Geochemical and mineralogical investigation of uranium in multi-element contaminated, organic-rich subsurface sediment

Applied Geochemistry 42 (2014) 77–85

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Geochemical and mineralogical investigation of uranium in multi-element contaminated, organic-rich subsurface sediment Nikolla P. Qafoku a,⇑, Brandy N. Gartman a, Ravi K. Kukkadapu a, Bruce W. Arey a, Kenneth H. Williams b, Paula J. Mouser c, Steve M. Heald d, John R. Bargar e, Noémie Janot e, Steve Yabusaki a, Philip E. Long b a

Pacific Northwest National Laboratory, Richland, WA 99352, United States Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States Ohio State University, Columbus, OH 43210, United States d Argonne National Laboratory, Argonne, IL 60439, United States e Stanford Synchrotron Radiation Lightsource, Menlo Park, CA 94025, United States b c

a r t i c l e

i n f o

Article history: Received 16 May 2013 Accepted 2 December 2013 Available online 19 December 2013 Editorial handling by M. Kersten

a b s t r a c t Subsurface regions of alluvial sediments characterized by an abundance of refractory or lignitic organic carbon compounds and reduced Fe and S bearing minerals, which are referred to as naturally reduced zones (NRZ), are present at the Integrated Field Research Challenge site in Rifle, CO (a former U mill site), and other contaminated subsurface sites. A study was conducted to demonstrate that the NRZ contains a variety of contaminants and unique minerals and potential contaminant hosts, investigate micron-scale spatial association of U with other co-contaminants, and determine solid phase-bounded U valence state and phase identity. The NRZ sediment had significant solid phase concentrations of U and other co-contaminants suggesting competing sorption reactions and complex temporal variations in dissolved contaminant concentrations in response to transient redox conditions, compared to single contaminant systems. The NRZ sediment had a remarkable assortment of potential contaminant hosts, such as Fe oxides, siderite, Fe(II) bearing clays, rare solids such as ZnS framboids and CuSe, and, potentially, chemically complex sulfides. Micron-scale inspections of the solid phase showed that U was spatially associated with other co-contaminants. High concentration, multi-contaminant, micron size (ca. 5–30 lm) areas of mainly U(IV) (53–100%) which occurred as biogenic UO2 (82%), or biomass – bound monomeric U(IV) (18%), were discovered within the sediment matrix confirming that biotically induced reduction and subsequent sequestration of contaminant U(VI) via natural attenuation occurred in this NRZ. A combination of assorted solid phase species and an abundance of redox-sensitive constituents may slow U(IV) oxidation rates, effectively enhancing the stability of U(IV) sequestered via natural attenuation, impeding rapid U flushing, and turning NRZs into sinks and long-term, slow-release sources of U contamination to groundwater. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The Department of Energy’s Integrated Field Research Challenge site (IFRC) at Rifle, Colorado (USA) overlies a shallow alluvial aquifer adjacent to the Colorado River contaminated with U, V, As, Se and other contaminants from former milling activities. Groundwater (GW) monitoring over the past decade shows varying patterns of dissolved metals behavior, with significant fluctuations in aqueous U, V, As, and Se associated with seasonal variations in water level and organic carbon addition during remediation activities. The

⇑ Corresponding author. Address: Pacific Northwest National Laboratory, 3335 Q Street, P.O. Box 999, MSIN P7-58, Richland, WA 99352, United States. Tel.: +1 (509) 371 6089. E-mail address: [email protected] (N.P. Qafoku). http://dx.doi.org/10.1016/j.apgeochem.2013.12.001 0883-2927/Ó 2014 Elsevier Ltd. All rights reserved.

U plume at the site was predicted to attenuate below drinking water standard in ca. 10 years based on a distribution coefficient (Kd) model coupled to a GW flow and transport model (DOE, 1999). However, the core of the plume has changed little over more than a decade and, at least in one area, aqueous U concentrations have increased. A key challenge at the site is unraveling the complex biogeochemical and hydrological processes that control contaminant mobility and gaining an understanding of variations in the solubility of such contaminants under changing redox conditions. Field sampling and observations of the shallow unconsolidated alluvial aquifer at the Rifle IFRC demonstrate that unusually high solid phase U concentrations are associated with the naturally reduced zones (NRZs), e.g., one of these zones contains up to 50 times more U than the typical alluvial sediment at Rifle (Campbell et al., 2012).

