U.S. EPA’s Office of Pesticide Programs (OPP) uses a tiered approach in its aquatic exposure assessments to determine whether pesticides that are applied according to label directions can result in concentrations in water that may adversely impact human health or aquatic organisms. In the initial tiers, watersheds are represented by combinations of soil, crop, weather, and hydrological factors that are expected to contribute to high-end pesticide concentrations in water. This is intended to distinguish between pesticides and/or uses that are not likely to be of potential concern and those that may exceed toxicity thresholds under certain conditions. OPP is updating the scenarios used to represent these watersheds for consistency and better representation of vulnerable conditions within the crop area. If estimated aquatic exposures indicate a potential for adverse effects, the assessment may need to characterize the likelihood of occurrence, including the range in magnitude of exposure, the frequency of exceeding toxicity thresholds, the likely locations of exposures that exceed the thresholds, and the degree of certainty in the risk assessment. The role of watershed models in characterizing the spatial and temporal magnitude and extent of exposure are explored.

Methods for representing watersheds in a tiered approach for pesticide risk assessments

2019 Fall National Meeting

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