AARST_Radon_Reporter_Q42025

THE RADON REPORTER | 47 VAPOR INTRUSION This prompted a deeper investigation using: • On-site analysis • Sewer video surveys • Revised conceptual modeling The data pointed to a conduit pathway as the likely source of vapor intrusion. Sewer vapor concentrations in a nearby manhole exceeded 1,000 µg/m³ for both TCE and cis-DCE—strong evidence that the sanitary sewer system was acting as a vapor conduit to the building. Further investigation revealed that the sanitary sewer line ran near an old waste disposal area, and that intermittent vapor intrusion events were likely driven by contaminant migration through this infrastructure. Mitigation Strategy: Targeting the True Pathway One of the most important takeaways from Beckley’s presentation was that standard mitigation approaches may not be effective when conduit pathways are involved. In this case, a traditional sub-slab depressurization system (SSDS) was deemed inappropriate. Instead, the team focused on interrupting the VOC transport route, targeting three key points: • Entry of VOCs into the sewer/conduit • Migration of VOCs within the sewer line • Migration of VOCs from the sewer into the building Mitigation actions included: • Identifying and abandoning historical sewer laterals near the disposal area • Replacing a portion of the sewer main • Lining the remaining sewer main and manholes in the area to prevent VOC infiltration The results were dramatic. Post-mitigation testing showed: Location Pre-Mitigation TCE Post-Mitigation TCE Sewer vapor (downstream manhole) up to 3,000 µg/m³ ND (<0.3 µg/ m³) — a 10,000x reduction Sewer liquid (downstream manhole) up to 15 µg/L 2 µg/L These outcomes underscore the importance of understanding the true source and pathway of vapor intrusion before selecting a mitigation strategy. Practical Implications for Environmental Professionals Beckley concluded with a series of practical insights: Most regulatory frameworks and testing protocols are still based on conventional CSMs , which may miss critical pathways. Site- and building-specific factors matter in data interpretation , including indoor sources, sewer configurations, and pressure differentials. Non-standard testing methods (e.g., building pressure control testing, sewer gas sampling) may be necessary to fully characterize VI risk. Mitigation is unlikely to succeed unless the source and pathway are correctly identified. These insights challenge practitioners to move beyond one-size-fits-all approaches. Vapor intrusion is a site- specific phenomenon, and investigative and mitigation strategies must reflect that complexity. Conclusion Beckley’s presentation at the Indoor Environments 2025 Symposium was a masterclass in integrating evolving science, regulatory expectations, and field experience into actionable strategies. Her emphasis on conduit pathways, especially sewer systems, adds a critical dimension to how vapor intrusion is assessed and mitigated. For environmental professionals, the message is clear: the right tools, data, and conceptual framework are critical to protecting occupants more effectively and designing smarter, site-specific solutions. Beckley’s work reinforces a central truth: understanding the pathway is essential to solving the problem. As vapor intrusion continues to gain attention across regulatory, business, and community domains, her insights offer a roadmap for more accurate diagnosis and more effective mitigation.