Migration of colloids in discretely fractured porous media: effect of colloidal matrix diffusion

Numerical simulations of colloid transport in discretely fractured porous m edia were performed to investigate the importance of matrix diffusion of co lloids as well as the filtration and remobilization of colloidal particles in both the fractures and porous matrix. To achieve this objective a finite element numerical code entitled COLDIFF was developed. The processes that COLDIFF takes into account include advective-dispersive transport of colloi ds, filtration and remobilization of colloidal particles in both fractures and porous matrix, and diffusive interactions of colloids between the fract ures and porous matrix. Three sets of simulations were conducted to examine the importance of parameters and processes controlling colloid migration. First, a sensitivity analysis was performed using a porous block containing a single fracture to determine the relative importance of various phenomen ological coefficients on colloid transport. The primary result of the analy sis showed that the porosity of the matrix and the process of colloid filtr ation in fractures play important roles in controlling colloid migration. S econd, simulations were performed to replicate and examine the results of a laboratory column study using a fractured shale saprolite. Results of this analysis showed that the filtration of colloidal particles in the porous m atrix can greatly affect the tailing of colloid concentrations after the co lloid source was removed. Finally, field-scale simulations were performed t o examine the effect of matrix porosity, fracture filtration and fracture r emobilization on long-term colloid concentration and migration distance. Th e field scale simulations indicated that matrix diffusion and fracture filt ration can significantly reduce colloid migration distance. Results of all three analyses indicated that in environments where porosity is relatively high and colloidal particles are small enough to diffuse out of fractures, the characteristics of the porous matrix that affect colloid transport beco me more important than those of the fracture network. Because the propertie s of the fracture network tend to have greater uncertainty due to difficult ies in their measurement relative to those of the porous matrix, prediction uncertainties associated with colloid transport in discretely fractured po rous media may be reduced. (C) 2001 Elsevier Science B.V. All rights reserv ed.