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1 <br /> 1.0 INTRODUCTION <br /> ' This report summarizes the results of the First Quarter 1998 groundwater monitoring <br /> activities conducted on March 16, 17, and 18, 1998 at the Shop 10 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 /> The following information was obtained from the Fourth Quarter 1996 groundwater <br /> sampling report prepared by Geocon Geotechnical and Environmental Consultants <br /> (Geocon) for the Shop 10 Maintenance Station "It is understood that four underground <br /> storage tanks (USTs) were removed from the site in August 1986 Fourteen on-site and <br /> four off-site borings were subsequently advanced between 1987 and 1991 to determine <br /> ' the extent of 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 (MW- <br /> 1 through MW-8) were constructed to evaluate the lateral limits of petroleum hydrocarbon <br /> impacted groundwater These wells were ultimately abandoned and replaced by fourteen <br /> additional monitoring and/or vapor extraction wells These wells have been monitored <br /> ' since 1990 with the results indicating elevated concentrations of TPH-G and benzene, <br /> toluene, ethylbenzene, and total xylenes (BTEX) in the groundwater samples adjacent to <br /> the former wells " The approximate locations of the monitoring wells are depicted in Figure <br /> ' 2 <br /> ' 3.0 GROUNDWATER MONITORING ACTIVITIES <br /> 31 GROUNDWATER ELEVATION AND HYDRAULIC GRADIENT <br /> On March 16, 1998, static groundwater elevations were measured in fourteen monitoring <br /> and/or vapor extraction wells (Figure 2) Groundwater depths were measured using a <br /> ' groundwater interface probe A summary of the depth-to-groundwater data collected <br /> during this monitoring event and previous monitoring events is presented in Table 1 <br /> Based on the groundwater data, the inferred groundwater flow direction beneath the site is <br /> to the southeast (Figure 2) with an average hydraulic gradient of 0 0034 meter per meter <br /> (0 0034 foot per foot) <br /> F Transfer%ReporilCALTRANSXCa1ty09717[0171gm198FIN <br />