Spatial variability of pesticide adsorption within the topsoil of a small agricultural catchment

We conducted a study of the adsorption of four herbicides (atrazine, isoproturon, metamitron and trifluralin) on the topsoil of a 135 ha catchment in the north of France. Fifty-one samples were taken to represent exhaustively the 31 agricultural fields and the 21 soil types of the catchment. Average Kds were 0.80, 0.85, 0.96 and 28.8 L/kg for atrazine, isoproturon, metamitron, and trifluralin, with coefficients of variation from 16 to 30%. Correlation analysis showed the Kds of each herbicide to be highly correlated to the organic carbon content (OrgC). Regression analysis showed that the relation between Kds and OrgC could be described well by a straight line, with a R-2 between 0.80 and 0.89. Koc distributions were found to be normal at a 5% level. Implications of Kd spatial variability in terms of pesticide fate modelling and pedotransfer functions development for risk assessment are discussed.