- Bioregional Assessment Program
- Gloucester subregion
- 3-4 Impact and risk analysis for the Gloucester subregion
The impact and risk analysis for the Gloucester subregion is a regional overview of potential impact on, and risks to, water resources and water-dependent ecological, economic and sociocultural assets due to coal resource development. Hydrological and ecosystem changes are quantified and impacts that are very unlikely (less than 5% chance) are ruled out.
The key finding of the impact and risk analysis indicates that the three proposed new coal mines and one proposed new coal seam gas (CSG) development are predicted to cause minimal impacts on water resources and water-dependent assets in the Gloucester subregion. These findings are explained briefly here and in detail in subsequent sections.
The Gloucester subregion spans an area of 348 km2 and is the smallest subregion in the Bioregional Assessment Programme. It is located north of the Hunter river basin in NSW. The subregion intersects the northerly flowing Avon and Gloucester rivers of the Manning river basin, which discharge to the Tasman Sea east of Taree, and the south-flowing Karuah River and tributaries, which discharge into Port Stephens. From a groundwater perspective, it is a closed system.
Coal resource developments
Bioregional assessments (BAs) consider two potential coal resource development futures:
- baseline coal resource development (baseline): a future that includes all coal mines and CSG fields that are commercially producing as at December 2012
- in the Gloucester subregion, the two baseline developments are both open-cut coal mines (Duralie Coal Mine in the south and Stratford Mining Complex in the north)
- coal resource development pathway (CRDP): a future that includes all coal mines and CSG fields that are in the baseline as well as the additional coal resource development (those that are expected to begin commercial production after December 2012)
- in the Gloucester subregion, the additional coal resource development includes the expansion of the two baseline coal mines (Duralie Coal Mine and Stratford Mining Complex), and a new open-cut coal mine at Rocky Hill in the north of the geological Gloucester Basin. AGL’s proposed CSG development, Gloucester Gas Project Stage 1, was included because the CRDP was finalised in October 2015 before AGL withdrew from this project in February 2016.
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. Potential hydrological changes due to these coal resource developments have been presented in companion products 2.6.1 (surface water) and 2.6.2 (groundwater); the risks to, and impacts on, water resources and water-dependent ecological, economic and sociocultural assets are summarised.
Zone of potential hydrological change
The zone of potential hydrological change covers an area of 250 km2 and includes 242 km of stream network. This represents 52% of the area and 70% of the stream length in the entire Gloucester assessment extent. The zone is the union of the groundwater zone of potential hydrological change and the surface water zone of potential hydrological change:
- The groundwater zone of potential hydrological change is defined as the area with a greater than 5% chance of exceeding 0.2 m of drawdown due to additional coal resource development in the regional watertable. It covers an area of 100 km2, comprising 88 km2 around Stratford mine, Rocky Hill mine and the Gloucester Gas Project Stage 1 and an additional 12 km2 around Duralie mine.
- The surface water zone of potential hydrological change contains those river reaches where a change in any one of the eight hydrological response variables used in the impact and risk analysis for the Gloucester subregion exceeds a specified threshold. The surface water zone covers an area of 187 km2, with approximately 117 km2 in the Gloucester river basin and 70 km2 in the Karuah river basin.
The zone was used to ‘rule out’ potential impacts on landscape classes and water-dependent assets within the Gloucester assessment extent. Water resources and water-dependent assets outside the zone are very unlikely (less than 5% chance) to be impacted. Within the zone, potential impacts due to the hydrological changes were assessed further.
Potential hydrological changes
Results from regional groundwater modelling show drawdown due to additional coal resource development of greater than 0.2 m is very likely for an area of almost 20 km2. This includes 17.2 km2 in the Gloucester river basin and 2.5 km2 near the Duralie Coal Mine in the Karuah river basin. It is very unlikely that more than 100 km2 will experience drawdowns of this magnitude due to additional coal resource development. Results for 2 m and 5 m drawdown extents suggest it is:
- very unlikely that more than 16 km2 will experience drawdown exceeding 2 m
- very unlikely that more than about 4 km2 will experience drawdown exceeding 5 m.
The modelled drawdowns close to open-cut mines are considered unreliable because of the very steep hydraulic gradients at the mine pit interface. A ‘mine pit exclusion zone’ was defined to identify the area of uncertain drawdown results. Within the zone of potential hydrological change, it encompasses an area of 14.5 km2 (of which 10 km2 is in the groundwater zone) and includes an area around each of the three mines at Rocky Hill, Stratford and Duralie. Drawdown numbers reported in Section 3.3 include this mine pit exclusion zone, but the area within this zone is not considered when evaluating potential impacts on landscape classes and ecological assets.
Within the zone, potential changes to surface water due to additional coal resource development were assessed using three hydrological response variables, chosen to represent low-flow, high-flow and average flow characteristics of streamflow.
