Solute Transport in Layered Soils

In this study, solute transport in multilayered soils where steady flow is dominant was investigated. We considered nonreactive as well as reactive solutes in layered soils with emphasis on nonlinear solute reactivity with the soil matrix. For individual soil layers, solute‐retention mechanisms considered were linear, nonlinear, Langmuir, first‐, second‐, and nth‐order kinetics, and irreversible reactions. The convective‐dispersive equation (CDE) for reactive solutes was solved using the finite difference method. First‐type and a combination of first‐ and third‐type boundary conditions (BCs) for the interface between soil layers were tested. Unlike the first‐type BC, the combined first‐ and third‐type BC always achieved a good solute mass balance in multilayered soils. Physical and chemical properties of each soil layer were assumed to differ significantly from one another. For all retention mechanisms used, our simulation results indicated that solute breakthrough curves (BTCs) were similar, regardless of the layering sequence in a soil profile. This finding is consistent with an earlier finding (Selim et al., 1977), where a linear adsorption mechanism was dominant and contrary to that of Bosma and van der Zee (1992) for nonlinear adsorption. Experimental results based on miscible displacements from soil columns of a two‐layer system [sand over Sharkey (very‐fine, smectitic, thermic Chromic Epiaquerts) clay and Sharkey clay over sand] support our simulation results. Specifically, BTCs for pulse inputs for tritium, as well as for the Ca–Mg system, support the above conclusion. All tritium and Ca–Mg BTCs were well predicted with our multilayered model where independently derived solute physical and retention parameters were implemented.