Transport of Reactive Solutes in Soils: A Modified Two‐Region Approach

A modified two‐region approach that accounts for chemical and physical nonequilibrium of solute behavior in soils was developed. Chemical nonequilibrium was described by a second‐order two‐site model, while physical nonequilibrium was represented by a two‐region (mobile‐immobile) approach. Model validity was based on predictions of atrazine [2‐chloro‐4‐(ethylamino)‐6‐(isopropylamino‐ s ‐triazine] miscible displacement experiments in a Sharkey clay soil (very fine, montmorillonitic, nonacid, thermic Vertic Haplaquept) for different aggregate sizes, flow velocities, column lengths, and flow interruption. Independently measured model parameters from kinetic batch experiments were used in model validation. Two model formulations were evaluated. Model I was based on the classical two‐region approach where the soil matrix was divided into two fractions. In Model II, we assumed that the rate of reaction within each soil region was a function of the total number of vacant sites in the soil. Thus, the partitioning coefficient f of the two‐region concept, which is difficult to measure, need not be specified and the amounts retained by each soil region is solely a function of reaction rate coefficients. Model I with f = F ( F = θ m /θ, mobile/total water contents) provided the worst atrazine predictions. Moreover, based on Model I predictions with f = 1 and f = 0, the significance of physical nonequilibrium was dependent on experimental constraints such as aggregate size and flow velocity. Based on root mean squares, however, the best overall predictions were obtained using Model II. We concluded that Model II, which requires fewer parameters, is superior to Model I in its prediction capability for transport results.