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S E C 0 R encountered in MW-2 from 16 to 20 feet bgs and sandy silt in MW-3 from 16 to 20 feet bgs. Soil boring logs and a geologic cross-section of the Site are included in Appendix B. 2.4 HYDROGEOLOGY Based on quarterly groundwater monitoring data from June 2000 to September 2006, the inferred shallow groundwater flow direction is generally to the east or southeast, at a gradient of 0.0004 to 0.009 feet per foot(URS, 2006). 2.5 PREVIOUS INVESTIGATIONS A summary of previous investigations is presented below. Historical soil and groundwater analytical data tables are presented in Tables C-1 to C-3 of Appendix C, respectively. September 1999 In September 1999, ten soil samples (D-1 to D-4 and PL-1 to PL-6) were collected beneath the dispensers and product lines during upgrading activities (URS, 2006). Analytical results indicated total petroleum hydrocarbons (TPH) as gasoline (TPHg) at a maximum concentration of 6,700 milligrams per kilogram (mg/kg) in sample D-1 collected at 3 to 3.5 feet bgs. Methyl tert-butyl ether (MTBE) was detected in one dispenser sample D-4 and one piping sample PL-2 at concentrations of 1.71 mg/kg (at 3 to 3.5 feet bgs) and 0.93 mg/kg (at 4.5 to 5 feet bgs), respectively. Benzene was not detected in any soil samples. However, toluene, ethylbenzene, and xylenes were detected at maximum concentrations of 47 mg/kg, 71 mg/kg, and 710 mg/kg (at 0-1), respectively. The most impacted soil samples were D-1, D-2, and PL-6. Based on these detected levels, approximately 50 cubic yards of petroleum-impacted soils (covering locations of samples D-1, D-2, PL-5, and PL-6) were over-excavated and disposed offsite. Post- excavation samples (samples ND4 [near sample D-1 and PL-61, ESW4, SPL4, NSW4, and SD4 [near sample D-2]) indicated the presence of up to 9,800 mg/kg of TPHg in sample SD4 collected at 4 to 4.5 feet bgs. Again, benzene was not detected in any soil samples, but toluene, ethylbenzene, and xylenes were detected at maximum concentrations of 52 mg/kg, 78 mg/kg, and 950 mg/kg (all at SD4), respectively. Toluene, ethylbenzene, and xylenes were also found at 26 mg/kg, 36 mg/kg, and 350 mg/kg at ND4), respectively. MTBE was detected in only one sample ND4 at a concentration of 0.41 mg/kg at 4 to 4.5 feet bgs. Historical soil data from this investigation are presented in Table C-1 of Appendix C. June 2000 In June 2000, three onsite wells(MW-1 to MW-3)were installed in the vicinity of the pump island(15 to 30 feet away) to assess the impact to groundwater at the Site (Delta, 2000). It is noted that the shallowest 3