AARST_Radon_Reporter_Q42025

46 | DECEMBER 2025 VAPOR INTRUSION Indoor Environments 2025 - Symposium Recap At Indoor Environments 2025, Lila Beckley, Senior Associate of GSI Environmental Inc. in Austin TX, delivered a compelling presentation that challenged conventional assumptions about vapor intrusion (VI). Her session, “Methods to Identify and Address Conduit Vapor Intrusion Preferential Pathways,” offered a technically rich and timely exploration of how volatile organic compounds (VOCs) can bypass traditional soil gas pathways and enter buildings through overlooked infrastructure— particularly sewer and utility conduits. Beckley’s insights underscored the importance of evolving conceptual site models to match the complexity of real-world sites. Her presentation combined regulatory context, historical perspective, multiple lines of evidence data interpretations, and a detailed case study to illustrate how conduit pathways are reshaping the field of VI assessment and mitigation. Vapor Intrusion: A Growing Priority Beckley began by outlining the regulatory, business, and community drivers that have elevated VI as a critical concern in environmental site assessments: Regulatory pressure is increasing nationwide. More than 40 U.S. states now have draft or final VI guidance in place. Federal oversight also remains central. USEPA’s 2015 Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway continues to shape national practice. VI is also a factor in the Hazard Ranking System and CERCLA five-year reviews. Business drivers include due diligence obligations under ASTM E1527 (Phase I Environmental Site Assessments) and ASTM E2600 (Vapor Encroachment Screening). Community concerns and litigation have further elevated the profile of VI. Beckley emphasized that vapor intrusion is no longer a niche technical issue—it is a mainstream environmental and public health concern with implications for land use, redevelopment, and occupant safety. A Historical Lens on Vapor Intrusion To contextualize current challenges, Beckley provided a brief history of vapor intrusion: • In the 1980s, the focus was on radon intrusion. • The 1990s saw increased attention to VOCs, with the development of the Johnson and Ettinger model. The 2000s brought expanded federal and state guidance, along with key studies such as those conducted in Redfield, Colorado. Between 2008 and 2013, ASTM formalized vapor encroachment and VI considerations in its standards. In 2015, USEPA finalized its VI guidance. In that timeframe, the scientific community also began recognizing the importance of sewer and conduit systems as preferential pathways, changing the way VI is conceptualized and investigated. This evolution reflects growing recognition that vapor intrusion is a dynamic phenomenon influenced by infrastructure, building design, and site-specific subsurface conditions. Conceptual Models: From Conventional to Complex Beckley’s presentation highlighted the limitations of the conventional vapor intrusion conceptual sitemodel (CSM), which assumes VOCs migrate from contaminated soil or groundwater through the vadose zone and into buildings via cracks in foundations. This model emphasizes diffusion and advection through porous media. Beckley presented more nuanced CSMs that can include preferential pathwayssuchassewer lines, utility conduits, and storm drains, which can act as low-resistance channels for vapor migration; direct infiltration, where contaminated media enters buildings through plumbing or structural breaches; and other scenarios. Different testing approaches and multiple lines of evidence may be needed to develop accurate CSMs. Accurate CSMs, in turn, are critical for effective mitigation. Case Study: Diagnosing a Conduit Pathway To illustrate these concepts, Beckley shared a case study involving a building located within 100 feet of a trichloroethylene (TCE) groundwater plume. Initial sampling using conventional methods—indoor air, outdoor air, and subslab soil gas—showed no detections of concern. However, when the building was tested under depressurized conditions, elevated concentrations of TCE, cis-1,2-dichloroethene (cis-DCE), and vinyl chloride (VC) were detected in indoor air, particularly in a garage with large floor drains. Rethinking Vapor Intrusion: How Conduit Pathways Are Reshaping Site Assessments