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ASSESSMENT OF VAPOR INTRUSION USING INDOOR AND SUB-SLAB AIR SAMPLING
Citation:
DIGIULIO, D. C. ASSESSMENT OF VAPOR INTRUSION USING INDOOR AND SUB-SLAB AIR SAMPLING. Presented at Soil Vapor Monitoring, Vapor Intrusion, and Indoor Air: A Workshop, San Diego, CA, September 26 - 27, 2005.
Impact/Purpose:
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Description:
The objective of this investigation was to develop a method for evaluating vapor intrusion using indoor and sub-slab air measurement and at the same time directly assist EPA’s New England Regional Office in evaluating vapor intrusion in 15 homes and one business near the former Raymark Superfund Site in Stratford, Connecticut. In this investigation, a VOC detected in indoor air was considered due to vapor intrusion if: (1) the VOC of interest was detected in ground-water and/or soil-gas in the “vicinity” of the house, and (2) the null hypothesis that the indoor/sub-slab concentration ratio of a VOC of interest was equal to the indoor/sub-slab concentration ratio of a conservative compound could not be rejected at a level of significance (p) less than or equal to 0.05. A conservative VOC was defined as a VOC detected in sub-slab air and known be associated only with subsurface contamination. 1,1-dichloroethylene and cis-1,2-dichloroethylene were considered conservative VOCs in this investigation because they are degradation products of 1,1,1-trichloroethane and trichloroethylene respectively and not commonly associated with any commercial products. The variance associated with each basement/sub-slab concentration ratio was calculated using the method of propagation of errors which incorporated the variance associated with both indoor and sub-slab air measurement. An average or overall indoor/sub-slab concentration ratio was computed using concentration ratios of all VOCs associated with vapor intrusion. The method of propagation of errors was then used to calculate the variance associated with the average indoor/sub-slab concentration ratio. As a result of this investigation, specific recommendations were developed regarding: (1) design of probes for multiple sample events, selection of materials for probe assembly, and method of probe installation, (2) sub-slab sampling for volatile organic compounds using SilcoCan canisters and EPA-Method TO-15 or Tedlar bags and on-site gas chromatography analysis, (3) sub-slab sampling for radon using scintillation cells, (4) selection of a minimum equilibration time after probe installation for air extraction, (5) selection of a minimum purge volume prior to sampling, (6) selection of extraction (purge + sample) volume, (7) selection of a flow rate during air extraction, and (8) methods of leak testing. Recommendations provided in this paper should minimize potential bias created by poor probe installation and sampling techniques, yield sample results representative of true sub-slab vapor concentration, and facilitate assessment of vapor intrusion.