Influence of viscous and buoyancy forces on the mobilization of residual tetrachloroethylene during surfactant flushing

The potential for nonaqueous phase liquid (NAPL) mobilization is one of the most important considerations in the development and implementation of surfactant-based remediation technologies. Column experiments were performed to investigate the onset and extent of tetrachloroethylene (PCE) mobilization during surfactant flushing. To induce mobilization, the interfacial tension between residual PCE and the aqueous phase was reduced from 47.8 to 0.09 dyn/cm by flushing with different surfactant solutions. The resulting PCE desaturation curves are expressed in terms of a total trapping number (N-T), which relates viscous and buoyancy forces to the capillary forces acting to retain organic liquids within a porous medium. The critical value of N-T required to initiate PCE mobilization fell within the range of 2 x 10(-5) to 5 x 10(-5), while complete displacement of PCE was observed as NT approached 1 x 10(-3). The interplay of viscous and buoyancy forces during PCE mobilization is illustrated in horizontal column experiments, in which angled banks of PCE were displaced through the columns. These results demonstrate the potential contribution of buoyancy forces to PCE mobilization and provide a novel approach for predicting NAPL displacement during surfactant flushing.