What are the potential hydrological changes?

Key finding 2

The zone of potential hydrological change (Figure 1 and Box 4) covers an area of 250 km2 including 242 km of streams. This represents 52% of the area and 70% of the stream length in the entire Gloucester assessment extent.


The area defined by the 250 km2 zone of potential hydrological change, including the mine pit exclusion zone (Box 3), is potentially impacted by additional coal resource developments.

Groundwater

Potential drawdown due to additional coal resource development occurs in two areas, associated with the Rocky Hill Coal Project, Stratford extension and Gloucester Gas Project Stage 1 in the north (Figure 3a), and the proposed Duralie extension in the south (Figure 3b).

The assessment investigated the maximum difference in drawdown (Box 3) between two potential futures (Box 1) to assess potential impacts on groundwater (Figure 7 and Figure 8). Results are reported for the regional watertable, which comprises the alluvial aquifer, as well as weathered and fractured rock aquifers. The largest additional drawdown generally occurs during or shortly after the active mining period. The time to maximum drawdown increases with distance from the mines (Peeters et al., 2018).

Key finding 3

The area with at least a 5% chance of greater than 0.2 m drawdown due to additional coal resource development is 100.1 km2. Under the baseline it is almost 140 km2 across the entire assessment extent. The area of overlap between these two is 52 km2, which is where there is potential for cumulative groundwater impacts between baseline and additional coal resource developments.


It is very likely that 19.7 km2 will experience at least 0.2 m of drawdown due to additional coal resource development, which includes 17.2 km2 in the Gloucester river basin and 2.5 km2 near Duralie Coal Mine in the Karuah river basin. It is very unlikely that more than 100.1 km2 will experience drawdown of more than 0.2 m due to additional coal resource development (Table 6 and Figure 16 in Section 3.3 of Post et al. (2018)).

Across both river basins, it is very unlikely that more than 15.8 km2 will exceed 2 m of drawdown, or that more than 4.3 km2 will exceed 5 m of drawdown due to additional coal resource development. Under the baseline, it is very unlikely that more than 17.2 km2 will exceed 2 m of drawdown or that more than 6.0 km2 will experience more than 5 m of drawdown (Table 6, Table 7, Figure 16 and Figure 17 in Section 3.3 of Post et al. (2018)).

In the Gloucester river basin, 50.2 km2 are predicted to have a 5% chance of at least 0.2 m drawdown under the baseline, while additional coal resource development is predicted to affect 88.1 km2 with the same probability. In the Karuah river basin, 31.6 km2 are predicted to have a 5% chance of at least 0.2 m drawdown under the baseline, while additional coal resource development is predicted to affect 12.0 km2 with the same probability.

Note that these numbers include the area covered by the mine pit exclusion zone, whereas these areas were excluded when impacts on ecosystems were assessed (see Box 3 and Box 7).

Box 3 Calculating groundwater drawdown

Drawdown is a lowering of the groundwater level, caused, for example, by pumping. The groundwater model predicted drawdown under the coal resource development pathway and drawdown under the baseline (baseline drawdown). The difference in drawdown between the coal resource development pathway and baseline futures (referred to as additional drawdown) is due to additional coal resource development. In a confined aquifer, drawdown relates to a change in water pressure and does not necessarily translate to changes in depth to the watertable.

The maximum drawdown over the course of the groundwater model simulation (from 2013 to 2102) is reported for each 0.25 km2 grid cell, and is expected to occur at different times across the area assessed. It is not expected that the year of maximum baseline drawdown coincides with the year of maximum additional drawdown. Therefore, simply adding the two figures will result in a drawdown amount that is not expected to eventuate.

Close to open-cut mines, confidence in the results of the groundwater model is very low because of the very steep hydraulic gradients at the mine pit interface. As a result, a ‘mine pit exclusion zone’ was defined. Groundwater drawdown inside this zone is not used in the assessment of ecological impacts.

