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Atmospheric nitrous oxide contributes directly to global warming, yet models of the nitrogen cycle do not account for bedrock, the largest pool of terrestrial nitrogen, as a source of nitrous oxide. Although it is known that release rates of nitrogen from bedrock are large, there is an incomplete understanding of the connection between bedrock-hosted nitrogen and atmospheric nitrous oxide. Here, we quantify nitrogen fluxes and mass balances at a hillslope underlain by marine shale. We found that, at this site, bedrock weathering contributes 78% of the subsurface reactive nitrogen, while atmospheric sources (commonly regarded as the sole sources of reactive nitrogen in pristine environments) account for only the remaining 22%. About 56% of the total subsurface reactive nitrogen denitrifies, including 14% emitted as nitrous oxide. The remaining reactive nitrogen discharges in porewaters to a floodplain where additional denitrification probably occurs. We also found that the release of bedrock nitrogen occurs primarily within the zone of the seasonally fluctuating water table and suggest that the accumulation of nitrate in the vadose zone, often attributed to fertilization and soil leaching, may also include contributions from weathered nitrogen-rich bedrock. Our hillslope study suggests that, under oxygenated and moisture-rich conditions, weathering of deep, nitrogen-rich bedrock makes an important contribution to the nitrogen cycle. Weathering of deep bedrock releases reactive nitrogen into the subsurface, which contributes to the flux of nitrous oxide to the atmosphere, according to a field study that combines soil, rock and groundwater data within a river catchment.

Recharge of alluvial aquifers is a key component in understanding the interaction between floodplain vadose zone biogeochemistry and groundwater quality. The Rifle Site (a former U-mill tailings site) adjacent to the Colorado River is a well-established field laboratory that has been used for over a decade for the study of biogeochemical processes in the vadose zone and aquifer. This site is considered an exemplar of both a riparian floodplain in a semiarid region and a post-remediation U-tailings site. In this paper we present Sr isotopic data for groundwater and vadose zone porewater samples collected in May and July 2013 to build a mixing model for the fractional contribution of vadose zone porewater (i.e. recharge) to the aquifer and its variation across the site. The vadose zone porewater contribution to the aquifer ranged systematically from 0% to 38% and appears to be controlled largely by the microtopography of the site. The area-weighted average contribution across the site was 8% corresponding to a net recharge of 7.5 cm. Given a groundwater transport time across the site of ~1.5 to 3 years, this translates to a recharge rate between 5 and 2.5 cm/yr, and with the average precipitation to the site implies a loss from the vadose zone due to evapotranspiration of 83% to 92%, both ranges are in good agreement with previously published results by independent methods. A uranium isotopic (234U/238U activity ratios) mixing model for groundwater and surface water samples indicates that a ditch across the site is hydraulically connected to the aquifer and locally significantly affects groundwater. Groundwater samples with high U concentrations attributed to natural bio-reduced zones have 234U/238U activity ratios near 1, suggesting that the U currently being released to the aquifer originated from the former U-mill tailings.

Increasing methane recovery rates from coal mines reduces greenhouse gas emissions and benefits mining safety. In recent years, raising the temperature of coal seams has been recognized as a potential method of improving methane recovery. However, the effect of elevated temperature (especially higher than 80 °C) on methane adsorption on coals is rarely studied. In this study, we investigated the effects of elevated temperature and pressure on methane adsorption on different types (ranking) of coal, and their thermal stability. We found that for all six coal samples measured, maximum methane adsorption capacities sharply decrease with increasing temperature, showing a linear relationship (r2 = 0.705–0.975), and that coals began to pyrolyze at 267–500 °C depending on the coal type. We defined the temperature-dependent decrease in methane adsorption of coal as its thermal desorption capacity, and found that coal types with higher fixed carbon content have a larger magnitude thermal desorption capacity. These results indicate that raising the coal seam temperature while capturing methane is a promising method for obtaining higher coalbed methane recovery rates, reducing greenhouse gas emissions, and improving mining safety.

