Distinguishing between vapor intrusion (VI) and indoor sources of volatile organic compounds (VOCs) is a significant challenge in site assessments, greatly increasing the cost and complexity of investigations. Rapid on-site analysis of indoor air samples using a portable gas chromatograph/mass spectrometer (GC/MS) allows users to understand the distribution of VOCs in real-time, supporting identification of the source while in the field. The objective of this demonstration was to develop and validate a step-wise investigation procedure using commercially available off-the-shelf (COTS) on-site GC/MS analysis with real-time decision making as a tool to distinguish between vapor intrusion and indoor sources of VOCs.
Use of on-site GC/MS analysis to distinguish between vapor intrusion and indoor sources of VOCs requires a field-portable analytical instrument with sufficient sensitivity to measure VOC concentrations in indoor air within the concentration range of regulatory concern (i.e., low µg/m3). A high degree of precision is also required because the protocol relies on measuring concentration gradients within a building to identify sources of VOCs. For the demonstration, a HAPSITE portable GC/MS instrument was utilized. Although specific procedures in the investigation protocol were developed using the HAPSITE, any on-site instrument with sufficient sensitivity and precision may be used.
The field investigation included application of the on-site GC/MS analysis protocol at four Department of Defense (DoD) sites. To evaluate the validity of the protocol, conventional vapor intrusion and compound-specific stable isotope analysis (CSIA; ESTCP project ER-201025) investigations were conducted at the study sites. Results from the three investigation methods were compared to evaluate the relative effectiveness of the different approaches.
The on-site analysis protocol performed as well as or better than the conventional investigation approach at all seven buildings evaluated. At six of the seven buildings, the results from the on-site analysis protocol were consistent with the overall evaluation of the vapor intrusion condition based on the results from all three of the investigation methods combined. At one building, the on-site results were consistent with the conventional program results, which suggested trichloroethylene (TCE) vapor intrusion; however, for this building, the CSIA result provided strong evidence of an indoor source. The scenario that best fits the results from all three investigation methods combined is that TCE was recently used in the building, but that the indoor source was removed prior to sampling.
In addition to this demonstration program, the on-site protocol has been used by the project team at a number of other sites for indoor source and vapor entry point identification. Overall, the on-site GC/MS analysis protocol has performed well under a wide variety of building conditions. The protocol includes an option to conduct on-site analysis while the building is pressurized or depressurized. This option can be used to get a better understanding of the VOC source as well as temporal variability and the susceptibility of a building to vapor intrusion.
This project developed and validated an on-site GC/MS analysis protocol to distinguish vapor intrusion from indoor sources of VOCs. The protocol can be used as a standalone investigation method or can be used within a larger investigation program.
Advantages of the protocol include:
Potential limitations on the use of the on-site GC/MS analysis protocol include: