Colloid-facilitated radionuclide transport in fractured porous rock

Abstract

Numerical methods have been applied for the prediction of colloid-facilitated radionuclide transport through water-saturated fractured porous rock. The presence of colloids may enhance the transport of radionuclides in groundwater by reducing retardation effects. The colloids existing in the groundwater act as carriers, adsorbing radionuclides in their large surface area and moving faster than the average water velocity. With colloids present, the system consists of three phases, i.e. an aqueous phase, a carrier phase and a stationary solid phase. In the basic model, one-dimensional advection in a single planar fracture of infinite extent is coupled with diffusion in the rock matrix perpendicular to the fracture. In this study, a full-equilibrium model was developed to describe the transport and fate of the radionuclides in the fracture. Sorption onto the rock matrix, fracture surface and sorption onto mobile and immobile colloids are included. The effect of colloidal particle size was also considered. Mass partition mechanisms between the colloids and solid matrix and between colloid and contaminant are represented by local equilibrium. In the three-phase system, the three parameters, the retardation coefficient, the hydrodynamic dispersion coefficient and fracture width, are modified to include the equilibrium distribution coefficient of a contaminant with a carrier. In the three-phase model, much smaller retardation and hydrodynamic dispersion coefficients are obtained and the effect of the fracture width is larger. Much faster transport of contaminant has resulted mainly because the radionuclides attached to colloidal particles are not subject to retardation by diffusion into the rock matrix. With the additional consideration of colloidal particle sizes, these effects become ever larger. Numerical solutions for the model were obtained using a fully implicit finite difference scheme. A significant sensitivity to model parameters was discovered, and, in particular, the equilibrium distribution coefficients between a contaminant and the carrier were found to be the most important factors.