[en] Although the accelerated transport of ⁹⁹Tc, ¹³⁷Cs, and ²³⁵U within the vadose zone beneath the 200-West Area of the Hanford tank-farm area has been recognized, the mechanisms responsible for the vertical migration of the radionuclides is unclear. Does horizontal stratification enhance the lateral movement of contaminants, which in turn enhances vertical preferential flow due to perched water dynamics? Do physical heterogeneities, such as stratification and pore regime connectivity, influence the retardation and degree of geochemical nonequilibrium during contaminant transport? Recent modeling efforts of the problem have failed to yield answers to this question since they are inadequately parameterized due to the lack of sufficient quality data. Fundamental experimental research is needed that will improve the conceptual understanding and predictive capability of radionuclide migration in the Hanford tankfarm environment. Since geochemical reactions are directly linked to the system hydrodynamics, coupled geochemical and hydrological processes must be investigated in order to resolve the key mechanisms contributing to vadose zone and groundwater contamination at Hanford. Our research group has performed extensive investigations on time-dependent contaminant interactions with subsurface media using dynamic flow techniques which more closely simulate conditions in-situ. Of particular relevance to this proposal is the work of Barnett (and others) (2000) who showed that U(VI) transport through Hanford sediments was highly retarded and extremely sensitive to changes in pH and total carbonate. What remains elusive are the geochemical mechanisms for uranium retention-necessary information for accurately simulating transport-and are thus the focus of this study. The experimental and numerical results from this research will provide knowledge and information in previously unexplored areas of vadose zone fate and transport to support EM's performance/risk assessment and decision-making process for Tank Farm restoration. By unraveling fundamental contaminant transport mechanisms in complex porous media, we provide an improved conceptual understanding and predictive capability of a variety of vadose issues within the DOE system

[en] Approximately 3 million liters of high temperature, Al-rich, hyperalkaline, and saline high-level waste fluids (HLWF) from the leaking tanks were deposited to the vadose zone at the Hanford Site, WA. The objective of this study was to investigate the effects of Al, hyperalkalinity and hypersalinity on dissolution in the Hanford sediments treated with solutions similar to HLWF..

[en] Research is investigating mineralogic and geochemical factors controlling the desorption rate of 137Cs+ from subsurface sediments on the Hanford Site contaminated with different types of high level waste. The project will develop kinetic data and models that describe the release rates of 137Cs+ from contaminated sediments over a range of potential geochemical conditions that may evolve during waste retrieval from overlying tanks, or in response to meteoric water infiltration. Scientific understanding and computational techniques will be established to predict the future behavior of sorbed, in-ground 137Cs+

[en] Aluminum-rich, hyperalkaline (pH > 13.5) and saline high-level nuclear waste (HLW) fluids at elevated temperatures (>50 deg C), that possibly contained as much as 0.41 mol L^-1Cr(VI), accidentally leaked to the sediments at the Hanford Site, WA. These extreme conditions promote base-induced dissolution of soil minerals which may affect Cr(VI)_aq mobility. Our objective was to investigate Cr(VI)_aq transport in sediments leached with HLW simulants at 50 deg C, under CO₂ and O₂ free conditions. Results demonstrated that Cr(VI)_aq fate was closely related to dissolution, and Cr(VI)_aq mass loss was negligible in the first pore volumes but increased significantly thereafter. Similar to dissolution, Cr(VI)_aq attenuation increased with increasing fluid residence time and NaOH concentration but decreased with Al concentrations in the leaching solutions. Aqueous Cr(VI) removal rate half-lives varied from 1.2 to 230 h with the fastest at the highest base concentration, lowest Al concentration, greatest reaction time, and lowest Cr(VI) concentration in the leaching solution. The rate of Cr(VI) removal (normalized to 1 kg of solution) varied from 0.83 x 10^-9 (+-0.44 x 10^-9) to 9.16 x 10^-9(+-1.10 x 10^-9) mol s^-1. The predominant mechanism responsible for removing Cr(VI) from the aqueous phase appears to be homogeneous Cr(VI) reduction to Cr(III) by Fe(II) released during mineral dissolution. Cr(VI)_aq removal was time-limited probably because it was controlled by the rate of Fe(II) release into the soil solution upon mineral dissolution, which was also a time-limited process, and other processes that may act to lower Fe(II)_aq activity

[en] In this project, the reduction rate of uranyl complexes with hydroxide, carbonate, EDTA, and Desferriferrioxamine B (DFB) by anthraquinone-2,6-disulfonate (AH2DS), a potential electron shuttle for microbial reduction of metal ions (Newman and Kolter 2000), is studied by stopped-flow kinetics techniques under anoxic atmosphere. The apparent reaction rates varied with ligand type, solution pH, and U(VI) concentration. For each ligand, a single largest kobs within the studied pH range was observed, suggesting the influence of pH-dependent speciation on the U(VI) reduction rate. The maximum reaction rate found in each case followed the order of OH- > CO32- > EDTA > DFB, consistent with the same trend of the thermodynamic stability of the uranyl complexes and ionic sizes of the ligands. Increasing the stability of uranyl complexes and ligand size decreased the maximum reduction rate. The pH-dependent rates were modeled using a second-order rate expression that was assumed to be dependent on a single U(VI) complex and AH2DS species. By quantitatively comparing the calculated and measured apparent rate constants as a function of pH, species AHDS3- was suggested as the primary reductant in all cases examined. Species UO2CO3(aq) , UO2HEDTA-, and (UO2)2(OH)22+ were suggested as the principal electron acceptors among the U(VI) species mixture in carbonate, EDTA, and hydroxyl systems, respectively

[en] The radioactive waste fluids stored in the tanks that have accidentally leaked into the vadose zone at the Hanford Site, WA, were high temperature, Al-rich, concentrated alkaline and saline solutions. In addition to dissolution, precipitation is likely to occur when these waste fluids contact the sediments..

