Effects of Adsorption Constant Uncertainty on Containment Plume Migration: One- and Two-Dimensional Numerical Studies (NUREG/CR-6780)

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Publication Information

Manuscript Completed: April 2002
Date Published: June 2002

Prepared by:
Louise J. Criscenti and Randall T. Cygan, SNL Geochemistry Department
Mehdi Eliassi, SNL Geohydrology Department
Carlos F. Jové-Colón, SNL Total Systems Performance Assessment Department

Sandia National Laboratories
Albuquerque, NM 87185-0750

E. O'Donnell, NRC Technical Monitor

Prepared for:
Radiation Protection, Environmental Risk, and Waste Management Branch
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code W6811

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Abstract

In this study, one- and two-dimensional (1-D and 2-D) reactive-transport models, with specific application to the hydrology and mineralogy of the Naturita uranium mill tailings site in Colorado, are used to examine variations in model predictions due to uncertainty in the model adsorption constants. This work demonstrates the importance of selecting the appropriate adsorption constants when using reactive-transport models to evaluate risk and pollution attenuation at contaminated sites. In our models, uranium is removed from uranium mill tailings leachate through adsorption onto smectite, an abundant clay mineral at the Naturita site. Uranium adsorbs to specific surface sites on both the basal planes and edges of the smectite. Because uranium adsorbs predominantly to the aluminum edge surface sites [>(e)AIOH], uncertainty was examined only in the equilibrium constants associated with these sites. One hundred pairs of equilibrium constant (log K) values for the surface species >(e)A10 and >(e)A1OUO₂⁺ were selected from normal distributions of each log K using the Latin Hypercube Sampling method. For the I-D simulations, two distinct groups of uranium breakthrough curves can be identified. In the first group, the breakthrough curves exhibited a classical sigmoidal shape whereas in the second group the breakthrough curves displayed higher uranium concentrations in solution over greater distances and times. These two groups are clearly separated by two different ranges of log K >(e)A10 values or two different ranges for the smectite point of zero charge. Preliminary 2-D simulations also demonstrate that predictions of both transverse and longitudinal plume migration are influenced by the choice of adsorption constants.