THE IMPORTANCE OF TRACE-METAL SPECIATION TO WATER-QUALITY CRITERIA

Because the bioavailability of a trace metal, and consequently its tox icity, is dependent on the physical and chemical form of the metal, we have presented a detailed assessment of how speciation of copper woul d be expected to affect its toxicity. Principles of chemical speciatio n are applied to demonstrate that inorganic forms will be in constant proportion to each other and to free copper ion during the course of t he titration of a sample of natural water with copper or in the variou s treatments in a toxicity test conducted at constant pH and alkalinit y. Binding of copper to dissolved organic matter or to suspended parti culate matter may render the copper nonbioavailable. We have considere d a simple complexation model to describe the complexation of copper t o soluble ligands. Naturally occurring dissolved organic matter is pre sent at concentrations only slightly greater than that of copper. Cons equently, titration of water with copper results in a nonlinear relati onship between the concentration of copper present as free copper ion plus inorganic copper species. The effects of stability constant of th e complex, concentration of ligand, and the total copper concentration are evaluated. We have related bioavailable copper to the concentrati on of free copper ion plus inorganic copper complexes, which is valid if the pH and alkalinity of the waters used to develop a criteria are not different. On the basis of limited field data for the complexation of copper in Narragansett Bay water, we do not expect that significan t differences in water quality criteria (WQC) would result if the crit eria were to be based on free copper ion plus inorganic copper complex es rather than total copper concentrations. We examined the effect of speciation of copper in different waters as related to empirical or th eoretically calculated water effect ratios (WER). We show that, on the basis of sound chemical principles, it would be expected that the mos t sensitive organisms would have the greatest WER. This prediction is confirmed by the empirical observations available. For insensitive org anisms, knowledge of the concentration of ligand is sufficient to reas onably predict the WER. However, for the more sensitive organisms that give higher WERs, it is necessary to measure or calculate the speciat ion of copper to predict the WER. Use of predicted WERs may replace us e of empirically derived WERs as is now part of regulatory guidance fo r derivation of site-specific WQC, if correspondence has been demonstr ated.