Determination of chlorpyrifos and acephate in tropical soils and application in dissipation studies

A rapid and accurate method for the extraction and determination of the two organophosphorus insecticides, chlorpyrifos and acephate in top- and subsoil materials of three tropical clayey soils from Sarawak has been developed. Soil samples were extracted with ethyl acetate and the pesticides were determined by GC-FPD. High recoveries of 76–102% and 76–100% were obtained for acephate and chlorpyrifos respectively, at fortification levels of 0.01, 0.1 and 1 mg kg^?1 with standard deviations below 9.0%. The addition of water prior to the extraction was important for obtaining high and reproducible recoveries. The method did not require clean-up of the extracts prior to GC analysis and could be detected down to 0.01 mg kg^?1. A field study was conducted using the modified method to measure the degradation kinetics and migration of acephate and chlorpyrifos in one of the soils over a period of 84 days. The degradation of acephate and chlorpyrifos were rapid with half-lives of 3.3 and 8.7 days, respectively. Both pesticides were detected in subsoils 2 h after application at the deepest (50 cm) soil layers examined and at concentrations up to 5.42 mg kg^?1. Subsoil concentrations of acephate were higher than for chlorpyrifos, and subsoil concentrations of acephate peaked after it had started to degrade in the top soil. The subsoil concentrations of the pesticides were attributed to transport with soil particles (chlorpyrifos) and via solution (acephate) through pores and cracks present in the soil profiles. The study demonstrates the high mobility of even strongly retained and fast degrading pesticides under tropical humid conditions.