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Hz}. <br /> Quik Stop Market No. 120 October 9, 1990 <br /> Project No.349120-3I Page I <br /> SOIL AND GROUNDWATER INVESTIGATION <br /> AT <br /> - QUIK STOP MARKET NO. 120 <br /> 9321 NORTH THORNTON ROAD <br /> S-1OCKTON, CALIFORNIA <br /> Exceltech, Inc„ recently completed a soil and groundwater investigation for Quik Stop <br /> -- Markets at 9321 North Thornton Road in the City of Stockton, San Joaquin County, <br /> California (Figure 1). This investigation was conducted as an emergency response of <br /> March 1990,due to the station's leak detection system being in the alarm mode. The scope <br /> of the work included drilling three exploratory borings,collecting soil samples at 5-foot <br /> depth intervals for identification and analysis, converting the borings to groundwater <br /> monitoring wells,collecting groundwater samples for laboratory analysis, and preparing <br /> this report. <br /> BACKGROUND <br /> - Quik Stop Market No. 120 currently utilizes four underground fuel-storage tanks on-site: <br /> two 10,000-gallon tanks, one containing regular leaded gasoline and one containing <br /> unleaded gasoline; and two 8,000-gallon tanks, one containing diesel and the other <br /> premium unleaded gasoline. In 1987, Exceltech installed a vadose monitoring system <br /> around the four tanks at the site, which revealed hydrocarbon contamination. Leaks were <br /> found in the fueling system that were determined to be confined to the product lines. The <br /> lines were then repaired, contaminated soil excavated and removed, and the excavation <br /> i.ry:,_.. <br /> u <br /> bac,c:,�, . <br /> Soil samples collected during the drilling of L%e boreholes for the vadose wells installed at <br /> the site in 1987 were submitted to a laboratory for analysis. Some of these samples,from <br /> the wells located west of the present tank complex, were found to contain petroleum <br /> hydrocarbons. Laboratory analyses of vapor samples collected from vadose wells east of <br /> the existing tank complex detected the occasional presence of petroleum hydrocarbon <br /> compounds. <br /> According to Mr. Deltier of the California Regional Water Quality Control Board <br /> (RWQCB), Central Valley Region, the depth to groundwater in the area of the site is <br /> approximately 50 feet (10 feet below sea level). The Spring 1989 San Joaquin Flood <br /> Control District map stated that the regional hydraulic gradient dips toward the south- <br /> southwest. Mr. Deltier stated that the hydraulic gradient at an adjacent site has been <br /> variable,but most recently dipped to the southeast. <br />