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} <br /> 1 <br /> 1 <br /> P/SHARPE/GWROD.31 1 <br /> 01/11/93 <br /> based groundwater quality standards using this approach. If it becomes <br /> apparent, during implementation or operation of the system, that contaminant <br /> levels have ceased to decline and are remaining constant at levels higher than the <br /> levels required by this ROD, the goal and remedy may be re-evaluated. <br /> The selected remedy will include groundwater extraction for a period of <br /> 16 years. During this period, system performance will be carefully and regularly <br /> monitored and adjusted as warranted by the performance data collected during <br /> operation. Modifications will include the following: <br /> 1. Discontinuing operation of extraction wells in areas where cleanup <br /> goals have been attained, <br /> 2. Alternating pumping at wells to eliminate stagnation points, and <br /> 3. Pulse pumping to allow aquifer equilibration and encourage adsorbed <br /> contaminants to partition into groundwater. <br /> The following sections describe the major components of the selected alternative. <br /> Engineering variables presented in the following sections, however, are subject to <br /> change based on the remedial design engineering process to be implemented <br /> following signature of the ROD by EPA and the State of California. <br /> Since initiation of regulatory review of this ROD, DDRW has proceeded with the <br /> design of the groundwater extraction and injection system. Remedial design <br /> information presented in the report entitled Remedial Well-Field Design Using <br /> Three-Dimensional Groundwater Flow and Transport Modeling supersedes <br /> information presented in this ROD. Specific areas impacted are details of the <br /> extraction and injection networks (locations of wells and flow rates) and <br /> estimated time needed to achieve aquifer cleanup levels. The number of <br /> treatment plants needed in the Central Area may also change. Information <br /> presented in this ROD is adequate for evaluating potential remedies and for <br /> selecting a remedy but will not be used as a basis for remedial design. Remedial <br /> designs will be based on the three-dimensional modeling work and on future <br /> design efforts. <br /> 9.1 GROUNDWATER EXTRACTION (RECOVERY) <br /> 9.1.1 Groundwater Recovery in North and South Balloon Areas <br /> Groundwater recovery systems implemented as interim response actions are <br /> currently in operation in the North and South Balloon Areas. Additional <br /> 57 <br />