Vapor Intrusion Emerges as a Major Cleanup Driver

When I started work as an environmental professional in 1986, I do not recall thinking about or even being aware of the risks posed by vapor intrusion. Vapor intrusion (sometimes referred to as “VI”) occurs when a volatile solvent or the volatile portion of a petroleum hydrocarbon from a release under or adjacent to a building migrates into the air inside a building.  Once in the indoor air, these volatile constituents can be readily inhaled by the building occupants, which is important because the lungs are the most efficient mechanisms by which chemicals enter the body.

How the VI Pathway Works

Volatile compounds are generally considered to include those that have boiling points less than that of water or vapor pressures greater than that of water, such as benzene from gasoline, industrial degreasers such as trichloroethylene (TCE), or perchloroethylene (PCE) dry-cleaning solvent.  These volatile compounds are the ones most likely to cause VI problems.   Following a release, these compounds are present in the ground partially in a liquid or solid phase, but they are also present partially in the vapor or gaseous phase.  As a gas or vapor, they can move readily through the soil to nearby buildings.

This mechanism is often intensified during winter heating conditions, when buildings are closed up, and heating systems vent to the outside, creating negative relative pressures inside buildings, sometimes called the ‘chimney effect.’  Even a relatively small relative negative pressure in a building significantly enhances the rate at which vapor intrusion occurs.

Comparing Exposures from VI to those from Soil and Groundwater

In the early days of waste site cleanup, we were focused on exposures and risks arising from soil and groundwater contamination.  While we are still concerned with the potential exposures to these media, limiting them is usually relatively simple (i.e. don’t eat or play in the soil and don’t drink the groundwater). In contrast, limiting exposures to indoor air containing volatile compounds is more problematic because everybody has to breathe continuously.  With experience has come the awareness that for volatile chemicals, VI is probably the most important exposure pathway to control for reducing risk when it is present.

Developing the Right Measurement Tools

While the Massachusetts Department of Environmental Protection became aware of VI issues in the late 1980’s, it wasn’t until they issued a  guidance document in 2002 that we had some state level agency guidance on how to proceed.  The guidance called for an initial field screening to evaluate whether soil gas was beneath a building which might result in vapor intrusion. With hindsight, has come the realization that the field screening methods of the day were not nearly sensitive enough to rule out vapor intrusion.

Vapor intrusion studies now usually rely on collection of soil gas samples in the field that are subsequently analyzed in a laboratory setting with highly sensitive instrumentation.  Alternately, portable gas chromatograph/mass spectrometers (GC/MS) can also be used.  In 2016, the Massachusetts Department of Environmental Protection issued final guidance, which describes the  state of the practice for “investigating, assessing, understanding, and mitigating vapor intrusion” in Massachusetts.

Remediating VI Conditions

The preferred remedial method for VI sites has become the installation of sub slab depressurization system or SSDS.  Borrowing from the radon remediation industry, regulators and environmental consultants realized that the mechanism by which radon gas enters buildings is almost identical to VI, and thus has a nearly identical solution. By capturing the vapor within the soil beneath a building, and venting it to the outside air before it can migrate into a building, we can almost eliminate the inhalation exposure that would otherwise occur in the building. This diagram presents a generalized depiction of an SSDS.


BHN blog figure

In designing an SSDS, environmental practitioners need to consider a number of site-specific factors such as how large an area the SSDS needs to cover, the permeability of the soil below a building, and the characteristics of the vacuum fan required.  A building underlain by a coarse sand or gravel might need only one vapor extraction point to cover the entire footprint, while a building underlain by finer grained soils might need more extraction points or horizontally laid slotted pipe to get the desired coverage.

At OTO, we typically perform an initial evaluation to help design an SSDS with an appropriately sized fan and geometry (i.e., a single extraction point or horizontal pipe). A small system for a residential fuel oil release under a portion of the basement might need a single extraction point with a small fan.  A system like this uses about the same amount of electricity as a standard light bulb.  In contrast, a large industrial property may need a larger system with multiple zones and stronger fans, and will result in a significantly higher electrical bill.

My first experience installing and operating an SSDS came in 2005, as a response to a relatively small residential fuel oil release. Since then, I have had a number of projects including SSDS’s ranging from relatively small and simple to quite complex.  To me, no other innovation in our industry has had a greater impact on our ability to reach acceptable endpoints for vapor intrusion than sub-slab depressurization systems.

As an environmental professional, many of the problems I have worked on have been difficult to solve. However, because of their relatively low cost, ease of installation and ability to improve conditions rapidly, solving vapor intrusion through installation of an SSDS has been a genuine bright spot in my career.




