Vapor Intrusion Fundamentals 48 – Sanitary Sewers, the Preferential Pathway, Part 2


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Vapor Intrusion Fundamentals 48 – Sanitary Sewers, the Preferential Pathway, Part 2

By: Craig A. Cox, CPG

As discussed in last month’s installment of Vapor Intrusion Fundamentals (Number 47), sanitary sewer lines are becoming recognized as an important, yet often overlooked, pathway for vapor intrusion (VI).  In this installment, we will review approaches used to evaluate sewer connections and deciphering the associated analytical data collected during VI assessments.                  

As a reminder, VI assessments can be thought of as a three-legged stool, with each leg representing a contributing source that must be understood:

  • contributions associated with soil gas,
  • contributions associated with indoor sources, and
  • contributions associated with preferential pathways.

Not understanding this third leg of VI assessment stool can lead to a misdiagnosis of the issues and implementation of ineffective mitigation efforts. 

The first step in the assessment of the vapor contribution associated with sewer gas is to develop a sound conceptual site model (CSM) that recognizes and anticipates this preferential pathway.  Trunk sewer lines, especially those is areas served by industries that used solvents, can transport volatile organic compounds over relatively long distances downgradient and contaminate environmental media (soil and groundwater) through degraded sections and defects in the lines. These areas should be thought of as secondary point or line sources.  Assessing the locations of these secondary sources can be done through detailed inspections of the lines using cameras. Many communities have done these studies in preparation for sewer improvement projects.

Understanding the potential effect of degraded sewers on your study area is an important aspect of the CSM and the overall evaluation of a neighboring buildings and/or houses, however, most investigations begin with an assessment of a single building.  You can get a sense of the potential contribution of sewer gas on your building through the collection of sewer gas samples from nearby manholes and cleanouts.  In addition to your chemicals of concern (e.g., chlorinated VOCs), you should also analyze for other chlorinated and brominated compounds (e.g., chloroform, bromoform, and bromodichloromethane), as these compounds are typically found in sanitary sewer gas as a result of the breakdown of household and industrial cleansers. 

As you begin your indoor air sampling campaign, review your sewer gas data closely.  If your chemicals of concern are found in sewer gas, then you should inspect the lavatories and areas containing floor drains as possible sampling locations.  If you collect samples from lavatories, conduct the sampling with the door shut and the fan running to induce air flow through plumbing leaks. After you receive the analytical results, review the dataset for the indicators of sewer gas (chloroform, bromoform, and bromodichloromethane) as well as the ratios of your chemicals of concern and compare them to your sub-slab and sewer gas data.  These comparisons will help you determine if the sanitary sewer represents a complete pathway for VI. In a recent study completed by Cox-Colvin, sewer gas was determined to be the primary pathway of VI at more than half of the buildings evaluated.

Accounting for the sewer gas VI pathway in your CSM provides you with additional insight into potential sources as well as appropriate means of mitigation. Sub-slab depressurization systems (SSDs) are very effective at interrupting the VI pathway associated with a sub-slab source; however, SSDs are typically not effective at mitigating the sewer gas source. If the VI pathway is dominated by the sanitary sewer line source, simple plumbing repairs could be enough to mitigate the VI issues.  

Published in Cox-Colvin’s April 2019 Focus on the Environment newsletter.


Craig Cox is a principal and co-founder of Cox-Colvin & Associates, Inc., and holds degrees in geology and mineralogy from the Ohio State University and hydrogeology from the Colorado School of Mines. Mr. Cox has over 30 years of experience managing large environmental project implemented under CERCLA and state voluntary action programs. Mr. Cox is the inventor of the Vapor Pin® and has developed a variety of software products including Data Inspector, an internet-enabled environmental database application. Mr. Cox is a Certified Professional Geologist (CPG) with AIPG and is a Certified Professional (CP) under Ohio EPA's Voluntary Action Program.