Theme: 09. Vapor Intrusion

Topic Area: 9a. Advances in Vapor Intrusion Investigations

Formatted Title:

Optimizing Site-Specific Ventilation-Based Mitigation Strategies Using Mass Discharge Test in Vapor Intrusion Studies

Background/Objectives:

In recent years, mass discharge tests (MDTs) of the unsaturated zone have been applied in vapor intrusion investigations in Denmark, both as an easy and useful tool for building a robust conceptual site model (CSM) and as a tool to evaluate the performance of future mitigation based on passive or active ventilation. The choice and robustness of a ventilation-based mitigation depends on the source strength of the contamination, both related to the total contaminant mass and the rate of contaminant diffusion from the hotspot geological matrix. Both factors can be investigated using the MDT approach presented in this study.

We hope that the presentation could lead to a greater use and a more uniform and well-documented application of MDTs in relation to a choice of mitigation strategy at a future site.

Approach/Activities:

The MDT is performed by application of a vacuum blower induced sub-slab vacuum while measuring air flow (m³/h) and concentration (µg/m³) as well as monitoring/logging the differential pressure across the floor.

The results presented are from more than 15 tests performed at sites contaminated with chlorinated solvents differing in geology and contaminant source strength. We will showcase test results from one site where a double MDT has been applied leading to an unusual but robust mitigation strategy.

Results/Lessons Learned:

The MDT can be divided into different test steps, each contributing with valuable knowledge regarding the CSM and possible mitigation strategies. The presentation will give a brief introduction to the various conceptualizations of the results that can be derived using different test procedures in different geologic settings.

The strength of the MDT to determine the contaminant mass in the unsaturated zone will be demonstrated. At the case site the method has shown to be particularly strong for proving the presence of significant contaminant mass in unsaturated zones. A mass that has not been documented/located by traditional soil sampling in relatively high resolution.

The presentation will also contain experiences and surprising results regarding slow rebound of the soil vapor contamination (a low rate of contaminant diffusion). After performing an MDT there is a risk that the soil air contamination will not represent steady-state for more than 300 days. The rate of contaminant diffusion from the geological matrix is found crucial for the design of a future mitigation system.

A diagram will be shown that combines the potential risk based on the steady-state mass discharge with the rate of contaminant diffusion. In the diagram more than 10 sites are mapped, making it possible to draw out some overall conclusions regarding mitigation strategies. This have shown to be a very useful tool for evaluating the potential and robustness of different mitigation strategies at future contaminated sites.

After the presentation the audience should be able to evaluate the site-specific potential of a ventilation-based mitigation looking at the results from a MDT, focusing on the steady-state mass discharge and the rate of contaminant diffusion, combined with the overall CSM of the site.

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Maria Hag