Migration of Contaminants in Fractured-Porous Media in the Presence of Colloids: Effects of Kinetic Interactions

Abstract

The presence of colloids in groundwater can enhance contaminant migration by reducing retardation effects. The field experiments performed in a water-conducting feature of a shear zone at the Grimsel Test Site in Switzerland indicate that the sorption processes for contaminants on mobile and immobile colloids are kinetically controlled and that colloid filtration occurs. To enable the modelling of those experiments, an appropriate model of colloid-facilitated contaminant transport in fractured-porous media is developed. The physical system is modelled as a single planar fracture with adjacent fully saturated porous rock matrix. Contaminants can diffuse into the rock matrix but colloids cannot. In the mathematical model, the 1D advective contaminant transport along the fracture is coupled with contaminant diffusion into the rock matrix perpendicular to the fracture. Radioactive decay and sorption processes for contaminants in the rock matrix (linear equilibrium), on the fracture surface (linear equilibrium and linear kinetic reactions) and on mobile and filtered colloids (linear kinetic approach), are considered. The model for colloid transport includes filtration of colloids in the fracture and their remobilization. A useful approach is developed that can be applied to adequately describe a natural system (crystalline rock) with our double-porosity model (single fracture integrated into porous rock). Numerical solutions are obtained using an implicit finite difference scheme and realized in the transport code COFRAME. The transport code is validated by the comparison of calculated results with field experiment data. Some sets of simulations are performed to study the effect of kinetics for interaction processes.