High Uranium Content Research Articles

Closure temperature of (U-Th)/He system in apatite obtained from natural drillhole samples in the Tarim basin and its geological significance

The apatite (U-Th)/He thermochronometry has been used to study the tectono-thermal evolution of mountains and sedimentary basins for over ten years. The closure temperature of helium is important for the apatite (U-Th)/He thermochronometry and has been widely studied by thermal simulation experiments. In this paper, the apatite He closure temperature was studied by establishing the evolutionary pattern between apatite He ages and apatite burial depth based on examined apatite He ages of natural samples obtained from drillholes in the Tarim basin, China. The study showed that the apatite He closure temperature of natural samples in the Tarim basin is approximately 88±5°C, higher than the result (∼75°C) obtained from the thermal simulation experiments. The high He closure temperature resulted from high effective uranium concentration, long-term radiation damage accumulation, and sufficient particle radii. This study is a reevaluation of the conventional apatite He closure temperature and has a great significance in studying the uplifting events in the late period of the basin-mountain tectonic evolution, of which the uplifting time and rates can be determined accurately.

Multivariate analysis of subsurface radiometric data in Rongsohkham area, East Khasi Hills district, Meghalaya (India): Implication on uranium exploration

Non-coring based exploratory drilling was under taken in the sedimentary environment of Rangsohkham block, East Khasi Hills district to examine the eastern extension of existing uranium resources located at Domiasiat and Wakhyn in the Mahadek basin of Meghalaya (India).Although radiometric survey and radiometric analysis of surface grab/channel samples in the block indicate high uranium content but the gamma ray logging results of exploratory boreholes in the block, did not obtain the expected results. To understand this abrupt discontinuity between the two sets of data (surface and subsurface) multivariate statistical analysis of primordial radioactive elements (K40, U238 and Th232) was performed using the concept of representative subsurface samples, drawn from the randomly selected 11 boreholes of this block.The study was performed to a high confidence level (99%), and results are discussed for assessing the U and Th behavior in the block. Results not only confirm the continuation of three distinct geological formations in the area but also the uranium bearing potential in the Mahadek sandstone of the eastern part of Mahadek Basin.

Did natural fission of 235U in the earth lead to formation of the supercontinent Columbia?

Steady decline in the percentage of 235U in terrestrial uranium made natural fission impossible after about 1.8Ga. Fission before 1.8Ga disturbed the lead isotope system at various places worldwide, such as Oklo, Gabon, and may have caused the first lead isotope paradox. Fission in areas of high uranium concentration may also have generated enough heat to localize sparse Archean and Paleoproterozoic UHT belts. The oldest widespread orogenic systems formed at approximately 2.0–1.8Ga after fission stopped contributing to the earth’s heat flow. These early orogenic systems partly created the supercontinent Columbia.

Eukaryotic life in biofilms formed in a uranium mine

The underground uranium mine Königstein (Saxony, Germany), currently in the process of remediation, represents an underground acid mine drainage (AMD) environment, that is, low pH conditions and high concentrations of heavy metals including uranium, in which eye-catching biofilm formations were observed. During active uranium mining from 1984 to 1990, technical leaching with sulphuric acid was applied underground on-site resulting in a change of the underground mine environment and initiated the formation of AMD and also the growth of AMD-related copious biofilms. Biofilms grow underground in the mine galleries in a depth of 250 m (50 m above sea level) either as stalactite-like slime communities or as acid streamers in the drainage channels. The eukaryotic diversity of these biofilms was analyzed by microscopic investigations and by molecular methods, that is, 18S rDNA PCR, cloning, and sequencing. The biofilm communities of the Königstein environment showed a low eukaryotic biodiversity and consisted of a variety of groups belonging to nine major taxa: ciliates, flagellates, amoebae, heterolobosea, fungi, apicomplexa, stramenopiles, rotifers and arthropoda, and a large number of uncultured eukaryotes, denoted as acidotolerant eukaryotic cluster (AEC). In Königstein, the flagellates Bodo saltans, the stramenopiles Diplophrys archeri, and the phylum of rotifers, class Bdelloidea, were detected for the first time in an AMD environment characterized by high concentrations of uranium. This study shows that not only bacteria and archaea may live in radioactive contaminated environments, but also species of eukaryotes, clearly indicating their potential influence on carbon cycling and metal immobilization within AMD-affected environment.

