Factors Affecting Bank Formation during Surfactant-Enhanced Mobilization of Residual NAPL

Two-dimensional flow cell experiments were used to investigate the flow dynamics and factors affecting tetrachloroethylene (PCE) mobilization and bank formation in an otherwise water-saturated porous medium. Aqueous phase injection rates and flow cell angles were varied to control both buoyancy and viscous forces, and both macroscopic- and pore-scale images were captured and analyzed to determine the effects of these forces on PCE transport characteristics. Results were interpreted in terms of a nondimensional bank number, NBa, which relates the forces on the trapped nonaqueous phase liquid (NAPL) ganglia parallel to the flow direction to those forces perpendicular to the flow. NBa, was found to predict bank formation well except for NBa ≈ 1, where other characteristics may have been important, such as droplet coalescence. Pore-scale observations showed that the mobilized PCE moved through the porous medium as noncoalesced droplets and that some of the trapped NAPL was mobilized through a dissolution/mobilization process.