What are the potential hydrological changes?


In the Clarence-Moreton bioregion, surface water and groundwater modelling was undertaken to investigate potential hydrological changes in the Richmond river basin due to additional coal resource development. To represent changes in the near-surface aquifer from which most ecological assets source water, a zone of potential hydrological change (Box 3) was developed based on the groundwater modelling. This identifies the area where additional coal resource development may affect water-dependent landscapes and assets due to hydrological changes in the near-surface aquifer. Outside the zone, CSG development is very unlikely (less than 5% chance) to have any appreciable impact on hydrology, and therefore on water-dependent landscapes or assets.

Groundwater

Key finding 4:

The zone of potential hydrological change (Box 3) covers an area of about 249 km2 (Figure 5) and extends no more than 10 km west of the West Casino Gas Project. Groundwater modelling found that within this zone, the maximum chance of exceeding the 0.2 m drawdown threshold (Box 3) due to additional coal resource development is estimated at 36%. It is very likely that the drawdown in the near-surface aquifer is less than 1 m across the entire assessment extent.


The simulated median annual groundwater extraction due to the West Casino Gas Project corresponds to 0.02% of the median annual recharge in the modelled part of the Richmond river basin for the time period 2013 to 2042. This very small percentage is attributed to the relatively small number of CSG wells, the hydraulic properties of the coal seams, and the high rate and volume of recharge to the Lamington Volcanics (most of which then discharges rapidly to streams and alluvial aquifers). Likewise, the predicted changes to surface water flows are insignificant because the streamflow rates in the Richmond river basin are overall very high (also driven to a large extent by the Lamington Volcanics) relative to the CSG groundwater extraction volumes.

Potential changes in drawdown were assessed for all hydrogeological layers shown in Figure 3, except for the Bundamba Group that underlies the Walloon Coal Measures.

Box 3 The zone of potential hydrological change

The predicted drawdown (Box 4) is used to define a zone to ‘rule-in’ or ‘rule-out’ potential hydrological change. The zone is the area with at least a 5% chance of greater than 0.2 m drawdown due to additional coal resource development (Figure 5). This threshold is consistent with the most conservative minimal impact thresholds in Queensland or NSW state regulations. Because impact and risk analysis was not undertaken for this bioregion, only groundwater hydrological changes were used to define the zone. The zone is defined by changes in the near-surface aquifer from which most ecological assets source water. Water-dependent landscapes 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 in an impact and risk analysis and smaller-scale analyses that take into account local conditions.

The zone of potential hydrological change can also be defined in deeper geological layers. Impact and risk analysis was not carried out for the Clarence-Moreton bioregion and hence impacts at these deeper layers were not assessed. Figure 42 of Cui et al. (2016b) shows the probability of exceeding 0.2 m drawdown at model nodes in deeper layers.


Box 4 Calculating groundwater drawdown

Drawdown is a lowering of the groundwater level, caused, for example, by pumping. The groundwater model predicts drawdown under the CRDP and drawdown under the baseline (baseline drawdown). The difference in drawdown between CRDP and baseline (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 direct changes in depth to watertable.

The groundwater model simulation is reported for each grid cell individually. The maximum drawdown of each grid cell occurs at different times across the area assessed and the year of maximum baseline drawdown does not necessarily coincide with the year of maximum additional drawdown. Therefore, adding the baseline drawdown and additional drawdown results in a drawdown that is not expected to eventuate.

Figure 5

Figure 5 The percent chance of exceeding 0.2 m drawdown in the near-surface aquifer due to additional coal resource development

This figure shows the drawdown due to additional coal resource development (ACRD, in this case West Casino Gas Project), which is obtained by subtracting drawdown under the baseline from drawdown under the coal resource development pathway. The ClarenceMoreton assessment extent is the area within which potential impacts to groundwater and surface water systems are investigated in this assessment. Only groundwater hydrological changes were used to define the zone of potential hydrological change.

Data: Bioregional Assessment Programme (Dataset 7)

Figure 6

Figure 6 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 and streamflow characteristics 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 6). This approach allows an assessment of the likelihood of exceeding a given magnitude of change, and underpins the assessment of the risk.

Hydrological models require information about physical properties such as the thickness of geological layers and how porous aquifers are. It is unknown how these properties vary across the entire assessment extent (both at surface and at depth), and therefore 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 about the result, which enables decision makers to anticipate potential impacts more precisely, and a wider range indicates less agreement and hence more uncertainty in the outcome.

The distributions created from these model runs are expressed as probabilities that hydrological variables (such as drawdown) exceed relevant thresholds, as there is no single ‘best’ estimate of change. In this assessment, the estimates of drawdown are shown as a 5%, 50% or 95% 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 the model was based on the best available information, if the range of parameters used was not realistic, or if the modelled system does not reflect reality sufficiently, these modelled probabilities might vary from the actual probability of exceeding thresholds.

Figure 7

Figure 7 Key areas for reporting probabilistic results

The assessment extent was divided into smaller square assessment units and the probability distribution (Figure 6) was calculated for each. In this synthesis, results are reported with respect to the following key areas (Figure 7):

  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 landscapes
  3. 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. 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

Key finding 5:

In the Richmond river basin, the predicted maximum decrease in annual streamflow due to additional coal resource development is several orders of magnitude smaller than the observed mean streamflow. Impacts are very likely to be minimal for all but the very lowest streamflows.


Analysis of the surface modelling results indicates that it is very likely (95% chance) that the reduction in annual streamflow would not exceed 100 ML/year in the Richmond River due to additional coal resource development. This maximum change is modelled to occur at Casino and represents less than 0.1% of the annual streamflow at that point.

For all simulated nodes, the median change in streamflow is less than 10 ML/year, which amounts to less than 0.02% of mean annual streamflow. This potential change would have almost no effect on total streamflow but would cause minor changes during low-flow periods in small tributaries such as Shannon Brook at Yorklea (Figure 5). For all but three simulated nodes, 90% of model runs have a change in streamflow less than 35 ML/year (see Figure 10 in Gilfedder et al. (2016)).

Accurate measurement and modelling of low flows is very challenging, and leads to high uncertainties in predictions. The largest effect on low flows (i.e. the lowest 1% of streamflow) is predicted in Shannon Brook, where the median result is a reduction in streamflow of 20%.

Due to the likelihood of very small hydrological changes to surface water and the near-surface aquifer, further impact and risk analysis was not carried out.

FIND MORE INFORMATION

Water balance assessment, product 2.5 (Cui et al., 2016a)

Surface water numerical modelling, product 2.6.1 (Gilfedder et al., 2016)

Groundwater numerical modelling, product 2.6.2 (Cui et al., 2016b)

Surface water modelling, submethodology M06 (Viney, 2016)

Groundwater modelling, submethodology M07 (Crosbie et al., 2016)

Surface water model (Dataset 8)

Surface water modelling, input and output data (Dataset 9)

Groundwater model (Dataset 10)

Groundwater modelling, input and output data (Dataset 7)

Last updated:
9 August 2017