Predicting solute transport in soils: Second-order two-site models

Failure of numerous efforts to describe the movement of reactive solutes in soils is often due to inaccurate identification of solute-soil interactions or to the lack of independently derived model parameters. The main objective of this work was to test the applicability of chemical vs. physical nonequilibrium approaches in describing the transport of solutes in soils. The models evaluated are based on second-order two-site approaches (SOTS) with and without consideration of physical nonequilibrium in soils. The capability of these approaches for predicting the transport of metolachlor [2-Chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] in Sharkey clay (very-fine, smectitic, thermic Chromic Epiaquerts) soil columns of different aggregate sizes (< 2, 2-4, and 4-6 mm) was examined. Moreover, two sets of model parameters were independently derived from the kinetic retention experiments, The first set was based on kinetic adsorption isotherms, and the second set utilized both adsorption and desorption kinetic retention (batch) results. Judging from data on the total root mean square error, parameters based on adsorption and desorption batch results provided breakthrough curve (BTC) predictions that are improved over those of parameters from adsorption kinetics only. The coupled physical and chemical nonequilibrium model (SOTS plus mobile-immobile [MIM]: SOTS-MIM) considerably improved ETC predictions for the 2- to 4- and 4- to 6-mm soil aggregate sizes. We conclude that the modified SOTS and SOTS-MIM methods provided an improved description of metolachlor transport in the Sharkey soil. Based on total root mean square errors, the modified SOTS-MIM with parameters derived from adsorption and desorption kinetic retention experiments provided best overall ETC predictions.