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NRZs are a common feature of these subsurface media and have been directly observed (through sediment sampling) at four U contaminated sites. In addition, their presence is indirectly suggested in a survey of GW data from U mill tailings sites in the Western U.S., which indicates that more than half of the subsurface reduced zones (11 out of 18) have exhibited persistently high levels of Fe(II) at one or more well locations on a given site (Bargar et al., 2011). However, given their heterogeneous distribution within the subsurface and no means for delineating their location prior to drilling, NRZs are among the least characterized zones within fluvial aquifer deposits. There is, therefore, a need for detailed characterization studies of these important U sinks and potential sources for GW contamination. Hexavalent U [U(VI)] interaction with oxidized sediments and their organic and inorganic sorbents has been in the center of many publications (Qafoku and Icenhower, 2008) and references therein). Contaminant U may interact with the organic matter (Bruggeman et al., 2012), Fe oxides (Bargar et al., 1999; Dodge et al., 2002; Duff and Amrhein, 1996; Ho and Miller, 1986; Hsi and Langmuir, 1985; Jang et al., 2007; Lefevre et al., 2006; Moyes et al., 2000; Reich et al., 1998; Rovira et al., 2007; van Geen et al., 1994; Villalobos and Leckie, 2001; Waite et al., 1994; Wazne et al., 2003), phyllosilicates (Catalano and Brown, 2005), and Fe sulfides such as pyrite (Aubriet et al., 2006; Eglizaud et al., 2006; Scott et al., 2007; Wersin et al., 1994). However, the presence of reducing conditions, high concentrations of organic matter, reduced Fe and S, and microbial activity may promote biogenic mineral formation within NRZs that are uncommon for oxidized sediments. For example, a previous study showed that one NRZ at Rifle contained a unique mineral and U host, i.e., framboidal pyrite (Qafoku et al., 2009). Studies are therefore needed to identify NRZs mineralogical components that may serve as contaminant hosts (sorbents and/or electron donors/ acceptors). Previous studies by Campbell et al. (2012) have also suggested that NRZs may have a variety of potential co – contaminants, which may compete with U for sorption sites and/or available electrons. However, information on the micron-scale spatial distribution and associations of U and other NRZ co – contaminants is scarce. In addition, U solubility and subsequent mobility change dramatically with the redox status, e.g., hexavalent U is relatively mobile although it may undergo some sorption onto reactive mineral surfaces, while tetravalent U [U(IV)] is nearly immobile. Again, the degree of U reduction and the identity of the U-bearing solid phases in NRZs are not known, further reinforcing the need for detailed characterization of these unique natural subsurface systems. A systematic, macroscopic, microscopic and spectroscopic study was conducted to: (i) demonstrate that NRZ sediment contains a variety of contaminants; and unique assortment of potential contaminant hosts (sorbent and/or electron donor or acceptor); (ii) investigate micron-scale spatial association of U with co – contaminants; (iii) determine solid phase associated U valence state and phase identity. The study involved macroscopic experiments (wet chemical batch extractions), bulk characterization efforts (Mössbauer spectroscopy and XRD), micron-scale inspections (SEM-EDS, l-XRF) and molecular scale interrogations (XANES and EXAFS). The important discoveries and trends presented in this paper help in gaining a better understanding of the processes and reactions occurring in NRZs, which affect or control U and other contaminants complex behavior and fate in contaminated subsurface environments. 2. Material and methods 2.1. Sediment collection and nomenclature Sediment cores were collected using a nitrogen gas borehole flush to avoid air (oxygen) contamination. Sediment samples were

