Anion Exclusion and Coupling Effects in Nonsteady Transport Through Unsaturated Soils: I. Theory

Abstract

AbstractA simulation model for the simultaneous transport of anions and water is developed, taking into account physico‐chemical interaction between the solutes and the soil matrix. The effects of convection, ionic diffusion, mechanical dispersion, anion exclusion (negative adsorption), and coupling phenomena are considered jointly. The dependence of the osmotic efficiency coefficient and anion exclusion on soil water content and solution concentration are estimated from diffuse double‐layer theory and hydrodynamic considerations. These estimates compare favorably with available data on clays and soils. The non‐steady flow equations are solved by a numerical approach that eliminates numerical dispersion. Based on an infiltration experiment which is characterized by relatively high water contents and rapid flow, data showed that osmotic gradients are of minor importance. A slight anion exclusion brought about some improvement in the comparison between the theoretical and experimental data.