Once sufficient credible real-world monitoring data has been accumulated for a pesticide (class), the resulting distribution of concentrations/detection frequencies provides a perspective on real world surface water exposure to provide context for Estimated Environmental Concentrations (EECs) generated using standard FIFRA Tier II models/scenarios. For pyrethroids, the monitoring data from agricultural catchments reflect worst-case ranges of exposure settings; the high percentile concentrations from these distributions are lower than the corresponding simulated Tier II EECs by several orders of magnitude for agricultural use patterns. Development of a pyrethroid-specific conceptual model of potential aquatic behavior indicates that some chemical class-specific environmental fate factors account for a portion of these discrepancies. However, detailed analyses of over 30 sources of potential uncertainty inherent in standard Tier II agricultural crop scenarios (including the percent of crops grown at the catchment scale, the density of crops treated and the intrinsic erosion/runoff vulnerability of scenarios) can be shown to contribute substantially to the over-prediction by Tier II exposure modeling approach. For agricultural uses, it can be shown that, when even a few of these factors co-occur in a catchment, even refined EECs will exaggerate real-world exposures by 5 to 20-fold and often by a much greater margin while still retaining considerable conservatism. A similar analysis was performed for potential sources of uncertainty in residential exposure assessments. One key finding is that the current modeled EECs are not incorrect, they simply reflect concentrations with a much lower probability of occurrence than the standard FIFRA protection goal of a return frequency of 1 year in 10 in a 90<sup>th</sup> percentile vulnerability use area for a particular crop. This highlights the importance of developing clear regulatory protection goals for surface water and ensuring that the standard models and scenarios reflect those goals. Equally, it is critical for risk managers to appreciate the real-world relevance of EECs generated using current Tier II scenarios.

Why are real-world surface water exposure distributions much lower than US EPA FIFRA Tier II EECs? An evaluation based on pyrethroid insecticides

253rd National Meeting (2017)

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