Preliminary results of combined ESR/U-series dating of fossil teeth from Longgupo cave, China

Longgupo Cave site, located in Wushan County, Chongqing, China has attracted continuous attention since its discovery of hominid remains in association with late Pliocene-early Pleistocene fauna and numerous lithic artefacts. In 2003–2006, new excavation was carried out on this site, allowing the description of a detailed stratigraphy of the highly complex cave infillings and the sampling of teeth for combined ESR/U-series analyses. Here we report preliminary dating results of seven herbivorous fossil teeth from different archaeological layers of the lowest geological unit (C III). Uranium-series analyses indicate that no obvious uranium leaching has occurred and all the teeth (except one) underwent a very recent uranium uptake history. The obtained US-ESR results show that the age of six teeth are basically consistent, between ∼1.4 and 1.8 Ma. At the same time, we observed an inverse correlation of two samples with the stratigraphical sequence. This could be caused by the distinct uranium uptake history of one sample, high uranium content in the enamel for another or bad estimation of external dose rate. Due to the complexity of the stratigraphic sequence, supplementary in situ gamma dose rate measurement should be performed for all the samples during the following excavations in order to confirm this preliminary ESR/U-series chronology.

Age of the oldest hominin settlements in Spain: Contribution of the combined U-series/ESR dating method applied to fossil teeth

This paper presents a synthesis of the combined Uranium-Series/ESR age results obtained on fossil teeth from several Early Pleistocene archaeological and/or palaeontological Spanish localities: Atapuerca Gran Dolina TD-6, Fuente Nueva-3, Barranco León, Venta Micena and Vallparadís. While the age results show some scattering, they are nevertheless encouraging and provide a significant contribution to the understanding of the first hominin settlements in Western Europe.These “old” samples are mainly characterised by high 230Th/234U activity ratios that may sometimes exceed secular equilibrium, and by high Uranium concentrations (several tens of ppm or more). The modelling of U-uptake is therefore really crucial for the age calculation process. In the case of the combined US–ESR approach, this uptake is derived from the U-series data measured in dental tissues, which seem to be strongly dependant on the hydro-geological context. Consequently, when dating “old” samples, a special attention has to be paid to the accuracy and the meaning of the U-series data collected from the dental tissues, which may be influenced by several factors such as spatial heterogeneity and recent U-uptake overprint. Some rapid scanning procedures should be developed in order to pre-identify the samples that are the most suitable for ESR dating.

High-uranium matrix effect in zircon and its implications for SHRIMP U–Pb age determinations

A correlation has previously been noted between extremely high-uranium concentrations in zircon and apparent U/Pb age as measured in ion microprobe analysis. Here we present data from two zircon populations with extremely high-uranium concentrations in an attempt to quantify effects related to the nature of the samples and/or instrumentally induced fractionation (instrument, analytical setup, and/or matrix effect). The high-uranium matrix effect is apparent in zircons from the 180Ma Tasmanian Dolerite, which typically shows an increase in Pb/U age of around 3% per thousand μg/g of uranium when measured on SHRIMP RG. One session on SHRIMP RG shows a correlation with 8% per thousand μg/g of uranium. Data from SHRIMP II show a weak correlation and no correlation is evident in data collected on SHRIMP I. High-uranium zircons from the 98Ma Mt Dromedary monzonite show little to no correlation between uranium concentration and U/Pb age. Analyses of younger (~20–50Ma) high-uranium zircons show a correlation between uranium concentration and an increase in apparent age, but this is less pronounced than the results obtained from the older Tasmanian Dolerite. Raman spectroscopy of the Tasmanian Dolerite zircons shows that these zircons are metamict. The link between uranium concentration and apparent age appears to be related to the degradation of the zircon matrix from radiation damage. The change in matrix enhances the emission of Pb+ relative to U and U oxide species. As this effect is associated with matrix and machine parameters, an external correction cannot be made assuming a constant correction value. With a secondary high-uranium standard, it might be possible to quantify the effect, but the matrix of this standard needs to be closely matched in age (radiation damage) and chemistry (U concentration); this solution is impractical. We instead propose that workers be aware of this matrix effect in SHRIMP analyses of zircon and to use Raman spectroscopy before or after SHRIMP analyses to ensure that the zircon is crystalline. These findings have important consequences for the use of SHRIMP data for zircon geochronology.

