CHARACTERIZATION OF U CONTAMINATION AND SOLID PHASE MINERALOGY AND GEOCHEMISTRY IN THE RIFLE SUBSURFACE SEDIMENTS

Several subsurface sediment samples were obtained during recent (2009 and 2010) drilling campaigns at the U.S. Department of Energy, Integrated Field Research Challenge Site located in Rifle, CO. The U plume at this site has not attenuated as previously expected due, most likely, to multiple sources of contaminant U. Both oxidized and naturally reduced zones are present in the Rifle subsurface. The naturally reduced zones (NRZ) usually contain high U concentrations, and, are likely significant contributors to the groundwater contamination. The objective of this study is twofold: i.) To characterize the subsurface sediments (NRZ and oxidized zones) in terms of U contamination and solid phase mineralogy and geochemistry; ii.) To study U interaction with various minerals and determine related sorption mechanisms. A variety of techniques such as XRD, µ-XRD, SEM-EDS, SEM-FIB, TEM-SAED, Mössbauer spectroscopy, EMPA, XRF and XANES, were or will be used to characterize the sediment samples from the La Quinta and Plot C experimental areas at the Rifle site. A series of extraction techniques were or will be employed to determine aqueous U(VI), Fe(II), Fe(III), labile U, acid volatile sulfide (AVS), inorganic and organic carbon contents and other co-contaminants. Initial results from chemical (acid and bicarbonate) extractions, microwave digestions and SEM/EDS measurements demonstrate that the solid phase concentrations of U and S were greater in the naturally reduced sediments. The presence of various co-contaminants, such as As, Zn, V, Cu and Se, was also confirmed in the La Quinta sediments. ZnS framboids and different sulfide minerals were present in the naturally reduced sediments. Initial spectroscopic analyses showed that areas of relatively high U concentrations were also present in contaminated sediments of the NRZs. U occurred as both U(IV) and U(VI). Although based on a small sample size, no strong correlations between Fe and U was found, suggesting that other minerals, in addition to Fe oxides and/or Fe sulfides, may host U in the naturally reduced sediments. Although the nature of U association with different minerals of both oxidized and NRZs is still under investigation, the initial results suggest that the secondary minerals may play an important role in determining U(VI) fate in the Rifle subsurface.