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UST Closure Report Sharpe Defense Distribution Region West Lathrop,California Page 27 John Deere loader. The piping trenches were backfilled with two lifts of 3/4 inch aggregate base fill. All lifts of fill above the drain rock were compacted with a Bomag compactor with an 84 inch- wide, vibratory, single `sheep's foot' roller. Kleinfelder, Inc. conducted sand-cone type compaction tests (ASTM D 1556 test method) at two widespread sites on each lift of backfill. Tests showed that the compaction achieved in the aggregate base fill ranged, in general, from 96.1 to 98.7%, and had an average of 97.5%. Tests on the two lifts of native sand yielded results of from 94.0 to 98.2% compaction, with an average of 95.4%. A minimum of 90% compaction was required for depths of fill below 50 inches, and 93% above 50 inches to surface grade. Two stockpiles of excavated material with MTBE contamination remain at a site 400 feet southeast of the UST excavation. Stockpile SPD-6 contains 19 cubic yards of native silty sand removed from the UST excavation. It was formed by removing from stockpile SPD, the soil delineated by the base environmental department around a sample (SPD-6) that contained 210 µg/kg MTBE and 200 mg/kg lead. The second stockpile, SPC-new, is 10 feet south of stockpile SPD-6, and comprises about 60 cy of silty sand excavated from the piping trenches. A composite of two soil samples from the stockpile SPC-4+SPC-2) contained 28 mg/kg TPH-diesel and 55 µg/kg MTBE. The stockpiles were placed on, and covered with, plastic sheets. An investigation is recommended to delineate in detail the lateral and depth extents of the petroleum hydrocarbon contamination detected in soil samples from the floor of the UST excavation. Preliminary analyses suggest the contaminants of concern are Total Petroleum Hydrocarbons-gasoline, Total Petroleum Hydrocarbons-diesel fuel, BETX, MTBE and possibly lead. Also recommended is an investigation of the groundwater quality below, and in the vicinity of, the UST excavation to delineate the potential extent of petroleum hydrocarbon contamination, especially benzene and MTBE. Finally, it is recommended that a new series of soil samples be collected from the contaminated soil in stockpiles SPC-new and SPD-6, and analyzed for Total Petroleum Hydrocarbons-diesel fuel, MTBE and lead. The stockpiled soil should be disposed of as soon as possible on the basis of the new analytical results. N:\PROJECTS\3546\Reports\3546 UST Closure Report Draft.doc March 24,1999