ISOTHERM NONLINEARITY AND NONEQUILIBRIUM SORPTION EFFECTS ON TRANSPORT OF FENURON AND MONURON IN SOIL COLUMNS

The influence of nonlinear sorption and prior sorbate/sorbent sorbent exposure time on transport of fenuron and monuron in laboratory soil c olumns was studied. Observed monuron breakthrough curves (BTCs) were w ell described using a two-region transport model with a single mass-tr ansfer rate coefficient, independent of prior sorbate/sorbent exposure history. However, levels of soil-resident postelution monuron increas ed considerably with increasing exposure time, indicating increased so rption nonequilibrium with aging. Significant sorption nonequilibrium was observed for fenuron in spite of relatively high water solubility, low sorption, and relatively short sorbate/sorbent exposure time (ca. 1-2 weeks). Sorption of fenuron was measured at solution concentratio ns up to the aqueous solubility limit. Batch data were reasonably well described by two distinct Freundlich isotherms depending on the conce ntration range. Accordingly, fenuron retardation factors varied signif icantly with concentration, displaying a minimum at intermediate conce ntration values. A comparison of linear and nonlinear transport modeli ng approaches suggests that failure to consider isotherm nonlinearity may be a source of considerable error when estimating nonequilibrium t ransport parameters from experimental BTCs.