Degradation of chlorpyrifos in soil: effect of concentration, soil moisture, and temperature

The dissipation of the organophosphorus insecticide chlorpyrifos was investigated in field and laboratory samples. Six field locations were selected within the United States, and included the states of Arizona, Florida, Hawaii, Iowa, Mississippi, and Texas. Five of the six locations were selected based on their suspected aggressive degradative capabilities towards this chemical;In the field study, chlorpyrifos was applied to surficial trenches prepared on the north and south side of each building and parallel to the foundation. In the soil samples from Florida, Hawaii, and Texas, concentrations decreased by more than 50% of the initial concentration within 3 mo of application. Arizona, Iowa, and Mississippi soil samples showed a 50% decrease within 12 mo of application. At the end of 24 mo environmental exposure, no statistically significant differences were detected between the concentrations of chlorpyrifos remaining in the soil. However, there may be biological differences. After 24 mo, only the Iowa field location would have a sufficient concentration of chlorpyrifos remaining to prevent termite penetration through the soil-chemical barrier. A single linear or exponential regression equation did not adequately describe the degradation kinetics. Two separate first-order reactions were observed, which proceed in two distinct phases. Using both degradation rates can aid in predicting the concentration of chlorpyrifos remaining in the soil at any time after application;In the laboratory experiment, soil samples from an urban Iowa soil were treated with 0.5 [mu] Ci [[superscript]14C]chlorpyrifos; 10, 500, or 1,000 [mu]g/g formulated Dursban TC; and sufficient water to establish moisture tensions of 0.03, 0.30, and 3.00 bar. Replicate samples were divided between two incubation chambers maintained at either 20° or 27°C. Concentration and moisture significantly affect the degradation of chlorpyrifos. Only in the samples treated with 10 [mu]g/g chlorpyrifos were there significant differences between soil moisture tensions. Concentration was most important in affecting the rate of chlorpyrifos degradation in soil samples treated with either 500 or 1,000 [mu]g/g. Incubation temperature did not significantly affect the degradation of the parent compound chlorpyrifos or soil bound residues. However, temperature did significantly interact with concentration and moisture tension in the mineralization of 3,5,6-trichloro-2-pyridinol (TCP)