Formatted Title
Benzene as a Tracer Gas for Assessing Preferential Pathways and Attenuation throughout a Multistory Structure
Background/Objectives
Regulatory agencies’ vapor intrusion guidance documents stress the importance of identifying preferential pathways for vapor migration. However, in cases where a well-designed vapor intrusion mitigation system (VIMS) has been installed with due care during building construction, the assessment of preferential pathways may be hindered by an absence of vapor detections in indoor air. Multi-year indoor air monitoring programs provide a unique opportunity to assess not only the performance of active or passive VIMS, but also to evaluate the characteristics of a structure that serve to either perpetuate or attenuate vapor migration, provided that a useful “tracer gas” exists or is introduced. For this study, eight rounds of monitoring data, encompassing 30 distinct monitoring locations, were reviewed to assess the migration and attenuation patterns of volatile compounds throughout a multistory structure.
Approach/Activities
In this case, perchloroethylene (PCE) and trichloroethylene (TCE) are the constituents of concern (COCs). Assessing vapor intrusion and migration by means of the PCE and TCE data has been extremely difficult because the PCE data have been, at times, skewed by an apparent indoor source; TCE is frequently not present at detectable levels; and both PCE and TCE data have increasingly been affected by contaminated sampling canisters being provided by analytical laboratories. However, although it is not a COC, benzene has been detected in every sample collected because it is continuously being released by the residents’ vehicles that are parked in the basement garage. Therefore, benzene serves as a useful “tracer gas” for the study of vapor migration and attenuation throughout the structure. Specifically, benzene concentrations in the basement garage; all four levels of the stairwells; the two residential-level hallways; a representative apartment; and ambient air were tabulated and then analyzed to assess how benzene migrates upward in the structure and is attenuated with increasing distance from the garage-level source.
Results/Lessons Learned
The data analysis indicates that volatile organic compounds (VOCs) can readily migrate upward through a structure by means of preferential pathways such as stairwells, but that their concentrations are attenuated by structural features, in particular doorways. Therefore, although benzene concentrations remain fairly constant throughout the four-story staircases (consistent with a preferential pathway), the concentrations drop appreciably at the doorways from the garage into a stairwell, from a stairwell into a hallway, and from a hallway into an apartment. The benzene levels attenuate by over 90% between the garage source area and the third-story apartment. Nevertheless, the benzene concentration in the apartment is consistently above that in ambient air, which in turn is above health-protective levels, as is typical in urban areas.