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1.0 INTRODUCTION <br /> This report summarizes the results of the Third Quarter 1998 groundwater monitoring <br /> activities conducted on September 28 and 29, 1998 at the Stockton Maintenance Station <br /> (site, Figure 1) The work presented herein was conducted in accordance with Task Order <br /> Number 10-911175-MA and Contract 43YO97 The purpose of this project is to continue <br /> quarterly sampling requirements <br /> 2.0 SITE HISTORY <br /> //1j W` <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 maintenance facility was built in <br /> approximately 1955 A fueling station was located at the northwest portion of the site It <br /> is understood that one 5,000 gallon diesel underground storage tank (UST), one 10,000- <br /> gallon unleaded gasoline UST, two fuel dispensing islands, and associated product piping <br /> were removed from the site in 1989 Petroleum hydrocarbons were detected in <br /> excavation soil samples indicating that the UST systems had impacted the surrounding <br /> soil Total Petroleum Hydrocarbons as Gasoline and Diesel (TPH-G and TPH-D) were <br /> reported in soil at concentrations of 2,000 and 58 milligrams per kilogram, respectively " <br /> To investigate soil and groundwater quality, 12 groundwater monitoring wells and 2 soil <br /> vapor extraction wells were installed at the site Groundwater samples collected from <br /> selected wells have shown high concentrations of TPH-G, TPH-D, and Benzene, <br /> Toluene, Ethylbenzene, and total Xylenes (BTEX) The approximate locations of the <br /> monitoring wells are depicted in Figure 2 <br /> 3 0 GROUNDWATER MONITORING ACTIVITIES <br /> 31 GROUNDWATER ELEVATION AND HYDRAULIC GRADIENT <br /> On September 28, 1998, static groundwater elevations were measured in monitoring <br /> wells MW-1 through MW-12, VW-1, and VW-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 00076 <br /> meter per meter (0 00076 foot per foot) <br /> F TransfeARepert\CALTRANSNCalty09A7iO17lSTOCKTON4gm398fin <br />