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WNMiEAS oESG'�Sagtul,,,W15 <br /> • 1.0 PROJECT BACKGR0i1ND <br />' This report presents the results of groundwater sampling conducted in December 1998 at the <br />' County of San Joaquin Human Services Agency building located at 145 South Sutter Street, <br /> Stockton, California Figure 1-1 presents the monitor well locations The following section <br /> presents a summary of previous investigations Appendix A presents historical soil and <br />' groundwater 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 /> �r 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 /> I wells Subsequent quarterly groundwater monitoring results indicated that the highest <br /> concentrations of hydrocarbons were present in down gradient monitor wells MW-7 and MW-8 <br /> IFollowing the Phase Il 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 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 ftOJ$CTMANJOQU{SAUISA_99 JAN 1 Jaasry 1999 <br />