immediately processed in a nitrogen – filled field – portable glove bag. Separate subsamples were taken from each predetermined depth which were triple – bagged with a chemical O2 – scrubber between the first and second bags and immediately placed on ice for overnight shipping. Upon arrival at the laboratory, the sediment samples were either frozen or processed in a glove bag. The subsurface area where the sediment samples were collected had not been impacted by previous organic carbon amendment experiments performed at the Rifle IFRC site (Williams et al., 2011) and references therein). In this study, sediments from the La Quinta (LQ) experimental gallery, which were recovered during the installation of the wells in this gallery, were used. Out of seven wells installed in the LQ gallery, five contained naturally reduced zones having a visibly reduced or dark color (Fig. S1, in the Supplementary Data (SD) file). The strongest NRZ zone was narrow (<2 m in width) and located on the center-line between wells 107, 112 and 117 (Fig. S2). Focused microscopic and spectroscopic inspections and interrogations were conducted with the following sediments: LQ 107, depth of 5.5 m below ground surface (bgs), just above the Wasatch formation (hereafter called LQ 107); LQ 112, depth 4.9–5.5 mbgs (hereafter called LQ 112); and LQ 117, depth 4.2– 4.9 mbgs (hereafter called LQ 117). 2.2. Sediment sample handling and extractions Sediment samples were stored inside a 80 °C freezer and subsamples were either air-dried on laboratory bench tops for size-fraction separation analyses and microwave digestion, or dried inside an anoxic chamber and used for chemical extractions and microscopic and spectroscopic studies. Seven size fractions were separated from the <2 mm sediment materials. A series of wet chemical extractions with double – distilled water and 0.5 M HNO3, as well as, microwave digestions with a combination of strong acids (such as HNO3, HCL and HF) were conducted to measure extractable concentrations of U and other co-contaminants. Details about these methods and step – by – step procedures are provided in the SD file. 2.3. Sediment characterization: Mössbauer spectroscopy, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) Mössbauer spectra were collected in the <53 lm size fraction using a 50 mCi (initial strength) 57Co/Rh source. The velocity transducer MVT-1000 (WissEL) was operated in a constant acceleration mode (23 Hz, ±12 mm/s). An Ar–Kr proportional counter was used to detect the radiation transmitted through the holder, and the counts were stored in a multichannel scalar (MCS) as a function of energy (transducer velocity) using a 1024 channel analyzer. Data were folded to 512 channels to give a flat background and a zero – velocity position corresponding to the center shift (CS) of a metal Fe foil at room temperature (RT). Calibration spectra were obtained with a 25 lm thick Fe(m) foil (Amersham, England) placed in the same position as the samples to minimize any errors due to changes in geometry. A closed – cycle cryostat (ARS, Allentown, PA) was employed for below RT measurements. The Mössbauer data were modeled with the Recoil software (University of Ottawa, Canada) using a Voight-based structural fitting routine. The coefficient of variation of the spectral areas of the individual sites generally ranged between 1% and 2% of the fitted values. Because U tends to interact mainly with Fe oxides, sulfides and clay minerals, which all are present in this NRZ, we collected evidence for the occurrence of those minerals that were difficult to detect with Mössbauer spectroscopy, using SEM/EDS. For the SEM and EDS measurements, polished sections and individual clasts of the <53 lm fraction were carbon coated to make them electrically conductive. They were examined using JEOL 6340f SEM. Images were

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then collected using a backscattered electron detector for atomic number contrast.

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3. Results and discussion 3.1. Sediment characterization: size-fractionation and extractions

