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The GAC system will use an effluent surge tank to facilitate delivery of the treated water from the GAC units to a discharge pipe that transmit the treated water to the sedimentation basin. The GAC system vessels will operate in series and will include periodic back washing to remove trapped suspended matter from the GAC bed. When the differential pressure through a GAC column is built-up and exceeds its operating range, its adsorption efficiency will degrade and a back wash operation will be required to restore effective operating conditions. The back-wash process will use water from the equalization tank to reverse flow and remove the suspended matter accumulating in the GAC bed. During back wash,the GAC system operation will be temporarily suspended. Since the filtration system will be installed ahead of the GAC columns, at most, monthly backwash of each unit is anticipated. The backwash operation typically requires about 15 to 30 minutes to complete (Table 13A). The cost to construct this option is estimated to be $720,813 (Table 1313). This cost also incorporates the anticipated construction costs for the groundwater extraction network. The anticipated yearly operating and maintenance cost for this option with groundwater extraction and including estimate energy costs, routine maintenance, GAC vessel replacement, monitoring, engineering support, and contingencies for this alternative is approximately$133,978 (Table 13C). • 5.0 EVALUATION OF REMEDIAL ALTERNATIVES The two in-situ, two source control, and two ex-situ remedial alternatives were quantitatively ranked and scored to determine the overall best alternative or set of alternatives to mitigate the release at the CHSL. Remedial alternatives were evaluated based on initial construction/engineering costs, annual operations and maintenance costs, technical effectiveness, constructability, and serviceability. 5.1 EVALUATION CRITERIA The groundwater remediation alternatives were ranked to assess their applicability to mitigate the impacted groundwater conditions at the CHSL. Using the criteria identified below, a numerical rating system of 0 to 10 was used to rate the individual alternatives. A weight factor of 0 to 10 was also assigned for each criterion, with a score of 10 indicating a critical factor and 2 indicating a less important selection criterion. 191 Engineering Feasibility Study Geo-Logic Page ■ 28 Corral Hollow Sanitary Landfill ASSOCIATES January 25,2013