Controlled Mobilization and Recovery of Dense Nonaqueous Phase Liquid From a Heterogeneous Porous Media System

Remediation of a persistent class of contaminants known as dense nonaqueous phase liquids (DNAPLs) from saturated subsurface systems is a complex environmental problem. As DNAPL migrates through porous media, it becomes entrapped in pore spaces due to capillary forces. This entrapped residual has neither a predictable nor a uniform distribution. If a sufficient volume is released, relatively long length-scale features, or pools, can form over a range of length scales. Although pools can be removed through pumping over extended periods of time, such approaches are usually not practical since a large fraction of DNAPL remains in the system as residual. To investigate this problem of remediating DNAPL, three-dimensional laboratory experiments were conducted in a saturated heterogeneous porous medium by using a combination of three remediation technologies: a surfactant, a brine barrier, and vapor extraction. Surfactant was used to mobilize entrapped DNAPL downwards by decreasing interfacial tension. A dense brine barrier was established below the source zone prior to the surfactant flush to control downward mobilization. The buoyancy forces associated with the brine barrier promoted pooling of mobilized DNAPL on top of the barrier. Approximately 72% of the initial trichloroethylene (TCE) mass was removed by extracting the DNAPL pool formed on the dense brine barrier. Dewatering of the system also resulted in further reductions of buoyancy forces. Vapor extraction was performed after dewatering, decreasing the remaining residual to less than 1% of the initial mass. The use of additional surfactant, implementation of a stepped down extraction point system and the use of a force-gradient approach were considered to decrease entrapment of mobilized DNAPL in fine sand regions and to increase recovery during the well extraction portion of the experiments. Well recovery using these techniques ranged from 76--86%.