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RESEARCH BRIEFING ON DEVELOPMENT OF A SUB-SLAB AIR SAMPLING PROTOCOL
Citation:
DiGiulio*, D C., A. Lee, K. Christensen, R. Cody*, R B. Mosley*, S. Clifford*, P. Kahn*, AND C J. Paul*. RESEARCH BRIEFING ON DEVELOPMENT OF A SUB-SLAB AIR SAMPLING PROTOCOL. Presented at Conference Call, Ada, OK, February 12, 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.
The primary purpose of this research effort is to develop a methodology for sub-slab sampling to support the EPA guidance and vapor intrusion investigations after vapor intrusion has been established at a site. Methodologies for sub-slab air sampling are currently lacking in referred literature. EPA's guidance on vapor intrusion lists five key questions in assessing the quality of sub-slab data. (1) Do analytical results meet the required detection thresholds? (2) Have ambient (outdoor) air or other sources of vapors been considered? (3) Does the data account for temporal variability? (4) Does the data account for spatial variability? (5) Is there any reason to suspect systematic error in sampling and analysis?
Sub-slab sampling was conducted at 11 homes near the Raymark Superfund in Stratford, Connecticut to assess these questions in detail and used to assist in protocol development. At least three sub-slab vapor probes were installed in each house. Indoor, outdoor, and sub-slab samples were collected in 100% certified 6-L Summa canisters and analyzed for a list of halogenated and non-halogenated compounds by EPA's New England Regional Laboratory using EPA Method TO-15. Sub-slab samples were also collected in 1-L Tedlar bags using a peristaltic pump and dedicated tubing and analyzed for a list of target compounds on-site by EPA's New England Regional Laboratory.
Compound identification and detection limits achieved through the use of EPA Method TO-15 (typically less than 0.1 ppbv) for outdoor, indoor, and sub-slab air sampling were adequate to differentiate sources of indoor air contamination. Elevated levels of 1,1-dichloroethene and trichloroethene in indoor air were attributable to ground-water contamination while other VOCs detected in indoor air such as benzene, toluene, and xylene were attributable to indoor and outdoor VOC sources. Sub-slab VOC concentrations exhibited substantial temporal and spatial variability (coefficient of variation exceeding 100%). Systematic sources of error (e.g., loss of VOCs through Tedlar bags, vapor probes as source of VOCs) were insignificant. Preparation of an EPA report is in progress which will provide detailed recommendations on sub-slab sampling and associated quality control.