Changes in aquatic toxicity of potassium dichromate as a function of water quality parameters

Abstract Potassium dichromate(K 2 Cr 2 O 7 ) is used as a general reference toxicant in aquatic toxicity testing, but relatively little is known regarding the effects of water quality parameters on K 2 Cr 2 O 7 toxicity to Daphnia magna. The acute toxicity of K 2 Cr 2 O 7 to D. magna was comparatively examined in one very hard (M4 medium for D. magna assay), four hard, one moderately hard and one soft dilution water samples. The 48-h EC 50 (50% effective concentration) of K 2 Cr 2 O 7 to D. magna was reproducible (coefficient of variation [CV]: 13%) in tests using the same dilution water sample, but reproducibility was poor (CV: 62%) in tests using seven different dilution water samples. The observed 48-h EC 50 value increased with increasing water hardness (28–250 mg CaCO 3 /L) and Na + concentration (4.3–19.7 mg Na/L). The effect of Ca 2+ and Mg 2+ on K 2 Cr 2 O 7 toxicity was equivalent in terms of molar concentration. The 48-h EC 50 for K 2 Cr 2 O 7 was determined according to OECD TG 202 by six contract laboratories using M4 medium and were shown to be reproducible (CV: 15%), indicating that the toxicity level can be determined with high accuracy if holding and dilution water samples are standardized. Multiple regression analysis revealed that the 48-h EC 50 was strongly correlated ( r 2 = 0.927) with the Ca 2+ , Mg 2+ , Na + concentration, and alkalinity of the dilution water samples. Detailed monitoring of water quality characteristics thus facilitates intra- and inter-laboratory comparisons of toxicity data and enables predictions of changes in the susceptibility of test animals.