ARCHIVED REPORTS XR0001315 - Laserfiche WebLink

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s., Tins Ms Lori Duncan Page 2 San Joaquin County Environmental Health Department July 26, 2004 California Regional Water Quality Control Board (RWQCB), and the groundwater monitoring wells at the Shell site were abandoned subsequently On February 10, 11, and 12, 1999, four groundwater monitoring wells (MW-1 through MW4) were Installed at the ARCO site under the direction of SECOR International, Inc (SECOR) Analysis of groundwater samples collected from the newly Installed wells Indicated that total petroleum hydrocarbons as gasoline (TPHg) was present in two wells (MW-1 and MW4) Benzene was reported in two wells (MW-1 and MW-4) MtBE was reported in monitoring wells MW-3 and MW4 Concentrations of MtBE have decreased in these wells since monitoring was initiated, suggesting that the source of contamination has been removed In June of 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 The wells were installed to help define the downgradient extent of gasoline-related compounds in site groundwater • In May of 2002, one dual-completion groundwater monitoring well set (MW-SM/D) was installed by SECOR on the western side of Center Street, downgradient from the former ARCO pump islands Two dual-completion groundwater monitoring well sets (MW-9M/D and MW-IOM/D) were installed on the eastern side of Center Street (Figure 2) Ongoing groundwater monitoring indicates that groundwater near MW-8M and 8D is impacted with petroleum hydrocarbons Hydrocarbon concentrations have decreased in MW-8M and 8D since groundwater monitoring was initiated Initial rounds of groundwater sampling indicated that groundwater in the vicinity of MW-9M/D and MW-IOMID also was impacted with petroleum hydrocarbons With the exception of low concentrations of MtBE, all analytes have been reported as below laboratory detection limits since the fourth quarter of 2002 in well MW-9M In MW-9D, all gasoline-related compounds and MtBE have been reported below laboratory detection limits since the first quarter of 2003 In MW-10M, all gasoline-related compounds and MtBE have been reported below laboratory detection limits 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 In November of 2002, URS supervised the installation of dual-completion groundwater monitoring well set MW-I IM/D downgradient from the site Groundwater analytical data suggest that groundwater in the vicinity of MW-11M is impacted with TPHg and MtBE MtBE has been detected in well MW-11D during four of five sampling events since the fourth quarter of 2002, but none of the detections has been above the California Secondary Maximum Contaminant Level (MCL) of 5 nucrograms per liter(p,g/L) K 1WprocessU5 BP ARCO\044931May Well 0414493 well installation report04b doc