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I <br /> wwceas oes+caexsca+aeruns <br /> WORK PLAN FOR PHASE IV <br />' SITE INVESTIGATION <br /> AND QUARTERLY GROUNDWATER SAMPLING <br />' AT <br /> SAN JOAQUIN COUNTY HUMAN SERVICES ADMINISTRATION <br />' 145 SOUTH SUTTER STREET <br /> STOCKTON, CALIFORNIA <br /> 1.0 INTRODUCTION <br />' This work plan and quality assurance plan presents the procedures for additional site assessment <br /> activities at the San Joaquin County Human Services Administration building located at 145 <br />' South Sutter Street, Stockton, CA The purpose of the Phase IV investigation is to obtain <br /> analytical data on groundwater upgradient of the site and to determine the extent of soil <br /> contamination in the vicinity of monitor well MW-2A The purpose of the quarterly <br /> groundwater sampling is to obtain groundwater contaminant concentration trends over time <br /> Data generated from the Phase IV investigation and the quarterly groundwater monitoring will <br /> be used to develop a corrective action plan <br />' 1.1 SITE BACKGROUND <br />' In April 1988, WESTON supervised the removal of nine underground storage tanks (USTs) from <br /> a former gasoline station located on the northwest corner of Washington and Sutter Streets <br /> Petroleum hydrocarbons were detected in soil samples collected from beneath the tanks <br /> Following the excavation of an additional 2,000 cubic yards of contaminated soil, the pit was <br /> backfilled with clean soil <br /> An initial soil and groundwater investigation (Phase I) was performed at the site by WESTON <br /> in late 1988 Dissolved hydrocarbons were detected in two of three monitor wells installed <br /> during this investigation (MW-2 and MW-3) The highest concentrations of hydrocarbons were <br /> detected in groundwater samples collected from monitor well MW-2 <br /> A Phase II groundwater investigation was completed in August 1991 which consisted of the <br /> installation of five additional monitor wells (MW-4 through MW-8), aquifer testing, and a tidal <br /> . influence study During this investigation, hydrocarbons were detected in all five new monitor <br /> IL%PR6JWr=AN10QWAWP 1 <br />