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Mr. Harlin Knoll, SJCEHD lune 18, 2007 Well Installation Work Plan A.Teichert and Sons Property, Stockton,CA Page 4 although the thickness of the strata appears to decrease with depth. The stratigraphic interpretation presented in Figures 3, 4, and 5 do not suggest sandy strata are acting as preferential pathways for vertical migration of dissolved hydrocarbons. Depth to groundwater beneath the site has been measured at depths ranging from approximately 37 feet bgs (1987) to 81 feet bgs (1992). 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. Groundwater was measured at approximately 44 feet bgs at the time of the first quarter 2007 well monitoring event. Prior to 2000, when the majority of the site monitoring wells were destroyed, the groundwater flow direction was variable, but predominately towards the east, southeast, and northeast, at low gradients. A rose diagram summarizing historical groundwater flow directions calculated during historical monitoring events is included on Figure 2. Extent of Petroleum Hydrocarbon Impact to Soil As described in the Remedial Actions section below, extensive efforts were undertaken by Teichert to excavate and remove petroleum hydrocarbon impacted soil from the site, including an excavation to the water table interface (38 feet bgs at the time of excavation in 1987). When the water table dropped in the early to mid 1990's, SVE was utilized to remove additional hydrocarbon mass. Limited soil analytical data is available following the SVE remedial actions. Four soil samples were collected during the advancement of boring B-17 in September 1999. Gasoline range organics (GRO) were reported in samples collected from 40 feet bgs (7.4 milligrams per kilogram [mg/Kg]), 49 feet bgs (9.4 mg/Kg), 85 feet bgs (2.1 mg/Kg), and 95 feet bgs (1.0 mg/Kg). Benzene and methyl tertiary butyl ether (MTBE) were not detected in the boring B-17 soil samples. The extensive excavation, lack of hydrocarbons in SVE influent samples, and very low concentrations reported in samples from boring B-17, suggest residual hydrocarbons have been effectively removed from the vadose zone. Extent of Petroleum Hydrocarbon Impact to Groundwater At the time of the third quarter 2006 groundwater sampling event (September 13, 2006), GRO was reported in wells MW-13 (2,000 micrograms per liter [pg/L]) and MW-14 (170 pg/L), and 1,2-DCA was reported in wells MW-14 (5.7 l.tg/L) and AS-1 (13 pg/L). Benzene or MTBE were not reported in any of the wells. GRO, benzene, and MTBE concentrations in all three wells show a decreasing trend. The 1,2-DCA concentrations in well MW-14 also indicate a decreasing trend, but the 1,2-DCA concentration in well AS-1 does not appear to be decreasing. KATcichertl.Stockton-1031Work Planslteichert-well install work plan 06-2007.doc r✓-7Rf17U5