Soil Gas & Vapour Intrusion
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 utility 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 within 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
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.