Degradation rates of eight pesticides in surface and subsurface soils under laboratory and field conditions

The rate at which a pesticide is degraded in both the surface and subsurface soils is an important factor in determining the groundwater contamination potential of the pesticide. Degradation rates of eight pesticides used commonly in Western Australia - chlorpyriphos, chlorthal dimethyl, fenamiphos, linuron, metalaxyl, metribuzin, prometryne and propyzamide - in the surface (0-25 cm) and subsurface (25-50 cm) layers of a sandy soil, were studied under controlled incubation conditions and were compared with those under field conditions. Field degradation rates were also simulated on the basis of degradation rates measured in the laboratory, taking into account the effects of temporal variations in field moisture and temperature. The incubation study showed that the eight pesticides had widely different degradation rates in both the surface and subsurface soils, with half-lives ranging from 23 to 142 days. Four of the pesticides had lower degradation rates in the subsoil than in the surface soil, but the reverse was observed for the other four pesticides. It is postulated that the nonuniform changes in degradation rates with soil depth were probably caused by interactive effects of changes in soil microbial activities and in organic matter content (and thus pesticide sorption) in the different soil layers. No consistent relationship was found between field degradation rates and those measured in the laboratory. The simulated field half-lives were all longer than those measured in the laboratory and did not match all the field-measured degradation rates.