SITE INFORMATION AND CORRESPONDENCE

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Kennedy/Jenks Consultants SUPPLEMENTAL SITE INVESTIGATION WORKPLAN 4555 NORTH PERSHING AVENUE STOCKTON, CALIFORNIA 1. INTRODUCTION The following Supplemental Site Investigation Workplan describes the proposed soil vapor and groundwater sampling to be performed in the general vicinity of the former Cardlock Gas Station Site at 4555 North Pershing Avenue, Stockton, California. The purpose of this investigation is to assess the possible off site migration of petroleum hydrocarbons, and to determine whether further off site investigations should be performed. The proposed sampling program includes collection of soil vapor samples from a total of seven sampling sites, five discrete sampling locations east of the site, and two locations on site (See Figure 1). Soil vapor samples will be collected and analyzed from depths of 16, 21, and 26 feet below the ground surface at each sampling location. Soil samples will be collected from each borehole and held pending the results of the mobile laboratory's soil vapor analyses. A grab groundwater sample will also be collected and analyzed from each sampling point. The proposed work is described in more detail in Section 3 of this workplan. 2. BACKGROUND The former Cardlock Service Station located at 4555 N. Pershing Avenue in Stockton sold gasoline and diesel fuel from 1979 until May of 1990. The site is approximately 140 feet long and 65 feet wide, and is bordered on the south by Rose Marie Lane, on the east by a pet hospital, on the north by the Venetian Square Shopping Center, and on the west by some retail businesses. The station opened in 1979 with three 12,000-gallon fuel tanks, one for diesel, one for leaded gasoline, and one for unleaded gasoline. In May 1987, the diesel tank and one of the gasoline tanks failed a precision tank test. In November 1989, the diesel tank again failed a precision tank test. The first site investigation was performed in May and June of 1991. The investigation consisted of a review of available agency records, and collection of soil samples from nine soil borings across the site. Analysis of soil samples indicated detectable petroleum hydro- carbons at depths of 21 to 27 feet across the site. In January 1992, the fuel tanks were removed from the site. The soil sampling results indicated relatively high petroleum hydrocarbon concentrations in the sample collected from beneath the southern end of the northernmost tank (the unleaded gasoline tank) at a depth of approximately 16 feet. The total petroleum hydrocarbon (TPH) as gasoline concentration in the sample was 785 parts per million (ppm). Petroleum hydrocarbons were not detected in samples collected from beneath the diesel and leaded gasoline tanks, nor from beneath the north end of the unleaded tank. g:Vobsk940549%ewkOan.doc 1