Search

Acid variable charge subsoils exhibit a positive charge, which retards the passage of anions through the soil profile. The objective was to study the NO{sub 3}{sup {minus}} adsorption isotherms and the effect of changing the subsoil positive charge on NO{sub 3}{sup {minus}} and Cl{sup {minus}} retention, estimate NO{sub 3}{sup {minus}} and Cl{sup {minus}} retardation in different subsoils, and quantify relationships between transport and adsorption parameters. Subsoils from different subtropical and tropical areas were used in column experiments. Four lime treatments and four leaching solutions were used to create different anion exchange capacities (AECs) in subsoils. In the Cecil subsoil (Fine, kaolinitic, thermic Typic Kanhapludult), NO{sub 3}{sup {minus}} isotherms were linear between 5 and 30 mmol NO{sub 3}{sup {minus}} L{sup {minus}1}. When forced through the origin, an L-curve NO{sub 3}{sup {minus}} isotherm was observed at native and low soil pH. The NO{sub 3}{sup {minus}} adsorption was largely affected by the changes in both pH and concentration of the leaching solution. Other variable charge subsoils have varying capacities to retard anions, which is reduced when either soil pH or leaching solution concentration is increased. Both parameters equally reduce the distribution coefficient K{sub d} and the retardation coefficient R. The AEC correlates closelymore » with R and K{sub d}. As a result, AEC can be successfully used in the solute transport models to estimate K{sub d} or R without measuring it to correct for the NO{sub 3}{sup {minus}} retardation.« less

Arsenic (As) is the biggest environment contaminant in most of the soils where fly ash is applied. Usually, it is not mobile and strongly adsorbed on to soil particles. However, in gypsum and phosphorus amended soils As may be much more mobile. A study in repacked columns was conducted to determine whether or not As becomes mobile when Ca(H2PO4)2and CaSO4are used as leaching solutions, and to compare the competitive interactions between PO4-AsO4and SO4-AsO4. Arsenic concentration in leachate was found to be approximately ten times greater when Ca(H2PO4)2was used to leach the columns as compared to CaSO4. A maximum concentration of 800 μg As L-1was found in the leachate in this case, which is much higher than the groundwater limit of 50 μg L-1for drinking water established by the United States Environmental Protection Agency. In fly ash, the portion of arsenate non-specifically adsorbed is believed to be much lower than that of specifically adsorbed. Sulfate anions were able to displace only non-specifically adsorbed arsenate. In this case the concentration of As in leachate was found to be within acceptable limits. On the other hand, phosphate can compete with arsenate for all available adsorption sites, non-specific and specific. Phosphate displacement of both forms of arsenates increases As mobility in both control and fly ash treatments.

This is a mini-review of the research work conducted by the authors with the objective of studying ion transport in variable charge subsoils collected from different areas around the world. An attempt is made in these studies to relate the unique behavior manifested during ionic transport in these subsoils with their mineralogical, physical and chemical properties, which are markedly different from those in soils from temperate regions. The variable charge subsoils have a relatively high salt sorption capacity and anion exchange capacity (AEC) that retards anions downward movement. The AEC correlates closely with the anion retardation coefficients. Ca2+ applied with gypsum in topsoil may be transported to the subsoil and may improve the subsoil chemical properties. These results may help in developing appropriate management strategies under a range of mineralogical, physical, and chemical conditions.

