Theme: 09. Vapor Intrusion

Topic Area: 9a. Advances in Vapor Intrusion Investigations

Formatted Title:

A New Look at Diffusion and Subslab Vapor Intrusion Sampling: Passive Adsorption Diffusion Samplers

Background/Objectives:

There are two primary transport mechanisms by which volatile organic compounds (VOCs) can migrate into a building via vapor intrusion (VI) through the slab/floor: molecular diffusion through the building slab or advective flow through cracks and other entry points. Currently, most VI investigation techniques are directed at evaluating the advective VI pathway, while diffusion through the building slab is largely dismissed. Furthermore, traditional sampling methods for VI, such as evacuated canisters or passive samplers, do not directly measure VI flux through the slab/floor. More specialized sampling techniques such as using a flux chamber can be logistically challenging to perform, time intensive, and expensive. A novel sampling device referred to as a passive adsorptive diffusion sampler (PADS) was developed and tested for the purpose of obtaining high-resolution data of the diffusive flux of VOCs through the slab directly, passively, and without drilling through the slab, while being simple to use and cost effective.

Approach/Activities:

This study was conducted at a vacant industrial warehouse where VI of trichloroethene (TCE) was concluded to be occurring based on traditional sampling methods. Existing passive and active subslab vapor data were used to calculate the theoretical diffusive flux through the slab by Fick’s First Law. The PADS devices were then deployed on the surface of the concrete slab at this building to evaluate if the PADS were capable of measuring the diffusive flux of TCE through the slab within the range of theoretical expectations. Additionally, the diffusive flux calculations and measurements were used to estimate TCE indoor air concentrations based on the building geometry and air exchange rates; these estimates were compared to existing indoor air data from samples collected using passive and active methods.

Results/Lessons Learned:

The building-wide diffusive flux of TCE through the building slab measured directly by the PADS varied by less than 10% from the theoretical diffusive flux estimated from existing subslab vapor data and indicated that diffusion was a significant pathway into this building. Using the diffusive flux measured with the PADS, the estimated indoor air TCE concentration varied by less than 3% from the average indoor air TCE concentration measured during the PADS deployment. The range and distribution of the PADS results also correlated well with the subslab vapor concentrations from previous sampling events. This study demonstrated that the PADS were capable of successfully measuring the diffusive flux through the slab and can be used to calculate the indoor air concentrations when diffusion is the dominant pathway. PADS can also be used as a screening tool to identify and prioritize slab areas with elevated VOC diffusive flux at a large building and select key locations where traditional samples may be required, as to inform if and where floor sealants are necessary to mitigate the risk from VI.

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Michael Niemet