Final Vapor Intrusion Guidance: An Overview


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Final Vapor Intrusion Guidance: An Overview

By: Martin (Mort) Schmidt, CPG & Craig A. Cox, CPG

“Vapor intrusion” is used to describe the processes in which chemical contaminants in the subsurface migrate toward buildings and enter the breathing zone.  US EPA (EPA) made a first pass at vapor intrusion guidance in 2002 with their Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air VI GuidancePathway from Groundwater and Soils.  The 2002 guidance was written in an awkward Question/Answer style, and it contained flow charts for conducting vapor-intrusion assessments that were worthy of Rube Goldberg.  Worse, the guidance remained in draft form for years, as EPA abdicated their responsibility to revise it, and instead referred to other documents, most often the Interstate Technology & Regulatory Council (ITRC), 2007, Vapor Intrusion Pathway: A Practical Guide.  This had the unfortunate effect of making some property owners feel that dealing with vapor-intrusion was optional.  Additionally, most states gave up on EPA’s ever finalizing the guidance, and wrote their own, leading to a profusion of conflicting requirements.

Finally, under pressure from EPA’s Office of Inspector General in 2009, EPA buckled down and started the process of revising the guidance.  Their update consisted of releasing a number of reports to the EPA Vapor Intrusion website, including:

  • Background Indoor Air Concentrations of Volatile Organic Compounds (2011)
  • Petroleum Hydrocarbons And Chlorinated Hydrocarbons Differ In Their Potential For Vapor Intrusion (2011)
  • Conceptual Model Scenarios for the Vapor Intrusion Pathway (2012)
  • EPA’s Vapor Intrusion Database: Evaluation and Characterization of Attenuation Factors for Chlorinated Volatile Organic Compounds (2012)

These reports, together with several other case studies written for EPA, laid out EPA’s thoughts on the fate and transport of chlorinated and non-chlorinated VOCs, and foreshadowed the 2013 Draft Final Guidance, and the June 2015 Final Guidance.  If you’ve followed these releases over the past several years, the Final Guidance won’t hold any surprises.  That said, a discussion of notable changes follows.

Petroleum Hydrocarbons (PHCs).  Members of the oil and gas community have long recognized that the standard vapor intrusion approach was far too conservative for non-chlorinated compounds.  The distance over which vapors migrate laterally was considered to be up to 100 feet from contaminated soil or groundwater.  This distance worked well enough for chlorinated compounds, but PHCs break down much more rapidly, especially in the presence of oxygen, soil moisture, and other conditions conducive to biological degradation.  The differences were great enough to warrant separate Petroleum Vapor Intrusion (PVI) guidance.  The PVI guidance is not a stand-alone document, but is a supplement to the Final Guidance.  The PVI guidance discusses at length the indicators of active breakdown.  If oxygen levels and other conditions are right, the default lateral distance for PHCs is only 30 feet.  Subsequently, petroleum underground storage tank (UST) sites may screen out much earlier than other types of sites, and may not require additional testing.

PHCs are also assigned vertical screening distances.  According to the PVI guidance, a vertical separation distance of six feet between a building’s lowest floor and dissolved PHC is sufficient, given the presence of sufficient oxygen and other conditions.  A vertical distance of 15 feet is adequate for free-phase hydrocarbons.  In contrast, there is no vertical distance for chlorinated VOCs considered great enough to assume no risk of vapor intrusion.  But to the dismay of many, EPA will not allow the PVI guidance to be used for all non-chlorinated compounds, just fuels in regulated USTs.  In fact, EPA excluded some PHC sites, including refineries, with the rationale that the potential contaminant mass is large enough to overwhelm microbial action, making the 30-foot distance inadequate.  The PVI guidance also doesn’t apply to some fuel additives, including tetraethyl lead and chlorinated lead scavengers, which were added to remove lead compounds from engine parts.

Attenuation Factors.   Attenuation factors are multipliers used to predict indoor vapor strength from subsurface concentrations.  EPA had long used default attenuation factors of 0.1 for sub-slab soil gas, 0.01 for deep soil gas, and 0.001 for groundwater.  This meant that vapor concentrations in indoor air were assumed to equal 10% of their concentrations in sub-slab soil gas (less than five feet below the floor), 1% of their concentrations in soil gas below 5 feet, and 0.1% of their concentrations in groundwater (after allowing for water-to-air partitioning as per Henry’s Law).  Information compiled in EPA’s database, and the results of computer modeling in their Conceptual Model report, indicated that their defaults were too conservative for sub-slab soil gas, but not conservative enough for deep soil gas.  The revised attenuation factor is 0.03 for soil gas, regardless of depth.  The groundwater attenuation factor remains at 0.001, but a more lenient attenuation factor of 0.005 can be used for some soil types.