The capacity for subsurface sediments to sequester radionuclide contaminants, such as uranium (U), and retain them after bioremediation efforts are completed is critical to the long-term stewardship of re-mediated sites. In U bioremediation strategies, carbon amendment stimulates bioreduction of U(VI) to U(IV), immobilizing it within the sediments. Sediments enriched in natural organic matter are naturally capable of sequestering significant U, but may serve as sources to the aquifer, contributing to plume persistence. Two types of organic-rich sediments were compared to better understand U release mechanisms. Sediments that were artificially primed for U removal were retrieved from an area previously biostimulated while detrital-rich sediments were collected from a location never subject to amendment. Batch incubations demonstrated that primed sediments rapidly removed uranium from the groundwater, whereas naturally reduced sediments released a sizeable portion of U before U(VI)-reduction commenced. Column experiments confirmed that U release persisted for 65 pore volumes in naturally reduced sediments, demonstrating their sink-source behavior. Acetate addition to primed sediments shifted the microbial community from sulfate-reducing bacteria within Desulfobacteraceae to the iron-reducing Geobacteraceae and Firmicutes, associated with efficient U(VI) removal and retention, respectively. In contrast, Geobacteraceae communities in naturally reduced sediments were replaced by sequences with similarity to Pseudomonas spp. during U release, while U(VI) removal only occurred with enrichment of Firmicutes. These investigations stress the importance of characterizing zones with heterogeneous carbon pools at U-contaminated sites prior to the determination of a remedial strategy to identify areas, which may contribute to long-term sourcing of the contaminants.

Experimental research work has been conducted and is undergoing at Pacific Northwest National Laboratory (PNNL) to address a variety of scientific issues related with the potential leaks of the carbon dioxide (CO2) gas from deep storage reservoirs. The main objectives of this work are as follows: • Develop a systematic understanding of how CO2 leakage is likely to influence pertinent geochemical processes (e.g., dissolution/precipitation, sorption/desorption and redox reactions) in the aquifer sediments. • Identify prevailing environmental conditions that would dictate one geochemical outcome over another. • Gather useful information to support site selection, risk assessment, policy-making, and public education efforts associated with geological carbon sequestration. In this report, we present results from experiments conducted at PNNL to address research issues related to the main objectives of this effort. A series of batch and column experiments and solid phase characterization studies (quantitative x-ray diffraction and wet chemical extractions with a concentrated acid) were conducted with representative rocks and sediments from an unconfined, oxidizing carbonate aquifer, i.e., Edwards aquifer in Texas, and a confined aquifer, i.e., the High Plains aquifer in Kansas. These materials were exposed to a CO2 gas stream simulating CO2 gas leaking scenarios, and changes in aqueous phase pHmore » and chemical composition were measured in liquid and effluent samples collected at pre-determined experimental times. Additional research to be conducted during the current fiscal year will further validate these results and will address other important remaining issues. Results from these experimental efforts will provide valuable insights for the development of site-specific, generation III reduced order models. In addition, results will initially serve as input parameters during model calibration runs and, ultimately, will be used to test model predictive capability and competency. The results from these investigations will provide useful information to support site selection, risk assessment, and public education efforts associated with geological, deep subsurface CO2 storage and sequestration.« less

Subsoils of highly weathered, acid soils usually contain appreciable quantities of anion-exchange capacity (AEC), which retards the passage of anions through the profile. This AEC varies in magnitude with ambient soil solution parameters such as pH and ionic strength. In order to be able to predict NO 3 - movement in such soils, estimates of the effect of subsoil charge characteristics on retardation are required. The effect of subsoil liming and ionic strength on the movement of NO 3 - in repacked Kanhapludult soil columns was studied for this purpose. Columns comprising topsoil (untreated) over subsoil limed to different levels to give a range of AEC were used. The soil was first leached with a solution of 10 mmol c , Ca(NO 3 ) 2 L -1 followed by a 10 mmol c CaCl 2 L -1 solution. Convection-dispersion equation (CDE) parameters for Cl - were determined using CXTFIT. Liming reduced the retardation coefficient (R) from 2.39 in the unamended subsoil to 1.12 in the treatment that received 2.08 g Ca(OH) 2 kg -1 . Both subsoil pH (r 2 = 0.96) and AEC (r 2 = 0.98) were correlated with R for Cl - . To test the effect of electrolyte concentration on retardation, columns of untreated subsoil were leached with solutions of Ca(NO 3 ) 2 and CaCl 2 at four input concentrations : 5, 10, 20, and 30 mmol c L -1 . Retardation coefficients for Cl - at these concentrations were 2.81, 2.25, 1.76, and 1.51, respectively. Anion retardation needs to be taken into account in modeling NO 3 - movement through acid subsoils.

