Soil Vapor Extraction Limitations and Enhancements

1. Soil Vapor Extraction Limitations and Enhancements LeeAnn Racz AgE 558 Semester Project April 2001

2. Outline • Theory • SVE Mass Removal Performance • Gas Extraction Methods • Application • Limitations to SVE • Enhancements to SVE • Remaining Uncertainties/Challenges

3. Theory • Removes soil gas under vacuum from soil matrix • Mass transfers from aqueous and sorbed phases to gas phase in order to re-establish equilibrium • Model assumes mass transfer between gas and solid phases occurs via continuous film of water (wetting fluid)

4. Mass Transfer Processes in the Vadose Zone (Armstrong et al. 1994) Advecting Air Air/Water Partitioning (Volatilization) Dissolved Contaminant Sorbed Contaminant Soil Moisture Soil Grain Water/Solid Partitioning (Desorption)

5. SVE Mass Removal Performance • First Stage • Removes pure product • System in equilibrium • High off-gas concentrations • Relatively short duration • Henry’s law dominates • High organic content can have partitioning between liquid and solid phases in equilibrium

6. SVE Mass Removal Performance • Henry’s law Pi = HiCi or Hi = CwCa where Pi = partial pressure in gas phase Ci and Cw = concentration in aqueous phase Hi = Henry’s law constant for phase partitioning of i Ca = concentration in gas phase

7. SVE Mass Removal Performance • Partitioning between liquid and solid phases • Expressed as linear Freundlich isotherm • Valid for soils with >0.1% organic carbon Kd = Cs/Cw and Kd = foc/Koc where Kd = distribution coefficient Cs = concentration in sorbed phase foc = mass fraction of organic carbon Koc = organic carbon partitioning coefficient

8. SVE Mass Removal Performance • Second Stage • Transition from first to third stages • System is in non-equilibrium • Quickly declining mass removal rates

9. SVE Mass Removal Performance • Third Stage • Also in non-equilibrium • Partitioning between soil gas, soil moisture and soil solids limit the mass transfer rate to mobile gas pathways • Non-zero asymptote

10. SVE Mass Removal Performance • Non-equilibrium mass transfer (second and third stages) • Rate limiting factors in mass transfer process • Modeled as first-order kinetic mass transfer relationships • Diffusive mass transfer between air and water driven by concentration gradient between average concentration in water phase and equilibrium concentration at water/air interface • Kinetically limited desorption from soil grains to water phase

11. Gas Extraction Methods • Active • Involves introducing fresh air into soil • Apply vacuum by mechanical means to draw soil gas from soil matrix • Passive • Screened well installed • Open to atmosphere • Gas flows from soil matrix out through open well when subsurface gas pressure greater than barometric pressure

12. Limitations to SVE • Well suited for: • Vapor removal from moist sand and granular soils • Soils with increased gas permeability • Removing VOCs and LNAPLs

13. Limitations to SVE • Not so well suited for: • Removing contaminants from capillary fringe • Low relative permeability to soil gas flow makes diffusion the rate-limiting process • Sites with high water tables • Mixed contamination • Includes nonvolatile compounds and DNAPLs • Sites without sufficient moisture • If too dry, increases sorption capacity of soil

14. Enhancements to SVE • Synergistic Effects • Pump and treat ground water • Leaves dewatered area treatable by SVE • SVE vacuum produces air flow • Enhances effects of aerobic microbial activity • Air sparging • Injects air into ground water and extracts volatile portion to unsaturated zone • Extracts gas and controls vapor migration

15. Enhancements to SVE • Soil Heating • Increases volatility of contaminant to gas phase • Reduces mass transfer limitations in non-equilibrium conditions • Useful for removing chlorinated compounds and compounds with higher boiling points • Methods • Hot air • Electrical heating • Microwave energy

16. Enhancements to SVE • Pulse Pumping • Theory: turn off vacuum at tailed portion of effluent curve and allow air phase concentrations to recover, then reapply vacuum • Intended to give lower energy costs and effluent treatment costs • However, slow but continuous pumping gives best performance

17. Enhancements to SVE • Passive SVE • Cap well with check valve to prevent air from flowing into subsurface through well • Lightweight ball in conical seat • Solenoid valve • Install surface cover around well • Prevents short-circuiting adjacent to well to increase horizontal flow to well • Prevents clean air from entering subsurface diluting contaminant concentrations • Increases differential between surface and subsurface gas pressures

18. Remaining Uncertainties/Challenges • Difficult to use in removing DNAPLs and other recalcitrant compounds • Difficult to use in certain soil types • Low porosity • High moisture content • Capillary fringe

19. Remaining Uncertainties/Challenges • Combine with other methods for synergistic effects • Nonzero asymptotic characteristic of nonequilibrium • Better measuring techniques to obtain data for better designs • Uncertainties in heterogeneous media • Otherwise, rely on further refined curve-fitted models

20. E-mail: leeann.racz@osan.af.mil