It is commonly true that a diluted colloidal suspension is more stable over time than a concentrated one because dilution reduces collision rates of the particles and therefore delays the formation of aggregates. However, this generalization does not apply for some engineered ligand-coated nanoparticles (NPs). We observed the opposite relationship between stability and concentration of NPs. We tested 4 different types of NPs: CdSe-11-mercaptoundecanoic acid, CdTe-polyelectrolytes, Ag-citrate, and Ag-polyvinylpyrrolidone. The results showed that dilution alone induced aggregation and subsequent sedimentation of the NPs that were originally monodispersed at very high concentrations. Increased dilution caused NPs to progressively become unstable in the suspensions. The extent of the dilution impact on the stability of NPs is different for different types of NPs. We hypothesize that the unavoidable decrease in free ligand concentration in the aqueous phase following dilution causes detachment of ligands from the suspended NP cores. The ligands attached to NP core surfaces must generally approach exchange equilibrium with free ligands in the aqueous phase; therefore, ligand detachment and destabilization are expected consequences of dilution. More studies are necessary to test this hypothesis. Because the stability of NPs determines their physicochemical and kinetic behavior including toxicity, dilution-induced instability needs to be understood to realistically predict the behavior of engineered ligand-coated NPs in aqueous systems. Environ Toxicol Chem 2018;37:1301-1308. © 2018 SETAC.

© 2017 American Chemical Society. Environmentally benign, economical, and effective surfactants and additives are needed in engineering subsurface energy recovery processes. Biosurfactants have some advantages over chemically synthesized surfactants, but the high costs of microbial-biosynthesis limit their applications in subsurface engineering. Here we propose to use naturally occurring biosurfactants contained within Earth's readily available and inexpensive humus deposits for subsurface engineering applications. We collected humus samples of different types from four different regions, and developed a simple method for extracting natural biosurfactant (NBS) using only four common chemicals. The average NBS extraction yields are 16 ± 3% of the raw humus. No significant differences in elemental composition and functional group chemistry were found among the NBS extracted from humus of different origins, suggesting that any humus deposit can be used for NBS extraction. Measurements of interfacial tensions between air-water and supercritical (sc) CO2-water interfaces indicate that the NBS is a highly effective surfactant. NBS has good foaming ability. Preliminary tests with only 0.5 mass % NBS in the aqueous phase (no other additives) yielded scCO2-in-water foams of 83% foam quality. The apparent viscosity of 13 cP measured at a shear rate of 2320 s-1indicates that much higher viscosities are achievable at lower shear rates. These results suggest that NBS merits further research and development for potential applications in subsurface energy production.

© 2017 American Chemical Society. Diffusion is important in controlling local solute transport and reactions in unsaturated soils and geologic formations. Although it is commonly assumed that thinning of water films controls solute diffusion at low water contents, transport under these conditions is not well understood. We conducted experiments in quartz sands at low volumetric water contents (θ) to quantify ion diffusion within adsorbed films. At the lowest water contents, we employed fixed relative humidities to control water films at nm thicknesses. Diffusion profiles for Rb+ and Br- in unsaturated sand packs were measured with a synchrotron X-ray microprobe, and inverse modeling was used to determine effective diffusion coefficients, De, as low as ∼9 × 10-15 m2 s-1 at θ = 1.0 × 10-4 m3 m-3, where the film thickness = 0.9 nm. Given that the diffusion coefficients (Do) of Rb+ and Br- in bulk water (30 °C) are both ∼2.4 × 10-9 m2 s-1, we found the impedance factor f = De/(θDo) is equal to 0.03 ± 0.02 at this very low saturation, in agreement with the predicted influence of interface tortuosity (τa) for diffusion along grain surfaces. Thus, reduced cross-sectional area (θ) and tortuosity largely accounted for the more than 5 orders of magnitude decrease in De relative to Do as desaturation progressed down to nanoscale films.

