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From Science tosolutions� <br /> ' HPP-BTR MP#220.7/223.4 <br /> Work Plans—Mountain House Development Site#3 <br /> DCN:C20-SAI-Moumnin-House-01-9583 <br /> October 5, 2006 <br /> Mr.M. Scott Mansholt <br /> Chevron Environmental Management Company <br /> ' 6001 Bollinger Canyon Road <br /> P.O. Box 6012 <br /> San Ramon,California 94583-2324 <br /> Subject: Addendum to the Work Plan for Additional Site Characterization <br /> Mountain House Development Site#3 <br /> Mountain House, California <br /> Dear Mr. Mansholt: <br /> Science Applications International Corporation(SAIC)provided a work plan for additional site <br /> characterization in the Soil and Groundwater Investigation Report(SGIR), dated October 5, <br /> ' 2005. The Work Plan was approved by the Regional Water Quality Control Board—Central <br /> Valley Region(RWQCB) in a letter dated November 7,2005,provided the agency's comments <br /> regarding the SGIR were addressed,as they were by SAIC in the Letter Notification of the <br /> Scheduling Plan, dated November 10,2005. SAIC has recently performed a review of project <br /> ' data, which included previously unavailable information, and obtained a better understanding of <br /> subsurface conditions. At the request of Chevron Environmental Management Company <br /> (CEMC), SAIC has revised the work plan to update the means by which the extent of affected <br /> groundwater and potential health risks associated with Mountain House Development Site#3 <br /> (Site#3)will be determined. <br /> Previously,polynuclear aromatic hydrocarbons (PAHs)in groundwater north of Byron Road at <br /> ' Site#3 were understood to originate from an unidentified third-party source, and the investigation <br /> was focused on impacts from the Chevron Old Valley Pipeline (OVP)south of Byron Road. <br /> However, after as-built drawings acquired from Union Pacific Railroad(UPRR) showed the <br /> former Tidewater Associated Oil Company(TAOC)pipelines in the UPRR right of way above <br /> the PAH plume, the source of the PAHs in groundwater appears to be more likely related to the <br /> TAOC pipeline. Furthermore,the project data suggested the presence of two sources of crude oil <br /> leaks, each contributing to crude oil constituents in groundwater that exceed RWQCB Region 5 <br /> Water Quality Objectives(WQOs): <br /> • An area of total petroleum hydrocarbons quantified as diesel(TPHd) and PAHs in <br /> groundwater near Condor soil borings GP-96, GP-97, GP-117, GP-173, and SAIC <br /> borings MH3-5, MH3-6, and MH3-7, which appears related to the OVP. <br /> • An area of TPHd in groundwater near SAIC borings MH3-1, MH3-3,MH34, MH3-8, <br /> ' and MH3-9, which appears related to the OVP. Down gradient from the source area, <br /> TPHd in groundwater from the OVP may commingle with the TAOC-affected <br /> groundwater. <br /> Science Applications International Corporation <br /> 1000 Broadway I Suite#675/Oakland,CA 946071 /tel:(510)433-0835/fax: (510)446-7919 1 saic.com <br />