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.




In Part I of this topic, we discussed the assessment and the identification of the cause of settlement of existing structures.  Once the causes have been identified, we can then provide alternatives to prevent on-going settlement, if needed.  This post will discuss a few of the engineering solutions that are available to mitigate a settlement problem of an existing structure.

It should be mentioned that we do not always propose mitigation or remediation.  For instance, if the settlement of the foundation appears to be due to the placement of compacted engineered fill in the 1960s (during construction) over soft compressible clay, OTO likely may recommend that the client delay large and expensive repairs and mitigation, and instead monitor the rate settlement over the next couple of years. In these instances, the rate of settlement often has decreased to negligible amounts and further significant settlement may be unlikely.  At that time, we often recommend that the owner proceed with larger structural and cosmetic repairs.

If settlement concerns appear to be due to improper drainage and the introduction of large amounts of water into the soil mass, OTO will provide recommendations for correcting the drainage problems.  We often can provide local contractor names, upon request, to help repair or install new drainage systems.   Often times, these repair or maintenance tasks can be performed by the owner or facilities manager.

If the cause of the building settlement is the presence of an unsuitable bearing layer, such as loose, non-engineered fill that may continue to compress, or a thick organic peat layer that may continue to degrade, we will recommend a mitigation alternative such as a deep foundation or a soil improvement technique.

A deep foundation system transfers loads through the unsuitable layer to a firm bearing layer, such as driving pilings through a clay layer to bear on a layer of dense sand or bedrock.  Deep foundation alternatives to mitigate the settlement of existing buildings may include helical piles or mini piles.  Helical piles consist of a central steel shaft with horizontal bearing plates (8 to 14 inches in diameter) welded to the shaft at spacings on the order of 12 inches, which are augered into the soil. Mini piles are drilled, cast in place, cement grouted shafts. The piles are constructed by drilling and advancing casing (three to ten inches in diameter) to a selected depth or bearing stratum, installing a steel reinforcing bar down the center of the casing, and injecting cement grout into the casing.  The grout is pumped into the borehole at high pressure, starting at the bottom of the casing and moving upward in order to displace drilling mud or any remaining soil cuttings from the borehole. As the grout is pumped into the borehole, the casing is pulled up to a selected depth at the top of the “bond zone,” allowing contact between the grout and the surrounding soil. Helical or mini-piles are typically connected to the existing footings using an underpinning bracket.

Soil improvement techniques, which improve the existing loose soil so that it can function as a suitable bearing layer, may include pressure or compaction grouting.  In compaction grouting, the soils within the improvement zone are densified and strengthened by a systematic, pressurized injection of controlled low mobility cement grout. The goal of the process is to achieve increased strength of the soil mass.

Compaction Grouting Ashley Blog II
Compaction grouting in progress at an industrial facility

Many factors must be considered in order to recommend the most appropriate engineered solution for settlement issues.  OTO will often discuss existing building and soil conditions and proposed mitigation techniques with specialty geotechnical contractors to evaluate possible alternatives costs.  OTO maintains relationships with most of the foundation specialty contractors in New England and often can provide two or three independent contractor contacts to the client so that competitive cost information can be obtained.  Once the mitigation alternative and contractor is chosen, OTO can assist during construction by documenting the installation and addressing any concerns that arise.

If you have other questions about building settlement, contact Ashley Sullivan at 413-276-4253 or to see how OTO can help!