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S E C 0 R Ms Lori Duncan ARCO Station 4493 Site Closure Report . July 31,2006 Page 2 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 TPH-g 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 sod concentrations of 1 52 mg/kg (MW-4 at 25 feet bgs) Lead had reported maximum soil concentrations of 141 mglkg in sod samples collected at MW-2 at 10 feet bgs (SECOR, 1999b) Soil analytical results are presented in Attachment A Analysis of groundwater samples collected from the newly installed wells indicated that total petroleum hydrocarbons as gasoline(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 Table 1 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 concentrations of hydrocarbons were reported soil samples from wells MW-5 through MW-7 Soil • analytical data is presented in Attachment Benzene, cis-1,2-dichloroethene,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 well 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 sod samples were collected during the drilling of MW-8MID 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 sod 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 sod 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 limit TPHg, 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) Ongoing groundwater monitoring indicates that groundwater near MW-8MID was impacted with petroleum hydrocarbons Hydrocarbon concentrations have decreased in MW-8M/D since groundwater monitoring was initiated Initial rounds of groundwater sampling indicated that groundwater In the vicinity of MW-9M/D 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 below the MCLS since the fourth quarter of 2002 In MW-10D, all gasoline-related compounds and MtBE have been reported below laboratory detection limits since the second quarter of 2003 (SECOR, 2002) }