Heavy metals and trace elements in atmospheric fall-out: Their relationship with topsoil and wheat element composition

The objectives of this study were to determine the average concentrations and deposition rates of 28 elements in atmospheric bulk deposition and to elucidate associations among topsoil, bulk deposition and wheat element composition. The fluxes of arsenic (As), copper (Cu), lead (Pb) and zinc (Zn) deposition in Córdoba were higher than in other agro-ecosystems, which reflects both natural (geochemistry and topsoil removal) and anthropogenic sources. High lanthanide, uranium (U) and thorium (Th) concentrations revealed the impact of an open cast uranium mine. The highest enrichment factors (EF) were those of Cu, Pb, Zn and nickel (Ni), with calcium (Ca) being the most prominent in the surroundings of a cement plant. Industries and the transport of airborne urban pollutants were the main anthropogenic sources for Ca, Cu, Ni, Pb, Zn, cadmium (Cd), iron (Fe), manganese (Mn) and antimony (Sb). The concentrations of metals in wheat grain were predicted using the topsoil and atmospheric fall-out composition with R2=0.90, with the latter being the best explanatory variable. The present study highlights the potential health hazards of wheat consumption (Environmental Protection Agency) by the assessment of heavy metals in bulk atmospheric deposition.

Natural radioactivity and radiation protection in use of phosphogypsum waste product samples

Phosphogypsum material is the phosphate fertilizer by-product usually used for soil fertility. This material contains high activity concentration of uranium (238U) and radium (226Ra and 228Ra) radionuclides. Addition of phosphate fertilizers to soils leads to accumulation of radium nuclides with other elements on soil. These nuclides can diffuse through soil layers and dissolve in groundwater components. Risks may be evolved from using groundwater for drinking purposes. The risks associated with radium nuclides and the exposure dose rate for soils and groundwater samples are assessed. Sand mineral is found to be a good adsorbent that can be used as a natural ion exchanger for the removal of radium and phosphate ions from aqueous solutions.

Origin of enriched uranium contained in Japanese atmospheric deposits

The chemical compositions of atmospheric deposits collected in Fukuoka, Japan, in 1964 and 2000, which are known to have high 235U/238U uranium isotope ratio were determined. High uranium contents relative to that of continental crust were observed for the deposition samples in 1964 together with the high 235U/238U ratios, which indicated that the origin of uranium is fallout materials due to the large-scale 1961- 1962 nuclear explosions conducted by US and former USSR. The deposits in 2000 were mainly composed of dusts from the East Asia. This suggested that uranium with a high 235U/238U ratio in the deposits in 2000 has been transported as a component of mineral dust particles contaminated with nuclear debris from nuclear test sites from the East Asian deserts and arid regions by the prevailing wind.

Natural Radioactivity and Radon Exhalation in Phosphate Fertilizers

238U, 232Th, 226Ra, 222Rn and 40K activity concentrations in fertilizer samples collected from a local market were determined. Radon activity concentrations ranged from 0.04–252 Bq kg−1 and were comparable with low activity concentrations ranges reported previously. Radon exhalation rates and effective 226Ra concentration were determined using CR-39 solid-state nuclear track detectors. Effective 226Ra concentrations ranged from 4.27–33.93 Bq kg−1. Two different 222Rn exhalation rates were determined; area dependent exhalation rates (0.044–0.351 Bq m−2 h−1) and mass dependent exhalation rates (0.023–0.180) Bq kg−1 h−1). Even without secular equilibrium, strong correlations were generally apparent among uranium concentrations, effective 226Ra concentrations and 222Rn exhalation rates in samples with high concentrations of uranium. 232Th and 40K concentrations ranged from 0–44.6 and 0.08–127 μg/g, respectively.

