Pesticide uptake from agricultural fields into runoff is the primary mechanism that risk assessors assume controls the amount of pesticide carried off site and into water bodies. Early uptake runoff models employed simple mixing cells set at the top of the soil surface. Later, laboratory-scale research suggested that runoff removes chemicals from the soil surface in a manner that decreases exponentially with the depth of the pesticide in the soil. This lab-derived evidence led to the inclusion of nonuniform chemical uptake routines into regulatory runoff models, albeit with little performance evaluations of the default assumptions, especially when evaluated at the field scale which is most relevant to regulatory work. In the present work, the uptake of soil-applied pesticide into runoff at the field scale (0.06 to 3.6 ha) was evaluated by re-examining data from previous model validation studies and comparing it to the results simulated by the USEPA PRZM5 model. The studies used for re-evaluation comprised 17 fields/chemical combinations and 117 runoff events. Modifications to PRZM5 facilitated easy optimization of the three uptake parameters that are critical to the nonuniform uptake model. Fitting optimizations were performed on the total mass delivered from each field/chemical combination as well as on the individual runoff events. Several weighting schemes were explored in an effort to derive the fairest evaluation, including unweighted, weighted by application mass, weighted by data value, and weighted by total mass off the field. When considering the overall mass removed from the fields, preliminary results suggest that the nonuniform uptake effect may not be apparent at the field scale and that a single uniform mixing cell can adequately model pesticide removal by runoff; however, when considering individual events, the nonuniform effect becomes more apparent. Overall, fitting of runoff uptake models indicated that a soil interaction depth of 1 to 2 cm with only about 10 to 20 percent of the runoff interacting with the soil was adequate to model the fields for either the uniform or the nonuniform case. The low interaction value (10-20%) implies that runoff uptake models should bypass the vast majority of runoff flow.

Pesticide uptake into runoff evaluated at the field scale

253rd National Meeting (2017)

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