Using vertical circulation wells for partitioning tracer tests and remediation of DNAPLs

Two different vertical circulation well (VCW) systems (Type A and Type B) were used to detect and subsequently remove tetrachloroethylene (PCE) spilled in a three-dimensional sand tank. The Type A system has one injection interval and one extraction interval. The Type B system has two injection intervals separated by an intermediate extraction interval. PCE was detected through partitioning tracer tests using ethanol and 2,2-dimethyl-3-pentanol as the non-partitioning and partitioning tracers, respectively. The concentration breakthrough curves for the tracers exhibited significant chromatographic separation due to partitioning of the 2,2-dimethyl-3-pentanol into the PCE. Because of the unique streamline distribution generated by the VCW systems, injected solutions did not sweep through the entire contaminated pore volume, which resulted in less than 100 percent detection of the spilled PCE. However, multiple tests along the borehole of the well were used to delineate the vertical profile of PCE residual saturation. Three food-grade surfactants were used to remediate the spilled PCE. For surfactant-enhanced solubilization, the PCE effluent concentrations for the VCW systems were approximately eight (Type A) and 13 (Type B) times higher than that of a pump-and-treat system using water alone. The VCW effluent concentrations are actually diluted because the extraction rate exceeds the injection rate. Removing the effects of dilution occurring in the borehole, the concentrations leaving the treatment zones around the VCW are found to be 32 (Type A) and 52 (Type B) times higher than using water alone. During surfactant-enhanced mobilization, the Type A VCW system was unable to capture all of the PCE-surfactant microemulsion as it migrated downward. The uncaptured PCE could have negative impacts on ground water quality due to its increased solubility. The TS,pe B system was found to be hydraulically more efficient than the Type A system both in terms of overall PCE removal and capturing mobilized microemulsions.