The Vapour Intrusion Issue: Remedying a Mist Opportunity!

The Vapour Intrusion Issue: Remedying a Mist Opportunity!

Over the years Vertex has published newsletters on innovative remedial methods for various soil and groundwater impacts.  But we recently realized that we’ve never before focused our information sharing on the vapour phase of subsurface contamination.  This newsletter article is an attempt to remedy this mist opportunity!

“Vapour intrusion occurs when there is a migration of vapour forming chemicals from any subsurface source into an overlying building” (US EPA definition).  Vapour intrusion and accumulation in buildings can lead to indoor air quality issues that can result in unacceptable health risks as well as physical safety hazards (i.e., explosions).  In the absence of physical contact with contaminated soils or ingestion of contaminated groundwater, indoor inhalation exposure often becomes the driving factor for sites undergoing risk assessments.  Vapour intrusion can occur on any site where volatile contaminants are present in the subsurface, and it arguably represents the greatest exposure risk to people who live or work on many impacted sites!

Figure 1: Migration of Soil Vapors to Indoor Air (courtesy of the US EPA)

Our awareness of the soil vapour intrusion issue goes back to the 1980s when the initial focus was on radon gas.  Over the years, the vapour intrusion field expanded to include risks presented by other common volatile contaminants such as petroleum hydrocarbons and chlorinated solvents.  Now in the present day, vapour intrusion mitigation systems are a common part of redevelopments plans, especially on Brownfield sites, as environmental professionals increasingly decide to manage contamination in place.  The most common combinations we see include a Risk Assessment, combined with a building Vapour Intrusion Mitigation System, sometimes combined with “hot spot” remediation or property boundary control using a permeable reactive barrier (PRB) (see How Do You Spell Remediation Success? C-O-M-B-I-N-E and PRB Design and Installation).  “Hot spot” remediation can include excavation and off-site disposal, but many in-situ remediation technologies allow for a more precise degree of treatment.

Getting back to the issue at hand, in this article, we want to shine the spotlight on the practical side of a couple different vapour intrusion mitigation methods by looking at a new building development as well as an existing building retrofit example.

Passive VMS at New Build Site

In late 2019 and early 2020, a general contractor constructing a 12,000 sqft new, slab on grade, commercial building in Ontario retained Vertex to install a passive vapour intrusion mitigation system at the site. Previous environmental investigations had identified the presence of F1 range petroleum hydrocarbons (PHCs) as well as tetrachloroethylene (PCE), trichloroethylene (TCE) and related degradation products at elevated concentrations in groundwater at the site.  Based on the facts that the groundwater impacts were located in the bedrock, and the bedrock was shallow (only approximately 1.0 m bgs), a Risk Assessment was completed for the site – ultimately concluding that a vapour intrusion mitigation system was required to prevent unacceptable human health risks via indoor air inhalation of the contaminants.

The consultant had designed a passive vapour intrusion mitigation system that comprised:

  • A permeable sub-slab material (i.e., coarse gravel / clear stone) beneath the floor slab
  • Perforated piping to collect the vapours that was in turn connected to riser piping and turbine vents
  • A composite EPRO membrane consisting of three layers:
    • Base layer comprised of HDPE film and geotextile fabric (e.base 205)
    • Spray-applied layer of polymer modified asphalt membrane (e.spray 205)
    • Protective layer that mechanically bonds with overlying concrete slab (e.shield 205)
  • The option to make the system active by the addition of blowers, if necessary based on planned indoor air quality monitoring

The design also included an extensive list of design, material and installation specifications including requirements for trained and licensed personnel to construct and install the sub-slab spray-on composite membrane according to the manufacturer’s instructions and Quality Assurance (QA) manual and requirements for completion of a detailed Quality Control (QC) testing program, including field sampling, smoke and other testing to confirm the membrane meets the manufacturer’s product and installation requirements.

Note that Vertex is a trained and authorized installer of EPRO vapour membrane systems as well as QA/QC smoke testing.

The design of the vapour intrusion mitigation system was submitted and approved by the Ontario Ministry of the Environment, Conservation and Parks (MECP) as being in compliance with the Certificate of Property Use (CPU).

The following series of photographs document the successful installation of the various stages of the vapour intrusion mitigation system at this site by Vertex.

Photo 1: Installation of Sub-Slab Vapour Collection Perforated Piping

Photo 2: Spray-application of the black e.spray 60 MIL barrier. Note the underlying grey e.base 205 membrane is visible in the foreground

Photo 3: Typical Seal Around a Penetration in the Vapour Barrier Membrane

Photo 4: Smoke Testing to Check for Leaks in the spray applied barrier (QC Testing)

Photo 5: Completed spray-applied barrier

Photo 6: Final layer of white e.shield 205, to protect the spray applied layer

Photo 7: Subsequent Repairs to Vapour Barrier after Installing Utility Trenches

For this installation Vertex also provided a final letter to confirm that the vapour intrusion mitigation system was installed as per manufacturer’s recommendations and as per the environmental consultant’s design.

Active VMS Retrofit of Existing Building

Usually more challenging projects than new-build sites are when we are retained to retrofit a vapour intrusion mitigation system in an existing building.  Not only is there usually no free and clear access, more often than not, the below-grade spaces are cramped, full of stored materials, shelving and other fixtures, contain multiple storage and divided rooms, and the vent piping must then traverse up through finished tenant spaces.  All of these factors require patience, attention to detail and creative problem solving.

Recently, Vertex was retained to install such a system in the basement of an older, multi-tenant commercial property.  This property had all of the above-noted challenges going for it.  Vertex’s scope of work was to:

  • Saw cut and break out trenches in the existing concrete basement floor slab to expose the underlying granular base material
  • Install vapour extraction system piping in the trenches, fill with clear stone and reinstate the concrete floor
  • Prepare and seal the existing concrete floor with two coats totaling 15 mil in thickness of a special epoxy sealant
  • Line the exterior walls with a chemical resistant vapour barrier
  • Seal all perforations through the floor and exterior walls with gaskets or sealant
  • Connect the vapour extraction piping to a discharge stack on the roof and equip with a blower

The following series of photographs document the successful installation of the various stages of this active, retrofitted vapour extraction system at the site by Vertex.

Photo 8: Basement Area with Limited Access Requiring Installation of an Active Vapour Intrusion Mitigation System

Photo 9: Saw Cut Trenches Through Basement Floor Slab to Install VMS Collection Piping

Photo 10: VMS Collection Piping Installed in Trenches and Backfilled with Clear Stone

Photo 11: Installing Vapour Barrier Material on Basement Exterior Wall

Photo 12: Sealing Around Perforations Near Utility Panel

Photo 13: Floor Cleanout fitted with a Gasket; Note Final Two Coat Epoxy Sealant on Concrete Floor

Photo 14: Rooftop Vent for Active Sub-Slab Vapour Extraction System

As noted, this system required a blower to produce sufficient negative pressure between the building’s interior and the sub-slab venting layer to make it effective.  In these instances, an Environmental Compliance Approval (ECA) is required for the system from the MECP to allow the actively extracted vapours to be discharged to the atmosphere.  Vertex is also able to provide assistance in obtaining site-specific ECAs for vapour intrusion mitigation systems, as needed.

So the next time your site requires a passive or active vapour intrusion mitigation system, Vertex has the training, certification and experience to assist.  Feel free to contact our office for more information on this service.