Vapor Intrusion: What You Can’t See Can Cost You

By Tony Anthony

Vapor intrusion can pose a significant risk to your existing or future development plans.

Vapor Intrusion (VI) is a topic that is becoming more and more prevalent when conducting and evaluating the overall environmental conditions of a property. In this post we’ll attempt to answer some of the major surrounding Vapor Intrusion:

  • What is Vapor Intrusion?
  • What can be done to determine if your site does have a VI issue?
  • If your site does have a VI issue, how does VI affect the site and/or future development plans?
  • What options are available to remedy or protect the existing or future building(s) and occupants?

Background

Vapor Intrusion has become a significant component in the comprehensive environmental evaluation of properties for regulators, developers, industrial leaders, and concerned residents.

The evaluation and thorough investigation of the vapor intrusion pathway in the past was often overlooked during environmental site investigations; however, the necessity to evaluate this pathway is crucial when determining both the environmental impact and the overall site development plan for a given property.

What is Vapor Intrusion?

Vapor intrusion is the migration or potential of migration of volatile chemicals from the subsurface into the potential breathing space of overlying buildings. Volatile chemicals can be found emanating from many different potential sources (i.e. fueling stations, dry cleaners, manufacturing facilities, landfills, etc.).

The potential of VI and the associated exposure risk depends upon several site specific variables. Therefore, a comprehensive approach to site characterization should be conducted to evaluate the VI potential. The following diagram presents a general depiction of how potential subsurface contamination may pose a risk to a buildings indoor air quality.

Vapor Intrusion Diagram

Vapor Intrusion Diagram (courtesy of MDEQ’s Petroleum Vapor Intrusion Workshop)

VI subsurface evaluation

Determining the presence of a source of contamination is often established through the evaluation of soil, groundwater, and soil gas laboratory analytical data.  A source could relate to a historical or current release or event associated with the former or current operations at the site (i.e. dry cleaning, manufacturing, fueling station, etc.). However, a source does not always have to be present onsite for there to be a potential VI risk.

Migrating subsurface vapors emanating from adjacent and adjoining properties can also pose a risk for vapor intrusion to your property. As detailed in the United States Environmental Protection Agency (USEPA) Draft Vapor Intrusion Guidance and the ASTM E2600 -10 Standard Guide for Vapor Encroachment Screening, any building structure located within 100 feet laterally or vertically of a known subsurface vapor source should be considered “near” for purposes of vapor intrusion investigations. All media (soil, groundwater, and soil gas (subsurface gas)) should be considered when evaluating the VI risk for your site. Comparison of the laboratory analytical data to the appropriate regulatory criteria and/or screening levels can assist in determining the “next steps” within the VI evaluation.

Each VI evaluation/investigation should be conducted on a site specific basis. When conducting the evaluation there are three key components to consider:

  • The type and concentration of contaminants present
  • The soil and groundwater conditions
  • The existing (or proposed) building layout and structure.

Continued Subsurface Investigation vs. Presumptive Remedy

Once the determination has been made that the site has a potential VI risk, what’s next? Continue with additional subsurface evaluation and investigation or move directly to a presumptive remedy? This again, should be considered on a site specific basis as both of these avenues could be the appropriate response. The variables could include a cost analysis and comparison of additional testing versus mitigation, the presence/absence of potentially liable parties, the timeframe in which the property owner is requesting an “end result”, or the site specific use of the property (i.e. manufacturing facility versus a child day care).  All of these options should be considered when moving along the VI evaluation pathway.

Presumptive Remedies

If the determination has been made that a potential VI does exist for your site, there are many options available to address the VI concern. Presumptive remedies such as active and passive subsurface depressurization systems, suction pits, vapor barriers, HVAC air flow exchange rate modifications, or a combination of one or more of these technologies are all proven mitigation techniques. It’s worth noting that all of the presumptive remedies listed above can be designed and applied to both new builds or retro-fitted to existing buildings.

So, which remedy will be most beneficial? It depends. The appropriate mitigation choice is dependent upon site specific conditions, mitigation goals, current/expected land use, etc.

Conclusion

Vapor intrusion can pose a significant risk to your existing or future development plans and should be considered and evaluated on a site specific basis. Contact us today to learn more about Vapor Intrusion and how it may affect your property and/or development.

Contributions to this article were made by Thomas Szocinski and Brett Shoaff.

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