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C. Scott Mansholt - ChevroO - 5 - 19 June 2008 Mountain House #4 concentration. U.S. EPA risk assessment guidance also stipulates that the exposure point concentration estimates should be based on the 95 percent UCL of the arithmetic mean to estimate a reasonable maximum exposure scenario. The 95 percent UCL for TPHc was determined to be 2,613 mg/kg, well below the construction/trench worker ESL of 15,000 mg/kg for TPHc. The report therefore concluded that TPHc at the site does not pose a threat to human health for the constructionitrench worker scenario. Based on their environmental risk evaluation, SAIC concluded that: • Affected soil and groundwater lie beneath Byron Road and within the ROW and there are no known plans to change the present land use and thus, the applicable health risk scenario is for a construction/trench worker. • Analytical results for BTEX and PAHs do not exceed the construction/trench worker ESLs but, the highest TPHc concentration of 19,000 mg/kg exceeds the ESL for TPHc. The calculated 95 percent UCL was 2,613 mg/kg, which is less than 15,000 mg/kg and thus poses no unacceptable risk for construction/trench workers. • A No FurtherAction Required determination and a site specific Soil Management Plan should be submitted. Based on my review, I find that: • Soil and groundwater impacts associated with the former OVP/TAOC pipelines at Site #4 have been adequately delineated and additional characterization work is not needed. • Removal of impacted soil is generally not feasible given that the majority lies beneath Byron Road. • Based on SAIC's environmental risk evaluation, I concur that the applicable scenario is for a construction/trench worker. • Elevated concentrations of TPHd, TPHg, BTEX, and PAH constituents reported in MH4-6 suggest the presence of refined petroleum hydrocarbon product(s) possibly leaking from the active refined product pipeline(s). • A request for No Further Action Required determination can be prepared and it should be in accordance with Central Valley Regional Water Quality Control Board's 16 April 2004 update to Appendix A of the Tri-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of Underground Tank Sites - No Further Action Required (NFAR). More specifically, it should follow Sections 6.5/6.6 - NFAR for Cases Exceeding Water Quality Objectives and Documentation. • SAIC needs to conduct a technological feasibility evaluation in accordance with State Water Resources Control Board Resolution No. 92-49, addressing the viability of implementing remedial options for affected soil and groundwater remaining at the site.