A PLATE MODEL FOR SOLUTE TRANSPORT THROUGH AGGREGATED SOIL COLUMNS .1. THEORETICAL DESCRIPTION

We have designed a plate model to simulate solute transport through aggregated soils. We assumed that the flow regime is composed of a mobile phase, an immobile phase, and a mixing phase as a boundary between the mobile and immobile phases, and that the diffusional mass transfer between mobile-mixing and immobile phases is proportional to the concentration difference between the two phases. The anion and cation concentration in each phase, which preserves the electroneutrality, depends on anion exclusion, anion adsorption and cation exchange effects. We have used Schofield's equation based on Gouy's theory to describe the anion exclusion effect, and we assumed that the anion adsorption effect is proportional to the concentration of the soil solution. Furthermore, we have assumed that the salt-sieving effect caused by anion exclusion in the mixing phase will determine the degree of the reduction of interdiffusion between mobile-mixing and immobile phases. We have described the cation exchange effect in terms of a binary cation exchange isotherm between monovalent and divalent cations with a uniform equilibrium constant. We have assumed that the calculation for the cation exchange effect should be conducted after the anion concentration in the soil solution has been determined, taking into account electroneutrality in the soil solution.