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_ -_ <br /> SEEM <br /> Ln KLEINFELOER <br /> 2 BACKGROUND <br /> The San Joaquin County Motor Pool, is located at 444 South Wilson Way, Stockton, <br /> California(PIate 1,Vicinity Map). The facility operated two underground waste oil storage <br /> tanks at two locations on the site. A 1000 gallon tank was located south of the tire shop <br /> and a 500 gallon tank was located north of the auto repair shop. The locations of the tanks <br /> and buildings are shown on the Site Map, Plate 2. <br /> - In May 1988, the two USTs were excavated and removed from the site by Thorpe Oil <br /> Company of Lodi,California. Petroleum hydrocarbon concentrations were detected in soil <br /> samples collected from the native soil at the bottom of the two excavations. The samples <br /> were collected and analyzed by Canonie Environmental of Stockton, California. The <br /> SJCEHD requested an assessment of the lateral and vertical extent of contamination in the <br /> w vicinity of the excavations. <br /> In November 1989, three soil borings were drilled around each of the two USTs at the San <br /> Joaquin County Motor Pool to assess the potential presence of petroleum hydrocarbons in <br /> the soil. Results of the soil investigation were presented in Kleinfelder's report "San <br /> - Joaquin Motor Pool, Soil Assessment Report, Stockton, California," dated December 22, <br /> t ' <br /> 1989. TPH and volatile aromatic concentrations were not detected in the bottom samples <br /> from three borings drilled around the former 1000-gallon UST. Petroleum hydrocarbons <br /> concentrations were detected at a depth of 11.5 feet in one of three borings (13-5) drilled <br /> around the former 500-gallon UST. The petroleum hydrocarbons were detected as diesel <br /> (53 mg/kg), oil and grease (234 mg/kg) and stoddard solvent (99 mg/kg). The petroleum <br /> hydrocarbons decreased to 12 mg/kg at a depth of 21.5 feet in the boring. <br /> Samples collected from the bottom of borings at both ;caste oil tank sites had detectable <br /> concentrations of chromium and zinc. The detected concentrations were below the <br /> California Administrative Codes Total Threshold Limit Concentration and are typical of <br /> background concentrations detected throughout the Sacramento and San Joaquin Valleys. <br /> Although the concentration of TPu detected in B-5 at a depth of 21,5 feet was Iow, the <br /> T -` ---- <br /> SJCEHD-requested-that-the vertical extent of TI?H migration in the vicinity of-boring-B-5----------- <br /> be <br /> f boring 8-5-- —be further assessed and background zinc and chromium concentrations be confirmed. The <br /> County's request was presented in a latter dated February 14, 1990, <br /> F13-90-60 2 <br />