CSG depressurisation, mine dewatering and the impacts of naturally occurring faults were represented in the modelling but their individual effects on groundwater drawdown were not differentiated.


Figure 7

Figure 7 Baseline drawdown (m) in the regional watertable (95%, 50% and 5% chance of exceeding given values of drawdown)

Baseline drawdown is the maximum difference in drawdown under the baseline relative to no coal resource development (Box 3). Results are shown as percent chance of exceeding drawdown thresholds (Box 5). These appear in Post et al. (2018) as percentiles. Areas reported for drawdown exclude the mine pit exclusion zones.

Data: Bioregional Assessment Programme (Dataset 1)

Figure 8

Figure 8 Additional drawdown (m) in the regional watertable (95%, 50% and 5% chance of exceeding given values of drawdown)

Additional drawdown is the maximum difference in drawdown between the coal resource development pathway and baseline, due to additional coal resource development (Box 3). Results are shown as percent chance of exceeding drawdown thresholds (Box 5). These appear in Post et al. (2018) as percentiles. Areas reported for drawdown exclude the mine pit exclusion zones.

Data: Bioregional Assessment Programme (Dataset 1)

Box 4 The zone of potential hydrological change

A zone of potential hydrological change (Figure 1) was defined to rule out potential impacts. It was derived by combining the groundwater zone of potential hydrological change with the surface water zone of potential hydrological change (see Section 3.3.1 in Post et al. (2018)). These zones were defined using hydrological response variables, which are the hydrological characteristics of the system that potentially change due to coal resource development – for example, groundwater drawdown or the number of low-flow days.

The groundwater zone is the area with at least a 5% chance of greater than 0.2 m drawdown (Box 3) due to additional coal resource development. This threshold is consistent with the most conservative minimal impact thresholds in NSW state regulations. The groundwater zone was defined by changes in the regional watertable from which most ecological assets source water.

The surface water zone contains those river reaches where there is at least 5% chance that a change in any one of eight surface water hydrological response variables used in the Gloucester subregion exceeds specified thresholds (see Table 5 in Post et al. (2018)).

Water-dependent ecosystems and ecological assets outside of this zone are very unlikely to experience any hydrological change due to additional coal resource development. Within the zone, potential impacts may need to be considered further. This assessment used regional-scale receptor impact models (Box 8) to translate predicted changes in hydrology within the zone into a distribution of ecological outcomes that may arise from those changes. However, to take account of local conditions, smaller-scale assessments may need to be undertaken.


Figure 9

Figure 9 Illustrative example of probabilistic drawdown results using percentiles and percent chance

The chart on the left shows the distribution of results for drawdown in one assessment unit, obtained from an ensemble of thousands of model runs that use many sets of parameters. These generic results are for illustrative purposes only.

Box 5 Understanding probabilities

The models used in the assessment produced a large number of predictions of groundwater drawdown or changes in streamflow rather than a single number. This results in a range or distribution of predictions, which are typically reported as probabilities – the percent chance of something occurring (Figure 9). This approach allows an assessment of the likelihood of exceeding a given magnitude of change, and underpins the assessment of risk.

Hydrological models require information about physical properties, such as the thickness of geological layers and how porous aquifers are. Because it is unknown how these properties vary across the entire assessment extent (both at surface and at depth), the hydrological models were run thousands of times using different sets of values from credible ranges of those physical properties each time. The model runs were optimised to reproduce historical observations, such as groundwater level and changes in water movement and volume.

A narrow range of predictions indicates more agreement between the model runs, which enables decision makers to anticipate potential impacts more precisely. A wider range indicates less agreement between the model runs and hence more uncertainty in the outcome.

The distributions created from these model runs are expressed as probabilities that hydrological response variables (such as drawdown) exceed relevant thresholds, as there is no single ‘best’ estimate of change.

