DEGRADATION RATES OF 8 PESTICIDES IN SURFACE AND SUBSURFACE SOILS UNDER LABORATORY AND FIELD CONDITIONS

The rate at which a pesticide is degraded in both the surface and subs urface soils is an important factor in determining the groundwater con tamination potential of the pesticide. Degradation rates of eight pest icides used commonly in Western Australia-chlorpyriphos, chlorthal dim ethyl, fenamiphos, linuron, metalaxyl, metribuzin, prometryne and prop yzamide-in the surface (0-25 cm) and subsurface (25-50 cm) layers of a sandy soil, were studied under controlled incubation conditions and w ere compared with those under field conditions. Field degradation rate s were also simulated on the basis of degradation rates measured in th e laboratory, taking into account the effects of temporal variations i n field moisture and temperature. The incubation study showed that the eight pesticides had widely different degradation rates in both the s urface and subsurface soils, with half-lives ranging from 23 to 142 da ys. Four of the pesticides had lower degradation rates in the subsoil than in the surface soil, but the reverse was observed for the other f our pesticides. It is postulated that the nonuniform changes in degrad ation rates with soil depth were probably caused by interactive effect s of changes in soil microbial activities and in organic matter conten t (and thus pesticide sorption) in the different soil layers. No consi stent relationship was found between field degradation rates and those measured in the laboratory. The simulated field half-lives were all l onger than those measured in the laboratory and did not match all the field-measured degradation rates.