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The high stability of calcium uranyl carbonate complexes in the circumneutral pH range has a strong impact on U(VI) sorption in calcareous soils. To quantify this influence, sorption of U(VI) to soils in the presence of naturally occurring calcium carbonate was investigated by conducting batch experiments in which either U(VI) concentration or solution pH was varied. Two soils containing different calcium carbonate concentrations were selected, one from Oak Ridge, TN, and another from Altamont Pass, CA. The results show that the presence of calcium carbonate in soils strongly affects U(VI) sorption. Higher concentrations of soil calcium carbonate lead to a pronounced suppression of the pH-dependent sorption curve in the neutral pH range because of the formation of a very stable neutral complex of calcium uranyl carbonate in solution. A surface complexation model considering both strong and weak sites for ferrihydrite and ionizable hydroxyl sites for clay minerals was compared with experimental results, and U(VI) binding parameters were reasonably estimated. Fair agreement was found between the model predictions and sorption data, which span a wide range of U(VI) concentrations and pH. The results also show that appropriate solution-to-solid ratios need to be used when measuring distribution coefficients in calcareous soils to avoid complete CaCO3 dissolution and consequent dilution of calcium uranyl carbonate complexes.

Chromium has become an important soil contaminant at many sites, and facilitating in situ reduction of toxic Cr(VI) to nontoxic Cr(III) is becoming an attractive remediation strategy. Acceleration of Cr(VI) reduction in soils by addition of organic carbon was tested in columns pretreated with solutions containing 1000 and 10 000 mg L(-1) Cr(VI) to evaluate potential in situ remediation of highly contaminated soils. Solutions containing 0,800, or 4000 mg L(-1) organic carbon in the form of tryptic soy broth or lactate were diffused into the Cr(VI)-contaminated soils. Changes in Cr oxidation state were monitored through periodic micro-XANES analyses of soil columns. Effective first-order reduction rate constants ranged from 1.4 x 10(-8) to 1.5 x 10(-7) s(-1), with higher values obtained for lower levels of initial Cr(VI) and higher levels of organic carbon. Comparisons with sterile soils showed that microbially dependent processes were largely responsible for Cr(VI) reduction, except in the soils initially exposed to 10 000 mg L(-1) Cr(VI) solutions that receive little (800 mg L(-1)) or no organic carbon. However, the microbial populations (< or = 2.1 x 10(5) g(-1)) in the viable soils are probably too low for direct enzymatic Cr(VI) reduction to be important. Thus, synergistic effects sustained in whole soil systems may have accounted for most of the observed reduction. These results show that acceleration of in situ Cr(VI) reduction with addition of organic carbon is possible in even heavily contaminated soils and suggest that microbially dependent reduction pathways can be dominant.

Colloid sorption onto air-water interfaces in a variety of natural environments has been previously recognized, but better quantification and understanding is still needed. Affinities of clay colloids for the air-water interface were measured using a bubble-column method and reported as partition coefficients (K). Four types of dilute clay suspensions were measured in NaCl solutions under varying pH and ionic strength conditions: kaolinite KGa-1, illite IMt-2, montmorillonite SWy-2, and bentonite. The K values of three types of polystyrene latex particles with different surface-charge properties were also measured for comparison. Kaolinite exhibited extremely high affinity to the air-water interface at pH values below 7. Illite has lower affinity to air-water interfaces than kaolinite, but has similar pH dependence. Na-montmorillonite and bentonite clay were found excluded from the air-water interface at any given pH and ionic strength. Positively and negatively charged latex particles exhibited sorption and exclusion, respectively, at the air-water interface. These results show the importance of electrostatic interactions between the air-water interface and colloids, especially the influence of pH-dependent edge charges, and influence of particle shape.

Recent studies have shown how water films at near-zero matric potentials on fracture surfaces could permit fast flow in unsaturated fractures. The present work is aimed at delineating the ranges of conditions necessary to permit film flow. The approximate matric potential needed to permit thickening of water films on fracture surfaces was estimated through correlating air-entry matric potentials and permeabilities for a wide range of porous media. An upper matric potential limit, associated with saturating the fracture, was related to the fracture aperture. These two limits, one dependent on matrix permeability and the other on fracture aperture, delineate the matric potential range within which thick water films develop on fracture surfaces. Rocks with matrix permeabilities less than ∼10−14 m2, and fractures with apertures greater than ∼30 μm permit some matric potential range over which thick water films can form. Highly transmissive films can develop on fracture surfaces only under near-zero matric and pressure potentials.