This study shows that acidic and alkaline wastes co-disposed with uranium into subsurface sediments have significant impact on changes in uranium retardation, concentration, and mass during downward migration. For uranium co-disposal with acidic wastes, significant rapid (i.e., hours) carbonate and slow (i.e., 100 s of hours) clay dissolution resulted, releasing significant sediment-associated uranium, but the extent of uranium release and mobility change was controlled by the acid mass added relative to the sediment proton adsorption capacity. Mineral dissolution in acidic solutions (pH2) resulted in a rapid (

Leakage from deep storage reservoirs is a major risk factor associated with geologic sequestration of carbon dioxide (CO2). Different scientific theories exist concerning the potential implications of such leakage for near-surface environments. The authors of this report reviewed the current literature on how CO2 leakage (from storage reservoirs) would likely impact the geochemistry of near surface environments such as potable water aquifers and the vadose zone. Experimental and modeling studies highlighted the potential for both beneficial (e.g., CO2 re sequestration or contaminant immobilization) and deleterious (e.g., contaminant mobilization) consequences of CO2 intrusion in these systems. Current knowledge gaps, including the role of CO2-induced changes in redox conditions, the influence of CO2 influx rate, gas composition, organic matter content and microorganisms are discussed in terms of their potential influence on pertinent geochemical processes and the potential for beneficial or deleterious outcomes. Geochemical modeling was used to systematically highlight why closing these knowledge gaps are pivotal. A framework for studying and assessing consequences associated with each factor is also presented in Section 5.6.

The goal of this chapter is to introduce the reader to the Fluidized Bed Steam Reforming (FBSR) process and resulting waste form. The first section of the chapter gives an overview of the potential need for FBSR processing in nuclear waste remediation followed by an overview of the engineering involved in the process itself. This is followed by a description of waste form production at a chemical level followed by a section describing different process streams that have undergone the FBSR process. The third section describes the resulting mineral product in terms of phases that are present and the ability of the waste form to encapsulate hazardous and radioactive wastes from several sources. Following this description is a presentation of the physical properties of the granular and monolith waste form product including and contaminant release mechanisms. The last section gives a brief summary of this chapter and includes a section on the strengths associated with this waste form and the needs for additional data and remaining questions yet to be answered. The reader is directed elsewhere for more information on other waste forms such as Cast Stone (Lockrem, 2005), Ceramicrete (Singh et al., 1997, Wagh et al., 1999) and geopolymersmore » (Kyritsis et al., 2009; Russell et al., 2006).« less

During the nuclear waste vitrification process volatilized (99)Tc will be trapped by melter off-gas scrubbers and then washed out into caustic solutions, and plans are currently being contemplated for the disposal of such secondary waste. Solutions containing pertechnetate [(99)Tc(VII)O(4)(-)] were mixed with precipitating goethite and dissolved Fe(II) to determine if an iron (oxy)hydroxide-based waste form can reduce Tc(VII) and isolate Tc(IV) from oxygen. The results of these experiments demonstrate that Fe(II) with goethite efficiently catalyzes the reduction of technetium in deionized water and complex solutions that mimic the chemical composition of caustic waste scrubber media. Identification of the phases, goethite + magnetite, was performed using XRD, SEM and TEM methods. Analyses of the Tc-bearing solid products by XAFS indicate that all of the Tc(VII) was reduced to Tc(IV) and that the latter is incorporated into goethite or magnetite as octahedral Tc(IV). Batch dissolution experiments, conducted under ambient oxidizing conditions for more than 180 days, demonstrated a very limited release of Tc to solution (2-7 μg Tc/g solid). Incorporation of Tc(IV) into the goethite lattice thus provides significant advantages for limiting reoxidation and curtailing release of Tc disposed in nuclear waste repositories.

