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III. Environmental Setting, Impacts,and Mitigations K. Hydrology, Geohydrology, and Water Quality These measures would minimize the potential for leakage of leachate from the new landfill materials over the active and post-closure lifetime of the project. Therefore,no significant impact is anticipated. Impact K.6. Project activities that increase leachate production from the existing landfill materials during various phases could alternately facilitate the remediation of local groundwater,or result in groundwater contamination. (SIGNIFICANT) Increased leachate production from the existing(unlined}landfill area could occur as a result of soil stockpiling during Phases I through IV of the proposed project, and during vertical expansion in Phases V and VI. The existing landfill will be closed under current landfill closure regulations(and in compliance with the groundwater remediation program of the approved CAP). Placing excavated materials on top of the existing landfill during Phases I through IV of the project would increase the rate of leachate production to be collected as part of the remediation program(CAP).This would be considered a beneficial effect of the proposed project,since it may increase the rate leachate from the existing landfill is recovered. Vertical expansion in Phases V and VI,would not occur until after the horizontal phases have been completed(approximately 35 years). Material excavated during construction of the Phase I to IV cells would be stockpiled on the closed existing landfill to preload the area,as described above. It is considered highly unlikely that the addition of weight to the existing landfill due to the added refuse in Phases V and VI would increase the leachate generation after the long intervening period(35 years)(Hetherington and Ingham, 1993). However, in the absence of quantitative modeling or other data to confirm that no additional leachate would be generated during Phases V and VI,the potential impact could occur. Mitigation Measure K.6. Identified by this EIR (a) To monitor and remediate for any increased leachate generation,the extraction wells developed as part of the CAP should be maintained and monitored during Phases V and VI, to be employed as needed for the extraction of any residual contamination by leachate. Implementation of this measure would minimize this impact to a less-than-significant level. III.K.17