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' s <br /> YNMfJJS OE9G160.Sttll9ATAtt5 <br /> 1.0 PROJECT BACKGROUND <br /> This report presents the results of groundwater samphng conducted in March 1999 at the County <br /> of San Joaquin Human Services Agency building located at 145 South Sutter Street, Stockton, <br /> California. Figure 1-1 presents the monitor well locations. The following section presents a <br /> summary of previous investigations. Appendix A presents historical soil and groundwater <br /> analytical results <br />' In April 1988, WESTON supervised the removal of nine underground storage tanks(USTs) from <br /> a former gasoline station located on the northwest corner of Washington and Sutter Streets <br /> Petroleum hydrocarbons were detected in soil samples collected from beneath the tanks. <br /> Following the excavation of an additional 2,000 cubic yards of contaminated soil, the pit was <br /> backfilled with clean soil <br /> An initial soil and groundwater investigation (Phase I) was performed at the site by WESTON <br /> in December 1988. Dissolved hydrocarbons were detected in two of three monitor wells <br /> installed during this investigation (MW-2 and MW-3) The highest concentrations of petroleum <br /> hydrocarbons were detected in monitor well MW-2 <br /> A Phase H groundwater investigation was completed in August 1991 which consisted of the <br /> installation of five additional monitor wells (MW-4 through MW-8), aquifer testing, and a tidal <br /> influence study During this investigation, hydrocarbons were detected in all five new monitor <br /> wells Subsequent quarterly groundwater monitoring results indicated that the highest <br /> concentrations of hydrocarbons were present in downgradient monitor wells MW-7 and MW-8 <br /> Following the Phase II investigation, monitor wells MW-2 and MW-3 went dry due to a drop <br /> in the regional groundwater table. These wells were replaced with deeper wells (MW-2A and <br />' MW-3A) in 1995 during the Phase III investigation Soil samples were collected from the bore <br /> holes of the new monitor wells During the installation of monitor well MW-2A free product <br /> was observed in soil and groundwater. Petroleum hydrocarbons were not detected in soil or <br /> groundwater samples collected from monitor well MW-3A <br /> A Phase IV investigation was conducted in August 1996 to collect additional soil contaminant <br /> concentration data in the vicinity of MW-2A and to collect upgradient groundwater analytical <br /> data As part of the planning of the Phase IV investigation San Joaquin County Capitol Protects <br /> and the San Joaquin County Public Health Services Environmental Health Division (EHD) <br /> L TR01FaFs,,SAN10QmAI"sA-99 APR 1 Apel 1999 <br />