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z r, Twelve groundwater monitoring wells (MW-1 through MW-12) and two vapor extraction wells (VW-1 and VW-2) were installed at the Site between 1992 and 1996. Well construction details are provided on Table 1. Groundwater monitoring has been conducted at the Site since April 1992 for wells MW-1, MW-2 and MW-3, since May 1995 for wells MW-4 through MW-8 and VW-1, and since February 1996 for wells MW-9 through MW-12 and VW-2. TPHg, TPHd and benzene concentrations up to 54,000, 11,000 and 13,700 micrograms per liter (µg/1), respectively, have been reported for the groundwater samples. Historical groundwater flow beneath the Site is directed toward the east and southeast. A cumulative summary of groundwater elevations and analytical data generated at the Site since 2002 is presented on Tables 2 and 3. The approximate locations of the groundwater monitoring wells are depicted on Figure 2. 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 hod is occurring.•A summary of the RNA data collected for the Site is presented on Table 3. In correspondence dated August 16, 2007, the SJCEHD requested the addition of analyses for 1,2- r dichloroethane (1,2-DCA) and 1,2-dibromoethane (EDB) and continued quarterly groundwater monitoring. Groundwater levels have risen approximately 20 feet since the wells were installed, resulting in submerged screens in wells MW-1 through MW-12. The SJCEHD further expressed i.+ concern regarding submerged well screen intervals and 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 and MW-9 and installation of replacement wells MW-13, MW-14 and MW-15 with shallower screened intervals. The well destruction and well installation activities were conducted between July 15 and 17, 2008. The reported contaminant concentrations for shallow-screened wells r MW-13, MW-14 and MW-15 are between one to two orders of magnitude higher than those reported for the deeper-screened wells at the Site. r In correspondence dated April 24, 2008, the SJCEHD requested preparation of a Site Conceptual Model (SCM). After two sampling events for wells MW-13 through MW-15, we prepared a SCM dated January 28, 2009. Based on the SCM, the results of the additional site investigation, and Third and Fourth Quarter — 2008 and First Quarter— 2009 groundwater monitoring results, it appeared that r monitoring data for the deep-screened wells at the Site may not be representative of current site conditions. We therefore recommended installation of three downgradient and one upgradient shallow- screened wells. In correspondence dated March 24, 2009, the SJCEHD approved our recommendation to install four additional groundwater monitoring wells. Wells MW-16 through MW-19 were subsequently installed on June 10 and 11, 2009. Results of the well installation activities were submitted in our Groundwater Monitoring Well Installation Report dated August 10, 2009. r In their March 24, 2009, correspondence, the SJCEHD also stated that the deeper-screened wells at the Site could be sampled on an annual basis. In correspondence dated July 1, 2009, the SJCEHD stated low that wells MW-16 through MW-19 should be sampled quarterly for one year, and sampled semi- annually thereafter. r r Stockton Maintenance Station,Task Order No.05 Caltrans Contract No.06AI 141,EA 10-ON8300 Project No.S9200-06-05A -2- December 28,2010 r