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r 1 0 INTRODUCTION This report summarizes the results of the Second Quarter 1997 groundwater monitoring ' activities conducted on May 27, 28, 29, 1997 at the Stockton 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 project is to continue ' quarterly sampling requirements ' 2 0 SITE HISTORY The following information was obtained from the fourth quarter 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 runderstood 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 soil 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 depicted in Figure 2 ' 3 0 GROUNDWATER MONITORING ACTIVITIES ' 31 GROUNDWATER ELEVATION AND HYDRAULIC GRADIENT On May 27, 1997, 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, the inferred groundwater flow direction beneath the site is to the east (Figure 2) with a hydraulic gradient of 0 001 meter per meter (0 001 foot per foot) t r Transier\Report\CALTRANSICs11y09T716171gm297 lin