Uranium-series disequilibrium in Australian soils and its effect on aerial gamma-ray surveys

Measurements of U and ²²⁶Ra in 445 soil samples collected from areas throughout Australia show that these soils have a mean U content of 2.0 mg kg⁻¹ and activity ratio (U/²²⁶Ra) of 1.09. There is a wide spread in individual values but, overall, more soils are U-rich than ²²⁶Ra-rich. Some indications of regional disequilibrium were found, with ²²⁶Ra-rich soils being noted in the Southern Cross-Kambalda area (WA), in south-eastern Eyre Peninsula (SA) and in an area north of Cobar (NSW). Uranium-rich soils were found in two Carboniferous volcanic terrains in north-eastern Queensland. The spread in activity ratio values indicates that disequilibrium will contribute to the noise in U channel data obtained by aerial gamma-ray surveying. The combination of statistical noise from low count rates (reflecting the low U concentrations in soils) and U-²²⁶Ra disequilibrium combine to give variances of 40–60% in typical U channel data. Data appearance can be improved to some extent by appropriate filtering.     

An analysis of naturally occurring radionuclides and <Superscript>137</Superscript>Cs in the soils of urban areas using gamma-ray spectrometry

This study of environmental radioactivity was carried out in the soils of an urban area. Naturally occurring gamma-emitting radionuclides and man-made ¹³⁷Cs were found in the soil profiles collected from four parks in the central Belgrade city area and the soil layer was examined every 10 cm and to a depth of 50 cm. Radioisotope activity concentrations (Bq kg^?1) in the samples of urban soil using the gamma-ray spectrometry method were in the range of 14–46 for ²³⁸U, 33–50 for ²²⁶Ra, 29–63 for ²¹⁰Pb, 1.2–3.4 for ²³⁵U, 28–50 for ²³²Th, 424–576 for ⁴⁰K and 0.7–35.8 for ¹³⁷Cs. Some of the basic physicochemical soil properties (pH, organic matter content, calcium-carbonate content, particle size distribution) were determined to investigate the impact on the vertical distribution of radionuclides. The results of this investigation showed that variations of activity concentration ratios of radionuclides that belong to the same (²³⁸U/²²⁶Ra) or different radioactive series (²³²Th/²²⁶Ra; ²³⁵U/²³⁸U), including ²¹⁰Pb/¹³⁷Cs ratios could well be explained by the properties of the soil. Alkaline pH reaction, the accumulation of organic matter in the uppermost and of carbonates in the deepest layers of urban soil had an effect on ²³⁸U/²²⁶Ra, and ²¹⁰Pb/¹³⁷Cs activity concentration ratio values, while ²³²Th/²²⁶Ra and partially ²³⁵U/²³⁸U ratios were associated with the particle sizes vertical distribution. A study of radionuclides in the samples of leaves of two deciduous tree species common for these parks was also conducted and ²¹⁰Pb and ⁴⁰K were found concentrated in leaves rather than other investigated radionuclides.     

The influence of fly and bottom ash deposition on the quality of Kastela Bay sediments

The objective of this study was chemical and radiological characterization of Kastela Bay sediments exposed to numerous anthropogenic sources like deposition of fly and bottom ash enriched in radionuclides and heavy metals, chemical plant, cement plant, iron plant, shipyard, electroplating facility, untreated industrial and domestic waste waters as well as heavy traffic. Totally, 33 samples of the mixture of fly and bottom ash, 12 sediment cores ranging from 0 to 40 cm and nine surface sediment samples were analyzed. Enrichment in heavy metals in the mixture of fly and bottom ash was ranging from 1.5 to 36 times compared to flysch soil while ²²⁶Ra and ²³⁸U were up to 50 times enriched compared to average activities characteristic for surrounding soils developed on the Middle and Upper Eocene flysch. Maximum ²³⁸U activity was approximately 32 times higher and ²²⁶Ra approximately 40 times higher in the Kastela Bay sediment compared to mean value determined for Adriatic sediments. The highest enrichment in sediment cores compared to background values were found for Zn (35.6 times), Pb (16 times), Cr (9.1 times) and Ni (4 times)     

Using short-lived nuclides of the U- and Th-series to probe the kinetics of colloid migration in forested soils

