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i I` 4. Shallow groundwater near the water table appears to be hydrologically connected with deeper water-bearing layers. The shallow groundwater migrates laterally toward the onsite production wells, and vertically downward toward these deeper layers. 5. Soils are contaminated with petroleum hydrocarbons in a small area of the tank I. pit. Most of the contamination appears to be restricted to a zone approximately 30 feet thick directly above the water table. The areal extent is probably less than 400 square feet. This area is the source for hydrocarbon contamination of groundwater. 6. The groundwater at the water table contains dissolved petroleum hydrocarbons in the tank pit area and at least 150 feet downgradient. The shallow groundwater does not contain petroleum hydrocarbons at distances of 500 feet �. or farther downgradient. Benzene has migrated downward with the groundwater to a depth of 140 feet in the downgradient area. 7. Concentrations of benzene in the groundwater range up to 32 mg/l in the source area. Benzene levels exceed the MCL in shallow groundwater at least 150 feet downgradient, and in deeper groundwater 500 ,feet downgradient from the source area.. The contamination exists as dissolved BTEX in groundwater, and non-aqueous phase liquids (i.e., free product) are absent. Concentrations of lead in groundwater exceed the MCL only in the source area. i Remedial Action Objectives Groundwater 1 Typically, target remediation levels for groundwater are consistent with drinking water regulations. As discussed in.Section 3 of this report, California MCLS exist for benzene (1 µg11), toluene (1,000 µg/1, effective 7192), ethylbenzene (680 µg11), and xylenes (1,750 µg/1). Benzene is the only known human carcinogen of the BTEX compounds and will be the focus of the groundwater remediation. The remedial objective, therefore, is to reduce the concentrations of benzene in groundwater below the California MCL of 1 µg11; if this objective is achieved, it is expected that toluene, ethylbenzene, and xylenes will also be reduced below their respective MCLS. Unsaturated Soil As discussed in Section 3, no standards have been promulgated for petroleum hydrocarbons in soil, since levels in soil required to protect groundwater may vary with local climatic and geologic conditions. The California Regional Water Quality Control Board LUFT Field Manual includes a leaching potential analysis for estimating concentrations of BTEX and TPH-Gas that may be left in place without threat to ( groundwater. Based on the LUFT leaching potential analysis (Appendix G), for SF0316041RP1007.51 4-2