March2011_ES&E D3_2010 01/04/11 10:05 PM Page 62
Public Health
Vapour intrusion from soil or groundwater: A challenge for property owners By Khaled Chekiri and Matthew Schroeder
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ssessing subsurface vapour intrusion (VI) to indoor air is one of the most complicated and challenging environmental assessment issues. Because of the potential environmental, health, safety and financial impact of VI exposure, it is also one of the most important considerations for owners, operators and managers of commercial properties. The science of vapour intrusion is very complex and includes a number of variables that can have a significant impact on the fate of vapours in subsurface conditions. Further, the regulating community may misunderstand or improperly characterize indoor air issues resulting from VI. Adding to the complexity is a wide disparity between the approaches of various regulatory bodies to tackling VI. There is also a significant challenge to find a good technical approach, which is also economically feasible, to quantify vapour intrusion. Several complex parameters affect the migration of vapours from the subsurface to indoor air. Considerations include screening methods, the use of models, collection of site-specific geological and hydrogeological data, information on building construction and building ventilation, sampling procedures, analytical methods, and data evaluation. The following is a review of the different choices in VI pathway assessment that are considered by the regulators, with the pros and cons for each alternative. A vapour intrusion primer Vapour intrusion is the process by which volatile chemicals in soil or groundwater migrate from the subsurface zone into the air space of a building. Notable cases of VI resulting from “toxic waste sites” include Love Canal in New York in the 1970s and, in 2008, the Bishop Street site in Cambridge, Ontario. Each of these cases points to the significance of VI. The concern surrounding VI is that vapours can have a significant impact on the health and safety of building occupants. Certain vapours associated with toxic chemicals can enter the bloodstream 62 | March 2011
Soil vapour and ambient air sampling setup.
more readily through inhalation than other exposure pathways, such as ingestion or dermal contact. Many toxicologists believe that chronic exposure to vapours, even at low concentrations, increases the risk of developing chronic and acute health problems. Safety concerns stem from the explosive nature of some vapours, such as gasoline components or methane vapours. VI challenges are both regulatory and technical in nature. The regulatory challenge lies in the lack of guidance from Canadian regulatory bodies. The following draft and guidance documents form the basis for investigations of vapour intrusion in Canada: • “Federal contaminated site risk assessment in Canada: Part VII: Guidance for soil vapour intrusion assessment at contaminated sites,” issued by Health Canada in October 2008 (currently in draft form). • A technical guidance document, “Vapour Investigation and Remediation,” issued by British Columbia’s Ministry of the Environment in July 2009. • “Rationale for the Development of Soil and Ground Water Standards for Use
at Contaminated Sites in Ontario,” Dec. 22, 2009. But for these documents, the regulated community is in the relative dark concerning requirements and procedures to investigate (including sampling protocols), assess and mitigate VI in Canada. It should be noted that in the United States, several guidance documents have been developed to address VI, at both the federal and state levels. Nevertheless, there is a need to assess the potential impact of vapours on properties. To conduct these assessments, there are basically three investigating approaches: mathematical modeling, media sampling, and “multiple lines of evidence.” Mathematical modeling Modeling consists of using subsurface geological and analytical data to mathematically predict the chemical concentration in indoor air. Modeling is generally performed as a screening tool to determine whether or not chemical concentration in soil and/or groundwater is likely to result in unacceptable concentrations in indoor air. The entry of soil vapours into buildcontinued overleaf...
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