ARCHIVED REPORTS_XR0012661 CASE 2

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KtEINFELDER 4 CONCLUSIONS AND RECOMMENDATIONS Petroleum hydrocarbons were not detected above laboratory reporting limits for samples collected at a depth of 11.5 feet and 51.5 feet in boring B-7,which was drilled adjacent to -- the 500-gallon UST near B-5. Petroleum hydrocarbons detected in B-5 at 12 mg/kg at a depth of 21.5 feet in the November 1989 soil assessment were slightly above the laboratory reporting limit of 10 mg/kg. The reason for TPH concentrations being below laboratory w reporting limits for the sample collected at 11.5 feet (B-7) may be due to natural degradation of the residual petroleum hydrocarbons, or an uneven distribution of petroleum hydrocarbons in the subsurface. Samples collected from the bottom of boring B-7 and the background boring B-8 had detectable concentrations of both chromium and zinc. The concentration of chromium detected in B-7 decreased from 44.9 mg/kg at 16.5 feet to 11.6 mg/kg at 51.5 feet. - Similarly, zinc decreased from 60 mg/kg to 39.5 mg/kg. The concentrations of chromium k and zinc detected in samples from B-7 indicate that elevated concentrations at 16.5 feet decrease to background concentrations at the termination depth (51.5 feet). The background concentrations of chromium (33.0 mg/kg)and zinc(35.8 mg/kg)were assessed by sampling the shallow boring B-8, which was drilled approximately 45 feet northeast of the former 500-gallon UST. Due to the depth of ground water in the area (approximately I.. 55-feet) the elevated concentrations of chromium and zinc beneath the former 500-gallon UST at a depth of 16.5 feet does not appear to pose a significant concern for impacting water quality. Based on the analytical results, the vertical extent of petroleum hydrocarbon migration 4 s_ from the 500-gallon UST appears to be limited to the general vicinity of the bottom of the former tank. The deepest TPH concentration detected (12 mg/kg) was 35 feet above i ground water, which was first encountered at a depth of 56.5 feet during the November 1989 soil assessment. Some petroleum hydrocarbons were previously detected at the bottom of the excavations and in shallow soil borings,but given the depth to water and the limited extent of vertical migration, Kleinfelder recommends that the Department of General Services Seel. approval-to backfill the excavation lvitli asphalt leaping the IimitCd-- _ residual amount of hydrocarbons in-place. Based on the current land use, no further assessment is required at this time. Kleinfelder's recommendations are consistent with F13-90-60 7