In–Well Vapor Stripping
In-well vapor-stripping is a remediation technology designed to remove dissolved volatile organic compounds from groundwater. The in-well vapor-stripping system comprises an engineered and a hydrologic component that operate in unison to form an in situ recirculation pattern. The engineered system is driven with compressed air, utilizing an air-lift pumping scheme that volatilizes dissolved organic compounds. The volatile vapors are removed from the gas stream above the ground surface and pumped water is infiltrated into the hydrologic system below the ground surface. This technology was demonstrated at Edwards Air Force Base (AFB) near Mojave, California, by collaborating researchers from Pacific Northwest National Laboratory(PNNL) and Stanford University as part of the interim cleanup activities at the base. Preliminary results on the performance of the demonstration system have been reported and indicate a significant reduction in the concentration of contaminant, dissolved trichloroethylene (TCE), around the demonstration well.
Numerical simulations and analyses were applied to the design, operational visualization, and performance prediction activities of the Edwards AFB in-well vapor-stripping demonstration. These numerical analyses were conducted to demonstrate and assess the utility of numerical simulation in the field installation and operation of in-well vapor-stripping technologies. All simulations were executed with the STOMP simulator, which was developed by the PNNL for the U.S. Department of Energy under the Arid Soils Integration Demonstration Program. Steady-flow simulations prior to the field design and installation were used to assess design concepts for the vapor-stripping well, to compute preliminary operational parameters, to locate monitoring equipment, and to predict system performance. These design simulations were based on a conceptual model of the hydrologic system, which was generated from core samples and laboratory analyses. Besides providing quantitative data on pumping/infiltration rates, zones of influence, water table mounding heights, recirculation flow rates, ponding heights, and equilibrium reduction rates, numerical simulations additionally provide a window to the subsurface through visualization of the recirculating flow fields, saturation mounds, and dissolved solute concentrations.
Numerical simulations to predict operational performance of the in-well vapor-stripping system at Edwards AFB were executed based solely on field data for ponding heights and air-to-water volumetric flow rate ratios. Redevelopment activities at the demonstration site to flocculate dispersive clays increased hydraulic conductivities of the vadose zone strata threefold between the period from January to April 1996. Three simulations of a continuous operational field test between January 16 and March 19, 1996 (63 days) were executed that differed in hydraulic conductivities for the vadose zone strata. These simulations were designed to bracket the operational performance of the demonstration system. Simulation results using low-permeability isotropic hydrologic properties showed close agreement in the timing sequencing of TCE concentration reductions at the monitoring wells and demonstrated the field observation that shallower zones displayed significantly lower dissolved TCE concentrations. Simulation results using high-permeability isotropic and anisotropic hydrologic properties showed advanced timing on the sequencing of TCE concentration reductions at the monitoring wells when compared against the field data. Both simulations predicted lower dissolved TCE concentrations within the recirculation zone, which indicates incomplete or nonequilibrium stripping of dissolved TCE during the air-lift pumping. Stripping rates of TCE from field results were estimated to be between 80% and 90%.
White, M. D., and T. J Gilmore. 1996. Numerical Analysis of the In-Well Vapor Stripping System Demonstration at Edwards Air Force Base. PNNL-11348, UC-2010, Pacific Northwest National Laboratory, Richland, Washington.
Gilmore, T. J, S. M. Gorelick, M. J. Pinto, O. Taban, M. D. White, F. A. Spane, Jr., and S. Ballard. 1996. Performance Assessment of the In-Well Vapor-Stripping System. PNNL-11414, UC-2010, Pacific Northwest National Laboratory, Richland, Washington.