Rapid Assessment of Potential Inhalation Risks Due to the Vapor Intrusion Pathway Using Building Pressure Control and High Volume Subslab Testing
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Vapor intrusion is frequently encountered in redevelopment of brownfield sites.

Inhalation risks due to vapor migration of volatile organic compounds (VOCs) from subsurface sources into occupied buildings, referred to as the vapor intrusion (VI) pathway, are challenging to assess using conventional indoor air sampling methods because of temporal and spatial variability in indoor air concentrations and contributions from background sources. Indoor air concentrations measured over periods of a few hours to a day can exhibit greater than 100-fold variability over time and space. Many background sources of VOCs exist in everyday use and vapor emissions from such sources can lead to a high bias in the estimate of VI-related inhalation exposures. This talk describes two innovative test methods for rapidly assessing potential inhalation risks due to the VI pathway: building pressure control (BPC) and high volume subslab sampling (HVS).

BPC provides a means of characterizing building susceptibility to VI and determining background (non-VI) contributions to indoor air VOC concentrations. The test procedure involves depressurizing a building to varying levels, which promotes VI, and subsequently pressurizing it, which hinders VI. Cross-building and cross-slab differential pressure is monitored throughout the test, concurrently with indoor air sampling under the various levels of imposed pressure conditions. Building susceptibility to VI and reasonable maximum exposures due to VI can be determined in a one or two days or less, depending on the size and complexity of the building. The results have been found to exhibit sufficiently low temporal variability (generally less than two-fold) that repeat sampling in different seasons is not needed to support reasonable risk management choices.

HVS provides a means of characterizing the VOC vapor mass loading under the building that could potentially impact indoor air quality through VI and deriving building-specific indoor/subslab attenuation factors. The test procedure involves measuring vapor concentrations as a function of the volume of gas extracted and measuring vacuum under steady and transient periods of vapor extraction from below the building foundation. These data are used to calculate the transmissivity of the subslab materials and the leakance of the slab, parameters that can be used to calculate a building specific attenuation factor. These tests also can be conducted in one or two days or less, depending on the size of the building. Multiple HVS tests at large buildings can be used to distinguish areas of high potential mass loading from areas that don't pose a VI risk and to design an efficient and effective subslab depressurization system to mitigate any VI threat.

Publication Summary

  • Geosyntec Authors: Helen Dawson
  • All Authors: Helen Dawson
  • Title: 2019 Southeast Brownfields Conference
  • Event or Publication: Event
  • Practice Areas: Vapor Intrusion Assessment and Remediation
  • Citation: David Latham, PG, Eric Sager, P.G. (Florida), Lydia Dorrance, Ph.D. (California), and Helen Dawson, Ph.D. (Washington D.C.), are presenting on multiple topics at the 2019 Southeast Brownfields Conference at the DoubleTree Universal Orlando in Orlando, Florida on October 27-30, 2019.
  • Date: October 27-30, 2019
  • Location: Orlando, Florida
  • Publication Type: Platform Presentation