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mg/Kg in MW-4 at approximately 60 feet bgs (Figure 4) In 1995, boring SB-1 was completed between MW-4 and the tank excavation SB-I, however, had a TPH as diesel concentration of only 1,200 mg/Kg at approxunately the same depth of 60 feet bgs as MW-4 (Figure 6) Boring SB-2 did not yield any detectable concentrations of TPH as diesel 8.0 Extent of Petroleum Hydrocarbons in Site Groundwater TPH as diesel was identified as the only contaminant of concern in the groundwater Fourteen groundwater sampling events between 1988 and 1996 indicate that impact to the groundwater and migration from the source appear to be limited to the immediate vicinity of the former USTs (Table 2) Three monitoring wells (MW-1, MW-2 and MW-3) were initially installed at the site, and when sampled in 1988, did not yield TPH as diesel at a detection lunit of 1,000 pg/L However, subsequent groundwater sample analyses at a lower detection limit (50 pg/L) yielded concentrations ranging from below detection limits to a maximum of 210 pg/L The average TPH as diesel concentration in the first, second, and third quarters of 1996 were 87 pg/L, less than 50 pg/L, and 50 pg/L, respectively These average concentrations were calculated assuming a concentration of 25 pg/L for "non-detect" concentrations (c50 pg/L) These average concentrations are consistent with the area- weighted average TPH as diesel concentration (44 pg/L) predicted in groundwater using mathematical models (IT, 1996b) 9.0 Remediation Activities and Effectiveness Remediation at the site has consisted primarily of source removal and intrinsic bioremediation The initial tank excavation and removal was conducted on May 26, 1986 In May 1995, approximately 330 cubic yards of impacted soil were removed by overexcavation of the location of the former USTs. The overexcavation was backfilled with clean backfill and paved (IT, 1995a) The excavated soil was found to be impacted with up to 6,500 mg/Kg TPH as diesel, and trace concentrations of toluene, ethylbenzene, and xylenes (Table 1) Sort borings in 1992 and 1993 indicated TPH as diesel contamination extending approximately 80 feet from the source, with soil concentrations ranging from 130,000 mg/Kg near the source (at a depth of 15 feet in MW4) to a maximum of 6 8 mg/Kg at 80 feet from the source (at a depth of 45 feet in B-104) Soil borings m 1995 indicated TPH as diesel contamination extending approximately 40 feet from the source, with soil concentrations MZ D&13-97/VDBF/97-0009 BK 5