Chemical element imaging for speleothem geochemistry: Application to a uranium-bearing corallite with aragonite diagenesis to opal (Eastern Siberia, Russia)

Two complementary image-based microanalysis methods, digital autoradiography of radioactive elements and X-ray fluorescence microspectroscopy (micro-XRF), were used to study the geochemistry of a speleothem from Eastern Siberia. Our objective was to show the interest of these micro-chemical methods to investigate the distribution of U and some other elements as Ca, Sr, and Si in speleothems. The sample studied was a corallite concretion that formed slowly by the precipitation of minerals from moisture that had condensed underground. Polished sections of the sample revealed alternating laminas of gray calcite and white aragonite. The concretion, which is older than 400Kyr (U/Th-TIMS), showed no signs of detrital contamination (230Th/232Th>10,000). Digital autoradiography and XRF analyses indicated exceptionally high uranium contents, ranging from 3–10 to more than 1300μg∙g−1 in different areas of the sample. Element maps for calcium (Ca), uranium (U), strontium (Sr) and silicon (Si) showed inverse correlations between Ca and U and between Ca and Si, but there is a strong correlation between Si and U. Under the optical microscope, the low Ca appears as corrosion voids, which were found to contain uranium-enriched amorphous silica replacing areas of aragonite and, in some cases, calcite. This postgenetic amorphous and uraniferous silica is composed of two kinds of opal: opal A formed by large microspheres of 30–40μm in diameter and a gray opal CT with irregular lepispheres. This late opal, dated around 385–412Kyr, is probably due to a special volcanic event formed by uraniferous glass ashes easily soluble by meteoric waters, as suggested by the presence of volcanic suite in the fine fraction of the opal. Opal precipitated after speleothem crystallization by evaporation or cryogenic supersaturation. The study of this exceptional concretion shows the value of direct chemical imaging methods, as they easily revealed the presence and distribution of U in the concretion, and allowed an unusual case of opal diagenesis in a carbonate speleothem to be described. Given the value of speleothems as geo-chronometers and recorders of environmental changes at different time scales, from season to isotopic stage, this methodology opens new perspectives in understanding the geochemical system and consequently the validity of radiometric datings, U/Th and U/Pb, especially in speleothem samples.

Zirconium alloys matrix as innovative material for composite fuel

A novel class of zirconium alloys is suggested as fuel matrix. They are “deep” ternary or quaternary eutectics having relatively low melting point i.e. from 963 to 1133K in comparison with pure zirconium and intended for use as a matrix of dispersion high uranium content fuel (CERMET and METMET) particularly for thermal reactors. For fast reactors and MA burning Zr–Ti based alloys are proposed that have resistant metallurgical bonds between fuel and steel cladding. Investigations have been carried out on the structure and properties of the alloys as well as the specific technologies of their fabrication, in particular via induction furnace melting. The alloys may be also produced in the amorphous state as granules and strips. It is shown that thanks to their capillary properties they might be applied in brazing dissimilar materials. Based on novel zirconium matrix alloys high uranium content fuel compositions were produced. They have high thermal conductivity and compatibility as well as 25–50% higher uranium content than for VVER and PWR fuels.Fuel pins are fabricated by capillary impregnation method. The use of dispersion fuel with novel zirconium matrix alloys improves neutronics characteristics of reactor cores and might lead to extension of burn-up, low operating temperatures and serviceability under transient conditions.

Influence of Uranium on Bacterial Communities: A Comparison of Natural Uranium-Rich Soils with Controls

This study investigated the influence of uranium on the indigenous bacterial community structure in natural soils with high uranium content. Radioactive soil samples exhibiting 0.26% - 25.5% U in mass were analyzed and compared with nearby control soils containing trace uranium. EXAFS and XRD analyses of soils revealed the presence of U(VI) and uranium-phosphate mineral phases, identified as sabugalite and meta-autunite. A comparative analysis of bacterial community fingerprints using denaturing gradient gel electrophoresis (DGGE) revealed the presence of a complex population in both control and uranium-rich samples. However, bacterial communities inhabiting uraniferous soils exhibited specific fingerprints that were remarkably stable over time, in contrast to populations from nearby control samples. Representatives of Acidobacteria, Proteobacteria, and seven others phyla were detected in DGGE bands specific to uraniferous samples. In particular, sequences related to iron-reducing bacteria such as Geobacter and Geothrix were identified concomitantly with iron-oxidizing species such as Gallionella and Sideroxydans. All together, our results demonstrate that uranium exerts a permanent high pressure on soil bacterial communities and suggest the existence of a uranium redox cycle mediated by bacteria in the soil.

Uranium in vegetable foodstuffs: should residents near the Cunha Baixa uranium mine site (Central Northern Portugal) be concerned?

