Reactive solute transport with a variable selectivity coefficient in an undisturbed soil column

The spatial distribution of major ion concentrations limits the predictability of solute transport processes in field soils. Therefore, it is important to analyze solute transport with chemical reactions based on results obtained from field soils and numerical simulation, A simulation model with cation-exchange reactions was developed and applied to solute-transport analysis of an undisturbed field soil. Chemical reaction terms in the convective-dispersive equation were estimated by the Levenberg-Marquardt nonlinear least-squares regression technique to satisfy physical and chemical processes simultaneously, The reliability of the model was tested with liquid-phase and solid-phase concentrations of measured spatial distributions of Ca2+, Mg2+, Na+, and K+ after continuous infiltration of KCI solution into an undisturbed soil column, The experimental results revealed that the selectivity coefficients for Ca-Na and Ca-Mg exchange could be kept constant, while those for Ca-K exchange increased with the equivalent fraction of K+ in the solid phase. The effects of the exchange selectivity coefficient on reactive solute transport are discussed based on the simulation results. When a constant selectivity coefficient was used, the model failed to predict the spatial distributions of cation concentrations in the solid phase, Thus, model predictions can be improved by use of variable instead of constant selectivity coefficients.