title: Thiocyanate, calcium and sulfate as causes of toxicity to Ceriodaphnia dubia in a hard rock mining effluent.
abstract: 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 (∼3000 mg l(-1)) total dissolved solids (TDS) composed primarily of Ca(2+) and SO(4)(2-). The effluent typically exhibits 6-12 toxic units (TUs) when tested with C. dubia. Phase I and II toxicity identification evaluations (TIEs) indicated Ca(2+) and SO(4)(2-) contributed only ∼4 TUs of toxicity, but this was likely an underestimate due to problems with simulating the supersaturated CaSO(4) concentrations in the effluent. Treatment of the effluent with BaCO(3) to precipitate Ca(2+) and SO(4)(2-) revealed that these ions contribute ∼6 TUs of the observed toxicity, but the remaining source(s) of toxicity (up to 6 TUs) remained unidentified. Subsequent investigations identified thiocyanate (SCN(-)) in the effluent at 100-150 μM. Toxicity tests reveal that C. dubia are sensitive to SCN(-) with an estimated IC25 of 8.3 μΜ for reproduction in moderately hard water suggesting between 12 and 18 TUs 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 10.4 TUs, suggesting that Ca(2+), SO(4)(2-) 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.
authors.affiliation: RSMAS, University of Miami, Miami FL, USA. kbrix@rsmas.miami.edu
authors.forename: Kevin V
authors.initials: KV
authors.lastname: Brix
authors.forename: Robert
authors.initials: R
authors.lastname: Gerdes
authors.forename: Martin
authors.initials: M
authors.lastname: Grosell
mesh_terms: D000818:Animals; D001464:Barium / Q000032:analysis / Q000633:toxicity; D002118:Calcium / Q000032:analysis / Q000633:toxicity; D002254:Carbonates / Q000032:analysis / Q000633:toxicity; D033361:Cladocera / Q000187:drug effects*; D003124:Colorimetry; D008906:Mining; D013431:Sulfates / Q000032:analysis / Q000633:toxicity; D013861:Thiocyanates / Q000032:analysis / Q000633:toxicity; D018675:Toxicity Tests; D014865:Waste Disposal, Fluid / Q000379:methods*; D014874:Water Pollutants, Chemical / Q000032:analysis / Q000633:toxicity*
chemical_list: D002254:Carbonates; D013431:Sulfates; D013861:Thiocyanates; D014874:Water Pollutants, Chemical; D001464:Barium; C006685:barium carbonate; C031760:thiocyanate; D002118:Calcium