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} INTRODUCTION GHH Engineering, Inc. (G�}is currently providing JAMAR Oil Distribution JAMAR) professional engineering services to assist them with environmental issues pertaining to their site located at 4075 East Main Street, Stockton, California, as shown on Figures 1 and 2. Mr. Jay McIlrath has authorized GHH to prepare this First Quarter, 2003, Groundwater Monitoring Report (QMR). This QMR presents the results of the quarterly groundwater monitoring and sampling completed at the site on March 13, 2003. This report has been prepared for submittal to the San Joaquin Environmental Health Department (County), the lead regulatory agency overseeing environmental issues at the site. BACKGROUND AND SITE HISTORY Site Description The site is currently an Exxon retail gasoline service station that is located on the northwest comer of East Main Street and Oro Avenue at 4075 East Main Street, Stockton, CA. The property houses one rectangular-shaped office/store building. The property is asphalt paved except in the area of the dispenser islands where it is paved with concrete. There is also a strip of dirt surface on the northern portion of the site. The site contains one underground storage tank (UST) and two dispenser islands as shown on Figure 2. The UST is a split 20,000 gallon tank that stores 12,000 gallons regular, unleaded gasoline and 8,000 gallons premium, unleaded gasoline. No diesel fuel tanks have been located on this site. An underground waste oil tank(300 gallon) was located behind the store building to the north and it was removed in 1995. It has been reported that this waste oil tank had been filled with diesel fuel for testing purposes approximately three times since it was installed. There were also three previous UST's that stored unleaded regular, leaded regular, and unleaded premium gasoline. These three UST's were removed in 1998. The location of the former waste oil tank and the previous UST's are shown on Figure 3. Geology and Hydrogeulogy Geologically, the property is underlain by the Quaternary Modesto Formation and the deeper Quaternary Riverbank formation, both of which consist mainly of arkosic alluvium derived predominantly from erosion of the Sierra.Nevada Mountains to the east. Both the Modesto and Riverbank formations are part of the Great Valley sedimentary sequence of rocks which fill the large Great Valley sedimentary basin which was originally formed as a forearc basin between.the Sierran arc to the east and the subduction zone to the west. Stockton is located within the San Joaquin Valley which consists of the southern two thirds of the Great Valley province. The northwesterly flowing San Joaquin River and its tributaries are the main drainage feature of the San Joaquin Valley. According to the USGS Stockton East, 7.5 Minute Quadrangle, which covers the subject site, the closest surface water body is Mormon Slough, which lies approximately 1400-feet south- southeast of the subject site and is crossed by Oro Avenue. Past quarterly monitoring at the subject site has shown the direction of groundwater flow to be south-southeast.