2.4. U Contamination characterization: X-ray fluorescence (l-XRF) elemental mapping, X-ray absorption near edge structure (XANES), and Extended X-ray Absorption Fine Structure (EXAFS) The l-XRF elemental mapping and XANES spectra were collected at the PNC/XSD beamline 20-ID-A [24] at the Advanced Photon Source. Rh coated K–B mirrors operated at 3 mrad incident angles were used to focus the beam to 3 lm  4.8 lm. Polished <53 lm samples were placed in the X-ray beam at 45° with respect to the incident X-ray beam direction. The sample fluorescence was measured using a four element Vortex detector from SII Nanotechnology. The signals for the various elements were separated using electronic regions of interest (ROI) around their corresponding peaks. These signals are corrected for deadtime and normalized to I0 measured in N2 gas filled miniature ion chamber located just in front of the sample. For the U peak in the spectrum there was significant overlap with the peaks due to Sr and Rb. The amount of this overlap was determined by scanning through the Sr and Rb edges, and then subtracted from the measured U to determine an estimated U signal. A U(VI) standard reference sample was measured using scattered radiation (Cross and Frenkel, 1998) with a uranyl nitrate standard. The peak of the first derivative of the uranyl standard was calibrated to 17171 eV. The XANES spectra from the sample were aligned to the standard spectra collected simultaneously. The spectra were then compared to the U(VI) standard and a natural uraninite as the U(IV) standard. The incident X-ray energy of 17200 eV was used for the l-XRF maps to excite U within the sample.

2.4.1. EXAFS and XANES analyses at SLAC Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy was employed to determine the physical/chemical identity of uranium in the LQ 117 sample. The sample was loaded in an Al sample holder with Kapton windows in an anaerobic chamber (2–5% hydrogen, balance nitrogen) at SSRL. Immediately prior to analysis, the sample assembly was mounted in a liquid N2 cryostat, placed under vacuum, and cooled to 77 K. U LII-edge fluorescence spectra were collected at SSRL beamline 11-2, using a Si (220) double-crystal monochromator detuned to reject higher harmonic intensity. Vertical slits in the experimental hutch were set to 1 mm during the measurement. Energy calibration was monitored continuously using a Y foil and no drift in calibration was detected. EXAFS spectra were background subtracted, splined and analyzed using ATHENA software (Ravel and Newville, 2005). Model compounds used were U(IV) species reduced by bacteria: biomass-bound monomeric U(IV) and biogenic UO2, obtained after bicarbonate extraction to remove loosely bound monomeric U(IV) (Alessi et al., 2012).

About 46–92% of the sediment samples consisted of <2000 lm diameter particles (Table S1). They were sandy and the <53 lm fraction (silt + clay) was present at 1.75–3.64%. Extractions performed with a 0.5 M HNO3 solution and the <2000 lm size fraction under anoxic conditions showed that acid extractable U in these sediments was detectable in the low lg g1 range (e.g., U = 4.0 ± 0.4 lg g1 in sediment LQ 112) (Table S2). Similar amounts of S were extracted from the sediments indicating that S was uniformly distributed throughout the NRZ (e.g., S = 99 ± 13 lg g1 in sediment LQ 112). In addition, significant amounts of As, Zn, Cu, and V were also extracted. Even greater amounts of these elements and potential contaminants were measured in the microwave digestion tests (Table S3). For example, the <53 lm fraction of the sediment samples LQ 107, 112 and 117 which were also used in detailed microscopic and spectroscopic analyses included in the following sections of this paper, contained a maximum (lg g1) of Co (65 ± 3), Cr (83 ± 1), As (181 ± 94), Cd (214 ± 6), Cu (247 ± 0), Se (267 ± 32), Zn (983 ± 17) and V (998 ± 3). These results show that, in addition to U, the sediment samples collected from the LQ experimental gallery had a variety of other elements and potential contaminants, implying that the U of this multi – contaminant subsurface system might exhibit a different behavior than that of the single contaminant system, i.e., in the absence of co – contaminants. Some redox sensitive co-contaminants of this NRZ, such as As, V, and Se, may occur in the aqueous phase as oxyanions, and, therefore, may compete with the U anionic species for sorption sites and/or available electrons, highlighting the complexity of the multi-element contaminated NRZs in terms of predicting the extent and rate of the U sorption, the extent of the redox reactions and the degree of interactions with NRZ minerals. 3.2. Sediment characterization: Mössbauer spectroscopy A qualitative modeling of the 4.2 K (liquid He) Mössbauer spectrum (Fig. 1) indicated that different Fe-(oxy)hydroxides, such as magnetite, hematite and goethite, of varying crystallinity and degree of Al substitution were present in the NRZ sediment. The well-defined sextets and the broad feature were due to the presence of these Fe-(oxy)hydroxides. Fe(II) bearing phyllosilicates