At the Hanford Site, chromate was used throughout the 100 Areas (100-B, 100-C, 100-D/DR, 100-F, 100-H, and 100 K) as a corrosion inhibitor in reactor cooling water. Chromate was delivered in rail cars, tanker trucks, barrels, and local pipelines as dichromate granular solid or stock solution. In many occasions, chromate was inevitably discharged to surface or near-surface ground through spills during handling, pipeline leaks, or during disposal to cribs. The composition of the liquids that were discharged is not known and it is quite possible that Cr(VI) fate and transport in the contaminated sediments would be a function of the chemical composition of the waste fluids. The major objectives of this investigation which was limited in scope by the financial resources available, were to 1) determine the leaching characteristics of hexavalent chromium [Cr(VI)] from contaminated sediments collected from 100-D Area spill sites; 2) elucidate possible Cr(VI) mineral and/or chemical associations that may be responsible for Cr(VI) retention in the Hanford Site 100 Areas through the use of macroscopic leaching studies, and microscale characterization of contaminated sediments; and 3) provide information to construct a conceptual model of Cr(VI) geochemistry in the Hanford 100 Area vadose zone that can be used for developing options for environmental remediation. The information gathered from this research effort will help to further improve our understanding of Cr(VI) behavior in the vadose zone and will also help in accelerating the 100 Area Columbia River Corridor cleanup by providing valuable information to develop remedial action based on a fundamental understanding of Cr(VI) vadose zone geochemistry. A series of column experiments were conducted with contaminated sediments to study Cr(VI) desorption patterns. Column experiments used the field size fraction of the sediment samples and a simulated Hanford Site groundwater solution. Periodic stop flow events were applied to evaluate the change in elemental concentration during time periods of no flow and greater fluid residence time. Sediments were characterized for the spatial and mineralogical associations of the contamination using some microscale techniques such as X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and Mossbauer spectroscopy.

This report summarizes the methodology used to test the sequestration of technetium-99 present in both deionized water and simulated Hanford Tank Waste Treatment and Immobilization Plant waste solutions.

Pacific Northwest National Laboratory has initiated a waste form testing program to support the long-term durability evaluation of a waste form for secondary wastes generated from the treatment and immobilization of Hanford radioactive tank wastes. The purpose of the work discussed in this report is to identify candidate stabilization technologies and getters that have the potential to successfully treat the secondary waste stream liquid effluent, mainly from off-gas scrubbers and spent solids, produced by the Hanford Tank Waste Treatment and Immobilization Plant (WTP). Down-selection to the most promising stabilization processes/waste forms is needed to support the design of a solidification treatment unit (STU) to be added to the Effluent Treatment Facility (ETF). To support key decision processes, an initial screening of the secondary liquid waste forms must be completed by February 2010.

This review provides insights on some unique properties of nanoparticles (NPs) that are present in soils. In addition, this review discusses the role of NPs in controlling or influencing single and/or coupled chemical, biological, and hydrological soil- and/or geo-processes, which directly or indirectly affect the mobility or may determine the ultimate fate of aqueous and sorbed (adsorbed or precipitated) chemical species of nutrients and contaminants in terrestrial ecosystems. The chapter is composed of five review sections, followed by another section on future research directions, the acknowledgments, and the list of the references. A brief introduction to nanotechnology, nanoscience and environmental soil nanoscience, and definitions of relevant terms and chapter objectives are provided in the first section. A discussion on size-dependent properties and controls, focusing initially on the differences in nanoscale versus bulk scale properties, and later on the properties that change within the nanoscale, is presented in the second section. The important topic of NP origin (natural or manufactured) and occurrence in soils is presented in the third section. The behavior of NPs in soils is discussed in the fourth section. Two subsections are included. In the first one, processes that may affect NPs behavior in soils, such as growth, stability, solid phase transformation, aggregation, and aging are discussed. In the second one, processes that may be affected by the presence of NPs in soils, such as contaminant and/or nutrient sorption, redox reactions, and their advective or diffusive mobility, are discussed. A brief discussion on NPs toxicity is presented in the fifth and last review section of this chapter.

