FACTORS INFLUENCING THE LOSS OF PESTICIDES IN DRAINAGE FROM A CRACKING CLAY SOIL

An experiment was established on plot lysimeters in autumn 1989 to examine the loss of pesticides from a cracking clay soil. Emphasis was placed on the movement of two autumn/spring-applied herbicides with contrasting properties - isoproturon and mecoprop. The first major drainflows in autumn/winter were important for the transport of these herbicides. During the dry autumns of 1989 and 1990, the development of substantial subsoil cracks led to rapid vertical movement of water, through the soil-cracks to the drainage system, before field capacity was reached. Isoproturon was detected in the resultant drainflow at concentrations of up to 50 mug l-1, well above the EC Directive for potable waters. Mecoprop was not detected. Measurements of herbicide degradation in soil indicated that isoproturon was rather persistent and hence available over many months for transport to the drainage system. In contrast, the long delay between application and the onset of drainflow, combined with the much faster degradation of mecoprop, resulted in this herbicide not occurring in drainflow. Although the site was some distance from a supply point for potable water, the loss of such concentrations of isoproturon in the early drainage flush could have undesirable consequences for water supplies and for the ecology of adjacent water courses.