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The Department of Energy/National Nuclear Security Administration (DOE/NNSA) and Sandia National Laboratories /New Mexico (SNL/NM) personnel submitted the Mixed Waste Landfill (MWL) Annual Long-Term Monitoring and Maintenance Report for the April 2017 through March 2018 reporting period to the New Mexico Environment Department (NMED) in accordance with the requirements of Section 4.8.1 of the MWL Long-Term Monitoring and Maintenance Plan (LTMMP). The LTMMP became effective on January 8, 2014.

The MWL is a 2.6-acre Solid Waste Management Unit (SWMU 76) located in the north-central portion of SNL/NM Technical Area III that has undergone corrective action. As of March 13, 2016, the NMED February 2016 Final Order became effective, granting the Class 3 Permit Modification to reflect that the MWL is Corrective Action Complete with Controls and incorporating the LTMMP in Attachment M of the SNL RCRA Facility Operating Permit. The LTMMP defines all monitoring, inspection, maintenance/repair, and reporting requirements for the MWL.

This fifth MWL Annual Long-Term Monitoring & Maintenance Report is comprised of 12 chapters that include monitoring results for air, surface soil, vadose zone soil moisture and soil-vapor, biota, and groundwater at the MWL. Inspection, maintenance, and repair activities for monitoring well networks and sampling equipment, the storm-water diversion structure, the evapotranspirative (ET) Cover system, and perimeter fence are also presented, along with a summary of MWL regulatory activities completed during the reporting period. The report includes seven annexes that contain supporting documentation for monitoring activities, forms used to document inspection and repair activities, and the annual biology report. The monitoring and inspection results presented in the report indicate the ET Cover and associated controls are performing as designed. No monitoring results exceeded LTMMP trigger levels and were consistent with historical MWL monitoring data. Site conditions continue to be protective of human health and the environment. 675 pgs.

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Sandia National Laboratories/NM