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n SITE BACKGROUND CANEPA'S CAR WASH, PARK STREET The Site is located in the downtown business district of Stockton, California One 12,000-gallon unleaded gasoline underground storage tank (UST), one 5,000-gallon leaded gasoline UST, and one 5,000-gallon diesel fuel UST were removed from the Site in 1990 According to WHF Environmental Consultants, Inc (WHF), soil samples collected from the UST excavation from approximately 16 feet below the ground surface (bgs) contained elevated concentrations of gasoline and gasoline constituents Between September 1993 and March 1994, WHF installed six soil borings and seven monitor wells at and around the Site Three of the monitor wells (MW-1, MW-2, and MW-3) were constructed with a screened interval from 45 to 65 feet bgs Monitor wells MW-4 through MW-7 were constructed with a screened interval from 40 to 60 feet bgs Beginning in April 1997, Condor implemented a quarterly groundwater monitoring program at the Site Following the July 1998 quarterly groundwater monitoring event, Mr Ron Rowe, formerly with San Joaquin County Environmental Health Department (SJCEHD) requested the installation of additional monitor wells The tops of the screened intervals of monitor wells MW-1 through MW-7 were submerged below the groundwater table during the period under study Condor prepared a work plan dated November 17, 1998, for the installation of four additional monitor wells (MW-8 through MW-11) In a letter dated September 7, 2001, however, Mr Jeffrey Wong of the SJCEHD requested that an additional work plan be submitted by October 20, 2001 to define the lateral and vertical extent of groundwater contamination and the vertical extent of soil contamination at the Site Mr Wong also requested submittal of a Sensitive Receptor Survey Report for a 2,000-foot radius around the Site by November 10, 2001 In response to Mr Wong's request, Condor submitted the Work Plan for Additional Soil and Groundwater Investigation, dated October 17, 2001 The work plan described several proposed direct push borings intended to investigate the lateral and vertical extent of contamination and assist in the siting of additional monitor wells in a subsequent phase of work, if necessary Mr Wong subsequently approved the work plan, with modifications, in a letter dated October 29, 2001 Site investigation activities pursuant to the above described work plan were performed from March 25 through March 28, 2002, details of the investigation are presented in Condor's Additional Soil and Groundwater Investigation Report, dated May 8, 2002 Investigation activities included the installation of two Cone Penetrometer test (CPT) borings, one soil boring, and twenty-one direct push (hydropunch) borings Laboratory analytical results of the soil samples indicated that relatively elevated concentrations of gasoline constituents were present down to the deepest sample collected (approximately 82 feet bgs) However, the lowest concentrations of gasoline constituents were detected in the sample collected at 82 feet bgs, indicating the potential for some attenuation of concentrations at that depth Methyl tertiary- butyl ether (MTBE) was not detected at or above the laboratory reported detection limit in any of the soil samples In addition, laboratory analytical results from the groundwater sample collected proximal to the soil samples indicated that the gasoline was partitioning into the groundwater phase with some difficulty, probably as a result of the relatively fine-grained soil texture The laboratory analytical results of the hydropunch groundwater samples indicated that relatively elevated concentrations of gasoline constituents were present at depths below the water table both proximal (CPT I) to and down-gradient (CPT2) from the location of the former USTs The results associated with CPT1 indicated an increasing trend of contaminant concentration with depth with benzene concentrations ranging from 65 µg/L at 79 to 83 feet bgs, to 1,900 ptg/ in the deepest sample collected at 119 to 123 feet bgs As opposed to CPT I, the results associated with CPT2 indicated a trend of decreasing groundwater