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# A Combined Analytical Study to Characterize Uranium Soil and Sediment Contamination: The Case of the Naturita UMTRA Site and the Role of Grain Coatings (NUREG/CR-6898)

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## Publication Information

**Manuscript Completed:** August 2005**Date Published:** February 2006

**Prepared by:**C. F. Jové Colón1, C. Sanpawanichakit2, H. Xu3,R.T. Cygan1, J.A. Davis4, D.M. Meece4, R.L. Hervig5

E. O'Donnell, 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 Y6464

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## Abstract

Composite sediment samples from the Uranium Mill Tailings Remedial Action site at Naturita, Colorado, were analyzed for uranium using a suite of microbeam analytical techniques encompassing Scanning Electron Microscopy—Energy Dispersive Spectrometry (SEM-EDS), Secondary Ion Mass Spectrometry (SIMS), High-Resolution Transmission Electron Microscopy (HRTEM), Micro-Synchrotron X-Ray Fluorescence (M-SXRF), and Micro-X-ray Absorption Near-Edge Spectroscopy (M-XANES). Two sets of alluvial sediment samples were considered in this study: an untreated composite sediment sampled from several uranium contaminated wells, and a carbonate-free (treated) composite sediment from an area significantly up-gradient from the contaminated portion of the site. The carbonate-free sample was treated with Na-acetate to remove carbonate material and then subsequently exposed to 10-5 molal uranyl (U6+) nitrate solution. The purpose of the treatment was to investigate uranium adsorption onto the sediment without complications resulting from carbonate complexation with uranyl ions. In all samples, SEM-EDS analysis showed the conspicuous presence of Fe-rich and Al-Si rich (clay) coating layers surrounding the periphery of soil/sediment grains. The bulk grains that serve as substrate to the overlying coatings are mostly quartz and detrital feldspar. The Fe-rich coatings are arranged conformably in both continuous and discontinuous modes, in some cases between the quartz interface and the clay-rich region. However, Fe-rich phases are also present as small scattered particles immersed in the clay layer. SIMS analysis on polished epoxy grain mounts of the naturally-contaminated composite sample reveals the presence of uranium diffusely distributed within the Al-Si rich clay layer. No clear association between Fe and uranium from materials collected in sampling wells with the highest level of uranium contamination was discerned using this analytical technique. However, M-SXRF analyses on the laboratory contaminated carbonate-free sample reveals a close association between uranium and Fe-rich domains on the grain surface. HRTEM analysis on a single grain from the carbonate-free sample characterized by this strong uranium-Fe spatial correlation indicate that these Fe-bearing phases are highly heterogeneous, composed mainly of mixed domains of hematite, goethite, and nanoporous and/or amorphous Fe-(oxy)hydroxides. These Fe-rich nanoporous and amorphous domains within larger Fe-bearing grains are identified as ferrihydrite on the basis of HRTEM observations. The arrangement of Fe-rich amorphous phase domains along with crystalline goethite resembles aggregated textural forms. However, HRTEM analysis indicates that these domains are structurally coherent and continuous, suggesting homogeneous transformation. Mixed layer illite/smectite (I/S) clays are to a great extent the main coating phases present hosting nanosized Fe and Ti oxides. On the basis of these combined analytical observations, mixed layer clays and Fe-rich coatings are the main sinks for uranium in the composite sediment material at Naturita.

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