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I <br /> 1.0 PROJECT BACKGROUND <br />! This report presents the results of groundwater sampling conducted in October 2001 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. f <br /> i <br /> In April 1988, WESTON supervised 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 /> s <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 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 /> i <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. Two deeper wells (MW-2A and MW-3A)were installed in 1995 during the Phase III <br /> investigation. Soil samples were collected from the bore holes of the new monitor wells. During the <br /> 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. i <br /> i <br /> I^I A Phase IV investigation was conducted in August 1996 to collect additional soil contaminant concentration r <br /> l� 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 i <br /> USTs. The fourth quarter of Phase IV groundwater sampling was completed in May 1997. The CAP was <br /> +l <br /> submitted to EHD in January 1998. <br /> L\PR0JECTS\SANJ0Q\RSA\1Q01 RPT.W PD 1 <br /> F1 <br />