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i <br /> MX4GEf5 OU". <br /> 1.0 PROJECT BACKGROUND <br /> This report presents the results of the second quarter of groundwater sampling at The County <br /> of San Joaquin Human Services Agency building located at 145 South Sutter Street, Stockton, <br /> California Tables 1-1 and 1-2 present historical soil and groundwater analytical results Figure <br /> 1-1 presents sample locations The following section presents a summary of previous <br /> investigations <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 /> IPetroleum 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 /> ibackfilled with clean soil <br /> An initial soil and groundwater investigation (Phase I) was performed at the site by WESTON <br /> iin 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 /> iwells 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 /> i Followingthe Phase II investigation, monitor wells MW-2 and MW-3 went d due to a drop <br /> g dry P <br /> i 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 /> iwas observed in soil and groundwater Petroleum hydrocarbons were not detected in soil or <br /> groundwater samples collected from monitor well MW-3A <br /> i A Ph IV investigation was conducted in August 1996 to collect additional soil contaminant <br /> Phase g g <br /> i 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 /> i and the San Joaquin County Public Health Services Environmental Health Division (EHD) <br /> agreed that one year of quarterly groundwater monitoring would be performed (Table 1-1) It <br /> iL iE-M.WCMCHSA 2N❑ 1 lmmry 1997 <br />