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Section 1 INTRODUCTION PURPOSE This report presents the results of a site investigation performed at Pacific Gas and Electric Company's (PG&E's) Lathrop Gas Dehydrator Station which is located in San Joaquin County, about 6 miles northeast of the city of Tracy,near the intersection of Undine and Willow Glen Roads (Figure 1). The purpose of this investigation is to further define the vertical and lateral extent of petroleum hydrocarbons in the subsurface soils at the site and to determine if groundwater in the uppermost water-bearing unit has been impacted. This investigation was requested by PG&E's Gas Production and Storage Department and was coordinated with San Joaquin County Public Health Services Department, Environmental Health Division (SJCPHSD). The scope of work for this study included the following field activities: advancing and sampling nine soil borings within the site boundaries and on a parcel adjacent to the site,the collection of in-situ groundwater samples from each soil boring,the installation and sampling of one groundwater monitoring well, chemical analyses of water and selected soil samples, measurements of depth to water in each boring, and a well head elevation survey to assist in determining groundwater flow direction. In addition to these tasks, available public records maintained by the State Department of Water Resources were reviewed to identify groundwater wells within a one-mile radius of the site and available geologic literature was evaluated to define subsurface conditions. All field methods, data, and information are presented in this report. BACKGROUND In the spring of 1988, during the construction of a foundation pad for a new reboiler unit at the Lathrop Gas Dehydrator Station,petroleum hydrocarbon odors were detected in the excavated soil removed from the area surrounding the reboiler's water collection sump and associated discharge line. As a result, PG&E removed approximately 100 cubic yards of soil from around the affected area. The soil was subsequently manifested and hauled to the Class I disposal facility in Kettleman Hills. On March 11, 1988, Twining Laboratories collected three soil samples (S-1, S-2, and S-3) from the excavation and one water sample(S-4)from the reboiler discharge line (Figure 2). These samples were analyzed by Twining Laboratories for total petroleum hydrocarbons, and benzene,toluene and xylenes. cca/08/19/93/9449arpt.doc/mkn4 1-1