Understanding the Role of Sewer Preferential Pathways in Vapor Intrusion Thomas McHugh and Lila Beckley (GSI Environmental)

Although the potential importance of preferential pathways is highlighted in many vapor intrusion regulatory guidance documents, there is little useful guidance on i) how to identify sites with preferential pathways and ii) how to test for the presence or absence of VOCs in such pathways.  As a result, there are no consistent procedures for evaluation of these pathways during vapor intrusion investigations. 

Through a research project funded by the Department of Defense ESTCP program, we have developed a protocol for evaluation of sewer preferential pathways at vapor intrusion sites.  This protocol includes i) preliminary screening to eliminate sites where sewers are unlikely to act as preferential pathways, ii) procedures for initial testing of sewers at locations most likely to be impacted by VOCs, and iii) delineation and building testing.  We have tested and refined this protocol through sampling at several sites where VOCs were known to be present in sewer lines.

In this presentation, we will share results from our research project to date, along with an updated conceptual model for sewer preferential pathway vapor intrusion.   VOCs can enter sewer lines through infiltration of contaminated groundwater, direct discharge of contaminated water into the sewer line, or, in some cases, infiltration of soil gas.  The spatial and temporal variability within the sewer line depends on a variety of factors including the type of sewer (storm, sanitary, combined), the source of vapors (discharge, groundwater, soil gas), season, and rainfall events.  As a result, the number of samples required to accurately characterize the presence or absence of VOCs within a sewer line can vary from site to site.  However, at many sites with chlorinated VOCs in groundwater, vapor concentrations within sewer lines can be above typical soil gas screening levels.  Tracer testing conducted as part of our research program showed that measurable gas exchange commonly occurs between sewer lines and connected buildings.  As a result, when VOCs are present at elevated concentrations within sewer lines, there is likely to be some risk to connected buildings.


Conference info - http://www.aehsfoundation.org/west-coast-conference.aspx 

For more information about vapor intrusion, or to request a copy of the presentation, please contact Lila Beckley at This email address is being protected from spambots. You need JavaScript enabled to view it.