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

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