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Site History The Mixed Waste Landfill (MWL) is located approximately 5 miles southeast of Albuquerque International Sunport, and 4 miles south of Technical Area (TA) 1. The site covers 2.6 acres in the north-central portion of TA 3. The MWL was established in 1959 as a disposal area for radioactive and mixed wastes generated at SNL research facilities. The landfill accepted low-level radioactive waste and minor amounts of mixed waste from March 1959 through December 1988. Approximately 100,000 cubic feet of radioactive wastes containing approximately 6300 Ci of activity (at the time of disposal) were disposed of at the MWL. The MWL consists of two distinct disposal areas. The classified area, occupying 0.6 acres, and the unclassified area, occupying 2.0 acres. Wastes known to have been disposed of in the classified area include solidified acids, solvents, and oils; DU; lead shielding; activation products; beryllium; sodium; lithium; neutron generator tubes; and empty liquid scintillation vials. Wastes known to have been disposed of in the unclassified area include assorted contaminated equipment; decontamination materials; lead shielding; construction debris; contaminated soils; and miscellaneous solid wastes. These wastes were disposed of in seven trenches, typically 15 ft deep, 20 ft wide, and 120 ft long. The exact depth of each trench is unknown. In 1967, approximately 271,000 gal of coolant waste water from the Sandia Engineering Reactor Facility was disposed of in Trench D. Approximately 1 Ci of total radioactivity, mainly short-lived radionuclides, was discharged into the trench with the cooling water. Because the disposal occurred in 1967, the short half-lived activation products have decayed to below detectable levels. Constituents of Concern Tritium is the contaminant of primary concern at the MWL. Tritium has been a consistent finding at the MWL since environmental studies were initiated in 1969. Tritium has been detected in surface and subsurface soils in and around the classified area of the landfill. Tritium has been detected above the background range in soils to depths of 110 ft. No tritium, or any other contaminants, have been detected in groundwater, which is approximately 500 ft below surface. Completed Work A Phase 1 RCRA Facility Investigation was conducted in 1989 and 1990 to begin characterizing the MWL and the nature and extent of contamination. This investigation included walkover surveys for radiation and VOCs; surface soil sampling; air sampling; and the drilling of 18 boreholes to depths of up to 150 ft around the landfill perimeter. Soil samples from the boreholes were analyzed for metals, radionuclides, VOCs and SVOCs. The Phase 1 investigation identified tritium as the contaminant of primary concern. A Phase 2 RCRA Facility Investigation was initiated in 1992. The Phase 2 characterization activities were completed in December 1995. Field characterization activities included determining background levels of metals and radionuclides in soils near the MWL; delineating pit and trench boundaries with surface geophysics; conducting passive and active soil gas surveys for VOCs; surface soils sampling for tritium and radionuclides; drilling 15 angled boreholes around the landfill perimeter; subsurface soil sampling for radionuclides, metals, and organic compounds; measuring vadose zone hydraulic and physical parameters in the field and laboratory; and conducting a baseline risk assessment. The Phase 2 investigation identified tritium as the contaminant of primary concern. The MWL Phase 2 RCRA Facility Investigation Report was submitted to the New Mexico Environment Department and the EPA in September 1996. The MWL was proposed for No Further Action and continued groundwater monitoring. NMED responded to the Phase 2 RCRA Facility Investigation Report by issuing a "Denial" in September 1997. NMED requested a formal RCRA Closure Plan, a Post-Closure Permit Plan Application, a RCRA Subtitle "C" Cap for the landfill, and responses to 85 technical comments. The 85 technical comments were addressed in two documents dated July 15, 1998 and January 28, 1999 and the MWL has been proposed for a vegetated soil cover. A vegetated soil cover would be an inexpensive alternative to a RCRA Subtitle "C" Cap. A vegetated soil cover design document was submitted to the NMED in September 1999. Technical comments from the NMED were received in June 2000. NMED comments are currently being addressed by the MWL technical team. A monitoring well network consisting of five wells has been installed at the MWL. The network includes one background well, three downgradient wells, and one angled well beneath Trench D in the northern half of the unclassified area. These wells are sampled annually for radionuclides, metals, and VOCs. Sampling of these wells has been conducted since 1990. No contaminants have been detected in groundwater to date. Two additional groundwater monitoring wells were installed at the MWL in 2000. These two wells were sampled on a quarterly basis for two years, then reduced to annual sampling. Groundwater monitoring at the MWL will continue for the foreseeable future. On October 11, 2001, the NMED directed the DOE and Sandia to conduct a Corrective Measures Study (CMS) for the MWL. The MWL CMS Report was submitted to the NMED on May 21, 2003 for technical review and comment. The purpose of the CMS was to identify, develop, and evaluate corrective measures alternatives and recommend the corrective measure(s) to be taken at the MWL. Based upon detailed evaluation and risk assessment using guidance provided by the EPA and the NMED, the DOE and Sandia recommended that a vegetative soil cover be deployed as the preferred corrective measure for the Mixed Waste Landfill. The NMED held a public comment period on the MWL CMS from August 11, 2004 to December 9, 2004. A public hearing was conducted on the MWL CMS on December 2-3 and 8-9, 2004. On May 26, 2005, the Secretary of the NMED selected a vegetative soil cover with bio-intrusion barrier as the remedy for the MWL. The selection was based on the administrative record and the Hearing Officer¹s report. The Secretary requested that a Corrective Measures Implementation Plan incorporating the final remedy be developed within 180 days following the selection of the remedy. The Corrective Measures Implementation Plan incorporates the final remedy selected by the NMED. The document contains a description of the selected remedy, the objectives for the remedy, detailed engineering design drawings and construction specifications, and a construction quality assurance plan and health and safety plan. The remedy, a vegetative soil cover, will consist of a thick layer of native soil. The design would rely upon soil thickness and evapotranspiration to provide long-term performance and stability, and would be inexpensive to build and maintain because of the availability of suitable soil in TA-3. This design has been formally submitted to the NMED for final closure of the MWL. The cover is a 3-foot-thick, vegetated soil cover. The cover will be underlain by a 1-foot-thick biointrusion barrier and a subgrade layer up to 40 inches in thickness. The proposed cover meets the intent of RCRA Subtitle C regulations, which include the following:
Performance of the cover will be integrated with the natural site conditions at TA-3, producing a "system performance" that will ensure that the cover protects both human health and the environment. The natural site conditions at the site include:
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