SITE INFORMATION AND CORRESPONDENCE

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No Further Action Required Determination -2- 6 August 2409 Mountain House Development Site County oil fields to refineries in the San Francisco Bay area. Soil impacts consisted primarily of total petroleum hydrocarbons as crude oil (TPHc) which may contain polynuclear aromatic hydrocarbons (PAHs) and benzene, toluene, ethylbenzene, and xylenes (BTEX). Soil and groundwater affected by crude oil have been delineated. Setting: Rural agricultural with some nearby residential properties under development. Source: Historical leak from crude oil pipeline(s) installed in the early 1900's and removed during the late 1960's and early 1970's. Actions: Preliminary Site investigations were conducted between 2001 and 2007. In 2001, five exploratory borings were advanced, and soil samples were analyzed for TPH constituents of concern. Affected soil with a TPHc concentration of 7,600 milligrams per kilogram (mg/kg) was noted in one boring in a sample from a depth of 16 feet bgs. Low concentrations of three polynuclear aromatic hydrocarbons (PAHs) were detected between 0.2 and 14.6 mg/kg. Groundwater, encountered at approximate depths of 8 and 17 feet bgs, was not sampled. • During 2005, an additional investigation included eight borings and samples for analysis to further delineate affected soil and groundwater. TPHc concentrations detected in soil samples from depths between 1 and 32 feet bgs ranged from 14 mg/kg to 1,100 mg/kg. Benzene, toluene, ethylbenzene, and xylenes (BTEX) were ND except for ethylbenzene and xylenes detected at slightly above the method detection limits (MDLs). PAHs were ND and/or detected at low concentrations slightly above respective MDLs. No BTEX or PAH constituent concentration exceeded respective ESLs for the construction/trench worker scenario. Analysis of two of five groundwater samples detected TPH as diesel (TPHd) at 350 and 670 micrograms per liter (Ng/L). The results for BTEX were ND as were PAHs except for two that were detected at or slightly exceeding the MDLs in one sample. The other three sample results were ND. • In 2007, three borings were drilled northeast of the Site to determine the downgradient extent of affected groundwater and evaluate water quality conditions. Results of soil and groundwater samples were ND for TPH constituents. As a result, the area of affected groundwater with approximate dimensions of 75 by 125 feet is slightly larger than the affected soil area. • SAIC completed an ERE in 2008 to evaluate exposure by comparing the analytical results to the 2008 ESLs for a construction/trench worker scenario. According to the investigative reports, TPHc, BTEX, and PAH constituent concentrations reported in samples taken in 2001, 2005, and 2007, did not exceed the ESLs. The highest TPHc concentration of 7,600 mg/kg detected in a soil sample from the 2001 investigation did not exceed the ESL of 15,000 mg/kg for a construction/trench worker scenario. Thus, SAIC concluded the TPHc concentrations and other TPH constituents detected do not pose a threat to human health for the construction/trench worker scenario. In September 2008, SAIC provided a No Further Action Required Request Report (N FAR) for consideration of site closure. The NFAR included documentation of site conditions in accordance with the 16 April 2004 Appendix A — Tri-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of Underground Storage Tank Sites, Section 6.5 NFAR for Cases Exceeding Water Quality Objectives.