Surfactant enhanced removal of PCE in a partially saturated, stratified porous medium

Abstract

Dense nonaqueous phase liquids (DNAPLs), such as perchloroethylene (PCE), pose a significant threat to the environment, specifically to our drinking water present in aquifers. In many instances, a DNAPL will become 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 large, two-dimensional flow container, packed with a fine sand layer surrounded by coarse sand, was therefore studied in this work. A 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 bottom 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 redistribute 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 simultaneously extracting solution at one or more locations at the bottom of the porous medium. Effluent samples were analyzed for PCE concentrations. Volumetric water and PCE content values were determined several times at a number of locations by means of dual-energy gamma radiation measurements. The coarse sand in the upper portion of the porous medium was cleaned rapidly by the surfactant flushing. Removal of PCE from 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 zones 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 force the surfactant solution through the entire fine layer, i.e., to hydraulically control the surfactant flushing, while simultaneously increasing the amount of solubilized PCE.