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I I <br /> I <br /> • 10 PROJECT BACKGROUND <br />' This report presents the results of groundwater sampling conducted in April 2002 at the County of San <br />' Joaquin Human Services Agency building located at 145 South Sutter Street,Stockton,California Figure 1-1 <br /> (figures follow the text)presents the monitor well locations The following section presents a summary of <br /> previous investigations Appendix A presents historical groundwater analytical results <br />' In April 1988 WESTON supervised the removal ofn <br /> me underground storage tanks(USTs)from a former <br />' gasoline station located on the northwest corner of Washington and Sutter Streets Petroleum hydrocarbons <br /> were detected in soil samples collected from beneath the tanks Following the excavation of an additional <br /> 2,000 cubic yards of contaminated soil, the pit was backfilled with clean soil <br /> An initial soil and groundwater investigation(Phase 1)was performed at the site by WESTON in December <br />' 1988 Dissolved hydrocarbons were detected in two ofthree monitor wells installed during this investigation <br /> (MW-2 and MW-3) The highest concentrations of petroleum hydrocarbons were detected in monitor well <br /> MW-2 <br /> A Phase II groundwater investigation was completed in August 1991 which consisted ofthe installation of <br />' five additional monitor wells(MW-4 through MW-8),aquifer testing,and a tidal influence study During this <br /> investigation,hydrocarbons were detected in all five new monitor wells Subsequent quarterly groundwater <br />' monitoring results indicated that the highest concentrations ofhydrocarbons were present in down gradient <br /> monitor wells MW-7 and MW-8 <br />' Following the Phase IT investigation,monitor wells MW-2 and MW-3 went dry due to a drop in the regional <br /> groundwater table These wells were replaced with deeper wells(MW-2A and MW-3A)in 1995 during the <br /> Phase III investigation Soil samples were collected from the boreholes ofthe new monitor wells During <br /> the installation of monitor well MW-2A free product was observed in soil and groundwater Petroleum <br /> hydrocarbons were not detected in soil or groundwater samples collected from monitor well MW-3A <br /> A Phase IV investigation was conducted in August 1996 to collect additional soil contaminant concentration <br />' data in the vicuuty ofMW-2A and to collect up gradient groundwater analytical data As part ofthe planning <br /> ofthe Phase IV investigation,San Joaquin County Capitol Projects and the San Joaquin County Public Health <br />' Services Environmental Health Division(EHD)agreed that one year of quarterly groundwater monitoring <br /> would be performed(Table 1-1) It was also agreed that a corrective action plan(CAP)would be prepared <br /> following the completion of one year of quarterly groundwater monitoring(Roy F Weston, Work Plan for <br />' Phase N Site Investigation and Quarterly Groundwater Sampling, February 1996) The Phase IV <br /> • investigation and first round of groundwater sampling confirmed that soil contamination is present in the <br /> Ivicinity ofM W-2A and that groundwater contamination is not present south(up gradient)ofthe former UST's <br /> 41PROJE;CTS\SANJOQQiSA\IQQIRPT WD l <br />