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7 0 PETROLEUM HYDROCARBON DISTRIBUTION 71 Petroleum Hydrocarbons in Soil I Prior to the initiation of remediation at the site, the highest concentrations of petroleum hydrocarbons were historically detected beneath the former middle and southern USTs at approximately 15 feet to 16 feet bgs (excavation samples TP2 and TP3), near the northwestern (SB-1) and southeastern boundaries (SB-3, W-2, and W-3) of the former UST excavation from approximately 13 feet to 30 5 feet bgs, and north of the product dispensers (W-8 and MW-1) from approximately 30 feet to 55 feet bgs TPHg and benzene concentrations were reported at site maximum concentrations of 8,900 mg/kg (TP3-NE) and 180 mg/kg (SB-2, 30 0-35 5') The ' petroleum hydrocarbon-impacted soil that was removed from the former UST area and the new UST excavation was combined and used as backfill within the former UST pit The petroleum hydrocarbon-impacted backfill and surrounding soil was remediated via the SVE/AS system ' from December 1999 through November 2004 Prior to remedial efforts at the site, the lateral and vertical extent of petroleum hydrocarbons had been defined by low to non-detectable concentrations at and above laboratory MRLs at a depth of approximately 41 feet to 46 feet bgs ' within the source area and across the site, with the exception of soil in the vicinity of well W-8 and MW-1 Petroleum hydrocarbons in deeper vadose zone and saturated zone soils in the vicinity of wells W-8 and MW-1 were vertically defined by low petroleum hydrocarbon concentrations at a depth of 101 feet bgs (MW-1) and laterally defined by low to non-detectable petroleum hydrocarbon concentrations in wells MW-2 through MW-5 Results of post-remediation confirmation sampling in November 2004 in the vicinity of the ' former USTs indicated the presence of residual petroleum hydrocarbons at approximately 30 feet to 35 feet bgs, with the highest concentration of TPHg reported near the northwestern boundary of the former UST excavation (1,300 mg/kg, S13-12) The presence of TPHg (29 mg/kg) near the southwestern boundary of the former USTs in boring SB-13 is significantly lower than historical pre-remediation TPHg concentrations at this depth in borings S13-3, W-2 and W-3 This reflects the effectiveness of remedial action at the site The effectiveness of ' remedial efforts at the site is further evidenced in soil in the vicinity of well MW-1 that display a decrease in petroleum hydrocarbons to comparatively minor (110 mg/kg TPHg) to non- detectable levels to the total depth explored of 805 feet bgs in boring SB-14 With the ' exception of xylenes detected at a maximum concentration of 18 8 mg/kg (SB-12@30), post- remediation confirmation sampling indicated the absence of benzene, toluene, ethylbenzene, and fuel oxygenates to the respective total depths explored of 40 5 feet (SB-12 and SB-13) and ' 80 5 feet bgs (SB-14) Sampling for the presence of fuel oxygenates in soil had not been performed at the site prior to post-remediation soil sampling activities conducted in November 2004 The non-detectable concentrations of benzene, toluene, and ethylbenzene in soil are below the Tier II SSTLs (Appendix D) established during the RBCA assessment, which are used ' as the cleanup goals for the site 72 Petroleum Hydrocarbons in Groundwater ' Since the initiation ofY uarterl groundwater sampling activities in January 1995, the highest 4 concentrations of petroleum hydrocarbons have historically been detected in wells MW-1 through MW-6 and MW-9 With the exception of well MW-1, which historically has contained Time Oil Asmt&SCM doc 1 1 March 17,2005 770T 41658 00 0015