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1.0 INTRODUCTION ' This report summarizes the results of the Fourth Quarter 1997 groundwater monitoring activities conducted on December 29, 1997 at the Shop 10 Maintenance Station (site, Figure 1) The work presented herein was conducted in accordance with Task Order ' Number 10-911175-AO and Contract 43YO97 The purpose of this protect is to continue quarterly sampling requirements 2 0 SITE HISTORY The following information was obtained from the fourth quarter 1997 groundwater sampling report prepared by Geocon Geotechnical and Environmental Consultants ' (Geocon) for the Shop 10 Maintenance Station "It is understood that four underground storage tanks (USTs) were removed from the site in August 1986 Fourteen on-site and four off-site borings were subsequently advanced between 1987 and 1991 to determine ' the extent of subsurface soil impacts beneath, and adjacent to, the site Total Petroleum Hydrocarbons as gasoline and diesel (TPH-G and TPH-D) were detected in soil samples collected from the soil borings at maximum concentrations of 3,800 and 4,500 milligrams per kilogram (mg/kg), respectively Between 1988 and 1991 five on-site and three off-site groundwater monitoring wells (MW- 1 through MW-8) were subsequently constructed to evaluate the lateral limits of petroleum hydrocarbon impacted groundwater These wells were ultimately abandoned and replaced by fourteen additional monitoring and/or vapor extraction wells These wells have been ' monitored since 1990 with the results indicating elevated concentrations of TPH-G and benzene, toluene, ethylbenzene, and total xylenes (BTEX) in the groundwater samples adjacent to the former wells " The approximate locations of the monitoring wells are ' depicted in Figure 2 ' 3 0 GROUNDWATER MONITORING ACTIVITIES 31 GROUNDWATER ELEVATION AND HYDRAULIC GRADIENT On December 29, 1997, static groundwater elevations were measured in fourteen monitoring and/or vapor extraction wells (Figure 2) Groundwater depths were measured ' using a groundwater interface probe A summary of the depth-to-groundwater data collected during this monitoring event and previous monitoring events is presented in Table 1 Based on the groundwater data, the inferred groundwater flow direction beneath ' the site is to the east (Figure 2) with an average hydraulic gradient of 0 0018 meter per meter (0 0018 foot per foot) 1 F Transfer%RepemCALTRANSICa"ty )7101NM497FIN