CAS RN: 60-51-5

Soil Adsorption / Mobility

The Koc values for dimethoate were measured to be 18 and 36 in a clay loam soil and clay soil, respectively(1). In other studies, the Koc was measured to be 5.2(2), 50(3) and 17(4) in unspecified soils. The Pesticide Properties Database lists the experimental Koc value for dimethoate as 20(5). According to a suggested classification scheme(6), these Koc values suggest that dimethoate will have very high mobility in soil. Average dimethoate losses due to leaching of various soil columns with the equivalent of 2.5 cm of rain ranged from 39.6% (clay) to 78.6% (sand)(7). In four soils containing less than 1% organic content, the soil TLC Rf values ranged from 0.89 to 0.97(8). The soil TLC Rf values in two other soils (0.35-1.05% OC) was 0.40-0.50 and was not affected by pH or salt concentration changes(9). Of 7 values sited in literature the mean Kd is 0.45(10). Kds were given for soil A (2.4 % clay, 0.8% silt, 96.4% sand, 2.1% organic material, pH 5.3) and soil B (13.4 % clay, 10.8% silt, 75% sand, 1.5% organic material, pH 6.4) as 0.08 and 0.05, respectively(11).
No dimethoate migration was observed below 3 inches in field sandy loam soil treated at a rate of 1 lb/acre dimethoate up to three times and analyzed for dimethoate up to 14 days after application(1). It has been reported that the downward movement of dimethoate in the field depends somewhat on the soil type; it increases with increasing soil moisture content(2). It was estimated that dimethoate could move approximately 2.5 cm in 1 month in soil containing 42.9% moisture; movement thereafter will probably decrease over time(3).
Soil sorption constants based on the organic carbon content of 15 pesticides were measured using 2 soils (clay loam and high clay) at 0.01, 0.1 and 1.0 ppm pesticide. The soil sorption coefficients ((ug pesticide/g soil)/(ug pesticide/g water)) for dimethoate were 0.8 + or - 0.3 in clay loam and 0.5 + or - 0.1 in high clay soil. The soil sorption constants were 18 and 36 respectively, with a mean of 27. Significant correlations were found between organic carbon content and water solubility, octanol/water partition coefficient, retention time in reversed phase high pressure liquid chromatography and molecular wt.
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