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

3.4.4.1 Description

Groundwater-dependent ecosystems (GDEs) are those that rely on the surface or subsurface expression of groundwater to meet all or some of their life-cycle requirements (Eamus et al., 2006). The dependence of GDEs on groundwater varies both spatially and temporally (Eamus et al., 2006). Ecosystems may be obligate GDEs, with a continuous or entire dependence on groundwater, or facultative GDEs, with an infrequent or partial dependence on groundwater (Zencich et. al., 2002). Plants that depend solely on moisture held within the soil profile are known as vadophytes and are not groundwater dependent (Sommer and Froend, 2010). In the Gloucester subregion, as in much of Australia, there is considerable uncertainty as to the nature of groundwater dependency for much terrestrial vegetation. The hydroclimatic environment of the Gloucester subregion is subtropical. Annual rainfall ranges from about 960 to 1400 mm/year, and is highest in summer when potential evaporation is also highest (companion product 1.1 for the Gloucester subregion (McVicar et al., 2014). Nonetheless, the region is still classified as being water limited inasmuch as potential evaporation (1400 to 1700 mm/year) exceeds rainfall in most months of the year. Rainfall is also highest along the margins of the subregion because this area is associated with the higher elevation regions, whereas the deficit of rainfall, relative to potential evaporation, is greater throughout much of the lowland areas of the subregion. The Gloucester Basin underlies the Gloucester subregion and is characterised as a closed hydrogeological system. Thus, water entering the system must leave as either surface water or groundwater discharge (companion product 2.3 for the Gloucester subregion (Dawes et al., 2018)). Groundwater recharge is estimated as up to 17% of rainfall under steady-state conditions and up to 28% of rainfall under transient conditions, with high values associated with alluvial aquifers (companion product 1.1 for the Gloucester subregion (McVicar et al., 2014)). This combination of rainfall deficit and surface water and groundwater recharge creates the potential for GDEs to exist within the Gloucester subregion.

The subregion has three main hydrogeological units (companion product 1.1 for the Gloucester subregion (McVicar et al., 2014)) relevant to sustaining GDE structure and function, which provide a useful conceptual framework for examining landscape classes dependent on groundwater:

The geomorphology of the Gloucester subregion has been described in detail elsewhere (companion product 1.1 (McVicar et al., 2014); companion product 2.3 (Dawes et al., 2018) for the Gloucester subregion), and only a brief summary is presented here as context (Figure 48). The Quaternary alluvial aquifers are developed in proximity to the rivers. Soils in these alluvial deposits are dominated by Tenosols and are composed of clay layers and highly permeable sediments with high hydraulic conductivities (up to 500 m/day). The thickness of the alluvia varies from 9 to 15 m and the watertable is shallow and responsive to rainfall and flood events close to the river.

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. These shallow-rock hydrogeological units are composed of interbedded sandstone, silt and claystone. Generally, hydraulic conductivities of these aquifers are low with a sluggish response to rainfall. However, these hydraulic conductivities are highly variable as a result of fracturing and fault zones within the formation. Soils of the fractured rock and weathered zone tend to be dominated by Kurosols. Typically, these soils have a sharp, abrupt boundary between the upper coarser-textured ‘A horizon’ and the finer-textured ‘B horizon’, which may provide a pathway for subsurface lateral flows of water.

The outcropping Alum Mountain Volcanics formations are generally considered to be impermeable but localised fractures may provide pathways for localised groundwater flow paths (companion product 1.1 for the Gloucester subregion (McVicar et al., 2014)). These may be expressed as springs along the margins of the basin, driven by localised circulation of meteoric water.

Figure 48 Conceptual model of the major groundwater processes in the Gloucester Basin

GDE = groundwater-dependent ecosystem

The water requirements of GDEs are poorly understood and there is large uncertainty as to the frequency, timing and duration of groundwater use within the Gloucester subregion. In general, transpiration of groundwater is expected to decline as the depth to groundwater increases, but there is very limited evidence to support this assumption within Australia. O’Grady et al. (2010) reviewed estimates of groundwater discharge in Australia and concluded that there is considerable variation in the relationship between transpiration of groundwater and depth to groundwater. Factors such as the rooting depth of a particular species (which is usually not known), hydroclimatic environment and groundwater salinity all impact on groundwater use by vegetation. Zolfaghar et al. (2014) examined the structure and productivity of eucalypt forest across a depth-to-watertable gradient in the upper Nepean catchment in NSW. They found that where groundwater was shallow, vegetation had significantly higher biomass and productivity than sites where groundwater was deeper than approximately 10 m. The relationships between depth to groundwater and the structural and functional attributes of the vegetation communities were highly non-linear, with steep declines in leaf area index and biomass over a range of 5 to 10 m depth to groundwater. However, it is important to note that the study was largely correlative in nature and did not quantify the groundwater requirements of the vegetation. Specific studies of GDEs within the Gloucester subregion are limited. Existing mapping of GDEs is based on a multiple-lines-of-evidence approach that incorporated existing vegetation mapping, modelled groundwater levels and remote sensing (Kuginis et al., 2012). Modelled depths to groundwater (Summerell and Mitchell, 2011) for the subregion are generally shallow (within 16 m of the ground surface). However, there is likely to be uncertainty in the mapping and this is indicated by relatively shallow groundwater modelled within the bordering Alum Mountain Volcanics.

