Density-induced advection of organic vapors has been demonstrated to be a significant transport process in granular porous media if the permeability is sufficiently high such that density-induced advection dominates over vapor diffusion. In the context of partially saturated fractured media, the presence of a network of open fractures may lead to rapid rates of density-induced advection of the vapors in the fractures, even though the matrix may have low permeability. To investigate the various factors which affect vapor migration in discretely fractured porous media, a two-dimensional finite element model has been developed whereby the porous matrix is represented by rectangular elements and the fractures are represented as one-dimensional line elements which are superimposed onto the rectangular grid. The cross-sectional model includes the processes of advection due to density and pressure gradients, and vapor and aqueous diffusion in both the fractures and the porous matrix. Phase partitioning between the vapor and aqueous phases and the aqueous and solid phases is assumed to be at equilibrium. Results from simulations involving a single vertical fracture indicate that density-induced advection decreases in importance as the fracture aperture decreases. In this case, there appears to be a critical fracture aperture, above which density-induced advection is the dominant process, and below which vapor diffusion dominates. Simulations that include a network of horizontal and vertical fractures show the importance of matrix properties such as air porosity and water content.

An increase in the matrix air porosity results in a higher storage capacity for vapor-phase contaminants and allows more diffusion to occur from the fractures to the matrix, whereas a higher matrix water content results in an increase in the degree of phase partitioning between the vapor and the water phases. Although these two processes act to retard the migration of the vapor plume, they can be problematic in the context of vapor extraction in fractured porous media. The reason for this is that these processes tend to increase the amount of contaminant mass that exists in the low-permeability matrix. As a result, vapor extraction in fractured porous media can be very difficult. ¿ American Geophysical Union 1994