Definitive Vapor Intrusion Investigations Using On-site GC/MS Analysis and Building Pressure Control

摘要

Rapid and accurate assessment of the vapor intrusion pathway is important from risk-management and community-relations perspectives. Vapor intrusion investigations using conventional approaches, however, do not efficiently address indoor sources of target volatile organic compounds (VOCs) or temporal variability in vapor intrusion. The conventional process includes a questionnaire to identify indoor VOC sources, followed by indoor and subslab sampling for off-site analysis with typical turn-around time of two weeks. When results are finally received, investigators must reconstruct field observations to attempt to decipher the potential sources of specific VOCs detected in the indoor air samples. Additionally, they are unable to efficiently evaluate how changes in building operating conditions may affect soil gas entry. As a result, multiple rounds of sampling are commonly required to complete the vapor intrusion investigation. We have developed a streamlined investigation approach that addresses these common challenges. Our approach involves on-site GC/MS analysis to map target VOC concentrations in indoor air, identify indoor VOC sources, and evaluate potential vapor entry points. Following baseline characterization, we induce negative and/or positive building pressure conditions to refine the understanding of VOC sources and susceptibility of the building to vapor intrusion under normal and "worst-case" conditions. The ability to accurately identify indoor VOC sources and vapor entry points eliminates the need to collect subslab samples to support the interpretation of the indoor air results. Additionally, vapor entry point identification supports development of interim measures (e.g., sealing of entry points) to reduce exposure. The investigation approach has been validated through application at a number of single-family residences and industrial buildings including buildings with difficult-to-locate indoor sources, a building with immovable indoor sources also found to have vapor intrusion, and a building with vapor intrusion through a sewer line. The investigation approach typically yields definitive results from a single 1/2-day to 1-day sampling program.