COMPARATIVE ACUTE TOXICITY OF A SYNTHETIC MINE EFFLUENT TO CERIODAPHNIA-DUBIA, LARVAL FATHEAD MINNOW AND THE FRESH-WATER MUSSEL ANODONTA-IMBECILIS

To determine if water quality-based effluent limitations recommended f or a proposed discharger would provide adequate protection of aquatic life, site-specific acute toxicity tests were performed. The proposed discharger studied was a mine to be located in northern Wisconsin. The mine's discharge would flow into the Flambeau River, a relatively pri stine river that harbors endangered freshwater mussels. The toxicity e ffects of synthetic effluents, representing a mixture of five differen t metals under contrasting conditions of hardness, were tested on seve ral different organisms. Results indicated that Ceriodaphnia dubia exh ibited a much greater sensitivity to various mixtures of cadmium, hexa valent chromium, copper, nickel, and zinc than did either larval fathe ad minnows (Pimephales promelas) or the freshwater mussel Anodonta imb ecilis. Mixtures of metals at the permitted discharge levels of indivi dual metals were also found to be consistently toxic to C. dubia. Beca use C. dubia was found to be much more sensitive than A. imbecilis, an effluent safe for C. dubia should also be safe for A. imbecilis and t he physiologically similar endangered mussels found in the river. Base d on such testing, aquatic life toxicity-testing conditions and criter ia suggested for the National Pollution Discharge Elimination System ( NPDES) permit should be protective of C. dubia and, hence, other aquat ic life such as endangered mussels. Overall, effluent limitations for proposed dischargers need to be based on the combined effect of comple x effluents. Aquatic toxicity testing using synthetic effluents that a pproximate the expected discharge characteristics provides a viable ap proach to determining the limits for proposed dischargers of multiple pollutants.