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Geochemical Controls on Contaminant Uranium in Vadose Hanford Formation Sediments at the 200 Area and 300 Area, Hanford Site, Washington

^a Pacific Northwest National Lab., Richland, WA 99354
^b Lawrence Berkeley Nation Lab., Berkeley, CA 94720
^c Argonne National Lab., Argonne, IL 60439

View larger version (104K): [in this window] [in a new window]   FIG. 1. Upper: Arial photograph of the 300 Area North Process Pond (NPP), in 2003, after excavation of the residual pond wastes. Samples described here were removed from the site after this photo was taken. Lower: Photograph of the 200 East Area BX tank farm under construction, 1944.

View larger version (48K): [in this window] [in a new window]   FIG. 2. Layout of the completed B-BX-BY tank farm. The 1951 spill occurred at tank BX-102. Locations of boreholes 299-E33-45, 299-E33-41, and 299-E33-18 are indicated.

View larger version (69K): [in this window] [in a new window]   FIG. 3. Uranium contaminant plume concentrations in the Hanford Site 300 Area. The excavated process ponds were located near the river shore. The North Process Pond (NPP) sample locations are indicated.

View larger version (61K): [in this window] [in a new window]   FIG. 4. Schematic diagrams for 299-E33-45. The stratigraphic column is within the Hanford formation and included a fine silty sand interval near the water table. The borehole intersected the uranium plume at approximately 43 m (140 ft), indicated by the leachate 238U concentrations and by the in situ gamma logs (sections logged through double casings indicated *). Moisture contents in 2001 were less than 10% w/w in the vadose zone, except in fine-grained, less-conductive sedimentary intervals.

View larger version (141K): [in this window] [in a new window]   FIG. 5. Uranium silicate mineralization in core samples was confined to intragrain microfractures in granitic lithic clasts. (a and b) Backscattered electron images of nm-scale uranyl silicate florettes in fractures; K: potassium feldspar, P: plagioclase feldspar, S: smectite secondary mineralization. (c) Cross-section in transmission electron microscope image of florettes on the fracture face; Pt: platinum coating applied by focused ion beam tool. (d) transmission electron image of microcrystalline uranyl silicate domains within a uranyl silicate.

View larger version (18K): [in this window] [in a new window]   FIG. 6. Observed and modeled uranyl silicate dissolution results. Model results assume several solid phases; results most nearly conform to sodium boltwoodite dissolution.

View larger version (59K): [in this window] [in a new window]   FIG. 7. Spectral gamma logging results for borehole 299-E33-18, showing the accumulation of vadose-zone uranium and cobalt. (Courtesy of S.M. Stoller Co.)

View larger version (39K): [in this window] [in a new window]   FIG. 8. Log-log plot of 235U vs. 238U for boreholes 299-E33-45, 299-E33-41, and 299-E33-18. 238U is derived from 234Pa activity. The ratio of 235U:238U apparently varies from 0.04 in 299-E33-45 to 0.08 in 299-E33-18. Results for 299-E33-41 are split according to depth (shown), and are intermediate in isotopic ratios and agree with the ratios of both other boreholes. (Courtesy of S.M. Stoller Co.)

View larger version (27K): [in this window] [in a new window]   FIG. 9. Results of timed imbibation experiments using Hanford formation sediment samples and simulated waste solutions. Experiments were conducted in columns and approximate the accumulation of uranium in the vadose zone.

View larger version (85K): [in this window] [in a new window]   FIG. 10. Electron microprobe comparisons of uranium, calcium, and silicon concentrations in process pond materials (NP 4-1) and in sediments excavated from beneath the process ponds. Uranium-rich solids are less abundant at depth. False-color scales are in X-ray intensity (counts per second per nA).

View larger version (121K): [in this window] [in a new window]   FIG. 11. Electron microprobe and X-ray microprobe analyses of copper and uranium in deeper North Process Pond (NPP) sediments. Disseminated copper and uranium, along with uranium-rich solids are present at a depth of 1.2 m (4 ft), but neither is detectible at a depth of 4.9 (16 ft) in Sample NPP 1-16, included for reference, even using the X-ray microprobe. Sample NPP 2-4 includes a backscattered electron image and electron microprobe elemental abundance maps, overlain by X-ray microprobe abundance maps. The red arrows indicate occurrence of uranium-rich mineralization; the green arrow indicates an area of disseminated copper and uranium.

View larger version (18K): [in this window] [in a new window]   FIG. 12. X-ray diffraction analysis over the two-theta range of 5 to 10 degrees; C: chlinochlore; M: muscovite.