Trihydro provides environmental consulting, engineering and surveying, air quality and process management, information technology, sustainable business solutions, and water and natural resources to public and private clients. From our initial startup as a two-person firm in 1984, Trihydro has grown into a dynamic company of over 300 employees with 13 offices nationwide.

 

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May 2011

Finalization of the United States Environmental Protection Agency's Vapor Intrusion Guidance

In 2002, the United States Environmental Protection Agency (USEPA) issued the Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils ("Draft 2002 VI Guidance", USEPA 2002). This guidance proposed collection and screening of groundwater and soil gas data as a primary means of assessing the VI pathway before proceeding with more intrusive monitoring (such as collection of indoor air samples) within structures. Since that time, the basis of understanding has grown substantially as a result of VI investigations across the country, compilation of data by the USEPA, and independent research. However, the Draft 2002 VI Guidance has not been updated to incorporate this growing body of knowledge. This has left the environmental industry with significant uncertainty and inconsistency on appropriate methods for investigating and mitigating sites with potential VI issues. In the interim, several state agencies and industry groups have published their own guidance documents, some of which the USEPA participated in developing (such as the Interstate Technology and Regulatory Council (ITRC) 2007 guidance). However, in many cases these documents are contradictory in their approach, adding to the confusion.

As a result, the USEPA's Office of the Inspector General (OIG) evaluated the lack of final guidance and released the evaluation report Lack of Final Guidance on Vapor Intrusion Impedes Efforts to Address Indoor Air Risks (http://www.epa.gov/oig/reports/2010/20091214-10-P-0042.pdf). The OIG had four overarching recommendations:

Based on these recommendations, the USEPA has committed to having a final guidance in place by November 2012 and is documenting this process on the web (http://www.epa.gov/oswer/vaporintrusion/).

 

What are some of the changes you can expect to see in the final guidance?

Generic Attenuation Factors – USEPA has compiled an extensive data set to evaluate the generic attenuation factors provided in the Draft 2002 VI Guidance.  These generic attenuation factors have been used to “screen out” sites from further assessment based on a single line of evidence (e.g., groundwater or soil-gas data).  The data that have been compiled (USEPA 2008) have been primarily collected at sites with chlorinated solvent contamination; therefore biodegradation, which has been shown to result in significant reduction of volatile constituent concentrations beneath petroleum release sites, is not being considered.  USEPA has indicated that the compiled data support the generic attenuation factors for groundwater and sub-slab soil gas provided in the Draft 2002 VI Guidance and these attenuation factors “remain valid.”  However, “external” soil gas data (collected outside of the footprint of a structure) appear to have a higher degree of variability both spatially and temporally.  Therefore, the USEPA is considering modifying the generic attenuation factor for “external” soil gas reportedly from 0.01 to 0.1.  This will mean a tenfold decrease in screening levels for soil gas and an increase in the number of sites where additional assessment will be required to evaluate the VI pathway.

Multiple Lines of Evidence – As previously discussed, the Draft 2002 VI Guidance allowed for” screening out” sites using a single line of evidence such as groundwater.  The final guidance will recommend a multiple lines of evidence approach (i.e., groundwater, soil gas, and sub-slab data, evaluation of background sources, tracer testing [e.g. radon] and modeling)to support evaluation of the VI pathway.  USEPA is also recommending collection of indoor air samples earlier in the investigation process.  The USEPA had previously shied away from indoor air sampling because of the potential for petroleum and chlorinated solvent sources within buildings and outdoor air to impact indoor air sampling results.  This change in guidance will result in more costly evaluations and will require both documenting and quantifying the contribution of background sources present in both indoor and outdoor air.

Petroleum vs. Chlorinated Solvent Releases – Due to the differences in fate and transport of petroleum and non-petroleum related chemicals, the OIG recommended that evaluation of the risks from petroleum release sites be handled differently than chlorinated solvent release sites.  These differences are primarily driven by the effectiveness of aerobic biodegradation in the subsurface at reducing volatile petroleum concentrations.  Separate guidance for petroleum release sites would streamline vapor investigations and potentially reduce the “do-loop” of vapor sampling. 

Future Building, Mixed Use, and Commercial Scenarios – The USEPA has indicated the final guidance will include an approach for evaluating both commercial buildings and non-traditional residential and non-residential structures such as apartment buildings and mixed-use buildings, as well as undeveloped property that may have future buildings. 

Preemptive Mitigation – In lieu of assessing the VI pathway, the USEPA will allow for preemptive installation of mitigation systems, which is of particular interest at Brownsfield sites.  However, there is uncertainty with this approach, as it is not clear what the USEPA will require for documenting the system’s effectiveness, as well as any long term operations and maintenance (O&M) obligations.  While installation of mitigation systems may result in lower costs during the site investigation, unforeseen future costs such as O&M, costs for optimization of the mitigation system, treatment of effluent, and documenting system effectiveness can add up over time making this preemptive approach more costly in the long term. 

Trihydro will continue to provide updates regarding finalization of the Draft 2002 VI Guidance as more information is made available.

Help is available … If you find this new guidance to be daunting, don’t worry, Trihydro’s Vapor Intrusion Specialists have been continuously tracking USEPA as well as state and industry VI guidance documents.  We have a highly qualified staff of engineers and geologists who have been working on assessment, mitigation, and remediation at both large and small project sites with VI concerns for more than 15 years.  If you have any questions or would like to talk to someone at Trihydro about vapor intrusion, send an email to Shannon Thompson at sthompson@trihydro.com or Paul Michalski at pmichalski@trihydro.com or call us at 307-745-7474.



References
ITRC (Interstate Technology and Regulatory Council, 2007. Vapor Intrusion Pathway: A Practical Guideline, January 2007.

Schuver, Henry. “Vapor Intrusion and USEPA Review of the 2002 Draft Guidance”, Presentation at the AWMA Vapor Intrusion Conference. Chicago, Il., September 29, 2010.

USEPA (United States Environmental Protection Agency), 2002. Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils.  Office of Solid Waste and Emergency Response, Washington, D.C. EPA/530/F-02-052. November 2002.

USEPA, 2008. U.S. EPA’s Vapor Intrusion Database: Preliminary Evaluation of Attenuation Factors. Office of Solid Waste, Washington, D.C. March 4, 2008.

USEPA, 2010. Lack of Final Guidance on Vapor Intrusion Impedes Efforts to Address Indoor Air Risks. Office of Inspector General, Washington, D.C. 10-P-0042. December 14, 2009.