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' M. Scott Mansholt - 2 - 12 November 2008 <br /> Chevron <br /> ' Mountain House Development Site #6 <br /> petroleum hydrocarbons as crude oil (TPHc) was detected at concentrations of 880 milligrams <br /> per kilogram (mg/kg) in a sample from 16 feet below ground surface (bgs) to 29,000 mg/kg at <br /> 12 feet bgs in another boring. Separate phase oil was reported in several of the 14 borings by <br /> Condor. Benzene, toluene, ethylbenzene, and xylenes (BTEX) was non detect (ND) in all soil <br /> ' samples. <br /> Following Condor's investigation, Chevron initiated separate soil and groundwater <br /> investigations for each of the six locations. Our office concurred with a March 2005 workplan <br /> ' for the Site, later amended 31 July 2007, to conduct the current soil and groundwater <br /> investigation. The field work was conducted in March and April 2008. <br /> ' CURRENT INVESTIGATION <br /> The purpose of the investigation was to identify and evaluate the extent of affected soil and <br /> ' groundwater identified by Condor in 2001. Additionally, SAIC completed a well survey of the <br /> Department of Water Resources (DWR) records to identify water supply wells within 2,000 feet <br /> ' of the Site. <br /> A total of 29 borings were completed to depths of 12 to 40 feet bgs. Twenty borings were <br /> advanced on the southwest side of the road and the former OVP. Eight were advanced on the <br /> opposite side of the road, northeast of the TAOC lines within the UPRR easement. An offsite <br /> boring was advanced on an adjacent property northeast, downgradient of the Site. <br /> Seventy-eight soil samples were submitted for analysis. Depth to groundwater ranged <br /> ' between 5 and 12 feet bgs and the gradient is northeast determined by monitoring well data <br /> from other nearby Mountain House Sites. <br /> ' Soil <br /> Samples collected at depths of between 4 and 40 feet bgs were submitted for analysis of the <br /> ' constituents of concern. Samples from depths of 10 feet bgs or less were analyzed primarily <br /> for future human health risk screening assessment evaluations. In addition, samples from <br /> depths greater than 10 feet bgs were analyzed to assess the magnitude of TPH constituents in <br /> soil, and from deeper depths to determine the vertical extent of impacted soil and/or <br /> groundwater. Analytical results indicated that TPHc was detected in soil samples from 4.5 to <br /> 9.5-foot bgs, ranging in concentrations from ND to 26,000 mg/kg. Analytical results for <br /> ' samples from depths greater than 13 feet bgs detected TPHc concentrations ranging from 740 <br /> to 2,900 mg/kg. The analysis of deeper samples from several borings did not detect TPHc at <br /> depths ranging from 24 to 40 feet bgs, the maximum depth sampled. <br /> According to SAIC's findings, the Site can be divided into three separate areas of affected soil <br /> and groundwater. The report describes Areas 1, 2, and 3, northwest to southeast with <br /> ' approximate dimensions of 150 feet, 300 feet, and 250 feet in diameter, respectively. <br /> Analytical results for samples from borings completed between the three Areas did not report <br /> the presence of TPHc or separate phase crude oil (SPO) in drill cuttings. <br /> ' Review of the lithologic logs indicated that SPO was observed in the soil cuttings from several <br /> borings beginning at the groundwater interface, often extending below the top of the water <br />