The distribution of metal contaminants such as chromium in soils can be strongly localized by transport limitations and redox gradients within soil aggregates. Measurements of Cr(VI) diffusion and reduction to Cr(III) were obtained in soil columns representing transects into soil aggregates in order to quantify influences of organic carbon (OC) and redox potentials on Cr transport distances and microbial community composition. Shifts in characteristic redox potentials, and the extent of Cr(VI) reduction to Cr(III) were related to OC availability. Depth profiles of Cr(VI, III) obtained with micro X-ray absorption near edge structure (micro-XANES) spectroscopy reflected interdependent effects of diffusion and spatially dependent redox potentials on reduction kinetics and microbial community composition. Shallow diffusion depths (2-10 mm) and very sharply terminated diffusion fronts in columns amended with OC (80 and 800 ppm) reflected rapid increases in Cr reduction kinetics over very short (mm) distances. These results suggest that Cr contamination in soils can be restricted to the outsides of soil aggregates due to localized transport and rapid reduction and that bulk sample characterization is inadequate for understanding the controlling biogeochemical processes.

Laboratory experiments were conducted to generate stable microbubbles and to measure their transport properties in porous media in order to evaluate their possible use in subsurface remediation. A mathematic model was developed as a special case of filtration theory for predicting microbubble transport in porous media. Several physical-chemical parameters were tested, including surfactant type, surfactant concentration, generation method, and pressure. A combination of an anionic surfactant and a hydrophobic nonionic surfactant was found to yield the highest concentrations of stable microbubbles (3 × 109 bubbles mL−1) in the size range of 0.7 to 20 μm. The specific surface areas of the microbubble suspensions were in the range of 50 cm2 mL−1, and specific air volumes were >0.07 mL mL−1. For 1 pore volume of injected bubble suspension, effluent recoveries of 100, 80, and 30% were achieved from columns of coarse (415–500 μm), medium (150–212 μm), and fine (53–106 μm) sands, respectively. Effluent recovery in the fine sand column increased to 63% following a 3 pore volume injection. Microbubble generation and injection under pressure were shown to minimize microbubble loss due to gas dissolution. Results from the modeling suggest that retention of microbubbles could be adequately described by filtration theory.

Although fractures in rock are well recognized as pathways for fast percolation of water, processes which permit fast flow along unsaturated fracture pathways remain to be identified and understood. Earlier aperture-based models of flow in partially saturated fractures permit fast flow only through a continuous network of locally saturated segments. Film flow was recently identified as a mechanism capable of sustaining fast flow along truly unsaturated fractures when the matric potential is very close to zero. Another mechanism for fast flow along unsaturated fractures is introduced in this study, “surface-zone flow,” which can be important when the permeability of the rock along fractures (fracture skin) is significantly greater than that of the bulk rock matrix. In such systems the fracture surface zone provides low resistance pathways through which fast flow (relative to the bulk matrix rock) can occur, even when the fractures are at very low water saturation. Initial experimental tests of surface-zone fast flow were performed. Surface-zone fast imbibition of water was measured on a welded tuff and a rhyolite. However, because (1) imbibition rates are also strongly influenced by rock wettability, (2) these initially air-dry rocks exhibited finite contact angles upon exposure to water, and (3) we lack methods to reliably measure permeabilities of thin regions on rock surfaces, quantification of permeability contrasts was not possible in these initial tests.

Transient film flow in unsaturated fractures was investigated conceptually and experimentally. By considering films on fracture surfaces as analogs to water in partially saturated porous media, the film hydraulic diffusivity and equation for transient film flow are obtained from their porous medium counterparts, the hydraulic diffusivity and the Richards equation. Experiments on roughened glass show that the average film thickness dependence on matric potential can be approximated as a power function. It is also shown that the film hydraulic diffusivity increases with increased film thickness (and with increased matric potential). Fast film flow (average velocities greater than 3×10−7 m s−1 under unit gradient conditions) was observed for average film thicknesses greater than 2μm and matric potentials greater than −1 kPa.

The effect of particle size (dp) to film thickness (ho) ratio on the motion of spherical particles in a stable liquid film flowing down an inclined flat surface is studied experimentally. Previously reported models that are based on force and torque balance are modified to predict the motion of particles that are smaller than film thickness. At low dp/ho values, particle velocity is observed to increase nearly linearly with particle size, reflecting the increasing influence of hydrodynamic drag as larger particles expose their surface to regions of higher fluid velocity. Good agreement between model predictions and experimental results is observed for small dp/ho ratios. When dp/ho is in the range of ∼0.7−1, particle velocities are observed to decrease rapidly with an increase in size. This may be attributed to the effect of the proximity of the free interface to the particle surface and also the deformation of the free surface induced by the moving particle. When dp/ho is in the approximate range of 1−1....

