Solute transport through saturated soils: a study of the physical non-equilibrium model

A two-region physical non-equilibrium transport model incorporating mobile and immobile regions, originally developed by Van Genuchten and Wieringa (1976), was used to predict the movement of solutes through saturated soils. Freundlich's non-linear isotherms were coupled with this transport model to account for the adsorption phenomena. A CSMP III (Continuous Syustem Modeling Program) program was used to solve this model numerically. The model was tested for 3 sets of experimental data having different soil and solute properties. The experimental curves were fitted by adjusting the 3 parameters, mass transfer coefficient (α), distribution factor for sorption sites in the mobile region (f), and the fraction of mobile water content (?). The model predicted well only for fairly conservative solutes. To predict the movement of highly reactive solutes such as Cd, the model needs further modification. Effects of dispersion coefficient (D), α, f, and ? on the breakthrough curves were studied for highly reactive solutes. Considerable influence was exerted by α and f. But D and ? showed little effect in highly reactive solutes (a term used for solutes with high adsorption coefficient K and high α), although these effects were significant for mild solutes. Solute concentration profiles for a semi-infinite column (as in the field situation) were predicted for three different cases and the penetration depths were compared with those obtained from Green and Ampt profiles. Green and Ampt profile was found to be a good approximation to find the penetration depths in environmental impact assessment studies for mildly reactive solutes exhibiting fairly sharp solute fronts.