Theme: 09. Vapor Intrusion

Topic Area: 9c. Vapor Intrusion Preferential Pathways

Formatted Title:

Vapor Intrusion through Sewers: Sample Collection and Mitigation Methods

Background/Objectives:

Preferential pathways, including sewers, have been identified as critical in the evaluation of the vapor intrusion (VI) pathway. Many state and federal guidance documents highlight the need to evaluate preferential pathways and some states have recently released guidance for documenting the investigation of human-made preferential pathways including utility corridors. Current research has highlighted the prevalence of preferential pathways in storm sewers, sanitary sewers and utility tunnels which may transport volatile organic compound (VOC) vapors into buildings at a rate greater than would occur from diffusion through subsurface soils. This presentation will provide a case study where VOC vapors, primarily trichloroethene (TCE), were identified in a sanitary sewer line near a contaminated site, triggering investigation of indoor air and sewer gas at nearby residential buildings and the installation of an active sewer venting system.

Approach/Activities:

Multiple rounds of sewer gas and indoor air sampling have been performed at sewer manholes, sewer cleanouts, and residential properties to evaluate the extent of sewer gas migration, as well as spatial and seasonal variability in concentrations. The presentation will discuss the advantages and disadvantages of using passive and active sampling methods to collect samples from sewers and indoor air. Active sewer venting has been implemented to mitigate sewer gas migration and an overview of the design, installation and post-monitoring results will be discussed.

Results/Lessons Learned:

This presentation will discuss sampling methods and provide a conceptual model for TCE migration through sewers on a neighborhood scale. Although TCE concentrations up to 400 µg/m³ had been identified within the sewer cleanouts in residential buildings, there were no exceedances of the indoor air criteria for TCE in these buildings due to the proper functioning of P-traps and other components of the plumbing systems. The active sewer venting system is effectively capturing sewer vapors, and sewer gas samples collected following installation suggest a decrease in VOC concentrations at the residential sewer cleanouts as a result.

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Greg Johnson