Transport of Chloride Ion in a Water‐unsaturated Soil Exhibiting Anion Exclusion

Abstract

AbstractMiscible displacement techniques were used to create Cl‐ concentration profiles in unsaturated laboratory columns of Delhi sand (Typic Xeropsamments), each having a nearly uniform water content. The three steady flow rates used resulted in three different, average water contents. Chloride concentrations near the top of the column were smaller and penetration of Cl‐ in the column was deeper than expected assuming that Cl‐ is a noninteracting solute. Such observations indicate the presence of anion exclusion. This interpretation is further substantiated by chloride and tritium breakthrough curves obtained from a saturated column of the same soil. The saturated experiments show that tritium occupies the entire measured pore volume of the column, but that Cl‐ is restricted to a smaller pore volume. The formulation of the conventional convection‐dispersion theory for solute transport in soil which includes anion exclusion resulted in model calculations that fitted the unsaturated Cl‐ concentration profiles quite well. The dispersion coefficients obtained for the unsaturated profiles increase with water velocity and are lower than those previously reported for comparable water velocities in the same but saturated soil. The dispersivity of the unsaturated soil is also smaller than that reported for the saturated soil. For the experimental conditions used, the effective Cl‐ exclusion volume was found to be independent of water content and velocity and occupied about ten percent of the unsaturated water content.