COPPER LETHALITY TO RAINBOW-TROUT IN WATERS OF VARIOUS HARDNESS AND PH

The 96 h median lethal concentration (LC50) of total dissolved copper varied from 20 μg 1-1 in soft acid water to 520 μg l-1 in hard alkaline water, in tests with hardness ranging from 30 to 360 mg l-1 as CaCO3 and pH from 5 to 9. The 3-dimensional response surface was complex, although an increase in hardness usually made copper less toxic. A good prediction of copper LC50 at usual combinations of hardness and pH was given by the equation: LC50 = antilog (1.933 + 0.0592 PT + 0.4912 HT + 0.4035 PTHT + 0.4813 P2T + 0.1403 H2T. The transformed variables are Hr= Log10 hardness-2.01671 0.62308 and Pr= pH -7.0 2.0. A somewhat less accurate equation is provided for extreme combinations of hardness and pH. Trout of 10 g weight were 2.5 times more resistant than 0.7 g trout. Effect of size was apparently the same at different combinations of hardness and pH, and was predictable by an equation of the form LC50 = Constant × Weight 0.348. Ionic copper (Cu2+) and two ionized hydroxides (CuOH+ and Cu2OH2+2) seemed to be the toxic species of copper, since they yielded the smoothest response surface with the best fit to measured LC50's. The sum of these ions produced LC50's ranging from 0.09 μg l-1 copper in soft alkaline water to 230 μg l-1 in hard acid water. The ions were different in relative toxicity, or became more toxic at high pH, or both. © 1978.