Future Directions for Assessing Vapor Intrusion

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Future Directionsfor Assessing Vapor Intrusion

• by
• Todd McAlary, GeoSyntec Consultants, Inc.
• AEHS VI Workshop
• October 19, 2004

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Outline

• Improved Protocols Whats Coming
• Qsoil
• Perimeter crack model vs 0.05gtQsoil/Qbldggt0.0001
• Barometric Pumping
• Implications for Data Variability
• Alternative Sampling Options
• More discrete in time and space
• More integrated

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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?

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Areas for Advancement

• Conceptual Models
• More tools in the toolbox
• Standardization
• QA/QC checks
• Interpretive Tools

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Qsoil Perimeter Crack Model
0.8 L/min gt Qsoil gt 0.05 L/min
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Qsoil Current Recommendation
0.05 gt Qsoil/Qbuilding gt 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 gt Qsoil gt 0.5 L/min (perhaps 50 to
0.5) Generally higher than perimeter crack
model OSWER Guidance uses 5 L/min, strictly
empirical
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Barometric Pumping

• Increasing B.P.

• Decreasing B.P.

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...
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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
• ?P barometric pressure change over time period
  tbp
• Po mean air pressure

0.02 gt ?P/Potbp gt 0.002 (Parker, 2002)
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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!
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Barometric Pressure Changes
Arrows represent approximately 24 hours,
during which BP may change a lot (gt200Pa in this
example) , or hardly at all
Compare this to 4Pa stack effect! (dP can be up
to 5,000 Pa)
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Temporal Variability
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Implications for Data Variability
How much of this is due to short-term barometric
pressure fluctuations?
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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 2More Discrete Data over time
• Real-time monitoring
• Vertical Profiling
• Monitor BP and delta P to establish correlation

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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

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Volume Integrated Samples
Representative Purge Volume
(DiGiulio, 2004)
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High Purge Volume Sampling
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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
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Is Bigger Always Better?
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Huntington Beach Soil Gas
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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