Using a Risk-based Approach for Derivation of Water Quality Guidelines for Sulphate

Sulphate is a major salt component in acid mine drainage and a crucial ecological concern in most coal and gold mining regions, globally. However, there remains a paucity of data on sulphate salinity toxicity on freshwater taxa. In this study, we hypothesised sensitivity differences for five freshwater species (Adenophlebia auriculata, Burnupia stenochorias, Caridina nilotica, Pseudokirchneriella subcapitata, and Oreochromis mossambicus) to increasing sulphate salinity concentrations after 240 h of exposure. Species sensitivity distributions (SSDs) were used to rank the sensitivity of tested species to the inorganic sulphate salts, which included magnesium sulphate (MgSO4), sodium sulphate (Na2SO4), and calcium sulphate (CaSO4) as models of mining salinisation in South Africa. The SSDs were also used to estimate appropriate protective concentrations of the salts for the tested species. Sensitivity differences were measured and Na2SO4 was the most toxic of the tested salts. A concentration of 0.020 g/L Na2SO4, 0.055 g/L CaSO4, and 0.108 g/L MgSO4 or a combined salts limit of 0.067 g/L would be protective of 95% of the populations of the five species tested; these all suggest that the 0.25 g/L compliance limit for South Africa is insufficient. Future studies should incorporate more species in the SSD approach to be coupled by field validations to further improve the ecological relevance of these findings.