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iMonitoring Well Installation and VET Report,and IRAP—Former Unocal Station#0187,Stockton CA Decernber 22 2000 Wellhead Survey Following installation of the wells, the top of casing and vault box elevations of all groundwater monitoring and vapor extraction 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, clay and sand to approximately 75 feet bgs At approximately 75 feet bgs, soil becomes mainly silt and clay to approximately 110 feet bgs, where soil becomes interbedded sand and silt with minor amounts of clay to the total depth explored A distinct color change occurs in soil between 45 to 55 feet bgs where soil is brown above that depth, and dark greenish gray below that depth Soil encountered during this investigation is similar to previous investigations at and in the vicinity of the site Groundwater was encountered at depths between 20 and 35 feet bgs, and stabilized at depths between 17 94 and 31 73 feet below top of casing Detailed descriptions of the soil encountered during drilling are presented on the boring logs in Appendix C The subsurface stratigraphy of the site is depicted on cross sections A-A' and B-B' (Figures 10 and 11) Cross section lines are shown on Figure 3 Based on the groundwater monitoring data collected on November 20, 2000, groundwater beneath the site appears to flow to the east to northeast at a gradient of approximately 0 003 (Figure 4 through 6) CHEMICAL ANALYTICAL RESULTS A total of 71 soil samples from the well borings, one composite soil samples from the stockpiled drill cuttings, and 31 groundwater samples were submitted under chain-of-custody for chemical analysis Analyses were performed by Sequoia Analytical (ELAP #1271 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 1 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), and tert-amyl methyl ether (TAME) 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 total lead by EPA Method 6010A Soil Analytical Results MtBE was not detected in soil samples from the well borings TPHg were not detected in wells U-15, U-16, U-17, U-19, or U-25, and benzene was not detected in wells U-15, U-17, U-25, or UV-2 TPHg were detected in wells U-21 (1 49 ppm at 51 5 feet bgs), U-23 (1 29 ppm at 81 feet bgs), UV-2 (3 44 ppm at 26 feet bgs), and UV-3 (33 7 and 14 0 ppm at 21 and 26 feet bgs, respectively) Benzene was detected in wells U-16 (0 0158 ppm at 56 5 feet bgs), U-19 (0 190 ppm at 119 5 feet bgs), U-21 (0 00506, 0 0296, 24004208-1 6