Assessing the Toxicity of Mine-Water Mixtures and the Effectiveness of Water Quality Guideline Values in Protecting Local Aquatic Species.

Six tropical freshwater species were used to assess the toxicity of mine waters from a uranium mine adjacent to a World Heritage area in northern Australia. Key contaminants of potential concern for the mine were U, Mg, Mn, and total ammonia nitrogen (TAN). Direct toxicity assessments were carried out to assess whether the established site-specific guideline values for individual contaminants would be protective with the contaminants occurring as mixtures. Metal speciation was calculated for contaminants to determine which were the major contributors of toxicity, with 84 to 96% of Mg predicted in the free-ion form as Mg2+ , and 76 to 92% of Mn predicted as Mn2+ . Uranium, Al, and Cu were predicted to be strongly bound to fulvic acid. Uranium, Mg, Mn, and Cu were incorporated into concentration addition or independent action mixture toxicity models to compare the observed toxicity in each of the waters with predicted toxicity. For >90% of the data, mine-water toxicity was less than predicted by the concentration addition model. Instances where toxicity was greater than predicted were accompanied by exceedances of individual metal guideline values in all but one case (i.e., a Mg concentration within 10% of the guideline value). This indicates that existing individual water quality guideline values for U, Mg, Mn, and TAN would adequately protect ecosystems downstream of the mine. Environ Toxicol Chem 2021;40:2334-2346.