TRANSPORT OF HERBICIDES AND NUTRIENTS IN SURFACE RUNOFF FROM CORN CROPLAND IN SOUTHERN ONTARIO

The risk of surface-water contamination by herbicides is greatest foll owing application to cropland when the active ingredients are at the m aximum concentration and the soil is the most vulnerable to erosion fo llowing cultivation. This study determined the magnitude of surface ru noff losses of herbicide and nutrients at, and subsequent to, applicat ion. The first of three weekly 10-min, 2.6-cm rainfalls were simulated on triplicated 1-m2 plots (a set) on which com had been planted and t he herbicide (metolachlor/atrazine, 1.5: 1.0) and fertilizer (28% N at 123 kg ha-1) had just been applied. Identical simulations were applie d to two other adjacent plot sets (protected from rainfall) 1 and 2 wk following herbicide application. Runoff (natural, simulated) was moni tored for soil, nutrient and herbicide losses. Concentrations of total phosphorus in surface runoff water and nitrate N in field-filtered sa mples were not significantly influenced by the time of the rainfall si mulation but exceeded provincial water-quality objectives. Atrazine an d metolachlor runoff losses were greatest from simulated rainfall (abo ut 5% loss) immediately following application. Subsequent simulated ra infall usually resulted in < 1% herbicide runoff losses. Herbicide con centrations in all plot runoff samples exceeded provincial drinking-wa ter quality objectives. Since herbicide surface transport is primarily in the solution phase (not via association with soil particles), wate r-management conservation technologies are the key to retaining these chemicals on cropland.