A model has been developed to account for the variability in trace-metal toxicity to fishes at different values of alkalinity, hardness, and pH. The model utilizes trace-metal speciation, gill surface interaction, and competitive inhibition to predict effective toxicant concentration (ETC). Copper, cadmium, lead, and zinc bioassay data have been utilized.

Copper (Cu) is highly complexed by carbonate and hydroxide ions in natural waters and this complexation determines the concentration of copper species in solution. Results of detailed equilibrium calculations on data from bioassays where alkalinity, pH, hardness, and total copper concentration are different indicate that copper(II) is the chemical species that is toxic to fishes and that alkalinity is the factor controlling copper(II) concentration.

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A soil fulvic acid bas been characterized and the complexation stability constants with cadmium have been evaluated. The average gram formula weight of the fulvic acid is 2775. Three complexes of stoichiometry CdFA, Cd2FA, and Cd3FA have been observed. The respective conditional stability constants at pH 5.7 are 105.3, 109.8, and 1014.0; at pH 6.7 they are 105.6, 1010.6, and 1015.5; and at pH 7.7 they are 106.0, 1010.7, and 1015.4.