Coal Tar – Yesterday’s Nuisance, Today’s Problem
OTO’s work includes a lot of remediation Massachusetts and Connecticut. One of the things we run into on occasion is coal tar, a viscous, black, smelly product of our industrial heritage. Coal tar is not one of the more common challenges encountered at MCP sites in Massachusetts, with gasoline, fuel oil, chlorinated VOCs and metals all coming up more often. It is, however, a complex and challenging mixture of contaminants. Thanks in part to the unfortunate experience with the Worcester Regional Transit Authority’s redevelopment project on Quinsigamond Avenue in Worcester, or the recent proposal to cap tar-contaminated sediments in the Connecticut River in Springfield, coal tar is back in the news after a period of relative obscurity.


Don’t worry– this isn’t in Massachusetts.

Coal tar is a challenge for three main reasons.

First, it can be a widespread problem. Most coal tar encountered in the environment is a legacy of the coal gasification plants that supplied Massachusetts’s cities and towns with heating and lighting gas before natural gas became available via pipeline in the 1950s. Virtually any city or substantial town before 1900 had a gasworks, and some towns had several. Gasworks were often historically built on the ‘wrong side of the tracks’ due to their historically noisome character—their smell and constant racket beggared description. Where such “city gas” plants were not available, mill or factory complexes like the Otis Mills Company in Ware often had their own private gas plants, some of which also sold surplus gas to local residents for gas lights and stoves, and in some cases essentially became the town’s gas company.

As byproducts of making gas out of coal, these plants produced coal tar, cyanide-laden spent purification media, and much else of a dangerous nature. Some coal tar could be refined into waterproofing pitch, paving materials, industrial solvents, and even the red, foul-tasting carbolic soap that nobody who has ever seen “A Christmas Story” will ever forget. Massachusetts was also home to several plants that reprocessed tar into these plants, some of which later became all too well known, like the Baird & McGuire site in Holbrook, MA or the Barrett plant in Everett and Chelsea.

In addition to historically releasing wastes to the environment at the gasworks, gas companies also historically created off-plant dumps for their wastes, creating hazardous waste sites that might be located miles from the gasworks, or even in a different town.
EPA historically kept a sharp eye out for coal gasification plants, and during the 1980s listed over a dozen coal tar sites in Massachusetts on CERCLIS. Many of the sites that most alarmed MassDEP in the ‘80s were also related to MGPs—for example, Costa’s Dump in Lowell or the former Commercial Point gasworks in Boston. In recent years, however, regulatory attention has taken on an increasingly narrow focus towards other concerns, most notably vapor intrusion from chlorinated VOCs.

The second important consideration about coal tar is that it is pretty dangerous stuff, and poses both cancer and noncancer risks. Coal tar is typically a heterogeneous mixture of something like 10,000 distinct identifiable compounds, ranging from low molecular weight, highly volatile compounds like benzene and styrene to massive “2-methyl chickenwire” asphaltene compounds. From an environmental and toxicological perspective, coal tar is most conspicuous for its high concentrations of polycyclic aromatic hydrocarbons (PAHs), as much as 10% PAHs by weight, which make it significantly more toxic than petroleum. Two of the coal tar’s signature PAHs are benzo [a] pyrene and naphthalene; some coal tar can be up to 3% naphthalene alone, which accounts for the distinct, penetrating ‘mothball’ odor at MGP remediation sites.

Coal tar was associated with occupational diseases ranging from skin lesions to scrotal cancer even during the mid-19th Century, and was the first substance to be conclusively shown to be a carcinogen (by the Japanese scientist Katsusaburo Yamagiwa in 1915). The British scientist E.L. Kennaway subsequently proved that benzo [a] pyrene was itself a carcinogen in 1933, the first individual compound to be so categorized. Coal tar also contains concentrations of lesser-known PAHs, some of which may have significantly greater carcinogenic potential than benzo [a] pyrene. Coal tar is also a powerful irritant; remediation workers and others exposed to it can expect hazards including painful irritation of the skin, and respiratory or vapor intrusion hazards including high levels of benzene and coal tar pitch volatiles.

The third consideration is that coal tar is very persistent in the environment; tars and other gasworks wastes are highly resistant to geochemical weathering (and also to many remediation technologies, such as in-situ chemical oxidation), and do not break down in the environment like gasoline and most fuel oils do, so that tar contamination can still create problems over a century after the material was released.

So, coal tar is still with us, and will be for a long time. On the bright side though, with effort and careful planning, these challenges can be overcome. Many of the “wrong side of the tracks” locations of former gasworks are now prime downtown real estate, and a number of Massachusetts gasworks have been redeveloped as shopping plazas, transportation hubs, and biotech research facilities. As land prices, urban real estate availability, and government incentives continue to drive brownfield redevelopment, hopefully most of the Commonwealth’s former gasworks will see a new life.

