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SAMPLING-BASED APPROACH TO INVESTIGATING VAPOR INTRUSION
Citation:
DIGIULIO, D. C. SAMPLING-BASED APPROACH TO INVESTIGATING VAPOR INTRUSION. Presented at Environmental Professionals of Connecticut Association, Hartford, CT, December 01 - 02, 2004.
Impact/Purpose:
To inform the public.
Description:
Vapor intrusion is defined as the migration of volatile organic compounds (VOCs) into occupied buildings from contaminated soil or ground water. EPA recently developed guidance to facilitate assessment of vapor intrusion at sites regulated by RCRA and CERCLA. The EPA guidance encourages sub-slab gas sampling prior to or in conjunction with indoor air sampling because sub-slab sampling can be used to differentiate indoor sources of VOCs from VOCs emanating from contaminated soil or ground water. There are however other notable benefits to sub-slab sampling. Sub-slab sampling provides measurement of vapors directly beneath a building. Uncertainty associated with interpolation or extrapolation of soil-gas and/or ground-water data based on measurements distant from a building is avoided. Additional uncertainty associated with the use of mathematical modeling to estimate indoor-air concentrations is avoided especially in heterogeneous subsurface media. Also, detection or non-detection of elevated levels of biodegradable compounds in sub-slab air verify or repudiate the effectiveness of biodegradation in reducing risk associated with vapor intrusion. Finally, when elevated levels of VOCs of concern are present in sub-slab gas, lack of detection or detection of VOCs below target levels in indoor air does not negate potential future exposure due to changes in air exchange rate (e.g., upgrading insulation) or foundation condition (e.g., development of cracks). In this case, it may be more cost-effective to take remedial action (e.g., sub-slab depressurization system) based on sub-slab concentrations alone rather than continue to sample in the future.