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1 0 INTRODUCTION <br /> This report summarizes the results of the Fourth Quarter 1998 groundwater monitoring <br /> ' activities conducted on December 2 and 3, 1998 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-MA and Contract 43YO97 The purpose of this project is to perform <br /> ' quarterly groundwater sampling <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 bonngs 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 presented in <br /> Figure 2 <br /> On August 3, 1998 Wells VW-1 and MW-26 were abandoned because they were located <br /> within the footprint of a future maintenance building at the site Under supervision of PSI <br />' personnel, V & W Drilling of Rio Vista, Califomia overdrilled the wells using a hollow-stem <br /> auger The wells were grouted to grade with a portland cement/bentonite mixture <br /> 1 <br /> 0 \TMI3PER\PRO.l=\GALT097\7I 017\SJOPIO\G"498 PIN Orr' <br />