Vapor Intrusion Audits

The following letter was prepared by Janine Commerford, Assistant Commissioner, MassDEP Bureau of Waste Site Cleanup, and published in the February, 2009 LSP Association Newsletter,

Vapor Intrusion Audits

Identifying and Mitigating Unacceptable Impacts to Indoor Air

MassDEP’s Bureau of Waste Site Cleanup is undertaking an evaluation of conditions at certain sites with Response Action Outcomes filed prior to April 2006, where the concentration of PCE (tetrachloroethylene) in groundwater may exceed the current GW-2 standard, to determine whether there may be vapor intrusion into nearby occupied buildings. Conditions at these sites may in fact be safe, but in light of the health concerns related to ongoing exposures to volatile chemicals in indoor air, MassDEP considers it necessary to determine whether vapor intrusion is occurring.

In 1993, Massachusetts became the first state in the country to establish regulations and cleanup standards for the assessment and cleanup of sites where vapor intrusion was a pathway of concern.  Since then, much more has been learned about how volatile organic compounds in groundwater and soil migrate through the subsurface and volatilize into buildings.  Updated information shows that for some volatile chemicals, including PCE, vapor intrusion at levels of concern occurs at much lower groundwater concentrations than originally understood.

In April 2006, MassDEP revised certain Method 1 GW-2 Standards to incorporate updated chemical-specific fate and transport information for the groundwater-to-air migration pathway. For PCE, the new GW-2 Standard (50 µg/l) is significantly lower than the previous standard (3,000 µg/l).  This means that some sites closed prior to the April 2006 regulation change may have concentrations of PCE that exceed the current groundwater standard, and that there could be indoor air impacts to nearby residences or other buildings that were not considered when the site was initially assessed.  As a result, some occupants of these previously closed sites may be exposed to PCE at unacceptable levels.

To address the potential for vapor intrusion at previously closed sites where PCE is a contaminant-of-concern, MassDEP has undertaken a review of approximately 600 sites that were closed prior to April 2006.  This review encompasses sites where PCE is a contaminant of concern, depth to groundwater is approximately 15 feet or less, PCE GW-2 levels are between 50 and 3,000 µg/l near occupied buildings, and the indoor air pathway had either not been evaluated, or had been ruled out in light of the previous (higher) GW-2 value or solely by modeling.   For about 500 sites (~80%), MassDEP concluded that conditions are unlikely to create an unacceptable exposure through the vapor intrusion pathway, based on information previously submitted.  For the remaining sites, a more in-depth review of site conditions is required through a comprehensive audit.  Many of these audits are either completed or already under way; we anticipate that the remainder will be initiated within the next few months.

Most audits conducted by MassDEP are initiated within 5 years of site closure.  However, per 310 CMR 40.1110 (4)(b), MassDEP has the authority to conduct audits outside that time-frame when “…a significant risk of harm to health, safety, public welfare or the environment may exist at a site, or in the vicinity of a site, for which a Response Action Outcome Statement has been submitted to the Department…

The audits of PCE sites will enable us to determine whether there may be unacceptable impacts to indoor air, or if additional data are needed to make that determination. As with any audit, with respect to compliance issues, site work will be evaluated in accordance with the standards in effect at the time the RAO Opinion was rendered. There are three possible audit outcomes:

  • MassDEP determines that site work was conducted in accordance with practice standards at the time, and that site conditions are unlikely to create an unacceptable exposure through the vapor intrusion pathway.
  • MassDEP determines that site work fell short of the practice standards in effect at the time the RAO was filed. The Department will follow its standard procedure for addressing noncompliance.  MassDEP may also require additional work to address the vapor intrusion pathway, if appropriate.
  • MassDEP determines that site work was conducted in accordance with practice standards at the time, but that unacceptable impacts to indoor air may be present based on site-specific conditions, including the revised PCE numerical/risk standard of 50 µg/l. MassDEP will notify the Potentially Responsible Parties for the site about the audit results, and the Department will ensure that appropriate steps are taken, either by the property owner, causally responsible party, MassDEP, or some combination of parties. These may include notification of building occupants, additional sampling and mitigation of indoor air impacts.

The results of audits completed thus far have ruled out indoor air impacts at some sites, but unfortunately confirmed them in others, in some cases at Imminent Hazard levels.  Examples of problems identified thus far include sites where adequate post-remediation sampling was not done to confirm the effectiveness of the remedy prior to RAO, adequate assessment of migration pathways was not conducted, fate and transport modeling assumptions were unsupported, and/or reliance solely on modeling in lieu of confirmatory sampling resulted in Imminent Hazard level exposures in residences.

MassDEP recognizes that vapor intrusion issues at 21E sites can pose considerable challenges.  These include the technical and logistical challenges involved in the assessment and mitigation of impacts, the risk communication challenges involved in presenting results to the public, and the challenges that uncertainty about regulatory closure present for property owners and developers.  MassDEP is committed to working through these challenges with the regulated community and has recently convened a vapor intrusion workgroup to assist in developing comprehensive, updated guidance for vapor intrusion situations.  Information about the workgroup can be found at

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