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Febr uai y 14 1997 1'a«e 4 Soil encountered in the area of the two removed USTs consisted piirnaifly of fill materials and native silts and clays Groundwater was encountered at a depth of appioxiinatel_y 9-112 feet below ground surface (bgs) in the fuel storage tank excavation The excavation was backfilled and compacted on November 26 1996 by Aronson A compaction test report was not provided to PACIFIC for inclusion in this letter report Underground Fuel Storage Tani. Abandonment On December 2, 1996, additional investigation was performed by Thomas to determine the ` presence or absence of a third UST, which had been known to exist in the vicinity of the two removed USTs Thomas drilled two shallow exploratory borings using a hand auger and reported the presence of the third UST to Unocal Also on December 2, 1996, PACIFIC visited the site on behalf of Unocal, and drilled four additional shallow hand- augured borings PACIFIC concurred with Thomas s conclusion that a third fuel tank remained in place to the west of the two removed fuel tanks On December 10, 1996, additional excavation activities were performed to document the presence or absence of the third fuel tank (Figure 2) Aronson was again contracted by Unocal to perform the excavation activities Excavation was observed by Tim Ripp of PACIFIC, Mike Infurna of SJCEHD, Ed Ralston of Unocal, and Tom Stoflet of Thomas . The third UST was identified in a location directly to the west of the two removed USTs The upper surface of the southern portion of the remaining UST was exposed within the second excavation The tlurd UST was located immediately beneath a subsurface storm drain conduit, and an above-grade traffic signal control box In a field meeting attended by Mr Mark Thrailkill of the City of Tracy, and by representatives of SJCEHD, Unocal and Wells Fargo, it was determined that closure in place was the most appropriate course of action On December 11, 1996, under the supervision of SJCEHD, the third UST was abandoned in place by filling the tank with concrete The upper surface of the south end of the UST was opened by Aronson using a spark resistant tool The UST was then examined and measured The UST was constructed of single-wall, riveted steel, and measured 42 inches in diameter and 90 inches in length Based on these measurements, the tank was determined to be of a nonunal 550-gallon capacity No holes or obvious damage to the UST were observed in the excavation Upon operung the UST, it was found to be approximately 3/4 full of water After the removal of the water by Erickson, the UST was filled in place using a slurry of sand, gravel and cement The excavation to expose the tank was then backfilled and compacted by Aronson A compaction test report was not provided t6 PACIFIC for inclusion in this letter report i 310097I BIREPORT DOC