A method was developed for fabricating transparent glass casts of fractured rock pairs. These glass casts provide reproduction of surface topography and roughness of natural fractures, optical clarity, and representative wettabilities of mineral surfaces. The glass casts are an improved tool for studying flow and transport in fractures compared to previous approaches. The surface of a clean glass fracture has a contact angle near zero, and more importantly, the surface wettability can be intentionally altered with chemical treatment. Glass casts of rock fracture surfaces may be used for visually and quantitatively studying various physical, chemical, and microbial processes occurring in rock fractures, especially when multiple fluid phases are involved.

Quantification of surface excesses of colloidal particles at gas-liquid interfaces under environmentally relevant conditions has not previously been reported, despite its importance in understanding and predicting partitioning, transport, and transformations of colloids and other organic and metal species complexed onto colloids in many natural environments and engineered systems. In this study, the authors developed a bubble column method for measuring partition coefficients of colloids at air-water interfaces. The method was validated by comparing the adsorption isotherm constant for a surfactant measured using the authors` method with that determined from conventional surface tension measurements and the Gibbs absorption equation. The first measured partition coefficients of kaolinite and humic colloids at the air-water interfaces are reported. The bubble column method can also be used to identify surface exclusion of colloids at the air-water interface. By extension, this method will permit quantification of surface activities of a wide range of inorganic, organic, and microbial colloids as well as molecular species complexed onto colloids.

A film-straining theory is introduced, which proposes that transport of suspended colloids can be retarded due to physical restrictions imposed by thin water films in partially saturated porous media. A quantitative, mechanistic model is provided to predict the film-straining efficiency. In this model, the concepts of “critical matric potential” and “critical saturation” are introduced, at which thick film interconnections between pendular rings are broken and film straining begins to become effective. The modeled magnitude of colloid transport through water films depends on the ratio of colloid size to film thickness and on flow velocity. Effective penetration of hydrophilic colloids through unsaturated porous media is predicted when a system is above the critical saturation value. For colloids smaller than the thickness of adsorbed thin water films, the model predicts that colloids can still be efficiently transported, even when the system matric potential and saturation are lower than their critical va...

This study shows that hydraulic properties of individual fracture surfaces can be meaningfully defined and measured, and that water film flow is a mechanism contributing to fast, unsaturated flow in fractures. The hydraulic conductivity of an unconfined block of Bishop Tuff was measured over a range of near-zero matric potentials, where differences between hydraulic conductivities obtained without and with wax sealing of its lateral sides allowed isolation of film flow effects. Tensiometer and flux measurements showed that surface film flow in this system was significant for matric potentials greater (more positive) than about −250 Pa. In this range the average film thickness was shown to be potential dependent and proportional to the observed enhanced hydraulic conductivity. Measured average surface film thicknesses ranged from 2 to 70 μm, with average film velocities in the range of 2 to 40 m d−1 (about 103 times faster than that of the pore water under unit gradient saturated flow). Our experiments demonstrate that hydraulic properties of macroscopic surfaces of porous media are quantifiable, related to surface roughness, and potentially important in the flow of water in vadose environments. This study further shows that contrary to existing conceptual models, unsaturated flow in fractures cannot generally be predicted solely on the basis of aperture distribution information. The high velocities of these surface films suggest that film flow can be an important mechanism contributing to fast flow in unsaturated fractures and macropores, especially in media characterized by low-permeability matrix and along regions of convergent flow in partially saturated fractures.

Recent studies by [Crist et al. (2004)][1]([2005][2]) attempted to provide pore-scale insights into mechanisms responsible for controlling colloid transport in unsaturated porous media. However, their key observation of colloids being trapped at air–water–solid (AWS) contact lines relied on

