Soil Vapor Extraction Limitations and Enhancements

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Soil Vapor Extraction Limitations and Enhancements

• LeeAnn Racz
• AgE 558
• Semester Project
• April 2001

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Outline

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

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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)

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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)
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SVE Mass Removal Performance

• First Stage
• Removes pure product
• System in equilibrium
• High off-gas concentrations
• Relatively short duration
• Henrys law dominates
• High organic content can have partitioning
  between liquid and solid phases in equilibrium

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SVE Mass Removal Performance

• Henrys law

Pi HiCi or Hi CwCa where Pi partial
pressure in gas phase Ci and Cw concentration
in aqueous phase Hi Henrys law constant for
phase partitioning of i Ca concentration in gas
phase
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SVE Mass Removal Performance

• Partitioning between liquid and solid phases
• Expressed as linear Freundlich isotherm
• Valid for soils with gt0.1 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
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SVE Mass Removal Performance

• Second Stage
• Transition from first to third stages
• System is in non-equilibrium
• Quickly declining mass removal rates

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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

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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

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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

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Limitations to SVE

• Well suited for
• Vapor removal from moist sand and granular soils
• Soils with increased gas permeability
• Removing VOCs and LNAPLs

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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

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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

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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

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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

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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

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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

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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

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• E-mail
• leeann.racz_at_osan.af.mil