New EPA Guidance Documents on Vapor Intrusion

In June 2015, the United States Environmental Protection Agency (“US EPA”) released two highly anticipated technical guidance documents regarding the assessment and mitigation of vapor intrusion. These separate but complementary documents replace the US EPA’s 2002 Draft Guidance that problematically failed to provide direction or assistance for managing petroleum vapor intrusion (“PVI”). The primary guidance, Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air (“OSWER VI Guidance”), applies broadly and assesses the vapor intrusion risk at all sites except for those associated with petroleum contamination. The secondary guidance, Technical Guide for Addressing Petroleum Vapor Intrusion at Leaking Underground Storage Tank Sites (“PVI Guidance”), complements the primary guidance by specifically assessing PVI resulting from leaking underground storage tanks (“USTs”).

Vapor intrusion is the phenomenon of toxic vapors and volatile chemicals migrating from contaminated groundwater or soil and rising into the air of the overlying buildings, often through cracked foundations, utility line openings or other pathways. The concern surrounding vapor intrusion has been recently amplified due to the potential for vapors accumulating at levels in overlying structures that may pose safety hazards, acute health effects, or noticeable fumes. Even if measured chemical concentrations are low or not present at detectable concentrations, long-term exposure to low-level chemicals may pose an unacceptable risk of chronic health effects.

The new Guidance documents provide a flexible framework to support risk management decisions regarding vapor intrusion at sites where vapor intrusion is an actual or potential concern by incorporating EPA’s current recommendations for identifying, evaluating, and managing vapor intrusion. Overall, these new Guidance documents take into account site-specific circumstances and promote consistency across the country; but more importantly, they announce the EPA’s confrontation of PVI at UST sites.

Petroleum vs. Non-Petroleum Vapor Intrusion

Separate Guidance is required to account for the dissimilar biodegradability and density of petroleum hydrocarbons (“PHCs”) that make PVI unique from the vapor intrusion of other chemicals. Soil microbes consume oxygen to degrade PHCs under aerobic environmental conditions, which can lead to different subsurface behavior that often reduces the potential for vapor intrusion and human exposure. As oxygen is depleted, contaminant concentrations become elevated and cause PHCs to biodegrade relatively rapidly over short distances.

Unlike PHCs, chlorinated solvents typically biodegrade under anaerobic conditions incompletely and more slowly. Consequently, chlorinated vapor plumes are often more extensive and more likely to result in vapor intrusion or the production of toxic degradation products, such as dichloroethylene and vinyl chloride. Furthermore, since PHC liquids are less dense than water, they can float and accumulate on the groundwater surface and spread laterally when released from a leaking UST. On the other hand, chlorinated solvents are denser than water and can penetrate the water table and collect as pools on the bottom of the aquifer. As a result, a separate PVI guidance is needed to adequately handle the effects unique to petroleum contamination.

General Vapor Intrusion Guidance

The OSWER VI Guidance provides comprehensive technical guidance that applies to all non-petroleum contamination sites being evaluated under federal, and state equivalent, remedial statutes. Although the document is complex and grapples with diverse circumstances, it recommends a broad and flexible framework to assess vapor intrusion at site-specific conditions and provides technical recommendations about monitoring and terminating building vapor mitigation systems.

The OSWER VI Guidance framework also supports risk management decisions by collecting and evaluating multiple lines of evidence. The approach for evaluating vapor intrusion varies for different chemicals but will ordinarily fall into two assessment levels: preliminary assessment and detailed investigation. The preliminary assessment relies on information easily available and ascertainable, which can lead to the recommendation of a detailed investigation. Based on the results of the assessment, the Guidance provides a variety of response actions to address and mitigate vapor intrusions posing unacceptable human health risks.

Petroleum Vapor Intrusion Guidance

The PVI Guidance covers leaking UST contamination sites. Although the PVI Guidance is less substantial, it is complementary to the OSWER VI Guidance and provides specific technical information for investigating and assessing PVI. The Guidance encourages appropriate site characterization, risk assessment, and corrective action activities until it is clear that human health and the environment are adequately protected from any adverse impacts of PVI.

The main criterion for the investigation process of PVI risks is screening the physical separation distances between vapor sources and potential receptors. The Guidance considers additional investigation to be generally unnecessary if the vertical separation distance between the overlying building and the dissolved contamination is greater than 6 feet or if the vertical separation distance is 15 feet from light non-aqueous phase liquid. When investigation commences, the process includes assessing and mitigating immediate threats to safety, conducting a site characterization and developing a conceptual site model, evaluating vapor source and attenuation of PHC vapors, and mitigating PVI.
The PVI Guidance applies to all sites where there is a potential for PVI. However, the Guidance excludes petroleum contamination found at sites that are not comparable to UST sites, such as refineries, pipelines, large-scale fueling and storage operations, and petrochemical plants. These contamination sites should be addressed under the OSWER VI Guidance.

Further Considerations

While these Guidance documents clarify US EPA’s position on the evaluation of vapor intrusion, the significance of vapor intrusion evaluation in the remedial and due diligence contexts under applicable federal and state statues remains uncertain. For example, CERCLA does not expressly deal with vapor intrusion or air quality but the Guidance documents require the assessment of the potential presence of vapor releases and intrusion. Furthermore, the extent of the costs accompanying vapor intrusion assessment or potential private litigation may be variable or extensive.

Additionally, since the protection of workers overlaps with OSHA jurisdiction, the extent to which the US EPA will continue to exercise authority over the protection of workers from potential exposure to indoor air contamination stemming from chemical releases also remains uncertain. The US EPA Guidance documents advise against using OSHA’s Permissible Exposure Limits (PELs) for indoor air vapor intrusion limits because the US EPA considers PELs outdated and inadequate for ensuring protection of worker health. This may in turn encourage OSHA to strengthen their PELs as to maintain authority and jurisdiction over contaminated indoor air exposure. Nonetheless, addressing vapor intrusion in accordance with these Guidance documents can be manageable from a cost perspective and result in significant short and long term health and safety benefits.


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