SORPTION OF ATRAZINE AND DICAMBA IN DELAWARE COASTAL-PLAIN SOILS - A COMPARISON OF SOIL THIN-LAYER AND BATCH EQUILIBRIUM RESULTS

The mobility and retention of atrazine and dicamba in six Atlantic Coa stal Plain soils were estimated by soil thin-layer chromatography (soi l-TLC). The soils studied were representative of the major agricultura l regions in Delaware and were sampled, by horizon, to the water table . Four horizons from each profile were leached simultaneously with dis tilled water on one soil-TLC plate. Two values were obtained from each plate: the ratio of the distance traveled by the herbicide center of mass over that traveled by the solvent front (R-m), and a sorption dis tribution coefficient (K-d). The R-m values ranged from 0.06 to 0.94 f or atrazine and from 0.80 to 0.94 for dicamba. Herbicide mobility was found to be greatest in coarse-textured soil horizons that contained l ow levels of organic matter, clay, and Fe and Al oxides. Correlation a nalysis indicated that effective cation exchange capacity, exchangeabl e acidity, exchangeable aluminum, and clay were useful predictive vari ables or both atrazine mobility and sorption. Organic matter was not u seful for predicting soil-TLC derived sorption estimates; however, it was correlated to K-d-batch estimates. Distribution coefficients calcu lated from soil-TLC data were found to be in general agreement with K- d values obtained for the same soils by batch equilibrium techniques. The average K-d-soil-TLC values for atrazine and dicamba were 2.09(+/- 2.24) and 0.03(+/-0.02), respectively. The ratio of the batch K-d to the soil-TLC K-d ranged from 0.1 to 19 ((x) over bar = 1.6, SD = 3.8) for atrazine and from 2.9 to 38 ((x) over bar = 12.6, SD = 8.7) for di camba. Thus, although for some horizons agreement between the two meth ods was good, for other horizons significant discrepancies existed. It is suggested that the soil-TLC gives results under non-equilibrium co nditions, whereas the batch procedure is, by definition, at quasi-equi librium. These fundamental differences may account for the observed di fferences between the two methods. It is also suggested that, due to t his difference, the soil-TLC procedure can provide additional informat ion relevant to herbicide partitioning in the field environment that i s not provided by traditional batch equilibrium techniques. (C) 1998 S ociety of Chemical Industry