Dissipation of pesticides by farm ditches

Abstract

The intensification of agriculture leads to nonpoint source pollution and contamination of surface water by pesticides. In order to limit this kind of pollution, many research programs have been developed to study and explain the different ways of pesticide transfer from fields to rivers. For example, typical landscape environments, such as buffer zones, can intercept surface runoff. Furthermore, farm ditches seem to play an important role in the transfer of nonpoint source pollution (especially in the West of France). Indeed they can either accelerate pesticide transport as hydraulic accelerators or reduce it, according to their physico-chemical characteristics (length, flow, plants, organic matter, etc.). Since 1998, Cemagref and CEH (Centre for Ecology and Hydrology) have studied the retention of pesticides by several natural ditches. Two different sites were studied. The French one is located at the ITCF (Institut technique des céréales et des fourrages) experimental farm at La Jaillière (Western France). A water solution containing three herbicides (diflufenican, diuron and isoproturon) and potassium chloride, a substance tracer, was introduced in each ditch for about five minutes. Water samples were collected in the ditches at two distances from the injection point. The second study site is located at the Oxford University farm at Wytham. Water samples were collected on a weekly basis (routine sampling) and an event basis during high water flow. The analysis of the samples showed a reduction of the maximum concentration, compared with a tracer, and of the accumulative mass of pesticides with distance. We noted that the retention of pesticides was also linked to their own physical and chemical properties. The preliminary studies proved that the surface and time of contact between pollutants and substratum are likely to play a major role in pesticide retention.