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, <i>N</i><sub>Ba</sub>, which relates the forces on the trapped nonaqueous phase liquid (NAPL) ganglia parallel to the flow direction to those forces perpendicular to the flow. <i>N</i><sub>Ba</sub>, was found to predict bank formation well except for <i>N</i><sub>Ba</sub> ≈ 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.