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EPA/600/R05/147
Assessment of Vapor Intrusion in Homes Near the Raymark Superfund Site Using Basement and Sub-Slab Air Samples (PDF)
(131 pp, 7.8MB, About PDF)Abstract
This report describes the results of an investigation conducted to assist EPA's New England Regional Office in evaluating vapor intrusion at 15 homes and one commercial building near the Raymark Superfund Site in Stratford, Connecticut. Methods were developed to sample sub-slab air and use basement and sub-slab air measurements to evaluate vapor intrusion on a building-by-building basis. A volatile organic compound (VOC) detected in basement air was considered due primarily to vapor intrusion if: (1) the VOC was detected in ground water or soil gas in the vicinity (e.g., 30 meters) of a building, and (2) statistical testing indicated equivalency between basement/sub-slab air concentration ratios of indicator VOCs and VOCs of interest. An indicator VOC was defined as a VOC detected in sub-slab air and known to be only associated with sub-surface contamination. Using this method of evaluation, VOCs detected in basement air due to vapor intrusion could easily be separated from numerous other halogenated and non-halogenated (e.g., petroleum hydrocarbons) VOCs present in basement air. As a matter of necessity, radon was used as an indicator compound at locations where an indicator VOC was not detected in basement air. However, when basement/sub-slab air concentration ratios were compared for radon and indicator VOCs, statistical non-equivalency occurred at three out of the four locations evaluated. Further research is needed to assess the usefulness of radon in assessing vapor intrusion.
Holes for sub-slab probes were drilled in concrete slabs using a rotary hammer drill. Probes were designed to allow for collection of air samples directly beneath a slab and in sub-slab media. Three to five probes were installed in each basement. Placement of a probe in a central location did not ensure detection of the highest VOC concentrations. Schematics illustrating the location of sub-slab probes and other slab penetrations (e.g., suction holes for sub-slab permeability testing) were prepared for each building to document sample locations, interpret sample results, and design corrective measures. Basement and sub-slab air samples were collected and analyzed for VOCs using six-liter SilcoCan canisters and EPA-Method TO-15. Sub-slab air samples were also collected in one-liter Tedlar bags using a peristaltic pump and analyzed on-site for target VOCs. Open-faced charcoal canisters were used to sample radon gas in basement air. Scintillation cells and a peristaltic pump were used to sample radon gas in sub-slab air.
Three methods were used to evaluate infiltration of basement air into sub-slab media during air extraction (purging + sampling). The first method consisted of sequentially collecting five one-liter Tedlar bag samples at a flow rate of 1 standard liter per minute and comparing vapor concentration of four VOCs associated with vapor intrusion as a function of extraction volume. This was performed at three locations with little effect on sample concentration. This testing also indicated the absence of rate- limited mass exchange during air extraction. Replicate canister samples representing extraction volumes of 5 to 9 and 10 to14 liters were compared at two locations with similar results. A second method was then employed which utilized a mass balance equation and sub-slab and basement air concentrations. When sensitivity of the method permitted, infiltration was shown to be less than 1% at sampled locations. A third method involved simulating streamlines and travel time in sub-slab media during air extraction. Air permeability testing in sub-slab media was conducted to obtain estimates of radial and vertical air permeability to support air flow simulations. Simulations indicated that less than 10% of air extracted during purging and sampling could have originated as basement air when extracting up to 12 liters of air. Overall, extraction volumes used in this investigation (up to 14 liters) had little or no effect on sample results.
To assess the need for an equilibration period after probe installation, advective air flow modeling with particle tracking was employed to establish radial path lengths for diffusion modeling. Simulations indicated that in sub-slab material beneath homes at the Raymark site (sand and gravel), equilibration likely occurred in less than 2 hours. Sub-slab probes in this investigation were allowed to equilibrate for 1 to 3 days prior to sampling. A mass- balance equation was used to estimate the purging requirement prior to sampling. Simulations indicated that collection of 5 purge volumes would ensure that the exiting vapor concentration was 99% of the entering concentration even if vapor concentration inside the sample system had been reduced to zero concentration prior to sampling.