The recent chemical dynamics of a podzolic forest soil section (from the Strengbach watershed, France) was investigated using U- and Th-series nuclides. Analyses of (²³⁸U), (²³⁰Th), (²²⁶Ra), (²³²Th), (²²⁸Ra) and (²²⁸Th) activities in the soil particles, the seepage waters, and the mature leaves of the beech trees growing on this soil were performed by TIMS or gamma spectrometry. The simultaneous analysis of the different soil (sl) compartments allows to demonstrate that a preferential Th leaching over Ra must be assumed to explain the (²²⁶Ra/²³⁰Th), (²²⁸Ra/²³²Th) and (²²⁸Th/²²⁸Ra) disequilibria recorded in the soil particles. The overall Ra- and Th- transfer schemes are entirely consistent with the prevailing acido-complexolysis weathering mechanism in podzols. Using a continuous open-system leaching model, the (²²⁶Ra/²³⁰Th) and (²²⁸Ra/²³²Th) disequilibria measured in the different soil layers enable dating of the contemporary processes occurring in this soil. In this way, we have determined that a preferential Th-leaching from the shallow Ah horizon, due to a strong complexation with organic colloids, began fairly recently (18 years ago at most). The continual increase in pH recorded in precipitations over the last 20 years is assumed to be the cause of this enhanced organic complexation. A lower soil horizon (50-60 cm) is also affected by preferential Th leaching, though lasting over several centuries at least, with a much smaller leaching rate. The migration of Th isotopes through this soil section might hence be used as a tracer for the organic colloids migration and the induced radioactive disequilibria demonstrate to be useful for assessing the colloidal migration kinetics in a forested soil.Ra and Th isotopic ratios also appear to be valuable tracers of some mineral-water-plant interactions occurring in soil. The (²²⁸Ra/²²⁶Ra) ratio enables discrimination of the Ra flux originating from leaf degradation from that originating from mineral weathering in shallow −10 cm seepage soil waters. It appears that, at least in some cases, the Ra-isotopic ratio measured in forest-soil seepage waters may not be representative of the Ra-isotopic ratio released from mineral weathering, indicating that the different origins of the dissolved ²²⁶Ra and ²²⁸Ra must be taken into account.     

Radionuclides in ground water of the Carson River Basin,western Nevada and eastern California,U.S.A.

Ground water is the main source of domestic and public supply in the Carson River Basin. Ground water originates as precipitation primarily in the Sierra Nevada in the western part of Carson and Eagle Valleys, and flows down gradient in the direction of the Carson River through Dayton and Churchill Valleys to a terminal sink in the Carson Desert. Because radionuclides dissolved in ground water can pose a threat to human health, the distribution and sources of several naturally occurring radionuclides that contribute to gross-alpha and gross-beta activities in the study area were investigated. Generally, alpha and beta activities and U concentration increase from the up-gradient to down-gradient hydrographic areas of the Carson River Basin, whereas²²²Rn concentration decreases. Both²²⁶Ra and²²⁸Ra concentrations are similar throughout the study area. Alpha and beta activities and U concentration commonly exceed 100 pCi/l in the Carson Desert at the distal end of the flow system. Radon-222 commonly exceeds 2,000 pCi/l in the western part of Carson and Eagle Valleys adjacent to the Sierra Nevada. Radium-226 and²²⁸Ra concentrations are <5pCi/l. Four ground water samples were analyzed for²¹⁰Po and one sample contained a high concentration of 21 pCi/l. Seven samples were analyzed for²¹⁰Pb; six contained <3pCi/l and one contained 12 pCi/l. Thorium-230 was detected at concentrations of 0.15 and 0.20 pCi/l in two of four samples.Alpha-emitting radionuclides in the ground water originated from the dissolution of U-rich granitic rocks in the Sierra Nevada by CO₂, oxygenated water. Dissolution of primary minerals, mainly titanite (sphene) in the granitic rocks, releases U to the water. Dissolved U is probably removed from the water by adsorption on Fe- and Mn-oxide coatings on fracture surfaces and fine-grained sediment, by adsorption on organic matter, and by coprecipitation with Fe and Mn oxides. These coated sediments are transported throughout the basin by fluvial processes. Thus, U is transported as dissolved and adsorbed species. A rise in the water table in the Carson Desert because of irrigation has resulted in the oxidation of U-rich organic matter and dissolution of U-bearing coatings on sediments, producing unusually high U concentration in the ground water.Alpha activity in the ground water is almost entirely from the decay of U dissolved in the water. Beta activity in ground water samples is primarily from the decay of⁴⁰K dissolved in the water and ingrowth of²³⁸U progeny in the sample before analysis. Approximately one-half of the measured beta activity may not be present in ground water in the aquifer, but instead is produced in the sample after collection and before analysis. Potassium-40 is primarily from the dissolution of K-containing minerals, probably K-feldspar and biotite. Radon-222 is primarily from the decay of²²⁶Ra in the aquifer materials. Radium in the ground water is thought to be mainly from alpha recoil associated with the decay of Th in the aquifer material. Some Ra may be from dissolution (or desorption) or Ra-rich coatings on sediments.     

