Assessments of chemical mixtures via toxicity reference values overpredicthazard to Ohio fish communities

A diverse array of environmental data from Ohio were placed into a geograph ical information system (GIS). This GIS allowed for the investigation of ap proaches and paradigms currently advocated for ecological risk assessment. The paradigm of chemical mixture additivity was investigated in this projec t. Toxic units (toxic unit = concentration of a chemical in an organism/che mical concentration causing a specified effect) for 12 organic and 11 metal contaminants were calculated from 2878 fish samples collected at 1010 site s throughout the state of Ohio. Additive analysis of TUs for organic chemic als based on regulatory-based protective limits (toxicity reference value = USEPA water quality criterion*bioconcentration factor) overpredicted adver se effects to individual fish and fish communities. However, addition of or ganic chemical molar units did not overpredict adverse effects, thus, suppo rting the concept of baseline toxicity. Molar units of organic chemicals wi th diverse modes of action may be added together, so long as they are at co ncentrations below levels deemed protective of most species (e.g., 95%, wat er quality criterion). Analysis of metal TUs benchmarked against regulatory /based limits overpredicted adverse effects, whereas benchmark concentratio ns from population response (survival, growth, reproduction) data from the literature and Ohio reference site fish community responses corresponded be tter to field observations. Of the factors analyzed, habitat quality is the best single predictor of fish community integrity in Ohio, not body burden s of metals or organic chemicals.