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Y ''r " Page 106 Miti ation K.4: (a)' Maximumre-loaded settlement ement of the existing landfill would provide the maximum period for settlement,and take advantage of anticipated improvements in geosynthetic technology that would occur over the next 35 years;.(b)the final design for the vertical expansion would,if necessary, include additional components in the liner system,depending on future differential settlement on the liner System; (c)the extraction wells developed as part of the CAP shall be maintained and monitored during Phases V and V1,to be employed as needed for the extraction of any residual contamination by leachate; (d)a review of the liner and leachate collection system for the vertical expansion over the existinglandfill shall be conductedappropriate re by theresponsible agencies upon the completion of the final design drawings. Impact K.5: Potential seepage of leachate from the landfill expansion would cumulatively affect water quality if not properly mitigated. p r Since the 1994 EIR, additional hydrologic and hydrochemical studies have better delineated the plume, using models to predict potential future seepage of leachate into the underlying aquifer(s). This is described in detail in new impact and mitigation SEIR-1,below. Fill Mitigation K.S: See 1994 mitigations 3 and 4,above, and new mitigation SEIR-1, below. f New Impacts and Mitigations Not Identified in the 1994 EIR Impact K-6: A V P UC-contaminated plume extends from the northern edge of the landfill border to approximately 4,000 feet northeast(downgradient)of the site. The extent of theund1 !y water plume associated with the landfill was underestimated in the 1994 EI)Z; various studies since 1994 have led up to the most recent April 1999 Corrective Action Engineering Feasibility Study Report. The plume is considerably larger,both vertically and laterally,than thought to be the case in 1994. The VOC-contaminated plume extends from the northern edge of the landfill border, to approximately 4,000 feet northeast(downgradient)of the site. The plume is thought to take more of a northerly direction near its leading edge. The ary t g g prim contaminants of concern are PCE and TCE,with ., concentration levels up to 69 pg/L and 48 µg/L,respectively. These levels are well above the drinking water MCLs of 6 pg/L for PCE and 5 pg/L.for TCE. The highest VOC concentrations present are present within a sand layer located at the 80-to 104-foot bgs interval. The investigation shows the plume ' concentrations decreasing outward from the landfill source as well as migrating deeper into lithologic units(CDM, 1999A). The plume velocity was estimated by CDM at approximately 4 feet per day(same as the groundwater velocity). Such a velocity is higher than expected in the Victor formation clays and silts,and most likely reflects preferential travel along channels. At the estimated velocity of nearly 1,500 feet per year,the . plume would have migrated well over 4,000 feet. The lack of definition beyond that point suggests either natural attenuation through dilution or the absence of data points which might identify additional parts of the plume. The 1994 EIR outlined a projected design capacity of 580,000 gallons per day(gpd)for the Groundwater Treatment System(GTS),with treatment consisting of water being passed through an air stripper and t 14 activated carbon units. 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. Actual extraction volumes of the current system are estimated'at about 400,000 gpd(Miller,personal communication, 1999), t r GR4SSETTI ENVIRONMENTAL CONSULTING F,