ARCHIVED REPORTS FINAL SITE INVESTIGATION

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FMS #24 Site Investigation Report California Army National Guard step -out boring to SB07, SB 11 (approximately 200 feet to the north of SB07), were all non -detect. The groundwater sample from SB06, located to the east of the former USTs, contained benzene at 330 µg/l, (MCL is 1 µg/L), in addition to a number of other BTEX compounds (ranging up to 3,200 µg/L) and TPHG at a concentration of 64,000 µg/L (EPA taste and odor threshold is 5 µg/L). Samples from the step -out boring to SB06, SB 10, were all non -detect. 3.4.3 Based on the results from the URS investigations, the UST source area contamination has reached groundwater and the contaminant plume has migrated to the northeast, but has not yet reached the facility boundary. The two phases of the URS investigation have determined the lateral extent of contamination at the former UST site at FMS #24. 3.5 Evaluation of Potential Health Risks from Vapor Intrusion 3.5.1 The analytical results for individual VOCs detected in the four soil -vapor samples collected at 8 feet bgs from the Stockton FMS #24 site (October 2007 URS investigation) were evaluated for the potential threat to human health via vapor intrusion into buildings at the site. As detailed below, there are no expected cancer or noncancer threats to human health from the VOCs detected in soil vapor, either for workers or potential residents at the site. 3.5.2 Health risks were evaluated by combining estimates of the chemical concentrations predicted to occur in indoor air, using standard health risk assessment procedures for scenarios representing current and potential land uses at the site. Current land use at the site is commercial/ industrial, while the future use of the site was considered to be residential, as a health -protective assumption for evaluating potential unrestricted land uses in the future. 3.5.3 Estimated VOC concentrations predicted to occur in indoor air were based on the California Environmental Protection Agency, Department of Toxic Substances Control's (DTSC's) Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air (DTSC, 2005). The maximum detected concentrations of benzene (6.5 ppbv at Location DP04) naphthalene (13 ppbv at Location DP01 and DP03) toluene (7.1 ppbv at Location DP01) and m,p-xylenes (19 ppbv at Location DPO1) were multiplied by a dimensionless attenuation factor "a" as estimates of the concentrations that may occur in indoor air. For preliminary screening evaluations, DTSC (2005) recommends that maximum detected soil -gas concentrations be used with default a values: a=0.001 for existing commercial slab -on -grade structures and a=0.0009 for future slab -on -grade residences. According to the DTSC, these default attenuation factors reflect reasonable worst-case conditions in California for degradation of indoor air quality due to intrusion of vapors migrating from the subsurface. DTSC derived these attenuation factors based on an empirical analysis of paired data, relating the relative magnitude of VOC concentrations in soil vapor and indoor air, from various sites across the United States. 3.5.4 The second health risk evaluation component uses the predicted indoor air concentrations in standard risk -assessment scenarios, which mathematically simulate the contact rates and activity patterns for people that might use the site (i.e., a current worker or a future resident). These standard processes assume, for example, the long-term use of the site (8 hours/day for 25 years) by an indoor office worker (weighing 70 kilograms [kg] and breathing at a normal [non -exertion] rate of 14 cubic meters of air per day [m'/day]) or the long-term use of the site (24 hours/day for 30 years, 6 years as a child and 24 years as an adult) by a resident (15 kg child breathing at 10 m'/day and a 70 kg adult breathing at 20 m'/day). The health risk evaluation is summarized in Table 3-7 (for current commercial -use conditions) and Table 3-8 (for hypothetical residential use). December 2007