3.3 Potential hydrological changes


The future development of seven large-scale thermal coal mines near the central-eastern margin of the Galilee Basin has the potential to affect the groundwater and surface water systems of this area. For bioregional assessment (BA) purposes, these seven proposed coal mines define the modelled component of the coal resource development pathway (CRDP) for the Galilee subregion. The potential hydrological changes due to these additional coal resource developments (i.e. the modelled CRDP) are the main focus of this section, as there are no baseline coal mines or coal seam gas (CSG) fields in the Galilee subregion. Potential impacts of the non-modelled CRDP, which comprises seven other potential coal mining projects and three CSG developments, are qualitatively assessed in Section 3.6 (and not further discussed in this section).

Probabilistic hydrological modelling has been used to define the zone of potential hydrological change for the Galilee subregion, which covers a total area of 14,030 km2. Within this zone changes in hydrology due to additional coal resource development exceed defined thresholds for either groundwater or surface water (or both). The groundwater zone of potential hydrological change is defined as the area with at least a 5% chance of drawdown exceeding 0.2 m in the near-surface (unconfined) aquifer system (Quaternary alluvium and Cenozoic sediments). The groundwater zone comprises two separate elongate areas (north and south), with the northern part centred around the cluster of coal mines at Hyde Park, China Stone and Carmichael, and the southern part centred on South Galilee, China First, Alpha and Kevin’s Corner. These separate parts of the zone indicate that drawdown from individual mines may overlap in both space and time, thereby leading to cumulative hydrological changes. The southern cumulative drawdown zone is the larger of the two areas and covers about 7898 km2, whereas the northern area is approximately 5466 km2. Overall, the combined area of the groundwater zone of potential hydrological change is 13,364 km2, which includes the 986 km2 area of the mine exclusion zone (where modelled drawdowns are considered unreliable due to the regional-scale nature of the BA modelling approach).

The groundwater modelling undertaken for the BA of the Galilee subregion is well suited to assessing regional-scale, cumulative impacts of the proposed coal resource developments, for example, as a means of defining the zone of potential hydrological change to then rule out impacts to any water resources and water-dependent assets outside of this area. However, the underpinning hydrogeological conceptualisation and modelling approach is recognised as being unsuitable for providing reliable drawdown results at some specific locations, such as springs, especially where the uppermost Quaternary alluvium and Cenozoic sediment aquifer does not exist. In these places, drawdown in the near-surface aquifer, and the underlying Clematis Group aquifer, will likely be overestimated. Applying an alternative modelling conceptualisation that better represents the likely hydraulic pathways that link areas of mining development to specified points in the landscape generates more plausible drawdown results in areas lacking Cenozoic sediments, such as those outcrop areas where the Triassic geological units of the Galilee Basin occur (e.g. the Rewan Group and Moolayember Formation). The variability of modelling results from the application of alternative hydrogeological conceptualisations highlights the importance of applying local knowledge to improve the regional-scale modelling outcomes of this BA at specific locations.

The surface water zone of potential hydrological change is largely restricted to the Belyando river basin, a headwater catchment of the larger Burdekin river basin. The surface water zone corresponds to the area along the modelled stream network where the change in at least one of eight surface water hydrological response variables exceeds a defined threshold due to additional coal resource development. The thresholds can be generally described as at least a 5% chance of a 1% (or 3 days) or greater change in a flow volume or yearly event frequency (e.g. increase in number of zero-flow days per year). The surface water zone also includes non-modelled streams in the groundwater zone that are likely to receive baseflow (i.e. available evidence supports some level of groundwater contribution), and any non-modelled streams that are directly affected by mining and mine-site infrastructure. In total, there are 6285 km of streams in the zone of potential hydrological change, with impacts to about 25% of this stream length not quantified (i.e. not included in the modelled surface water network).

The potential impacts to surface water hydrology due to additional coal resource development are evaluated across the spectrum of the low-flow (using zero-flow days), high‑flow and annual flow regime. The most substantial modelled surface water changes are for increases in zero-flow days, which mostly affect the main channel of the Belyando River, and the Suttor River downstream of its junction with the Belyando. In particular, the approximately 250 km stretch of this river network from downstream of the Native Companion Creek junction northwards to Lake Dalrymple (Burdekin Falls Dam) is very likely (greater than 95% chance) to experience substantial increases in the number of zero-flow days per year. At the 95th percentile of all model simulations, increases in zero-flow days along this river stretch exceed 200 days/year for all model nodes and interpolated stream links. These results indicate that increases in zero-flow days can aggregate from individual mines and thus result in cumulative impacts that extend beyond the mine areas along the main Belyando River channel. Other smaller streams that may experience substantial increases in the number of zero-flow days due to additional coal resource development include Tallarenha Creek, Lagoon Creek, Sandy Creek, Alpha Creek and Native Companion Creek, all of which are proximal to the cluster of four mines in the south of the zone. North Creek is the main surface water drainage likely to experience increases in zero-flow days in the area of the northern mining cluster.

