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May 02, 2013
Guidance on UST vapor intrusion

In response to a report issued by EPA’s Office of Inspector General in 2009, the Agency has issued a draft guidance document intended to assist the regulated community and federal and state environmental and public health bodies in assessing and reducing risks to human health posed by hazardous vapors from leaking underground petroleum storage tanks (USTs).  Mainly, the guidance details steps agencies should take to determine if a threat exists, protect building inhabitants if it does, define the extent of the vapor release, and remediate the problem.

Integral to response action

Petroleum hydrocarbon (PHC) vapors that enter buildings can pose a range of threats to occupants, including the short-term potential for fires and explosions as well as longer-term harm from inhaling toxic chemicals such as benzene from gasoline.  PHC vapor intrusion (PVI) has been a borderline issue for federal and state environmental agencies because indoor air pollution is generally not a regulated area.  But the draft guidance notes that assessing the potential for PVI is an “integral part of the normal response to a suspected or confirmed release from a regulated UST. 

“At any leaking UST site, it is important to have a thorough understanding of the release (i.e., source, composition, and magnitude) and other factors that may influence the distribution and transport of contaminants and impact human safety and health,” states the EPA.

Response measures

According to the draft guidance, PVI investigations at leaking USTs should contain the following actions, at a minimum:

  • Assess and mitigate immediate threats to safety.  If residents of nearby buildings reported odors or visible signs of PHC contamination, alert first responders to ensure that residents are evacuated until threats are mitigated.
  • Conduct site characterization.  All information related to the site should be integrated into a conceptual site model (CSM).  Site characterization should include a determination of the full extent and location of contamination, the nature and characteristics of the contamination, and the hydrologic (e.g., soil moisture) and geological characteristics underneath and near buildings in the lateral inclusion zone.
  • Delineate a lateral inclusion zone.  Assess the potential for PVI for each of the buildings within the lateral inclusion zone.
  • Identify preferential transport pathways within the inclusion zone.  Assess whether preferential contaminant transport pathways are present and could result in PVI into nearby buildings either inside or outside the lateral inclusion zone.
  • Determine whether contamination underlies any buildings within the lateral inclusion zone.  If contaminated groundwater is within the vertical separation distance (6 feet) between the contamination and the foundation, slab, or basement of any building(s), the EPA recommends subslab vapor sampling to assess the potential for PVI.
  • Mitigate PVI, as appropriate.  Select a remedial design that is appropriate for the building and site.  As necessary, establish institutional controls to limit or prohibit access to affected areas.

Community engagement

The draft guidance also emphasizes the importance of early community engagement.  “Depending on site circumstances, obtaining meaningful community input is a sound approach that may result in better-informed decisions,” says the EPA. 

Click here for EPA’s draft, Guidance For Addressing Petroleum Vapor Intrusion At Leaking Underground Storage Tank Sites.