The specified threshold for low-flow days is a 5% chance of an increase of three or more low-flow days per year. Low-flow days are likely to increase at a number of locations across the assessment extent. There is at least a 5% chance of increases of more than 3 additional low-flow days per year in Mammy Johnsons and Karuah rivers due to the additional coal resource development at Duralie. Modelled hydrological changes in the Karuah river basin are generally smaller, more localised and of lower likelihood than in the Avon River catchment. At most locations, the modelled increases in low-flow days are less than the interannual variability seen under the baseline. However, results at three model nodes near the Rocky Hill coal mine, indicate at least a 5% chance of increases in number of low-flow days per year that are comparable to or greater than interannual variability, potentially leading to changes in low-flow characteristics outside what has previously been experienced.
The impact on number of high-flow days and annual flow due to additional coal resource development tends to be smaller, but the same streams identified as at risk from changes in the number of low-flow days are likely to be affected.
Potential changes in hydrology could lead to changes in water quality, but these were not modelled. A number of regulatory requirements are in place in NSW to minimise potential water quality impacts from coal resource developments. Changes in water quality due to additional coal resource development are considered unlikely in the Gloucester subregion, as none of the proposed developments are licensed to discharge mine water off site and modelled changes in flow regime are relatively small.
Impacts on, and risks to, landscape classes
The vast majority (247 km2 or 99%) of the zone of potential hydrological change comprises landscape classes from the ‘Non-groundwater-dependent ecosystem (GDE)’ and ‘Economic land use’ landscape groups. The ‘Non-GDE’ landscape group is not considered to be impacted by coal resource development because of a lack of dependence on water other than rainfall. The ‘Economic land use’ landscape group is considered in impacts on, and risks to, economic assets.
The following are very unlikely to be impacted and were not considered further because they are located outside the zone of potential hydrological change:
- the ‘Estuarine’ landscape group, which includes 38 km2 in the estuarine reaches of the Karuah River
- the ‘Freshwater wetlands’ landscape class (1.1 km2) within the ‘GDE’ landscape group
- 65 km of perennial - gravel/cobble streams and 3 km of intermittent - gravel/cobble streams, mainly along the Karuah River.
There are 242 km of stream length in the ‘Riverine’ and 3.3 km2 extent in the ‘GDE’ landscape groups in the zone of potential hydrological change that are subject to potential hydrological changes due to additional coal resource development. Results are presented for the potential hydrological changes that may impact the modelled landscape classes within these groups, and using qualitative mathematical models and receptor impact models the associated response of selected ecological indicators is reported.
‘Riverine’ landscape group
The zone of potential hydrological change includes 76% (133 km) of perennial - gravel/cobble streams and 96% (78 km) of intermittent - gravel/cobble streams in the assessment extent. To investigate ecological changes in these landscape classes, qualitative mathematical models and receptor impact models were constructed. The receptor impact models determined the potential impact of hydrological changes, specifically groundwater drawdown, change in baseflow index and increased zero-flow days per year, using these variables:
- annual mean percent canopy cover of woody riparian vegetation (perennial streams)
- mean number of larvae of the Hydropsychidae family (net-spinning caddisflies) in a 1 m2 sample of riffle habitat (perennial streams)
- mean number of the eel-tailed catfish (Tandanus tandanus) in a 600 m2 transect whose long axis lies along the mid-point of the stream (perennial streams)
- mean hyporheic invertebrate taxa richness, where hyporheic invertebrate taxa are the organisms found where surface water and groundwater mix below the bed of a stream (intermittent streams).
Overall, the median estimate from modelled changes in the 60-year period (2042 to 2102) showed that there would be no change along any reach in the four receptor impact variables listed above due to additional coal resource development. This is because, at a regional scale, the modelled hydrological changes in these streams are very minor.
‘Groundwater-dependent ecosystem (GDE)’ landscape group
The ‘GDE’ landscape group includes those ecosystems that rely on the surface or subsurface expression of groundwater to meet all or some of their life-cycle requirements. There are 3.2 km2 of the mapped ‘GDE’ landscape group in the zone of potential hydrological change, including forested wetlands, wet sclerophyll forests, rainforests and dry sclerophyll forests landscape classes. About 0.4 km2 is in the mine pit exclusion zone.
Of the 2.8 km2 of GDEs not in the mine pit exclusion zone, there is a 5% chance that 1.1 km2 will experience drawdown due to additional coal resource development of more than 0.2 m. Most of this coincides with areas of forested wetlands. It is very unlikely (less than 5% chance) that any of the GDEs will be subject to more than 2 m of drawdown.
Potential ecological changes are difficult to model because the water requirements of GDEs are poorly understood and there is large uncertainty as to the frequency, timing and duration of groundwater use by GDEs within the Gloucester subregion. Qualitative mathematical models were developed for the ‘Forested wetlands’, ‘Wet sclerophyll forests’ and ‘Dry sclerophyll forests’ landscape classes. The models focus on the role that forest canopies play as a food source and habitat and their response to a simultaneous decrease in shallow and deep groundwater. They reflect the understanding that drawdown has negative impacts on all vegetation-related variables, including overstorey and understorey (ground layer) cover, and recruitment. The models do not represent the magnitude or likelihood of potential impacts due to a change in drawdown. Thus this assessment identifies where these GDEs coincide with areas of modelled drawdown, and what the likely direction of change in ecosystem components is, but not how big an impact the changes will have.
