Environmental fate and transport models were used to supplement monitoring data to characterize potential concentrations of chlorpyrifos (CPY) in water and sediment for three watersheds in the United States: Orestimba Creek in the San Joaquin River basin in California, Dry Creek in the Flint River basin of southwest Georgia, and Cedar Creek in the Betsie-Platte River system in northwestern Michigan. The watersheds were selected by evaluating the relative runoff potential of CPY across the US under different use practices. Runoff of CPY was estimated using the European version of the Pesticide Root Zone Model (WinPRZM) based on its ability to simulate the dissipation of CPY via volatilization, microbial degradation, and transport in runoff, eroded soil, and from furrow/flood irrigation tail water. Loadings of CPY to water from spray drift were estimated with the AgDRIFT model. Concentrations in receiving streams were estimated using the Surface Water Assessment Tool (SWAT). Up to 30 years of CPY use were simulated to account for CPY runoff under a variety of weather conditions. The upper 90th centile 96-hour concentrations in water, determined from the annual maximum series, were estimated to be 1.32, 0.020, and 0.018 �g/L for the California, Georgia, and Michigan watersheds, respectively, based on the more persistent of two values used for CPY metabolism in soil (96.3-day half-life). Durations of concentrations exceeding 0.1 �g/L were generally less than 48 h. Monitoring from three relatively intensive sampling programs totaling 119,455 samples from 2002-2010 have reported median, 90th, and 99th percentile concentrations under 0.03, 0.3, and 0.9 �g/L, respectively.

Exposures to aquatic organisms from the use of chlorpyrifos in North America

246th National Meeting (2013)

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