A Numerical Study of Barometric Pumping

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A Numerical Study of Barometric Pumping

• Jeff Sondrup
• AgE 588
• Fluid Mechanics of Porous Materials
• April 11, 2001

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Presentation Outline

• Introduction
• Gas Transport Barometric Pumping
• Model Description
• Model Results
• Conclusions

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Subsurface Disposal Area, INEEL
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VOC Background at the SDA

• VOCs first discovered in GW near SDA in 1987
• Soil gas survey confirmed SDA pits and trenches
  were a VOC source
• Inventory search indicated sludges containing
  VOCs from Rocky Flats buried in SDA (1966-70)
• Primarily carbon tetrachloride (CCl4) with TCE,
  PCE, and TCA
• Vadose zone vapor sampling indicates a large
  plume
• GW concentrations ND to slightly above MCL
• Modeling estimates GW concentrations to peak
  decades in the future at several times MCL
• ROD signed in 1994, Soil Vapor Extraction (SVE)
  preferred alternative
• Five extraction wells began operating 1996,
  removed 75,000 lbs TVOCs, 48,000 lbs CCl4

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Gas Transport Mechanismsin the Vadose Zone

• Advection (contaminants travel with the bulk
  movement of air)
• Natural water displacement, barometric pressure
  changes, density
• Induced drilling, soil vapor extraction (SVE)
• Diffusion (random motion of molecules)
• Sorption (contaminants adhere to the rock/soil)
• Vapor-Liquid Partitioning (contaminants move into
  and out of air-water)

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Barometric Pumping
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Barometric Pressure Data
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Model Geometry Grid
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Hydraulic Properties of Fractured Basalt and
Sediment
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Properties ofCarbon Tetrachloride
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Barometric Pressure Data
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Barometric PressureSine Wave Approximation
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Base Case Simulation(No Barometric Pumping)
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Barometric Pumping(Square Wave Approximation
Dt1 day)
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Barometric Pumping(Square Wave Approximation
Dt10 day)
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Barometric Pumping(Sine Wave Approximation Dt1
day)
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CCl4 Vertical Profile (1 year)
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CCl4 Vertical Profile (5 years)
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CCl4 Mass Remaining in VZ
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CCl4 Mass Accounting(Barometric Pumping, Square
Wave, Dt1 day)
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Conclusions

• Time step important when simulating BP
• Square wave approximation is reasonable if
  pressure patterns predictable and repeatable
• BP impact small but can be important
• Impact is site and event specific (depends on
  contaminant, location, pressure patterns,
  subsurface)
• Diffusion is the dominant mechanism
• BP important for passive soil venting (gas
  extraction)