Problems Associated with the Use of Solute-Transport Models and Vulnerability Assessments for Predicting the Behavior of Pesticides in the Subsurface.

Jack E. Barbash, U.S. Geological Survey, 345 Middlefield Rd., Mail Stop 470, Menlo Park, CA 94025.

A recent trend in the regulation of pesticide use has involved an increased reliance on solute-transport models to predict the transport and fate of pesticides in the subsurface. This approach assumes that existing models accurately predict solute behavior in the unsaturated and saturated zones. The few simulation studies which have compared model predictions or ground-water- vulnerability assessments with actual field behavior (as opposed to adjusting model parameters to fit field data) have found, however, that the models generally underestimate the distance of pesticide transport. All of the simulation models used in making these predictions for pesticides have assumed "local equilibrium" during transport. Research by numerous investigators has suggested that these disparities between prediction and observation arise largely from the combined influence of non-equilibrium solute exchange (between sorption sites and the bulk solution) and preferential flow on pesticide movement. Such effects therefore must be accounted for in order to obtain accurate predictions of pesticide behavior in the subsurface.