Samples of a naturally bioreduced, U-contaminated alluvial sediment were characterized with various microscopic and spectroscopic techniques and wet chemical extraction methods. The objective was to investigate U association and interaction with minerals of the sediment. Bioreduced sediment comprises approximately 10% of an alluvial aquifer adjacent to the Colorado River, in Rifle, CO, that was the site of a former U milling operation. Past and ongoing research has demonstrated that bioreduced sediment is elevated in solid-associated U, total organic carbon, and acid-volatile sulfide, and depleted in bioavailable Fe(III) confirming that sulfate and Fe(III) reduction have occurred naturally in the sediment. SEM/EDS analyses demonstrated that framboidal pyrites (FeS(2)) of different sizes ( approximately 10-20 microm in diameter), and of various microcrystal morphology, degree of surface weathering, and internal porosity were abundant in the

Column experiments combined with geochemical modeling, microscopic inspections, spectroscopic interrogations, and wet chemical extractions were used to study sediment-dependent Cr(VI) desorption, physical location, mineral association, and attenuation mechanism(s) in four freshly or naturally aged contaminated sediments exposed to concentrated Cr(VI) waste fluids. Results showed that majority of Cr(VI) mass was easily removed from the sediments (equilibrium site K(d) varied from 0 to 0.33 mL g(-1) and equilibrium site fraction was greater than 95%). Long tailings of time-dependent Cr(VI) concentrations above maximum contaminant level of 1.9 micromol L(-1) were also observed (kinetically controlled fraction K(d) and desorption reaction half-lives varied from 0 to 45 mL g(-1), and from 76.1 to 126 h, respectively). Microscopic and spectroscopic measurements confirmed that Cr was concentrated within fine-grained coatings in small areas mainly rich in phyllosilicates that contained both Cr(III) and Cr(VI). However, Cr(VI) reduction was neither significant nor complete. Under slightly alkaline and oxic conditions, contaminant Cr in the sediments occurred: (i) In the highly mobile pool (over 95% of total Cr); (ii) In the slow and time-dependent releasing pool, which served as long-term source of contamination; (iii) As reduced Cr(III) which most likely formed during Cr(VI) reaction with aqueous, sorbed, or structural Fe(II).

The long-term interactions of carbon tetrachloride (CCl4) and chloroform (CHCl3) with sediments that are low in organic matter (OM) are not well studied and documented in the literature. In this study, CCl4 and CHCl3 were mixed with supercritical carbon dioxide (CO2) and loaded onto columns packed with two sediments with low OM content and different textures, to establish contamination and achieve expedited artificial aging. The columns were subsequently leached with a simulated groundwater under hydraulically saturated conditions. Scanning electron microscopy was used to inspect the morphology of sediment single particles, determine the degree of particle association in aggregates and qualitatively estimate porosity and the length of possible diffusional pathways that might affect the overall contaminant desorption rates in sediments with low sorption capacity. Results demonstrated that most of contaminant inventories were rapidly released in the first pore volume of effluent, although a small portion of contaminants’ total mass exhibited time-dependent desorption. The calculated Kd values of CCl4 or CHCl3 partition were negligibly small. The transport behavior of both contaminants was similar and it was simulated well with a distributed (multiple)-rate (DR) statistical model, which accounted for the apparent contaminant mass transfer through diffusional pathways of different lengths, towards the advectivemore » pores. The distribution of contaminant mass between equilibrium and kinetic fractions, the distribution of the individual rate constants, and the average rate constants calculated with the parameters of the γ-distribution function (β and η) of the DR model, were sediments (texture) dependent, indicating that contaminant desorption during the late stage of leaching was driven by concentration gradients (i.e., diffusion) within sediment matrix porosity.« less

[1] Column experiments were performed to investigate the scale-dependent desorption of uranyl [U(VI)] from a contaminated sediment collected from the Hanford 300 Area at the U.S. Department of Energy (DOE) Hanford Site, Washington. The sediment was a coarse-textured alluvial flood deposit containing significant mass percentage of river cobble. U(VI) was, however, only associated with its minor fine-grained (

