in Table 1) for developing a conceptual model of causal pathways. It identifies the:

• key system components, processes and interactions, which essentially define pathways over and through which water can move (Section 2.3.2)
• ecosystems in the Hunter subregion in terms of landscape classes and their dependence on water (Section 2.3.3)
• baseline and coal resource development pathway (CRDP) (Section 2.3.4)
• causal pathways from coal resource developments using an Impact Modes and Effects Analysis (IMEA) hazard analysis approach (Section 2.3.5).

Summary of key system components, processes and interactions

The Hunter subregion is a little over 17,000 km², and contains portions of the Western, Hunter and Newcastle coalfields. The subregion is not considered to have any groundwater connections with areas to the north-east due to the geological basin divides that define its boundary, although the Hunter river basin extends north-east of the subregion boundary. Surface water catchments define the subregion boundary to the south and west of the subregion, but regional-scale groundwater connections with the Sydney Basin and Northern Inland Catchments bioregions may exist where suitable gradients and conductive aquifers exist. Although cross-boundary flows are not expected to be significant to the water balance of the Hunter subregion, suitable margins and boundary conditions are required in numerical modelling to minimise possible edge effects. Regional groundwater systems extend east of the coastline, which defines the eastern boundary.

The surface water catchment of the Hunter subregion contains rivers that flow into the Hunter river basin and Macquarie-Tuggerah lakes basin. Salinity of the Hunter River has been a significant issue; however, since the introduction of the Hunter River Salinity Trading Scheme (HRSTS), this has improved considerably. The Hunter River lies within an extensive alluvial aquifer, which has exchanges with the river as both a source and sink, and with the underlying fractured rocks where permeable pathways exist. Under HRSTS rules, industries may release saline water to the river under high-flow conditions as long as this does not elevate river salinity above target levels. The use of dam releases and the HRSTS have tended to reduce some of the variability in river flow and stage in the regulated reaches of the river.

Ecosystems

The landscape classification describes the main ecological and human systems (including agricultural production systems, industrial and urban uses), and provides a high-level conceptualisation of the subregion at the surface. Most assets are related to one or more landscape classes, which are defined for BA purposes as ecosystems with characteristics that are expected to respond similarly to changes in the groundwater and/or surface water due to coal resource development. The Assessment team refined the landscape classification and high-level conceptualisation of the subregion following discussions at the ‘Conceptual modelling of causal pathways’ workshop held in August 2015. The landscape classes were grouped into five broad landscape groups, defined to reflect different connections to surface water and groundwater systems:

These landscape groups are expressed as a percentage of the preliminary assessment extent (PAE) area, which in the Hunter subregion coincides with the subregion boundary. Of the approximately 17,000 km² of the Hunter PAE, 620 km² (or 3.6%) fell within the ‘GDE’ and ‘Coastal lakes and estuaries’ landscape groups. In addition, there is 15,000 km of rivers and streams. Just over 10,400 km² of native vegetation within the Hunter PAE is not classified as GDE. The remainder of the PAE is classified as the ‘Economic land use’ landscape group.

Coal resource development

Coal mining has been occurring in the Hunter subregion for over 100 years. To quantify impacts of coal resource development in the Hunter subregion, two potential futures are considered:

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 Hunter subregion, including expansions of baseline operations that are expected to begin commercial production after December 2012.

In December 2012, there were 42 mining operations in the Hunter subregion, comprising 22 open-cut mines and 20 underground mines. As of September 2015, 22 proposals for coal resource developments were identified for the Hunter subregion, including 3 new open-cut coal mines, 3 new underground mines and 16 projects of baseline mining operations. A further ten proposed coal resource developments were deemed to be too exploratory or unlikely to proceed and do not comprise the additional coal resource development. As of May 2015, there is no CSG production in the Hunter subregion, nor any proposals for CSG development in the future. Therefore, the CRDP includes the 42 mining operations in the baseline, plus the 22 coal resource developments, which are new or expanded coal mines.

