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Application of Surface Complexation Modeling to Describe Uranium(VI) Adsorption and Retardation at the Uranium Mill Tailings Site at Naturita, Colorado (NUREG/CR-6820)

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Publication Information
Abstract

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NUREG/CR-6820 (PDF - 691 KB)

Publication Information

Manuscript Completed: April 2003
Date Published: December 2003

Prepared by
J.A. Davis, G.P. Curtis
U.S. Geological Survey
Menlo Park, CA 94025

J.D. Randall, NRC Project Manager

Prepared for
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code W6813

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Abstract

The objective of this study was to demonstrate a surface complexation modeling approach at the field scale for estimating K_d values and the retardation of a sorbing radionuclide with complex aqueous chemistry. The Uranium Mill Tailings Remediation Act (UMTRA) site near Naturita, Colorado was chosen for study, because it had a well-developed and definable uranium(VI) plume in a shallow alluvial aquifer and had spatially variant chemical conditions that we believed would be important in influencing U(VI) transport and retardation. It was shown in laboratory batch and column experiments with Naturita sediments that the adsorption and retardation of U(VI) by the Naturita sediments was strongly influenced by the dissolved carbonate concentration (alkalinity). A Generalized Composite surface complexation model (GC-SCM) was developed for the Naturita aquifer background sediments (NABS) based on fitting batch U(VI) adsorption data. With only two surface reactions (four surface species), the GC-SCM without electrical double layer terms was able to accurately simulate K_d values for U(VI) adsorption on the Naturita aquifer sediments over the observed range of pH and dissolved carbonate and U(VI) concentrations. For the range of Naturita aquifer chemical conditions, alkalinity was more important than either variable pH or U(VI) concentration in influencing U(VI) mobility. K_d values ranged from 0.29 to 22 mL/g when calculated for all Naturita groundwater analyses using the SCM. Low K_d values were associated with portions of the U(VI) groundwater plume containing high concentrations of dissolved U(VI) and alkalinity. Higher K_d values were associated with low concentrations of dissolved U(VI) and alkalinity.