Microscale experiments can provide mechanistic insights into larger-scale flow and transport phenomena. Studies of the microscale mechanics involved in preferential flow in general, and unsaturated fast flow paths in particular, require the development of new experimental techniques. A new method for constructing glass micromodels has been developed which permits direct visualization and quantification of flow and transport phenomena in fractured porous media. In the fracture-matrix micromodels a sequential etching procedure was developed in order to provide the necessary contrast of depths between matrix pores and fracture apertures. This high contrast in etching depths ensures that very different capillary properties are associated with micromodel "fractures" and "matrix" blocks. Improved techniques were also developed for reducing the pore sizes of the matrix to a natural fine-grained sandstone pore scale. The improved micromodel pattern designs allow for previously unachievable control of boundary conditions. Various saturated and unsaturated fracture flow and transport processes can be visually and quantitatively studied with these micromodels. A method for directly measuring pore-scale flow velocity distribution through tracing trajectories of suspended fluorescent microspheres was also developed. Examples of applications include measurements of velocity profiles in fractures, imbibition, fracture-matrix transient flow, and matrix diffusion. In general, the improved micromodel method provides a unique tool for exploring some of the previously unrecognized flow and transport processes in fractured porous media. This research is directed at providing microscale explanations to some currently unresolved flow and transport issues important in predicting the larger-scale flow processes.

Numerous previous studies of bacterial transport in groundwaters and to deep aquifers and sediments have either neglected, or regarded as insignificant, the potential contribution of bacterial sedimentation. This study examines the potential significance of sedimentation as a mechanism for bacterial transport. A simple model is developed to predict the behavior of particles (bacterial or inorganic colloids) sedimenting through granular porous media under hydrostatic conditions. The model indicates that tortuosity-limited sedimentation velocities through porous media consisting of large, well-rounded grains can proceed at velocities close to (≈90% that of) free sedimentation in water columns when particle-grain interactions involve only tortuosity. The assumption of neutral buoyancy of bacteria was demonstrated to be invalid through buoyant density measurements on 25 subsurface bacterial strains (using Percoll density gradient centrifugation). An average buoyant density of 1.088 Mg m−3 was obtained (range from 1.040 to 1.121 Mg m−3). The two nonmotile bacterial strains selected for sedimentation experiments were Arthrobacter globiformis B672 (isolated from the Middendorf aquifer, 259-m depth), and OYS3, a streptomycin-resistant strain isolated from shallow groundwaters at Oyster, Virginia. All experiments were carried out under nongrowth conditions. Stokes' law sedimentation velocities for the two bacterial strains calculated from measurements of buoyant densities and characteristic sizes were 5.8 and 40 mm d−1, respectively. Direct measurements of free sedimentation of Arthrobacter B672 and OYS3 through water columns (21°C) yielded median velocities of 7.1 and 42 mm d−1, respectively, in good agreement with Stokes' law calculations. The Arthrobacter B672 and OYS3 strains sedimented through saturated sand columns (quartz sand, 300–420 μm diameter) under hydrostatic conditions at median velocities of 7 and 17 mm d−1. Thus the sedimentation model is consistent with sand column observations on Arthrobacter B672 and too simplistic in the case of OYS3. Bacterial breakthrough by sedimentation exhibited trends consistent with first-order attenuation with distance. Bacterial deposition coefficients for this first-order model were in the range of 0.008–0.012 mm−1. Surface physical-chemical interactions, grain and pore size distributions, and grain surface microtopography can be very important in controlling the effectiveness of bacterial sedimentation as a transport mechanism. This research suggested that if timescales are sufficiently long, spanning many generations, sedimentation can become a significant mechanism for bacterial transport.

Waste disposal into seepage basins has generated groundwater contaminant plumes at many locations. At the F Area within the Savannah River Site, Pu was extracted from depleted U from 1955 to 1988, with wastewater discharged into seepage basins. Basin 3 was the largest F-Area seepage basin, receiving acidic wastewater containing radionuclides (including 3 H, 129 I, and multiple isotopes of U, Pu, Sr, and Cs), elevated NO 3 , and some metals. Contaminants transported into the groundwater migrate toward Fourmile Branch, a tributary to the Savannah River. We developed a two-compartment model and used 20 yr of groundwater quality data to estimate the post-closure drainage of waste solutions through its vadose zone into the aquifer. Tritium, NO 3 − , and specific conductance were used as tracers in the model to estimate drainage rates. Our calculations indicate that early stages of post-closure waste drainage occurred with high water fluxes (~0.5 m yr −1 ) and quickly declined. Even 20 yr after basin closure, however, drainage continues at several centimeters per year. While the magnitude of this late-stage drainage rate is low, its impact is large because of the high concentrations of contaminants it continues to supply to the groundwater. These estimated drainage fluxes constrain predictions on the waste plume behavior, especially with respect to its trailing gradient and time scales suitable for monitored natural attenuation. Our methodology requires only groundwater monitoring data and a small number of well-constrained input quantities. This approach can be useful for understanding contaminant dissipation at other locations as well, especially where the hydrogeological setting is relatively simple.