Hazard analysis

A dedicated hazard analysis, using IMEA, is used to systematically identify coal resource development hazards with the potential to impact hydrology. Details of the IMEA are outlined in companion submethodology M11 (as listed in Table 1) for hazard analysis. A large number of hazards were identified, many of which are beyond the scope of an Assessment or are assumed to be adequately addressed by site-based risk management processes and regulation. Hazards for CSG were not considered in the Hunter subregion as there are no proposals for CSG development as of May 2015. The four highest ranked hazards (listed with the syntax [Activity]:[Impact mode]) in the Hunter subregion are in the production life-cycle stage and include:

The hazards are grouped according to the four causal pathway groups (refer to Appendix B in companion submethodology M05 (as listed in Table 1) for developing a conceptual model of causal pathways):

Causal pathways

The ‘Surface water drainage’ causal pathway group involves the physical disruption and disturbance of surface topography and near-surface materials (vegetation, topsoil, weathered rock). Most disruptions to surface water drainage arise from changes directly at the surface, but can also be caused indirectly, such as from subsidence. Mine subsidence is a lowering of the land surface due to collapse of the regolith above an underground mine. The collapsed zone is highly fractured and often has enhanced hydraulic conductivity and storage. In the Hunter Valley, the following have all been affected by subsidence: Eui Creek, Wambo Creek, Bowmans Creek, Fishery Creek and Black Creek. Damage has occurred in the form of enhanced streambed erosion leading to degraded water quality, loss of streamflow and death of riparian vegetation.

The ‘Operational water management’ causal pathway group involves the modification of water management systems to facilitate sourcing, storing, using and disposing water at the coal resource development site. For salt concentrations related to mine water releases in the Hunter Regulated River above Singleton, the process is covered by the HRSTS. The quantitative models used in the Assessment do not model water quality parameters; however, rules governing the discharge of water from industry to the Hunter River under the HRSTS have been incorporated into the river model. Monitoring of water quality for other pollutants, such as heavy metals or BTEX (benzene, toluene, ethylbenzene and xylene), and their removal or disposal, is the responsibility of individual mining companies under their environment protection licences (EPLs).

The ‘Subsurface depressurisation and dewatering’ causal pathway group arises when coal mines and coal seam gas (CSG) operations intentionally dewater and depressurise subsurface hydrostratigraphic units (such as coal seams and aquifers) to permit coal resource extraction. This causal pathway group includes degradation of the water resource in terms of availability or quality in the surface water system, and conjunctively in the alluvial aquifer. Under the Water Sharing Plan for the Hunter Unregulated and Alluvial Water Sources, there are specific rules under low- and no-flow conditions to prevent pumping that may stress the alluvial aquifer under drying conditions. For example, at the Mount Arthur Coal Mine, the change of direction of the groundwater gradient toward Permian rocks and away from the Hunter River alluvial aquifer through depressurisation has occurred; however, this has been without a change to water levels in the alluvial aquifer.

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. Surface and underground mine operations may cut through several aquifer layers and their intervening layers. This may also lead to connection of layers that were previously separated. Furthermore, leakage may now occur of other material introduced by industrial activities, through these connected systems. Minerals, including sulfides, which are exposed by mining to both oxygen and water can create sulfuric acid, and the resulting drainage water is termed ‘acid rock drainage’. There are several inactive mine sites in the Lower Hunter that are leaking, or have leaked, acid-rock drainage to surface water courses at Aberdare East, Testers Hollow, Neath, Dagworth, Greta and Rothbury. Groundwater modelling has indicated that filled-in mining voids and lake pits can be a groundwater sink for decades or centuries, and that mine closure is a whole-of-landscape development.

Gaps

A good conceptual understanding of a system is underpinned by good data. In the Hunter subregion, there is a good streamflow monitoring network, with many gauging stations also collecting salinity readings. Exchanges between surface water and groundwater are less well known, as they are not routinely monitored. The various methods for estimating baseflow produce widely varying estimates of its contribution to streamflow.

Although the location of major faults are known, the extent to which they act as conduits of water between strata over different depths is not well understood. Many smaller geological features have not been mapped, but while they may be an important control on local groundwater flows, for regional scale modelling, they are less important and therefore not a significant knowledge gap.

For some additional coal resource developments, the availability of data or scale of proposed changes means they cannot be represented in the numerical modelling. While inferences about their effects on surface water and groundwater can be made, for the purposes of the numerical modelling, this is a knowledge gap.

Subsidence is an inevitable consequence of longwall mining, but the effects on hydrology can be hard to predict. Depth of mining and properties of the inter-burden influence the extent of subsidence at the surface and magnitude of changes in hydraulic properties due to fracturing. Hydraulic properties can be varied in a groundwater model to reflect this uncertainty.

Further work

The causal pathways described 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.