Persistence of parathion in six California soils under laboratory conditions

Abstract

Persistence curves obtained for parathion in six California soils are partially dependent on soil type. The possibility that long-term, low-level parathion soil residues can exist is confirmed. Each soil was fortified at 20 ppm, kept in an enameled tray, and maintained at 30°C with a soil moisture level of approximately 40 percent of saturation. The parathion residue in Laveen loamy sand drops rapidly to about 0.2 ppm in 30 days. In Mocho silt loam, Linne clay, and Madera sandy loam, it drops to between one and two ppm in 30 days and gradually decreases to about 0.2 ppm in 130 days. The rapid parathion residue decline is attributed to microbial degradation. In Windy loam, the residue after eight months remains above three ppm. In two experiments differing slightly in soil moisture, the residue in Santa Lucia silt loam is about 1.5 ppm after eight months in one experiment and about 0.5 ppm after six months in the other. The latter two soil types give linear semi-logarithmic persistence curves, suggestive of degradation by hydrolysis.

Aminoparathion, above the detectable level of one ppm, is not found in Madera sandy loam after fortification with parathion at 200 ppm. However, a sharp decline of the parathion residue after ten days suggests microbial degradation. Three ppm aminoparathion were recovered after seven days when Madera sandy loam was fortified at 20 ppm with parathion and submerged under water. In contrast, the degradation of parathion remaining in Windy loam (3.2 ppm) and Santa Lucia silt loam (2.2 ppm) 7.7 months after fortification is not greatly accelerated when flooded with water.