Elevated concentrations of U in contaminated environments necessitate understanding controls on its solubility in groundwaters. Here, calculations were performed to compare U(VI) concentrations expected in typical oxidizing groundwaters in equilibrium with different U(VI) minerals. Among common U(VI) minerals, only tyuyamunite (Ca(UO(2))(2)V(2)O(8)·8H(2)O), uranophane (Ca(UO(2))(2)(SiO(3)OH)(2)·5H(2)O), and a putative well-crystallized becquerelite (Ca(UO(2))(6)O(4)(OH)(6)·8H(2)O) were predicted to control U concentrations around its maximum contaminant level (MCL = 0.13 μM), albeit over narrow ranges of pH. Given the limited information available on uranyl vanadates, room temperature Ca-U-V precipitation experiments were conducted in order to compare aqueous U concentrations with tyuyamunite equilibrium predictions. Measured U concentrations were in approximate agreement with predictions based on Langmuir's estimated ΔG(f)°, although the precipitated solids were amorphous and had wide ranges of Ca/U/V molar ratios. Nevertheless, high initial U concentrations were decreased to below the MCL over the pH range 5.5-6.5 in the presence of newly formed CaUV solids, indicating that such solids can be important in controlling U in some environments.

Wettability of reservoir minerals and rocks is a critical factor controlling CO(2) mobility, residual trapping, and safe-storage in geologic carbon sequestration, and currently is the factor imparting the greatest uncertainty in predicting capillary behavior in porous media. Very little information on wettability in supercritical CO(2) (scCO(2))-mineral-brine systems is available. We studied pore-scale wettability and wettability alteration in scCO(2)-silica-brine systems using engineered micromodels (transparent pore networks), at 8.5 MPa and 45 °C, over a wide range of NaCl concentrations up to 5.0 M. Dewetting of silica surfaces upon reactions with scCO(2) was observed through water film thinning, water droplet formation, and contact angle increases within single pores. The brine contact angles increased from initial values near 0° up to 80° with larger increases under higher ionic strength conditions. Given the abundance of silica surfaces in reservoirs and caprocks, these results indicate that CO(2) induced dewetting may have important consequences on CO(2) sequestration including reducing capillary entry pressure, and altering quantities of CO(2) residual trapping, relative permeability, and caprock integrity.

This paper explores the significance of the gas-water interface on colloid sorption and transport. Three types of common saturation conditions were simulated in packed sand columns: (1) a completely water-saturated condition, (2) gas bubbles trapped by capillary forces as a nonwetting residual phase (15% gas), and (3) gas present as a continuous phase (46% gas), or in other words, a vadose zone situation. Different saturations provided different interfacial conditions. Two types of polystyrene latex particles (0.2 μm), hydrophilic and hydrophobic, were used in each of the three saturations. Relative surface hydrophobicity of latex particles was characterized by contact angle measurements. Each experiment was repeated five times. A total of 30 columns with good reproducible packing and gas content gave reproducible particle breakthrough curves. The retention of both hydrophilic and hydrophobic colloids increased with gas content of the porous medium. Colloids preferentially sorbed onto the gas-water interface relative to the matrix surface. The degree of sorption increased with the increase of colloid surface hydrophobicity. The results validate and quantify direct microscopic scale visualization observations made in two-dimensional pore network glass micromodels (Wan and Wilson, 1994). These findings suggest an additional mechanism for filtration and for particulate transport in the subsurface environment, whenever more than one fluid phase is present. The results also give clear evidence that the presence of inadvertently trapped residual gas could help explain the discrepancy between water-saturated laboratory column experiments and related theory.

This paper exposes the significant role played by the gas-water interface in the fate and transport of colloids in porous media and also introduces a micromodel method to allow direct visualization of colloid behavior in pore networks. The gas-water interface was created by trapping air in the pore space. Various types of latex and clay particles, as well as bacteria, were studied. The results suggest that the gas-water interface sorbs not only hydrophobic but also hydrophilic particles. The degree of sorption is controlled by particle surface hydrophobicity, solution ionic strength, and particle charge sign. Sorption increases with increasing particle hydrophobicity and solution ionic strength, while positively charged particles have a very strong affinity for the gas-water interface. The sorption on the gas-water interface is essentially irreversible, in that the capillary free energy provides a large attractive force to hold particles on the gas-water interface after its rupture. These findings reveal a mechanism of vadose zone transport: A static gas-water interface behaves as a sorbent phase retarding the transport of particulate contaminants. The visualization method developed in this research is very useful for the investigation of particulate contaminant behavior and interface-related transport, especially in the context of bioremediation. 32 refs., 8more » figs., 1 tab.« less

