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• INTRODUCTION GHH Engineering, Inc. (GHH) 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 Figure 1. Mr. Jay McIlrath has authorized GHH to prepare this Third Quarter 2003 Groundwater Monitoring Report (QMR) for the site. This report has been prepared for submittal to the San Joaquin County Environmental Health Department (County), the lead regulatory agency overseeing environmental issues at the site. This QMR documents the September 11, 2003 groundwater monitoring event. BACKGROUND AND SITE HISTORY Site Description The site is currently an Exxon retail gasoline service station that is located on the northwest corner of East Main Street and Oro Avenue at 4075 East Main Street, Stockton, CA. A rectangular off ce/store'building is located near the center of the property. The property is asphalt paved except in the area of the pump dispenser islands where it is paved with concrete, and the northern portion of the site where there is a strip of dirt. The site contains one underground storage tank (UST) and two dispenser islands as shown in 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) located behind the store building to the north and a 500 gallon underground kerosene tank were removed in 1995. The waste oil tank had reportedly been filled with diesel fuel for testing purposes approximately three times since installation. Three previous UST's that stored unleaded regular, leaded regular, and unleaded premium gasoline were removed in 1998. The location of the former waste oil tank and the previous UST's are shown on Figure 3. Geology and Hydrogeology 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. The subject site lies at an elevation of approximately 29 feet above Mean Sea Level (MSL).