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1.0 INTRODUCTION <br /> ' This report summarizes the results of the Third Quarter 1997 groundwater monitoring <br /> activities conducted on September 25, 1997 at the Shop 10 Maintenance Station (site, <br /> ' Figure 1) The work presented herein was conducted in accordance with Task Order <br /> Number 10-911175-AO 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 Shop 10 Maintenance Station "It is understood that four underground storage tanks <br /> (USTs) were removed from the site in August 1986 Fourteen on-site and four off-site <br />' borings were subsequently advanced between 1987 and 1991 to determine the extent of <br /> subsurface soil impacts beneath, and adjacent to, the site Total Petroleum <br /> Hydrocarbons as gasoline and diesel (TPH-G and TPH-D) were detected in soil samples <br /> collected from the soil borings at maximum concentrations of 3,800 and 4,500 milligrams <br /> per kilogram (mg/kg), respectively <br /> Between 1988 and 1991 five on-site and three off-site groundwater monitoring wells <br /> (MW-1 through MW-8) were subsequently constructed to evaluate the lateral limits of <br /> petroleum hydrocarbon impacted groundwater " These wells were ultimately abandoned <br /> and replaced by fourteen additional monitoring and/or vapor extraction wells These wells <br /> have been monitored since 1990 with the results indicating elevated concentrations of <br /> TPH-G and benzene, toluene, ethylbenzene, and total xylenes (BTEX) in the <br /> groundwater samples adjacent to the former wells 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 25, 1997, static groundwater elevations were measured in fourteen <br /> monitoring and/or vapor extraction wells (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, the inferred groundwater flow direction beneath <br /> the site is to the east (Figure 2) with a hydraulic gradient of 0 003 meter per meter (0 003 <br /> foot per foot) <br /> F TransferYRepc4riCALTRANSICaity09717t017%gm297 fin <br />