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Future Directions for Assessing Vapor Intrusion

Future Directions for Assessing Vapor Intrusion. by Todd McAlary, GeoSyntec Consultants, Inc. AEHS VI Workshop October 19, 2004. Outline. Improved Protocols – What’s Coming Qsoil Perimeter crack model vs 0.05>Q soil /Q bldg >0.0001 Barometric Pumping Implications for Data Variability

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Future Directions for Assessing Vapor Intrusion

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  1. Future Directionsfor Assessing Vapor Intrusion by Todd McAlary, GeoSyntec Consultants, Inc. AEHS VI Workshop October 19, 2004

  2. Outline • Improved Protocols – What’s Coming • Qsoil • Perimeter crack model vs 0.05>Qsoil/Qbldg>0.0001 • Barometric Pumping • Implications for Data Variability • Alternative Sampling Options • More discrete in time and space • More integrated

  3. Improved Protocols • API - Collecting and Interpreting Soil Gas Samples from the Vadose Zone: A Practical Strategy for Assessing the Subsurface Vapor-to-Indoor-Air Migration Pathway at Petroleum Hydrocarbon Sites – Final Draft, July 2004 • Health Canada – Soil Vapour Intrusion Guidance for Health Canada Screening Level Risk Assessment (SLRA) – Final Draft, October 2004 • EPRI - Best Practices Manual For Evaluating Subsurface Vapor Intrusion to Indoor Air – Preliminary Draft, Sept 2004 • ITRC – Workshop next week to develop an outline • Others?

  4. Areas for Advancement • Conceptual Models • More tools in the toolbox • Standardization • QA/QC checks • Interpretive Tools

  5. Qsoil: Perimeter Crack Model 0.8 L/min > Qsoil > 0.05 L/min

  6. Qsoil: Current Recommendation 0.05 > Qsoil/Qbuilding > 0.0001 (Johnson, 2002) (perhaps 0.01 to 0.0001 is more common) For a typical residence: V ~ 600m3, AER ~0.5 hr-1 Qbuilding ~ 300 m3/hr or 5,000 L/min 250 L/min > Qsoil > 0.5 L/min (perhaps 50 to 0.5) Generally higher than perimeter crack model OSWER Guidance uses 5 L/min, strictly empirical

  7. Increasing B.P. Decreasing B.P. Barometric Pumping Ground Surface Ideal Gas Law: P1V1 = P2V2 Gas is compressible, so changes in B.P. either compress soil gas or allow it to expand. No net flow in the long-term, but short term...

  8. Transient Effect of Barometric Pumping • Qsoil-BP = transient soil gas flow rate induced by B.P. change • A = area of the footprint of the building • a = air-filled porosity • xbp = depth of barometric pressure propagation • ΔP = barometric pressure change over time period “tbp” • Po = mean air pressure 0.02 > ΔP/Potbp > 0.002 (Parker, 2002)

  9. Qsoil: Perimeter Crack Model vs Delta BP Magnitude agrees well with empirical estimate of 5 L/min But in the short-term, Qsoil can be -5 to 5 L/min!

  10. Barometric Pressure Changes Arrows represent approximately 24 hours, during which BP may change a lot (>200Pa in this example) , or hardly at all Compare this to 4Pa stack effect! (dP can be up to 5,000 Pa)

  11. Temporal Variability

  12. Implications for Data Variability How much of this is due to short-term barometric pressure fluctuations?

  13. Alternative Sampling Options • Option 1: More Integrated Data • Time-Integrated: sample over several barometric cycles (e.g. 7 days) to smooth temporal variability • Volume-Integrated: to smooth spatial variability • Option 2:More Discrete Data over time • Real-time monitoring • Vertical Profiling • Monitor BP and delta P to establish correlation

  14. Time-Integrated Samples • Summa Canisters • Smaller critical orifice • Solenoid valve “timer” • ATD Tubes • Lower Flow Rate over Longer Time • VOST Samplers – borrow stack testing tools? • PUF: TO-13 designed for 300m3 samples • 10m by 10m house, 10 m vadose zone, 30% air-porosity… • Or Qsoil for 24 hours… Radon analogy: passive electret samplers

  15. Volume Integrated Samples Representative Purge Volume (DiGiulio, 2004)

  16. High Purge Volume Sampling

  17. High Purge Volume Sampling Q = 15 scfm PID readings ~ 1,000 ppmv over 1,700,000 L (3 days) indicates concentrations are spatially uniform Total mass removed ~ 10 kg (10-6 risk needs only 0.001 kg!) Combine with pneumatic testing, assess gas K

  18. Is Bigger Always Better?

  19. Huntington Beach Soil Gas

  20. Summary & Conclusions • New and Improved Protocols are coming – data quality • Qsoil from BP changes is probably not negligible • What is the contribution to data variability? • $$ to find out? • Risk assessment requires long-term average exposures • Time- or volume-integrated samples may help • Vertical profiling needed for assessing biodegradation

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