In this section, groundwater quality is reported in terms of either salinity (mg/L) or electrical conductivity (EC; µS/cm) as reported in the referenced documents. The readily available data pertaining to alluvial aquifers and basin aquifers are summarised in Table 6 and Table 7, respectively. A more comprehensive discussion on groundwater quality and groundwater chemistry will follow in the future report: Conceptual Modelling of the Clarence-Moreton bioregion. Groundwater quality data for the alluvial aquifers is more abundant and was compiled on a river basin basis as shown in Table 6. Data for the river basins in south-east Queensland were primarily sourced from two hydrogeological investigations conducted for the National Action Plan for Salinity and Water Quality (Pearce et al., 2007a, 2007b) as well as from the Queensland Department of Natural Resources and Mines database (2013). According to the Bureau of Meteorology (2010) no quality assured data is available for the New South Wales region. However, New South Wales groundwater quality data were compiled from a number of relevant studies (e.g. McKibbin and New South Wales Department of Land and Water Conservation, 1995; Brodie, 2007; Metgasco, 2007; New South Wales groundwater database, 2013).
Groundwater quality throughout the alluvial, volcanic and basin aquifers in the Clarence-Moreton bioregion exhibits a strong degree of spatial and temporal variability.
Alluvial and coastal aquifers
Typically, groundwater in the alluvium is fresher than groundwater in the sedimentary bedrock (Table 7), although there are areas in the alluvium where salinity can reach levels similar to those observed in the bedrock aquifers (e.g. Walloon Coal Measures). This is mainly due to discharge of groundwater from the underlying formations into the alluvial aquifers, longer residence time of infiltrating groundwater in the unsaturated zone during groundwater recharge within low permeability sediments and evapotranspiration from shallow watertables (Pearce et al., 2007a). Groundwater quality in the Lockyer Valley alluvium is spatially and temporally highly variable ranging from fresh to very saline (Pearce et al., 2007a). The primary controls of the spatial variability are the nature of connectivity of the alluvial aquifer with surface water and the underlying bedrock.
Near the headwaters of the Lockyer Creek and in the upper parts of the tributaries to the Lockyer Creek, groundwater is commonly fresh, marking the strong influence as a major source of recharge to the alluvial aquifer of the Main Range Volcanics which generally contain good quality groundwater (Raiber and Cox, 2012; Watkinson et al., 2013). Further down gradient, groundwater becomes more saline due to discharge from the underlying basin aquifers. Water in the Bremer/Warrill alluvium is mostly fresher than groundwater from the underlying sedimentary units (here primarily Walloon Coal Measures). A gradual increase in the EC of alluvial groundwater is observed in the down gradient river direction, that is, alluvial groundwater quality is better in the upstream direction (Pearce et al., 2007b). Similar to the patterns observed in the Lockyer Valley, the observed spatial changes in the Bremer/Warrill alluvial systems are due to the dominance of recharge to the alluvium from the Main Range Volcanics in the headwaters near the Great Dividing Range, and the progressively increasing influence of the connectivity with the Walloon Coal Measures down gradient.
The typical salinity in the Logan/Albert River alluvium is in the range of 500 to 2600 mg/L with a significant increase since 1950s in some parts (Queensland Water Resources Commission, 1991). The groundwater quality in the Richmond River and Clarence River alluvium is generally good (State of Catchments, 2010), however, this assessment is based on only limited quality assured data. In the Casino area, the highest beneficial groundwater use is for stock and agricultural purposes (Metgasco, 2007). According to Beale et al. (2004), although groundwater salinity data are not available for most of the Clarence River alluvium, some measured high salinity samples in creeks, such as Tooloom Creek, Washpool Creek, and Peacock Creek, corresponds with local high groundwater salinities.
