HERBICIDE TRANSPORT FROM LAND TO SURFACE WATER IN AN ARTIFICIALLY DRAINED CATCHMENT

Pesticide use is often associated with a number of negative impacts on the environment and on water quality. In this thesis the processes contributing to herbicide transport from land to surface water were examined, using a combination of existing data analysis, modelling, and monitoring. Field investigations of herbicide transport were conducted on an artificially drained field dominated by heavy clay soil (Denchworth soil association), in the Upper Cherwell catchment (UK). The main drain was monitored over five months during November 2009 to March 2010, when the field was in oilseed rape (OSR) and was treated with propyzamide and carbetamide. In the UK recent attention has focused on these two herbicides, which are used to control black grass in OSR. Both chemicals were detected at very high concentrations (up to 55.7 μg l-1 and 694 μg l-1 for propyzamide and carbetamide, respectively). The concentration pattern clearly followed drain discharge, with rapid increase on the rising hydrograph limb and a quasi-exponential decline on the recession limb. The MACRO pesticide fate model, which was applied to represent field observations, supported the hypothesis that herbicide transport to drains is a very quick process and suggested that preferential flow is a major transport mechanism. This analysis has contributed to the development of a preliminary model of catchment-scale pesticide transfers, which integrates hillslope responses through the river network to simulate flow and herbicide losses at the catchment outlet. The application of this model tends to corroborate the hypothesis that rapid transport to drains play a major role in herbicide contamination of surface water at the catchment outlet. Herbicides seem to peak about one day after rainfall events. The results also suggest that the first few rain events following herbicide application are very critical in terms of chemical losses.