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T � <br /> f <br /> y - <br /> T <br /> 1k� <br /> 22 <br /> 1 <br /> rNTRODUCTrON <br /> .`� This report <br /> Investiga4ianprconductedthe <br /> atethEt�ri/Valof dditlonal si.ibsurface <br /> 26200 Nowell Road, Thornton, California y Growers Plant(Figure e <br /> purpose of this investigation was 1), to further define the <br /> groundwater contamination detected in monitoring well MW-4 <br /> and, 2), sample tank pit A to determine if all contaminated <br /> soil had been excavated (Figure 2). <br /> 1 Five underground tanks: two 550, one 5,000, and one 10,000 <br /> gallon gasoline, and one 50,000 gallon no. six fuel oil <br /> were removed in February 1997. Samples collected during the <br /> ;.q tank removal reported soil and ' <br /> gasoline hydrocarbons groundwater contamination by <br /> contamination was not �etectediinttank ank pit A. Hydrocarbon <br /> B. <br /> excavation and surface aeration of Solistfrom tankdlpional <br /> were <br /> performed by Exceltech, Inc. A composite sample oftthe <br /> excavated material subsequent to aeration did not d <br /> recor - <br /> gasoline hydrocarbons above method detection limit . Four <br /> -� groundwater monitoring wells were installed b <br /> Inc. on August 20, 1987 and January 29, 1988y Exsceltech, <br /> hydrocarbon contamination was detected in MW-4 only. <br /> groundwater sampling results ate summarized in Table . The <br /> " San Joaquin Local Health District requested the groundwater <br /> contamination plume be defined to the east, north and <br /> northwest of MW-4 and tank pit A be sampled to confirm all <br /> contaminated soil had been excavated <br /> Installation of three additional Prior to backfilling, <br /> wells was proposed, but groundwater monitoring <br /> Property to the north couldinot vbetobtained.on Asesuch, two <br /> wells were installed on the Tri/Valley Growers <br /> Permission was received to collect a water sample from?ry± <br /> irrigation well on the adJacent <br /> for Irrigation only since the residenceyIs connected. This well <br /> do Uthe <br /> City water supply. A 500 <br /> is located on thio gallon underground gasoline tank <br /> stated that they arerPlann ngty Ftouremove theetank,�erbuthaat <br /> this time it is still in glace. <br /> 2.0 FIELD INVESTIGATION <br /> 2.1 Soil Borings <br /> On March 23, 1989,-two 5011 borings were advanced to depths - <br /> _ . <br /> Of approximately 25 feet below grade for the installation of <br /> groundwater monitoring wells (Figure 2) . The wells were <br /> located basertheastavious monitorings which showed a site <br /> specific gradient <br /> groundwater gradient Is northwestalthoutowards the Mokelumne <br /> River. Drilling was conducted with a Mobil B-53 drilling <br /> rig using seven inch diameter hallow stem augers. The <br />