Ion Transport during Unsteady Water Flow in an Unsaturated Clay Soil

Abstract

AbstractThe transport of many solutes through soils is strongly affected by their reaction with the solid constituents. One of the most common of such reactions is cation exchange. It is, therefore, important to be able to predict the effects of cation‐exchange reactions on cation transport. The purpose of this study was to examine experimentally the effects of two different types of cation exchange on the transport of cations through soil, and to compare the results with simple mathematical predictions. The experiments also permitted an examination of the effect of cation exchange on anion transport via its effect on anion exclusion. The effects of both K‐Ca and Na‐Ca exchange were studied because these have quite different isotherm shapes, Na‐Ca exchange being much less favorable than K‐Ca exchange. The spatial distributions of water, tritium, Cl, Na, K, and Ca were measured following constant‐flux horizontal infiltration of tritiated NaCl or KCl solutions into a Ca‐saturated clay soil. The concentrations of cations both in solution and in the sorbed phase were measured independently. Soil water fluxes throughout the experiments were small enough to ensure that physical and chemical equilibrium was attained. Anion exclusion caused the Cl front to move faster than the tritium front (which moved with the average pore‐water velocity), while exchange resulted in the monovalent cations moving more slowly than tritium. The position of the Cl front was independent of the cation in the invading solution, being determined by the anion‐exclusion properties of the soil in equilibrium with the cation in the initial solution. The shape of the cation profiles, the relationship between the position of the cation fronts in the solution and sorbed phases, and the average position of the cation fronts depended on the shape of the exchange isotherm and the inflowing solution concentration. The penetration of the center of mass of the combined cation fronts (total cation mass per unit soil volume) was successfully predicted using the effective distribution coefficient and a moving coordinate based on the water.