Evaluating non-equilibrum herbicides transport in undisturbed grassed buffer strips soil columns

Possible contamination of water resources by applied pesticides (including insecticides and herbicides) is a problem currently. Grassed buffer strips have been considered to limit pesticide transfer to surface waters by reducing run-off and erosion. As reduction of run-off mainly involves infiltration in the grassed strip soils, possibility of groundwater contamination by rapid transfer of pesticides via the numerous root channels and macropores presents in grassed soils can not be excluded so far. The beneficial impact of high pesticide retention in the superficial horizons of these soils, related to their high organic matter content, could therefore be strongly reduced by the occurrence of preferential flows. Displacement studies on leaching of bromide (water tracer) and two herbicides, metribuzin (weakly adsorbed) and isoproturon (moderately adsorbed) were conducted in two undisturbed soil columns (length 30 cm, diameter 14 cm) collected from La Jaillière (Loire Atlantique, France) under non saturated conditions. The infiltration fluxes, delivered through a rainfall simulator, were 1.5 and 3 mm/h for one of the columns and 0.6, 1.5 and 3 mm/h for the second column. We obtained asymmetrical shapes of the breakthrough curves, with early breakthrough and increased tailing, qualitatively indicating the presence of preferential flow and confirming the risk of pesticide vertical transport through grassed strips superficial soil horizons. The results showed that leaching of both herbicides increased with the intensity of rainfall applied. This could be explained by a decrease of herbicides sorption due to a lower residence time in the soil.