Modeling colloid transport and deposition in saturated fractures

Abstract

A model is developed to describe the transport of colloids in a saturated fracture with a spatially variable aperture, accounting for colloid deposition onto fracture surfaces under various physicochemical conditions. The fracture plane is partitioned into unit elements with different apertures generated stochastically from a log-normal distribution. The model also accounts for colloid size exclusion from fracture elements with small apertures. Both equilibrium and kinetic colloid deposition onto fracture surfaces are investigated. Colloid surface exclusion is incorporated in the dynamics of kinetic deposition. The impact of deposited colloids on further colloid deposition is described by either a linear or a non-linear blocking function. The resulting system of governing partial differential equations is solved numerically using the fully implicit finite difference method. Model simulations illustrate the presence of preferential colloid transport in the fracture plane. It is shown that size exclusion increases the dispersion of colloids and leads to earlier breakthrough, especially for large-size particles. Furthermore, it is demonstrated that surface exclusion enhances colloid transport, and the assumption of clean-bed media may underestimate liquid-phase colloid concentrations.