MODIFICATION OF ATRAZINE DESORPTION DURING FIELD INCUBATION EXPERIMENTS

The effect of field aging on atrazine desorption was determined. C-14-ring-labelled atrazine was applied to the top of soil columns in field plots. The columns were periodically removed from the field for 16 months after application. The desorption isotherms of [C-14]atrazine residues were determined on the surface soil using the batch equilibration method with 0.01 M CaCl2 solution. The distribution of atrazine residues in soil and liquid phases was determined by total C-14 analysis and HPLC with a radiactive flow detector to identify the [C-14] atrazine derivatives. In laboratory experiments, desorption coefficients were dependent on the adsorbed atrazine concentration; low concentrations were retained to a greater extent and were more difficult to desorb than the higher concentrations. For the field aged residues, desorption K(f) values increased slightly with aging. However, the values were lower than would be expected from the laboratory values if equal adsorbed concentrations are compared. It is not possible, however, to predict the desorption behavior of atrazine in the field samples from the simple adsorption-desorption laboratory studies. It is necessary to take into account the change in the atrazine-soil interactions with aging.