National characterization of pesticide runoff and erosion potential to put USEPA standard ecological scenarios in context for pyrethroids.

Decision-making for pesticide registration by the US Environmental Protection Agency (USEPA) relies upon crop-specific scenarios in a tiered framework. These standard modeling scenarios are stated to represent "…sites expected to produce runoff greater than would be expected at 90% of the sites for a given crop/use." This study developed a novel approach to compare the pesticide runoff + erosion (Sum ) mass flux potential of a hydrophobic chemical using 36 of these ecological regulatory scenarios with national-scale distributions of modeled Sum from over 750 000 USA-wide agricultural catchments to provide real-world context for the simulated transport predictions used for regulatory decision-making. For the standard scenarios and national scale modeling, "edge of field" Sum mass flux was estimated using regulatory guidance for a hypothetical pyrethroid. The national-scale simulations were developed using publicly available soil, hydrography, and crop occurrence /regional timings databases. Relevant soil and crop combinations identified by spatial overlay along with weather data were used in a regulatory model to generate daily Sum estimates, which were assigned to the catchments. The resulting average annual total Sum mass fluxes were ranked to produce distributions to compare with the standard regulatory scenario outputs. These comparisons showed that Sum flux from 25 of the 36 USEPA ecological regulatory crop-specific scenarios modeled ranked above the 99 percentile of pyrethroid runoff + erosion vulnerability from any catchment growing that crop; Sum flux from six scenarios was more severe than any catchment. For 12 USEPA regulatory scenarios, the resulting eroded sediment corresponds to highly erodible land (HEL), which the US Department of Agriculture mandates should not be cropped without substantial additional erosion prevention controls for sustainability. Since the pesticide regulatory framework already incorporates many acknowledged assumptions to ensure it conservatively meets protection goals, these HEL observations suggest that the standard scenarios overestimate potential aquatic exposure and that the regulatory process is more protective than intended. Integr Environ Assess Manag 2023;19:175-190. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).