Degradation of Selected Organophosphate Pesticides in Water and Soil

Laboratory studies on the breakdown of several organophosphate pesticides both in aqueous solution and moist soil were conducted. The hydrolysis rates of phosmet, dialifor, malathion, methyl chlorpyrifos, dicapthon, chlorpyrifos, and parathion were measured at 20 and 37.5 °C (pH 7.4) in an aqueous system.A similar studywas carried out at 20 °C and pH 6.1. The half-lives at 20 °C (pH 7.4) range from 7.1h for phosmet to 130 days for parathion; the corresponding rates at 37.5 °C are approximately five-seven times greater than those at 20 °C. The rate equations at pH 7.4 were calculated from the 20 and 37.5°C data in an Arrhenius form : k = A exp(-Ea/RT). In moist soil (pH 6.2), degradation rates were measured at pesticide concentrations of approximately 1.0 and 0.1 ppm in a Willamette clay loam soil at a moisture level of 50% of field capacity. A comparison of the 20 °C half-lives for phosmet and dialifor in water and in moist soil at comparable pH indicates an appreciable increase in persistence for these two compounds, but little for the others in the soil-water system. This study was intended to evaluate the stabilities of several agricultural pesticides.