FACTORS AFFECTING THE HYDROLYTIC DEGRADATION OF CHLORPYRIFOS IN SOIL

The abiotic hydrolysis of the organophosphorus insecticide chlorpyrifo s was examined in 37 different soils, which were chosen to represent a wide variety of physicochemical characteristics (e.g., pH 3.8-8.5). S amples of soil were sterilized via gamma-irradiation, treated with [C- 14]chlorpyrifos at 10 mu g/g, and incubated under standardized conditi ons (25 degrees C, field moisture capacity, darkness) for up to 4 mont hs. Chlorpyrifos hydrolysis proceeded at a slow rate (<0.008 day(-1)) in acidic soils (pH less than or equal to 7). In alkaline soils, howev er, hydrolytic rate constants varied greatly (0.004-0.063 day-l). Corr esponding hydrolytic half-lives for acidic and alkaline soils ranged f rom 92 to 341 and 11 to 200 days, respectively. Correlation analyses i ndicated that soil pH was the independent variable displaying the stro ngest association with hydrolytic rate constant (r = 0.55), but multip le regression models based on combinations of this parameter with othe r soil properties, including phosphatase enzyme activities, did not of fer strongly predictive models for explaining the variability in kinet ics observed (best fit r(2) = 0.59). Incubation of chlorpyrifos with b oth sterile and nonsterile soils revealed that although both microbial and hydrolytic mechanisms contributed to chlorpyrifos degradation in all soils, there were clearly soils in which hydrolysis constituted th e major route of degradation. Chlorpyrifos hydrolysis was greatly acce lerated under low moisture conditions, both in acidic and alkaline soi ls. Additional experiments in several soils that displayed rapid chlor pyrifos hydrolysis at 10 mu g/g provided evidence that the hydrolytic reaction was inhibited at higher concentration (1000 mu g/g). Results highlight the importance but also the complex nature of the hydrolytic breakdown of chlorpyrifos in soil. Under certain conditions (e.g., so me alkaline soils, air-dry soils) hydrolysis may be the driving factor modulating chlorpyrifos persistence.