Many aquifers contaminated by U(VI)-containing acidic plumes are composed predominantly of quartz-sand sediments. The F-Area of the Savannah River Site (SRS) in South Carolina (USA) is an example. To predict U(VI) mobility and natural attenuation, we conducted U(VI) adsorption experiments using the F-Area plume sediments and reference quartz, goethite, and kaolinite. The sediments are composed of ∼96% quartz-sand and 3-4% fine fractions of kaolinite and goethite. We developed a new humic acid adsorption method for determining the relative surface area abundances of goethite and kaolinite in the fine fractions. This method is expected to be applicable to many other binary mineral pairs, and allows successful application of the component additivity (CA) approach based surface complexation modeling (SCM) at the SRS F-Area and other similar aquifers. Our experimental results indicate that quartz has stronger U(VI) adsorption ability per unit surface area than goethite and kaolinite at pH ≤ 4.0. Our modeling results indicate that the binary (goethite/kaolinite) CA-SCM under-predicts U(VI) adsorption to the quartz-sand dominated sediments at pH ≤ 4.0. The new ternary (quartz/goethite/kaolinite) CA-SCM provides excellent predictions. The contributions of quartz-sand, kaolinite, and goethite to U(VI) adsorption and the potential influences of dissolved Al, Si, and Fe are also discussed.

Understanding the fate and transport of engineered nanoparticles (ENPs) in subsurface environments is required for developing the best strategy for waste management and disposal of these materials. In this study, the deposition and release of quantum dot (QD) nanoparticles were studied in saturated sand columns. The QDs were first deposited in columns using 100 mM NaCl or 2 mM CaC12 solutions. Deposited QDs were then contacted with deionized (DI) water and/or varying Na(+) concentrations to induce release. QDs deposited in 100 mM Na(+) were easily reversible when the column was rinsed with DI water. Conversely, QDs deposited in the presence of Ca(2+) exhibited resistance to release with DI water. However, significant release occurred when the columns were flushed with NaCl solutions. This release behavior was explained by cation exchange (Ca(2+) in exchange sites were replaced by Na(+)) which resulted in the breakdown of calcium bridging. We also studied the effect of aging time on the QD release. As the aging time increased, smaller amounts of QDs were released following cation exchange. However, deposited QDs were subsequently released when the column was flushed with DI water. The release behavior was modeled using a single first-order kinetic release process and changes in the maximum solid phase concentration of deposited QDs with transition in solution chemistry. The results of this study demonstrate that the presence of carboxyl groups on ENPs and divalent ions in the solution plays a key role in controlling ENP mobility in the subsurface environment.

© 2016 In order to understand the transport and humification processes of dissolved organic matter (DOM) within sediments of a semi-arid floodplain at Rifle, Colorado, fluorescence excitation–emission matrix (EEM) spectroscopy, humification index (HIX) and specific UV absorbance (SUVA) at 254 nm were applied for characterizing depth and seasonal variations of DOM composition. Results revealed that late spring snowmelt leached relatively fresh DOM from plant residue and soil organic matter down into the deeper vadose zone (VZ). More humified DOM is preferentially adsorbed by upper VZ sediments, while non- or less-humified DOM was transported into the deeper VZ. Interestingly, DOM at all depths undergoes rapid biological humification process evidenced by the products of microbial by-product-like (i.e., tyrosine-like and tryptophan-like) matter in late spring and early summer, particularly in the deeper VZ, resulting in more humified DOM (e.g., fulvic-acid-like and humic-acid-like substances) at the end of year. This indicates that DOM transport is dominated by spring snowmelt, and DOM humification is controlled by microbial degradation, with seasonal variations. It is expected that these relatively simple spectroscopic measurements (e.g., EEM spectroscopy, HIX and SUVA) applied to depth- and temporally-distributed pore-water samples can provide useful insights into transport and humification of DOM in other subsurface environments as well.

