Transport of two sulfonylurea herbicides in runoff from border dyke irrigation

Sulfonylurea herbicides have been detected in reservoirs and wetlands in the North American Great Plains. However, since little is known about the transport characteristics of these herbicides, the mechanism of contamination has not been established. In order to assess the potential contribution of surface runoff, transport of two sulfonylurea herbicides (thifensulfuron-methyl and tribenuron-methyl) was measured in surface runoff from a flood-irrigated field in the Outlook Irrigation District of Saskatchewan, Canada. Following herbicide application, the field was flood irrigated, and surface runoff was monitored. Surface drain flow rates and herbicide concentrations were measured from three 0.85 ha (2.1 ac) plots in the field. Although the rate of application of tribenuron-methyl (4.9 g ha(-1) [0.07 oz ac(-1)]) was half that of thifensulfuron-methyl (9.8 g ha(-1) [0.14 oz ac(-1)]), concentrations of the herbicides in surface runoff water were similar. The highest concentrations (0.98 mu g L-1 for thifensulfuron-methyl and 0.81 mu g L-1 for tribenuron-methyl) were measured during periods of low flow at the beginning of the runoff events. For both herbicides, concentrations decreased as flow in the drain increased but then recovered somewhat as flow receded. Total loss of tribenuron-methyl in runoff water,five days after herbicide application averaged 1.9% of the amount applied, which was significantly greater than thifensulfuron-methyl losses that were approximately 0.8% of the application. These results confirm postapplication transport in surface runoff-as a significant pathway for contamination of surface waters by thifensulfuron-methyl and tribenuron-methyl.