2.5. Statistical analyses Differences between focused (60 lm  60 lm) l-XRF elemental distributions were assessed between two classes of samples (one class where the U was present mainly in the reduced oxidation state, and the other one where U was partially oxidized) using the Mann-Whitney test in SPSS version 19 (IBM Corporation, Armonk, NY) at the a = 0.005 level. Multivariate correlations in XRF data were evaluated using principal components analysis in JMP version 9 (SAS Institute, Inc., Cary, NC). Additional details about the statistical analyses are provided in SD.

Fig. 1. Results from the Mossbauer Spectroscopy analyses (sediment LQ 107).

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were also present. Usually, all these minerals occur in the Rifle subsurface sediments, e.g., the oxidized background sediment (Rifle Aquifer Background Sediment, RABS) (see Campbell et al., 2012 for the Mössbauer spectroscopy results of the RABS). Conversely, the unique mineralogy of the NRZ sediment distinguishes it from oxidized zones. For example, siderite was found in this NRZ. Importantly, the modeled siderite parameters agreed well with those obtained for laboratory-synthesized biogenic siderite (Chistyakova et al., 2010), suggesting a microbial mediated process leading to siderite formation. Features of pyrite and other sulfides (such as mackinawite) were not readily discernible in the spectrum because they overlap with those of Fe(II) phyllosilicates. However, the presence of pyrite and other sulfides was suggested in the SEM/EDS results presented below. An unidentified, Fe-bearing phase (the features are indicated by * in Fig. 1), was also present in this NRZ. The spectral features of this phase were significantly different from those of other Fe sulfides, e.g., greigite [an Fe(II)/Fe(III) sulfide mineral which serves

as precursor for pyrite (Qafoku et al., 2009)]. These features suggest the presence of other, structurally complex Fe sulfides. Structural elemental substitutions and minor changes in chemical composition, which may have occurred in this sediment (suggested in the SEM/EDS analyses and measurements presented below), significantly affect Mössbauer spectral features of Fe-sulfides, and identification of these phases using Mössbauer spectroscopy is difficult. 3.3. Sediment characterization: SEM inspections and EDS chemical composition Fe oxides occurred as discrete, relatively small (micron-size), particles on surfaces of other larger particles (Fig. 2A). Iron sulfides occurred as small particles (Fig. 2B), and, mainly, as coatings (such as the bright area depicted in Fig. 2C). Small, single crystal pyrite particles were also present (clearly depicted in Fig. S3). The presence of other sulfide particles and/or coatings, such as Zn sulfide (Fig. S4), is also suggested. Some of the sulfides had detectable

Fig. 2. Fe oxides (A), and complex sulfide [(Fe, Cu)S (B), and (Fe, Ni, Cu, Zn)S (C)] located in reduced sediment LQ 107.

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amounts of two or more elements, such as the followings: [(Fe, Cu)S, Fig. 2B, Fig. S5; (Fe, Ni, Cu, Zn)S, Fig. 2C; (Cd, Fe, Co, Ni, Cu, Zn)S, Fig. S6]. Note, however, that some of these metals could have been adsorbed to sulfide surfaces rather than being part of the sulfide crystal structure. The positive identification of these minerals using, for example, bulk XRD is challenging because they occur at much smaller concentrations than other primary and secondary minerals. Another important finding was the discovery in the NRZ of solids such as Zn sulfide framboid (Fig. 3) and cupric selenide (CuSe) (the relatively bright area depicted in Fig. 4). The unique structure of Zn sulfide framboid had similar morphological features, such as high reactive surface area and substantial porosity, to those of framboid pyrites of another NRZ at Rifle (Qafoku et al., 2009). Little is known about the origin and formation pathways of framboids of elements other than Fe, and many aspects of their possible interactions with contaminant U or other NRZ co-contaminants are currently unknown. The formation of CuSe would require reduction of Se from seleð4þÞ 2 nate [Seð6þÞ O2 O3 ] and finally to selenide [Se2] 4 ] to selenite [Se as a result of each Se atom accepting 8 electrons, and serving as a micron-scale electron source and/or sink, in the localized areas where its concentration could be substantial. Although other Cu and Se solids may be formed, e.g., cupric selenate (CuSeO4), cupric selenite dehydrate [CuSeO3  2(5)H2O], or cupric selenate pentahydrate (CuSeO4  5H2O), their formation is, however, unlikely because oxygen is scarce in the reduced areas and it is preferentially consumed by reduced Fe and S phases that are more abundant in

