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.

 

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.