What is Vapour Intrusion?
Vapour intrusion (VI) into buildings can affect short and long-term indoor air quality and the health of occupants. Volatile organic compounds (VOCs) within the subsurface can intrude into a building via preferential pathways in the foundation such as cracks and control joints, floor drains and other utilities penetrations.
Recognition of the health and safety risks of VI into buildings spaces began in the 1980s with the identification of carcinogenic radon gas present in the indoor air of structures erected over uranium-bearing geologic deposits, and during studies of methane migration from solid-waste landfills pooling in basements to explosive concentrations. Since then, there has also been an increasing awareness that refined or man-made chemicals such as light fraction petroleum hydrocarbons (PHC F1-F2) and chlorinated solvents present in contaminated soil and groundwater could volatilize, and negatively impact indoor air quality via the VI pathway. PHCs F1-F2 and benzene, toluene, ethylbenzene and xylene (BTEX) are commonly associated with fuel spills. Chlorinated solvents are commonly associated with in dry cleaning chemicals.
As a result, VI assessments are on the increase due to strengthening regulatory requirements or as a preventative strategy. Pre-emptive assessment is required where environmental conditions are present that could lead to VI concerns.
A VI assessment approach involves compiling multiple lines-of-evidence to supplement uncertainties in Site conditions including:
- Building/foundation construction (i.e., basement depth and thickness, condition, crawl spaces, and sumps),
- Building use (whether or not potential contaminant sources exist within the building itself),
- Soil type and environmental condition,
- Groundwater depth and environmental condition,
- Climate effects (seasonal variation), ventilation system and air exchange rate, and
- Regional baseline outdoor air quality.
In this regard, VI assessments tend to be highly site-specific.
Should I be Concerned about VI?
Although concentrations of VOCs, are often low and unnoticeable, certain compounds may be carcinogenic and can result in chronic health effects even at low levels. In other more severe cases, build-up of contaminants could lead to potentially explosive conditions. Assuming a Phase I or II Environmental Site Assessment (ESA) has identified a need for follow-up assessment, full characterization and delineation of potential sources of impacts in soil, groundwater, and soil vapour should be completed. All of the information collected is then used to develop an informed Conceptual Site Model (CSM) that is used as a basis to evaluate health risks and design of remedial solutions. The CSM demonstrates the various vectors for contaminant of concern migration that may ultimately affect human health
Regulatory Framework
In the US, the Environmental Protection Agency (EPA) has developed various VI guidance documents and many individual states have incorporated VI into their generic cleanup standards. The American Society for Testing and Materials (ASTM) has developed a standard for vapour encroachment screening.
Several jurisdictions in Canada have VI regulations and/or guidance. On a federal level, Health Canada has provided guidance on vapour intrusion in their contaminated site guidance manual. Provincially, British Columbia and Ontario have also developed soil vapour assessment protocols and standards. Quebec allows a proponent to evaluate soil vapour using the Ontario or Canadian Council of Ministers of the Environment (CCME) approach. Of course there are also occupational exposure standards to consider, but these are almost always less stringent that risk-based standards, occasionally upwards by a few orders of magnitude.
How are VI Samples Collected and Analyzed?
One key challenge of assessment is many VOCs are ubiquitous and evident in indoor air due to above slab sources within the building. For example, concentrations of toluene and several chlorinated solvents have been found at measurable levels in a large proportion of residences in one recent study in North America. Prior to air sample collection, an audit of potential indoor sources should be conducted to help screen for potential contaminants that may be present from chemicals stored within the building.
Air and vapour sampling methodologies typically include the use of specially-prepared canisters, desorption tubes or passive samplers, depending on the contaminants and their concentration. Soil vapour and sub-slab samples (for example, below a basement floor slab) are collected from discretely-screened soil vapour probes. Indoor and background air samples are also often collected to support the evaluation and the CSM and any modelling that may have been completed. Tracer gases can also be used to obtain a better understanding of the VI pathway.
Collected samples are then analyzed using laboratory gas chromatography/mass spectrometry (GC/MS) and compared against regulatory standards. In some cases, portable GC/MS laboratory equipment can be used to collect continuous results and gain a clearer picture over time. A well designed field and lab Quality Assurance/Quality Control (QA/QC) program is crucial to ensure data is meaningful, representative, and reproducible.
Compound-Specific Isotope Analysis (CSIA) is one technique that may be used to distinguish between VI and indoor sources. The gaseous compound must be reliably detectable in both sampled substrates (i.e. soil vapour, sub-slab soil vapour, and/or indoor air) for a direct comparison of the carbon isotopes within the contaminants of concern. Review of the different carbon isotopes can identify different sources. Typically, sorption tubes are used to collect samples for CSIA because a larger sample volume is required to obtain sufficient a quantity of the given contaminant for isotope comparison.
Mitigation
Once a problem has been identified, what can be done about it? There are various strategies and solutions to reduce risk and mitigate VI issues depending on whether the building is proposed or existing. In some cases, the VOC source can be remediated, but this often takes time and can be disruptive and expensive.
In other cases, an engineered solution is developed to permit proposed or continued use that protects human health. If a new building is being constructed over a known area of soil or groundwater contaminants, the building can be designed to block and/or divert vapours to the atmosphere. In these cases, all building perforations are sealed and commercially-available barriers are applied beneath the concrete floor slab (typically geosynthetic or spray-on bitumen membranes). Often this approach is used in conjunction with buried piping in granular material to permit sub-slab depressurization (SSD). SSD relies on changing the pressure difference between the sub-slab and the inside of the building to prevent migration. Passive SSD systems typically rely upon small rooftop wind-driven turbines and active systems rely on powered mechanical ventilation systems. Alternatively, an aerated floor can be installed beneath the floor slab during construction.
If the building is an existing structure, often the most cost-effective technique is the sealing of all cracks and joints in the floor slab and foundation walls, followed by the application of commercially-available epoxy products to limit continued migration and to protect the surface. An SSD approach using sump pits and header piping can also be used if sealing alone is not entirely effective. Alternatively, building pressurization for some buildings utilizing HVAC systems, with or without air filtration for treatment, may also be effective but can result in increased energy costs.
Despite typically low concentrations present in subsurface vapours, regulatory approvals are commonly required to operate a VI remedial system. Regular VI assessments may be required to satisfy regulatory requirements and demonstrate that a VI system performs adequately and is operating as designed.
John Cracknell is a Senior Environmental Engineer at Premier Environmental with over 25 years of consulting experience, primarily focusing on environmental due diligence and on the assessment, remediation and redevelopment of contaminated sites. For more information, please contact John at (514) 608-7202 or jcracknell@premiercorp.ca
