Vapor Intrusion 101

If you have come across a petroleum or solvent plume at a site you are assessing, there is a likelihood that you may not only be dealing with a soil and groundwater concern, but also a possible vapor intrusion (VI) issue for surrounding structures. What’s a VI issue? Well, vapor intrusion occurs when vapor-forming chemicals migrate from a subsurface source (soil and groundwater contamination, underground storage tanks, etc.) into an overlying building. It’s a similar issue to radon gas seeping into home. It is important to note that an enclosed structure has to exist over or near a source in order for a VI pathway to be present. However, if a future structure is being proposed over the location of a petroleum or solvent plume, then a post construction evaluation or pre-emptive mitigation is highly recommended.

So what do you do when you find out a petroleum or solvent plume is underneath your building or your proposed building? The first question that you have to answer in the evaluation of the VI pathway is what are the chemicals of concern at the site? Are you dealing with petroleum compounds or maybe chlorinated compounds? The way you assess the chemicals is similar but there are some deviations.

In the case of dissolved-phase petroleum compounds, let’s assume you have a plume in which analytical data demonstrates that groundwater concentrations exceed the applicable Vapor Intrusion Screening Levels (VISL) and no light non-aqueous phase liquid (LNAPL) is present. In this example too, let’s assume that there is at least 6 feet of non-impacted soils between groundwater and the foundation surface. In this scenario, the VI pathway would be considered incomplete. However, it’s important to note that there is no set “clean” soil thickness determination for chlorinated compounds in evaluating a VI pathway. Therefore, taking the previous scenario and inserting trichloroethylene was the contaminant of concern, would trigger further assessment of the VI pathway. Surrounding structures also need to be taken into consideration while assessing the VI pathway. A VI evaluation would be recommended for enclosed structures that are above the known plume and any enclosed structure located within 100 feet of the known plume edge.

Once it is determined that assessment is needed to rule out a VI pathway, the next step is to perform a soil gas survey (SGS). During a SGS, soil borings are typically advanced approximately 3 to 4 feet below ground surface. These soil gas points are placed near slab and should not be confused with sub-slab soil gas points. Soil gas samples are collected and then compared to the EPA’s VISL values for determining if further VI assessment is necessary. If soil gas sampling indicates the need for further assessment, the more intrusive sub-slab soil gas sampling (SSGS) is triggered. SSGS consists of boring a hole through the concrete slab of the structure and installing a sample point for the collection of a soil gas sample. The sub-slab soil gas samples are compared to the VISL values to determine if further assessment is necessary. It is imperative that the borehole created during the sub-slab soil gas sampling event is completely patched/repaired as the borehole may create a conduit for contaminants and/or moisture into the structure.


The reason we never suggest moving immediately to sub-slab sampling is to avoid damaging a structure and also to avoid impacting an effective barrier to the subsurface.

If sub-slab soil gas analytical results indicate further assessment is required, the last step in evaluating the VI pathway is triggered – indoor air sampling. Indoor air sampling is the last step in the process for a multitude of reasons – the main reason being its potential to result in false positives and the fact that it is invasive, often requiring the structure to be shut down during the sampling process. Prior to indoor air sampling, an inventory of chemicals utilized in the structure has to be performed, and any chemicals found that may impact the results of the testing are removed and the facility is allowed an “airing out” period prior to sample collection. Indoor air samples are typically collected over an 8-hour period to mimic a work day but can be collected at other durations depending upon structure usage. The indoor air samples are then compared to the applicable VISL values to determine if the VI pathway is complete. Depending upon the indoor air analytical results, there is the possibility of immediate action being required by regulatory agencies to ensure health and safety of the structure’s occupants.

Should a VI issue be found that must be remediated, there are several remediation technologies that can be implemented, but that’s an article for another time!

Encountering an environmental issue under your facility is never a welcomed find. However, we at PPM are skilled in VI assessments and remediation technologies – like sub-slab depressurization systems and vapor barriers. If you are worried about a potential VI issue at your site, don’t hesitate to give us a call.


Contributed by:

Ben Lightsey, P.E.

Project Manager

(601) 953-8233