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r <br /> 1.0 INTRODUCTION <br /> This report summarizes the results of the Fourth Quarter 1997 groundwater monitoring <br /> r activities conducted on December 29, 1997 at the Stockton Maintenance Station (site, <br /> Figure 1) The work presented herein was conducted in accordance with Task Order <br /> Number 10-911175-AO and Contract 43YO97 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 soil <br /> at concentrations of 2,000 and 58 milligrams per kilogram, respectively " Subsequently, <br /> 12 monitoring wells and 2 vapor extraction wells have been installed at the site <br /> Groundwater samples collected from selected wells have shown high concentrations of <br />' TPH-G, TPH-D, and benzene, toluene, ethylbenzene, and total xylenes (BTEX) The <br /> 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 December 29, 1997, static groundwater elevations were measured in monitoring wells <br /> MW-1 through MW-12, VW-1, 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 Based on the groundwater data, a groundwater flow direction beneath the site <br /> could not be inferred Previous quarter monitoring shows the flow direction to be to the <br />' east with a hydraulic gradient of 0 001 meter per meter (0 001 foot per foot) <br /> Pei 1 <br />' F 7ransferlReportYCALTRANSTa11yO97571017%gm497tm <br /> i <br />