Theme: 09. Vapor Intrusion

Topic Area: 9c. Vapor Intrusion Preferential Pathways

Formatted Title:

Preferential Pathways: Responding to Changes in the Vapor Intrusion Conceptual Site Model

Background/Objectives:

Background/Objectives: While vapor Intrusion (VI) has been evaluated for approximately 30 years, this exposure pathway is still in its scientific and regulatory infancy. Over the past decade, significant scientific advancements and an improved understanding of subsurface vapor transport have resulted in a cascade of most State and Federal regulatory VI guidance documents. One of these advancements is the growing recognition of preferential pathways and their importance in managing sites affected by VI. Guidance documents now recognize that preferential pathways need to be understood and adequately characterized but provide little information about how this can be achieved. Much of this is attributed to site-specific conditions that govern when preferential pathways may exist. In the absence of regulatory guidance for the investigation and mitigation of preferential pathways, moving sites through the closure process in a consistent manner has become a significant challenge for projects across the United States.

Approach/Activities:

Approach/Activities: Standard VI transport models assume volatilization of compounds from a steady state subsurface source (groundwater and/or soil) and are reliant on applicable industry assumptions that have been investigated via standard investigation practices over the past 30 years. Preferential pathway vapor transport adheres to very different assumptions, ultimately negating the usefulness and validity of standard VI models and screening methods. Applying innovative sampling methods and mitigation strategies is critical to understanding the evolving science around preferential pathways and successfully managing them as part of the site closure process. Case studies will be presented that highlight a variety of applicable tools for use in understanding the sewer preferential pathway, particularly under complex site conditions. Tools that will be discussed include grab and time-integrated canister sampling, passive sampling techniques, and real-time monitoring.

Results/Lessons Learned:

Results/Lessons Learned: Real-time screening instruments have proven successful in providing adaptive sampling capabilities in the field to overcome transient sewer conditions and allow for collection of vapor data over a more frequent timeframe. Sewer mitigation strategies, including lining of the sewers and use of backflow preventers have also been crucial and will be discussed in the case studies.

Sarah Jonker

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Preferential Pathways: Responding to Changes in the Vapor Intrusion Conceptual Site Model