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the subsurface. Clearly, a variety of metal sulfides or other exotic minerals may form in the NRZs most likely as a result of the microbial mediated reduction driven by the abundant natural organic matter (NOM). Future research should study the extent and rate of U(VI) interaction (i.e., adsorption and reduction) with rare minerals morphologies and other complex sulfides of the NRZs and study association of U with these unique hosts which may increase U(IV) phase stability by providing an oxidative buffering capacity capable of preferentially titrating molecular oxygen or other potential oxidants from solution. 3.4. U Contamination characterization: l-XRF elemental mapping This effort was conducted in two phases. First, we inspected and interrogated relatively large areas (about 1000 lm  1000 lm, with a step size of 10 lm) of the polished-sections prepared with sediment subsamples (Fig. S7). The objective was to locate areas or spots of high U concentrations. Sediment LQ 107 contained many small, micron size U-rich spots, which were distributed throughout the area interrogated with the X-ray beam. Typically, the detection limit for U is about one lg g1. The concentration we are seeing is much higher than this, but not characteristic of an actual U containing mineral. Note that the U signal is less than 1% of the Fe signal which is probably coming out of a Fe containing phase, i.e., Fe oxide or sulfide. Because of various factors, we cannot directly compare these two signals, but these measurements suggest that the U signal is either from a concentrated phase (i.e., UO2) that is only a small fraction of the hot spot particle or the U

Fig. 3. Back scattered electron image of a Zn sulfide framboid and the EDS spectra collected at different spots (sediment LQ 117).

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Fig. 4. SEM micrographs and EDS spectra taken in sediment LQ 117.

is present throughout the particle but at a much lower, yet detectable, concentration. We conducted similar l-XRF elemental analyses in smaller areas (60 lm  60 lm, with a step size of 5 lm) around the U hot spots, and studied micron-scale elemental distribution and association patterns for the following elements: U, Fe, Se, V, Mn, Cu, Zn, As, Ca, K, and Cr (Figs. 5, S8, S9, S10 and S11). A number of important, micron-scale, trends were revealed in the l-XRF elemental maps. Firstly, we were able to detect U that occurred in concentrated hot spots, which were about 5 to 30 lm in size. These relatively large U concentrated areas, or sinks, may contribute via desorption and/or oxidative dissolution reactions to the GW contamination. Secondly, enriched concentrations of co-contaminants, such as Se, V, Cr, As, Cu, and Zn, were also detected in the sediments. Thirdly, in some high concentration areas, U shared the same space with other redox sensitive co-contaminants. Patterns of elemental associations both similar and dissimilar to those presented above and in the SD section were discovered in other areas. Statistical analyses indicated that the distributions of elements in the l-XRF maps differed between the two classes of data (data from areas of partially oxidized U or reduced U based on the XANES analyses). With the exception of U (no significant difference was observed between U distributions in these areas), elemental concentrations such as Cr, V and Zn in partially oxidized areas had significantly higher mean rankings (p < 0.005) than elements in reduced areas (see Fig. S15 and other materials included in SD). Most likely, Fe oxides, which are better sorbents than Fe sulfides, were more abundant in these areas. Closer inspection of these distributions revealed that low frequencies of enriched elemental concentrations skewed the distributions in partially oxidized elements to higher rankings; whereas high frequencies of low level