Of the five GDE landscape classes that were identified as likely to be groundwater dependent, four were present in the zone of potential hydrological change: ‘Forested wetlands’, ‘Wet sclerophyll forests’, ‘Rainforests’ and ‘Dry sclerophyll forests’. GDEs occur within each of the three hydrogeological units described previously but they are predominantly associated with the weathered/fractured rock zone and alluvial aquifers (Table 21). Few GDEs are present above the Alum Mountain Volcanics.

Table 21 Area of groundwater-dependent ecosystem landscape classes within each of three hydrogeological units across the assessment extent

The wet sclerophyll forests of NSW occur on moderately fertile soils in high rainfall areas, and are characterised by a tall, open, sclerophyllous tree canopy and a luxuriant understorey of soft-leaved, mesophyllous, shrubs, ferns and herbs. Many understorey plants are rainforest species or have close rainforest relatives. Rainforests may be embedded within a matrix of wet sclerophyll forest and the two often blend together as intermediate forms. More than 30% crown cover of emergent, non-rainforest species (including eucalypts, brushbox and turpentine) results in a classification of wet sclerophyll forest rather than rainforest (DECC, 2007). As discussed in Section 2.7.2 of companion product 2.7 for the Gloucester subregion (Hosack et al., 2018b), a single conceptual model was developed for both the ‘Wet sclerophyll forests’ and ‘Rainforests’ landscape classes in the Gloucester subregion. The main vegetation communities are described in Table 22.

Table 22 Main vegetation communities within the ‘Wet sclerophyll forests’, ‘Rainforests’ and ‘Dry sclerophyll forests’ landscape classes

Qualitative mathematical models for the ‘Dry sclerophyll forests’ and ‘Forested wetlands’ and ‘Wet sclerophyll forests’ landscape classes are presented in Section 2.7.4 of companion product 2.7 for the Gloucester subregion (Hosack et al., 2018b)).

3.4.4.2 Potential hydrological change

The area of GDE landscape classes within the zone of potential hydrological change is 3.2 km² of which 0.4 km² is in the mine pit exclusion zone (Table 23). Of the remaining 2.8 km² of GDEs, 1.2 km² is potentially subject to groundwater drawdown of more than 0.2 m under the baseline future based on the 95th percentile, compared to 0.6 km² based on the 50th percentile and 0.4 km² based on the 5th percentile (Table 23). The majority of the GDE area potentially subjected to groundwater drawdown under the baseline future is forested wetland. The majority of the GDE area potentially subjected to groundwater drawdown under the baseline future is subject to a drawdown of less than 2 m; 0.3 km²of forested wetland is potentially subject to a drawdown of 2 to 5 m under the baseline future.

Of GDEs, 1.1 km² is potentially subject to additional groundwater drawdown as a result of additional coal resource development based on the 95th percentile, compared to 0.5 km² based on the 50th percentile and 0.1 km² based on the 5th percentile (Table 24). The majority of the GDE area potentially subjected to additional groundwater drawdown is in the ‘Forested wetlands’ landscape class. No rainforest GDE is potentially subject to additional groundwater drawdown. All of the GDE area potentially subjected to additional groundwater drawdown is subject to a drawdown of less than 2 m.

3.4.4.3 Potential ecosystem impacts

Based on the modelling of the 95th percentile presented in Table 23 and Table 24, it was concluded that approximately an additional 1.1 km² of GDEs would be subjected to a groundwater drawdown of greater than 0.2 m but less than 2 m, and that most of the impact would be in the ‘Forested wetlands’ landscape class. As noted in Section 3.4.4.1, the water requirements of GDEs are poorly understood and there is large uncertainty as to the frequency, timing and duration of groundwater use within the Gloucester subregion. In general, transpiration of groundwater is expected to decline as the depth to groundwater increases, but the relationships between depth to groundwater and the structural and functional attributes of the vegetation communities are highly non-linear, with steep declines in leaf area index and biomass over a range of 5 to 10 m depth to groundwater.

Hence, for the ‘GDE’ landscape group in the Gloucester subregion, qualitative mathematical models of the ‘Forested wetlands’, ‘Wet sclerophyll forest’ and ‘Dry sclerophyll forest’ landscape classes were developed to make qualitative predictions about the impact of coal resource development on GDEs (see Sections 2.7.4.2, 2.7.4.3 and 2.7.4.4 of companion product 2.7 for the Gloucester subregion (Hosack et al., 2018b) for details of the models). GDE vegetation may form habitat for several threatened plant species included in the Gloucester subregion asset register (see companion product 1.3 for the Gloucester subregion (McVicar et al., 2015); Bioregional Assessment Programme, 2017; Bioregional Assessment Programme, Dataset 11), including the Charmhaven apple (Angophora inopina), white-flowered wax plant (Cynanchum elegans), leafless tongue orchid (Cryptostylis hunteriana), slaty redgum (Eucalyptus glaucina) and trailing woodruff (Asperula asthenes). In addition, GDE vegetation can form habitat for a range of vertebrate and invertebrate fauna. Examples of vertebrate fauna from the Gloucester subregion asset register listed under the Commonwealth’s Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) that might use GDE vegetation, either for habitat or feeding, include the grey-headed flying-fox (Pteropus poliocephalus), red goshawk (Erythrotriorchis radiatus), regent honeyeater (Anthochaera phrygia), swift parrot (Lathamus discolor), giant barred frog (Mixophyes iteratus), stuttering frog (Mixophyes balbus), Hastings River mouse (Pseudomys oralis) and the koala (Phascolarctos cinereus). Riparian vegetation, in particular, provides migration corridors for aquatic and terrestrial fauna and habitat for a range of threatened species, such as the spot-tailed quoll (Dasyurus maculatus ssp. maculatus).