SITE INFORMATION AND CORRESPONDENCE

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M. Scott Mansholt - 2 - 4 April 2008 Mountain House Site #2 The risk evaluation summarized laboratory analysis and compared the results to the Preliminary Remediation Goals *1 (PBGs), relevant Environmental Screening Levels *z (ESLs), and/or Water Quality Objectives *3 (WQOs) to assess various potential exposure scenarios for the site COPCs. There are health risk based ESLs provided in the listed references for certain PAHs and benzene for different exposure scenarios that can be used to evaluate potential risk depending on the possible future site use(s). Based on the various investigative findings, it was determined that affected soil and groundwater is limited in extent within an area of approximately 75 by 100-feet, and has been adequately delineated. The area of affected soil and groundwater occurs within the Byron Road ROW easement, generally beneath the former OVP lines and Byron Road. Adjacent property use is agricultural and/or residential; the UPRR easement is adjacent to the east. The future use of the site likely will remain unchanged and continue as a public utility corridor and for Byron Road. SAIC's report provides documentation to support a No Further Action Request closure for the site 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 Requests. Depth to groundwater ranges from approximately 12 to 18 feet and based on data from nearby OVP sites, the gradient is northeast and relatively flat. A well search of Department of Water Resources (DWR) records completed by SAIC identified three water supply wells within one- half-mile. The affected groundwater plume is defined and appears to-be stable. and considering the distance from the site, it is unlikely these wells would be affected. According to the DWR records, the wells identified had at least a 35-foot sanitary surface seal and thus unlikely to be impacted by OVP crude oil leaks. Since the site is limited to within the public roadway ROW easement, it is unlikely that new water supply wells would be installed within the project area. During an earlier investigation, groundwater samples were obtained and submitted for total dissolved solids (TDS) determination. The TDS values were up to 1,410 milligrams per liter (mg/L); the Water Quality Control Plan for the Sacramento River Basin and the San Joaquin River Basin (Basin Plan) considers groundwater a potential source of drinking water provided it does not exceed TDS of 3,000 mg/L and yields at least 200 gallons per day. However, the TDS values reported are greater than the U.S. EPA Secondary Maximum Contaminant Level (MCL) of 500 mg/L for TDS, which is based on taste and odor and staining and corrosion characteristics. *1--Preliminary Remedial Goals US EPA Region 9, San Francisco, October 2004. *2- Screening for Environmental Concerns at Sites with contaminated Soil and Groundwater, Interim Final, San Francisco RWQCB, November2007. *3- The US EPA, "Suggested No Adverse Response Levels' 1980, describes 100 pg/L as the taste and odor water quality objective (WQO)threshold for diesel in groundwater.