DDD TRACK HIGHLIGHT: Tips for Assessing Vapor Migration Risk

In October, we hosted three Due Diligence at Dawn/Dusk events in Detroit, Edison and Arlington. One track, titled Vapor Migration Case Studies in Phase I ESAs, was presented by Anthony Buonicore, P.E., BCEE, QEP, Principal, The Buonicore Group and Chairman of the ASTM Vapor Intrusion Task Group. During Buonicore’s track, he reviewed several real-world case studies and highlighted eight important tips for assessing vapor migration risk during the conduct of a Phase I ESA.


Awareness about vapor migration risk has grown significantly in recent years to the point that our pre-DDD survey results show that more than 80 percent of EPs are “always” considering the potential for vapor migration risk at or near the target property. When doing so, a significant 83 percent rely on the guidance in the ASTM E 2600-15 standard.

Vapor Migration Risk Survey

As you’re working on projects that involve assessing the potential for vapor migration issues, keep in mind these tips from Buonicore’s DDD tracks earlier this month:

  1. Beware of PERC.

In your research, any discovery that perchloroethylene, or PERC, has been used at or around the target property should raise a red flag. If you’re dealing with operations involving the use of PERC, strong odds are that there has been a release that could lead to vapor migration risk.

  1. Dry cleaners and gas stations are high-risk operations for vapor migration.

There are literally hundreds of thousands of dry cleaners and gas stations (present and former) across the U.S. that potentially present vapor migration risk. Assume dry cleaners using PERC had releases – even if there were no records of releases –as experience has shown that almost all have had releases (e.g., from accidental spills, equipment and piping leaks and malfunction, separator or filter rinse water releases to landscaping or storm water drains, leaking sanitary sewer lines, etc.).  Other high-risk operations to look for include: industrial sites (particularly those using chlorinated solvents for degreasing and parts cleaning), former manufactured gas plant sites, former hazardous waste disposal sites, and present and former garbage landfills.

  1. Vapors do not necessarily follow groundwater flow.

Vapors do not necessarily follow the direction of groundwater flow; rather, they follow the path of least resistance, which in some cases, may be the opposite of groundwater flow direction. This is why it’s so important to pay attention to soil types. Sandy or gravelly soil types, for instance, are highly permeable and will allow for easier movement of vapors than denser, more impermeable soil types such as clay and silty-clay soils that retard vapor migration. Appropriate search distances for different soil conditions in the target property area should be established by your firm’s geologists.

  1. Local expertise is critical.

Local knowledge of groundwater flow direction and soil types, in particular, can be invaluable in evaluating vapor migration risk. If you’re making assumptions in your report, document them. Whenever you provide a professional opinion, always qualify it (i.e., identify the basis upon which you are forming your opinion). Whenever you state a fact, always reference it. Make sure your report clearly identifies what assumptions you made–and what the basis was for them. Did you assume that groundwater flow follows surface topography? Or that the surface soil type found in soil survey maps (SSURGO/STATSGO2) is representative of soil in the entire vadose zone? All of these assumptions should be clearly documented in your report in order to avoid potential liability exposure.

  1. Stay up to speed on states’ vapor intrusion guidance.

Check to see if the state you’re working in has issues vapor intrusion guidance. Many states have developed their own vapor intrusion guidance that might assist you in identifying the area of concern around your target property. If your research reveals that a release has or might have occurred, state vapor intrusion guidance may be relevant in your assessment.

  1. Residential uses deserve the most conservative vapor risk analysis.

If there’s a situation deserving of a more conserve analysis for vapor migration risk, it is when the target property is residential. Most of the major vapor intrusion litigation involves residential or multifamily properties.

  1. The AOC distances in E 2600-15 are Conservative.

The Area of Concern (AOC) distances in ASTM E 2600-15 are very conservatively based upon 90th percentile plume lengths. These are default distances that are typically adjusted based upon the environmental professional’s judgment and experience with respect to local area conditions. The results of our pre-DDD survey show that more than half of EPs (56 percent) work at firms that have established AOC distances from a target property boundary where vapor migration may be a concern. Consider developing and standardizing your own  AOC distances for the various conditions (e.g., when groundwater flow is known or can be inferred, for different soil types in the vadose zone, etc.).

  1. Don’t ignore underground utility corridors.

The existence of major underground structures (e.g., utility corridors or piping networks) can intercept vapor releases and redirect them either towards the target property or away from it. If you do not have any information on the presence of major underground utilities in the area, be sure to include an assumption in your vapor migration analysis that you are assuming they do not exist.