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Effect of Cation Exchange on Calculated Hydrodynamic Dispersion Coefficients¹

ABSTRACT

A semiempirical approach based on cation exchange theory is used to express the reaction term of the transport equation for unsaturated transient flow of water into soil. The system described is the horizontal infiltration of a 1M solution of KCl into an initially air-dry, Ca-saturated silty clay loam soil. Values of the dispersion coefficient were calculated using an expression for the reaction term which assumes the adsorbed concentration of a cation to be a function of its concentration in solution, as well as of the negative charge concentration of the solution. These dispersion coefficient values were compared to those obtained with a reaction term which assumed the adsorbed concentration to be a function of the concentration in solution only. The results indicate that the dispersion coefficient is affected only slightly by the two choices of the expression for the reaction term. The fact that the dispersion coefficients calculated for the cation using the two models are of the same order of magnitude suggests that the reaction is described adequately by both models. The dispersion coefficient values obtained for Cl^- and K⁺ are about one order of magnitude higher than expected. This cannot be explained by the choice of the reaction term and may be due to the presence of immobile water and/or to anion exclusion.

¹ Contribution from Department of Land Resource Science, Univ. of Guelph, Guelph, Ontario, Canada. The authors acknowledge support from the Natural Sciences and Engineering Research Council of Canada and the Ontario Ministry of Agriculture and Food.

² Graduate Student (now Research Assistant, Dept. of Earth Sciences, Univ. of Waterloo, Ontario) and Professor, respectively.

Received for publication November 10, 1983. Accepted for publication July 18, 1984.