© 2016 American Chemical Society. The wettability of rocks and minerals significantly affects the safety and efficiency of energy-related subsurface operations. Salinity is an important controlling factor in terms of wettability but has received limited attention. We studied the effects of salinity-induced chemical reactions on biotite's wettability changes under relevant subsurface conditions. Biotite was reacted at 95 °C and 102 atm of CO2for 70 h in solutions with salinities of 0, 0.1, 0.5, and 1.0 M NaCl. Then, static and dynamic water contact angles on reacted biotite basal surfaces were measured using a captive drop method. As a result of enhanced biotite dissolution at higher salinities, increased roughness, more negatively charged surfaces, and higher densities of hydroxyl groups on the biotite surfaces made biotite basal surface more hydrophilic. These results provide new information about the interplay of chemical reactions and wettability alterations of minerals, providing a better understanding of CO2transport in subsurface environments.

© 2016, US Government. Flood plains play a potentially important role in the global carbon cycle. The accumulation of organic matter in flood plains often induces the formation of chemically reduced groundwater and sediments along riverbanks. In this study, our objective is to evaluate the cumulative impact of such reduced zones, water table fluctuations, and temperature gradients on subsurface carbon fluxes in a flood plain at Rifle, Colorado located along the Colorado River. 2-D coupled variably-saturated, non-isothermal flow and biogeochemical reactive transport modeling was applied to improve our understanding of the abiotic and microbially mediated reactions controlling carbon dynamics at the Rifle site. Model simulations considering only abiotic reactions (thus ignoring microbial reactions) underestimated CO2partial pressures observed in the unsaturated zone and severely underestimated inorganic (and overestimated organic) carbon fluxes to the river compared to simulations with biotic pathways. Both model simulations and field observations highlighted the need to include microbial contributions from chemolithoautotrophic processes (e.g., Fe+2and S−2oxidation) to match locally-observed high CO2concentrations above reduced zones. Observed seasonal variations in CO2concentrations in the unsaturated zone could not be reproduced without incorporating temperature gradients in the simulations. Incorporating temperature fluctuations resulted in an increase in the annual groundwater carbon fluxes to the river by 170 % to 3.3 g m−2d−1, while including water table variations resulted in an overall decrease in the simulated fluxes. We conclude that spatial microbial and redox zonation as well as temporal fluctuations of temperature and water table depth contribute significantly to subsurface carbon fluxes in flood plains and need to be represented appropriately in model simulations.

At the Savannah River Site's F-Area, wastewaters containing radionuclides were disposed into seepage basins for decades. After closure and capping in 1991, the U.S. Department of Energy (DOE) has being monitoring and remediating the groundwater plume. Despite numerous studies of the plume, its persistence for over 20 years has not been well understood. To better understand the plume dynamics, a limited number of deep boreholes were drilled to determine the current plume characteristics. A mixing model was developed to predict plume tritium and nitrate concentrations. We found that the plume trailing edges have emerged for some contaminants, and that contaminant recharge from the basin's vadose zone is still important. The model's estimated time-dependent basin drainage rates combined with dilution from natural recharge successfully predicted plume tritium and nitrate concentrations. This new understanding of source zone influences can help guide science-based remediation, and improve predictions of the natural attenuation timeframes.

