Sewers and Utility Tunnels as Preferential Pathways for Volatile Organic Compound Migration into Buildings: Risk Factors and Investigation Protocol

Dr. Thomas McHugh | GSI Environmental, Inc.



In the 1990s, vapor intrusion (VI) was identified as a potential exposure pathway but was not routinely evaluated during site investigations because there were no accepted and validated evaluation procedures. Today, the same is true for sewer/utility preferential pathways at VI sites. Although not typically tested as part of the VI investigation process, sewers and utility tunnels have been identified as important transport pathways at a small but growing number of sites. It is likely that additional VI sites have sewer/utility preferential pathways that have not yet been identified. The objectives of this project are to: (1) develop and validate an effective protocol to determine the presence or absence of a sewer/utility tunnel preferential pathway during a VI investigation (i.e., is there a sewer/utility that is transporting volatile organic compounds (VOCs) from a subsurface source causing unacceptable buildings impacts?), (2) apply the validated protocol at additional VI sites to evaluate how often sewer/utility preferential pathways play an important role in transport of VOCs into buildings, and (3) utilize the results to develop a detailed conceptual model for preferential pathways that identifies the types of sites at risk and the key mechanisms and processes involved in VOC transport through preferential pathways.


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Technology Description

The protocol for identifying and evaluating sewer and utility tunnel preferential pathways at VI sites will include the following: (1) initial screening, (2) field investigation of sewer/utility tunnels, and (3) building testing. The protocol will provide a clear, stepwise process with early exit ramps and will serve to minimize false positive and false negative results. It will allow sites that are more likely to have sewer preferential pathways to be identified, sewers to be evaluated early in the VI investigation process resulting in more efficient investigations, and more rapid and effective mitigation of vapor intrusion at sites with these pathways.

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Development of a validated protocol for identifying and evaluating sewer and utility tunnel preferential pathways will yield more accurate and efficient investigations of VI sites, more timely identification of sites needing mitigation, and improved ability to select effective mitigation technologies. Today, sewer/utility preferential pathways are not fully considered during many VI investigations. Sewer and utility tunnel preferential pathways may be particularly important at DoD facilities with shallow groundwater such as those located adjacent to large water bodies. Soil gas and sub-slab testing may fail to identify buildings impacted by VOCs transported through preferential pathways. For large buildings, proper evaluation of sewers and utility tunnels may identify impacts in parts of the building displaced from the known area of soil or groundwater contamination. When impacts are identified by indoor air testing but the preferential transport pathway is not identified, traditional mitigation technologies (e.g., sub-slab depressurization) are often not effective. This demonstration will increase awareness of sewer/utility tunnel preferential pathways and provide reliable methods for investigation and mitigation. (Anticipated Project Completion - 2019)

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McHugh, T., P. Loll, and B. Eklund.  2017. Recent Advances in Vapor Intrusion Site Investigations. Journal of Environmental Management, available online 22 Feb 2017.

McHugh T. and L. Beckley. 2017. Evidence of a Sewer Vapor Transport Pathway at the USEPA Vapor Intrusion Research Duplex. Science of the Total Environment, 598:772-779.

McHugh T., L. Beckley, and P. Philp. 2016. Utility of Compound-Specific Isotope Analysis for Vapor Intrusion Investigations. Groundwater Monitoring and Remediation, 10.1111(gwmr.12185). 

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Points of Contact

Principal Investigator

Dr. Thomas McHugh

GSI Environmental, Inc.

Phone: 713-522-6300

Fax: 713-522-8010

Program Manager

Environmental Restoration