THE INFLUENCE OF SURFACE SORPTION ON DISPERSION IN PARALLEL-PLATE FRACTURES

Solute transport in a parallel plate fracture is simulated using a ran dom walk model which accounts explicitly for sorption onto the fractur e walls. With the hypothesis that solute must move into the vicinity o f the fracture wall in order to participate in the sorption process, t hree implications follow. (1) In comparison to a nonreactive solute, a sorbing solute requires a greater entrance length along the fracture before transverse homogenization is established. The increase in this entrance length is proportional to the surface retardation factor. (2) At short transport distances, surface sorption leads to a high degree of non-uniform retardation. The transport distance required to establ ish uniform retardation of the entire solute mass varies greatly with geometric conditions and sorption strength. (3) Surface sorption resul ts in enhanced longitudinal spreading of the solute mass in transport regimes which favor advective transport along the fracture relative to transverse diffusion across the fracture aperture. At distances great er than that required for transverse homogenization, an effective long itudinal dispersion coefficient can be defined that describes this enh anced dispersion for a wide range of fluid velocities. The magnitude o f enhanced dispersion increases with sorption strength. (C) 1997 Elsev ier Science B.V.