Large uranium accumulations in vegetable foodstuffs may present risks of human health if they are consumed. The objective of this study was to evaluate the uranium concentrations in different vegetable foodstuffs and grown in agricultural soils, which are then consumed by the residents of the village of Cunha Baixa (Portugal),--located in an former uranium mining area. This study was conducted to address concerns expressed by the local farmers as well as to provide data for uranium-related health risk assessments for the area. Soils, irrigation water and edible tissues of lettuce, potato, green bean, carrot, cabbage, apple and maize (Latuca sativa L., Solanum tuberosum L., Phaseolus vulgaris L., Daucus carota L., Brassica oleracea L., Malus domestica Borkh, Zea mays L., respectively) were sampled and uranium determined. High uranium concentrations were found in some soils (U(total) > 50 mg/kg), in irrigation waters (218 to 1,035 μg/l) and in some vegetable foodstuffs (up to 234, 110, 30, 26, 22, 16 and 1.6 μg/kg fresh weight for lettuce, potato with peel, green bean pods, cabbage, corn, carrot and apple, respectively). However, the results of the toxicity hazard analysis were reassuring the estimated level of uranium exposure through the ingestion of these vegetable foodstuffs was low, suggesting no chemical health risk (hazard quotient <1) to this uranium exposure pathway for a local residents during their lifetime, even for the most sensitive part of the population (child).

Determination of uranium concentrations and its activity ratios in coal and fly ash from Philippine coal-fired thermal power plants using ICP-MS and TIMS

Abstract The specific activity of 238U as a technologically enhanced naturally occurring radioactive material (TENORM) in feed coal, bottom and fly ash samples from four major coal-fired thermal power plants in the Philippines have been measured using high-resolution gamma-ray spectroscopy system equipped with a high-purity germanium (HPGe) detector. The uranium concentration has been determined from same samples using inductively coupled plasma mass spectrometry (ICP-MS). There was a good correlation between the measured uranium using both methods and has been estimated to be 0.98. Uranium from coal, bottom and fly ash samples were chemically separated and activity ratio (234U/238U) and 235U/238U ratio was measured using a thermal ionization mass spectrometer (TIMS). The highest concentration of uranium was found in fly ash and lowest was for feed coal. Uranium isotopic composition plays an important role in studying its biogeochemical behavior and is a good tracer on the sources of uranium in the environment.

Uranium and radium diffusion in organic-rich sediments (sapropels)

Among the late Quaternary Mediterranean sapropels, the S5 (125 ka) is one of the best preserved due to its high organic carbon content that has limited postdepositional oxidation. The high uranium content in this sapropel, >40 dpm g−1, makes this layer interesting for studying uranium series disequilibrium in organic-rich sediments. For this reason, the present work provides isotopic measurements of the U decay series in a S5 sapropel by applying more precise mass spectrometric methods, TIMS/MC-ICPMS, and gamma spectrometry. Assuming that U in the sapropel mostly originated from seawater the (234U/238U), (230Th/238U), (226Ra/230Th) and (231Pa/235U) activity ratios show systematic deviations from the theoretical values for a closed-system evolution of the U series over the 125 ka since sapropel formation. The radiogenic 234Urad and 226Ra show clear evidence of migration in the sapropel with modeled diffusion coefficients of (7.1 ± 1.1) × 10−12 cm2 s−1 and (1.6 ± 0.2) × 10−10 cm2 s−1, respectively. The diffusion of 234Urad cannot explain the high (230Th/238U) and (231Pa/235U) activity ratios observed in the sapropel. Two possible mechanisms or a combination of both are proposed for explaining the irregular (230Th/238U) and (231Pa/235U) activity profiles in sapropel S5. The first one is an enhanced export flux of 230Thxs and 231Paxs excesses exceeding the production rate in seawater, during the time of sapropel formation, and the second one is diffusion of authigenic Uauth in the sapropel. However, the ambiguous determination of 230Thxs and 231Paxs in the sapropel and the poorly understood processes that might lead to Uauth migration in anoxic sediments still limit a final explanation for the deviation of (230Th/238U) and (231Pa/235U) activity ratios from their expected theoretical values.