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The XANES analysis conducted at APS (an average spectrum of at least five XANES spectra taken in each of the U-rich spots) confirmed that U(IV) was the predominant valence state present in all hot spots where the XANES data were collected (Figs. 6, S12, S13 and S14). The U(IV) percentages in the hot spots varied between 67% and 84% in sediment LQ 117, and 53–100% in sediment LQ 107 of the total U amount in these spots. Similar measurements conducted at SSRL revealed that 88% of U in the hot spot of sediment LQ 117, was U(IV). Collecting EXAFS spectra in these sediments is challenging because of the relative low U concentration in these samples. However, the data collected at SSRL were of good quality. Uranium LIII – edge EXAFS spectra of the LQ 117 sample was fitted using a linear combination of EXAFS spectra of biomass-bound monomeric U(IV) and biogenic uraninite (UO2). The results of the fits are shown in Fig. 7A, B and C. Best fit had 82 ± 10% of the uranium as biogenic UO2, and 18 ± 10% as biomass – bound monomeric U(IV). This fit reproduces all major and most minor features of the sample’s spectrum. Normalized XANES of the sample and of U(IV) and U(VI) standards, as UO2 and autunite, respectively, are presented in Fig. 7C. These findings demonstrated that U(VI) reduction has naturally occurred in the sediments of this NRZ, and provided unequivocal evidence that natural attenuation, i.e., biotically induced reduction, is an important pathway contributing to decreasing U mobility in

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Fig. 5. Select XRF elemental maps [60  60 lm, U (Sr and Rb signals were removed), Fe and Se], in the vicinity of the U hot spot (sediment LQ 107, thin section). Additional elemental maps (V, Mn, Cu, Zn, As, Ca, K, and Cr) from this spot are provided in Fig. S13.

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A

Fig. 6. XANES analysis for U hot spot located in the XRF elemental map of the thin section of sediment LQ-107. The sample (green) line represents the average of 5 XANES measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

subsurface; this is also found in other studies conducted with sediments from the Rifle site (Campbell et al., 2012; Mouser et al., submitted for publication). NRZs usually contain elevated concentrations of NOM, Fe(II), and inorganic sulfides, all of which can scavenge oxidants introduced during infiltration by rain water or snow melt and/or following seasonal incursion of (sub)oxic GW. This creates a unique suburface chemical and biological system, in which NOM drives microbial mediated reduction of U(VI), promoting retention of U(IV). Other attenuation mechanisms, such as adsorption to microbial or inorganic surfaces (N’Guessan et al., 2008) and homogeneous and/or heterogeneous abiotic reduction (Hua and Deng, 2008), may be operational, too. Uranium(VI) was present in some of the XANES interrogated hot spot areas, indicated by the rise of the absorption edge between the U(VI) and U(IV) standards (Figs. S12 and S13). In addition to the hot spots, sorbed U(VI) could have been broadly distributed in these sediments, albeit at concentrations below the detection limit of the l-XRF method. The NRZ U(VI) pool may contribute to the GW contamination. The results from a hydraulically saturated column experiment conducted with sediment LQ 107 leached with GW collected from the LQ experimental gallery showed a peak U concentration of about 2000 lg L1 in the first pore volume (PV) (Mouser et al., submitted for publication), suggesting that U(VI) was indeed present and mobile in the sediments. In addition, substantial increases of U concentration were observed after each stop-flow event indicating that the U release was controlled by a time dependent reaction and/or process, following a similar trend to that observed in experiments conducted with sediments from the Hanford site, WA (Ilton et al., 2008; Liu et al., 2008; Qafoku et al., 2005), although a different mechanism than adsorption most likely controlled U release from the Rifle sediment [e.g., slow oxidation of U(IV) and subsequent release of U(VI)]. Furthermore, results from the column experiments also showed that this sediment was able to sustain for a relatively long time, i.e., about 65 PV, much greater concentrations of U than those of the U.S. Environment Protection Agency’s Maximum Contaminant Level (MCL) for drinking water of 30 lg L1 (0.126 lmol L1), confirming that the NRZ is contributing to the U plume persistence and prolonged GW contamination. XANES analyses combined with l-XRF elemental maps helped in revealing another important trend. Some elemental maps showed that when U occurred alone and was spatially unassociated with other elements (e.g., Fig. S9), both U(IV) and U(VI) were

