Comparative Studies of Uranium Sorption Pathways and Redox Behavior in the Rifle (CO) and Hanford (WA) Subsurface.

Uranium (U) contaminated soils and subsurface media, such as the ones at the Rifle (CO) and Hanford (WA) sites, are present throughout U.S. and around the world.  The subsurface plumes at these sites have not dissipated as previously predicted.  The complex U interactions within soil and subsoil matrixes are difficult to predict because of the presence of an intricate network of sequential and/or simultaneous (often time dependent) geochemical, biological and hydrological reactions and processes, and the effects of scale-dependent, solid phase physical, mineralogical (and chemical) heterogeneous domains.  Subsurface sediment samples were obtained from both Rifle and Hanford sites.  Sediments from the naturally reduced zones (NRZ) at Rifle were also collected.  For the Rifle site, the objective was to characterize the subsurface sediments in terms of U contamination and solid phase mineralogy and chemistry, study U interaction with various minerals and determine related sorption mechanisms.  For the Hanford site, in addition to the objectives above, U sorption was investigated under a range of pH values and background electrolyte concentrations to mimic the conditions currently existing at the Hanford subsurface (slightly basic pH), or those created after the highly acidic or basic waste liquids were discarded.  A variety of techniques (XRD, µ-XRD, SEM-EDS, SEM-FIB, TEM-SAED, Mössbauer spectroscopy, EMPA, XRF and XANES), were used in conjunction with a series of extraction techniques and batch and column experiments.  Collectively, the results from this study will help to understand the nature of micron-scale U associations with different minerals in the presence of other co-contaminants or under extreme pH conditions; unravel mineral – fluid interface complexity and dynamics by identifying and estimating the role of key geochemical and hydrological reactions and processes controlling U behavior; develop conceptual models and apply predictive models of contaminant behavior to support development, implementation and monitoring of effective and sustainable remediation approaches.