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t <br /> 1 0 INTRODUCTION <br /> This report summarizes the results of the Second Quarter 1998 groundwater monitoring <br /> activities conducted on June 17 and 26, 1998 at the Stockton Maintenance Station (site, <br /> Figure 1) The work presented herein was conducted in accordance with Task Order <br /> Number 10-911175-MA and Contract 43Y097 The purpose of this project is to continue <br /> quarterly sampling requirements <br /> 2.0 SITE HISTORY <br /> The following information was obtained from the Fourth Quarter groundwater sampling <br /> report prepared by Geocon Geotechnical and Environmental Consultants (Geocon) for <br /> the Stockton Maintenance Station "The Stockton Yard was built in approximately 1955 <br /> and utilized as a fueling station located within the northwest portion of the site It is <br /> understood that one 5,000 gallon diesel underground storage tank (UST), one 10,000- <br /> gallon unleaded gasoline UST, two pump islands and associated product piping were <br /> removed from the site in 1989 Petroleum hydrocarbons were detected in excavation soil <br /> samples indicating that the UST systems had impacted the surrounding soil Total <br /> } Petroleum Hydrocarbons as Gasoline and Diesel (TPH-G and TPH-D) were reported in <br /> soil at concentrations of 2,000 and 58 milligrams per kilogram, respectively " <br /> Subsequently, 12 monitoring wells and 2 vapor extraction wells have been installed at the <br /> site Groundwater samples collected from selected wells have shown high <br /> concentrations of TPH-G, TPH-D, and benzene toluene, ethylbenzene, and total xylenes <br /> (BTEX) The approximate locations of the monitoring wells are depicted in Figure 2 <br /> 3 0 GROUNDWATER MONITORING ACTIVITIES <br /> 31 GROUNDWATER ELEVATION AND HYDRAULIC GRADIENT <br /> On r measured r <br /> 4 June 17, 1998, static groundwater elevations were measu a �n monitoring wells MW- <br /> 1 through MW-12, VW-11, and VW-2 (Figure 2) Groundwater depths were measured <br /> using a groundwater interface probe A summary of the depth-to-groundwater data <br /> collected during this monitoring event and previous monitoring events is presented in <br /> Table 1 The groundwater flow direction is to the east with a hydraulic gradient of 0 001 <br /> meter per meter (0 001 foot per foot) <br /> F TransfeAReportlCALTRANS\CaIry097710171STOCKTON1gm299fin 1 <br />