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Jeff Whitworth - 2 - 7 August 2008 ` <br /> in the segregated soil were 390 mg/kg and 75 mg/kg, respectively. BTEX compounds were <br /> not detected. <br /> To evaluate the human health risk of soil vapor migration to indoor air, soil gas samples were <br /> collected from six of the soil excavation areas (Areas B2, B3, B6, B8, B9 and B12). Soil gas <br /> samples were collected 1 foot below the base of the previously excavated areas and at depths <br /> of 5, 10, and 15 feet bgs northeast of the excavations in the presumed downgradient <br /> groundwater flow direction. The samples were analyzed for BTEX, naphthalene, and <br /> tetrafluoroethane (Freon 134a, a leak detection compound) using EPA Method TO-15. <br /> Maximum concentrations of BTEX were 33, 24, 5.0 and 12 micrograms per cubic meter <br /> (Ng/m3), respectively, in Area B3 at sample location SVB3-1 at 25 feet bgs. Naphthalene and <br /> Freon 134a were not detected in any of the samples above the respective reporting limits. <br /> Comparison of the shallow soil gas benzene data (maximum concentration of 3.1 ug/m3 at <br /> location SVB3-2) with California Human Health Screening Levels (CHHSLs) established by the <br /> Office of Environmental Human Health Assessment indicated no human health risk with <br /> respect to the indoor air intrusion pGthway. Use of the maximum benzene concentration data <br /> in the Johnson and Ettinger Model for further evaluation of the indoor air intrusion pathway <br /> resulted in an incremental cancer risk of 3.4 x 10-8, which does not exceed the acceptable <br /> threshold level of 1 x 10-6. <br /> Shell concluded that the direct contact pathway for future residents or site workers had been <br /> removed by the remedial action and that the soil vapors do not pose a threat for residential <br /> land use. Shell also concluded that no further action is required regarding soil cleanup in <br /> Neighborhood B of the Mountain House residential development. Based on the data provided <br /> in the RAR, Regional Water Board staff concurs with Shell that development of the Mountain <br /> House community residences overlying the former Shell pipeline areas does not pose a <br /> significant health risk from direct contact with soil or a threat to human health due to soil gas <br /> migration to indoor air. <br /> Regional Water Board staff appreciates the significant effort Shell made to remove the <br /> impacted soil and to evaluate the potential risks of soil vapor migration to future residents. In <br /> addition to mitigating the potential risks associated with direct soil contact or soil vapor <br /> intrusion, groundwater plume delineation and potential groundwater remediation will also be <br /> necessary before a full no further action determination can be granted for the site. By <br /> 10 October 2008, please submit a work plan for further groundwater plume delineation of <br /> previously identified areas of potential concern located within Neighborhood B. If you have <br /> any questions regarding this letter, you may contact me at (916) 464-4665 or <br /> ncasebeer@waterboards.ca.gov. <br /> Nathan Casebeer, P.G. <br /> Engineering Geologist <br /> Private Sites Cleanup Unit <br />