Hydrological pathways of metolachlor export from an agricultural watershed

Hydrological pathways of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] transport from a 5.5 ha−1 corn field in southern Ontario were examined. Metolachlor was applied as an early post-emergence surface broadcast treatment at a rate of 334.1 mg active ingredient m−2. Persistence in soils decreased exponentially following application, with slower dissipation in footslope areas. Metolachlor concentrations in runoff from experimental plots also decreased exponentially with time, and were strongly associated with herbicide levels in surrounding soils. Differences between concentrations and yields in saturation overland flow and Horton overland flow during the monitoring period were controlled by near-stream water table conditions. Total herbicide losses in runoff from the plots were in the order of 1% of applied a.i. Metolachlor was rapidly leached to depths of 1 m, such that peak concentrations and instantaneous metolachlor loads in tile drainage were observed within 14 days following application. Peak concentration in streamflow (125 µg L-1) occurred during the first major runoff event 13 days following application; peak concentrations in succeeding events followed an exponential decay during the growing season. Export of metolachlor from the field was in the order of 1% of applied a.i., with overland flow serving as the dominant transport mechanism from site of application. Near-stream hydrologic conditions may determine whether field exports of herbicide enter receiving water courses.