By Alexander Gordon of Aon Risk Solutions
With the continued growth of Brownfield redevelopment projects and other revitalization efforts across the country, vapor intrusion —the migration of volatile chemicals from contaminated groundwater or soil into above surface buildings— is increasingly seen as a significant pathway for environmental contamination. When evaluating the risks of vapor intrusion a number of variables must be weighed, this includes the source of contamination, the concentration of chemical vapors in the air, the age of occupants exposed to vapors, and the amount of time occupants spend in the affected building. While in rare cases vapor intrusion can pose an immediate or short-term hazard, more often, the risks of vapor intrusion come from inhaling low-levels of contaminated air over long periods of time. As the EPA has acknowledged, the expropriation of the senses entailed in an odorless and invisible threat demand clear and proactive guidelines for identifying the source of contamination and containing the hazard.
Although the idea of vapor intrusion is not new, the science and techniques for modeling the risk have continued to evolve over the past ten years. In response, the EPA has continued to work with stakeholders and experts to update the 2002 ‘Draft Guidance for Evaluating the Vapor Intrusion to Indoor Pathway from Groundwater and Soils’. After a round of public comments on the 2002 draft, the final version of the EPA’s Subsurface Vapor Intrusion Guidance was set to be released by November 30, 2012, yet the publication has now been delayed until 2013. As the EPA awaits comments from regional offices, some stakeholders have suggested that the EPA should open up the revised document to a new round of formal public commenting. The EPA has yet to respond to these requests.
Once completed the EPA’s final Subsurface Vapor Intrusion Guidance will present new technical and policy recommendations for assessing vapor intrusion risks in the cleanup of chemicals like dry-cleaning solvents, degreasers, polychlorinated biphenyls (PCBs), and other vapor forming chemicals. It is also believed that the EPA will include a companion document that will address specific issues related to petroleum-based vapor intrusion. Insiders anticipate that the new guidance will contain more conservative recommendations for two key metrics used in screening for vapor intrusion: soil depth and lateral inclusion zone. According to sources who have viewed the latest draft, the EPA has proposed to extend clean soil screening depths from 15 to 30 feet, while also widening the lateral inclusion zone, or area from which chemical vapors can be expected to travel or escape.
Since the release of the EPA’s 2002 draft guidance document, federal and state cleanup agencies have gained extensive knowledge from investigating contaminated sites of varying sizes, contaminants, and degrees of exposure. With an improved understanding of how to assess and manage vapor intrusion, it will be no surprise that the final guidelines on subsurface vapor intrusion will be more detailed and prescriptive than the 2002 draft document. In particular, the final guidance is anticipated to include more rigorous indoor air sampling methods and a new emphasis on preemptive mitigation as a cost-effective technique for controlling vapor intrusion prior to having fully demonstrated that currently unacceptable vapor intrusion exists in a specific building under consideration. Until the final guidance is released, the EPA has suggested that the approach for dealing with vapor intrusion described in the 2002 Draft Vapor Intrusion Guidance would generally remain appropriate and valid.
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