Influence of adsorption-desorption phenomena on pesticide run-off from soil using simulated rainfall

The surface run-off of a number of pesticides (diuron, isoproturon, atrazin e, alachlor, aclonifen, trifluralin, lindane and simazine), chosen for thei r range of adsorption behaviours, was studied using simulated rainfall appl ied to small plots over a short time (one hour). Pesticides were applied to gether onto bare soil using two different sandy loam soils from Jailliere a nd Coet Dan sites. The surface run-off samples were collected throughout th e running of the event and concentrations of pesticides were measured in bo th liquid and solid phases. Sorption isotherms for isoproturon and diuron o n Jailliere soil as well as eroded particles were measured under equilibriu m conditions and compared to their partitioning during surface run-off. At the rainfall intensity used, both soils generated a large load of eroded particles. The average run-off flow rate increased with time for the Jalli ere soil, while it remained relatively constant at a higher level for the C oet Dan soil. The concentrations of each pesticide in the run-off samples d ecreased as the experiments proceeded. The pesticides were classified into two types by their partitioning between the solid and liquid phases. Atrazi ne, simazine, diuron, isoproturon and alachlor were mainly transported in s urface run-off water. By contrast, 90% of trifluralin and aclonifen was ads orbed onto eroded particles. Lindane was intermediate, with a 37% adsorptio n level. When the contribution of eroded particles was minor, the agrochemi cal concentrations were inversely proportional to the water flow rate. We h ave proposed a model that describes the mass of chemicals extracted from so il into surface water during a surface run-off event of a given average dur ation and flow rate. This model takes into account the dilution of the soil solution and the desorption of chemicals through two parameters called, re spectively, the dilution factor and the extraction retardation factor. The desorption kinetic was the limiting step in the surface run-off of weakly s orbed chemicals, such as isoproturon. (C) 1999 Society of Chemical Industry .