Theme: 09. Vapor Intrusion

Topic Area: 9d. Vapor Intrusion Risk Assessment and Site Management

Formatted Title:

Mitigating a Phantom Source: Where Are These Vapors Coming From?

Background/Objectives:

Evaluate tools and strategies that can be employed to critically look at data on a site-specific basis and support the development of a mitigation strategy that is efficient, protective, and site-specific. Vapor intrusion screening levels should be considered just that, a screening tool, and not the only factor considered when developing a mitigation strategy.

Approach/Activities:

As is typical for the development of any potentially environmentally impacted site, Langan conducted a soil vapor investigation in September 2022 in support of a new terminal development at a major international airport. What wasn’t typical were the results of said soil vapor investigation. Elevated concentrations of chlorinated, not petroleum, compounds were found throughout the footprint of the planned development. Subsequent investigations were completed to not only duplicate the results, but also define the extent of soil vapor impacts and identify the source. While the supplemental sampling better defined the lateral extents of the impacts, a source, whether it be site soils and/or groundwater, was not identified. This initiated a series of additional supplemental sampling events, as well as the concurrent performance of Johnson & Ettinger modeling to evaluate the need for mitigation measures and the design of a passive sub-slab mitigation system, all during active redevelopment and under the stopwatch of the construction schedule.

Based on the initial results, Langan’s working theory identified the existing subsurface utilities (i.e., preferential pathways) as the likely source for the observed soil vapor impacts. As such, Langan completed supplemental soil vapor sampling throughout construction, as existing utilities were abandoned and/or removed. Over time, the results became more and more compelling and pointed the finger at an active thermal distribution system (TDS) line. However, as long as the line was operating, the theory could not be definitively tested. As such, Langan had to move forward concurrently with evaluating the need and developing the design for a mitigation system, so that it could be properly coordinated with the design team and integrated into the development, if determined needed.

The impacts were not associated with a known spill and, as such, were not beholden to the local state agency – the evaluation was strictly technical and dependent on Langan’s experience and professional recommendation. As such, in support of a final recommendation, Langan completed Johnson and Ettinger modeling, accounting for building-specific characteristics, considering sensitivity of certain input assumptions, and evaluating the effectiveness of a contaminant vapor barrier. In fact, manufacturer-published diffusion coefficients for the waterproofing membrane specified for the development were incorporated into the model and demonstrated compliance with indoor air guidance of the local state agency. This, in conjunction with an observed decreasing trend in soil vapor concentrations in the areas in which utilities had been abandoned and/or removed and overall positive pressure throughout the majority of the first floor of the development, supported a vapor barrier only mitigation strategy.

Results/Lessons Learned:

This presentation will provide a summary of the investigations completed, the evaluations performed, and the design developed, all in the context of a fast-paced redevelopment with many different stakeholders. The investigation and subsequent evaluation and design phases were made further difficult as a result of special access requirements and the coordination required to work alongside the redevelopment activities onsite. The project team was recently authorized to temporarily disable the TDS line, such that soil vapor samples could be collected to confirm (or deny) the source theory – the results of this sampling will also be reviewed. Due to Langan’s experience, particularly as it relates to implementing remediation strategies in support of redevelopment, and technical knowledge, we were able to look beyond the face-value of the results and value engineer a strategy that will be protective of human health, while being cost and schedule efficient.

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Sarah Baker