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1 ; <br /> 1 <br /> 10 PROJECT BACKGROUND <br />' This report r <br /> p presents the results of groundwater sampling conducted in February 2002 at the County of San <br />' Joaquin Human Services Agency building located at 145 South Sutter Street, Stockton, California Figure <br /> 1-1 (figures follow the text)presents the monitor well locations The following section presents a summary <br /> of previous investigations Appendix A presents historical groundwater analytical results <br />' In April 1988 WESTON supervised p p wised the removal of nine 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 androundwater investigation vestigation(Phase I)was performed at the site by WESTON in December <br />' 1988 Dissolved hydrocarbons were detected in two of three 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 of the installation of <br />' five additional monitor wells (MW-4 through MW-8), aquifer testing, and a tidal influence study During <br /> this investigation, hydrocarbons were detected in all five new monitor wells Subsequent quarterly <br /> groundwater monitoring results indicated that the highest concentrations of hydrocarbons were present in <br /> 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 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 bore holes of the 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 /> fA Phase IV investigation was conducted in August 1996 to collect additional soil contaminant concentration <br /> data in the vicinity of MW-2A and to collect up gradient groundwater analytical data As part of the planning <br /> of the Phase IV investigation, San Joaquin County Capitol Projects and the San Joaquin County Public <br /> Health Services Environmental Health Division (EHD) agreed that one year of quarterly groundwater <br /> monitoring would be performed(Table 1-1) It was also agreed that a corrective action plan (CAP)would <br /> be prepared following the completion of one year of quarterly groundwater monitoring(Roy F Weston, Work <br /> Plan for Phase IV 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 /> . vicinity of MW-2A and that groundwater contamination is not present south (up gradient) of the former <br /> USTs The fourth quarter of Phase IV groundwater sampling was completed in May 1997 The CAP was <br /> submitted to EHD in January 1998 <br /> UPR0JECMSAN10Q1HSAUQ01RPT WPD 1 <br />