Indoor sources of volatile organic compounds (VOCs) are ubiquitous, resulting in detectable concentrations in indoor air, often at levels exceeding regulatory screening criteria. At corrective action sites with potential vapor intrusion concerns, the presence of indoor VOC sources significantly complicates the exposure pathway evaluation. Because of these indoor sources, the detection of a site-related VOC in a potentially affected building does not necessarily indicate a vapor intrusion impact. However, because conventional investigation methods often do not clearly identify the source of VOCs, additional rounds of sampling are commonly required.
The objective of this demonstration was to validate use of compound-specific stable isotope analysis (CSIA) to distinguish between vapor intrusion and indoor sources of VOCs. As part of this project, a step-by-step protocol has been developed that can be used to provide an independent line of evidence to determine whether or not buildings are impacted by vapor intrusion.
Many elements, such as carbon, occur as different isotope species, differing in the number of neutrons present in their nucleus. For example, 12C, with 6 neutrons, is the most abundant form of carbon. 13C, with 7 neutrons, makes up a small fraction (~1%) of the carbon in the environment. Isotopic ratios (13C/12C) of a specific compound (e.g., trichloroethylene [TCE]) can vary as a result of differences in their source material or compound synthesis or due to transformation in the environment. Differences in the isotopic ratio measured in organic contaminants present in environmental samples can be used to (1) distinguish between different sources of the contaminants and (2) understand biodegradation and other transformation processes occurring in the environment.
While CSIA has been applied to groundwater investigations, its applicability to vapor intrusion assessments has only recently been explored. As part of this project, the applicability of CSIA for vapor intrusion was evaluated and a step-by-step protocol was developed for investigations using CSIA. This protocol includes a decision matrix to guide users who may be unfamiliar with isotope analyses.
The field investigation included application of the CSIA protocol at four Department of Defense (DoD) sites. To evaluate the validity of this approach, investigations were conducted using conventional vapor intrusion and on-site gas chromatography/mass spectrometry (GC/MS) analysis (ESTCP project ER-201119) at the same buildings. In two of the four buildings, the CSIA approach yielded results consistent with the other investigation methods. A spray can was planted in a closet of the third building; the CSIA approach correctly identified an indoor source as being the source of VOCs in indoor air. In the fourth building, the CSIA approach provided clear and strong evidence of an indoor source while the other methods yielded ambiguous results. Overall, the demonstration results validated the CSIA protocol as a useful tool for distinguishing between vapor intrusion and indoor sources of VOCs.
The CSIA protocol for vapor intrusion is not a standalone investigation approach. It is most useful in buildings that have previously been sampled with investigation results showing VOC concentrations near or above regulatory screening levels. In these buildings, differentiating between indoor and subsurface sources becomes critical for site and risk management.
Advantages of the CSIA protocol include:
Limitations of the CSIA protocol include: