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Mr. Harlin Knoll, SJCEHD September 29,2005 Site Investigation Work Plan 103 North E Street, Stockton,CA Page 4 event. Based on discussions with SJCEHD personnel, depth to groundwater in the site vicinity may have been as low as approximately 105 feet bgs in 1979. Historical depth to groundwater information is included in Appendix B. Prior to 2000, when all remaining groundwater monitoring wells except MW-1, MW-13, MW-14, and AS-1 were properly abandoned, groundwater flow beneath the site was variable, but predominantly toward the northeast and east with low flow gradients. Petroleum Hydrocarbon Impact to Groundwater Groundwater beneath the site has been impacted with total petroleum hydrocarbons as gasoline (TPHG), and benzene, toluene, ethylbenzene, and xylenes (BTEX compounds). The fuel additive methyl tertiary butyl ether (MTBE) has been reported in samples collected from well MW-13. MTBE concentrations have consistently been reported below the 13 1tg/L primary MCL established by the State of California. Tertiary butyl alcohol (TBA) has also been reported in the well MW-13 samples, at concentrations up to 65 pg/L. 1,2-DCA has been analyzed in groundwater samples since October 2001. Concentrations are consistently reported in wells MW-14 (1.8 to 31 µg/L) and AS-1 (5.1 to 11 µg/L), and have been reported once (June 4, 2002) in well MW-13 (6.0 pg/L). Historical groundwater analytical data is included in Appendix B. PROJECT APPROACH As discussed above, the vertical and lateral characterization of hydrogeologic conditions and dissolved 1,2-DCA is incomplete. Stratus proposes two phases of investigation to further characterize site conditions and to move the site toward NFA status. The first phase will consist of installing eleven CPT borings to refine our understanding of the subsurface conditions, and to identify potential migration pathways that might be missed by conventional hollow-stem drilling and sampling techniques. Depth-discrete grab groundwater samples will be collected during this phase to further characterize the extent of impact. Following completion of this first phase, Stratus will discuss the hydrogeologic and chemical data with SJCEHD. Stratus will then prepare a work plan outlining a second phase of investigation that will consist of installing monitoring wells targeted at specific migration pathways. These wells will provide repeatable data on groundwater flow and 1,2-DCA concentrations, allowing verification of plume extent and groundwater flow conditions. Data from the CPT borings and the wells will be used to perform a risk analysis of the site. The outcome of this risk assessment will determine if a risk-based closure of the site is feasible, or if other actions are necessary. The proposed scope of work for the first phase of investigation has been subdivided into tasks 1 through 4. Details are provided for the activities associated with each task. All K:%Teicher1\Stoc1cton-1031Work Planslteichert workplan 09-2005.doe OTRATu5