Mitigation of Permethrin in Irrigation Runoff by Vegetated Agricultural Drainage Ditches in California

As organophosphate use has decreased in California, a concomitant increase in their replacement insecticides (pyrethroids) has occurred. Although the probability of off-site movement of pyrethroids is less than their predecessors (organophosphates), transport of pyrethroids to aquatic receiving systems is still a potential threat. To mitigate possible harm, several in-field and edge-of-field management practices have been proposed, including conservation tillage, stiff grass hedges, riparian buffers, and constructed wetlands. By incorporating several individual components of these management practices, vegetated agricultural drainage ditches (VADD) have been proposed as a potential economical and environmentally efficient management practice to mitigate effects of pesticides in irrigation and storm runoff. A field trial was held in Yolo County, California, where three ditches (U-shaped vegetated; V-shaped vegetated; and V-shaped unvegetated) were constructed and amended for 8 h each with a mixture of permethrin and suspended sediment simulating an irrigation runoff event. Spatial and temporal collections of water, sediment, and plant samples were analyzed for cis and trans permethrin concentrations. Because the cis- isomer of permethrin is considered more toxic than the trans- isomer, only cis-permethrin results are reported herein. Cis-permethrin half-lives in water were similar between ditches ranging from 2.4-4.1 h. The differences between half-distances (distance required to reduce initial pesticide concentration by 50%) among the V-shaped vegetated and unvegetated ditches were two times more efficient with vegetation, indicating importance of vegetation in mitigation. Cis-permethrin half-distances ranged from 22 m (V-vegetated) to 50 m (V-unvegetated). These studies are being used to validate a computer simulation model that is being developed to design VADD for site-specific implementation. Utilizing features already present in the agricultural landscape, such as drainage ditches, will provide farmers with an economical alternative that still is protective of the receiving aquatic environment.