Vapor Intrusion: Risks and Challenges

Vapor Intrusion (VI), defined as the migration of volatile contaminants from the subsurface into the indoor air of overlying buildings, is a major and challenging environmental issue for both regulators and the regulated community—one that continues to elicit heated debate among stakeholders. Eight factors associated with the VI pathway that contribute to its challenging nature are discussed below.

WHAT IS The Magnitude of the Problem?  The number of contaminated sites in the United States is not known with certainty, but it’s undoubtedly large. The U.S.  Environmental Protection Agency’s (EPA) 2002 VI guidance document1 references a total of 374,000 contaminated sites, the National Research Council reports that the number may be as high as 439,000,2 and an often cited total in brownfields redevelopment literature is 500,000 sites. The fraction of these contaminated sites with conditions favorable for VI also is not known with certainty, but will depend, in part, on the number of sites that contain volatile organic compounds (VOCs). Volatile contaminants have been reported at approximately one-half of all Superfund and similar cleanup sites. Preliminary estimates suggest that approximately one-half of volatile-contaminated sites have conditions that could be favorable for VI. This suggests, therefore, that VI may be an issue at one-quarter of the total number of contaminated sites in the United States.  VI analysis and control has not been approached in a systematic way at the universe of potential sites. Instead, assessments have likely been biased toward those sites with a higher potential for VI problems (e.g., sites with off-property groundwater plumes). If this is the case, the results to date may overestimate the extent of the problem. Nonetheless, these assessments indicate that a disturbing proportion of individual sites have a “complete” exposure pathway (i.e., VI is occurring). Frequently, the resulting exposures exceed risk-based targets and are therefore classified as being “unacceptable.”

Conversely, existing VI assessments may underestimate the magnitude of the problem. The assessments to date have focused on sites with groundwater ingestion threats. VI, however, can be a threat from other types of contamination, such as sites with nonpotable aquifers, perched (nonaquifer) waters, and/or soil contamination. Furthermore, these conditions may be most common in urban areas where there are more buildings and larger populations. In addition, existing estimates of VI potential may underestimate impacts to nearby and adjacent properties if the groundwater plumes continue to spread or actually extend beyond the assumed boundaries. In some cases, VI risks may exist a mile beyond the supposed plume boundary.

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