Effect of water hardness on the chronic toxicity of nitrate to Ceriodaphnia dubia and Oncorhynchus mykiss

We conducted chronic toxicity testing to evaluate the effect of water hardness on the toxicity of nitrate to freshwater invertebrate (Ceriodaphnia dubia) and fish (Oncorhynchus mykiss) test species in formulated soft laboratory waters (≤40 mg/L as CaCO3) and field-collected base waters of high to very high hardness (167-709 mg/L as CaCO3). The most sensitive endpoints were C. dubia reproduction (20% effects concentration [EC20] 2.7-63 mg/L as N) and O. mykiss normal larval development (EC20 11-79 mg/L as N). Both species exhibited reduced nitrate toxicity with increasing hardness. However, both species also exhibited additional unexplained variation in sensitivity among test waters and between our field-collected base waters and previous studies in formulated waters. We explored the hypothesis that differences among test waters and between studies could be explained by a second toxicity-modifying factor, superimposed on the effect of hardness, that further ameliorated nitrate toxicity. Our results demonstrate the magnitude of error that can occur in adopting generic laboratory test results to characterize risk at a site and highlight the value of testing in site waters to reduce that error.