Herbicide transport to surface waters at field and watershed scales in a Mediterranean vineyard area

The contamination of soil and runoff water by two herbicides, diuron [N'-(3 ,4-dichlorphenyl)-N,N-dimethylurea] and simazine (6-chloro-N,N'-diethyl-1,3 ,5-triazine-2,4-diamine), were monitored on two fields, one no-till and one tilled. Experiments were carried out in a 91.4-ha watershed in southern Fr ance during the 1997 growing season in order to understand the patterns of pesticide transport from field to watershed. The persistence of the herbici des in soil was prolonged due to the climatic conditions. At the field scal e, annual herbicide loads were due to overland now and amounted to 65.6 and 6.3 g ha(-1) of diuron for the no-till and tilled field, respectively, and to 29.6 and 1.83 g ha(-1) of simazine. Maximum herbicide concentrations ex ceeded 580 mug L-1 during the first storm event after application and decre ased thereafter but remained for 8 mo above 0.1 mug L-1. At the watershed o utlet, estimated annual loads amounted to 4.12 g ha(-1) of diuron and 0.56 g ha(-1) of simazine. Among them, 96% of the losses in diuron and 83% of th ose in simazine were caused by the fast transmission through the network of ditches of the overland now exiting the fields. For diuron, which was spra yed over most of the vineyards, its in-stream concentrations during storm n ow were close to those at the outlet of the fields. The herbicide loads in baseflow were smaller than 0.2 g ha(-1). The patterns of the loads at the f ield and watershed scales suggested that a major part of the herbicides lea ving the fields reinfiltrated to the groundwater by seepage through the dit ches, and was there degraded or adsorbed.