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UST Closure Report Sharpe Defense Distribution Region West Lathrop,California Page 26 concentrations of TPH-G, TPH-D and benzene (7.1 µg/kg). The concentration of benzene, however, exceeds the USEPA National Ambient Water Quality Criteria of 1.2 µg/kg for Human Health and Welfare Protection for a drinking water source. Thirty-eight of the 46 samples from the stockpiles of soil removed from the UST excavation contained from 2.4 to 88 mg/kg TPH-D, or an average of 17 mg/kg. The average concentration of TPH-D was about the same in the samples from stockpiles SPA and SPB as that in the composite, mixed stockpile SPD. The concentrations of the BETX compounds were all below the method detection limits in all the 20 samples from the composite stockpile SPD. The concentrations of MTBE were greater than the method detection limit of 10 µg/kg in 82% of the samples from the older stockpiles SPA and SPB, but only in 20% of the samples from the composite, mixed and aerated stockpile SPD. MTBE ranged in concentration from 14 to 210 µg/kg in the 4 soil samples from stockpile SPD in which it was detected. The sample with 210 µg/kg MTBE also contained very high concentrations of total lead (200 mg/kg). About 19 cubic yards surrounding the site of sample SPD-6 were delineated by the base environmental department. A new stockpile, SPD-6,was formed from the delineated soil. Two of the samples collected from the site of the two fuel dispenser islands contained low concentrations of MTBE (12 and 23 gg/kg). TPH-D was detected in concentrations of 3.9 to 68 mg/kg in 7 of the 11 samples collected from the floor of the product and vapor return pipe Ip excavation. Three of these were from the sites of fuel dispensers; four were from areas of 90° bends in the piping direction. The composite of two samples from the 14 cubic yard stockpile of soil excavated from the piping trenches contained 28 mg/kg TPH-D, 55 µg/kg MTBE and 19 mg/kg total lead. The UST excavation was backfilled with a total of 8 lifts of fill materials. On December 31, 1998 and January 4, 1999, 116 tons of 1 t/2 inch drain rock were placed, on the UST hold-down concrete slab in the base of the excavation up to a depth of 9 feet below surface grade. The drain rock spans the groundwater table. Overlapping sheets of 10 mil-thick plastic were placed above the drain rock to delineate it from overlying backfill. The remainder of the excavation was backfilled on March 4 and 5, after CAL INC received authorization from the U.S. Army Corps of Engineers to return the native fill materials excavated from it and stored in stockpile SPD. The first lift of backfill above the drain rock comprised 23 inches (compacted) of 3/4 inch aggregate base fill (about 260 cy). Over this, two lifts, 20 inches and 29 inches thick, respectively, were formed from the native silty sand in stockpile SPD (about 780 cy total). However, it was necessary to dry the sand to achieve an acceptable degree of compaction. The soil was spread and formed into windrows, then reformed into new windrows with a bulldozer for 10 hours. This reduced the moisture content from about 14% to 12.5%, or only 1% above that at which maximum field compaction could be obtained (upon advice from Kleinfelder, Inc.). Although good compaction of the sand was obtained (94.0 to 98.2%) was obtained, the poorly graded material tended to `pump' in restricted areas when heavy equipment passed over it. Three successive lifts, 8 inches, 8 inches and 12 inches thick, of 3/4 inch aggregate base (589 tons total) were placed over the two lifts of native sand. The final lift of aggregate was graded to match the existing grades surrounding the excavation by backblading with a Volvo front-end loader and a N:\PROJECTS\3546\Reports\3546 UST Closure Report Draft.doc March 24,1999