SURFACTANT ENHANCED REMOVAL OF PCE IN A PARTIALLY SATURATED, STRATIFIED POROUS-MEDIUM

Dense nonaqueous phase liquids (DNAPLs), such as perchloroethylene (PC E), pose a significant threat to the environment, specifically to our drinking water present in aquifers. In many instances, a DNAPL will be come trapped and form pools on top of confining layers while migrating through the vadose zone. Residual DNAPL and DNAPL pools form sources of long term contamination, which are difficult to remove by classical pump-and-treat remediation. A physically simulated PCE spill into a l arge, two-dimensional flow container, packed with a fine sand layer su rrounded by coarse sand, was therefore studied in this work. ii water table was maintained near the bottom of the flow container, such that the coarse sand just below the fine layer was unsaturated, but the bot tom of the fine sand layer was at or near saturation. A PCE spill was applied at the center of the porous medium's surface and allowed to re distribute until static equilibrium was reached. The porous medium was then flushed with a surfactant solution (Triton X-100, 4.0% by volume ), using the same application configuration as for the spill, while si multaneously extracting solution at one or more locations at the botto m of the porous medium. Effluent samples were analyzed for PCE concent rations. Volumetric water and PCE content values were determined sever al times at a number of locations by means of dual-energy gamma radiat ion measurements. The coarse sand in the upper portion of the porous m edium was cleaned rapidly by the surfactant flushing. Removal of PCE f rom the fine sand layer, however, proved to be more difficult because most of the surfactant solution bypassed the PCE. Lateral spreading of the surfactant plume occurred near the saturated/nearly saturated zon es in the fine and coarse sand. This lateral spreading, unfortunately, allowed for solubilized PCE, the concentration of which was enhanced by micelle formation, to move into regions previously uncontaminated. The pumping configuration was subsequently changed in an attempt to fo rce the surfactant solution through the entire fine layer, i.e., to hy draulically control the surfactant flushing, while simultaneously incr easing the amount of solubilized PCE. (C) 1998 Elsevier Science B.V. A ll rights reserved.