357 North Main Street
Memphis, TN 38103

By Luke Hall, PG, geologist 

Last year around this time we discussed vapor intrusion, which is the migration of potentially hazardous vapor from groundwater or soil contamination into indoor spaces. Nowadays, vapor intrusion is more consistently being included as part of the site assessment process. Volatile contamination in soil or groundwater can produce vapors that can migrate through the subsurface and build up under the slab of a building. This vapor can then intrude into a structure where it can pose a risk to people through inhalation. While property can be at risk based upon past use, the technology of vapor mitigation can remove the risk from inhalation of contaminated vapors.

Vapor mitigation may be required depending on the type of building associated with the contamination. For example, contamination identified under a school or residence is handled with a much more conservative approach than if it were identified at a factory. At a school or residence, it is assumed that children will be exposed, while exposure at a factory is limited to an adult employee for the duration of a typical work shift.

Vapor mitigation strategies are varied in both the effort of installation and the cost of installation and maintenance. The main goal of vapor mitigation is to remove the risk of contaminated vapors from the interior of a building. When determining what type of mitigation may be required at a property, consultants often reference a Tennessee Department of Environment and Conservation memorandum titled Vapor Intrusion Mitigation Risk Criteria for New Construction. This memorandum details a range of concentrations of vapor contamination and the recommended mitigation strategy. While designed specifically for new construction, the document can be useful in determining the potential severity of vapor contamination at an existing structure as well.

Based on the range of vapor contamination identified, mitigation strategies are split into two categories: passive and active. Passive mitigation strategies work without machinery while active mitigation strategies generally require a system of pumps and motors to remove vapor.

Passive vapor mitigation strategies are less expensive and are generally applied when identified vapor contamination is present but of a lower range of severity. One method for vapor mitigation appropriate for new construction is the design of an open-air parking garage on the lowest level of a building. This allows contaminated vapors to disperse into the atmosphere and pose minimal risk to people. Mitigation at new construction can also include installation of vapor barriers below the slab during construction to prevent vapor intrusion. Similar products can be installed on the top of the slab of an existing building that has an identified risk when renovation without removal of the slab is desired.

Active vapor mitigation systems are for more severe contamination and can be considerably more expensive and complex. Stay tuned for when we discuss active vapor mitigation and how we can help mitigate the risk from vapor intrusion at your property. In the meantime, if you are unsure if your facility is exposed to vapor contamination, contact us today.