A column study on U(VI)-contaminated vadose zone sediments from the Hanford Site, WA, was performed to investigate U(VI) release kinetics with water advection and variable geochemical conditions. The sediments were collected from an area adjacent to and below tank BX-102 that was contaminated as a result of a radioactive tank waste overfill event. The primary reservoir for U(VI) in the sediments are micrometer-size precipitates composed of nanocrystallite aggregates of a Na-U-Silicate phase, most likely Na-boltwoodite, that nucleated and grew within microfractures of the plagioclase component of sand-sized granitic clasts. Two sediment samples, with different U(VI) concentrations and intraparticle mass transfer properties, were leached with advective flows of three different solutions. The influent solutions were all calcite-saturated and in equilibrium with atmospheric CO2. One solution was prepared from DI water, the second was a synthetic groundwater (SGW) with elevated Na that mimicked groundwater at the Hanford site, and the third was the same SGW but with both elevated Na and Si. The latter two solutions were employed, in part, to test the effect of saturation state on U(VI) release. For both sediments, and all three electrolytes, there was an initial rapid release of U(VI) to the advecting solution followed by slower near steady-state release. U(VI)aq concentrations increased during subsequent stop-flow events. The electrolytes with elevated Na and Si depressed U(VL)aq concentrations in effluent solutions. Effluent U(VI)aq concentrations for both sediments and all three electrolytes were simulated reasonably well by a three domain model (the advecting fluid, fractures, and matrix) that coupled U(VI) dissolution, intraparticle U(VI)aq diffusion, and interparticle advection, where diffusion and dissolution properties were parameterized in a previous batch study.

The feasibility of applying a combination of H2S and NH3 gases for Tc-99 remediation in vadose zone sediments evaluated in laboratory experiments showed a significant mobility decrease from 75 to 95% leachable Tc-99 in untreated sediments to 15 to 48% leachable Tc-99 after gas treatment. Individual H2S or NH3 gas treatment of sediments had little lasting effect. For the combined gas treatment, the H2S gas created reducing conditions at the pore water/mineral interface that pertechnetate was temporarily reduced/precipitated. The NH3 gas created alkaline pore water, which induces some mineral dissolution and subsequent aluminosilicate precipitation as the pH neutralizes, which may coat Tc-99 surface phases and render them less mobile. Surface phase analysis showed Tc-99 associated with weathered basalt clasts and sulfur, possibly from the precipitation of TcSx. Treatment performance was nearly the same at different Tc-99 concentration (34 to 6500 pCi/g), water content (1 to 8%), and gas injection rate, but was sensitive to gas concentrations. Low gas concentrations ( 30%) formed NH4SH precipitate. The 15 - 23% mobile Tc-99 remaining after gas treatment may be caused by limited time for aluminosilicates to precipitate in experiments. Nitrogen species concentrationsmore » showed no nitrate or nitrite production and a significant decrease in the sediment microbial population over three months, suggesting nitrification of added ammonia was inhibited. Overall, this study showed that combined H2S and NH3 gas treatment of low water content sediments was a robust technology to significantly decrease Tc-99 mobility.« less

Soils in the Oxisols, Ultisols, Alfisols, Spodosols and Andisols orders that are rich in constituents with surface reactive groups with amphoteric properties are considered variable charge soils (VCS). They have developed under intensive weathering in subtropical and tropical regions or from volcanic ash parent material. The magnitude and sign of the surface charge of variable charge constituents depend on the chemistry of the contacting solution (pH and ionic strength). The mineralogical, physical and chemical characteristics of these soil systems are different from those observed in soil systems of temperate regions. In this chapter, the mineralogy, chemistry and management of VCS are reviewed and discussed, focusing on the chemistry of these systems.

Substantial amounts of self-boiling, Al-rich, hyperalkaline, and saline high-level waste fluids (HLWF) were deposited to the vadose zone at the Hanford Site, in Washington State. The objective of this study was to investigate the effects of similar fluids on the extent of dissolution and precipitation in the sediments. Metal- and glass-free systems were used to conduct batch experiments at 323 K under CO2 and O2 free conditions. Base-induced dissolution of the soil minerals was rapid in the first 48 h as indicated by immediate releases of Si and Fe into the soil solution. Potassium release lagged behind and dissolution of K-bearing minerals (mica and K-feldspar) proceeded faster only after 2 to 3 d of the experiment. Silicon and Fe release exhibited high dependence on aqueous [Al] (rate orders-1), because Al decreased free OH concentration in the contact solution and probably inhibited soil mineral dissolution. Initial K release exhibited low dependence on [Al] (fractional rate orders). Initial dissolution rates calculated based on Si release varied with aqueous [Al] from 29.47 to 4.35 x 10(-12) mol m(-2) s(-1). Aluminum participated in the formation of the secondary phases (precipitation rates of 10(-8) mol s(-1)) but the overall precipitation rate of alumino-silicate secondary phases was probably controlled by aqueous [Si] (rates of 10(-9), and rate constants between 0.0054 and 0.0084 h(-1)). The changes in the soil solution chemistry (release of K, Si, Fe, and other elements) may play a significant role in the fate of radionuclides and contaminants like Cs, Sr, Cr, and U in the Hanford sediments.