The mobility of an acidic uranium waste plume in the F-Area of Savannah River Site is of great concern. In order to understand and predict uranium mobility, U(VI) adsorption experiments were performed as a function of pH using background F-Area aquifer sediments and reference goethite and kaolinite (major reactive phases of F-Area sediments), and a component-additivity (CA) based surface complexation model (SCM) was developed. Our experimental results indicate that the fine fractions (≤45 μm) in sediments control U(VI) adsorption due to their large surface area, although the quartz sands show a stronger adsorption ability per unit surface area than the fine fractions at pH5.0. Kaolinite is a more important sorbent for U(VI) at pH4.0, while goethite plays a major role at pH4.0. Our CA model combines an existing U(VI) SCM for goethite and a modified U(VI) SCM for kaolinite along with estimated relative surface area abundances of these component minerals. The modeling approach successfully predicts U(VI) adsorption behavior by the background F-Area sediments. The model suggests that exchange sites on kaolinite dominate U(VI) adsorption at pH4.0, goethite and kaolinite edge sites cocontribute to U(VI) adsorption at pH 4.0-6.0, and goethite dominates U(VI) adsorption at pH6.0.

The transport of polyacrylic acid capped cadmium telluride (CdTe) quantum dots (QDs), carboxylate-modified latex (CML), and bare silica nanoparticles (NPs) was studied in packed columns at various electrolyte concentrations and cation types. The breakthrough curves (BTCs) of QDs and CML particles in acid-treated Accusand showed significant amounts of increasing deposition with 0.5, 1, and 2 mM Ca 2+ , but only minute deposition at 50 and 100 mM Na + . Negligible QD and CML deposition occurred at 2 mM Ca 2+ in columns packed with ultrapure quartz sand that was similar in size to the Accusand. These observations are not consistent with interpretations based on Derjaguin–Landau–Verwey–Overbeek (DLVO) calculations of interaction energies. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis demonstrated that there were regions on the acid-treated Accusand covered with small amounts of clay that were absent on the ultrapure quartz sand. A salt cleaning method was therefore used to remove the clay from the acid-treated Accusand. The BTCs of QDs and CML in this acid + salt treated Accusand exhibited much less deposition at any given Ca 2+ concentration compared to those obtained from the acid-treated sand. SEM images showed that most of the QD deposited in acid-treated Accusand occurred on clay surfaces. Unlike our results with QDs and CML, negligible deposition of bare silica NPs occurred at 5 and 10 mM Ca 2+ in acid-treated Accusand. The high deposition of QDs and CML particles was therefore attributed to bridging complexation in which Ca 2+ serves as a bridge between the cation exchange locations on the clay and carboxyl functional groups on the QD and CML particles, which were absent on the bare silica NPs. Our results suggest that the transport of carboxylic ligand-modified NPs may be limited in subsurface environments because of the ubiquitous presence of clay and divalent cations.

Acidic uranium (U) contaminated plumes have resulted from acid-extraction of plutonium during the Cold War and from U mining and milling operations. A sustainable method for in-situ immobilization of U under acidic conditions is not yet available. Here, we propose to use humic acids (HAs) for in-situ U immobilization in acidic waste plumes. Our laboratory batch experiments show that HA can adsorb onto aquifer sediments rapidly, strongly and practically irreversibly. Adding HA greatly enhanced U adsorption capacity to sediments at pH below 5.0. Our column experiments using historically contaminated sediments from the Savannah River Site under slow flow rates (120 and 12 m/y) show that desorption of U and HA were non-detectable over 100 pore-volumes of leaching with simulated acidic groundwaters. Upon HA-treatment, 99% of the contaminant [U] was immobilized at pH < 4.5, compared to 5% and 58% immobilized in the control columns at pH 3.5 and 4.5, respectively. These results demonstrated that HA-treatment is a promising in-situ remediation method for acidic U waste plumes. As a remediation reagent, HAs are resistant to biodegradation, cost effective, nontoxic, and easily introducible to the subsurface.

