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Contaminant transport in a fracture with spatially variable aperture in the presence of monodisperse and polydisperse colloids

Chrysikopoulos Constantinos, Scott C. James, Tanya K. Bilezikjian

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Summary

A quasi-three-dimensional particle trackingmodel is developed to characterize the spatial and temporaleffects of advection, molecular diffusion, Taylordispersion, fracture wall deposition, matrix diffusion,and co-transport processes on two discrete plumes(suspended monodisperse or polydisperse colloids anddissolved contaminants) flowing through a variableaperture fracture situated in a porous medium. Contaminantstravel by advection and diffusion and maysorb onto fracture walls and colloid particles, as well asdiffuse into and sorb onto the surrounding porous rockmatrix. A kinetic isotherm describes contaminant sorptiononto colloids and sorbed contaminants assume theunique transport properties of colloids. Sorption of thecontaminants that have diffused into the matrix is governedby a first-order kinetic reaction. Colloids travel byadvection and diffusion and may attach onto fracturewalls; however, they do not penetrate the rock matrix. Aprobabilistic form of the Boltzmann law describes filtrationof both colloids and contaminants on fracturewalls. Ensemble-averaged breakthrough curves of manyfracture realizations are used to compare arrival times ofcolloid and contaminant plumes at the fracture outlet.Results show that the presence of colloids enhancescontaminant transport (decreased residence times) whilematrix diffusion and sorption onto fracture walls retardthe transport of contaminants

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