Dissolved and colloidal contaminant transport in a partially clogged fracture

A water‐saturated fracture, partially clogged with porous material coating the fracture surfaces, is considered. Fluid flow and contaminant transport in this fracture are significantly altered relative to an unclogged fracture. Analytical expressions are developed for the water velocities in the clogged and the unclogged regions in the fracture and the asymptotic longitudinal dispersion coefficient for the system. For highly adsorbing dissolved contaminants or large colloids, the slow diffusion within the porous region causes enhanced dispersion. In a standard tracer test, colloidal contaminants will arrive earlier than dissolved tracers for either one of two reasons: (1) colloids confined to the unclogged portion of the fracture will have a larger average velocity or (2) colloids that can diffuse into the porous region have very low Brownian dififusivities, resulting in a large longitudinal dispersion due to the inverse relationship between Taylor dispersion and Brownian diffusion. The average velocity or asymptotic longitudinal dispersion coefficient can be orders of magnitude greater than that for a molecular tracer.