Persistence, Mobility, and Degradation of14C-Dimethoate in Soils

Abstract

Dimethoate-carbonyl-14C was applied to 3 types of soil (silty loam, loamy sand, and clay loam) contained in 10-inch-diameter cylinders sunk into the ground to give concentrations in the top one inch of 0.056 and 0.169 ppm in 2 separate experiments. Thereafter, samples were taken periodically, extracted, fractionated, and analyzed for residues of dimethoate and its metabolites. The same metabolites were found in all 3 soil types, although in varying quantities. Dimethoate was converted to its oxygen analogue and 2 unknown metabolites. Conversion to dimethoxon was faster in the more moist soils, and levels were generally greater. Dimethoate carboxylic acid was the only hydrolytic metabolite identified. No major difference in total recovery of applied radioactivity was noted between the 3 soil types. However, less radioactivity was recovered from all soils in the 2nd set of experiments where more moist soils were encountered. The ratios between radio-activity recovered in the chloroform and water fractions and the unextracted soil residues were relatively constant in all 3 soil types in both experiments. Distribution of radioactivity appeared to be influenced somewhat by soil type. Downward movement was slightly more extensive in the loamy sand soil. Increased soil moisture content also promoted downward movement in all 3 soil types and more rapid disappearance of the dimethoate-14C-equivalents. The quantity of unextracted radioactivity remaining in the soil was not significantly different between the 3 soil types. The nature of these soil residues, accounting for a high proportion of the applied dose, remains unknown. It was established that these materials are not neutral phosphorus esters.