Screening Levels.  Screening levels in the draft 2002 guidance were tied to residential exposures.  Subsurface screening levels were derived by dividing indoor screening levels by the default attenuation factors of 0.1, 0.01, or 0.001, for shallow soil gas, deep soil gas, or groundwater, respectively.  Unfortunately, those screening levels are long out of date.  They are also too conservative for commercial/industrial settings, since people typically spend fewer hours at work, and can safely breathe vapors at several times higher concentrations.  The default screening levels for groundwater in the earlier guidance were also too conservative for groundwater in most cases, because groundwater-to-vapor partitioning was assumed to take place at 25 degrees centigrade, which is far too high in northern states.  Screening levels are now drawn from the Vapor Intrusion Screening Level Calculator, which is updated regularly and which allows adjustments for commercial/industrial settings and groundwater temperature.  The VISL calculator also contains pages that simplify the process of adding cumulative risk from multiple compounds, and is a useful source of various physical constants and risk.  Notice that a number of poly-aromatic hydrocarbons (PAHs) have been added to the June 2015 list of screening levels.  PAHs are highly toxic, but not terribly volatile, so the overall effect on vapor-intrusion assessments may be small, but could be important at sites containing PAHs.

Multiple Lines of Evidence (MLE).  EPA’s final guidance is stronger than ever on MLE, and says that, “Multiple lines of evidence are particularly important for supporting “no-further-action” decisions regarding the vapor intrusion pathway (e.g., pathway incomplete determinations) to reduce the chance of reaching a false-negative conclusion.”  A site can no longer be screened out with a single piece of data.  The guidance does indicate, however, that the absence of subsurface contaminants in the vicinity of an existing or future building is sufficient to rule out vapor intrusion.

Indoor Air Last.  The reluctance to measure indoor air in the course of investigation is eroding.  EPA conceded that, “Experiences since 2002 illustrate the value of collecting indoor air samples earlier in the investigations.  The ‘indoor air last’ approach has been updated, which will allow more flexibility in the sequencing of subsurface and interior/indoor sample collection.”  Screening levels were, and to a point still are, too conservative to screen out sites on the basis of subsurface data, which inevitably leads to indoor air sampling.  This makes it easier to move on to indoor sampling and getting it over with.

Preferential Pathways.  Some clarification was also offered on the issue of preferential pathways.  Preferential pathways are paths through which vapors can migrate unusually rapidly, such as in the sand or gravel surrounding a sewer line.  Preferential pathways can allow vapors to travel past the 100 foot lateral limit, and make it difficult or impossible to screen out a site on the basis of distance from the source.  Unfortunately, some got carried away with preferential pathways and regarded any building with indoor plumbing as having a preferential pathway.  But now, according to EPA, “preferential migration routes are distinguished from adventitious and intentional openings in a building that may also facilitate vapor entry from the subsurface (see Section 2.3), but which are expected to typically be present in all buildings…”

Sampling Procedures.  A number of details regarding field sampling are formalized in the guidance.  One small victory for Cox-Colvin involves sample tubing.  EPA now says that that various types of sample tubing, including nylon, and even copper (which was once anathema), are acceptable for constructing soil-gas probes.  Cox-Colvin has long held that nylon tubing works as well as Teflon, at half the price, so we sell nylon tubing for use with the Vapor Pin.  Although we don’t use pure copper tubing for sampling, this should also put to rest any questions about the acceptability of brass Vapor Pins or other hardware.  But as always, local guidance might say otherwise.

The new guidance has plenty to like, and plenty to dislike, but in any case it’s a huge improvement over what went before.

Published in Cox-Colvin’s August 2015 Focus on the Environment newsletter.


Mort Schmidt is a Senior Scientist with Cox-Colvin & Associates, Inc. He received his BS and MS degrees in Geology and Mineralogy from The Ohio State University, and has been a Cox-Colvin & Associates employee since 1997. His areas of expertise include vapor intrusion and contaminant investigation and analysis, and he currently serves as Cox-Colvin's Practice Leader - Vapor Intrusion Services. Mort is a Certified Professional Geologist with AIPG and is a registered Geologist in Indiana. Craig Cox is a principal and co-founder of Cox-Colvin & Associates, Inc., and holds degrees in geology and mineralogy from the Ohio State University and hydrogeology from the Colorado School of Mines. Mr. Cox has over 30 years of experience managing large environmental project implemented under CERCLA and state voluntary action programs. In addition, Mr. Cox has developed a variety of software products including Data Inspector, an internet-enabled environmental database application. Mr. Cox is a Certified Professional Geologist (CPG) with AIPG and is a Certified Professional (CP) under Ohio EPA's Voluntary Action Program.