SITE INFORMATION AND CORRESPONDENCE

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r M. Scott Mansholt - 2 - 3• April 2008 Mountain House Site #1 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 150 by 200-feet, and has been adequately delineated. The site is located in the Byron Road right-of-way (ROW), the Union Pacific Railroad (UPRR) ROW, and beneath two active refined petroleum product pipelines. Adjacent property use is residential. The future use of the site likely will remain unchanged and continue as a utility corridor and for the railroad and Byron Road usage. 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 completed by SAIC identified water supply wells at least '/4-mile from the site. Since the affected groundwater plume is defined and appears to be stable, it is unlikely they would be affected. San Joaquin County requires at least a 50-foot sanitary surface seal for new domestic water supply wells and thus unlikely to be impacted by the OVP/TAOC crude oil leaks. Since the site is within the railroad easement and public roadway, 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 1,350 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 m91L 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 mg1L for TDS, which is based on taste and odor and staining and corrosion characteristics. SAIC stated that the future use of affected groundwater is not likely since the site is within the Byron Road and UPRR easement ROW's, is of poor quality, and in fine grained soils that typically yield low flow wells. Considering that the site use will not likely change, SAIC conducted the environmental risk evaluation based on the most likely potential exposure - for a construction/trench maintenance worker. SAIC's environmental risk evaluation considered total petroleum hydrocarbons as crude oil (TPHc), PAHs, and BTEX component analytical results in soil. The results were compared to the ESLs for a construction/trench worker scenario to evaluate potential risk exposure at the site. The BTEX and PAH constituent concentrations did not exceed respective ESLs with most results below the ESLs or non detect (ND). SAIC also evaluated TPHc concentrations relative to the ESL using the U.S. EPA software, ProUCL, to establish a 95-percent upper confidence limit (UCL) for all TPHc data collected,