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iIll. Environmental Setting, Impacts, and Mitigations <br /> K. Hydrology,Geohydrology, and Water Quality <br /> Impact K.7. Surface and/or groundwater contamination could result if differential <br /> settlement damaged the liner and/or leachate collection system of the vertical expansion <br /> portion of the landfill(Phases V and VI). (SIGNIFICANT) <br /> Differential settlement under the vertical expansion area could damage the liner and/or leachate <br /> collection systems and result in degradation of surface and/or groundwater quality. <br /> Over time,differential settlement of the underlying(closed)landfill materials under static <br /> conditions or seismic loading could detrimentally effect the integrity of the liner and leachate <br /> collection systems of the Phase V and VI vertical expansion. Although the planned pre-loading <br /> of the existing landfill surface is intended to maximize the opportunity for a stable foundation,it <br /> is uncertain if this will have happened by the time Phase V and VI are initiated. R.W.Beck and <br /> Associates describe two complicating factors: (1)a lack of data on landfill compaction and <br /> settlement rates, and(2)lack of design history and standards for vertical expansions(R.W. Beck <br /> and Associates, 1992b). <br /> Mitigation Measure K.7. <br /> Proposed as Part of the Project <br /> a The project design philosophy allows for maximum re-loaded settlement of the existing <br /> ' landfill to provide the maximum period for settlement, and would take advantage of <br /> anticipated improvement.-,in geosynthetic technology that would occur prior to the <br /> construction of Phase IV(over the next 35 years). <br /> (b) The final design for the vertical expansion may include additional components in the liner <br /> system, such as geogrids for structural support. Specific design criteria for the vertical <br /> expansion would take into account the effect of potential future differential settlement on <br /> the liner system. <br /> Identified by this EIR <br /> (c) To monitor and remediate for any groundwater contamination,the extraction wells <br /> developed as part of the CAP should be maintained and monitored during Phases V and VI, <br /> to be employed as needed for the extraction of any residual contamination by leachate. <br /> (d) A review of the liner and leachate collection system for the vertical expansion over the <br /> existing landfill should be conducted by the appropriate responsible agencies upon <br /> completion of the final design drawings. <br /> III.K.18 <br />