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1.0 PROJECT BACKGROUND This report presents the results of groundwater sampling conducted in December 2004 at the County of San Joaquin Human Services Agency building located at 145 South Sutter Street, Stockton, California Monitoring well locations are presented in Figure 1-1 The following section presents a summary ofprevious ' investigations Appendix A presents historical groundwater analytical results In April 1988,Weston Solutions,Inc (WESTON)supervised the removal of nine underground storage tanks ' (USTs) from a former gasoline station located on the northwest corner of Washington and Sutter Streets Petroleum hydrocarbons were detected in soil samples collected from beneath the tanks Following the excavation of an additional 2,000 cubic yards of contaminated soil,the pit was backfilled with clean soil ' An initial soil and groundwater investigation(Phase 1)was performed at the site by WESTON in December 1988 Dissolved hydrocarbons were detected in two of three monitoring wells installed during this investigation(MW-2 and MW-3) The highest concentrations of petroleum hydrocarbons were detected in ' monitoring well MW-2 A Phase II groundwater investigation was completed in August 1991,and consisted of the installation of five ' additional monitoring wells(MW-4 through MW-8),aquifer testing,and a tidal influence study During this investigation, hydrocarbons were detected in all five new monitoring wells Subsequent quarterly groundwater monitoring results indicated that the highest concentrations of hydrocarbons were present in ' downgradient monitoring wells MW-7 and MW-8 Following the Phase II investigation, monitoring wells MW-2 and MW-3 went dry due to a drop in the regional groundwater table Additional deeper wells(MW-2A and MW-3A)were installed in 1995,during the Phase III investigation MW-3 was abandoned during this investigation Soil samples were collected from the boreholes of the new monitoring wells During the installation of monitoring well MW-2A, free ' product was observed in soil and groundwater Petroleum hydrocarbons were not detected in soil or groundwater samples collected from monitoring well MW-3A ' A Phase IV investigation was conducted in August 1996 to collect additional soil contaminant concentration data in the vicinity of MW-2A and to collect upgradient groundwater analytical data As part of the planning of the Phase IV investigation, San Joaquin County Capital Projects and the San Joaquin County Public ' Health Services Environmental Health Division (EHD) agreed that one year of quarterly groundwater monitoring would be performed according to the schedule presented in Table 1-1 It was also agreed that a corrective action plan (CAP) would be prepared following the completion of one year of quarterly groundwater monitoring (Roy F Weston, Inc,Work Plan for Phase IV Site Investigation and Quarterly ' Groundwater Sampling,February 1996) The Phase IV investigation of groundwater sampling confirmed that soil contamination is present in the vicinity of MW-2A and that groundwater contamination is not present south(upgradient)of the former USTs The fourth quarter of Phase IV groundwater sampling was completed in May 1997 The CAP was submitted to EHD in January 1998 Quarterly groundwater sampling has been conducted since 1996 This report is the Fourth Quarter 2004 ' Groundwater Monitoring Report,which represents the fourth report in the current fiscal year groundwater sampling effort and presents the results of continued groundwater sampling following the schedule presented in Table 1-1 1