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1.0 PROJECT BACKGROUND <br /> This report presents the results of groundwater sampling conducted in February 2001 at the <br /> County of San Joaquin Human Services Agency building located at 145 South Sutter Street, <br /> Stockton, California. Figure 1-1 <br /> g (figures follow the text) presents the monitor well locations. <br /> k The following section presents a summary of previous investigations. Appendix A presents <br /> historical groundwater analytical results. <br /> In April 1988, WESTON supervised the removal of nine underground storage ge 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 in <br /> December 1988. Dissolved hydrocarbons were detected in two of three monitor wells installed <br /> 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 11 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 monitoringresults is indicated that the highest <br /> concentrations of hydrocarbons were present in down gradient 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 in <br /> the regional groundwater table. These wells were replaced with deeper wells (MW-2A and MW- <br /> 3A) in 1995 during the Phase III investigation. Soil samples were collected from the bore holes <br /> of the new monitor wells. During the installation of monitor well MW-2A free product was <br /> observed in soil and groundwater. Petroleum hydrocarbons were not detected in soil or <br /> r 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 up gradient groundwater analytical <br /> data. As part of the planning of the Phase IV investigation, San Joaquin County Capitol Projects <br /> L\PR0JECTSISAN30Q\1iW1Q01 RPr.WPp 1 <br />