CONTAMINANT/COLLOID CO-TRANSPORT IN A VARIABLE APERTURE FRACTURE

A three-dimensional particle tracking model is developed to characterize the spatial and temporal effects of advection, molecular diffusion, Taylor dispersion, fracture wall deposition, matrix diffusion, and co-transport on two discrete plumes (colloids and aqueous phase contaminants) flowing through a variable aperture fracture. Contaminants travel by advection and diffusion and may sorb onto fracture walls and colloids, as well as diffuse into and sorb onto the surrounding porous rock matrix. Colloids also travel by advection and diffusion and may sorb onto fracture walls, but do not penetrate the rock matrix. A probabilistic form of the Boltzmann law is used to describe attachment of colloids and contaminants onto fracture walls. For colloids that have diffused into the matrix, a linear distribution coefficient governs their sorption; an irreversible kinetic isotherm is employed to describe contaminant sorption onto colloids. Ensemble averaged breakthrough curves of many fracture realizations are used to compare arrival times of colloid and contaminant plumes at the fracture outlet. Results show that the presence of colloids enhances contaminant transport (decreased residence times) while matrix diffusion and sorption onto fracture walls retard the transport of contaminants.