In contrast to the effects on zero-flow days, changes to high-flow days and annual flow volumes due to the seven modelled coal mines are generally less substantial, and also tend to have a greater effect on the smaller tributary network within the zone (i.e. in smaller catchments that are closer to the mines) rather than the main river channel. For example, the largest decreases in high-flow days per year occur on Tallarenha Creek, Lagoon Creek and Sandy Creek in the south, due to their proximity to the southern mining cluster. In the north, the main impacts are modelled for North Creek and Bully Creek. However, unlike for zero-flow days, these high-flow changes do not accumulate downstream in the Belyando River, such that the Suttor River downstream of the Belyando junction is very unlikely (less than 5% chance) to experience decreases in high-flow days of more than 10 days per year. About 269 km of the modelled stream network is very likely (greater than 95% chance) to experience reductions in annual flow volume of 5% to 20%. These decreases in annual flow affect the same streams that are expected to experience reductions in high-flow days. There is only one surface water model node, on Tallarenha Creek downstream of the proposed South Galilee Coal Mine, where reductions in annual flow volume may exceed 20%.

Any change in hydrology could result in changes to stream water quality; however, this was not modelled as part of the BA. A range of regulatory requirements are in place in Queensland that are intended to minimise potential salinity impacts from coal resource development. Groundwater is typically more saline than surface water runoff, which suggests that any reductions in baseflow are more likely to lead to decreases in stream salinity. However, the actual effects depend very much on local conditions, and increases in stream salinity cannot be ruled out.

Users can visualise more detailed results for hydrological changes in the Galilee subregion using a map-based interface on the BA Explorer, available at www.bioregionalassessments.gov.au/explorer/GAL/hydrologicalchanges.

In the BA for the Galilee subregion, potential hydrological changes due to additional coal resource development are summarised using hydrological response variables based on results from the surface water and groundwater modelling. These modelling results are reported, respectively, in companion product 2.6.1 (Karim et al., 2018b) and companion product 2.6.2 (Peeters et al., 2018) for the Galilee subregion. The hydrological response variables have been specifically defined to represent the maximum difference between the CRDP and baseline futures for groundwater drawdown and a range of streamflow characteristics. They have also been used to define the zone of potential hydrological change – the main focus of this impact and risk analysis (Section 3.3.1). As previously discussed, the baseline for the Galilee subregion does not have any coal resource development, as there were no commercially producing coal mines or coal seam gas (CSG) fields in the subregion as of December 2012. Consequently, the CRDP is here defined simply by the seven coal mines (Table 3) that comprise the additional coal resource development modelled in this BA.

Potential changes to groundwater and surface water systems within the zone of potential hydrological change are presented in Section 3.3.2 and Section 3.3.3, respectively. Areas within the zone are identified as being ‘more at risk’ of experiencing hydrological changes, and hence potentially adverse impacts, based on variations in the magnitude of modelled hydrological change, coupled with the probability of such change occurring. Conversely, the areas of the Galilee assessment extent that occur outside of this zone are effectively ruled out from further consideration in this BA (i.e. the impact and risk analysis is not undertaken in these areas). Importantly though, the regional-scale analysis of the BA for the Galilee subregion means that, to further refine the assessment of risk and determine appropriate management responses for particular assets or ecosystems within the zone, local-scale data and information may be further required to develop locally specific and development-scale impact predictions. Additionally, while changes in water quality were not part of the hydrological modelling undertaken for this BA, the potential for changes in water quality due to additional coal resource development in the Galilee subregion is considered qualitatively in Section 3.3.4 .

Additional hydrological response variables have been defined for input into the landscape class qualitative models and receptor impact models developed for the Galilee subregion as part of this BA. These modelling approaches focus specifically on predicting ecosystem-level responses to modelled hydrological changes, and are discussed in detail in companion product 2.7 for the Galilee subregion (Ickowicz et al., 2018). They represent key water dependencies in these systems and are based on average differences over 30-year (2013 to 2042) and 90-year (2013 to 2102) periods. The application of the receptor impact models to landscape classes, using the hydrological response variables that were considered to best reflect key dependencies between ecosystems and hydrological systems, are presented as part of the impact and risk analysis in Section 3.4.

Last updated:
6 December 2018
Thumbnail of the Galilee subregion

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