Dissipation of Chlorpyrifos from Dry Soil Surfaces1

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Abstract
Hydrolysis and volatility contributed to loss of 14C-ring-labeled chlorpyrifos on air-dried surfaces of SOils and soil clays in the laboratory. Dissipation rates were influenced by temperature, composition of the substrate, and level of hydration. Chlorpyrifos degraded rapidly via a temperature-dependent catalytic hydrolysis process on Sultan silt loam. Times for 50% degradation at 5, 15, 25, 35, and 45°C were >20, >20, 8, 3, and 1 day, respetively, when ambient RH levels were 93, 67, 38, 30, and 10%, respectively. The insecticide degraded even more rapidly on air-dried surfaces of calcium homoionic clays (illite, vermiculite, adkaolinite) and clay separates isolated from Sultan silt loam and Chehalis clay loam. Degradation was slower on a sandy loam, on montmorillinite, and on an organic soil and negligible on pulverized peat moss and quartz sand. An increae in the sorbed water content of air-dried substrates decreased the rate of the clay-catlyzed hydrolysis reaction and increased the volatilization rate of 14C from mineral substrate surfaces. 3,5,6-trichloro-2-pyridinol was the only 14C-breakdown product found in acetone extracts of aged, chlorpyrifos-treated soils and homoionic clays.

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