Analytical Considerations for Air Samples for Vapor Intrusion Investigations
The term “vapor intrusion” refers to the migration of volatile chemicals from subsurface contaminated sources into overlying residential or commercial structures. “Historically, it was thought that vapor intrusion was only an issue where the source of the contaminants was very shallow and the magnitude of the contamination was very great. It is now known that the previous assumptions about the mechanisms that could lead to exposure to vapor intrusion were not complete (NYS DEC DER Vapor Intrusion Guidance).” For a growing number of federal, state and local agencies, as well as environmental consultants and laboratories, vapor intrusion could emerge as the next major environmental challenge.
Vapor intrusion is not a new phenomenon— for some environmental experts, it has been recognized as a potential pathway of contamination for almost 20 years. In the late 1980s, the first vapor intrusion studies were carried out to evaluate potential health effects from chronic exposure to volatile organic compounds. Presently, vapor intrusion is of growing concern to the environmental community due to a number of factors, such as increased recognition of it as a potential pathway for exposure and the risks associated with that exposure, as well as the location and the number of potential sites for investigation and remediation. With this increased focus comes ongoing debate regarding the mechanism of the exposure pathway, compliance concentrations of contaminants, identification of sites, sampling approaches, analytical methodology, use and validity of current models, screening approaches, and risk assessment, among other topics.
What this has meant for many laboratories specializing in air analyses is an upward trend in the number of ambient air, indoor air, soil gas and sub-slab samples submitted each year for volatile organic compound (VOC) analyses. The primary compounds of concern are often chlorinated VOCs. Trichloroethene (TCE) and tetrachloroethene (PCE), in particular, are common targets of the investigations due to the health risks associated with these compounds and their breakdown products.
In instances where the project specific objectives of the vapor intrusion investigation call for sampling, several kinds of air samples can be collected: soil vapor, ambient (outdoor) air, indoor air and sub-slab vapor. The timing of the collection, as well as the number, placement and combination of samples will all vary depending on the client-defined sampling protocol, which ultimately relies on local, state or federal requirements.
So, keeping in mind the client’s project-specific objectives and the underlying regulatory requirements, here are some of the factors to be taken into consideration for the analytical portion of a vapor intrusion investigation:
EPA Method TO-15 is the most frequently requested method for the analysis of VOCs for the range of air samples associated with vapor intrusion investigations. The method uses gas chromatography (GC) to achieve sample separation and a mass spectrometer (MS) for identification and quantitation.
TO-15 was a new method added to the Second Edition of the Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air in January 1999. Although very similar to EPA Method TO-14A, EPA Method TO-15 is larger in scope and better defined for the analysis of VOCs in air and other gaseous matrices than TO-14A (which is a revised and updated version of the former Method TO-14).
A wide range of compounds may be analyzed by EPA TO-15 including alkanes, alkenes, aromatics, halogenated VOCs, ketones, esters and some alcohols. Some aldehydes and sulfides may also be evaluated using this method.
EPA TO-15 does not specify a target compound list in the method. As a result, there is some variation among commercial environmental laboratories in the compound lists that are available for VOCs. Target compound lists may include anywhere from 40 to 60 compounds or more, and may provide results in μg/m3, ppbV, or both.
Compound lists can usually be tailored to meet project-specific objectives. This is true especially in the case of indoor air, which typically involves a subset of the laboratory’s standard target compound list.
Method Reporting Limits
Target method reporting limits (MRLs) will vary depending on the data quality objectives of the investigation, which should take into account any state or federal regulatory documents or guidances that may apply.
For soil gas or sub-slab samples, it is usually sufficient to analyze the samples in the normal operating mode of the GC/MS (SCAN), which yields MRLs from sub-parts per billion up to parts per million levels.
For indoor or ambient air analyses, investigators are often considering the potential risk to human health, so they are typically interested in lower MRLs, down to the single digit part per trillion levels. Indoor air and ambient air analyses are frequently performed by GC/MS in Selective Ion Monitoring (SIM) mode to achieve these ultra-low level MRLs.
The risk-based levels for the contaminants of concern are, in many cases, less than the typical or observed background levels in the indoor or outdoor environment, creating a challenge for many vapor intrusion investigations. The best approach for assessing and dealing with this issue continues to be discussed and debated by experts in the field.
For soil gas determinations, samples may be collected using either passivated stainless steel canisters (such as Summa canisters) or Tedlar bags. Each sampling medium has its advantages and disadvantages, as summarized in the table below.
For soil gas sampling, 1L Summa canisters can offer certain benefits over the larger 6L canisters. They fill faster, reducing time in the field for the investigators. They are smaller and lighter, so they are easier to transport, handle and ship. The smaller volume reduces the likelihood of ambient air intrusion, especially when sampling more densely packed soils. The trade off is that a smaller sample portion can be withdrawn from the canister for the analysis, which results in higher MRLs than those achievable from samples collected in a larger canister, typically 3 to 5 times higher.
For indoor air or ambient air sampling, 6L Summa canisters are the recommended sampling medium. Canisters may be either batch certified clean or individually certified clean—the selection depends, again, on the data quality objectives of the project and on any regulatory specifications. In situations where it may be valuable to have documentation for every canister, (e.g. potential litigation, risk assessment), then individual certification can be requested. In either case, the canisters will be cleaned and certified below the target MRLs.
Columbia Analytical’s Air Quality Laboratory has extensive experience performing analyses of indoor and ambient air, as well as sub-slab and soil gas samples. Specializing in the analysis of volatile and semi-volatile organic compounds, sulfur compounds and other hazardous substances in a wide variety of air and vapor matrices since 1988, the lab has performed tens of thousands of analyses from its southern California location near Los Angeles, and it successfully serves clients in all 50 states and around the globe.