Comprehensive understanding of the transport and deposition of engineered nanoparticles (NPs) in subsurface is required to assess their potential negative impact on the environment. We studied the deposition behavior of functionalized quantum dot (QD) NPs (CdTe) in different types of sands (Accusand, ultrapure quartz, and iron-coated sand) at various solution ionic strengths (IS). The observed transport behavior in ultrapure quartz and iron-coated sand was consistent with conventional colloid deposition theories. However, our results from the Accusand column showed that deposition was minimal at the lowest IS (1 mM) and increased significantly as the IS increased. The effluent breakthrough occurred with a delay, followed by a rapid rise to the maximum normalized concentration of unity. Negligible deposition in the column packed with ultrapure quartz sand (100 mM) and Accusand (1 mM) rules out the effect of straining and suggests the importance of surface charge heterogeneity in QD deposition in Accusand at higher IS. Data analyses further show that only a small fraction of sand surface area contributed in QD deposition even at the highest IS (100 mM) tested. The observed delay in breakthrough curves of QDs was attributed to the fast diffusive mass transfer rate of QDs from bulk solution to the sand surface and QD mass transfer on the solid phase. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis were used to examine the morphology and elemental composition of sand grains. It was observed that there were regions on the sand covered with layers of clay particles. EDX spectra collected from these regions revealed that Si and Al were the major elements suggesting that the clay particles were kaolinite. Additional batch experiments using gold NPs and SEM analysis were performed and it was observed that the gold NPs were only deposited on clay particles originally on the Accusand surface. After removing the clays from the sand surface, we observed negligible QD deposition even at 100 mM IS. We proposed that nanoscale charge heterogeneities on clay particles on Accusand surface played a key role in QD deposition. It was shown that the value of solution IS determined the extent to which the local heterogeneities participated in particle deposition.

Bioreduction-based strategies for remediating uranium (U)-contaminated sediments face the challenge of maintaining the reduced status of U for long times. Because groundwater influxes continuously bring in oxidizing terminal electron acceptors (O2, NO3(-)), it is necessary to continue supplying organic carbon (OC) to maintain the reducing environment after U bioreduction is achieved. We tested the influence of OC supply rates on mobility of previously microbial reduced uranium U(IV) in contaminated sediments. We found that high degrees of U mobilization occurred when OC supply rates were high, and when the sediment still contained abundant Fe(III). Although 900 days with low levels of OC supply minimized U mobilization, the sediment redox potential increased with time as did extractable U(VI) fractions. Molecular analyses of total microbial activity demonstrated a positive correlation with OC supply and analyses of Geobacteraceae activity (RT-qPCR of 16S rRNA) indicated continued activity even when the effluent Fe(II) became undetectable. These data support our hypothesis on the mechanisms responsible for remobilization of U under reducing conditions; that microbial respiration caused increased (bi)carbonate concentration and formation of stable uranyl carbonate complexes, thereby shifted U(IV)/U(VI) equilibrium to more reducing potentials. The data also suggested that low OC concentrations could not sustain the reducing condition of the sediment for much longer time. Bioreduced U(IV) is not sustainable in an oxidizing environment for a very long time.

Leakage of saline-alkaline tank waste solutions often creates a serious environmental contamination problem. To better understand the mechanisms controlling the fate of such waste solutions in the Hanford vadose zone, we simulated reactive transport in columns designed to represent local site conditions. The Pitzer ion interaction module was used, with principal geochemical processes considered in the simulation including quartz dissolution, precipitation of brucite, calcite, and portlandite, multi-component cation exchange, and aqueous complexation reactions. Good matches were observed between the simulated and measured column data at ambient temperature ( approximately 21 degrees C). Relatively good agreement was also obtained at high temperature ( approximately 70 degrees C). The decrease of pH at the plume front is examined through formation of secondary mineral phases and/or quartz dissolution. Substantial formation of secondary mineral phases resulting from multi-component cation exchange suggests that these phases are responsible for a decrease in pH within the plume front. In addition, a sensitivity analysis was conducted with respect to cation exchange capacity, selectivity coefficient, mineral assemblage, temperature, and ionic strength. This study could serve as a useful guide to subsequent experimental work, to thermodynamic models developed for the concentrated solutions at high ionic strength and to other types of waste plume studies.

