20E020 - Building Specific Attenuation Factors for Vapor Intrusion Assessments
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Background/Objectives: In vapor intrusion (VI) assessments attenuation factors are the ratio of the indoor concentration of a contaminant to the subsurface concentration of a contaminant and account for reductions in concentrations which occur during transport of vapors from beneath a building to indoor air.

When evaluating the potential risk to occupants within a building, attenuation factors are used to estimate the indoor air concentrations based on subsurface concentrations. The current National Environment Protection (Assessment of Site Contamination) Measure (ASC NEPM) incorporates a default attenuation factor of 0.1 when deriving the interim Health Investigation Levels (HILs) for chlorinated hydrocarbons. Empirical data from the USEPA indicates that this value could result in over 99% of vapor intrusion sites requiring additional investigations. A screening value that screens in almost every location is not practical. There are several approaches for developing more accurate building attenuation factors which will be discussed in this presentation.

Approach/Activities: Australia has a limited data set on attenuation factors thereby resulting in the selection of an overly conservative value in the NEPM. One of the key aspects for the entry of vapors to a building are the various building specific factors, like building pressure, cracks, air exchange rates etc. Therefore, the determination of a building specific attenuation factor is a key to evaluating VI risk to occupants. There are some simple and robust methods which could be adopted by industry to better calculate attenuation factors. Approaches to building specific attenuation factors will be presented:

- The Johnson and Ettinger model and the range of assumed building values

- USEPA empirical attenuation factor database and selection of a conservative value of 0.03

- Indoor air and subsurface data forensics to determine building specific attenuation factors and differentiating background sources

- Subsurface pneumatic testing to calculate a building specific attenuation factor

- Radon as a potential tracer compound

Results/Lessons Learned: The presentation will focus on the need for a change in policy with respect to the current NEPM attenuation factor value of 0.1 for subslab to indoor air. Building specific attenuation factors are generally orders of magnitude higher than the NEPM values and simple methods are available for developing building specific attenuation factors to show they are in the range of 0.03 to 0.0003.

Publication Summary

  • Geosyntec Authors: Paul Nicholson
  • All Authors: Paul Nicholson
  • Title: EcoForum 2020 eConference
  • Event or Publication: Event
  • Practice Areas: Vapor Intrusion Assessment and Remediation
  • Citation: Geosyntec will deliver five presentations and one keynote at EcoForum 2020 eConference on September 14-18, 2020.
  • Date: September 14-18, 2020
  • Location: eConference
  • Publication Type: Platform Presentation