This past October I attended the Soils Conference at the University of Massachusetts and was fortunate enough to sit in on a number of excellent presentations. Despite this positive experience, I was planning to leave early on the last day of the conference, but as I thought about my cluttered desk and pile of unanswered messages back at the office, I decided to stay a little longer and hear three presentations on vapor intrusion assessment.  This proved to be a very good decision as each of them was outstanding.

New VI Assessment Method

The first of these presentations was given by Ms. Lila Beckley of GSI Environmental in Austin Texas; her title was “Vapor Intrusion (or Not): Sniffing out the Source of VOCs in Indoor Air”.   She described a new method for assessing vapor intrusion (new to me anyway) and gave an eye opening description of their techniques.  Her firm has developed a vapor intrusion assessment method that uses real time sampling and analysis of indoor air while the pressure within the building is adjusted from positive to negative.  Based on the pattern of change observed in VOC air concentrations as the pressure is reduced, it was possible to determine whether the VOCs were truly from vapor intrusion or whether they actually originated from a source within the building. 

This technique represents a significant improvement over the methods previously used to assess vapor intrusion. 

A Specialized Application of the Same Technique

Following a short break, there was a presentation on vapor intrusion assessment by Mr. David Shea of Sanborn, Head & Associates that used the same general technique described by Beckley, but with the additional objective of identifying specifically how and where vapor intrusion was taking place in a large building located over a known groundwater contaminant plume.  The example used in Shea’s presentation nicely complemented and expanded on the methods illustrated in the previous talk. 

In summary, both presenters described a highly credible method that appears to quickly and accurately assess vapor intrusion.  The method requires only a 1-2 day assessment, but does need to be conducted with sensitive specialized field instruments.  The method seems capable of differentiating between indoor sources of VOCs in air and those originating from a sub-slab source.

How Good is the Correlation between Sub-Slab VOC Concentrations and Vapor Intrusion?

The last of these three talks was given by Mr. Ben Matrich of Geosyntec Consultants’ Anchorage Alaska office.  His title was “The Case for Less Emphasis on Sub-Slab Data in Decision Making for the Vapor Intrusion Pathway”.  The thrust of his presentation was that investigators have come to place too much emphasis on sub-slab soil gas measurements without adequately understanding their limitations.  The scientific basis for his talk comes from the comprehensive vapor intrusion research the USEPA has sponsored at Sun Devil Manor, a house purchased for this purpose in Layton, Utah.  Investigators are using the Sun Devil Manor (which is located over a contaminated aquifer) to study the detailed mechanisms of how vapor intrusion takes place; the results have upset much of the old thinking about vapor intrusion.

Details of the findings go beyond what I have space for here, but here are some examples.  EPA found that at Sun Devil Manor sub-slab soil gas concentrations can vary more than expected over time, as can the indoor air concentrations caused by vapor intrusion.  Temperature, barometric pressure and wind speed can all strongly affect vapor intrusion rates as well as the resulting indoor air VOC concentrations. Of course these findings all come from a single location and may not be generally applicable.


The simple vapor intrusion models used to develop the early regulatory efforts to limit the vapor intrusion pathway are being revised based on new studies and assessment techniques.  It is reasonable to expect that the approach to vapor intrusion will continue to evolve as additional research is published.

Recently we were discussing a pending real estate purchase with a client who was surprised when we commented that the best way to reduce environmental risk was to purchase a property that was already known to have soil and/or groundwater contamination that had been reported to the state.  Now let me add a couple of provisos: 1) we were talking about Massachusetts, where property purchasers are not automatically punished by the state for purchasing and trying to cleanup contaminated property; and 2) we were assuming that there was already enough testing that no big surprises were still lurking in the ground.

The advantage of purchasing contaminated property is that there are reduced expectations by the seller, buyer, lender and the state.  With a supposed “clean” property, surprises can only go in one direction, down.  Surprises with “dirty” property are often positive; it often turns out that dirty properties can be safely reused for any number of purposes.

Now there are some watch-outs, among which vapor intrusion (VI) may be the the most important.  Many properties achieved satisfactory closures and are now being reexamined by the state because the ground rules related to VI have changed.  As a result, many otherwise attractive properties that may have VI issues are now languishing.

Another possible watch-out lurking on the horizon is PCBs (polychlorinated biphenyls) in building materials.  So far this is an issue that has mostly caused problems when found in schools.  The potential clearly exists for this issue to spread beyond schools into commercial and residential structures.

Once beyond VI and PCBs, contaminated property looks like a a winner as long as the property works for the intended uses; it comes down to a case of the devil you know being better than the devil you don’t know.