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August 2019 Vapor Pin Article


Vapor Intrusion Fundamentals 47 – Sanitary Sewers, the Preferential Pathway, Part 1

By: Craig A. Cox, CPG

In general, Vapor Intrusion (VI) assessments can be thought of as a three-legged stool, with each leg representing a contributing source that must be understood; including: contributions associated with soil gas, contributions associated with indoor sources, and contributions associated with preferential pathways.

Until recently, most assessment efforts (and modeling) have focused on the first two legs – understanding the contributions from soil gas and the contributions from indoor air. The third leg, contribution from preferential pathways, is generally given lip service but rarely evaluated in a rigorous manner.

In the past, the preferential pathway has been considered the trenching and permeable backfill associated with underground utilities.  However, when you consider that a sanitary sewer is essentially an air-filled conduit providing a direct vapor pathway into every inhabited building, more often than not, the sanitary sewer will be the preferential pathway of concern.

Not understanding this third leg of VI assessment stool can lead to a misdiagnosis of the issues and implementation of ineffective mitigation efforts.  This is a game changer when you consider that: sanitary sewers have been an integral part of the industrial waste elimination systems for years through permitted discharges to publicly-owned treatment works (POTWs), the “drain” has been an easily accessible disposal route for those that have used solvents and may not have obtained discharge permits, sanitary sewers used by industry are, in many instances, the same ones used by homes in the same neighborhood, sanitary sewers are typically plagued by failures or penetrations from tree roots, sanitary sewers may extend below the water table (before entering a lift station), and sanitary sewers contain a great deal of air-filled space that is essentially connected to every home and business along its route.

Plumbing traps (P-Traps) and seals (e.g., wax toilet rings) in our plumbing systems typically prevent the intrusion of sewer gas into our homes and businesses, and vent pipes on the soil stack allow these vapors to vent to the atmosphere.  However, P-Traps can dry out, wax rings can fail, and stacks within the home or business can develop cracks or other failures allowing vapors within the sewer to enter the indoor air space.

So how does this result in a misdiagnosis of the issue?  Let’s say you collect sub-slab soil gas and concurrent indoor air samples above VOC contaminated groundwater and find that similar compounds are detected in both.  This could look like a line of evidence indicative of VI. The tendency may be to mitigate with a sub-slab depressurization (SSD) system.  However, if the plume intersects the sanitary sewer line, or the bed of the sanitary sewer line is impacted by a history of solvent disposal, then the vapors may be coming into the indoor space through the sewer connection which will not be mitigated by SSD.  Instead, ensuring that the P-Traps are filled, replacing defective toilet seals and repairing sewer line breaks could mitigate the issue. In the end, it is important keep in mind all of the potential migration pathways when you develop your conceptual site model and take steps to evaluate each pathway before you move to mitigation.  In the next installment, we will review approaches used to evaluate sewer connections and deciphering the associated analytical data.


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.