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i <br /> 1.0 PROJECT BACKGROUND <br /> This report presents the results ofgroundwater sampling conducted in September 2005 at the County <br /> of San Joaquin Human Services Agency building located at 145 South Sutter Street, Stockton, <br /> California Monitoring well locations are presented in Figure 1-1 The following section presents <br /> ' a summary of previous investigations Appendix A presents historical groundwater analytical <br /> results <br /> ' In April 1988, Weston Solutions, Inc (WESTON) supervised the removal of nine underground <br /> storage tanks(USTs)from a former gasoline station located on the northwest corner of Washington <br /> ' and Sutter Streets Petroleum hydrocarbons were detected in soil samples collected from beneath <br /> the tanks Following the excavation of an additional 2,000 cubic yards of contaminated soil,the pit <br /> was backfilled 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 monitoring wells installed <br /> ' during this investigation(MW-2 and MW-3) The highest concentrations of petroleum hydrocarbons <br /> were detected in monitoring well MW-2 <br /> ' A Phase II groundwater investigation was completed in August 1991, and consisted of the <br /> installation of five additional monitoring wells (MW-4 through MW-8), aquifer testing, and a tidal <br /> influence study During this investigation, hydrocarbons were detected in all five new monitoring <br /> wells Subsequent quarterly groundwater monitoring results indicated that the highest concentrations <br /> of hydrocarbons were present in downgradient monitoring wells MW-7 and MW-8 <br /> ' Following the Phase 11 investigation,monitoring wells MW-2 and MW-3 went dry due to a drop in <br /> the regional groundwater table Additional deeper wells (MW-2A and MW-3A) were installed in <br /> ' 1995, during the Phase III investigation MW-3 was abandoned during this investigation Soil <br /> samples were collected from the boreholes of the new monitoring wells During the installation of <br /> monitoring well MW-2A, free product was observed in soil and groundwater Petroleum <br /> hydrocarbons were not detected in soil or groundwater samples collected from monitoring well MW- <br /> 3A <br /> A Phase IV investigation was wnducted In August 1996 to collect additional soil contaminant <br /> ' concentration data in the vicinity of MW-2A and to collect upgradxent groundwater analytical data <br /> As part of the planning of the Phase IV investigation, San Joaquin County Capital Projects and the <br /> ' San Joaquin County Public Health Services Environmental Health Division(EHD)agreed that one <br /> year of quarterly groundwater monitoring would be performed according to the schedule presented <br /> in Table 1-1 It was also agreed that a corrective action plan (CAP) would be prepared following <br /> ' the completion of one year of quarterly groundwater monitoring(Roy F Weston, Inc,Work Plan for <br /> Phase IV Site Investigation and Quarterly Groundwater Samphng,February 1996) The Phase fV <br /> ' investigation of groundwater sampling confirmed that soil contamination is present in the vicinity <br /> of MW-2A and that groundwater contamination is not present south (upgradxent) of the former <br /> U STs The fourth quarter of Phase TV groundwater sampling was completed in May 1997 The CAP <br /> was submitted to EHD in January 1998. <br /> 1 <br />