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Closure Plan Deficiencies <br /> 1 . Additional soil sampling has been conducted to determine the <br /> extent of soil contamination using total lead as the indi- <br /> cator constituent . After evaluation of the results and <br /> presentation to Board staff of the sample locations and <br /> results additional samples may be necessary. <br /> 2 . Closure as a landfill with residual contamination left in <br /> place is no longer proposed. We believe that contamination <br /> can be removed to background concentrations and propose <br /> clean closure of the impoundment by excavation of contami- <br /> nated soils to background. <br /> 3 . If after further excavation, it is determined that all <br /> contamination cannot be removed, the excavation will be <br /> backfilled and closed by capping with an engineered cap. <br /> The cap design will be submitted for review and approval of <br /> the Regional Board as part of the Closure and Post Closure <br /> Plan for the surface impoundment. <br /> 4 . If it is determined that all residual contamination cannot <br /> be excavated, a cap will be designed pursuant to Chapter 15 <br /> requirements or an equivalent engineered alternative will be <br /> proposed. Depending on the contaminants and contaminant <br /> concentrations remaining in place, a Class I , II , or III <br /> closure may be appropriate. <br /> 5. If contaminants are to be left in place, the Designated <br /> Level Methodology for Waste Classification and Cleanup Level <br /> Determination (Central Valley Regional Water Quality Control <br /> Board Staff Report, June, 1989 update ) will be utilized. <br /> The use of deionized water to perform the WET rather than <br /> citric acid extraction should be acceptable . Since the <br /> purpose of the citric acid extraction is to simulate the <br /> acidic environment of a landfill , and the soils left in <br /> place would only be subjected to normal surface runoff with <br /> neutral to slightly alkaline pH, WET using a deionized water <br /> extraction would be more appropriate in evaluating threat to <br /> water quality. <br /> 6. Soils used to backfill the excavation will be clean imported <br /> fill or on-site soils which do not exceed background con- <br /> stituent concentrations . Backfill will be compacted to <br /> minimize infiltration of surface runoff and waters from <br /> Mormon Slough to the subsurface. <br /> 5 <br />