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Well Installation Re ort August 17,2001 ARCO Facility No. 5469 Stockton, California • Page 5 June 25, 2001 was calculated to be towards the east-northeast at a gradient of 0.0022 ft/ft. Historic and second quarter 2001 groundwater elevations are presented in Table 1. Laboratory Analysis The groundwater and selected soil samples were analyzed by Sequoia Analytical, Inc. (Sequoia), a California Department of Health Services-certified analytical laboratory. Strict chain-of- custody procedures were followed from the time the soil samples were collected until the time the samples were delivered to the laboratory. The groundwater samples and selected soil samples were analyzed for TPHG, benzene, toluene, ethylbenzene, and total xylenes (BTEX) using USEPA Method 8015/8020 DHS/LUFT, and for MTBE, TBA, di-isopropyl ether (DIPE), ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME), and 1,2-dichloroethane (1,2-DCA) using USEPA Method 8260A. Groundwater samples were additionally analyzed for ethanol and ethylene dibromide (EDB) using USEPA Method 8260A. Groundwater analytical results are presented in Table 1 and Table 2 and soil analytical results are presented in Table 3. Certified analytical reports with chain-of-custody documentation are presented in Appendix E. Concentrations of TPHG, benzene, and MTBE for monitoring wells MW-1 through MW-10 during the second quarter 2001 are shown on Figure S. • Benzene was detected in soil samples collected from boring MW-10 at 51 feet bgs (0.25 mg/kg), 56 feet bgs (0.016 mg/kg) and 73.5 feet bgs (0.009 mg/kg). MTBE was detected in the soil sample collected from boring MW-10 at 51 feet bgs (1.0 mg/kg). No petroleum hydrocarbons were detected in the soil samples collected from boring MW-9. Benzene was detected in the groundwater samples collected from monitoring wells MW-8 (2300 µg/1) and MW-10 (51 µg/1). MTBE was detected in the groundwater samples collected from monitoring wells MW-8 (1600 µg/1) and MW-10 (260 pg/1). 1,2-DCA was reported in the samples collected from monitoring wells MW-9 and MW-10 at concentrations just above laboratory method detection limits. No petroleum hydrocarbons were detected in the sample collected from monitoring well MW-7. Summary of Findings • Benzene was detected in soil samples collected from boring MW-10 at decreasing concentrations beginning at 51 feet bgs [(0.25 mg/kg), 56 feet bgs (0.016 mg/kg) and 73.5 feet bgs (0.009 mg/kg)]. MTBE was only detected in the soil sample collected from boring MW-10 at 51 feet bgs (1.0 mg/kg). No petroleum hydrocarbons were detected in the soil samples collected from boring MW-9. • TPHG, benzene and MTBE were not detected in groundwater samples collected from monitoring wells MW-7 and MW-9. C:1ARC0154691REP0RTS15469WeH Installation 0601.doe