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6.0 DISCUSSION OF INTERIM SOIL REMEDIAL OPTIONS Environmental studies performed at the site indicate that a TPHg plume remains in soil generally beneath the UST excavation. Presented below are discussions of the remedial options typically evaluated for sites with soil impacted with TPHg. Remedial options associated with the current groundwater impacts are not discussed herein. Source removal of the residual TPHg in soil should be performed to evaluate potential benefits to groundwater associated with operation of the VES. After removal of the source hydrocarbons from soil, the remaining groundwater plume can be evaluated from arisk-based perspective for a possible low risk and no further action classification. This evaluation should be performed after final policy relative to low risk site criteria is issued by the State of California Water Resources Control Board. Such policy is currently under development. 6.1 No Remediation Alternative Due to the plume of TPHg-impacted soil beneath the site contacting groundwater, there exists the potential for continued leaching of petroleum hydrocarbons from soil to groundwater. Such leaching could further degrade the current groundwater quality and contribute to further downgradient groundwater plume expansion. Due to the potential for continued leaching of petroleum hydrocarbons to groundwater beneath the site, the "no remediation" alternative will not be given further consideration. 6.2 Excavation of Soil The removal of soil impacted with petroleum hydrocarbons would require extensive excavation beneath the UST excavation. Due to the depth of soil impacts being on the order of 65 feet, excavating impacted soil would not be a cost-efficient means of remediation. This option is considered non-permissible and will not be given further consideration. 6.3 On-site Soil Aeration,Off-site Soil Treatment These options would require the direct excavation of soil and are not feasible for the same reason presented in Section 6.2. 6.4 Bioremediation The design of an in-situ soil bioremediation system typically includes provisions for the injection of air, water, and nutrients into the plume for the purpose of enhancing the biological growth of naturally occurring microorganisms. The existence of residual TPHg in soil in proximity to groundwater poses risks associated with the injection of water for bioremediation. The leaching of Project No.S8100-06-11C -7- December 17,1996