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Case Study

National Modeling of Runoff, Erosion and Drift Potential for Surface Water in the US

We teamed up with the Pyrethroid Working Group to gain understanding of the potential for pyrethroids to enter flowing surface water. By doing so, we identified and characterized national distributions of relevant factors for exposure modeling of agricultural uses of pyrethroids.

Our Findings

Using the National Hydrography Dataset Plus (NHDPlus) as a framework, we characterized 2.5 million catchments. Each represented a single stream segment for parameters critical to higher tier landscape-level exposure modeling.

Upon analyzing this data, we developed distributions of potential worst-case exposure to drift and runoff/erosion entry of pyrethroids into flowing water at a watershed scale. Using these real-world distributions, we modeled exposure modeling of drift entry (using AgDRIFT/RegDisp) and runoff/erosion entry (using PRZM / VFSMOD) using US EPA’s standard pond (using AGRO-2104) with a set of 111 scenarios covering the range of environmental variables, and representing more than 10 specific crops and their associated growing locations over the last 5 years.

The EEC distributions of the 111 scenarios were weighted with the probability of occurrence in the landscape based on spatial data covering crop location, specific crop/soil relationships and 30 years of weather data to create a probabilistic exposure distribution. The resulting 3,330 modeled concentrations (111 scenarios x 30 years or annual maxima) were used to select an environmental exposure value, or set of values, to be used in the risk assessment.


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