Unraveling the Soil System Complex Behavior: Mineral and Migrating Fluids Interactive Processes and Reactions.

Sediment samples from a naturally bioreduced zone of aquifer and from the oxidized vadose zone were obtained during recent drilling campaigns at the U.S. Department of Energy, Integrated Field Research Challenge Site located in Rifle, CO.  The objective was to study the contaminant uranium hosting minerals, and to determine their role on uranium attenuation.  The sediment samples were or will be characterized with XRD, Mössbauer spectroscopy, XANES, XRF, SEM-EMPA, SEM-FIB, and TEM-SAED.  A series of extraction techniques were also employed to determine extractable amounts of U(VI) and different  cations and anions.  Results indicated that Fe(II) and S contents were substantially higher in sediments that were naturally bioreduced.  U occurred as both U(IV) and U(VI) and it was associated with framboidal pyrite and Fe-oxides.  These minerals may play an important role in determining U(VI) fate in the sediments. Further characterization and additional experimental work is underway with sediments from other galleries and experimental areas at Rifle.