Challenges in ecological risk assessment, including the evaluation of endangered species, require understanding exposure throughout networks of flowing water systems at the watershed scale. In these complex flowing water networks, the potential for exposure is largely driven by the connectivity and travel times of the upstream treated areas to the flowing water segments of interest. Current methods used in regulatory aquatic pesticide exposure modeling are based on a simplistic conceptual model of a single treated field adjacent to a static receiving water body. In flowing water bodies, however, upstream treated agricultural fields can be a significant distance to the receiving water body of interest, directly impacting time-dependent exposure profiles. This paper will focus on a methodology that takes advantage of nationally available NHDPlus watershed connectivity and hydrology data to reduce the uncertainty in potential runoff and drift contributions to aquatic exposure throughout flowing water networks. The methodology considers treated agricultural areas within NHDplus catchments independently to account for travel time of the source loadings to the flowing water segment of interest to realistically account for time delay and decay of upstream pesticide loadings. The approach represents a significant conceptual improvement in comparison to the existing screening level methodology and a simplification of process-based pesticide stream routing models with the advantage of using standard regulatory aquatic exposure models. Comparisons of simulated concentrations with monitoring data will demonstrate the more realistic results achieved. The approach developed can be combined with the nationwide available Cropland Data Layer to parameterize landscape-level PRZM model simulations and make refined, spatially explicit estimates of exposure in headwater and downstream catchments for use in ecological and endangered species risk assessments.

Opportunities and challenges of using NHDPlus connectivity data in refined modeling of aquatic exposure in flowing water bodies at the watershed scale

256th National Meeting (2018)

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