Comparative acute toxicity of a synthetic mine effluent to Ceriodaphnia dubia, larval fathead minnow and the freshwater mussel Anodonta imbecilis

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Abstract
To determine if water quality‐based effluent limitations recommended for 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 pristine river that harbors endangered freshwater mussels. The toxicity effects of synthetic effluents, representing a mixture of five different metals under contrasting conditions of hardness, were tested on several different organisms. Results indicated that Ceriodaphnia dubia exhibited a much greater sensitivity to various mixtures of cadmium, hexavalent chromium, copper, nickel, and zinc than did either larval fathead minnows (Pimephales promelas) or the freshwater mussel Anodonta imbectlis. Mixtures of metals at the permitted discharge levels of individual metals were also found to be consistently toxic to C. dubia. Because 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 the physiologically similar endangered mussels found in the river. Based on such testing, aquatic life toxicity‐testing conditions and criteria suggested for the National Pollution Discharge Elimination System (NPDES) permit should be protective of C. dubia and, hence, other aquatic life such as endangered mussels. Overall, effluent limitations for proposed dischargers need to be based on the combined effect of complex effluents. Aquatic toxicity testing using synthetic effluents that approximate the expected discharge characteristics provides a viable approach to determining the limits for proposed dischargers of multiple pollutants.

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