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:

• Less intrusive than an intensive (manual) source identification and removal effort, which is commonly used in conventional investigations.
• Less training needed to implement the protocol, as compared to other source identification methods (i.e., on-site GC/MS analysis [ER-201119]).

Limitations of the CSIA protocol include:

• Sample collection methods. Sample collection using adsorbent tubes and pumps is slightly more complicated than sample collection using Summa canisters. This limitation can be mitigated by identifying a sampling team with prior experience using USEPA Method TO-17.
• Potential for inconclusive results. Interpretation of CSIA results is largely a matter of pattern-matching. If the isotope composition of subsurface VOCs is within the range commonly observed for VOCs in consumer products, the data interpretation becomes more uncertain. Because of this limitation, the investigation protocol recommends characterization of the subsurface source either prior to collection of indoor air samples or in conjunction with sampling at the first one or two buildings included in a site investigation. The investigation method should be applied as part of a larger indoor air sampling program only when the subsurface source has been found to be distinct from most potential indoor sources. 
• Issues with hydrocarbon sites. At chlorinated hydrocarbon sites, two isotope ratios can be developed (δ13C and δ37Cl from TCE), providing more data for interpretation. At petroleum hydrocarbon sites, it may not be practical to analyze for both relevant isotope ratios (δ13C and δ2H from benzene). CSIA for hydrogen requires a large sample mass which, in turn, may require an overly long sample collection period. Other potential issues include saturation of the sorbent tubes and interference from other hydrocarbon compounds, which may complicate the laboratory analysis. Coordination with the analytical laboratory is important to mitigate these risks.
• High concentrations of VOCs in indoor air. In some buildings, indoor sources may cause indoor air concentrations to exceed screening levels by a large margin (e.g., >10x screening levels). In these buildings, additional CSIA sampling may be helpful after indoor source removal, to account for uncertainty in isotope mixing and potential low-level vapor intrusion.