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uNL1GEll4 ESfI£RSCbxAUNnS <br />' 1.0 PROJECT BACKGROUND <br />' This report presents the results of the fourth quarter of groundwater sampling at The County of <br /> P <br /> San Joaquin Human Services Agency building located at 145 South Sutter Street, Stockton, <br />' California. Figure 1-1 presents sample locations The following section presents a summary <br /> of previous investigations. Appendix A presents historical soil and groundwater analytical <br />' 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 /> I <br /> An initial soil and groundwater investigation (Phase I) was performed at the site by WESTON <br />' <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 II 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 monitonng results 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 <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 Dunng 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 up gradient groundwater analytical <br /> data As part of the planning of the Phase IV investigation San Joaquin County Capitol Projects <br /> . and the San Joaquin County Public Health Services Environmental Health Division (EHD) <br /> L TROJECYS%SANEJOQLSICHSA 4rH 1 July IM <br />