An alternative cover design consisting of a monolithic layer of native soil is proposed as the closure path for the Mixed Waste Landfill at Sandia National Laboratories, New Mexico. The proposed design would rely upon soil thickness and evapotranspiration to provide long-term performance and stability, and would be inexpensive to build and maintain. The proposed design is a 3-ft-thick, vegetated soil cover. The alternative cover meets the intent of RCRA Subtitle C regulations in that a) water migration through the cover is minimized; b) maintenance is minimized by using a monolithic soil layer; c) cover erosion is minimized by using erosion control measures; d) subsidence is accommodated by using a "soft" design; and e) the permeability of the cover is less than or equal to that of natural subsurface soil present.Performance of the proposed cover is integrated with natural site conditions, producing a "system performance" that will ensure that the cover is protective of human health and the environment. Natural site conditions that will produce a system performance include a) extremely low precipitation and high potential evapotranspiration; b) negligible recharge to groundwater; c) an extensive vadose zone; d) groundwater approximately 500 ft below the surface; and e) a versatile, native flora that will persist indefinitely as a climax ecological community with little or no maintenance. The Mixed Waste Landfill (MWL) is located 4 miles south of SNL/NM's central facilities and 5 miles southeast of Albuquerque International Sunport. The landfill is a fenced, 2.6-acre compound in the north-central portion of Technical Area (TA)-3. The MWL was established in 1959 as a disposal area for low-level radioactive and mixed waste generated by SNL/NM research facilities. The landfill accepted low-level radioactive and minor amounts of mixed waste from March 1959 through December 1988. Approximately 100,000 cubic feet (ft) of lowlevel radioactive and mixed waste containing approximately 6,300 curies of activity were disposed of in the landfill.The MWL consists of two distinct disposal areas. The classified area occupies 0.6 acres and the unclassified area occupies 2.0 acres. Low-level radioactive and mixed waste was disposed of in each of these areas. Classified wastes were buried in unlined, cylindrical pits in the classified area. Unclassified wastes were buried in shallow, unlined trenches in the unclassified area.A Phase 1 Resource Conservation and Recovery Act (RCRA) Facility Investigation (RFI) was conducted in 1989 and 1990 to determine if a release of RCRA contaminants had occurred at the MWL. The Phase 1 RFI indicated that tritium had been released to the environment. A Phase 2 RFI was conducted from 1992 to 1995 to determine the contaminant source, define the nature and extent of contamination, identify potential contaminant transport pathways, evaluate potential risks posed by the levels of contamination identified, and provide remedial action alternatives for the landfill.The Phase 2 RFI confirmed that...
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