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S E C 0 R Ms. Lori Duncan ARCO Station 4493 Site Closure Report October 31, 2006 Page 2 and removed. The four USTs were hauled by ECI to their Richmond, California facility for disposal. Excavation and sampling activities were observed and approved by the San Joaquin County Public Health Services-- Environmental Health Division (SECOR, 1999a). In February 1999, four groundwater monitoring wells (MW-1 through MW-4)were installed at the ARCO site under the direction of SECOR. These wells are shown on Figure 2. The maximum detected concentration of total petroleum hydrocarbons as gasoline(TPHg)was 1.80 milligrams per kilogram (mg/kg) in MW-4 at 20 feet below ground surface (bgs). Benzene had a maximum soil concentration of 0.118 mg/kg (MW-4 at 25 feet bgs). MtBE had reported maximum soil concentrations of 1.52 mg/kg (MW-4 at 25 feet bgs). Lead had reported maximum soil concentrations of 141 mg/kg in soil samples collected at MW-2 at 10 feet bgs (SECOR, 1999b). Soil analytical results are presented in Attachment B. Analysis of groundwater samples collected from the newly installed wells indicated that TPHg and benzene were present in two wells (MW-1 and MW-4). MtBE was reported in monitoring wells MW-3 and MW-4. Concentrations of MtBE have decreased in these wells since monitoring was initiated, suggesting that the source of contamination has been removed. Historical groundwater analytical data is presented in Tables 1 and 2. In June 2000,shallow monitoring wells MW-5 through MW-7 were installed at the site,downgradient from the former source area. The wells are located on a north-to-south line on the eastern side of Center Street, across from the former source area (Figure 2). The wells were installed to help define the downgradient extent of gasoline-related compounds in site groundwater. No detectable '-1 concentrations of hydrocarbons were reported soil sar I nples from wells MW-5 through MW-7. Soil analytical data is presented in Attachment B. Benzene, cis-1,2-dichloroethane,and trichloroethene (TCE)were reported in groundwater samples collected from well MW-5 at concentrations of 2.75 parts per billion (ppb), 5.46 ppb and 20.2 ppb, respectively. Benzene,MtBE,cis-1,2-dichloroethene, and TCE were reported in groundwater samples from ell MW-6 at concentrations of 1.87 ppb, 171 ppb, 17.6 ppb, and 3.28 ppb, respectively. Well MW-7 was non detect for all constituents analyzed (SECOR, 2000). Groundwater analytical data is presented in Tables 1 and 2. In May 2002, one dual-completion groundwater monitoring well set (MW-8M/D) was installed by SECOR on the western side of Center Street, downgradient from the former ARCO pump islands. No soil samples were collected during the drilling of MW-8M/D. Two dual-completion groundwater monitoring well sets(MW-9M/D and MW-10M/D)were installed on the eastern side of Center Street. Benzene and ethylbenzene were detected in soil samples from the boring for MW-9 at 42 feet bgs at concentrations of 0.0072 mg/kg and 0.024 mg/kg, respectively. MtBE was also detected in soil samples from MW-9 at 25 feet bgs (0.020 mg/kg) and 42 feet bgs (0.026 mg/kg). Other fuel oxygenates were not detected in soil samples from the boring of MW-9 above the laboratory detection limits. TPHg, benzene, toluene, ethylbenzene, and xylenes compounds (BTEX), MtBE, and other fuel oxygenates were not detected in soil samples from the boring of MW-10 above the laboratory detection limit(SECOR, 2002). Hydrocarbon concentrations have decreased in MW-8MID since groundwater monitoring was initiated. Initial rounds of groundwater sampling indicated that groundwater in the vicinity of MW- 9M/0 and MW-10M/D was impacted with petroleum hydrocarbons. In MW-9D,all gasoline-related compounds and MtBE have been reported below the maximum contaminant level (MCL)since the first quarter of 2003. In MW-10M, all gasoline-related compounds and MtBE have been reported ` }f below the MCLs since the fourth quarter of 2002. In MW-1 OD,all gasoline-related compounds and