Simultaneous assessment of sources, processes, and factors influencing herbicide losses to surface waters in a small agricultural catchment.

To take appropriate measures to minimize agricultural herbicide inputs into surface waters, detailed knowledge is required about all the factors that control the losses of a given compound from point sources (i.e., farmyards) as well as from the diffuse sources (i.e., the fields) within a given catchment. In this and in a companion paper, we present the results of a comprehensive field study, in which the temporal and spatial variability of the losses of three herbicides (atrazine, dimethenamid, and metolachlor) into the surface waters within a small catchment (2.1 km2) were investigated on different scales (i.e., field scale to whole catchment) after a controlled application of the compounds. In this paper, we discuss the loss dynamics of the three herbicides (and some of their metabolites) from the whole catchment over a period of 67 d after application. An identical mixture of the three herbicides was applied on 13 cornfields within 12 h, allowing for a comparison of their losses under identical meteorological conditions. Thanks to a high temporal sampling resolution, it was possible to distinguish between losses from a farmyard and losses from the fields. Farmyard losses contributed less than 20% to the total loads but caused the highest concentrations. The major herbicide losses from the agricultural fields occurred during the first two rain events after application that led to significant surface runoff and preferential flow into tile drains. In the soils of all fields, dimethenamid declined somewhat faster than atrazine and metolachlor, whereas atrazine was mobilized most effectively to runoff water. Relative losses of the three compounds did not vary by more than a factor of 3 (0.82, 0.27, and 0.41% of the mass applied for atrazine, dimethenamid, and metolachlor, respectively). Highest peak concentrations at the outlet of the catchment were found for atrazine (i.e., approximately 8 microg L(-1) for a short period (<2 h) due to point source losses and between 1 and 3.5 microg L(-1) during more than 24 h due to diffuse losses).