A more stable and extensive analysis of climate is necessary to simulate long-term impacts associated with climate change. The exponential dispersion model (EDM) family of distributions, a popular choice when characterizing precipitation levels and temperature in different climate environments, is being considered for its applicability to near-surface disposal performance assessments. In this study, the EDM family is examined to determine if there is a preferred distributional form within the family for these parameters using data from two sites whose climate environments are quite different. One site is in a semiarid environment and the other is in a humid environment. In addition, the merits of selecting a different distributional form to represent INTRODUCTIONThe US Department of Energy (US DOE) is responsible for the cleanup of nuclear waste at former nuclear weapons sites across the United States. Cleanup activities include the containment of radioactive and hazardous chemical waste materials in near-surface disposal facilities, such as landfills, trenches, and vaults (US EPA, 1989). With the abundance of sites across the United States and the variability in operational management at each site, the US DOE introduced DOE Order 435.1, Radioactive Waste Management, in 1999 to assess the performance of these facilities. While the order requires uncertainty analyses, it may be unclear to users with respect to whether these requirements address important long-term features associated with climate (Ho et al., 2001 entire United States may be impacted by a changing climate, the extent to which certain effects are prevalent should be determined on a regional basis. Therefore, any approach to understanding how long-term features will affect environmental performance must be performed at a regional level using numerical models that assess the design integrity and performance of disposal facilities. Since these models require temperature and precipitation inputs, they are directly impacted by climate change. Traditional approaches to evaluating near surface facility performance neglect the effects of increases in average temperatures or the occurrence of more frequent and extreme weather conditions (Ho et al., 2001). Typically, records are examined from earliest records to the present. Common weather patterns are identified, as well as extreme occurrences (e.g., wettest year). These extreme events are taken to be a worst-case scenario and are used as "design year" conditions. A disadvantage of using solely historical data is that the lengths of recorded time periods are typically small relative to the forecast period. In addition, worst-case scenarios are developed based on precipitation, ignoring extreme-temperature episodes, such as hotter than normal months. Research has 100Remediation DOI: 10.1002/rem c⃝ 2013 Wiley Periodicals, Inc. REMEDIATION Autumn 2013Exhibit 3. Monthly precipitation distribution for Monticello, Utahshown that near-surface disposal facility cover systems rely on plants to remove water from the s...

DEDICATIONThis work is dedicated to my loving husband, Richard, who has never ceased in his support of me, regardless of the sacrifice. Thank you for helping me to reach my dreams.

The ability of near‐surface disposal facility cover designs to meet percolation performance criteria is influenced by degradation occurring over long periods of time. This study was conducted to determine the effect of degradation on percolation based on probabilistic distributions derived from historical climate data. Water‐balance predictions were evaluated using the HELP model, employing several variations of degradation in a traditional Resource Conservation and Recovery Act disposal facility cover design over a 100‐year simulation period. Analysis results were evaluated relative to two different selected thresholds for annual percolation (1 mm and 3 mm). Approximately 20 percent of the results did not exceed both the 1‐mm and 3‐mm thresholds, while 10 percent of the realizations exceeded the 1‐mm threshold but not the 3‐mm threshold, with remaining cases exceeding the 3‐mm threshold. These results demonstrate the importance of considering degradation in designing near‐surface disposal facilities, especially given the very long performance periods desired by different regulators. © 2013 Wiley Periodicals, Inc.