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UST Closure Report Sharpe Defense Distribution Region West Lathrop,California Paye 7 supplied with a GMC C7H042 water tank truck. That same day Delta Oilfield Services vacuumed 2,500 gallons of rinsate from the USTs and transported it for treatment and disposal. All above-ground piping over the USTs was dismantled and removed. 3.2.3 UST Inerting Crushed solid carbon dioxide (`dry ice') was placed inside the USTs on December 22 to temporarily purge flammable vapors from, and reduce the atmospheric oxygen concentration in, them. Dry ice was added to the diesel fuel USTs at a rate of about 15 lbs per 1000 gallons of capacity, and to the gasoline USTs at about 30 lbs per 1000 gallons. The atmospheres in the excavation area and within the USTs were monitored for flammable or combustible vapor concentrations with a Gastec combustible gas detector(Model GT 302) during excavation and up to the time the tanks lifted from the excavation. Detector measurements showed that both the percentage of the lower explosive limit (LEL) and the concentration of oxygen (02) were less than 10% (see Tank Closure Certification Form, Appendix 5). 3.2.4 Excavation The immediate area of the USTs was covered with gravel. Therefore, it was not necessary to demolish any paving prior to excavating and removing the USTs. Fill materials were excavated from above and around the sides of the eight USTs on December 23, 1998 with a 325 Caterpillar excavator. The northern part of the UST excavation had dimensions of 30 by 50 feet, the southern part, 45 by 70 feet. It was 12 to 13 feet deep. The excavated fill comprised a poorly graded silty sand. The log from the 1959 borehole near the future site of UST No. 26 indicates that the original UST excavation was backfilled with native sediments. The UST removals were conducted under the direction of Mr. Eric Travena, an inspector with the San Joaquin County Environmental Health Division (SJCEHD). He noted that groundwater was intersected in the excavation below USTs Nos. 21 through 27 (Appendix 8). He also noted that those USTs were held down to a concrete slab with metal straps. Mr. Travena did not note any holes or areas of poor structural integrity on the steel tanks. 3.2.5 Removal of USTs Additional dry ice was added to the USTs just prior to their transport from the site. A total of 1,500 lbs of dry ice had been placed inside the USTs prior to their removal. The tanks were lifted from excavation and loaded onto six flatbed tractor-trailers with a 60-ton rubber-tired crane on December 23, 1998. The USTs were transported from the site for ® treatment and disposal by Trident Truck Lines on December 23 and 24. The steel straps that had held down the USTs were cut from the concrete slabs in which they were anchored for removal and disposal. N:\PROJECTS\3546\Reports\3546 UST Closure Report Draft.doc March 24,1999