Vapor Encroachment Explained

In a Phase I Environmental Site Assessment (ESA) report, these days you are likely to see an evaluation of vapor encroachment conditions at the site. If you haven’t, you will. The upcoming update to ASTM’s Phase I Standard proposes to treat vapor encroachment like any other source of contamination, and thus a routine component of environmental due diligence.  So, what is it?

Think of vapor encroachment as a cousin of vapor intrusion.

According to the EPA, vapor intrusion occurs when there is a migration of volatile chemicals from contaminated groundwater or soil into an overlying building. Evaluation of vapor intrusion identifies contamination sources on- site. Vapor encroachment, on the other hand, also considers off-site sources. Think of vapor encroachment as a screening tool.

Vapor encroachment screening identifies releases in the vicinity of the subject property and, based on the contaminant plume, evaluates the likelihood of vapors migrating to the subsurface of the subject property. Vapors, in this case, consist of any chemical of concern (COC); and their presence or likely presence constitutes a vapor encroachment condition (VEC). The ASTM standard for this screening (E2600-10) uses a two-tiered approach (not to be confused with the EPA’s three-tiered vapor intrusion guidance!)

Tier 1 of vapor encroachment screening uses state and federal database records to identify those sites with the potential to affect subsurface vapor conditions. Much like the radius search for a Phase I report, Tier 1 employs a 1/3 mile radius for releases of non-petroleum products, and a 1/10 mile radius for releases of petroleum products. If a VEC cannot be ruled out, i.e. there are open sites within the search distance, then on to Tier 2.

Tier 2 starts out as a non-invasive records review. If you’re lucky, the state will have regulatory files for sites identified in Tier 1, and those files will contain the location of the source and contaminant plume. Based on the relation of the plume to the subject property, it is possible to rule out a VEC. If you don’t have access to plume info, or, if a VEC cannot be ruled out non-invasively, the next option is sampling – of soil, soil gas, and/or groundwater.

Ultimately, there are four possible outcomes of a vapor encroachment screen: 1) VEC exists; 2) VEC likely exists; 3) VEC cannot be ruled out; or 4) VEC can be ruled out. In the context of a Phase I ESA, the environmental professional determines if a VEC represents a recognized environmental condition (REC) for a specific site.

As vapor intrusion receives more attention from regulators, vapor encroachment screening can be a useful tool to delineate vapor concerns originating on-site and off. The intent is to provide an evaluation of vapor in the soil, thus refining the determination of vapor that may end up indoors. Some agree, some disagree, either way it is here to stay. Have you found vapor encroachment screening to provide insight or headaches? Leave a comment below, and please feel free to contact me at with your vapor encroachment questions.