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(21-01) Risk Assessment of Mixtures: Probabilistic Risk Assessment Approaches and Microcosm Validation. Solomon, Keith*,1, Chappel, Mark2, George, Tara3, Sibley, Paul1, Liber, Karsten3, 1 Centre for Toxicology, Guelph, ON, Canada2 Ontario Ministry of the Environment, Toronto, ON, Canada3 Centre for Toxicology, Saskatoon, SK, Canada ABSTRACT- Ecological risk assessment of mixtures requires consideration of both the sensitivity of receptor organisms to the substances as well as the spatial and temporal co-occurrence of these substances. Toxic equivalents (TEs) can be used to estimate total potency of mixtures with similar mechanism of action such as the triazine herbicides and the organophosphorus insecticides. Data for field measurements were used to determine total TEs which were then analyzed as distributions. For triazine herbicides, contribution of each of the components to the total potency of the mixture was estimated by subtracting the TEs for that particular substance from the total TEs for all triazines. The relative contribution of the each triazine to the total TEs differed by sampling region as a result of different use patterns. Based on all the sites, atrazine contributed only 36% of the total TEs. For the NAWQA sites in California, atrazine contributed <1% to the total TEs while, in Texas, atrazine contributed 91% to the total TEs. Similar approaches were use to assess toxicity of mixtures of chlorpyrifos, diazinon, and azinphos-methyl to fish and zooplankton in 12,000-L microcosms. Field measurements were used to characterize distributions of exposure which were used to generate greater exposure concentrations based on centiles >90. Zooplankton and fish in three replicated treatments of ternary mixtures, each dominated by a different insecticide were assessed for effects. The TE approach predicted morbidity and AChE inhibition in fish but population responses in different species of zooplankton responded differently, depending on which pesticide dominated the mixture. The implication of these results for PRA will be discussed. This research was supported by the Canadian Network of Toxicology Centres. Key words: mixtures, PRA, risk assessment, pesticides |