The oxidation status of uranium in sediments is important because the solubility of this toxic and radioactive element is much greater for U(VI) than for U(IV) species. Thus, redox manipulation to promote precipitation of UO2 is receiving interest as a method to remediate U-contaminated sediments. Presence of Fe and Mn oxides in sediments at much higher concentrations than U requires an understanding of their redox status as well. This study was conducted to determine changes in oxidation states of U, Fe, and Mn in U-contaminated sediments from Oak Ridge National Laboratory. Oxidation states of these elements were measured in real-time and nondestructively using X-ray absorption spectroscopy on sediment columns supplied with synthetic groundwater containing organic carbon (OC, 0, 3, 10, 30, and 100 mM OC as lactate) for over 400 days. In sediments supplied with OCor = 30 mM, 80% of the U was reduced to U(IV), with transient reoxidation at about 150 days. Mn(III,IV) oxides were completely reduced to Mn(II) in sediments infused with OCor = 3 mM. However, Fe remained largely unreduced in all sediment columns, showing that Fe(III) can persist as an electron acceptor in reducing sediments over long times. This result in combination with the complete reduction of all other potential electron acceptors supports the hypothesis that the reactive Fe(III) fraction was responsible for reoxidizing U(IV).

Costly disposal of uranium (U) contaminated sediments is motivating research on in situ U(VI) reduction to insoluble U(IV) via directly or indirectly microbially mediated pathways. Delivery of organic carbon (OC) into sediments for stimulating U bioreduction is diffusion-limited in less permeable regions of the subsurface. To study OC-based U reduction in diffusion-limited regions, one slightly acidic and another calcareous sediment were treated with uranyl nitrate, packed into columns, then hydrostatically contacted with tryptic soy broth solutions. Redox potentials, U oxidation state, and microbial communities were well correlated. At average supply rates of 0.9 micromol OC (g sediment)(-1) day(-1), the U reduction zone extended to only about35-45 mm into sediments. The underlying unreduced U(VI) zone persisted over 600 days because the supply of OC was diffusion-limited and metabolized within a short distance. These results also suggestthat low U concentrations in groundwater samples from OC-treated sediments are not necessarily indicative of pervasive U reduction because interior and exterior regions of such sediment blocks can contain primarily U(VI) and U(IV), respectively.

Nuclear weapons and fuel production have left many soils and sediments contaminated with toxic levels of uranium (U). Although previous short-term experiments on microbially mediated U(VI) reduction have supported the prospect of immobilizing the toxic metal through formation of insoluble U(IV) minerals, our longer-term (17 months) laboratory study showed that microbial reduction of U can be transient, even under sustained reducing conditions. Uranium was reduced during the first 80 days, but later (100-500 days) reoxidized and solubilized, even though a microbial community capable of reducing U(VI) was sustained. Microbial respiration caused increases in (bi)-carbonate concentrations and formation of very stable uranyl carbonate complexes, thereby increasing the thermodynamic favorability of U(IV) oxidation. We propose that kinetic limitations including restricted mass transfer allowed Fe-(III) and possibly Mn(IV) to persist as terminal electron acceptors (TEAs) for U reoxidation. These results show that in-situ U remediation by organic carbon-based reductive precipitation can be problematic in sediments and groundwaters with neutral to alkaline pH, where uranyl carbonates are most stable.

Summary The retention, mobility, and bio-availability of U(VI) in contaminated soils depend strongly on release of U(VI). Laboratory batch experiments were performed to evaluate the factors (ionic strength of the extraction solution and solution volume to solid mass ratio) controlling the release of U(VI) from contaminated soil at Oak Ridge, Tennessee. Increase in ionic strength shows an evident effect on U(VI) release as indicated by the increase in release rates and associated release mass of U(VI). The ratio of solution volume to solid mass (V/M) has a significant impact on the release of U(VI). Increase in the V/M ratio shows a negligible effect on the U(VI) release over a 4-day period. However, at Day 30 and Day 120, larger V/M ratios cause greater U(VI) release. The maximum U(VI) concentrations observed in the release experiments are in the range of schoepite solubility estimated under conditions relevant to the experiments, suggesting that schoepite solubility primarily controls the U(VI) release, but that solubilization and desorption effects cannot be distinguished using macroscopic methods.

