Surfactant enhanced aquifer remediation at neutral buoyancy

The two most important outstanding issues regarding surfactant enhanced aquifer remediation of DNAPL sites are the risk of downward migration of the DNAPL to lower, uncontaminated groundwater or underlying clean aquifers and the impact of aquifer heterogeneity. The first of these problems has been addressed and solved by this research. The main objective was to prevent vertical migration of microemulsion (and thus dense contaminants) when using surfactant solutions to recover DNAPLs from the subsurface and still remove essentially all of the DNAPL with a reasonable amount of surfactant. When surfactant solution solubilizes DNAPL, the resulting microemulsion is denser than the surrounding fluid and thus tends to migrate downward. Unless a good capillary barrier exists below the DNAPL, vertical migration can cause contamination of clean groundwater below the DNAPL and should be avoided. Since many sites do not have a known lower confining layer with cenainty, this problem needed to be addressed so that surfactant enhanced aquifer remediation could be more widely used and used with acceptable risk and effectiveness.;The DNAPLs used in these experiments were trichloroethylene and perchloroethylene, and the surfactant was sodium dihexyl sulfosuccinate. Isopropanol was used in the surfactant solution to lower the density of the microemulsion that formed when the DNAPL was solubilized and has other benefits as well. This approach, termed neutral buoyancy, was used to prevent vertical migration.;To test the concept, several major steps were taken. First, a suitable surfactant exhibiting classical Winsor phase behavior was selected. Density measurements were made of the microemulsions formed when the surfactant solution solubilized DNAPL, using various alcohols in the solution. Next, column experiments were done to ensure that the surfactant solution exhibited good behavior when used to recover the DNAPL from a sandpack. Finally, a model aquifer was constructed, and experiments designed and performed. Three tank experiments were made to demonstrate the neutral buoyancy concept. The first experiment was designed so that the microemulsion would exhibit a small degree of vertical migration but would still be recovered by the extraction well of the model. The second experiment was designed with a large degree of vertical migration that would render the microemulsion unrecoverable before it reached the bottom of the tank. The third tank experiment used a neutrally buoyant design to demonstrate that DNAPL can be removed from a coarse sand layer on top of a clean fine sand layer that would not be an adequate barrier for downward migration of microemulsion (and thus contaminant) if it were not neutrally buoyant.;The main contribution of this research work was solving the problem of vertical migration of microemulsion so that surfactant solutions could be used at field sites that are not underlain by a capillary barrier. The neutral buoyancy concept was developed and was verified in these experiments.