Hydrochemical solute transport model in subsurface environment: Model development and laboratory tests

This paper presents a numerical model for predicting solute transport with multicomponent chemical reactions in subsurface. A nonlinear least-squares estimation by the Levenberg-Marquardt method is adopted for obtaining numerical solution of the chemical reaction term in the advection-dispersion equations. To test the reliability of the model, a laboratory experiment is performed with continuous injection of KCl solution into an undisturbed soil column. The values of cation exchange capacity of the soil used in the experiment have spatial distribution with a range from 15 meq/100g in the upper part of the column to 5 meq/100g in the lower part. Based on the numerical and experimental results, the effect of the choice of selectivity coefficient on the cation concentrations is investigated. When the exponential function with respect to the equivalent fraction of K⁺ on sorption site is employed for selectivity coefficient between Ca²⁺ and K⁺, the calculated results by the model developed here are in good agreement with the measured vertical distributions of the cation concentrations in solution and sorption sites. The use of fixed selectivity coefficient results in poor prediction accuracy. It is concluded that the accuracy of model predictions can be improved by suitable choice of selectivity coefficient for the soil being considered.