Uranium contamination of soils and sediments often originates from acidic or alkaline waste sources, with diffusion being a major transport mechanism. Measurements of U(VI) diffusion from initially pH 2 and pH 11 solutions into a slightly alkaline Altamont soil and a neutral Oak Ridge soil were obtained through monitoring uptake from boundary reservoirs and from U concentration profiles within soil columns. The soils provided pH buffering, resulting in diffusion at nearly constant pH. Micro X-ray absorption near edge structure spectra confirmed that U remained in U(VI) forms in all soils. Time trends of U(VI) depletion from reservoirs and U(VI) concentration profiles within soil columns yielded Kdvalues consistent with those determined in batch tests at similar concentrations (approximately 1 mM) and much lower than values for sorption at much lower concentrations (nM to microM). These results show that U(VI) transport at high concentrations can be relatively fast at non-neutral pH, with negligible surface diffusion, because of weak sorption.

Leakage of highly saline and alkaline radioactive waste from storage tanks into underlying sediments is a serious environmental problem at the Hanford Site in Washington State. This study focuses on geochemical evolution of tank waste plumes resulting from interactions between the waste solution and sediment. A synthetic tank waste solution was infused into unsaturated Hanford sediment columns (0.2, 0.6, and 2 m) maintained at 70°C to simulate the field contamination process. Spatially and temporally resolved geochemical profiles of the waste plume were obtained. Thorough OH − neutralization (from an initial pH 14 down to 6.3) was observed. Three broad zones of pore solutions were identified to categorize the dominant geochemical reactions: the silicate dissolution zone (pH > 10), pH-neutralized zone (pH 10 to 6.5), and displaced native sediment pore water (pH 6.5 to 8). Elevated concentrations of Si, Fe, and K in plume fluids and their depleted concentrations in plume sediments reflected dissolution of primary minerals within the silicate dissolution zone. The very high Na concentrations in the waste solution resulted in rapid and complete cation exchange, reflected in high concentrations of Ca and Mg at the plume front. The plume-sediment profiles also showed deposition of hydrated solids and carbonates. Fair correspondence was obtained between these results and analyses of field borehole samples from a waste plume at the Hanford Site. Results of this study provide a well-defined framework for understanding waste plumes in the more complex field setting and for understanding geochemical factors controlling transport of contaminant species carried in waste solutions that leaked from single-shell storage tanks in the past.

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.

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.

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....

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...

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.

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.

Highly saline and caustic tank waste solutions containing radionuclides and toxic metals have leaked into sediments at U.S. Department of Energy (DOE) facilities such as the Hanford Site (Washington state). Colloid transport is frequently invoked to explain migration of radionuclides and metals in the subsurface. To understand colloid formation during interactions between highly reactive fluids and sediments and its impact on contaminant transport, we simulated tank waste solution (TWS) leakage processes in laboratory columns at ambient and elevated (70 degrees C) temperatures. We found that maximum formation of mobile colloids occurred at the plume fronts (hundreds to thousands times higher than within the plume bodies or during later leaching). Concentrations of suspended solids were as high as 3 mass %, and their particle sizes ranged from tens of nanometers to a few micrometers. Calcium carbonate is always one of the dominant phases of the plume front colloids, while the other phases varied with solution pH and temperature. During infiltration of the leaked high-Na+ waste solution, rapid and completed Na+ replacement of exchangeable Ca2+ and Mg2+ from the sediment caused accumulation of these divalent cations at the moving plume front. Precipitation of supersaturated Ca2+/Mg2+-bearing minerals caused dramatic pH reduction atthe plume front. In turn, the reduced pH caused precipitation of other minerals. This understanding can help predict the behavior of contaminant trace elements carried by the tank waste solutions and could not have been obtained through conventional batch studies.

