PESTICIDE MOVEMENT INTO SUBSURFACE DRAINS ON A LOAMY SILT SOIL

Pesticide losses to subsurface drain were monitored in field trials on an agriculturally important loamy silt soil in the ''Soester Borde'', a typical crop growing area in Germany. Over a one year period, the drainflow was measured and the drain water was analysed for four applied herbicides. Significant differences were observed comparing spring and autumn application. The herbicides applied in spring 1992 were only found in concentrations less than or equal to 0.15 mu g/l (chloridazon, metamitron) in the drain water; pendimethalin was not detected in the drains during this spring/summer period. Following the autumn application, maximum concentrations of about 62 mu g/l for isoproturon and 0.7 mu g/l for pendimethalin were observed. Between 0.09 % and 0.4 % of the applied isoproturon, but less than 0.001 % of pendimethalin leached to the drains from November 1992 to April 1993. The experimental results indicate that both preferential now and non-equilibrium sorption were occurring on a field-scale