Completed by EPA contractors in August, the study will be incorporated into forthcoming EPA guidelines detailing vapor intrusion monitoring requirements and could even be applied retroactively to completed or ongoing site assessments if they rely on near-slab soil gas monitoring instead of sub-slab monitoring, according to Brian Schumacher, an EPA staff scientist who conducted the study. The study was completed at a site with known trichloroethylene (TCE) contamination and found that the samples taken within a few feet of the slab underestimated the concentration of TCE in the air as compared to the samples taken from under the slab, meaning that air samples taken from outside a building may not be as accurate as more intrusive sampling methods, such as sampling underneath or through the slab itself.

Although the science and policy related to vapor intrusion is rapidly changing, frustration is occurring throughout the regulated community as indicated by the US EPA Office of Inspector General (OIG).

The OIG issued a report on December 14, 2009 titled Lack of Final Guidance on Vapor Intrusion Impedes Efforts to Address Indoor Air Risks. The report from the OIG concluded that:

-EPA's efforts to protect human health at sites where vapor intrusion risks may occur have been impeded by the lack of final Agency guidance on vapor intrusion risks. EPA's 2002 draft vapor intrusion guidance has limited purpose and scope, and the science and technology associated with evaluating and addressing risk from vapor intrusion is evolving. EPA's draft also contains outdated toxicity values for assessing risk to humans from chemical vapors in indoor air.

-EPA's draft guidance does not address mitigating vapor intrusion risks or monitoring the effectiveness of mitigation efforts. The draft guidance also does not clearly recommend that multiple lines of evidence be used in evaluating and making decisions about risks from vapor intrusion. The draft guidance is not recommended for assessing vapor intrusion risks associated with petroleum releases at Underground Storage Tank sites. EPA's outdated toxicity values allow for the use of widely different, nonfederal toxicity values and have caused delays in work to address possible risks.

-EPA has not finalized its guidance, according to EPA managers and staff, because the 2007 Interstate Technology Regulatory Council guidance addressed many issues that EPA would have addressed in a final guidance, and because finalizing EPA's guidance would take a long time in light of the emerging scientific issues in the field. Also, previous administrative review requirements for Agency guidance were perceived as barriers to issuing timely guidance in a rapidly changing environment. These requirements were revoked by the current Administration, but significant guidance remains subject to some administrative review.

-Seven years later, EPA is developing a roadmap of technical documents that will update its draft guidance. However, technical documents may not be effective for conveying and representing Agency policy. EPA has also made some progress in updating toxicity values for some contaminants most frequently associated with vapor intrusion.

-The OIG recommended that EPA issue final guidance to establish current Agency policy on the evaluation and mitigation of vapor intrusion risks. The Agency should also finalize toxicity values for trichloroethylene and perchloroethylene – common contaminants associated with vapor intrusion.

Suction Cavity with HDPE piping

Delays by EPA, however, have not slowed down the development of State-lead vapor intrusion guidance documents and/or policy, as it is reported that a least 26 States have developed their own guidance document/policy.

SSDS Extraction Point Detail

The drawback to this, however, is that there is inconsistency from State to State and large variations in the toxicity values used for determining risk leading to reliance on values which may not be universally viewed as protective. Additionally, ASTM International is working on a vapor intrusion guidance document which is expected to be voted on by its members in 2010.

In the past several years, there has been an increase in the number of projects at RT which are requiring the evaluation of the vapor intrusion pathway and an increase in the number of remediation systems (passive and/or active) being installed at sites. Sub-slab depressurization systems utilizing radon type mitigation fans have been installed at several sites and are planned for at least one other site in the coming month. Retrofit projects are also becoming a hot topic as State Agencies evaluate and update guidance and more Brownfield sites are redeveloped. We've recently worked on a New Jersey dry-cleaning site where their was historic soil impacts beneath the slab which required the installation of an active sub-slab venting system in an existing building to control vapor impacts to adjacent tenant spaces. Larger retrofit projects can be needed on some projects where large extraction trenches are needed to effectively control vapor intrusion from beneath the slab, similar to a project RT completed in Maryland.

Additionally, passive remedial sub-slab venting systems have been installed at many sites which include installation of venting piping and a plastic (PVC or HDPE) vapor barrier (some similar to a landfill lining system) beneath the floor slab prior to construction. Passive venting systems are not connected to fans and only prevent vapors from accumulating beneath the slab of a building.

Until EPA finalizes the draft guidance for evaluating the vapor intrusion at sites, the regulated community will be forced continue to use the readily available State developed guidance as a basis for making decisions related to evaluation and remediation of vapor intrusion issues. RT will continue to monitor the developments in the EPA guidance and keep its clients informed as the Agency moves toward finalization of the guidance. Should you have further questions related to evaluating and/or remediating vapor intrusion, please contact RT and ask for Walter H. Hungarter, III or Gary R. Brown, P.E.

Sources: Environmental Reporter Volume 40, Number 50; December 14, 2009 US EPA - At a Glance; December 14, 2009

Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils (Subsurface Vapor Intrusion Guidance); November 2002 EPA530-F-02-052; November 2002