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Draft Environmental Impac R t a IV.F-25 Rep ort Page Forward Inc. Landfill Expansion differential settlement. The landfill design attempts to mitigate this by placing daily soil cover over the refuse. The potential for differential settlement affecting the landfill leachate collection system is limited to the Austin Road portion of the combined landfill where the new leachate collection system is to be placed on top of the existing landfill. Differential settlement could damage the liner and/or leachate collection system and affect water quality. The following measures are proposed as part of the project, as described in the Project Description: • A settlement analyses was performed to evaluate how landfill grades could change in the future as refuse materials biodegrade and consolidate. • Maximum pre-loaded settlement of the combined landfill is projected to provide the maximum period for settlement, and take advantage of anticipated improvements in geosynthetic technology. • Future design for the vertical expansion will, if necessary, include additional components in the liner system, depending on future differential settlement on the liner system. • The appropriate responsible agencies,the CIWMB and RWQCB, shall conduct a review of the liner and leachate collection system for the vertical expansion over the existing landfill(s) proposed in the upcoming JTD update. This has for the most part been constructed. The design for the last part of it was approved this may and will be constructed this summer. Implementation of these procedures would reduce the impact to a less-than-significant level. Mitigation Measure F.5: None required. Impact F.6: A VOC-contaminated plume extending northeast from the northern edge of the landfill border could affect water quality in offsite private water supply wells. The extent of the groundwater plume associated with the landfill was underestimated in previous studies. This was documented in the 1994 Austin Road EIR, as updated in 2002 Austin Road Expansion EIR and the review of more current data and corrective action assessments. The water quality data show the contaminant plume associated with the area north of Austin Road Landfill unit to extend north to the NCYCC wells as already described. The recent RWQCB Order 2008-0714 addresses the need for further articulation of remedial action to remedy the offsite plume. The VOC-contaminated plume extends from the northern edge of the landfill border,to approximately 5,500 feet north/northeast(downgradient)of the Austin Road unit. The plume is thought to take more of a northerly direction near its leading edge. Concentrations in some key downgradient monitoring wells, such as MW-11, has demonstrated the existing system is very effective in reducing the VOC in the border area where the extraction wells are most effective (see Table IV-F1). Further northward of the infiltration basin the capture of the plume is more problematic. The April 1999 CDM report estimates the probable need to extract up to 1,570,000 gpd based on the extended pumping capture zone model. This compares with actual extraction volumes of the current system estimated at about 400,000 gpd. The volume of groundwater extracted could be more that the CDM projections based on the AEE model, depending upon which combination of remedies are selected. The planned artificial recharge would also have the