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, <br /> r � <br /> Request for Site Closure Page 2 <br /> Chevron U.S.A. Products Company,3246 East Fremont Street, Stockton, CA January 3, 1995 <br /> j I <br /> Due to elevated levels of hydrocarbons detected in soil samples collected at the bottom of the UST <br /> pit, three monitoring wells were installed and sampled in March 1987 (Groundwater Technology, Inc., <br /> 1987). Soil excavated from the former UST pit was returned to the UST cavity in December 1987, <br /> after samples collected from the aerated stockpile were found to contain hydrocarbon <br /> concentrations below laboratory detection limits. Due to the presence of nearby pumping wells, <br /> additional investigation was conducted to determine the threat to nearby potential receptors. <br /> Domestic/irrigation well records searches, well sampling, contaminant fate and transport modelling, <br /> and risk assessment evaluation were conducted to determine the immediate threat to nearby usable <br /> water sources. No immediate impact was reported in water samples collected from the nearby <br /> wells. Modelling conservatively predicted potential contaminant migration to the nearby wells in <br /> concentrations above State maximum contaminant levels (MCLs) (EA Engineering Science and <br /> Technology, Inc., 1987). Due to the mathematically perceived threat to the surrounding wells, four <br /> additional groundwater monitoring wells were installed in February 1988 along the perimeter of the <br /> site to monitor the migration of dissolved hydrocarbons (EA-1 through 4). Two additional <br /> groundwater monitoring wells (EA-5 and 6) were installed in August 1990 to replace wells now dry <br /> due to failing water levels. Wells MW-1, 2 and 3 were destroyed at this time. Methods and results of <br /> well installation/destruction are summarized in the Report of Soil and Groundwater Investigation <br /> (EA Engineering Science and Technology, Inc., 1990). <br /> Further investigation at the site consisted of drilling and sampling one soil boring in the former UST <br /> pit (SB-1) in November 1991, installation of two additional groundwater monitoring wells (MW-4 and <br /> MW-5) in May 1993, and product piping removal and soil sampling in May 1993. The methods and <br /> results of the investigations are summarized in Environmental Assessment Reports (Groundwater <br /> Technology, Inc, 1991 and 1993). <br /> EXISTING SITE CONDITIONS <br /> The site is currently an undeveloped, graded, fe ed lot. Stockpiled soils from UST pit excavation <br /> and product line removal have been used to backfill cavities associated with each excavation event. <br /> Prior to backfilling, each stockpile was sampled to confirm that no impact from petroleum <br /> hydrocarbons was present. As a result of continued site investigation, eight groundwater monitoring <br /> wells are present on site (MW-4, MW-5, and EA-1 through EA-6). As of June 2, 1994, water levels <br /> were approximately 71-73 feet below ground surface (SGS) at the site. Water is present in only the <br /> deepest wells (approximately 81-85 feet deep) on site (MWs-4 and 5, and EA-5 and 6). No dissolved <br /> petroleum hydrocarbons have been detected in groundwater since water levels were approximately <br /> 30 feet higher in February 1988. <br /> 0072clos.rpt <br /> ❑❑® GROUNDWATER <br /> ❑❑❑ TECHNOLOGY <br />