A series of Toxicity Identification Evaluations (TIEs) to identify the cause(s) of observed toxicity to Ceriodaphnia dubia have been conducted on a hard rock mining effluent. Characteristic of hard rock mining discharges, the effluent has elevated (~3000mgl-1) total dissolved solids (TDS) composed primarily of Ca2+ and SO42-. The effluent typically exhibits 6-12 toxic units (TUs) when tested with C. dubia. Phase I and II toxicity identification evaluations (TIEs) indicated Ca2+ and SO42- contributed only ~4TUs of toxicity, but this was likely an underestimate due to problems with simulating the supersaturated CaSO4 concentrations in the effluent. Treatment of the effluent with BaCO3 to precipitate Ca2+ and SO42- revealed that these ions contribute ~6TUs of the observed toxicity, but the remaining source(s) of toxicity (up to 6TUs) remained unidentified. Subsequent investigations identified thiocyanate (SCN-) in the effluent at 100-150γM. Toxicity tests reveal that C. dubia are sensitive to SCN- with an estimated IC25 of 8.3γ for reproduction in moderately hard water suggesting between 12 and 18TUs of toxicity in the effluent. Additional experiments demonstrated that SCN- toxicity is reduced in the high TDS matrix of the mining effluent. Testing of a mock effluent simulating the major ion and SCN- concentrations resulted in 10.4TUs, suggesting that Ca2+, SO42- and SCN- are the three toxicants present in this effluent. This research suggests SCN- may be a more common cause of toxicity in mining effluents than is generally recognized.
- Ceriodaphnia dubia
- Toxicity identification evaluation
ASJC Scopus subject areas
- Health, Toxicology and Mutagenesis
- Public Health, Environmental and Occupational Health