Methods for remediating groundwaters contaminated with uranium (U) through precipitation under oxidizing conditions are needed because bioreduction-based approaches require indefinite supply of electron donor. Although strategies based on precipitation of some phosphate minerals within the (meta)autunite group have been considered for this purpose, thermodynamic calculations for K- and Ca-uranyl phopsphates, meta-ankoleite and autunite, predict that U concentrations will exceed the Maximum Contaminant Level (MCL = 0.13 microM for U) at any pH and pCO2, unless phosphate is maintained at much higher concentrations than the sub-microM levels typically found in groundwaters. We hypothesized that potassium uranyl vanadate will control U(VI) concentrations below regulatory levels in slightly acidic to neutral solutions based on thermodynamic data available for carnotite, K2(UO2)2V2O8. The calculations indicate that maintaining U concentrations below the MCL through precipitation of carnotite will be sustainable in some oxidizing waters having pH in the range of 5.5 to 7, even when dissolution of this solid phase becomes the sole supply of sub-microM levels of V. Batch experiments were conducted in solutions at pH 6.0 and 7.8, chosen because of their very different predicted extents of U(VI) removal. Conditions were identified where U concentrations dropped below the MCL within 1-5 days of contact with oxidizing solutions containing 0.2-10 mM K, and 0.1-20 microM V(V). This method may also have application in extracting (mining) U and V from groundwaters where they both occur at elevated concentrations.

A saline-alkaline brine containing high concentration of U(VI) was accidentally spilled at the Hanford Site in 1951, introducing 10 tons of U into sediments under storage tank BX-102. U concentrations in the deep vadose zone and groundwater plumes increase with time, yet how the U has been migrating is not fully understood. We simulated the spill event in laboratory soil columns, followed by aging, and obtained spatially resolved U partitioning and speciation along simulated plumes. We found after aging, at apparent steady state, that the pore aqueous phase U concentrations remained surprisingly high (up to 0.022 M), in close agreement with the recently reported high U concentrations (up to 0.027 M) in the vadose zone plume (1). The pH values of aged pore liquids varying from 10 to 7, consistent with the measured pH of the field borehole sediments varying from 9.5 to 7.4 (2), from near the plume source to the plume front. The direct measurements of aged pore liquids together with thermodynamic calculations using a Pitzer approach revealed that UO2(CO3)3(4-) is the dominant aqueous U species within the plume body (pH 8-10), whereas Ca2UO2(CO3)3 and CaUO2(CO3)32- are also significant in the plume frontvicinity (pH 7-8), consistent with that measured from field borehole pore-waters (3). U solid phase speciation varies at different locations along the plume flow path and even within single sediment grains, because of location dependent pore and micropore solution chemistry. Our results suggest that continuous gravity-driven migration of the highly stable U02(CO3)34 in the residual carbonate and sodium rich tank waste solution is likely responsible for the detected growing U concentrations in the vadose zone and groundwater.

Remediation of uranium-contaminated sediments through in situ stimulation of bioreduction to insoluble UO2 is a potential treatment strategy under active investigation. Previously, we found that newly reduced U(IV) can be reoxidized under reducing conditions sustained by a continuous supply of organic carbon (OC) because of residual reactive Fe(III) and enhanced U(VI) solubilitythrough complexation with carbonate generated through OC oxidation. That finding motivated this investigation directed at identifying a range of OC supply rates that is optimal for establishing U bioreduction and immobilization in initially oxidizing sediments. The effects of OC supply rate, from 0 to 580 mmol of OC (kg of sediment)(-1) year(-1), and OC form (lactate and acetate) on U bioreduction were tested in flow-through columns containing U-contaminated sediments. An intermediate supply rate on the order of 150 mmol of OC (kg of sediment)(-1) year(-1) was determined to be most effective at immobilizing U. At lower OC supply rates, U bioreduction was not achieved, and U(VI) solubilitywas enhanced by complexation with carbonate (from OC oxidation). At the highest OC supply rate, the resulting highly carbonate-enriched solutions also supported elevated levels of U(VI), even though strongly reducing conditions were established. Lactate and acetate were found to have very similar geochemical impacts on effluent U concentrations (and other measured chemical species), when compared at equivalent OC supply rates. While the catalysts of U(VI) reduction to U(IV) are presumably bacteria, the composition of the bacterial community,the Fe-reducing community, and the sulfate-reducing community had no direct relationship with effluent U concentrations. The OC supply rate has competing effects of driving reduction of U(VI) to low-solubility U(IV) solids, as well as causing formation of highly soluble U(VI)-carbonato complexes. These offsetting influences will require careful control of OC supply rates in order to optimize bioreduction-based U stabilization.

