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f Problem Assessment War[c Plan CsUS—Multi-Campus Regional Center Tammy 23,2403 Pagc•6- f. { TABLE 1 € LaboratoryAnal tical Methods for Groundwater Sa les i CONSTITUENT METHOD DETECTION U IIT ' BTEX 8020 0.5 pt l (B,T,E) 1.0µg/L(X) TPH-G 8015M 50µ9/L TBPH 8015M 50 Ltg/L(D)250 jtg/L(K)500 µg/L(MO) Gasoline Additives/Oxygenates 8260 0.5 µg/L-13µglL I D=diesel K—kerosene MO=motor oil 6.0 REMEDIAL FEASIBILITY EVALUATION' i; 6.1 DESCRIPTIONS, JUSTIFICATIONS, AND TIME FOR PROPOSED' CORRECTIVE ACTION STRATEGIES Three alternatives for remediation of soil and groundwater contamination are considered: 1. Over-excavation and removal of contaminated soil with accelerated bioremediation (application of oxygen release compound) of groundwater contamination. 2. Soil vapor extraction with groundwater pump and treat. 3. No action. Over-excavation and removal of contaminated soil would involve the excavation and removal of approximately 1,800 cubic yards of soil from the vadose zone. The estimated lateral extent of hydrocarbon contamination in soil is shown in Figure 2, Appendix A. The contaminated soil would be removed to a designated waste facility and replaced with clean fill. In previous investigations, first groundwater was encountered in a sand bed at a depth of approximately 40 feet bgs. The groundwater rapidly-rose to approximately 37 feet bgs in borings that penetrated the top of the sand bed. Groundwater encroachment into the excavation is expected to limit the depth of the excavation to approximately 38 feet bgs. As a result, approximately 900 cubic yards of contaminated soil would be left in place in the saturated zone. Due to the proposed depth of the excavation and the proximity of buildings to theproposed excavation, j shoring of the excavation sidewalls will be necessary to prevent collapse of the excavation walls. Soil over-excavation would not directly address groundwater plume containment nor rapidly decrease ? ' contaminant concentrations in groundwater. This alternative will rely on the natural attenuation of - dissolved gasoline constituents over time. The application of Oxygen Release Compound (ORC) to the excavation before backfilling may accelerate bioremediation(natural attenuation or decay) of the gasoline constituents remaining in the saturated zone. The bioremediation will require quarterly effectiveness monitoring for an extended period of time (months to years). This alternative is potentially justified because it will eliminate the source of additional contaminant loading in groundwater that will occur each year as the groundwater rises and falls through contaminated soil (an ongoing source of down-gradient groundwater contamination). This alternative is also potentially justified because it has a very;.1iznited temporal impact to the site. The time to arrange logistics and complete the soil over-excavation is estimated i 3 to be between four and six months. The over-excavation activities will be quite disruptive at the site but far i t OW I CONDOR i