SITE INFORMATION AND CORRESPONDENCE

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} • • I Ms.Lori Duncan November 15,2011 RE:2911 E. Fremont St,Stockton,CA Page 10 of 14 Based upon the two lines of evidence summarized above,it appears that indoor air concentrations are due in part to vapor intrusion from subsurface contaminants. Risk Evaluation Due to detected concentrations of gasoline constituents in indoor and outdoor air samples collected at the site, L&P evaluated the potential risk of contaminants in both indoor and outdoor air. Tier 1 Risk Evaluation As an initial evaluation,L&P compared detected concentrations of TPHg,benzene,toluene,ethylbenzene, xylenes,and naphthalene to CHSSLs. The CHSSLs are concentrations of selected hazardous chemicals in soil,soil gas,or indoor air that the California Environmental Protection Agency(Cal/EPA)considered to be below thresholds of concern for risks to human health. The thresholds of concern used to develop the CHHSLs are an excess lifetime cancer risk of one-in-a-million(10-6)and a hazard quotient of 1.0 for non- cancer health effects. The CHHSLs were developed using standard exposure assumptions and chemical toxicity values published by the U.S. Environmental Protection Agency (USEPA) and Cal/EPA (CaUEPA-OEHHA, 2005). In general, if a contaminant is present in soil, soil gas, or indoor air at concentrations below their respective CHHSLs,it is assumed that they do not pose a significant health risk to people who may work at the site. If a contaminant is present at a concentration exceeding its respective CHHSL, however, it does not necessarily indicate that adverse impacts to human health are present or will occur; it suggests that further evaluation is warranted and mitigation may or may not be necessary. Further evaluation could include additional sampling and analysis, consideration of ambient samples, and/or using site specific parameters to calculate potential human health risk at the site. As summarized in Table 3,TPHg,benzene,toluene,ethylbenzene,and xylenes were detected in all four air samples collected at the site, regardless of whether the samples were indoor air or ambient air samples. A comparison of concentrations of contaminants in air with the respective CHHSLs are summarized below. TPHg: No CHHSL has been established for TPHg. As tve, L&P compared concentrations of TPHg in air to Environmental Screening Levels (ESL5) f r indoor alr n commercial/industrial settings,as established by the California Regional Water Quality Cool Board RWQCB),San Franciso Bay Region (May 2008). Similar to the CHHSLs, the ESLs establish conservative reference concentrations for comparison to actual site data for a conservative evaluation of the possible health risk Concentrations of TPHg in ambient air samples were below the ESL of 14 micrograms per cubic meter µg/m3 while concentrations of TPHg in indoor air were above the ESL. It should be noted that the ESL for TPHg is based upon conservative noncancerous exposure (hazard quotient of 0.2) rather than the hazard quotient of 1.0 utilized by Cal/EPA to establish CSHHLs. Benzene: Concentrations of benzene in ambient air and indoor air samples exceeded the CHHSL of 0.14 µg/_ m',although the concentrations in indoor air(1.3 to 1.6µg/m3)exceeded those in ambient air(0.45 to 0.71 µg/m3). 10759-EXC SUMMARY LTR