Atrazine and metolachlor losses in runoff events from an agricultural watershed: the importance of runoff components

Losses of atrazine and metolachlor in runoff events were investigated at the outlet of an agricultural watershed. The streamflow hydrographs were decomposed into surface runoff, interflow, and baseflow. The majority of losses of the applied herbicides occurred in surface runoff and interflow. The combined losses in surface runoff and interflow accounted for up to 75% of atrazine and 65% of metolachlor, of the total loss. The vast majority of the losses occurred within 70 days of the application and during a large storm event, shortly after herbicide application. Herbicide concentrations showed a steady disappearance with pseudo first-order half-lives of 54 days for atrazine and 50 days for metolachlor. The depletion ratios determined from the runoff events, were 0.989, 0.943 and 0.939 for baseflow, interflow, and surface runoff, respectively. The high depletion ratios of the runoff components characterized slow withdrawal of water from the watershed storage. The slow withdrawal of water from the storage allowed time necessary for herbicide adsorption-desorption processes. The losses of atrazine in surface runoff, interflow and baseflow of that applied, per hectare, were 0.073, 0.038 and 0.035%, respectively and the losses of metolachlor in surface runoff, interflow and baseflow, per hectare, were 0.038, 0.029 and 0.037%, respectively.