Description of atrazine transport in soil with heterogeneous nonequilibrium sorption

Numerous studies have shown that a single sorption rate constant is insufficient to describe nonequilibrium sorption and transport of organic solutes in soil. To investigate the effect of sorption rate (or, more generally, sorption site) heterogeneity on solute transport, we proposed a model assuming a gamma distribution for both sorption rate and partitioning coefficient (abbreviated as GS for gamma-distributed sites). The model was tested by a series of column leaching and batch sorption experiments conducted with atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) over various time scales. Observed asymmetry and a left-hand shift of measured breakthrough curves (BTCs) with increasing how velocity (0.51 to 21.3 cm/h) or decreasing column length (15.5 to 6.0 cm) were successfully predicted by GS with sorption parameters obtained from an independent experiment. However, two other distinct sorption site models, the two-site first-order sorption (TSFO) and the two-site spherical diffusion (TSSD), were not able to describe the observed BTCs unless parameter values were adjusted for each case. In batch sorption experiments, increasing equilibration time (ranging from 1 to 237 h) was found to generally increase the isotherm-derived partitioning coefficient (KD). The increasing trend was independently well-predicted using the GS model with optimized parameters from the column studies. The TSSD was less successful and TSFO the least able to reconcile the entire range of change in K-D. The weak time-scale dependency of GS found in both column and batch experiments supports the assumption that a wide range of heterogeneous sorption sites or domains have been involved in the overall sorption process but to a variable degree as time elapsed. Potential alteration of soil sorption properties by gamma irradiation was also observed.