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Declining COC concentrations in groundwater are apparent in most wells. The <br /> historical groundwater monitoring data also indicate that the COC plumes are <br /> collapsing to their cores and not migrating. No water production wells or surface water <br /> bodies are at risk. <br /> Vapor intrusion is considered an incomplete pathway. <br /> Additionally, a deed restriction can be filed with the County Assessor's Office as an <br /> administrative control for protecting potential future receptors. <br /> The correlation of historical MTBE concentrations to TBA concentrations is an indication <br /> of attenuation. <br /> Consistent with the draft low-threat closure policy's media specific criteria, site-specific <br /> analysis shows contamination poses no current or near-term future threat to human <br /> health, safety, and the environment, and WQOs will be achieved within a reasonable <br /> time frame. "Reasonable time frame" for plumes of limited extent is multiple decades or <br /> longer as determined in State Water Board closure order WQ 98-04. <br /> For comparison and assuming the draft low-threat closure policy is adopted by the State <br /> in 2012, CRA estimates four years of groundwater monitoring would be necessary to <br /> validate projections and plume stability/retraction. Assuming an average annual <br /> groundwater monitoring cost of approximately $20,000, the total groundwater <br /> monitoring cost is $80,000. A closure request and well decommissioning by <br /> pressure-grouting is estimated to cost $97,000. Therefore, the total life-cycle cost for <br /> MNA is $177,000. For further comparison, groundwater monitoring could continue for <br /> more than 50 years before equaling the spend of the AS/SVE or excavation scenarios. <br /> Last, NINA has the smallest environmental footprint of the options considered. MNA <br /> mini*nizes use of resources, generation of waste, and generation of greenhouse gas <br /> emissions. <br /> 5.3.4.3 RECOMMENDATION <br /> There are no plans to change the existing use of the site as a bulk fuel terminal, and no <br /> receptors are at risk from residual impacts. MNA is feasible, cost-effective,and the most <br /> environmentally sustainable approach. CRA recommends implementing MNA as the <br /> final remedial approach, supplemented with limited/cost-controlled excavation of <br /> 241729(33) 23 CONESTOGA-ROVERS$ASSOCIATES <br />