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Shell Mountain House Pipeline -4 - 3 November 2Q11 <br /> Mountain House Parkway and ,- _nt Line Road, Tracy <br /> Table 4 - Greatest Soil Vapor Concentrations [in micrograms per cubic meter(pg/m3)] <br /> Benzene Toluene T <br /> Ethyl-benzene Total Xylenes Naphthalene <br /> 570 790 170 250 <260 <br /> Shell detected elevated petroleum hydrocarbon concentrations in soil gas in the vicinity of the <br /> proposed Mountain House High School in Neighborhood D. Therefore, Shell installed a <br /> permanent soil vapor monitoring probe, SVP-1-6/16, screened at 5 and 16 feet bgs at the <br /> proposed school location in February 2009 at the request of the Department of Toxic <br /> Substances Control (DTSC). Volatiles were not detected during the only sampling event <br /> conducted in SVP-1-6/16 on 15 July 2010. <br /> RISK EVALUATION <br /> Two searches identified eighteen wells within 2,000 feet of the Site: a Department of Water <br /> Resources (DWR) well search identified 15 domestic wells and an Environmental Data <br /> Resources (EDR) well search identified three additional wells with the closest well about 400 <br /> feet west of the Site. The 18 wells were not field verified. Wells are associated with <br /> residential dwellings and older commercial structures in the area. <br /> Shell's Sensitive Receptor Survey shows two surface water bodies, the "120" and "165" <br /> Canals, in the western portion of Neighborhood C. Additionally, West Side Irrigation District's <br /> Upper Main Canal is at the eastern property boundary of Neighborhood B. Mountain House <br /> Preschool and Childcare Facility is about 150 feet north of Neighborhood D. Petroleum <br /> hydrocarbons released from the Site are not expected to impact these sensitive receptors. <br /> Although LPH was initially present on the water table, petroleum hydrocarbons were only <br /> detected in seven of more than 150 groundwater samples collected from monitoring wells after <br /> Shell completed soil cleanup. The seven detections were below water quality objectives. <br /> Shell conducted Tier 1 Human Health Risk Assessments by comparing soil vapor sampling <br /> results from a direct push soil vapor survey to California Human Health Screening Levels <br /> (CHHSLs). None of the samples from five feet bgs or shallower exceeded the respective <br /> CHHSL for shallow soil gas for vapor intrusion. Benzene concentrations were further analyzed <br /> using the Johnson and Ettinger (J&E) Model for Evaluating Subsurface Vapor Intrusion into <br /> Buildings. The J&E model results show that benzene concentrations in soil vapor are not a <br /> human health risk when considering vapor intrusion into buildings. <br /> There does not appear to be complete exposure pathways for petroleum hydrocarbons in soil, <br /> soil vapor, or groundwater at the Site, and therefore, are unlikely to pose a threat to human <br /> health under an unrestricted use exposure risk scenario. <br /> CONCLUSIONS <br /> Shell detected elevated petroleum hydrocarbon concentrations in soil, soil vapor, and <br /> groundwater beneath the Site and excavated about 165,000 cubic yards of soil between July <br /> and November 2006, and June through July 2007. Shell transported approximately 77,000 <br /> cubic yards of soil off Site for disposal at the Altamont Landfill facility. Petroleum hydrocarbons <br /> were detected during only one of five monitoring events and the seven detections were below <br /> WQOs. The results of Tier 1 and 2 HHRAs for soil and soil vapor show that residual <br />