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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 Directionsfor 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>Qsoil/Qbldg>0.0001 • Barometric Pumping • Implications for Data Variability • Alternative Sampling Options • More discrete in time and space • More integrated
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?
Areas for Advancement • Conceptual Models • More tools in the toolbox • Standardization • QA/QC checks • Interpretive Tools
Qsoil: Perimeter Crack Model 0.8 L/min > Qsoil > 0.05 L/min
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
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...
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)
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!
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)
Implications for Data Variability How much of this is due to short-term barometric pressure fluctuations?
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
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
Volume Integrated Samples Representative Purge Volume (DiGiulio, 2004)
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
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