Assessing the Runoff Potential of Diuron Following Application to Irrigation Canal Access Roads in Fresno County
Abstract
Diuron is a pre emergent soil herbicide that is widely used in agricultural production and on rights-of way (ROW) to control weed growth in Fresno and Tulare counties. ROW applications generally occur in the fall-winter season to comply with label recommendations for incorporation of residues into the soil by rainfall. A field study was conducted in Riverdale, Fresno County, California to evaluate the potential for off-site movement of diuron caused by rainfall after a ROW application. The study was conducted in cooperation with the Riverdale Irrigation District, (RID) and the study site was located on an access road that straddled an irrigation canal in Riverdale. Soil at the site was classified as a sandy loam with 2.9% slope and it was compacted due to vehicular traffic. Diuron (Diuron 80DF) was applied at a rate of 21.1 kg ai/ha, corresponding to 1474 mg ai applied to the experimental plot. Simulated rain events were applied at 0, 14, and 28 days after application and run-off water samples were collected from each event. Intensity of simulated rainfall was set to 38 mm per hour to represent a worst-case scenario and the average total runtime was 78 minutes. For the first simulated rain event, 125 mg of diuron was removed, which represented 8.5% of the total amount of diuron applied on the plot. The second and third events resulted in an additional removal of 73 and 28 mg of diuron, respectively. Total amount of diruon removed in runoff from all three events accounted for 15.3% of the initial applied amount. The amount of diuron measured in the runoff water generated from the compacted access road in this study was similar to the mass measured in runoff investigations conducted for ROW applications made to strips of land located adjacent to major highways that also were compacted. Incorporation by rainfall on compacted soil could result in runoff of a significant portion of the application, potentially affecting nearby ecological systems, contaminating surface or ground water, or causing crop injury.