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Wendy Cohen - 3 - 15 November 2007 Former Mobil Oil Company Bulk FL ?lant, Tracy Dermal Contact Hazard The remaining residual hydrocarbons are in soils greater than 10 feet bgs. Based on the depth of the residual hydrocarbons and the residual concentrations, there does not appear to be a threat to human health through dermal contact at the Site. Threat to Groundwater Excavation activities removed the majority of the threat to groundwater quality. TPHd concentrations in downgradient monitoring wells have declined to below or near detection limits. The nearest drinking water well is about 300 feet east of the site (cross-gradient) and is screened from 340 to 920 feet bgs. In addition, Leprino has an operating groundwater extraction system that would capture any groundwater pollution from the Mobil Site. Based on this information, the Site does not pose a threat to beneficial use of groundwater quality. SUMMARY Site structures were removed in 1988 and included one 1,000-gallon UST, four 10,000-gallon ALTs, and associated piping. In 1990, about 6,000 cubic yards of TPH-contaminated soil were excavated and removed from the Site. Mobil estimated the remaining mass of TPHd in soil following excavation to be about 650 gallons at a depth of 10.5 to 12.5 feet bgs. Soil borings completed following the excavation contained little to no TPH contamination below 16 feet bgs. Twelve groundwater monitoring wells were installed during the investigations at the Site. Groundwater is at a depth of about 11 feet bgs with a flow direction of north-northeast. Leprino continues to monitor three monitoring wells downgradient of the Site. Leprino operates a groundwater extraction system on its site, which is downgradient of the Site, so any TPH migrating onto the Leprino property will be captured by that system. Alton concluded closure of the site is warranted because of the following: • Excavation activities were able to remove most of the hydrocarbon contaminated soil. • Confirmation soil borings drilled in 1994 showed that TPHd concentrations in soil remained at 10.5 to 12.5 feet bgs, and they decreased with depth. • Natural attenuation has likely reduced the TPH concentrations in groundwater. • No public or domestic wells are threatened. • There is no threat to human health through vapor intrusion or dermal contact. • Any residual hydrocarbons in the groundwater that have migrated offsite will be captured by the groundwater extraction system operating at the Leprino site.