Abstract
The salinisation of freshwater systems is an urgent ecological issue (Cañedo Argüelles et al., 2013; Cañedo-Argüelles et al., 2016; Cañedo-Argüelles et al., 2019; Kefford et al., 2016b). Anthropogenic sources such as dryland salinity, industrial and agricultural runoff, and coalbed methane (CBM) produced waters are increasingly contributing to this problem (Williams et al., 1999; Sauer et al., 2016; Cañedo Argüelles et al., 2013; Peck and Hatton, 2003). These sources can negatively impact stream species and family richness, especially invertebrates, which are highly sensitive to changes in salinity (Cañedo Argüelles et al., 2013; Hart et al., 1991). CBM produced waters, in particular, pose a significant threat due to their unique ionic composition, which includes higher bicarbonate concentrations compared to other saline effluents (Patz et al., 2004; Veil et al., 2004; Jackson and Reddy, 2007; Hamawand et al., 2013; Wright, 2012).This thesis addresses critical knowledge gaps concerning the acute toxicity of CBM-produced waters to freshwater invertebrates. Through a series of experiments, we investigated the specific ionic contributions of these saline effluents to receiving streams. Our findings indicate that the major ions in CBM discharge waters, especially bicarbonate, are more toxic to freshwater invertebrates than other commonly studied sources of salinity, such as synthetic marine salts (SMS) and NaCl (Gros et al., 2008a; Vera et al., 2014b). The toxicity of these waters increases with higher bicarbonate concentrations, highlighting the need for toxicological studies specific to the ionic compositions of CBM waters.
We hypothesised that waters containing high bicarbonate concentrations would be more toxic to freshwater invertebrates than SMS. Our experiments supported this hypothesis, showing that bicarbonate-based waters were consistently more toxic than either SMS or NaCl. Additionally, we hypothesised that bicarbonate was the primary driver of toxicity in CBM waters. While this hypothesis was supported, we also found that moderate concentrations of magnesium and chloride could mitigate the toxicity of bicarbonate to freshwater invertebrates.
The lack of water-quality guidelines for bicarbonate-rich saline waters currently impedes decision-making processes by both regulators and industries worldwide. Reliable toxicity data with sensitive local species are essential to improve water-quality guideline values, guiding industry practices and providing confidence to regulatory authorities and the community that risks are appropriately managed. Our research contributes to this by demonstrating that bicarbonate-based waters are significantly more toxic to freshwater invertebrates than those based on NaCl or seawater. This finding underscores the necessity for guidelines that account for specific ions and ionic proportions to ensure desired ecological outcomes (Nielsen et al., 2003; Prasad et al., 2014; Cañedo-Argüelles et al., 2016; James et al., 2003).
Furthermore, our work has important implications for managing coal seam gas (CSG) extraction and conventional coal mining, both of which produce large volumes of often saline wastewater (Davies et al., 2015; Hamawand et al., 2013). These waters are sometimes released into waterways without desalination, posing significant risks to aquatic ecosystems.
Even when desalinated, the potential negative impacts on biota are not well understood. Our findings suggest that measuring salt compositions, including bicarbonate alkalinity, hardness, and chloride, as additional regulatory parameters for saline water discharges, would offer more protection for aquatic invertebrate assemblages in receiving streams compared to conductivity limits alone.
The ecotoxicological data presented in this thesis also contribute to the Species Sensitivity Distribution (SSD) database for bicarbonate, which is valuable in risk assessments and determining appropriate guidance values for protecting species within ecosystems. Our SSDs include a representative group of taxa commonly found across eastern Australia, providing a robust basis for regulatory decisions.
Additionally, we identified a strong direct relationship between the acute toxicity of SMS and NaHCO3 to several freshwater invertebrate species. This suggests a potential predictive relationship between the toxicity of bicarbonate based waters associated with coal-bed geologies and more understood seawater-based waters. This correlation presents a valuable starting point for future research and regulation, potentially allowing for quicker and less resource-intensive management approaches in the early phases of research.
While our work primarily involved acute laboratory toxicological studies, these findings lay the groundwork for longer-term studies in natural or semi-natural conditions. Further research should evaluate the potential impact on freshwater invertebrates at different life stages and consider the interactions with other stressors, such as heavy metals, which are often key constituents of mine water discharges.
By collaborating with the NSW Department of Planning, Industry, and Environment, this thesis not only contributes to the scientific knowledge base but also increases the likelihood that our findings will be utilised to improve environmental management practices. This new knowledge will enable regulatory authorities to form solid, objective decisions on whether to license discharges and under what conditions, ultimately enhancing the protection of freshwater ecosystems.
| Date of Award | 2025 |
|---|---|
| Original language | English |
| Supervisor | Ben KEFFORD (Supervisor) & Bill MAHER (Supervisor) |