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1 FlYDROGEOLOGIC ASSESSMENT AND QUARTERLY MONITORING REPORT FORMER ECONO GAS STATION ' 4325 EAST WASHINGTON STREET STOCKPON, CALIFORNIA ' 1.0 INTRODUCTION This report presents the results of Du Pont Environmental Remediation Services' (DERS) subsurface ' exploration at Conoco Inc's (Conoco) former Econo Gas Station located in the City of Stockton, California The investigation site is located at 4325 East Washington Street (see the Location Map, t Figure 1) The site is currently vacant and has been out of service since 1985 The subsurface exploration was performed from September 23, 1991 through October 16, 1991 and was coordinated with Mr Gregory P Fletcher of Conoco (Houston) The DERS field investigation was performed according to the DERS work plan dated July 12, 1991, and according to changes by Conoco The purpose of this assessment was to determine the groundwater gradient, to redefine the occurrence of ' petroleum hydrocarbon concentrations in the groundwater, to better define the horizontal and vertical petroleum hydrocarbon concentrations in the native soils at the subject site, and to install additional vapor extraction wells This study was performed ui response to a request from the San Joaquin County Public Health Services P q q tY Environmental Health Division (PHS/EHD) in their letter dated January 2, 1991, requesting groundwater monitoring at the subject site Our scope of services included a site visit, review of pertinent geologic and hydrogeologic literature and maps, review of previous reports, subsurface ' exploration, laboratory testing, geologic analysis, and the preparation of this report Selected soil samples collected from the exploratory borings during our investigation, and groundwater samples ' collected from all the monitoring wells at the site were sent to Trace .Analysis Laboratory (Hayward) for chemical analyses 20 BACKGROUND ' The former Econo Gas Station is located at the northwest corner of the intersection of Washington and Sinclair Streets in Stockton, California The site's lot size is approximately 108 x 120 feet It is bounded ' on the east by Sinclair Street, on the south by Washington Street, and the north and west by private residences (see the Site Pian Showing Lures of Cross Sections, Figure 2) The only remaining structure on the site is the former cashier's office The pump islands, canopy, and much of the asphalt and ' concrete pavement have been removed The surface is nearly flat at an elevation, of approximately 27 feet above mean sea level To date, a total of 48 wells have been drilled on and near the site to investigate and remediate soil and groundwater contamination Thirty-one of these wells were originally drilled as groundwater monitoring wells, three wells were drilled as "bio" wells, and fourteen wells were drilled as vapor extraction wells 400511 i Rev i Du Pont Environmental Remediation Services ' Pnnted an Recycled Paper