Interactive effects of water hardness and humic acid on the chronic toxicity of cadmium to Daphnia pulex

Abstract

The chronic toxicity of Cd was reduced by an increase in water hardness from 58 to 116 mg/l; a further increase in hardness to 230 mg/l, however, had no further effect on chronic Cd toxicity. The most consistent and sensitive index of chronic Cd stress was the abortion of young. Based on this index, 0.75 or 1.50 mg/l humic acid had no effect on the chronic toxicity of Cd in either the soft or medium-hard test-waters. This was also true for 0.75 mg/l humic acid in hard water; however, the addition of 1.5 mg/l humic acid to hard water resulted in an increase in the chronic toxicity of Cd. In conjunction with Cd exposures greater than the chronic no-effect concentration, the addition of either 0.75 or 1.50 mg humic acid per liter to water of any of the three hardnesses usually increased the mortality rate over a 42-day exposure period. The mechanism by which humic acid increases the abortion rate in hard water and increases mortality rate in soft, medium-hard and hard water is unknown.