2.7.4 'Groundwater-dependent ecosystem (GDE)' landscape group


The subregion has three main hydrogeological units relevant to sustaining groundwater-dependent ecosystem (GDE) structure and function, which provide a useful conceptual framework for examining landscape classes that are dependent on groundwater: (i) alluvial aquifers along major creek lines; (ii) relatively shallow weathered/fractured rock aquifers; and (iii) impermeable Alum Mountain Volcanics that underlie these hydrogeological units.

The Quaternary alluvial aquifers are developed close to the rivers. The Permian fractured rock and weathered zone is up to 150 m thick. It underlies the alluvial system and extends to the edges of the subregion. The outcropping Alum Mountain Volcanics are generally considered to be impermeable but localised fractures may provide pathways for localised groundwater flow paths.

GDEs occur within each of the three hydrogeological units described above but they are predominantly associated with the weathered/fractured rock zone and alluvial aquifers. Few GDEs are present above the Alum Mountain Volcanics. Of the five GDE landscape classes that were identified as likely to be groundwater dependent in the Gloucester subregion, qualitative models were developed for three landscape classes: ‘Forested wetlands’, ‘Wet sclerophyll forests’ and ‘Dry sclerophyll forests’. A qualitative model for rainforests was not developed owing to its small area within the subregion and lack of proximity to coal resource development.

The qualitative mathematical model for the ‘Forested wetlands’ landscape class focused on the role that forest canopies play in providing flower nectar (a food resource for insect, bird and mammal consumers), habitat structure for nesting and general habitat for various predators. The model recognises the possibility for coal resource development to impact the supply of both deep and shallow groundwater to these forest communities, and in some instances, new coal mines could lead to further fragmentation of the remaining forested wetlands in the Gloucester subregion.

A single impact scenario focusing on simultaneous reduction in deep and shallow groundwater generally indicates an ambiguous or negative response prediction for biological variables within the forested wetlands community as a consequence of decreases in available nectar production and quality of forest habitats. Ambiguous responses arise from positive effects associated with the potential release from predation or competitive dominance, matched by negative effects resulting from reduced nectar production.

The qualitative mathematical model for the ‘Wet sclerophyll forests’ landscape class is based on the model structure for forested wetlands, with the overstorey providing the same ecosystem functions as forested wetlands. An added feature of this model is the effect of the overstorey in creating a microclimate that supports ground-layer and mid-storey vegetation. The model identifies deep groundwater as being important to the survival of the trees but it is recognised that this is likely only happening in the alluvial deposits of river floodplains or on lower slopes. The qualitative mathematical predictions for a single impact scenario (simultaneous decrease in shallow and deep groundwater) are the same as for the ‘Forested wetlands’ landscape class.

The qualitative mathematical model for the ‘Dry sclerophyll forests’ landscape class is based on the model structure of the ‘Wet sclerophyll forests’ landscape class. All features of this model are the same as for the ‘Wet sclerophyll forests’ landscape class, but with added uncertainty in the role of microclimate in supporting understorey vegetation components. The model also captures (via alternative model structures) additional uncertainty as to whether or not dry sclerophyll trees can effectively access stores of deep groundwater outside of alluvial deposits, and also whether the microclimate created by the overstorey is beneficial to mid-storey and ground-layer vegetation. The model predictions for a single impact scenario (simultaneous decrease in shallow and deep groundwater) match those for the ‘Forested wetlands’ and ‘Wet sclerophyll forests’ landscape classes, except for the predicted zero (no change) response of ground-layer and mid-storey vegetation in the second model.

No receptor impact models were developed for the ‘Groundwater-dependent ecosystem (GDE)’ landscape group due to time restrictions and the limited availability of suitable experts.

Last updated:
13 November 2018