Low water content sediments were treated with NH 3 gas to evaluate changes in U mobility as a potential field remediation method for vadose zone contamination. Injection of NH 3 gas created high dissolved NH 3 concentrations that followed equilibrium behavior. High NH 3 concentration led to an increase in pH from 8.0 to 11 to 13, depending on the water content and NH 3 concentration. The increase in pore water pH resulted in a large increase in pore water cations and anions from mineral-phase dissolution. Minerals showing the greatest dissolution included montmorillonite, muscovite, and kaolinite. Pore water ion concentrations then decreased with time. Simulations based on initial pore water ion concentrations indicated that quartz, chrysotile, calcite, diaspore, hematite, and Na-boltwoodite (hydrous U silicate) should precipitate. Electrical resistivity and induced polarization tomography (ERT/IP) was able to nonintrusively track these NH 3 partitioning, dissolution, and precipitations processes through changes in conductivity and chargeability. Ammonia treatment significantly decreases the amount of U present as adsorbed and aqueous species in field-contaminated sediments. In contrast, sediments containing a large fraction of U associated with carbonates generally showed little change. Uranium leaching from sediments containing high Na-boltwoodite decreased significantly by NH 3 treatment, but x-ray absorption near-edge structure/extended x-ray absorption fine structure showed no change in the Na-boltwoodite concentration. Therefore, NH 3 treatment of contaminated sediment acts to decrease the highly mobile aqueous and adsorbed U by incorporation into precipitates and appears to decrease mobility of some existing U precipitates (Na-boltwoodite) as a result of mineral coating.

Naturally bioreduced zones with considerable sorbed U were recently identified at a former U mining and processing site at Rifle, CO, USA. Most of the sorbed U appears to be associated with Fe minerals. Variably reduced sediment samples were analyzed by suite of techniques, primarily by room temperature Mossbauer spectroscopy. Fe-oxides of different types and crystallinity, and Fe(II)/Fe(III)-containing clays are dominant in all the sediments. The amounts of poorly crystalline Fe(III)-oxide, however, was lower in the reduced samples. In addition, framboidal pyrites with sorbed U were common in the highly reduced sediments. Overall, the information gained from this work may help develop design field strategies for immobilization and stabilization of U(VI) in contaminated subsurface environments.

A previously developed system to measure NH 3 volatilization in the field consists of a vacuum pump, volatilization chambers, and acid traps to capture NH 3 . The vacuum pump of the system draws air through each chamber at a constant rate, which may be faster or slower than the wind speed outside the chamber. Because wind speed affects NH 3 volatilization, it would be desirable to draw air through the chambers at a wind speed similar to that outside the chambers. The objective of this work was to improve the existing design by adding the capability of adjusting the speed at which air is drawn through each chamber. A hot needle anemometer was placed next to each chamber to measure wind speed at 1 cm above the soil surface. Wind speed was used to adjust an electronic valve so that the rate at which air was drawn through the chamber would match the external wind speed. Three N recovery studies were conducted with a sand-CaCO 3 mixture in the field to determine if the system could be used to obtain quantitative estimates of NH 3 volatilized. The NH 3 volatilized in each study was estimated from (i) the loss of soil inorganic N and (ii) the N captured in the acid traps. In all three studies, the NH 3 volatilized estimated from the acid traps (10-15% of applied N) was not different from the NH 3 volatilized estimated from the loss of soil inorganic N. These results indicate that the system can be used to obtain quantitative measurements of NH 3 volatilized under the conditions of these studies.