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Chronic toxicity tests using Ceriodaphnia dubia are conducted across the United States by facilities releasing wastewaters into bodies of freshwater, as well as by the appropriate agencies governing environmental protection. Until recently, the chronic values used for decision making were the “No Observable Effect Concentration” (NOEC), the “Lowest Observable Effect Concentration” (LOEC) and the “Chronic Value” (ChV). Currently, “Inhibition Concentrations of a Given Percent” (ICp) are being considered, for effluent permitting, by a number of states. The ICp value is the concentration at which the reproduction of C. dubia is inhibited by “p” percent. It has been asserted that the IC25 is approximately equivalent to the NOEC and that the IC50 is similar to the ChV. Using data collected in a series of chronic toxicity tests, these values were compared. The results came from 3-brood Ceriodaphnia Survival and Reproduction tests that had been performed with 10 different metal cations. Results showed that the IC25 values were similar to the NOEC, LOEC and the ChV. However, the IC25 correlated best with the ChV, having only 7 (28%) of the values differing by more than 20%. In contrast, the LOEC and NOEC had 12 (48%) and 18 (72%) values, respectively, that were not within 20% of the IC25. Likewise the LOEC and IC50 were most comparable to each other, with only 2 (8%) values differing by a factor of two or more, while the ChV had 9 (36%) and the NOEC had 12 (48%) such values. Finally the IC10 was the most similar to the NOEC for these experiments, with only 6 (24%) values differing by more than 20%. This implies that the more recent philosophy of regulating to the IC25, instead of the NOEC, may not be as protective of aquatic life, if the wastewater contains metals.
chronic values, Ceriodaphnia, metals, inhibition concentrations, hypothesis testing, regulations
Assistant Professor, Columbus State University, Columbus, GA
Professor, University of Kentucky, Lexington, KY