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I � i v { � ATC l ` A S 'S O C I A T E S I N C S I Summary Report of Additional Subsurface Investigation Transportation Center/Corporation Yard 1932 El Pinal Drive Stockton, California ATC Project No. 17292.0004 i 1.0 INTRODUCTION ATC Associates Inc. has prepared this report, on behalf of Stockton Unified School District L; (SUSD) to summarize findings of the additional subsurface investigation performed at the District's Transportation Center/Corporation Yard, 1932 El Pinal Drive, Stockton, California (Figure 1). The purpose of the subsurface investigation was to evaluate whether or not petroleum hydrocarbons have migrated through soil and into groundwater. This work was performed at the request of San Joaquin County Public Health Services, Environmental Health Division (PHS/EHD). The findings of the subsurface investigative study are summarized in the following paragraphs. 1.1 Site Location i The site is located on the east side of El Pinal Drive, Stockton, California, as shown on Figure 1. The location is primarily commercial. I 1.2 Background C Remote Fill Pipeline and Diesel Tanks ' C In December 1996, three-diesel underground storage tank (USTs) and an associated remote fill pipeline were removed (see Figure 2). Soil samples were collected from the tank excavation and from a depth of 2 feet below the bottom of the pipeline trench and at 20-foot intervals. Due to staining noted near the southern-most diesel UST, the base of the excavation near the near the tank was overexcavated to a depth of approximately 18 feet below ground surface (bgs), the maximum depth the backhoe could reach. Following sample collection, the excavation was backfilled with clean pea gravel and Class AIB subbase was imported to complete the backfilling of the diesel tank excavation and pipeline trench. w:1172421reportslsumwe1.doc 1 I