Differential transport and dispersion of colloids relative to solutes in single fractures.

This work employed numerical experiments simulating colloid and solute transport in single parallel-plate fractures, using the random walk particle tracking method, to demonstrate that (1) there exists an aspect ratio of the colloid radius to half the fracture aperture, delta(o), where the average velocities of colloids and solutes are similar. When delta>delta(o), the velocity distribution assumption is satisfied, and the fact that the ratio of the colloid transport velocity to the solute transport velocity, tau(p), decreases as delta increases is well documented in the literature. However, when delta<delta(o), the velocity distribution assumption is violated, and tau(p) increases as delta increases and (2) the Taylor dispersion coefficient and its extension by James and Chrysikopoulos [S.C. James, C. V. Chrysikopoulos, J. Colloid Interface Sci. 263 (2003) 288] will overestimate the colloid dispersion coefficient significantly. Additionally, numerical experiments simulating colloid and solute transport in variable-aperture fractures demonstrated that tau(p) and D(L,)(coll)/D(L,)(solute) decrease with increasing CoV, and the anisotropy ratio only plays a minor role compared to the CoV. These observations have important implications towards the interpretation of colloid transport in both porous and fractured media.