MODELING COLLOID TRANSPORT AND DEPOSITION IN SATURATED FRACTURES

A model is developed to describe the transport of colloids in a satura ted fracture with a spatially variable aperture, accounting for colloi d deposition onto fracture surfaces under various physicochemical cond itions. The fracture plane is partitioned into unit elements with diff erent apertures generated stochastically from a log-normal distributio n. The model also accounts for colloid size exclusion from fracture el ements with small apertures. Both equilibrium and kinetic colloid depo sition onto fracture surfaces are investigated. Colloid surface exclus ion is incorporated in the dynamics of kinetic deposition. The impact of deposited colloids on further colloid deposition is described by ei ther a linear or a non-linear blocking function. The resulting system of governing partial differential equations is solved numerically usin g the fully implicit finite difference method. Model simulations illus trate the presence of preferential colloid transport in the fracture p lane. It is shown that size exclusion increases the dispersion of coll oids and leads to earlier breakthrough, especially for large-size part icles. Furthermore, it is demonstrated that surface exclusion enhances colloid transport, and the assumption of clean-bed media may underest imate liquid-phase colloid concentrations. (C) 1997 Elsevier Science B .V.