Radionuclide and colloid co-transport in a coupled fracture-skin-matrix system

A numerical model is developed for studying the transport of colloids accompanied by radionuclides in a coupled fracture-skin-matrix system. The radionuclides and the colloids are assumed to decay, sorb onto the fracture surface, as well as diffuse into the fracture-skin and rock-matrix. The sorption of the radionuclides onto the mobile and immobile colloids within the fracture is assumed to be linear. The governing equations describing the radionuclides and colloid transport along the fracture and diffusion into fracture-skin as well as rock-matrix, normal to the fracture axis are coupled with each other. The coupled non-linear equations are solved numerically with fully implicit finite difference method. Constant concentration is assumed at the inlet of the fracture for both colloids and radionuclide. A varying grid is adopted at the fracture and skin interface to capture the flux transfer. Sensitivity analysis is performed to investigate the effect of various colloid properties on the colloid concentration in the fracture-matrix coupled system in the presence of fracture skin. Results suggests that as the colloid velocity increases the diffusion of colloids into the fracture-skin decreases due to the low residence time available for the colloids, resulting in higher colloidal concentration in the fracture. It is observed that as the filtration coefficient increases, the colloidal concentration in the fracture decreases as more colloids are filtered from the aqueous phase. The reverse mechanism occurs in the case of remobilization coefficient. Further, the effect of colloidal dispersion coefficient and diffusion into the fracture-skin on colloid migration was analysed. The concentration profiles for various diffusion coefficients of colloids in the fracture-skin pose an unusual behavior.