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Ranter
<br /> December 8, 2010
<br /> Page 5 of 9
<br /> Each sample was collected by first withdrawing the probe by about 6 inches to allow exposure
<br /> of the probe tip to the subsurface. Then polyethylene tubing with a threaded adapter at the end
<br /> was inserted down the center of the rod. Once attached to the probe tip, the tube/adapter
<br /> assembly forms a seal. One volume of soil gas was purged through each tube/adapter
<br /> assembly, and then the actual soil gas samples were collected by attaching a clean, one-liter
<br /> Tedlar bag to each polyethylene tube via a Teflon tube regulated by a control valve. To obtain
<br /> the samples, the Tedlar bag was placed inside a sealed vacuum box; as vacuum was applied to
<br /> the box, the Tedlar bag inflated with the soil gas sample. Once filled, the valve was locked and
<br /> the bag sample was removed from the box for transport in an insulated container to Acculabs,
<br /> Inc. located in Davis, California. The soil gas samples were shipped under chain-of-custody
<br /> (COC) protocol and analyzed for BTEX, MTBE, and TPH-g by EPA Methods 8020/modified
<br /> 8015, and for MTBE, TBA, DIPE, ETBE, TAME, ethylene dibromide (EDB), and 1,2-
<br /> dichloroethane (1,2-DCA) by EPA method 8260 (Table 3).
<br /> ANALYTICAL RESULTS
<br /> Soil-Gas Samples
<br /> Benzene was non-detect for all samples except SG-9, which contained 41 pg/L (41,000 pg/m3).
<br /> Concentrations of TPHg were also non-detect for all samples except SG-9, which contained
<br /> 2,500 pg/L'(2,500,000 pg/m3).'For MtBE, all locations were non-detect, except for SG-8, SG-9,
<br /> SG-10, and SG-11; levels in all of those locations were less than 9 pg/L(9,000 pg/m3), except
<br /> SG-9, which contained 2,200 pg/L(2,200,000 pg/m).
<br /> Laboratory results for the soil-vapor samples collected during the September 18, 2000
<br /> investigation are included in the Report on Additional Site Assessment included in Attachment
<br /> B.
<br /> VAPOR INTRUSION SCREENING LEVEL HUMAN HEALTH RISK ASSESSMENT
<br /> Potential vapor intrusion risks resulting from hydrocarbons in soil gas were quantified by
<br /> comparing the concentrations to California Human Health Screening Levels (CHHSLS; Cal-
<br /> EPA, 2005) or CRWQCB Region 2 Environmental Screening Levels (ESLs; CRWQCB, 2008).
<br /> When both a CHHSL and an ESL were available, the lesser of the two values was used to
<br /> provide a more conservative evaluation. For chemicals with no CHHSL (i.e. TPHg and
<br /> ethylbenzene), ESLs were used as the screening level.
<br /> Because the ESLs for noncarcinogens are based on a hazard quotient (HQ) of 0.2 and CHHSLs
<br /> are based on the Cal-EPA and USEPA acceptable HQ of 1.0, the ESLs were multiplied by 5 to
<br /> provide an equal comparison. The ESLs are based on an HQ of 0.2 to account for the presence
<br /> of up to five chemicals and for exposure via multiple pathways. In this assessment, the
<br /> potential risks resulting from exposure to multiple chemicals were accounted for by summing the
<br /> HQs for the individual chemicals and comparing the result (known as a hazard index or HI) to
<br /> the acceptable HI of 1.0.
<br /> Both CHHSLs and ESLs for carcinogens are based on a one in a million (1 E-06) excess cancer
<br /> risk (ECR); however, no CHHSLs have been established to account for the noncarcinogenic
<br /> effects of these chemicals. Therefore, ESLs based on an HQ of 1.0 were used to quantify the
<br /> noncarcinogenic risks of carcinogens (i.e. benzene, MTBE, and naphthalene).
<br /> 19976 Vapor Risk Assessment Report.doc
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