Vapor Intrusion: Subsurface Vapor Intrusion Guidance

A Quick Look
Vapor intrusion is the migration of volatile chemicals from the subsurface into overlying buildings.
Vapor intrusion is an emerging concern at brownfields sites.
Several strategies have been developed to reduce or eliminate indoor air contaminant concentrations using source control of the contaminant of concern, improving ventilation in buildings, air treatment methods, and land use controls.
Savings can be realized when controls are addressed prior to redevelopment.

Vapor intrusion is the migration of volatile chemicals from the subsurface into overlying buildings. Vapor intrusion is typically associated with releases of volatile organic compounds (VOCs) such as petroleum products and chlorinated solvents to soil and groundwater from former gas stations, dry cleaners, automobile repair shops, and industrial facilities. VOCs in buried wastes and contaminated groundwater can emit vapors that may migrate through the subsurface into air spaces of overlying buildings through foundation cracks, holes in concrete floors, and small gaps around pipes and utility lines. Certain site characteristics such as the presence of a high water table or fractured bedrock can increase the likelihood of vapor intrusion.

Vapor intrusion is an emerging concern at brownfields sites. The underground contaminant vapors can act as hidden sources of contamination that may not be detected during the due diligence process for property transfers. Redevelopment of older buildings with damaged foundations can increase the potential for vapor intrusion. In extreme cases, the vapors may accumulate in dwellings or occupied buildings to levels that may pose near-term safety hazards, acute health effects or aesthetic problems. In most cases, however, the chemical concentrations are low, or depending on site-specific conditions, vapors may not be present at detectable concentrations.

Some states along with EPA have developed methods to screen for sites with potential vapor intrusion concerns. Several strategies have been developed to reduce or eliminate indoor air contaminant concentrations including source remediation, as well as building-specific engineering (e.g., improved ventilation) and land use controls. While all exposure controls may need ongoing operation, maintenance and monitoring, considerable savings can be realized when controls such as vapor barriers, or active and passive venting systems, are included prior to redevelopment.

EPA Subsurface Vapor Intrusion Guidance
On November 29, 2002 (67 FR 71169), EPA published the Draft Guidance For Evaluating The Vapor Intrusion to Indoor Air Pathway From Groundwater And Soils (Subsurface Vapor Intrusion Guidance). The draft guidance is intended to provide a tool to conduct a screening evaluation as to whether or not the vapor intrusion exposure pathway is complete and to determine whether it poses an unacceptable risk to human health. It is not intended to provide recommendations for delineating extent of risk or eliminating risk. The draft guidance provides a three-tiered approach to evaluating the vapor intrusion pathway: Primary Screening, Secondary Screening, and Site-Specific Pathway Assessment. Each tier contains a set of questions that aid in the determination of whether a vapor intrusion pathway is complete. A complete pathway means that humans are exposed to vapors originating from site contamination. For those sites determined to have a complete pathway, the draft guidance provides ways to evaluate whether the pathway poses a potential significant risk to human health. If a site is determined to have an incomplete vapor intrusion pathway, further consideration of the current site situation generally should not be needed. In addition to brownfields sites, this draft guidance is suitable for use at RCRA Corrective Action and Superfund sites. A copy of the draft guidance, a fact sheet, and other background information is available at: www.epa.gov/epaoswer/hazwaste/ca/eis/vapor.htm.

For additional information, visit: www.epa.gov/epaoswer/hazwaste/ca/eis/vapor.htm, www.itrcweb.org/Documents/BRNFLD-1.pdf, and http://iavi.rti.org.:

Monday, December 1, 2008