In this assessment, results are shown as a 95%, 50% or 5% chance of exceeding thresholds. Throughout this synthesis, the term ‘very likely’ is used to describe where there is a greater than 95% chance that the model results exceed thresholds, and ‘very unlikely’ is used where there is a less than 5% chance. While models are based on the best available information, if the range of parameters used is not realistic, or if the modelled system does not reflect reality sufficiently, these modelled probabilities might vary from the actual changes that occur in reality. These regional-level models provide evidence to rule out potential cumulative impacts due to additional coal resource development in the future.

Figure 10

Figure 10 Key areas for reporting probabilistic results

The assessment extent was divided into smaller square assessment units and the probability distribution (Figure 9) was calculated for each. In this synthesis, results are reported with respect to the following key areas (Figure 10):

  1. outside the zone of potential hydrological change, where hydrological changes (and hence impacts) are very unlikely (defined by maps showing the 5% chance)
  2. inside the zone of potential hydrological change, comprising the assessment units with at least a 5% chance of exceeding the threshold (defined by maps showing the 5% chance). Further work is required to determine whether the hydrological changes in the zone translate into impacts for water-dependent assets and ecosystems
  3. assessment units with at least a 50% chance of exceeding the threshold (i.e. the assessment units where the median is greater than the threshold; defined by maps showing the 50% chance)
  4. assessment units with at least a 95% chance of exceeding the threshold (i.e. the assessment units where hydrological changes are very likely; defined by maps showing the 95% chance).


Surface water

The zone of potential hydrological change in the Gloucester subregion has 242 km of stream network. Hydrological modelling shows surface water changes will be relatively small. Most streamflow changes are predicted to occur in the north of the subregion in Avondale Creek, Dog Trap Creek, Waukivory Creek, Oaky Creek and the Avon River (Figure 3), near where two of the three coal mines and most of the CSG wells are located.

Maximum changes in low-flow days, high-flow days and annual flows due to additional coal resource development are the hydrological response variables (Box 4) chosen to represent the modelled changes in overall streamflow. Changes in these variables indicate the dominant hydrological drivers: low flows are sensitive to both the interception of surface runoff and the cumulative impact on baseflow over time caused by groundwater drawdown, while high flows are more sensitive to interception of surface runoff (Zhang et al., 2018). Changes in other hydrological response variables can be viewed on the BA Explorer at www.bioregionalassessments.gov.au/explorer/GLO/hydrologicalchanges.

Key finding 4

It is very unlikely that low-flow days will increase by more than 20 days per year in the Avon River near Rocky Hill, Stratford and Gloucester Gas Project Stage 1, due to additional coal resource development. These changes are similar to, or greater than, the interannual variability under the baseline, which is more likely to move the system outside the range of conditions previously encountered.

Changes in high-flow days and annual flow are predicted to occur in a much smaller length (8.5 and 1.7 km of streams, respectively) and are both less than the interannual variability under the baseline at most locations.


Low-flow days

Regional modelling quantified the median change in the number of low-flow days due to additional coal resource development for 251 km of the 344 km of streams covered by the assessment.

Results indicated that it is very unlikely that more than 92 km of streams will experience increases in low-flow days of more than 3 per year. There is a 5% chance that 47 km of modelled streams in the zone will experience 20 or more additional low-flow days per year, and a 5% chance that 5.7 km of these will experience 80 or more additional low-flow days per year.

The median result indicates increases in low-flow days of between 3 and 20 days per year in the Avon River between its junctions with Avondale Creek and the Gloucester River, with no increases of more than 20 days per year expected in any streams (Figure 11). There are no modelled streams where increases in the number of low-flow days due to additional coal resource development are very likely. See Figure 11 for details.

It was not possible to quantify the median change in low-flow days for 93 km of streams, including Avondale, Dog Trap and Waukivory creeks and part of Mammy Johnsons River, which flow close to the mine sites (see Figure 11). This was either because of their proximity to the mines or due to difficulties in extrapolating results from model nodes to links. For further explanation, see Section 3.2.3 of Post et al. (2018). Potential changes in these streams cannot be ruled out.