U-series disequilibria in Kick’em Jenny submarine volcano lavas: A new view of time-scales of magmatism in convergent margins

We present data for U and its decay series nuclides ²³⁰Th, ²²⁶Ra, ²³¹Pa, and ²¹⁰Po for 14 lavas from Kick’em Jenny (KEJ) submarine volcano to constrain the time-scales and processes of magmatism in the Southern Lesser Antilles, the arc having the globally lowest plate convergence rate. Although these samples are thought to have been erupted in the last century, most have (²²⁶Ra)/(²¹⁰Po) within ±15% of unity. Ten out of 14 samples have significant ²²⁶Ra excesses over ²³⁰Th, with (²²⁶Ra)/(²³⁰Th) up to 2.97, while four samples are in ²²⁶Ra-²³⁰Th equilibrium within error. All KEJ samples have high (²³¹Pa)/(²³⁵U), ranging from 1.56 to 2.64 and high ²³⁸U excesses (up to 43%), providing a global end-member of high ²³⁸U and high ²³¹Pa excesses. Negative correlations between Sr, sensitive to plagioclase fractionation, and Ho/Sm, sensitive to amphibole fractionation, or K/Rb, sensitive to open system behavior, indicate that differentiation at KEJ lavas was dominated by amphibole fractionation and open-system assimilation. While (²³¹Pa)/(²³⁵U) does not correlate with differentiation indices such as Ho/Sm, (²³⁰Th)/(²³⁸U) shows a slight negative correlation, likely due to assimilation of materials with slightly higher (²³⁰Th)/(²³⁸U). Samples with ²²⁶Ra excess have higher Sr/Th and Ba/Th than those in ²²⁶Ra-²³⁰Th equilibrium, forming rough positive correlations of (²²⁶Ra)/(²³⁰Th) with Sr/Th and Ba/Th similar to those observed in many arc settings. We interpret these correlations to reflect a time-dependent magma differentiation process at shallow crustal levels and not the process of recent fluid addition at the slab-wedge interface.The high ²³¹Pa excesses require an in-growth melting process operating at low melting rates and small residual porosity; such a model will also produce significant ²³⁸U-²³⁰Th and ²²⁶Ra-²³⁰Th disequilibrium in erupted lavas, meaning that signatures of recent fluid addition from the slab are unlikely to be preserved in KEJ lavas. We instead propose that most of the ²³⁸U-²³⁰Th, ²²⁶Ra-²³⁰Th, and ²³⁵U-²³¹Pa disequilibria in erupted KEJ lavas reflect the in-growth melting process in the mantle wedge (reflecting variations in U/Th, daughter-parent ratios, fO₂, and thermal structure), followed by modification by magma differentiation at crustal depths. Such a conclusion reconciles the different temporal implications from different U-series parent-daughter pairs and relaxes the time constraint on mass transfer from slab to eruption occurring in less than a few thousand years imposed by models whereby ²²⁶Ra excess is derived from the slab.     

