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i <br /> +M , <br /> 1.0 PROJECT BACKGROUND <br /> This report presents the results of groundwater sampling conducted in May 2001 at the County <br />' of San Joaquin Human Services Agency building located at 145 South Sutter Street, Stockton, <br /> California Figure 1-1 (figures follow the text)presents the monitor well locations The following <br /> section presents a summary of previous investigations Appendix A presents historical <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 />' backfilIed 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 mvestiganon (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 monitoring 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 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 /> 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 /> 1 L1PR0JRM\SAN]OQIHSAI[QD1RPT WPD 1 <br /> V <br />