The use of Runoff and Surface Water Transport and Fate Models in the Pesticide Registration Process

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Abstract
In support of the registration and reregistration processes under FIFRA ‘88, numerical models are used to estimate the dissolved runoff and soil erosion loadings of pesticides to surface waters and the subsequent fate and transport of pesticides in the receiving waters. Uses for simulations include: 1. Helping to determine whether additional studies on the fate and distribution of a candidate chemical in the environment and/or ecological effects may be needed when full chemical characterization is incomplete. 2. Helping to more fully integrate data submissions of laboratory and field observations. 3. Estimating probable fate and distribution of an agrochemical after a severe runoff event. 4. Comparing alternative chemical application rates and methods for the same chemical/soil/crop/environmental combinations. 5. Comparing different soil/crop/environmental combinations representing different geographical areas with the same chemical. 6. Evaluating preliminary designs of proposed field studies. 7. Gaining insight into the environmental fate of modern chemicals that are applied at 1 to 2% of the rates of older chemicals when sampling designs and analytical methods are not available. The Office of Pesticide Programs (OPP) commonly employs the SWRRB (45, 46) and PRZM (8, 9) models for runoff and EXAMS II (5), a fate and transport model, for surface waters. These three models are used in conjunction with appropriate databases for soils, chemical properties, and meteorological and other environmental parameters. Registrants are encouraged to clearly state assumptions, identify values for parameters by citation, report results of simulations with summary tables and graphics, and interpret their results in relation to current scientific disciplines.

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