The soil-water partitioning coefficient (Kd) is an important parameter for predicting the transport of herbicides in soils. We studied the sorption behavior of two s-triazines [atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5 triazine) and prometon (2,4-bis(isopropylamino)-6-methoxy-s-triazine)] and two thiocarbamates [EPTC (s-ethyl dipropylthiocarbamate) and triallate (s-(2,3,3-trichloroallyl)diisopropyl-thiocarbamate)] on two soils [Hanford sandy loam (HSL) (coarse-loamy, mixed, nonacid, thermic Typic Xerorthents) and Tujunga loamy sand (TLS) (mixed, thermic Typic Xeropsamments)] with varying organic matter content and soil texture. The Kd-values were determined with a mass balance equilibrium batch technique in which the solution as well as the sorbed phase concentration was directly measured. Two grams of soil were equilibrated with 20 mL of aqueous herbicide solution for 24 h at 25 ± 1 °C. Four concentrations of each herbicide at one-half, one-fourth, and one-sixth, and one-eighth of their respective aqueous solubilities were selected for the experiments. Sorption constants were calculated with both linear and nonlinear Freundlich isotherms. The exponents 1/n of the Freundlich isotherm varied between 0.767 and 0.984, thus indicating considerable nonlinearity in some of the isotherms. The K∞-values (soil-water partitioning constants based on the organic C fraction of a soil) were calculated as the ratio of the measured Kd-values and the soil organic fraction of the HSL and TLS soils. Using the mass balance equilibrium method, the average K∞-values of atrazine, EPTC, prometon, and triallate for the two soils were found to be 54, 196, 76, and 2838 m3/Mg, or 0.42, 0.50, 0.21, and 0.88 times those estimated from measured soil solution concentrations only. Since the latter values are much closer to reported literature values (most of which were determined from measured solution concentrations only), we believe that the variability and ambiguity of current literature K∞-values may be due largely to inaccurate determination of Kd-values. We also estimated the K∞-values from basic soil physical and chemical properties by using previously reported predictive equations.
Contribution from the USDA-ARS, and the Univ. of California, Riverside, CA 92521.