AN ECOLOGICAL RISK ASSESSMENT METHOD FOR SPECIES EXPOSED TO CONTAMINANT MIXTURES

The method developed here provides a quantitative, objective measure o f ecological risk for natural populations exposed to mixtures of chemi cal contaminants. It is founded on generally accepted risk assessment concepts: use of toxic units to assess the joint toxic effects of mixt ures and expression of ecological risk as a relationship between toxic ological end points and estimated environmental concentrations. Toxico logical end points may be regulatory levels with zero variance and spe cies-dependent concentrations with estimates of variance. Risk is the probability that a linear combination of toxic units exceeds 1, which expresses the probability that a measurement end point (e.g., 50% mort ality in 96 h) will occur. Computations have three variations. One add resses concentration addition, in which chemicals act independently to produce similar biological effects. For noninteractive joint action w ith no addition, in which the biological response to the mixture is no t significantly different from the response to the most toxic componen t, the method reduces to an analysis of extrapolation error. For other noninteractive joint action-antagonism, partial addition, and supra-a ddition - a correction factor similar to Konemann's mixture toxicity i ndex is applied. An initial validation using published data indicated that increased in situ striped bass mortality was generally associated with elevated risk estimates. The method is applicable to many organi sms and toxicant mixtures.