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1.0 INTRODUCTION GHH Engineering, Inc. (GHH) is currently providing USA Gasoline Corporation (USA) professional engineering services to conduct environmental monitoring and remediation at their retail gasoline station#65 located at 2500 West Lodi Avenue, Lodi, California, as shown on Figures 1 and 2. Mr. Srikanth Dasappa of USA has authorized GHH to prepare this "Third Quarter, 1998, Groundwater Monitoring Report" (QMR) for the site. Investigations and ongoing monitoring and sampling activities conducted at the site are under the direction of the San Joaquin County Public Health Services, Environmental Health Division (County). 2.0 BACKGROUND AND SITE HISTORY A brief discussion of project background and history is presented in the following section. 2.1 Site Description The site is located at 2500 West Lodi Avenue, in the City of Lodi, California, and consists of a retail gasoline station and car wash. The site borders a shopping center/parking area to the west and south, West Lodi Avenue to the north and South Main Canal and Lower Sacramento Road to the east. Three 12,000-gallon underground storage tanks (USTs) and three fuel dispensing islands are present at the site. The tanks and dispensers store and dispense regular, plus and premium grade unleaded gasolines and diesel, respectively. One of the USTs is separated into two 6,000/6,000- gallon compartments, which store premium unleaded gasoline and diesel. The surface elevation of the site is approximately 37 to 38-feet above mean sea level (MSL). The locations of the USTs and fuel dispensing islands are shown on Figure 3. 2.2 Geology and Regional Hydrogeology The site is located within the south central portion of the Sacramento Valley. The valley is a broad northwest trending structural trough bounded by the Sierra Nevada to the east, the coast range to the west, the cascades to the north and the Transverse Range to the south. The site is underlain by the Pleistocene Modesto Formation. Deposits of the Modesto Formation were laid down as a series of coalescing alluvial fans and consist mostly of gravel, sand and silt. According to March and Allwardt (1977), numerous channels are present within the Modesto Formation that .--result in rapid lateral changes in.texture. The site is in the Western San Joaquin County Groundwater Basin. The regional groundwater gradient in the Eastern San Joaquin Groundwater Basin is to the southwest (California Department of Water Resources, 1980). The Mokelumne River is approximately 1-mile north of the site. The South Main Canal on the eastern edge of the property is used to seasonally transport irrigation water from the Mokelumne River. Water in the canal flows to the south in the vicinity of the site.