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Historical groundwater flow beneath the Site is directed toward the east and�since ast. A cumulative summaof groundwater e evatlons and analytical data generated at the Site2002 is presented on Tables 5 and 6. The approximate locations of the groundwater monitoring wells are depicted on Figure 3. Historical data is presented in Appendix B. .r Groundwater samples were analyzed for sulfate, nitrate, dissolved iron, dissolved manganese and methane to evaluate remediation by natural attenuation (RNA) during six groundwater monitoring events conducted between March 2007 and May 2008. Field measurements including dissolved oxygen, oxidation reduction potential and pH were also collected. The historical RNA data collected for the Site suggests that active biodegradation of petroleum hydrocarbons through natural attenuation is occurring. A summary of the RNA data collected for the Site is presented on Table 6. Groundwater levels have risen approximately 20 feet since wells MW-1 through MW-12 were installed, resulting in submerged screens in the wells. In correspondence dated August 16, 2007, the San Joaquin County Environmental Health Department (SJCEHD) expressed concern regarding the well screen intervals at the Site, and concern that the current monitoring data may not be providing accurate information regarding the groundwater impacts in shallow groundwater beneath the Site. In a phone conversation with SJCEHD personnel, Caltrans proposed the destruction of wells MW-7, MW-8 two and MW-9 and installation of replacement wells MW-13, MW-14 and MW-15 with shallower screened intervals. We performed the well destruction and new well installation activities in July 2008. Results of the Fourth Quarter—2008 groundwater monitoring event show that the highest concentrations of petroleum hydrocarbons were reported for newly installed shallow-screened wells MW-13 through MW-15.The reported concentrations for the shallow-screened wells were one to two orders of magnitude higher than those reported for the deep-screened wells within the same area. Near laboratory test method reporting limit (MRL) concentrations of petroleum hydrocarbons were reported for the samples from the deep-screened wells. The estimated limits of petroleum hydrocarbons in groundwater are depicted on Figure 3. Petroleum hydrocarbon concentrations from the most recent groundwater monitoring event are depicted on Figure 4,Petroleum Hydrocarbons in Groundwater—October 2008. In correspondence dated April 24, 2008, the SJCEHD requested preparation of a Site Conceptual Model (SCM). We have prepared this SCM in response to the SJCEHD request. 1.3 Site Geology/Hydrogeology The site geology was inferred from boring logs prepared during well installation activities at the Site and is generalized in the schematic cross-sections presented on Figures 5 and 6, Cross-Section A-A' and B-B',respectively. The surface of the Site is paved with 4 to 6 inches of asphalt concrete. Beneath the pavement, alluvial deposits consisting of interbedded silty clay, clayey silt, silty sand, sand and clayey sand were encountered to the total depth explored(approximately 80 feet). .r Project No.Stockton Yard,Task Order No.45 -2- Caltrans Contract 03AI368,EA No.I O-ON8300 Project No.S9300-06-45 January 28,2009 V