Five Steps for a Vapor Intrusion Investigation, Continued

Yesterday we talked about reasons why you should be concerned about the potential of vapor intrusion (VI) on your property or property you may want for future development. We also reviewed the first two steps you should follow in a vapor intrusion investigation, as outlined in a process developed by the Minnesota Pollution Control Agency (MPCA). Today we review the next steps if the first two steps show that there is a VI risk in any of your buildings.

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Five Steps in Investigating VI

The MPCA offers a five-step process for conducting vapor investigations and a mitigation decision framework for determining if mitigation is necessary. This process can help anyone, anywhere, in identifying and considering key factors related to VI assessments, risk management decisions, and mitigation.

The first two steps suggested that you evaluate your site and building and collect samples of known or suspected vapor sources. Here are the next three steps if you suspect a VI risk.

Step 3: Determine VI area of concern

The VI area of concern (AOC) is the area where building specific investigations are necessary. That is, where sub-slab and/or soil gas concentrations are detected above 33X ISVs. If the VI AOC extent is not defined, meaning that perimeter concentrations are above 33X ISV in all directions, continue collecting samples outward in the undefined directions until the vapor impacts are defined. If you are a nonresponsible party conducting VI investigation for something such as a brownfields redevelopment, the VI AOC should be the property boundaries and all buildings, both existing and proposed, within the property boundaries.

Step 4: Make vapor mitigation decisions

If vapor mitigation is necessary, the extent of the mitigation depends on the use of the buildings affected, i.e., is the building residential or commercial/industrial. Building VI surveys should be conducted to determine the proper method of determining if mitigation is required. In some cases the use of the 33X ISV may not be valid for a specific building because there may be significant openings to the subsurface, such as unlined crawl spaces or sumps, that facilitate VI into the building, or the building may have a deteriorating basement or dirt floor, which generally provide poor barriers to vapor entry.

In addition, in making the mitigation decision, you will have to determine if expedited actions are necessary for that building to protect human health. Expedited actions could be necessary depending on the contaminant (especially the presence of trichloroethylene [TCE]) and sensitive individuals, including women of child-bearing age, children, the elderly, or people with chronic diseases or compromised immune systems.

Step 5: Establish the vapor mitigation area

The vapor mitigation area defines the area where active mitigation is required for a building. The MPCA suggests three scenarios and their appropriate actions:

  • If the extent and magnitude of VI are defined and soil gas and groundwater plumes are demonstrably stable, prepare a Mitigation Area Map.
  • If the extent and magnitude of VI are not defined, and/or all sources and potential sources have not been evaluated, conduct additional investigation to evaluate soil, groundwater, and soil-vapor.
  • If the soil gas plume and/or groundwater plumes are not stable, install and sample sentinel groundwater and soil gas monitoring points quarterly to monitor plume stability and evaluate the potential VI risk to buildings outside the vapor mitigation area.