At the Hanford Site in Washington State, the pH values of contaminant plumes resulting from leaking of initially highly alkaline-saline radioactive waste solutions into the subsurface are now found to be substantially neutralized. However, the nature of plume pH neutralization has not previously been understood. As a master geochemical variable, pH needs to be understood in order to predict the fate and transport of contaminants carried by the waste plumes. Through this laboratory study, we found that the plume pH values spanned a broad range from 14 (within the near-source region) down to the value of 7 (lower than the pH value of the initial soil solution) while the plume was still connected to an actively leaking source. We defined two zones within a plume: the silicate dissolution zone (SDZ, pH 14-10) and the neutralized zone (NZ, pH 10-7). Quartz dissolution at elevated temperature and precipitation of secondary silicates (including sodium metasilicate, cancrinite, and zeolites) are the key reactions responsible for the pH neutralization within the SDZ. The rapid and thorough cation exchange of Na + replacing Ca 2+ /Mg 2+ , combined with transport, resulted in a dynamic Ca 2+ /Mg 2+ -enriched plume front. Subsequent precipitation of calcite, sodium silicate, and possibly talc led to dramatically reduced pH within the plume front and the neutralized zone. During aging (after the plume source became inactive), continued quartz dissolution and the secondary silicate precipitation drove the pH value lower, toward pH 11 at equilibrium within the SDZ, whereas the pH values in the NZ remained relatively unchanged with time. A pH profile of 11 from the plume source to pH 7 at the plume front is expected for a historical plume. This laboratory-based study provided realistic plume pH profiles (consistent with that measured from borehole samples) and identified underlying mechanisms responsible for pH evolution.

Biogeochemical transformations of redox-sensitive chemicals in soils can be strongly transport-controlled and localized. This was tested through experiments on chromium diffusion and reduction in soil aggregates that were exposed to chromate solutions. Reduction of soluble Cr(VI) to insoluble Cr(III) occurred only within the surface layer of aggregates with higher available organic carbon and higher microbial respiration. Sharply terminated Cr diffusion fronts develop when the reduction rate increases rapidly with depth. The final state of such aggregates consists of a Cr-contaminated exterior, and an uncontaminated core, each having different microbial community compositions and activity. Microbial activity was significantly higher in the more reducing soils, while total microbial biomass was similar in all of the soils. The small fraction of Cr(VI) remaining unreduced resides along external surfaces of aggregates, leaving it potentially available to future transport down the soil profile. Using the Thiele modulus, Cr(VI) reduction in soil aggregates is shown to be diffusion rate- and reaction rate-limited in anaerobic and aerobic aggregates, respectively. Thus, spatially resolved chemical and microbiological measurements are necessary within anaerobic soil aggregates to characterize and predict the fate of Cr contamination. Typical methods of soil sampling and analyses that average over redox gradients within aggregates can erase important biogeochemical spatial relations necessary for understanding these environments.

Uranium(VI) sorption onto a soil collected at the Melton Branch Watershed (Oak Ridge National Laboratory, TN) is strongly influenced by the pH of the soil solution and, to a lesser extent, by the presence of calcium, suggesting specific chemical interactions between U(VI) and the soil matrix. Batch experiments designed to evaluate factors controlling desorption indicate that two anionic surfactants, AOK and T77, at concentrations ranging from 60 to 200 mM, are most suitable for U(VI) removal from acidic soils such as the Oak Ridge sediment. These surfactants are very efficient solubilizing agents at low uranium concentrations: ca. 100% U(VI) removal for [U(VI)] o,sorbed = 10 -6 mol kg -1 . At greater uranium concentrations (e.g., [U(VI)] o,serbed = ca. 10 -5 mol kg -1 ), the desorption efficiency of the surfactant solutions increases with an increase in surfactant concentration and reaches a plateau of 75 to 80% of the U(VI) initially sorbed. The most probable mechanisms responsible for U(VI) desorption include cation exchange in the electric double layer surrounding the micelles and, to a lesser extent, dissolution of the soil matrix. Limitations associated with the surfactant treatment include loss of surfactants onto the soil (sorption) and greater affinity between U(VI) and the soil matrix at large soil to liquid ratios. Parallel experiments with H 2 SO 4 and carbonate-bicarbonate (CB) solutions indicate that these more conventional methods suffer from strong matrix dissolution with the acid and reduced desorption efficiency with CB due to the buffering capacity of the acidic soil.