Major inorganic anions (i.e. SO42-, HCO3-, Cl-) and cations (i.e. Na+, Ca2+, Mg2+, K+) are ubiquitous in freshwater. The proportion of major ions present in freshwater rivers typically reflects underlying geology, although can be influenced by human activities such as the discharge of industrial wastewaters and inappropriate land management practices. Accordingly, the composition of major ions in freshwater can vary spatially and temporally. Freshwater biota occurs within an optimal range of ionic strength. An increase or decrease in ionic strength from their preferred range requires biota to balance their internal ion concentrations against an external gradient, placing them under osmotic stress. The toxicity of major ions varies between ions and their toxicity can be modified when present as a mixture.
Salinity impacts can be managed by establishing water quality guidelines within a risk assessment framework though spatial and temporal variation in major ion composition in surface waters and associated toxicity creates uncertainty when assessing the environmental risk posed by salinity. To be able to evaluate salinity risks effectively there is a need for an improved understanding of how changes in ionic composition and strength affect freshwater biota. This thesis considers key knowledge gaps relevant to the management of anthropogenic salinity impacts in Northeast Australia. The focus areas of this thesis are:
1. the ability of macroinvertebrates to adapt to elevated background ionic strength and composition as well as the tolerance of macroinvertebrates to salinity,
2. the evaluation of the differences between the use of standard test exposures and a simulated ionic composition representative of natural waters to assess the tolerance of freshwater macroinvertebrates to salinity increases,
3. the toxicity of sulfate as the dominant ion present in coal mine effluent discharge to streams, and
4. the possible sub-lethal effects of water (e.g. from Coal Seam Gas discharges) with low electrical conductivity to the freshwater prawn Macrobrachium australiense.
Based on current understanding of the factors influencing the tolerance of freshwater biota to major ions, it was postulated that species tolerance may be influenced by their prior exposure to a particular salinity regime (i.e. tolerance is not consistent between catchments with different background electrical conductivity and ion composition). As a group, macroinvertebrates are generally more salinity sensitive than other taxonomic groups such as vertebrates or plants. Deriving thresholds of effect for macroinvertebrates is therefore likely to provide protection for other groups and accordingly, they were selected as appropriate test organisms for use in this study. To investigate the ability of macroinvertebrates to adapt to elevated background ionic strength and composition, it was hypothesised that macroinvertebrate tolerance would not be consistent between Northeast and Southeast Australia due to the influence of background salinity. To test this hypothesis, the 72-h acute salinity tolerance of 102 aquatic macroinvertebrates representing a diverse range of invertebrates present in two different climatic regions across Queensland was determined and compared with previous studies in Southeast Australia. This allowed a comparison of macroinvertebrate salinity tolerance between Northeast and Southeast Australia where both the salinity and ionic composition of freshwaters can be different. The tolerance of macroinvertebrate species were found to be consistent between regions of Australia therefore toxicity data collected for a species in one region will be applicable to the assessment of salinity risk in another regardless of collection location. However, as macroinvertebrate communities in each region can comprise of different species, it will be necessary to develop species sensitivity distributions representative of the species present in any region to provide adequate protection at the community level.
As the composition of ions is known to vary spatially in freshwaters and the toxicity of major ions is also dependent on the type of ions present, it was hypothesised that the use of standard test exposures such as synthetic marine salts would be different compared with a test exposure representative of surface water ion compositions at the river basin scale. A test diluent representative of observed surface water ionic composition was defined and its toxicity was evaluated. Results showed that ionic composition of simulated natural waters can differ from standard marine salts hence a test exposure representative of observed ionic composition, provides a useful basis for evaluating salinity risk.
The toxicity of sulfate was assessed as it is commonly present in high concentrations in coal mine effluents and there is a lack of understanding of its potential impacts on freshwater biota in the Southern Hemisphere. Accordingly there is a need to develop toxicity trigger values for sulfate to provide a basis for its management in those aquatic ecosystems. It has been well established in the literature that sulfate toxicity is affected by water hardness. However, the reported effect of water hardness on sulfate toxicity is different between some toxicity test end-points and species hence there is a need for more toxicity information to support the development of a hardness correction for sulfate. In order to develop trigger values for sulfate there is a need to consider the background ionic composition and water hardness relevant to these waters. Therefore the use of standard test diluents may not be representative when the results are applied to assess the risk of increased sulfate concentrations in natural waters. To account for variation in major ions present in natural waters, this study simulated patterns of surface water ion composition to define an environmentally representative test exposure to test the toxicity of sulfate. A toxicity trigger value was derived using water hardness and calcium concentrations typical of the Fitzroy River basin and was found to be comparable to those derived internationally when considering water hardness. This suggests the use of an ion composition representative of natural waters as used in this study provides an effective means of accounting for factors modifying sulfate effects including hardness.
The potential impacts of low electrical conductivity and ion deficiency associated with the discharge of treated Coal Seam Gas (CSG) water permeates to streams was also assessed. The impacts of low electrical conductivity are not well understood and there is a lack of targeted monitoring tools that may be used to detect such impacts. It was hypothesised that decreased ion strength and altered ion composition associated with CSG water post mixing with river water would have minimal sub-lethal effects. This hypothesis was tested using oxidative stress biomarkers in the native freshwater prawn Macrobrachium australiense. Specimens of M. australiense were collected upstream and downstream of two CSG permeate discharge points to streams and exposed to a simulated CSG permeate using laboratory-based toxicity testing. Results showed a biomarker response in the field environment, though the same response was not observed under laboratory conditions. This suggests that M. australiense did not have an oxidative stress response to the primary stressor associated with CSG permeates and were not suitable as a monitoring tool. The response of M. australiense in the field may have been due to the combined effect of stressors; however, further research is required to confirm this. There is also a need to evaluate the response of additional test species and biomarkers to water with low electrical conductivity.