Analysis of one-dimensional multispecies transport experiments in laboratory soil columns

Abstract

A numerical scheme based on the mixing cell concept is used to analyse multispecies solute transport under the condition of steady flow. Explicit examples are presented for the case of two chemical species that are reacting during transport. Also, unlike the standard mixing cell approach, the improved method used here maintains second-order accuracy. The mixing cell scheme is applied to various data sets from multispecies transport experiments. First, the scheme is used to analyse the snow-plow effect as observed in a laboratory experiment. This phenomenon results from large concentration differences in the solution and solid phases within the soil profile. The analysis revealed that the solute exchange process is time dependent, rather than being in equilibrium. In another case, the scheme was used to analyse the observed snow-plow peaks of Ca resulting from ion exchange reactions between Ca and Mg in a soil profile. The analysis revealed that the selectivity coefficient changes with total cation concentration in the system. The results demonstrate that nonlinear reactions in multispecies transport can be analysed efficiently, accurately, and quickly using the improved mixing cell model.