Impacts on, and risks to, water-dependent assets
The Gloucester subregion has 116 ecological assets in the assessment extent. Of these, 52 are in the zone of potential hydrological change and are therefore subject to potential hydrological changes due to additional coal resource development. They include 17 ‘Surface water feature’ subgroup assets, 3 ‘Groundwater feature (subsurface)’ subgroup assets and 32 ‘Vegetation’ subgroup assets. The vegetation subgroup assets include:
- Environment Protection and Biodiversity Conservation Act 1999
- 1 threatened ecological community, Lowland Subtropical Rainforest
- 10 point locations of known platypus occurrence
- 7 fish biodiversity hotspot monitoring locations.
Based on regional-scale modelling of hydrological change and associated ecological responses, impacts on ecological assets in the ‘Riverine’ landscape group due to additional coal resource development are considered very unlikely.
There is some potential for impacts on ecological assets associated with GDE landscape classes, although median estimates of areas impacted are less than 1.0 km2. Species with extensive potential habitat distributions (e.g. regent honeyeater) are more likely to be identified as subject to potential hydrological change because there is greater likelihood their potential distribution will intersect an area of potentially significant hydrological change. However, the magnitude of the impact, if any, is uncertain; it will depend on local-scale factors, including whether or not the species is actually present in the ‘at risk’ area and the sensitivity of the species to the modelled hydrological changes. It is very unlikely that any areas of the Lowland Subtropical Rainforest threatened ecological community are impacted.
There are five surface water sources and two groundwater sources containing 339 bores and surface water extraction points in the zone of potential hydrological change. When non-alluvial bores in the surface water zone and monitoring bores are filtered out, there remain 246 bores and surface water extraction points where the potential for an economic impact cannot be ruled out.
The potential for economic impacts from modelled groundwater changes is assessed in terms of numbers of bores in areas where drawdowns are predicted to exceed minimal impact consideration thresholds. Under the NSW Aquifer Interference Policy, if a proposal to extract water from an aquifer is likely to impact a licensed water holder’s access to their entitlement, then there is provision for the licence holder to be recompensed. There are five bores outside the mine pit exclusion zone that have at least a 5% chance of experiencing a drawdown greater than 2 m, the minimal impact threshold for aquifers in the Gloucester subregion, due to additional coal resource development. Four are monitoring bores and thus drawdown is unlikely to result in an economic impact; one is a production bore, owned by AGL and considered unlikely to lead to an economic impact.
The impact on water availability in water sources of the Gloucester subregion was assessed in terms of the modelled reductions in mean annual flow due to additional coal resource development. Modelling indicates reductions of less than 1.6 GL/year between 2013 and 2042 in both the Upper Gloucester River and Avon River water sources. These 1% to 2% reductions in mean annual flow from the baseline are less than the interannual variability due to climate.
Potential surface water changes show that the reliability of surface water supply in the Gloucester assessment extent is very unlikely to be affected by additional coal resource development. The reliability of water supply to licence holders in the subregion, as indicated by an increase in the number of cease-to-pump days per year, was found to not significantly change in any water source: for the Karuah River water source, where the potential changes are greatest, reductions of more than 3 days are very unlikely. Cease-to-pump rules apply to most water sources in NSW to ensure sufficient water is retained in unregulated rivers to meet environmental requirements.
There are 19 water-dependent sociocultural assets identified in the assessment extent. The Washpool at the Karuah River north of the town site of Washpool is the one water-dependent sociocultural asset in the zone of potential hydrological change, but due to very small changes in hydrology at this location, it is not likely to be impacted.
Gaps and opportunities
Any local-scale studies of changes to surface water and groundwater should focus on the northern part of the Gloucester subregion to determine potential impacts of additional coal resource development, specifically the area north-east of Stratford and including Avondale Creek, Dog Trap Creek, Waukivory Creek, Oaky Creek and the Avon River.
Research in the Gloucester subregion should focus on the role of faults, mapping the depth to groundwater, carrying out additional mapping of vegetation, and ground-truthing GDE locations.
Future assessments of the cumulative impacts of coal resource developments on water resources and water-dependent assets and ecosystems in the Gloucester subregion should focus on incorporating the impacts of baseline coal mines, changes in other land uses, and climate variability and climate change.
The full suite of information, including information for individual assets, is provided at www.bioregionalassessments.gov.au. Users can explore detailed results for the Gloucester subregion using a map-based interface at www.bioregionalassessments.gov.au/explorer/GLO.
- 3.1 Overview
- 3.2 Methods
- 3.3 Potential hydrological changes
- 3.4 Impacts on and risks to landscape classes
- 3.5 Impacts on and risks to water-dependent assets
- 3.6 Commentary for coal resource developments that were not modelled
- 3.7 Conclusion
- Contributors to the Technical Programme
- About this technical product