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ARCADIS GERAGHTY&MILLER In the opinion of ARCADIS Geraghty & Miller, previous work by UNOCAL's consultants has resulted in delineation of the lateral extent of groundwater impact in the shallow zone. The scope of work proposed herein is designed to delineate the vertical extent of site groundwater impacts and to finalize the database upon which a subsequent remedial strategy will be developed and a corrective action plan work plan written. OBJECTIVES The objectives of the limited subsurface investigation are to: 1. Vertically delineate the dissolved-phase petroleum hydrocarbon plume downgradient of wells with elevated concentrations of dissolved-phase petroleum hydrocarbons; and 2. Evaluate the vertical groundwater gradient. Completion of these objectives is expected to provide sufficient data to support completion of the delineation of the vertical and lateral extent of impact to groundwater and to form the basis for development of a corrective action plan. SITE BACKGROUND AND SETTING The site is a vacant lot situated on the northwest corner of East Miner Avenue and California Street(Figure 1)in the City of Stockton, California. The site is situated in an area of retail stores and hotels. Topography in the site vicinity is flat-lying, at an elevation of approximately 15 feet above mean sea level (MSL). McLeod Lake and the Stockton Deep Water Channel are situated approximately one-half mile to the west. The site was an operating UNOCAL Service Station from 1922 until 1992. Two 1,000-gallon gasoline and one 550-gallon waste oil underground storage tanks (USTs) were removed from the southeast comer of the subject site in 1967 (collectively referred to hereafter as "the 1967 tanks"). Data related to soil testing at the time of UST removal are not available. These USTs were replaced with two 10,000-gallon gasoline and one 280-gallon waste oil USTs (collectively referred to hereafter as the "the 1992 tanks") situated in a common excavation in front of the station building. The most recent service station consisted of a three-bay service station building,three product dispenser islands (D-1 through D-3), and two gasoline and one waste oil USTs in a common excavation. The site is currently an unpaved vacant lot. Pertinent features of the former service station are shown on the Site Plan(Figure 1).