[en] The effect of caustic NaNO3 solutions on the sorption of 137Cs to the Hanford site micaceous subsurface sediment was investigated as a function of time, temperature (10 C or 50 C), and NaOH concentration. At 100C and 0.1 M NaOH, the slow evolution of [Al]aq was in stack contrast to the rapid increase and subsequent loss of [Al]aq observed at 50 C (regardless of base concentration). At 50 C, dissolution of phyllosilicate minerals increased with [OH], at 1 and 3 M NaOH solutions, almost complete dissolution of clay-sized phyllosilicates occurred. At 0.1 M NaOH, a zeolite (tetranatrolite) precipitated after about 7 days, while an unnamed mineral phase (Na2Al2Si3O10-2H2O) precipitated after 4 and 2 days of exposure to 1 M and 3 M NaOH solutions. At 100C there was no conclusive evidence of secondary mineral precipitation. The effect of base dissolution on Cs+ sorption by the Hanford sediment was investigated via (1) Cs+ sorption over a large concentration range (10-9 - 10-2 mol/L) to sediment after exposure to 0.1 M NaOH for 56, 112, and 168 days, (2) Cs+ sorption to sediment in the presence of NaOH (0.1 M, 1 M, and 3 M NaOH) at Cs+ concentrations selected to probe high affinity, transition, and low affinity cation exchange sites, and (3) the application of a two-site numeric ion exchange model (Zachara et al. 2002a). No effect on Cs+ sorption to the Hanford sediment was observed during the 168 days sediment was exposed to 0.1 M NaOH, at 10 C; Cs+ sorption in the presence of base was well described by the ion exchange model when enthalpy effects were considered. In contrast, at 50 C, there was a trend toward slightly lower (log ∼ 0.25) conditional equilibrium exchange constants over the entire range of surface coverage, and a slight loss of high affinity sites (15%) after 168 days of exposure to 0.1 M base solution. However, model simulations of Cs+ sorption to the sediment in the presence of 0.1 M base for 112 days were good at the lower Cs+ surface densities. At the higher surface densities, model simulations under predicted sorption by 57%. This under prediction was surmised to be the result of tetranatrolite precipitation, and subsequent slow Na - Cs exchange. At higher OH concentrations, Cs+ sorption in the presence of base for 112 days was unexpectedly equal to, or slightly greater than that expected for a pristine sediment. The presence of neoforms, coupled with the fairly unique mica distribution and quantity across all size-fractions in the Hanford sediment, appears to mitigate the impact of base dissolution on Cs+ sorption

[en] The uptake of Eu3+ by elongating oat plant roots was studied by fluorescence spectroscopy, fluorescence lifetime measurement, as well as laser excitation time-resolved confocal fluorescence profiling technique. The results of this work indicated that the initial uptake of Eu(III) by oat root was most evident within the apical meristem of the root just proximal to the root cap. Distribution of assimilated Eu(III) within the roots differentiation and elongation zone was non-uniform. Higher concentrations were observed within the vascular cylinder, specifically in the phloem and developing xylem parenchyma. Elevated levels of the metal were also observed in the root hairs of the mature root. The concentration of assimilated Eu3+ dropped sharply from the apical meristem to the differentiation and elongation zone and then gradually decreased as the distance from the root cap increased. Fluorescence spectroscopic characteristics of the assimilated Eu3+ suggested that the Eu3+ exists a s inner-sphere mononuclear complexes inside the root. This work has also demonstrated the effectiveness of a time-resolved Eu3+ fluorescence spectroscopy and confocal fluorescence profiling techniques for the in vivo, real-time study of metal[Eu3+] accumulation by a functioning intact plant root. This approach can prove valuable for basic and applied studies in plant nutrition and environmental uptake of actinide radionuclides

[en] Nine projects have been recently selected by the US Department of Energy (EM-22) to address groundwater contaminant migration at the Hanford Site. This paper summarizes the background and objectives of these projects. Five of the selected projects are targeted at hexavalent chromium contamination in Hanford 100 Area groundwater. These projects represent an integrated approach towards identifying the source of hexavalent chromium contamination in the Hanford 100-D Area and treating the groundwater contamination. Currently, there is no effective method to stop strontium-90 associated with the riparian zone sediments from leaching into the river. Phytoremediation may be a possible way to treat this contamination. Its use at the 100-N Area will be investigated. Another technology currently being tested for strontium-90 contamination at the 100-N Area involves injection (through wells) of a calcium-citrate-phosphate solution, which will precipitate apatite, a natural calcium-phosphate mineral. Apatite will adsorb the strontium-90, and then incorporate it as part of the apatite structure, isolating the strontium-90 contamination from entering the river. This EM-22 funded apatite project will develop a strategy for infiltrating the apatite solution from ground surface or a shallow trench to provide treatment over the upper portion of the contaminated zone, which is unsaturated during low river stage.

No abstract available