Soil Vapour Intrusion: A Complex and Challenging Process

Posted by on Jun 20, 2018 in Blog | 0 comments

Last year, my colleagues “set out” to compile vapour intrusion (VI) standards in each Canadian Province, as well as in each US State.  This effort, we soon found out, was not as easy as we had envisioned.  More on these findings later.

Understanding Vapour Intrusion

Let’s begin by establishing the VI framework.  Vapour intrusion is a process by which chemicals from soil and groundwater volatilize and migrate into buildings.  These underlying vapours can find their way into buildings through cracks, sump pumps, and utility corridors.

VI was initially recognized as a potential human health and environmental concern in the 1980s with radon intrusion.  It was then found that there were many vapour-forming chemicals, primarily volatile organic compounds (VOCs), from gas bars, dry cleaners, and manufacturing.

Gas Bars, Dry Cleaners, and Manufacturing

With documented historical releases of volatile chemicals to the subsurface, regulators in Canada and the United States focused their attention on gas bars, dry cleaners, and manufacturing companies that used chemicals that contained VOCs (e.g., trichloroethylene [TCE]) in their operations or production processes.

Knowing where there may be a VI concern is part of the challenge (for the regulators and for those buying commercial properties).  Another concern is understanding how these vapours “behaved” or moved in the subsurface to potential receptors.

In an effort to understand the science of VI, mathematical models have been developed using hydrogeological/geological and chemical-input parameters.  These models were developed for predicting the concentrations of volatile chemicals that may be present in the sub-slab vapour beneath a building when groundwater and/or soil concentrations are known.

It should be noted that regulators, such as the United States Environmental Protection Agency, suggest that mathematical models should be used cautiously, as they have a limited role in evaluating the VI pathway.  Specific site data should be gathered to support your VI assessment.

VI is an Important Factor in Business Decisions

As with other potential environmental issues, it’s important to put VI into proper perspective so business risks can be better understood.  But, suffice it to say, the potential for VI poses a human health, environmental, and business risk. The cost for mitigation or environmental remediation associated with VI can be expensive and can carry significant liability from third-party lawsuits.

The potential for soil vapour intrusion is one of many business-environmental risks to consider (Photo by rawpixel on Unsplash).

The potential for soil vapour intrusion is one of many business-environmental risks that companies and senior management have to take into consideration (Photo by rawpixel on Unsplash).

The challenge for the regulated community is evident in the range of standards developed across North America, as there is no consensus on regulatory standards.

VISLs in Canada and the United States

Back to the survey of VI standards for each province and state.  We evaluated the vapour intrusion screening levels (VISLs) developed across North America.  We used the term “VISL” as a default term to describe the regulatory standards across jurisdictions.  The survey assessed VISLs from every province and territory across Canada and every state within the United States.

The survey considered data from each province/state for sub-slab soil vapour and indoor air for both residential and commercial properties.  The survey focused on data for four common VI chemicals: benzene, tetrachloroethylene, trichloroethylene, and vinyl chloride.

As previously mentioned, the standards vary widely, especially when you take the United States into consideration.

Hypothetical Manufacturing Sites – Vapour Intrusion Standards

Only considering provincial standards (or in many cases, the lack of standards), they pose a regulatory challenge as you move across jurisdictions.

Assume you have a manufacturing facility in Ontario and a similar operation in Nova Scotia.  Let’s further assume that both facilities have a concern about the potential for VI from a historical spill of gasoline from an underground fuel storage system.

In evaluating benzene in indoor air in the commercial buildings, you find both facilities have detected benzene at 20 micrograms per cubic metre (µg/m3).  For your facility in Nova Scotia (VISL standard is 25 µg/m3), you are in compliance.  For your facility in Ontario (VISL standard is 1.63 µg/m3), however, you are significantly out of compliance.

To further complicate this issue, the standards are changing, in some cases quickly and without sufficient notification to the regulated community.  The VISLs in Ontario changed within weeks of the initial publication of our reference guide (we have since updated this reference).

Changing VISL

The same is true in the United States; the VISLs routinely change, typically to more stringent guidance.  These changing standards can affect your business decisions as more stringent VISLs affect everything, including your understanding of environmental/business risks, potential capital costs, and current or future remediation efforts.

Because of the “fluid nature” of the VI standards, we have signed up for a number of electronic newsletters so we can update the data as soon as we learn of the changes. We also added footnotes to each province/state when the data were updated.

The other thing (not a surprise) that makes it difficult for businesses evaluating risks is that many Canadian Provinces have no standards.  In some cases, they refer to a federal guidance document from 2010, “Federal Contaminated Site Risk Assessment in Canada, Part VII: Guidance for Soil Vapour Intrusion Assessment at Contaminated Sites.”

Variation in Provincial Standards

Understanding how there could be such a wide range of standards for the same chemical is complex.  However, the variability is likely reflective of how “young” the science of VI is compared to the science of groundwater contamination.  Like other environmental issues, it may be reflective of local politics and how governing bodies view risk.

Vapour intrusion is complicated and can be a challenge to address and put into proper scientific and business perspective.  This complexity is a key factor in why so many regulatory bodies have kept their guidance documents or protocol in a “draft” format.

Finally, if you are buying/selling commercial property and you are assessing VI, (1) consult with your legal and technical advisors and (2) document your results and the regulatory standards at the time of your assessment.

Thanks to my colleague, Katherine Rey, E.P., for her help in drafting this blog.

If you have question regarding vapour intrusion, Phase I/II Environmental Site Assessment, remediation, or other environmental issues, please contact Katherine or me at 519-979-7300.