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A Numerical Study of Barometric Pumping

Explore the impact of barometric pumping on vadose zone contamination using numerical modeling. Investigate gas transport mechanisms, barometric pressure data, and CCl4 vertical profiles. Conclude with insights on simulation accuracy and site-specific implications.

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A Numerical Study of Barometric Pumping

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  1. A Numerical Study of Barometric Pumping Jeff Sondrup AgE 588 Fluid Mechanics of Porous Materials April 11, 2001

  2. Presentation Outline • Introduction • Gas Transport & Barometric Pumping • Model Description • Model Results • Conclusions

  3. Subsurface Disposal Area, INEEL

  4. 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

  5. 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)

  6. Barometric Pumping

  7. Barometric Pressure Data

  8. Model Geometry & Grid

  9. Hydraulic Properties of Fractured Basalt and Sediment

  10. Properties ofCarbon Tetrachloride

  11. Barometric Pressure Data

  12. Barometric PressureSine Wave Approximation

  13. Base Case Simulation(No Barometric Pumping)

  14. Barometric Pumping(Square Wave Approximation Dt=1 day)

  15. Barometric Pumping(Square Wave Approximation Dt=10 day)

  16. Barometric Pumping(Sine Wave Approximation Dt=1 day)

  17. CCl4 Vertical Profile (1 year)

  18. CCl4 Vertical Profile (5 years)

  19. CCl4 Mass Remaining in VZ

  20. CCl4 Mass Accounting(Barometric Pumping, Square Wave, Dt=1 day)

  21. 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)

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