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• Monitoring Well Installation and VST Report—Former Unocal Station#2859,Stockton,CA July 30,2001 the wells Purge water generated during development and sampling procedures have been removed from the site and transported to Tosco's Rodeo refinery for disposal Copies of the monitoring well observation summary sheets are included in Appendix D Monitoring data are summarized in Table 2 Wellhead Survey Following installation of the wells, the top of casing and vault box elevations of the newly installed groundwater monitoring wells were surveyed by Virgil Chavez Land Surveying of Vallejo, CA (license #6323) The horizontal locations of all wells were measured The surveyor's report is included in Appendix E Well elevations are summarized in Table 2 RESULTS OF THE SUBSURFACE INVESTIGATION Soil encountered during this investigation consisted of interbedded silt and clay to the total depth explored with a few interbedded sand and silty sand horizons from approximately 65 feet bgs to the total depth explored In the northern half of the site and extending downgradient, a sand and silty sand horizon is also present between approximately 25 to 40 feet bgs, and in the southern half of the site a silty sand horizon is present between approximately 15 to 25 feet bgs Soil color varied from olive brown to dark yellowish brown in all well borings Soil encountered during this investigation is similar to previous investigations at the site Groundwater was encountered at depths between 20 and 45 feet bgs, and stabilized at depths between 11 74 and 28 28 feet below top of casing Detailed descriptions of the soil encountered during drilling are presented on the bortng logs in Appendix C Based on the groundwater monitoring data collected on dune 5, 2001, groundwater beneath the site appears to flow to the northeast at a gradient of approximately 0 002 in Zone A, and flow is to the southeast at a gradient of approximately 0 027 in Zone B(Figures 2 and 3) CHEMICAL ANALYTICAL RESULTS A total of 20 soil samples from the well borings, one composite soil sample from the stockpiled drill cuttings, and nine groundwater samples were submitted under chain-of-custody for chemical analysis Analyses were performed by Sequoia Analytical (FLAP #2360 and #1624) Copies of the laboratory reports and chain-of-custody forms are included in Appendix F Soil and groundwater chemical analytical data are summarized in Tables l and 2,respectively Chemical Analytical-Procedures All soil and groundwater samples were analyzed for TPHg, BTEX, and MtBE by DHS LUFT Method Groundwater samples were also analyzed for tert-butyl alcohol (TBA), MtBE, di-isopropyl ether(DIPE), ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME), 1,2-dibromoethane (1,2-DBA), and 1,2-dichloroethane (DCA) by EPA Method 8260B As required by the disposal facility, the stockpiled drill cutting's composite sample was analyzed for TPHg, BTEX, and MtBE by DHS LUFT Method, and for total lead by EPA Method 6010A Soil Analytical Results TPHg, benzene,or MtBE were not detected in sail samples from the well borings. 24007204-1 4