- Bioregional Assessment Program
- Compiling water-dependent assets
- 3 Determining and applying the preliminary assessment extent
3.1 Determining the preliminary assessment extent
Undertaking a bioregional assessment (BA) requires the determination of the preliminary assessment extent (PAE). The PAE is defined as the geographic area associated with a bioregion or subregion in which the potential water-related impact of coal resource development on assets is assessed. The purpose of the PAE is to provide a first step in the process of determining whether a water-related link is possible between coal resource development and the assets. It is intended to be a realistic yet inclusive estimate of the land surface area where potential impacts might occur. As the model-data analysis, impact analysis and risk analysis components of a BA are completed it will be possible to more closely characterise and quantify impacts in terms of their extent and their likelihood.
The PAE is developed by the Assessment team for each bioregion or subregion and, when finalised, is reviewed by a group consisting of the Bioregional Assessments Projects Director, the Science Directors of CSIRO and Geoscience Australia and then submitted to the Principal Science Advisor of the Office of Water Science for approval.
The PAE is derived from the intersection of surface water hydrology features, groundwater management units, mining development leases and/or coal seam gas (CSG) tenements, and directional flows of surface water and groundwater (Barrett et al., 2013). The process of defining the PAE is undertaken via consideration of the vertical and horizontal proximity and scale of surface water and groundwater connectivity pathways and potential for depressurisation or dewatering.
The following information sources, underpinned by spatial datasets, are used to define the PAE in any bioregion or subregion:
- Bioregion or subregion boundary. The PAE might extend beyond a bioregion or subregion boundary. The BA methodology should be consulted where ‘far-field’ water-related impacts on assets outside a bioregion or subregion boundary may be important and these are incorporated into the PAE where necessary.
- Geology and the coal resource. CSG tenements and large coal mining development leases, as defined in Section 3.3 of the BA methodology (p. 38–39), must be included inside the PAE. Further consideration should be given to known stratigraphic relationships between coal measures, adjacent aquifers and their spatial extent to ensure as accurately as possible that boundaries of the PAE enclose potential important causal pathways between subsurface processes and surface features. The boundary should enclose important geologic features including aquifer recharge and discharge locations. The boundary should enclose any area that, based on the available evidence and expert advice, is likely to be affected by coal resource development occurring within the subregion, though limited to developments that are either occurring now or anticipated to occur within the next 10 to 15 years. Reasons supporting these judgments are to be recorded in product 1.3.
- Surface water hydrology. There are a number of mechanisms by which coal mine and CSG developments can affect surface water resources. One mechanism is by direct extraction of water from or discharge of water to nearby streams. A second mechanism is through impacts of disturbed areas on surface runoff generation and retention. A third mechanism is through changes in baseflow from parts of the landscape where the development has a significant impact on groundwater levels and where there is surface water – groundwater connectivity. In general, areas associated with the third mechanism will also be included in the groundwater PAE (item 4, below), however, surface water PAE assessors should ensure that there are not streams outside the groundwater PAE with substantial baseflow generation. This section therefore focuses on the first and second impact mechanisms.
Direct impacts on surface water resources from coal mine and CSG developments are likely to occur only at and downstream of the developments, though in rare circumstances, upstream effects may need to be considered. To a first approximation the surface water PAE should therefore include only those streams which produced water can potentially reach or with flows that might be reduced by extraction. The key question, then, is in establishing how far downstream the PAE should extend. To achieve this, different approaches are likely to be applied in different subregions, depending on local climate variability and on the availability of gauged streamflows at suitable locations and details of water management plans. In general, though, they will involve estimates of the proportion of the streamflow in a given downstream reach that is derived from the development locations. In reaches where this proportion is considered significant (typically this will mean about 5 to 10%), the reach should be included in the PAE. In many cases this assessment will result in a surface water PAE extending downstream of the bioregion boundary. A second important consideration is in choosing the width of the buffer zone around the stream. This will depend on local conditions, including geomorphology, topography and typical extents of floodplain inundation. Buffer zones should include potential water-dependent assets near but not in the river. The buffer may extend up to 10 km or further in braided networks with wide floodplains.
- Groundwater hydrology. The PAE must include the extent of any groundwater system that intersects with coal resource development. The groundwater system is defined by the best available knowledge and may be refined based on further information about the geology and stratigraphy from (2) above. In bioregions or subregions where groundwater management units have been defined, these should be used to define the extent of the groundwater system for the purpose of determining boundaries of the PAE. Where uncertainty in groundwater management units exists, the boundaries of the PAE must be positioned at a point where, in the Assessment team’s judgment, impacts from coal resource development outside this boundary are not likely. Reasons supporting these judgments are to be recorded in the Assessment’s workflow and provided to the Assessment team. Where a river reach is connected to a groundwater system that overlies or underlies (or otherwise is potentially connected to) coal resource development, the groundwater system associated with that river reach must also be considered. For groundwater systems impacted by coal resource development where there is an interaction with a river that has not already been considered, the in-bank area from the interaction location downstream must be considered.
- Flow paths. Known available information on gradients of pressure, watertable height, stream direction, surface water – groundwater connectivity and any other available data that provide information on surface and subsurface flow paths and the potential effects of coal seam depressurisation or dewatering on these flows must be considered in defining the PAE for a bioregion or subregion. For example, a zero-flow boundary provides a hard constraint on the extent of the PAE. Potential for flow reversal in an alluvium will potentially require the entire geomorphological unit to be considered.
- Infrastructure. The prescribed boundary of the PAE will not take into consideration landscape impacts of infrastructure related to coal resource development such as pipelines, conveyors, roads and railways that have no direct connection with impacts on water resources. Where potential impacts are identified, such as effects of infrastructure on overland flow, these are to be considered in determining the PAE as far as possible. However, in most cases uncertainties in the location and extent of infrastructure development will preclude this consideration in determining the boundary of the PAE.
Based on this information, the PAE is defined by the intersection of the spatial datasets described above. The resulting map of the PAE boundary is approved and deemed as fit for purpose for determining the asset list.
3.2 Selecting assets within the preliminary assessment extent
This process is conducted by the Assets and Receptors Project team. Assets with spatial extents that overlap with, or are fully contained within, the PAE are included in the asset database for that bioregion or subregion. Assets made up of multiple elements may have a mixture of elements within and outside the PAE. Only elements that overlap with, or are fully contained within, the PAE are included in the asset list for that bioregion or subregion. Once assets have been reduced to only those within or intercepting the PAE they are labelled as such and, together, form the asset list. Next, the asset list is assessed by the Assessment team for potential water-related impacts, as described in Section 4 .
Barrett DJ, Couch CA, Metcalfe DJ, Lytton L, Adhikary DP and Schmidt RK (2013) Methodology for bioregional assessments of the impacts of coal seam gas and coal mining development on water resources. A report prepared for the Independent Expert Scientific Committee on Coal Seam Gas and Large Coal Mining Development through the Department of the Environment. Department of the Environment, Australia. Viewed 20 November 2014, http://iesc.environment.gov.au/publications/methodology-bioregional-assessments-impacts-coal-seam-gas-and-coal-mining-development-water.
METHODOLOGY FINALISATION DATE
- 1 Background
- 2 Compiling the asset database
- 3 Determining and applying the preliminary assessment extent
- 4 Assessing water dependence of assets
- Appendix A Simple descriptions of key tables within the asset database
- Appendix B Identifying economic assets
- Contributors to the Technical Programme
- About this submethodology