Colloid‐facilitated contaminant transport in discretely fractured porous media: 1. Numerical formulation and sensitivity analysis

Abstract

A two‐dimensional numerical model is developed that incorporates the mechanism of colloid‐facilitated transport in discretely fractured porous media. The numerical model accounts for aqueous phase contaminant transport in the fractures and the porous matrix, colloid transport in the fractures, and sorption of the solute. Deep‐bed filtration of the colloids is accounted for, and the solute is allowed to sorb on both the mobile and filtered colloids. The numerical formulation allows for either equilibrium or kinetic sorption reactions onto the fracture walls, the matrix solids, and the mobile and filtered colloids according to either a Langmuir or a Freundlich isotherm. The results of a series of simulations involving a system of parallel fractures explore the importance of mobile colloids on contaminant migration and indicate that if sorption onto the colloids is a slow kinetic process, then the mobile colloids may lead to significantly enhanced contaminant migration.

Mathematical Formulation Requirements and Specifications for the Process Models

“…The factor f (1 ≤ f ≤ 1.5) accounts for the velocity of the colloidal particle flow being larger than that of water (Ibaraki and Sudicky, 1995). This larger particle flow results from the relatively large size of the colloids, which tends to concentrate them in the middle of the pores where the water velocity is larger than the bulk average velocity.…”

“…This larger particle flow results from the relatively large size of the colloids, which tends to concentrate them in the middle of the pores where the water velocity is larger than the bulk average velocity. The factor f v tends to increase with decreasing ionic strength, but cannot exceed 1.5 because colloids cannot move faster than the maximum water velocity, which occurs at the middle of the pores and is equal to 1.5 times the average pore velocity (Ibaraki and Sudicky, 1995).…”

“…From De Marsily (1986) and Ibaraki and Sudicky (1995), the following expression for the k + coefficient can be derived: (6.6) where f is the filter coefficient of the porous medium , f v is a velocity adjustment factor, q is the Darcy velocity , and G B is a dynamic blocking function that describes the variation of the porosity and specific surface with ξ i (James and Chrysikopoulos, 1999). The factor f (1 ≤ f ≤ 1.5) accounts for the velocity of the colloidal particle flow being larger than that of water (Ibaraki and Sudicky, 1995).…”

“…Pseudocolloids are all other colloidal particles, i.e., natural colloids that can be inorganic (e.g., clay, iron oxyhydroxides, silica) or organic (microbes and humic acids (Ibaraki and Sudicky, 1995). Pseudocolloids become carriers of contaminants (including radionuclides) when the corresponding solutes sorb onto them.…”

“…Once attached, colloid detachment (declogging) is generally slow to irreversible (Ibaraki and Sudicky, 1995). Sorption of radionuclides on colloids is controlled by a range of chemical processes such as ion exchange, surface complexation, and organic complexation (Ding et al, 2006;EPRI, 1999).…”