SORPTIVE REVERSIBILITY OF ATRAZINE AND METOLACHLOR RESIDUES IN FIELD SOIL SAMPLES

Predictions of the fate and transport of organic compounds in soils depend on sound sorption models. The impacts of slowly reversible (nonequilibrium) sorption on compound fate are not well quantified. Soil samples containing residues of the herbicides atrazine (2-chloro-4-ethylamino-6-isopropyl-1,3,5-triazine) and metolachlor (2-chloro-N-[2-ethyl-5-methylphenyl]-N-[2-methoxy-1-methylethyl] -acetamide) were collected from fields 2 to 15 mo after their last application to assess the sorptive reversibility of these residues. The apparent sorption constant of the native herbicide, K(app), was determined from the sorbed and solution concentrations after suspension of the sample in water for 24 h. The "equilibrium" sorption constant, K(d), was determined in the same samples from 24 h sorption isotherms of freshly added herbicide, taking into account the fraction of labile native herbicide (f(L)). The ratio K(app)/K(d) varied from 2.3 to 42 and was directly related to the "age" of the residue (i.e., time between sampling and last application). The value of f(L) ranged from 0.056 to 0.60 and was inversely related to the age of the residue. The results indicate that contaminated samples collected from the field can contain a large fraction of contaminant in a slowly reversible sorbed state, and that this fraction increases with time.