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Jeff Whitworth - 2 - 8 May 2007 <br /> per 1,000 cubic yards to evaluate suitability for reuse. Maximum TPHcr and TPHd <br /> concentrations in the segregated soil were 200 mg/kg and 110 mg/kg, respectively. BTEX <br /> compounds were not detected. <br /> Soil gas samples were collected from eight of the soil excavation areas (Areas 1, 3, 4, 5, 6 and <br /> 7, 9 and 10, 11, and 13) to provide data to evaluate human health risk related to the soil vapor <br /> to indoor air migration pathway. Excavation Areas 3, 4 and 5 are located within the parcel <br /> planned for construction of Mountain House High School. Soil gas samples were collected for <br /> vertical profiling at 5 feet, 10 feet, and 15 feet bgs northeast of the excavations, the presumed <br /> downgradient groundwater flow direction, and 1 foot above the base of the excavations within <br /> the excavation areas. 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 570 micrograms per cubic meter (ug/m3), 790 ug/m3, <br /> 170 ug/m3, and 250 ug/m3, respectively, in Area 11 at sample location SV24 at 18 feet bgs. <br /> Naphthalene was not detected above the reporting limit. Freon 134a was detected in several <br /> samples; however, concentrations of BTEX detected in those samples were of similar <br /> magnitude to detections in other samples from a similar depth indicating that the leaks did not <br /> diminish BTEX concentrations. Comparison of the shallow soil gas benzene data (maximum <br /> concentration of 8.9 ug/m3 at location SV26) with California Human Health Screening Levels <br /> (CHHSLs) established by the Office of Environmental Human Health Assessment indicated no <br /> human health risk with respect to the indoor air intrusion pathway. Use of the maximum <br /> benzene concentration data in the Johnson and Ettinger Model for further evaluation of the <br /> indoor air intrusion pathway resulted in an incremental cancer risk of 4.4 x 10-'. <br /> Shell concluded that the remaining petroleum hydrocarbon concentrations in soil do not pose <br /> a significant risk to human health due to direct contact with soil and that the soil gas data <br /> collected do not indicate a significant cancer risk. Shell also concluded that no further action <br /> is required regarding soil and groundwater cleanup in Neighborhoods C and D of the Mountain <br /> House residential development. <br /> REGIONAL WATER BOARD AND DTSC REVIEW COMMENTS <br /> Shell made a significant effort to remove soil that contained visible SPH or TPH concentrations <br /> in excess of 5,000 mg/kg. Soil containing SPH was removed down to the water table in Areas <br /> 3, 6 and 7, and 11. Based on the confirmation soil sample data, Regional Water Board staff <br /> concurs that residual soil TPH concentrations in the vadose zone should not pose a significant <br /> threat to groundwater degradation or human health risk due to direct contact. <br /> The soil vapor sampling program and subsequent evaluation of the soil vapor to indoor air <br /> migration pathway indicated that the highest benzene concentration resulted in an incremental <br /> cancer risk of 4.4 x 10-' which does not exceed the acceptable threshold level of 1 x 10-6. <br /> Regional Water Board staff concurs that there appears to be no significant risk to human <br /> health from the intrusion of soil vapor to indoor air associated with the remedial soil excavation <br /> areas. <br /> DTSC provided review comments for Neighborhood D excavation Areas 3, 4, and 5 where <br /> Mountain House High School is planned for construction. DTSC's general comments <br /> requested a review of future groundwater chemical data as significant TPH and BTEX <br />