A hybrid equilibrium model of solute transport in porous media in the presence of colloids

Abstract

Groundwater contaminants adhered to colloid surfaces may migrate to greater distances than predicted by using the conventional advective—dispersive transport equation. Owing to various factors such as changes in soil properties or pumping, colloids are released and aqueous phase contaminants are transported by mobile solid particles. A mathematical hybrid equilibrium model is developed to describe the simultaneous transport and fate of a contaminant and colloids in porous media. The problem is represented by a three-phase model with two solid phases, i.e. colloids and a stationary solid matrix, and an aqueous phase. Sorption and migration processes of the contaminant and colloids are formulated by mass balance equations of respective phases and sorption relations. The interaction of colloids with the contaminant and porous matrix is included in the three-phase transport model by employing the local equilibrium assumption for colloid—contaminant mass partitioning and first-order kinetic relations for colloid—matrix mass partition mechanisms. Numerical solutions are obtained by use of a finite difference scheme to provide estimations of the contaminant and colloid concentrations. A significant sensitivity to model parameters, particularly the contaminant equilibrium distribution coefficients with mobile and immobile colloids, is found. This conclusion has practical implications in fine-grained aquifers with low permeability where the flow rate is slower than the rate of sorption.