Progress in Predicting the Aquatic Toxicity of Mixtures of the Major Ions Using Mechanistically Based Models

This exploratory work addresses the aquatic toxicity of mixtures of the seven major ions Ca, Mg, Na, K, Cl, SO4, and HCO3. Attempts to develop a water quality criterion for the concentration of any single ion (for example, Cl) and attempts to develop a criterion for the summation of ion concentra-tions (for example, conductivity or total dissolved solids) are confounded by the influence of the mixture composition on toxicity. Searching for unifying principles for explaining how composition affects toxicity, this work describes further progress in using transepithelial potential (TEP) calculated from Nernst Equation concepts, specifically the Spangler modi-fication of the Goldman, Hodgkin-Katz (GHK) Equation. It also considers whether the Strong Ion Difference approach might have potential for application. Results will be presented for model applications to a few thou-sand toxicity test treatments from studies of mayfly, daphnids, and fathead minnow. Model strengths and limitations will be described.