Modelled increases in the number of low-flow days are less than the interannual variability seen under the baseline in most locations and for most probabilities of change (Figure 12). However, at some locations near the Rocky Hill and Stratford mines, there is a 5% chance that some of these increases are similar to or even greater than the interannual variability seen under the baseline (Figure 12), which is more likely to move the system outside the range of conditions previously encountered. For more information, see Section 3.3.3.1 of Post et al. (2018).

Figure 11

Figure 11 Maximum increase in the number of low-flow days due to additional coal resource development (95%, 50% and 5% chance of exceeding given values of change)

The coal resource development pathway includes baseline and additional coal resource developments (ACRD). The difference in low-flow days between the coal resource development pathway and baseline is due to additional coal resource development (ACRD). Results are shown as percent chance of exceeding given values of change (Box 5). These appear in Post et al. (2018) as percentiles.

Data: Bioregional Assessment Programme (Dataset 1)

Figure 12

Figure 12 Ratio of maximum increase in number of low-flow days due to additional coal resource development to the interannual variability in the number of low-flow days (95%, 50% and 5% chance)

The coal resource development pathway includes baseline and additional coal resource developments (ACRD). The difference in low-flow days between the coal resource development pathway and baseline is due to additional coal resource development. Results are shown as percent chance (Box 5). These appear in Post et al. (2018) as percentiles.

Data: Bioregional Assessment Programme (Dataset 1)

High-flow days

Additional coal resource development is more likely to affect low flows than high flows, reflected by the shorter length of streams likely to experience changes in high-flow days (Figure 23 and Table 10 in Section 3.3 of Post et al. (2018)).

It is very unlikely that more than 46 km of streams in the 344 km in the assessment extent will experience decreases of more than 3 high-flow days per year. There is a 5% chance that 8.5 km of these streams might experience a reduction of 10 or more high-flow days per year. There is a 5% chance that 1.7 km of the Avon River might experience a reduction of 20 or more high-flow days per year. Under the median result, modelling indicates that 8.5 km of the Avon River will experience a reduction of 3 to 10 high-flow days per year, with no streams experiencing more than this reduction. In some sections of Dog Trap Creek, it is very likely that there will be a reduction of between 3 and 10 high-flow days per year.

Thirty-one km of streams may see median decreases of more than 3 high-flow days per year, but these streams were unable to be quantified, either because of their proximity to mine sites, or due to difficulties in extrapolating results from model nodes to links.

These decreases in the number of high-flow days are less than the interannual variability seen under the baseline in most locations and for most probabilities of change (Figure 25 in Section 3.3 of Post et al. (2018)).

Annual flow

Modelling predicted that it is very unlikely that more than 55 km of streams within the assessment extent will experience decreases of more than 1% in annual flow.

Immediately downstream of mine sites, 26 km of streams are very likely to experience reductions in annual flow of more than 1%, and 1.7 km of Dog Trap Creek is very likely to experience reductions of more than 5%. See Figure 26 and Table 11 in Section 3.3 of Post et al. (2018) for more information.

Another 46 km of streams may see median decreases in annual flow of greater than 1%, but these streams were unable to be quantified, either because of their proximity to mine sites, or due to difficulties in extrapolating results from model nodes to links.

These decreases in annual flow are less than the interannual variability seen under the baseline in most locations and for all probabilities of change (Figure 28 in Section 3.3 of Post et al. (2018)).

Water quality

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. See Section 3.3.4 of Post et al. (2018) for more detail. All four additional coal resource developments are operating under a ‘no discharge’ rule which means all water is to be retained and reused on site. Because of this, potential impacts on water quality from additional coal resource development are considered unlikely in the Gloucester subregion. Streamflow and groundwater level data suggest that any reduction in baseflow due to drawdown from additional coal resource developments is likely to lead to a decrease in stream salinity, whereas reductions in catchment runoff could lead to increases. Section 1.5.2 of Rachakonda et al. (2015) provides details of stream and groundwater salinities in the Gloucester subregion. To quantify the likely effect requires more local data and modelling.

Last updated:
11 January 2019