B

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Fig. 7. A. Uranium LIII-edge EXAFS data for sample LQ 117 and the corresponding fit. B. Fourier transform of v(k) data of sample, model compounds and fit. C. Normalized XANES of the sample LQ 117, and U(IV) and U(VI) standards, as UO2 and autunite, respectively [(12% U(VI) and 88% U(IV)].

present (Fig. S17) implying incomplete reduction of passingthrough and/or diffusive U(VI), or partial re-oxidation of only a fraction of the U(IV). This was also suggested from the results in other hot spots (Figs. S10 and S11).

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One possible explanation would be that U(IV) is ‘‘shielded’’ by other redox sensitive co-contaminants, in areas where they share the same space. For example, XANES analyses revealed that almost all U in the hot spot depicted in Fig. 5 was U(IV) (Fig. 6); in this spot, U and Se bearing solid phase(s) (or coatings) were sharing the same space, or a Se bearing coatings could be covering U(IV) phase protecting it from reoxidation. In some other hot spots, U occurred with redox sensitive elements and/or co-contaminants (such as Zn and Cu), both U(IV) and U(VI) were present. The amount of U(VI) is smaller in the hot spot of Fig. S15 (XANES results are included in Fig. S18), where U was associated with As. It was even smaller in the hot spot depicted in Fig. S11 (XANES results are presented in Fig. S14) where U was associated with Se, As and V. It is quite possible that complex redox reactions (electron transfer reactions among redox sensitive elements), and/or unusual and not well studied U – involving reactions and processes (in the presence of other Fe(II) bearing phases and sulfides of different types, as well as, co – contaminants, solid phases of these co-contaminants, and/or U bearing solid phases of these co – contaminants), may occur in the NRZs 4. Conclusions and implications The results from this characterization study demonstrate that sediments from the NRZ in Rifle, CO have significant amounts of U and other potential co-contaminants suggesting competing sorption reactions and more complex responses to transient redox conditions than single contaminant systems. Sediments were also rich in many potential contaminant hosts (sorbents and/or electron donors), such as Fe oxides, siderite, Fe(II) bearing clays and rare solids such as CuSe, ZnS framboids and chemically complex sulfides. High concentration, multi – contaminant, micron size (ca. 5–30 lm) areas with mainly U(IV) (53–100%) as biogenic UO2 (82%), or biomass – bound monomeric U(IV) (18%), were discovered within sediment matrix. The presence of U(IV) in the NRZ not only confirms the reduction of U(VI) via natural attenuation processes, but also demonstrates that the U(IV) is relatively stable towards re – oxidation during seasonal changes in GW redox status, associated with (a) recharge by rainfall and snow melt, and (b) entrapment of vadose zone molecular oxygen during water table excursions. The NRZ, may therefore, serve as a long-term source of contaminant U to GW. The results from our study demonstrate that the Rifle NRZs may provide an important contribution to U plume maintenance and long-term contamination and persistence, among the factors listed above. In addition, the valuable information on the spatial correlations among different co-contaminants can and will be used to explain temporal variations in dissolved contaminants’ concentrations at the Rifle subsurface (or other similar sites), and to develop remediation strategies based on this information. Acknowledgments This research was supported by the U.S. Department of Energy (DOE), SC and BER, through the IFRC at Rifle, CO. Pacific Northwest National Laboratory (PNNL) is operated for the DOE by Battelle Memorial Institute under the Contract DE-AC06-76RLO 1830. Lawrence Berkeley National Laboratory (LBNL) is operated by the University of California under contract DE-AC02-05CH11231. A portion of this work was supported by the LBNL’s Sustainable Systems Scientific Focus Area. The research presented in this paper was conducted in part in the Environmental Molecular Sciences Laboratory located at PNNL and the Stanford Synchrotron Radiation Laboratory, which are both national scientific user facilities respectively operated by Battelle Memorial Institute on behalf of

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