MODELING OF COLLOID AND COLLOID-FACILITATED CONTAMINANT TRANSPORT IN A 2-DIMENSIONAL FRACTURE WITH SPATIALLY-VARIABLE APERTURE

Mathematical models are developed for two-dimensional transient transp ort of colloids, and cotransport of contaminant/colloids in a fracture -rock matrix system with spatially variable fracture aperture. The ape rture in the fracture plane is considered as a lognormally distributed random variable with spatial fluctuations described by an exponential autocovariance function. Colloids are envisioned to irreversibly depo sit onto fracture surfaces without penetrating the rock matrix; wherea s, the contaminant is assumed to decay, sorb onto fracture surfaces an d onto colloidal particles, as well as to diffuse into the rock matrix . The governing stochastic transport equations are solved numerically for each realization of the aperture fluctuations by a fully implicit finite difference scheme. Emphasis is given on the effects of variable aperture an colloid and colloid-facilitated contaminant transport. Si mulated breakthrough curves of ensemble averages of several realizatio ns show enhanced colloid transport and more pronounced fingering when colloids are subject to size exclusion from regions of small aperture size. Moreover, it is shown that an increase in the fracture aperture fluctuations leads to faster transport and increases dispersion. For t he case of contaminant/colloids cotransport it is shown, for the condi tions considered in this work, that colloids enhance contaminant mobil ity and increase contaminant dispersion.