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ARCAUIS BBL Mr. Brian Taylor <br /> July 27,2007 <br /> This indicates that given this particular exposure scenario, potential vapor intrusion <br /> does not present an unacceptable health risk to individuals working in the onsite <br /> office building. The following sections describe the methods and information that <br /> were used to arrive at this conclusion. <br /> Site Description <br /> Currently, the onsite office building is used by Chevron employees and their <br /> contractors. The building is constructed with a slab-on-grade foundation. There is a <br /> shower located within the building. However,the shower is only used for emergency <br /> dousing if a worker is sprayed by petroleum product. The future use of the building is <br /> expected to remain the same. <br /> COPCs have been detected in groundwater collected beneath the office building. <br /> Groundwater monitoring well MW-69UA,and water supply well WSWA are located <br /> in close proximity to the building(See Figure 1). MW-69UA has been sampled <br /> quarterly since December 14,2004. WSWA has been sampled since 1994, and has <br /> been sampled semi-annually since March 15,2005. <br /> Of the suite of chemicals analyzed, the specific chemicals detected in groundwater <br /> include benzene,toluene, ethylbenzene, and xylenes (BTEX), and oxygenates <br /> including methyl-tert-butyl ether(MTBE),tertiary butyl alcohol (TBA)and tertiary amyl <br /> methyl ether(TAME).Table 1 presents these chemicals and their maximum and <br /> arithmetic average concentrations. BTEX and MTBE were identified as COPCs in <br /> groundwater. TBA and TAME were not selected as COPCs in groundwater. Neither <br /> Cal-EPA's Office of Environmental Health Hazard Assessment(OEHHA) nor USEPA <br /> have evaluated the inhalation toxicity of these chemicals. <br /> Exposure Assessment <br /> The COPCs in groundwater have the potential to volatilize.Therefore,the most likely <br /> exposure pathway for building occupants is inhalation of vapors which have migrated <br /> from groundwater through soil into the building. <br /> Following Cal-EPA Department of Toxic Substances Control(DTSC,2004)guidance, <br /> it was assumed that the maximum concentrations of COPCs detected in the <br /> groundwater collected beneath the onsite office building could serve as the vapor <br /> source. The groundwater dataset used for this evaluation comprised the groundwater <br /> analytical data collected by ARCADIS BBL from June 13,2006 to March 6,2007, <br /> e.g.,the last four sampling events for MW-69UA and last two for WSW-1.The <br /> maximum concentrations of BTEX and MTBE in groundwater are: 310;5;36; 95; and <br /> 240 micrograms per liter(pg/L), respectively.All of the maximum detected <br /> Page: <br /> c:iav nmcomao�mtow�mm ttrinernE .mc 2/7 <br />