Vapor Intrusion
by Ben Thompson
There is a large human health risk based on the intrusion of volatile organic compounds into homes and businesses from subsurface sources of contaminants. Understanding of this vapor intrusion (VI) pathway and the associated risks is constantly evolving. Additionally, regulatory agencies and consultants are continuing to refine the policies on and processes for studying and defining the VI pathway.
The VI initiative has been a high priority for CH2M HILL and has grown rapidly over the past several years. CH2M HILL consultants, engineers, and scientists are well suited to deliver these projects through a considerable amount of project experience. As a part of this important work, ASL has been supporting a wide variety of VI projects across the country through our laboratory and consulting services.
ASL was at the forefront of the VI sampling and analytical methodology development. Our lab was one of the first to be able to achieve the low part-per-trillion levels required for indoor air risk assessments. Having the analytical and technical resources to draw from has enabled ASL personnel to develop many field procedures for sampling related to VI. These include soil-gas, sub-slab, and ambient air sampling, as well as temporary and semi-permanent vapor well installation.
The sampling procedures address processes like using tracer gas to assess leakage of ambient air into soil gas probes and wells. Many of these procedures are part of the CH2M HILL best practices compendium, and they are all compliant with many regulatory agencies across the nation.
A recent VI project employed several of these techniques to provide multiple lines of evidence to investigate the potential for a VI pathway at a site contaminated with chlorinated solvents. Soil gas, crawlspace, indoor air, outdoor air, and groundwater samples were taken simultaneously from residences near the site. Additionally, background samples were taken around the study area and a weather station was set up onsite to assess potential bias from offsite sources.
Results from the sampling events indicate similar concentrations in outdoor air and crawlspace samples at all locations, including background. However, at times, indoor air samples were significantly higher. One possible explanation for this were unknown indoor air sources causing positive bias in the results. Common contaminated-site chlorinated solvents, such as trichloroethylene (TCE) and perchloroethylene (PCE), can be found in items as simple as recently dry-cleaned clothing, some aerosol gun cleaners, or stainless steel cleaner.
These sources are not always obvious while conducting building surveys prior to sampling. For that reason, it is problematic to use indoor air samples alone in assessing human health risk. Because of these multiple lines of evidence, a clear case could be made that the data collected thus far on this project indicates that no current human health risk exists due to the VI pathway at this particular site.