Off-site transport of pesticides from treated agricultural fields to receiving water bodies from spray drift can be an important potential pathway for aquatic exposure. Current methods used in regulatory exposure modeling are based on a simplistic conceptual model of a single treated field adjacent to a static receiving water body. New challenges in ecological risk assessment, including the evaluation of endangered species, require understanding exposure in more complex water body and landscape configurations at the watershed scale. In these more complex environmental conditions, the potential for spray drift exposure is largely driven by the spatial characteristics of the agricultural landscape relative to the receiving water system.  This paper will focus on a methodology that takes advantage of nationally available Cropland Data Layer and NHDPlus watershed/ hydrography data to reduce the uncertainty in potential drift contributions to aquatic exposure in flowing water bodies. The methodology considers individual crop years independently to minimize the potential for over-representing crop extents while accounting for the year-to-year variability in cropping practice. Using a raster-based spatial analysis approach, the proximity from individual water body grid cells to the nearest treated crop of interest is assessed over the entire extent of the flowing water body and connected impoundments. These proximity distances are translated to drift fractions using drift versus distance curves applicable to the application method for each crop of interest. A drift deposition mass at each grid cell can then be calculated based on the pesticide application rate and drift fraction, then summed to represent the entire water body. Receiving body width at the sub-grid cell scale is determined through estimation channel width versus streamflow relationships using average annual flow estimates from the NHDPlus dataset. The approach developed can be combined with stream connectivity data to make refined estimates of drift-based exposure in headwater and downstream catchments.

Analysis of spatial data to reduce the uncertainty of pesticide spray drift contributions to aquatic exposure at the watershed scale

256th National Meeting (2018)

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