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

An experiment was established on plot lysimeters in autumn 1989 to exa mine the loss of pesticides from a cracking clay soil. Emphasis was pl aced on the movement of two autumn/spring-applied herbicides with cont rasting properties - isoproturon and mecoprop. The first major drainfl ows in autumn/winter were important for the transport of these herbici des. During the dry autumns of 1989 and 1990, the development of subst antial subsoil cracks led to rapid vertical movement of water, through the soil-cracks to the drainage system, before field capacity was rea ched. Isoproturon was detected in the resultant drainflow at concentra tions of up to 50 mug l-1, well above the EC Directive for potable wat ers. Mecoprop was not detected. Measurements of herbicide degradation in soil indicated that isoproturon was rather persistent and hence ava ilable over many months for transport to the drainage system. In contr ast, the long delay between application and the onset of drainflow, co mbined with the much faster degradation of mecoprop, resulted in this herbicide not occurring in drainflow. Although the site was some dista nce from a supply point for potable water, the loss of such concentrat ions of isoproturon in the early drainage flush could have undesirable consequences for water supplies and for the ecology of adjacent water courses.