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' 1.0 INTRODUCTION This report summarizes the results of the First Quarter 1998 groundwater monitoring activities conducted on March 16, 1998 at the Stockton Maintenance Station (site, Figure ' 1) The work presented herein was conducted in accordance with Task Order Number 10- 911175-MA and Contract 43YO97 The purpose of this project is to continue quarterly ' groundwater monitoring ' 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 Stockton Maintenance Station "The Stockton Yard was built in approximately 1955 and utilized as a fueling station located within the northwest portion of ' the site It is understood that one 5,000 gallon diesel underground storage tank (UST), one 10,000-gallon unleaded gasoline UST, two pump islands and associated product piping were removed from the site in 1989 Petroleum hydrocarbons were detected in ' excavation soil samples indicating that the UST systems had impacted the surrounding soil Total petroleum hydrocarbons as gasoline and diesel (TPH-G and TPH-D) were reported in sod at concentrations of 2,000 and 58 milligrams per kilogram, respectively " Subsequently, 12 monitoring wells and 2 vapor extraction wells have been installed at the site Groundwater samples collected from selected wells have shown high concentrations of TPH-G, TPH-D, and benzene, toluene, ethylbenzene, and total xylenes ' (BTEX) The approximate locations of the monitoring wells are presented in Figure 2 ' 3.0 GROUNDWATER MONITORING ACTIVITIES 31 GROUNDWATER ELEVATION AND HYDRAULIC GRADIENT On March 16, 1998, static groundwater elevations were measured in monitoring wells MW-1 through MW-12, VW-1 , and VW-2 (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, a groundwater flow direction beneath the site is to the east-southeast with a hydraulic gradient of 0 0018 meter per meter (0 0018 ft /ft } This is consistent with previous measurements ' 1 F Transler�NojecACalty697t8G0131STOCKTO"m198FIN