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- - - - <br /> 1.0 INTRODUCTION <br /> This report summarizes the results of the Fourth Quarter 1998 groundwater monitoring <br /> activities conducted on December 2, 1998 at the Stockton Maintenance Station located at <br /> 1504 South B Street, Stockton, California (site; Figure 1). The work presented herein was <br /> conducted in accordance with Task Order Number 10-911175-MA and Contract 43Y097. <br /> The purpose of this project is to perform quarterly sampling requirements. <br /> 2.0 SITE HISTORY <br /> =i The following information was obtained from the Fourth Quarter groundwater sampling <br /> report prepared by Geocon Geotechnical and Environmental Consultants for the Stockton <br /> Maintenance Station. "The Stockton Yard was built in approximately 1955. A fueling <br /> -` station was located in the northwest portion of the site. It is understood that one 5,000 <br /> gallon diesel underground storage tank (UST), one 10,000-gallon unleaded gasoline UST, <br /> Y vi two pump islands, and associated product piping were removed from the site in 1989. <br /> Petroleum hydrocarbons were detected in excavation soil samples indicating that the UST <br /> systems had impacted the surrounding soil. Total Petroleum Hydrocarbons as Gasoline <br /> and Diesel (TPH-G and TPH-D) were reported in soil at concentrations of 2,000 and 58 <br /> milligrams per kilogram, respectively." <br /> Subsequently, 12 monitoring wells and 2 vapor extraction wells were installed at the site. <br /> � Groundwater samples collected from selected wells have shown high concentrations of <br /> TPH-G; TPH-D; and Benzene, Toluene, Ethylbenzene, and total Xylenes (BTEX). The <br /> approximate locations of the monitoring wells are presented in Figure 2. <br /> r; 3.0 GROUNDWATER MONITORING ACTIVITIES <br /> 3.1 GROUNDWATER ELEVATION AND HYDRAULIC GRADIENT <br /> n <br /> On December 2, 1998, static groundwater elevations were measured in monitoring wells <br /> MW-1 through MW-12, VW-1, and VW-2. Groundwater depths were measured using a <br /> =' groundwater interface probe. The average groundwater elevation increased 0.57 meters <br /> (1.85 feet) compared to last quarter. A summary of the depth to groundwater data <br /> collected during this monitoring event and previous monitoring events is presented in <br /> x Table 1. The well and groundwater elevation data is presented in Figure 2. The <br /> groundwater flow direction is to the east with a hydraulic gradient of 0.001 meter per meter <br /> (0.001 foot per foot). <br /> i � <br /> t; F:TransfE!AReporllCALTRANSkCaIlyO97171O171STOCKTONlgm498fin <br /> �'1 <br />