Uranium speciation in river sediments contaminated by phosphate ores

Large amounts of phosphate ores with high concentrations of uranium were dumped by a phosphate plant into the Flix water reservoir in the Ebre River, Catalonia, NE Spain. These phosphate wastes have been mixed over the years with effluents from other industries as well as with the sediments of the river, resulting in a complex mixture of solid wastes and sediments. No investigations on uranium speciation in such sediments were made because of the complexity of the sediments composition as well as the relatively low uranium content. However, these studies are necessary in order to predict the release of the uranium to the river waters. Here, we studied uranium speciation in sediments from two sampling points of the Flix water reservoir and at depths from 5 to 113 cm. We used room temperature time-resolved laser fluorescence spectroscopy and a three-step sequential extraction procedure described by the Standards, Measurements, and Testing Programme of the European Union. We found that uranium was mainly present in the sediment samples as meta-autunite [Ca(UO2)2(PO4)2·10–12H2O], whose low solubility will result in a low release of uranium to the river waters. In addition, we found that some uranium was linked to sediments by forming surface complexes. We therefore made the first study of uranium speciation in the sediments of the Flix water reservoir.

Timing of Variscan HP-HT metamorphism in the Moldanubian Zone of the Bohemian Massif: U-Pb SHRIMP dating on multiply zoned zircons from a granulite from the Dunkelsteiner Wald Massif, Lower Austria

In an attempt to better constrain the timing of Variscan HP-HT metamorphism in the SE Bohemian Massif we have dated zoned zircons from a garnet-kyanite granulite of granitic composition from the Dunkelsteiner Wald Massif, Lower Austria, by means of sensitive high-resolution ion microprobe (SHRIMP) technique. In order to combine isotopic information with crystal growth textures, CL and BSE images were systematically taken from the dated zircons. A characteristic threefold concentric zoning was found in many zircons. This involves pre-Variscan protolithic cores followed by two distinct metamorphic/anatectic overgrowth shells of Variscan age. The inner overgrowth shell is characterized by a weak CL but bright BSE signal, and yields high contents of uranium (0.1 to 0.2 wt.%). A pooled U-Pb Concordia age for this zone is 342.0 ± 3.0 Ma (n = 11, MSWD = 0.12). The second, outer, overgrowth shell is always bright in the CL image, dark in the BSE image, and has generally low uranium contents (mostly <500 ppm). A pooled U-Pb Concordia age for this zone is 337.1 ± 2.7 Ma (n = 11, MSWD = 0.22). These results imply that the Variscan HT crystallisation history of the Moldanubian granulites took place over a period of a few million years and was not an extremely rapid subduction-exhumation process. SHRIMP measurements in the protolithic cores yield a cluster of (sub)concordant ages between ∼390 and 460 Ma and a few outliers at higher ages mostly represented by cores in cores. Core domains, which are large, homogeneous and with undisturbed igneous oscillatory zoning, yielded preferentially ages between 430 and 460 Ma. We therefore consider that granitic protolith formation took place at that time. The still older inner cores are interpreted as inherited into the granitic melt.

Innovative mathematical modeling in environmental remediation

There are two different ways to model reactive transport: ad hoc and innovative reaction-based approaches. The former, such as the Kd simplification of adsorption, has been widely employed by practitioners, while the latter has been mainly used in scientific communities for elucidating mechanisms of biogeochemical transport processes. It is believed that innovative mechanistic-based models could serve as protocols for environmental remediation as well. This paper reviews the development of a mechanistically coupled fluid flow, thermal transport, hydrologic transport, and reactive biogeochemical model and example-applications to environmental remediation problems. Theoretical bases are sufficiently described. Four example problems previously carried out are used to demonstrate how numerical experimentation can be used to evaluate the feasibility of different remediation approaches. The first one involved the application of a 56-species uranium tailing problem to the Melton Branch Subwatershed at Oak Ridge National Laboratory (ORNL) using the parallel version of the model. Simulations were made to demonstrate the potential mobilization of uranium and other chelating agents in the proposed waste disposal site. The second problem simulated laboratory-scale system to investigate the role of natural attenuation in potential off-site migration of uranium from uranium mill tailings after restoration. It showed inadequacy of using a single Kd even for a homogeneous medium. The third example simulated laboratory experiments involving extremely high concentrations of uranium, technetium, aluminum, nitrate, and toxic metals (e.g., Ni, Cr, Co). The fourth example modeled microbially-mediated immobilization of uranium in an unconfined aquifer using acetate amendment in a field-scale experiment. The purposes of these modeling studies were to simulate various mechanisms of mobilization and immobilization of radioactive wastes and to illustrate how to apply reactive transport models for environmental remediation.