Risk-Based Probabilistic Performance Assessments of Long-Term Cover Systems for Waste Isolation


A probabilistic, risk-based performance-assessment method has been developed to assist designers, regulators, and stakeholders in the selection, design, and monitoring of long-term covers to isolate subsurface contaminants. Current landfill-cover design guidelines, such as those stated in the Resource Conservation and Recovery Act, are not risk-based and do not consider long-term site-specific influences such as climate, vegetation, and soils. These design guidelines may not address important long-term features, events, and processes at the site that may contribute to the long-term risk of groundwater contamination and human exposure. In addition, traditional design guidelines for covers often rely on deterministic models of flow and transport processes that neglect uncertainty inherent in actual contaminant transport.

aerial view of a landfill cover
Ariel view of the double composite-liner system at the base of the Monticello Mill tailings repository

A probabilistic, risk-based performance-assessment methodology considers regulatory requirements, site-specific parameters, engineering-design parameters, and long-term verification and monitoring requirements. Because many of the contaminants are long-lived, this methodology also considers changes in the environmental setting (e.g., precipitation, temperature) and cover components (e.g., liner integrity) for long time periods (>100 years). Uncertainty and variability in important site-specific parameters are also incorporated through stochastic simulations in this method.

Performance-Assessment Process

As defined by DOE M 435.1-1, a performance assessment is An analysis of a radioactive waste disposal facility conducted to demonstrate there is a reasonable expectation that performance objectives established for the long-term protection of the public and the environment will not be exceeded following closure of the facility. In addition, DOE M 435.1-1 also states that the method used for the performance assessment must include uncertainty analyses. A method that addresses these requirements has been used for the Waste Isolation Pilot Plant (DOE, 1996), the Yucca Mountain Project (DOE, 1998), and the intermediate-depth Greater Confinement Disposal Boreholes (Cochran et al., 2001) to assess the long-term performance of nuclear waste repositories. Uncertainty analyses and probabilistic approaches have also been used for decommissioning of contaminated sites (Meyer and Gee, 1999; Meyer and Taira, 2001). A similar systematic approach is proposed here to conduct performance assessments for long-term covers. The approach is outlined as follows:

  • Develop and screen scenarios based on regulatory requirements (performance objectives) and relevant features, events, and processes
  • Develop models of relevant features, events, and processes
  • Develop values and/or uncertainty distributions for uncertain input parameters
  • Perform calculations and sensitivity/ uncertainty analyses
  • Document results and provide feedback to previous steps and associated areas to improve calculations, as needed
graphic of the process for modeling landfill covers

Performance-Assessment Demonstrations

Alternative Landfill Cover Demonstration

The ALCD was a research program conducted to investigate the performance of various landfill cover systems including alternatives that may be well suited for arid and semi-arid climates. This research involved the design, construction and monitoring of six large-scale cover systems at a test facility located at Sandia National Laboratories in Albuquerque, NM. Water balance and vegetation aspects were monitored beginning in 1995 and continued through 2002 for each cover system. Soil hydraulic properties were measured in the laboratory and field. Numerical simulations of the water balance for each cover system were conducted to assess current design approaches and gain insight into the cover performance.

More about the ALCD

Reports and Presentations
DOE Relevance

Long-term cover systems are needed at U.S. Department of Energy (DOE) sites to assist in isolating contaminants from the biosphere at near-surface landfills, waste-disposal sites, and high-level radioactive waste tanks. The long-term covers are considered to be a vital remedial option for DOE 2006 Accelerated Cleanup Plan (DOE/EM-0362), which intends to clean up more than 90 percent of the contaminated sites in DOE Environmental Management Program. In addition, DOE Order 435.1 states that performance assessments are to be conducted for low-level radioactive waste disposed after September 26, 1988, and that performance objectives should be evaluated for a 1,000-year period to determine potential risk impacts to the public and environment.

The use of performance assessments for long-term cover systems also provides the following benefits:

  • Quantification of uncertainty in the simulated performance metrics
  • Identification of parameters and processes most important to performance for prioritization of site characterization and long-term monitoring activities
  • Comparison of alternative designs to optimize cost and performance while ensuring that regulatory requirements are met