Theme: 09. Vapor Intrusion

Topic Area: 9c. Vapor Intrusion Preferential Pathways

Formatted Title:

VI and Preferential Pathways: Assessment Needs to Expand the Understanding Regarding Receptor Risks

Background/Objectives:

Vapor intrusion (VI) science has evolved significantly since the issuance of ITRC’s various VI guidance documents (2007, 2018, 2020), ITRC’s mitigation fact sheet (2020), EPA’s core guidance document (2015), along with various state guidance documents. Regarding preferential or atypical pathways, core publications, such as ESTCP’s Sewers and Utility Tunnels As Preferential Pathways For Volatile Organic Compound Migration Into Buildings: Risk Factors And Investigation Protocol (2015), have been published to summarize the generally recommended approaches to evaluating sewers as preferential pathways. While the collection of analytical data within preferential pathways has been well documented, the understanding of the mechanisms and associated data needed to assess the risks around preferential pathways requires additional evaluation and publication.

Approach/Activities:

The presentation will briefly summarize the accepted approach for sampling preferential pathways in several scenarios (e.g., sewers above, near, or through sources, etc.) and present a summary of recent publications regarding preferential pathways. After sewer data are collected for the appropriate contaminants of concern (COCs) at a contaminated site, the data need meaning to understand whether there is soil gas to pipe intrusion and subsequent flow within the pipe. The resulting data analysis often indicates that a correlation between site COCs and contaminants inside the sewer can often be similar due to the nature of sewer discharge contents where there may not be a true vapor intrusion source. The challenge is discerning whether contamination inside a sewer is connected to other media or the result of the influence of impacted entering or near the sewer lines.

The core elements of this presentation will include a thorough evaluation of the following elements:

Understanding of sewer gas flow through main sewer lines and lateral connections, including other main sewer lines and residential/commercial sewer laterals
Soil gas migration and the potential influence of “backfill” around sewer conduits
The mechanisms that could potentially cause soil gas to enter sewer pipes and flow preferentially from the source
Information required to mitigate sewer vapors, where necessary, and potential pitfalls around certain mitigation techniques used to interrupt the source-receptor pathway
Layout and construction of the sewer lines which serve a critical role in the manner in which sewer water and sewer gas flows throughout the sewer network

Results/Lessons Learned:

The goal of this presentation is to advance the scientific understanding of preferential pathways and VI beyond what is currently understood and influence what is based in science into the guidance used by the regulatory agencies. To fully evaluate and determine whether the pathways are complete, significant data are needed to understand other variables beyond traditional information gathered from soil gas, groundwater, and soil media. The presentation will present a detailed evaluation of how to understand sewer gas migration potential within sewers and propose methods to collect this data to further confidence in establishing the risk profile from preferential pathways.

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Andrew Wallace Wallace