There is growing recognition that preferential pathways can play an important role at sites affected by vapor intrusion (VI). Although this pathway is often mentioned in regulatory guidance documents, there is little information detailing the conceptual model or assessment methods. The goal of this project was to obtain a better understanding of sewers and utility tunnels as preferential pathways for VI. Specifically, the project involved studying the conceptual model, identifying risk factors, and developing and validating an investigation protocol for sewer/utility tunnel VI. Key questions addressed in this study included:

• what types of samples to collect (liquid vs. vapor),
• the significance of temporal variability, and
• the significance of spatial variability.
   

This project has resulted in refinement of the conceptual model for sewer/utility tunnel VI and development of an investigation protocol:

•  Volatile organic compound (VOC) contaminated sites can be categorized as higher risk for sewer/utility tunnel vapor intrusion (sites where a sewer or utility tunnel directly intersects a VOC groundwater plume or vadose zone nonaqueous phase liquid (NAPL) source) or lower risk (sewer or utility tunnel in the vadose zone above a plume). Usually, background sources of VOCs in sewer vapor will not confound the vapor intrusion investigation. Although VOCs are commonly detectable in sewers at background locations away from known subsurface source areas, background concentrations in sewer vapor are usually lower than screening levels recommended for evaluation of VI. VOCs can move from sewer vapor into buildings through a variety of mechanisms that are difficult to predict without site-specific data (e.g., open pipes, plumbing defects).
•  The protocol includes initial desktop screening to classify sites as higher risk or lower risk. Early field testing of sewers/utility tunnels is recommended for higher risk sites while conventional VI investigation (including indoor air testing) is recommended for lower risk sites.

This project validated the various aspects of the conceptual model and protocol. Routine implementation of the initial steps of the protocol is not expected to be cost prohibitive and can result in more standardized evaluation of sewers and utility tunnels during VI investigations.

Advantages of the protocol include: i) provides a standardized framework for evaluating sewers and utility tunnels as potential preferential pathways for VI; ii) provides a decision logic for testing based on potential risk of the presence of sewer/utility tunnel VI; and iii) recommends sampling procedures that are practical and relatively simple to implement. Limitations of the protocol include: i) relies on indoor air testing to identify VI impacts at lower risk sites; and ii) does not provide detailed guidance on sewer/utility tunnel mitigation.

Beckley, L. and T. McHugh. 2020. A Conceptual Model for Vapor Intrusion from Groundwater through Sewer Lines. Science of the Total Environment, 698:134283. doi.org/10.1016/j.scitotenv.2019.134283.

McHugh, T., P. Loll, and B. Eklund. 2017. Recent Advances in Vapor Intrusion Site Investigations. Journal of Environmental Management, 204(2):783-792. doi.org/10.1016/j.jenvman.2017.02.015.

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. doi.org/ 10.1016/j.scitotenv.2017.04.135.

McHugh T., L. Beckley, and P. Philp. 2016. Utility of Compound-Specific Isotope Analysis for Vapor Intrusion Investigations. Groundwater Monitoring and Remediation, 36(4):31-40. doi.org/10.1111/gwmr.12185.