Within any endangered species risk assessment framework, quantitative pesticide exposure and risk modeling is a powerful tool to make the important distinction between threatened and endangered species that are likely to be or not likely to be adversely affected by the action (registration of a pesticide). A multi-tiered approach for aquatic exposure modeling in national scale endangered species risk assessments was developed, where the types of waterbodies evaluated range from small ponds with a single field fed by a 10 ha watershed to very large flowing waterbody systems with watersheds greater than 10,000 km<sup>2</sup>. The process begins with a  highly conservative, Tier 1, approach. At Tier 2, spatial crop and usage data is incorporated into the exposure modeling in three refinement steps. The first refinement accounts for potential pesticide use sites, or percent cropped area (PCA). The second refinement step is based on best available pesticide usage data resulting in multiple realizations of use footprints that are incorporated into the exposure models as percent crop treated (PCT) values. In both of these refinement steps, watershed specific estimated environmental concentrations (EECs) are calculated by summing the PCA or PCT adjusted 1-in-15-year annual maximum use pattern-specific concentrations to determine the multi-use pattern aggregated EECs for the watershed and waterbody. While this approach is efficient because no new model runs have to be conducted, it conservatively assumes that the peak events from different use pattern and crops are occurring at the same time (maxima are added even if they are not coincident in time). Thus, the third refinement step of Tier 2 accounts for the differences in application timing between use patterns by first creating a PCA/PCT-adjusted time series of the loadings entering the waterbody originating from multiple use patterns throughout the watershed and then using it as input for the receiving water model. This approach is in line with the highest tier assessment proposed by EPA when integrating PCA and PCT into drinking water assessments. This presentation will provide an overview of the impact of this refinement step. The results show that this step can be a significant refinement but is dependent upon the number of different use pattern present in the watershed, often a function of the watershed size.

Aggregation of multiple pesticide use patterns to estimate aquatic exposure at different scales

ACS Fall 2021

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