The results of a field experiment designed to test the effectiveness of a novel approach for long-term, in situ bioimmobilization of toxic and soluble Cr(VI) in groundwater using a hydrogen release compound (HRC)--a slow release glycerol polylactate--are described. The field experiment was conducted at the Hanford Site (Washington), a U.S. Department of Energy nuclear production facility, using a combination of hydrogeological, geophysical, geochemical, and microbiological measurements and analyses of water samples and sediments. The results of this experiment show that a single HRC injection into groundwater stimulates an increase in biomass, a depletion of terminal electron acceptors O2, NO3-, and SO4(2-), and an increase in Fe2+, resulting in a significant decrease in soluble Cr(VI). The Cr(VI) concentration has remained below the background concentration in the downgradient pumping/ monitoring well, and below the detection limit in the injection well for more than 3 years after the HRC injection. The degree of sustainability of Cr(VI) reductive bioimmobilization under different redox conditions at this and other contaminated sites is currently under study.

Effect of Saline Waste Solution Infiltration Rates on Uranium Retention and Spatial Distribution in Hanford Sediments Jiamin Wan 1* , Tetsu K. Tokunaga 1 , Yongman Kim 1 , Zheming Wang 2 , Antonio Lanzirotti 3 , Eduardo Saiz 4 , and R. Jeffrey Serne 2 1. Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 2. Pacific Northwest National Laboratory, Richland, WA 99352 3. University of Chicago, Chicago, IL 60637 4. Material Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 ABSTRACT The accidental overfilling of waste liquid from tank BX-102 at the Hanford Site in 1951 put about 10 metric tons of U(VI) into the vadose zone. In order to understand the dominant geochemical reactions and transport processes occurred during the initial infiltration and help understand current spatial distribution, we simulated the waste liquid spilling event in laboratory sediment columns using synthesized metal waste solution. We found that, as the plume propagating through sediments, pH decreased greatly (as much as 4 units) at the moving plume front. Infiltration flow rates strongly affect U behavior. Slower flow rates resulted in higher sediment-associated U concentrations, and higher flow rates ( 5 cm/day) permitted practically unretarded U transport. Therefore, given the very high K sat of most of Hanford formation, the low permeability zones within the sediment could have been most important in retaining high concentrations of U during initial release into the vadose zone. Massive amount of colloids, including U-colloids, formed at the plume fronts. Total U concentrations (aqueous and colloid) within plume fronts exceeded the source concentration by up to 5-fold. Uranium colloid

In situ reduction of toxic Cr(VI) to less hazardous Cr(III) is becoming a popular strategy for remediating contaminated soils. However, the long-term stability of reduced Cr remains to be understood, especially given the common presence of Mn(III, IV) oxides that re-oxidize Cr(III). This 4.6 year laboratory study tracked Cr and Mn redox transformations in soils contaminated with Cr(VI), which were then treated with different amounts of organic carbon (OC). Changes in Cr and Mn oxidation states within soils were directly and nondestructively measured using micro-X-ray absorption near-edge structure spectroscopy. Chromate reduction was roughly first-order, and the extent of reduction was enhanced with higher OC additions. However, significant Cr(III) re-oxidation occurred in soils exposed to the highest Cr(VI) concentrations (2560 mg kg(-1)). Transient Cr(Ill) re-oxidation up to 420 mg kg(-1) was measured at 1.1 years after OC treatment, followed by further reduction. Chromate concentrations increased by 220 mg kg(-1) at the end of the study (4.6 years) in one soil. The causal role that the Mn oxidation state had in re-oxidizing Cr was supported by trends in Mn K-edge energies. These results provide strong evidence for long-term dependence of soil Cr oxidation states on balances between OC availability and Mn redox status.

Bioreduction of U in contaminated sediments is an attractive strategy because of its low cost, and because of short-term studies supporting its feasibility. However, any in-situ immobilization approach for U will require assurance of either permanent fixation, or of very low release rates into the biosphere. Our previous long-term (2 years) laboratory experiments have shown that organic carbon (OC) based U(VI) bioreduction to UO2 can be transient even under sustained reducing (methanogenic) conditions. The biogeochemical processes underlying this finding urgently need to be understood. The current research is designed to identify mechanisms responsible for anaerobic U oxidation, and identify conditions that will support long-term stability of bioreduced U. We are investigating: (1) effects of OC concentration and supply rate on remobilization of bioreduced U, (2) the roles of Fe- and Mn-oxides as potential U oxidants in sediments, and (3) the role of microorganisms in U reoxidation, and (4) influences of pH on U(IV)/U(VI) redox equilibrium.