The influence of hydrological pathways on the transport of the herbicide, isoproturon, through an underdrained clay soil

This paper reports the findings of a 12 month field monitoring programme at Wythara (Oxfordshire, UK), which examined the leaching of the phenylurea herbicide, isoproturon, from an agricultural clay hillslope to the adjacent ephemeral stream Isoproturon was shown to reach the drains (at 50 cm depth) through a combination of vertical and lateral macropore flow through wormholes and inter-pedal cracks. Seasonal changes in hydrological response were also observed at the site, with lateral flow through inter-pedal cracks providing an important route by which herbicide was transported during autumn and early spring when the clay soils were wetting up. Concentrations of isoproturon in drainflow frequently exceeded the Environment Agency's (EA) nonstatutory Environmental Quality Standard (EQS) of 20 μg/l for isoproturon in surface waters. Even trader low intensity rainfall (< 1 mm/hr) the saturated region at the base of the hillslope was shown to conduct macropore flow containing significantly high concentrations of herbicide to the drain. The results of this study suggest that those regions close to surface waters which are areas of converging hydrological flow may be key contributors to the problem of isoproturon contamination of surface waters.