NUREG/CR-6346 PNL-10843 Hydrologic Evaluation Methodology for Estimating Water Movement Through the Unsaturated Zone at Commercial Low-Level Radioactive Waste Disposal Sites Manuscript Completed: November 1995 Date Published: January 1996 Prepared by P. D. Meyer, M. L. Rockhold, W. E. Nichols, G. W. Gee Pacific Northwest Laboratory Richland, WA 99352 T. J. Nicholson, NRC Project Manager Prepared for Division of Regulatory Applications Office of Nuclear Regulatory Research U. S. Nuclear Regulatory Commission Washington, DC 20555-0001 NRC Job Code L2466 Abstract This report identifies key technical issues related to hydro- Water flow in a LLW disposal facility must be evaluated logic assessment of water flow in the unsaturated zone at before construction of the facility. In addition, hydrologic low-level radioactive waste (LLW) disposal facilities. In performance must be predicted over a very long time frame. addition, a methodology for incorporating these issues in For these reasons, the hydrologic evaluation relies on the the performance assessment of proposed LLW disposal use of predictive modeling. In Chapter 4, the evaluation of facilities is identified and evaluated. The issues discussed unsaturated water flow modeling is discussed. A checklist of fall into four areas: items is presented to guide the evaluation. Several computer simulation codes that were used in the examples (Chapter 6) (1) Estimating the water balance at a site (i.e., infiltration, are discussed with respect to this checklist. The codes used runoff, water storage, evapotranspiration, and include HELP, UNSAT-H, and VAM3DCG. recharge); (2) Analyzing the hydrologic performance of engineered To provide a defensible estimate of water flow in a LLW components of a facility; disposal facility, the uncertainty associated with model pre- (3) Evaluating the application of models to the prediction dictions must be considered. Uncertainty arises because of of facility performance; and the highly heterogeneous nature of most subsurface envi- ronments and the long time frame required in the analysis. (4) Estimating the uncertainty in predicted facility perfor- Sources of uncertainty in hydrologic evaluation of the mance. unsaturated zone and several approaches for analysis are An estimate of recharge at a LLW site is important since discussed in Chapter 5. The methods of analysis discussed recharge is a principal factor in controlling the release of include a bounding approach, sensitivity analysis, and contaminants via the groundwater pathway. The most com- Monte Carlo simulation. mon methods for estimating recharge are discussed in Chap- To illustrate the application of the discussion in Chapters 2 ter 2. Many factors affect recharge; the natural recharge at through 5, two examples are presented in Chapter 6. The an undisturbed site is not necessarily representative either of first example is of a below ground vault located in a humid the recharge that will occur after the site has been disturbed environment. The second example looks at a shallow land or of the flow of water into a disposal facility at the site. burial facility located in an arid environment. The examples Factors affecting recharge are discussed in Chapter 2. utilize actual site-specific data and realistic facility designs. At many sites engineered components are required for a The two examples illustrate the issues unique to humid and LLW facility to meet performance requirements. Chapter 3 arid sites as well as the issues common to all LLW sites. discusses the use of engineered barriers to control the flow Strategies for addressing the analytical difficulties arising in of water in a LLW facility, with a particular emphasis on any complex hydrologic evaluation of the unsaturated zone cover systems. Design options and the potential perfor- are demonstrated. mance and degradation mechanisms of engineered compo- The report concludes with some final observations and rec- nents are also discussed. ommendations. iii NUREG/CR-6346 Contents Abstract ..................................................................................................................................................................................... iii Executive Summary ................................................................................................................................................................... xi Foreword ................................................................................................................................................................................... xv Acknowledgments.................................................................................................................................................................. xvii Abbreviations ........................................................................................................................................................................... xix Symbols.................................................................................................................................................................................... xxi 1 Introduction ............................................................................................................................................................................... 1 1.1 Regulatory Issues.............................................................................................................................................................. 1 1.1.1 Near-term vs. Long-term Perspective...................................................................................................................... 1 1.2 Technical Issues: Hydrology ............................................................................................................................................ 2 1.2.1 Hydrologic Evaluation Methodology ...................................................................................................................... 2 2 Fundamental Hydrologic Concepts........................................................................................................................................... 5 2.1 Water Balance .................................................................................................................................................................. 5 2.1.1 Precipitation............................................................................................................................................................. 5 2.1.2 Infiltration ................................................................................................................................................................ 6 2.1.3 Runoff ...................................................................................................................................................................... 6 2.1.4 Soil Water Storage................................................................................................................................................... 7 2.1.5 Evapotranspiration................................................................................................................................................... 7 2.1.6 Recharge .................................................................................................................................................................. 7 2.2 Factors Affecting Recharge.............................................................................................................................................. 7 2.2.1 Climate..................................................................................................................................................................... 8 2.2.2 Soil........................................................................................................................................................................... 8 2.2.3 Vegetation................................................................................................................................................................ 9 2.2.4 Engineered Barriers ............................................................................................................................................... 10 2.3 Estimating Recharge....................................................................................................................................................... 10 2.3.1 Governing Equations ............................................................................................................................................. 10 2.3.2 Recharge Estimation Methods............................................................................................................................... 12 2.3.2.1 Water Budget Analysis .................................................................................................................................. 12 2.3.2.2 Lysimetry ....................................................................................................................................................... 12 2.3.2.3 Zero-Flux Plane ............................................................................................................................................. 13 2.3.2.4 Unit Hydraulic Gradient ................................................................................................................................ 13 2.3.2.5 Tracers...........................................................................................................................................................