The pH of groundwater in the alluvial aquifers is mostly around 7.0. However, in some areas, groundwater contained in sediments where acid sulfate soils in coastal or estuarine areas have developed can be characterised by low pH of approximately 3.5 (Johnston et al., 2004, 2009; Santos and Eyre, 2011). These ASS materials are often found at 1 to 2 m under the ground surface (Tulau, 1999). It has been confirmed that some pollution has been caused by the acid sulfate soil materials in the Richmond and Clarence river floodplains (Tulau, 1999; Santos et al., 2011). When the acid sulfate soils are exposed to air due to watertable fluctuation, sulfuric acid will be generated. Consequently, the pH of groundwater in these areas will decline. Furthermore, the decrease of pH can increase the solubility of many minerals and consequently may cause water salinity variation. However, this influence should be limited to shallow groundwater and surface water systems.
Bedrock aquifer systems
Water quality in bedrock aquifers ranges from fresh to very saline with typically higher variability than that in the overlying alluvial aquifers (Table 6). For example, groundwater contained in the Main Range Volcanics is generally fresh due to thin soil coverage and rapid recharge through the well-developed fracture network which only allows a limited degree of evapotranspiration to occur. In contrast, the salinity of groundwater in the Walloon Coal Measures can vary from 750 to 19,475 mg/L (Metgasco, 2007). The salinity distribution can be affected by a number of factors, such as lithological variability and the relative position within the basin, recharge processes, depth, residence time and interaction with surface water.
Near the Great Dividing Range, where bedrock aquifers are overlain by the Main Range Volcanics, groundwater quality in the Walloon Coal Measures and other bedrock aquifers is typically similar to the groundwater quality in the Main Range Volcanics, highlighting that recharge of the basin aquifers occurs primarily through the overlying Main Range Volcanics in these areas. Elsewhere, salinity in the basin aquifers is generally higher than that observed in alluvial aquifers as recharge primarily occurs through the thick clay-rich regolith profiles, which allow only small rates of recharge and which promote a high degree of evapotranspiration of infiltrating rainwater prior to recharge.
A notable exception to this is the Woogaroo Subgroup, which mostly contains groundwater of drinking water quality (Pearce et al., 2007b; Raiber and Cox, 2012). However, the data currently available cannot depict a complete spatial and temporal quality evolution across the Clarence-Moreton bioregion. This is due to the lack of groundwater observation bores especially in the deeper parts of the bedrock aquifers.
Only very limited information is available on the groundwater quality of basement aquifers. Electrical conductivities ranging from 1,000 to 10,000 μS/cm have been reported for the Neranleigh Fernvale Beds in south-east Queensland (Metgasco, 2007). Depending on the fracture network and regolith developed on the rocks, groundwater quality is likely to be highly variable.
An assessment of temporal variability of groundwater in the alluvial aquifers is currently limited to the Lockyer Valley. Here, data from Watkinson et al. (2013) and the groundwater database (Department of Natural Resources and Mines, 2013) demonstrate a distinct change in groundwater quality of the alluvial systems of the Lockyer Valley following the 2011 floods. During the drought, groundwater salinity had progressively increased in some areas due to a lack of surface water recharge and upwards discharge from the underlying bedrock into the alluvium induced by continuous pumping (Watkinson et al., 2013). Following the break of drought and the subsequent flooding in 2011, alluvial groundwater has become fresher in most parts of the river basin. For example, in the lower Lockyer Valley near the outlet, the electrical conductivity has increased to approximately 25,000 µS/cm, and following the flood, the electrical conductivity at the same location has decreased to approximately 2500 µS/cm (Watkinson et al., 2013). A similar influence of episodic flood events on groundwater recharge has been noted in other similar river basins in south-east Queensland (e.g. King et al., in press) and is also likely for other river basins with similar lithology within the Clarence-Moreton bioregion. In contrast, groundwater quality in the bedrock aquifers is less likely to be influenced by short-term climatic patterns.
Table 6 Salinity summary of the bedrock aquifers
Table 7 Salinity summary of the alluvial aquifers
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- 1.1.1 Bioregion
- 1.1.2 Geography
- 1.1.3 Geology
- 1.1.4 Hydrogeology and groundwater quality
- 1.1.5 Surface water hydrology and surface water quality
- 1.1.6 Surface water – groundwater interactions
- 1.1.7 Ecology
- Contributors to the Technical Programme
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