Naturally occurring radionuclides and their geochemical interactions at a geothermal site in the North German Basin

The activities of the most common, naturally occurring radionuclides ²³⁸U, ²²⁶Ra, ²¹⁰Pb, ²²⁸Ra, ²²⁸Th, and ⁴⁰K were measured by gamma-ray spectrometry in samples from reservoir rocks, geothermal fluids, and mineral precipitates at the geothermal research site Groß Schönebeck (North German Basin). Results demonstrated that the specific activity of the reservoir rock is within the range of the mean concentration in the upper earth crust of <800 Bq/kg for ⁴⁰K and <60 Bq/kg for radionuclides of the ²³⁸U and ²³²Th series, respectively. The geothermal fluid showed elevated activity concentrations (up to 100 Bq/l) for ²²⁶Ra, ²¹⁰Pb, and ²²⁸Ra, as compared to concentrations found in natural groundwater. Their concentration in filter residues even increased up to 100 Bq/g. These residues contain predominantly two different mineral phases: a Sr-rich barite (Sr, BaSO₄) and laurionite (PbOHCl), which both precipitate upon cooling from the geothermal fluid. Thereby they presumably enrich the radionuclides of Ra (by substitution of Ba) and Pb. Analysis of these precipitates further showed an increased ²²⁶Ra/²²⁸Ra ratio from around 1–1.7 during the initial months of fluid production indicating a change in fluid composition over time which can be explained by different contributions of stimulated reservoir rock areas to the overall produced fluid.     

Precise two chronometer dating of Pleistocene travertine: The Th/U and Raex/Ra(0) approach

In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the ²³⁰Th/²³⁴U ingrowth system. Additional analyses of further radionuclides within the ²³⁸U decay chain, i.e. ²²⁶Ra and ²¹⁰Pb, showed that the Th/U chronometer started with insignificant inherited ²³⁰Th over the entire formation period of the travertine setting (i.e. ²³⁰Th(0)=0). A contribution from detrital impurities with ²³⁰Th/²³⁴U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise ²³⁰Th/²³⁴U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the ²³⁸U-decay series, based on decay of excess ²²⁶Ra normalized to the initial, i.e.²²⁶Ra_ex/²²⁶Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the ²³⁰Th/²³⁴U chronometer. Analytical solutions of the coupled ²³⁴U/²³⁰Th/²²⁶Ra radionuclide system predicted that the ²²⁶Ra_ex/²²⁶Ra(0) chronometer is independent of the actual ²³⁰Th activity build up from decay of ²³⁴U, if the systems starts with zero inherited ²³⁰Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated ²²⁶Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of ²²⁶Ra. This is concluded because (i) the ²²⁶Ra_ex/²²⁶Ra(0) ages agreed well with those derived from ²³⁰Th/²³⁴U, (ii) all data plot within uncertainty on the ²²⁶Ra_ex/²²⁶Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such boundary conditions hold, ²²⁶Ra_ex/²²⁶Ra(0) should be applicable to materials which are suitable for ²³⁰Th/²³⁴U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support ²³⁰Th/²³⁴U for samples that have been formed a few thousand years ago.     

Organic complexation and translocation of ferric iron in podzols of the Negro River watershed. Separation of secondary Fe species from Al species

The development of podzols in lateritic landscapes of the upper Amazon basin contributes to the exportation of organic carbon and associated metals in the black waters of the Negro River watershed. We have investigated the distribution of Fe^III in the clay-size fraction of eight organic-rich horizons of waterlogged plateau podzols, to unravel the weathering conditions and mechanisms that control its transfer to the rivers. The speciation and amount of Fe^III stored in residual mineral phases of laterites, or bound to organic compounds of weakly and well-expressed podzols, were determined by electron paramagnetic resonance spectroscopy combined with chemical analyses.Reducing conditions restrict the production of organo-Fe complexes in the subsoil B-horizons of waterlogged podzols and most of the Fe²⁺ released from the dissolution of Fe-oxides is exported to the rivers via the perched groundwater. However, significant amounts of diluted Fe^III bound to organic ligands (Fe^III_OM) and nano Fe-oxides are produced at the margin of the depression in the topsoil A horizons of weakly expressed podzols due to shorter periods of anoxia. The downward translocation of organically bound metals from topsoil A to subsoil B-horizons of podzols occurs in shorter distances for Fe than it does for Al. This separation of secondary Fe species from Al species is attributed to the physical fractionation of their organic carriers in texture contrasted B-horizons of podzols, as well as to the effect of pH on metal speciation in soil solutions and metal binding onto soil organic ligands (mostly for Al). This leads us to consider the topsoil A horizons of weakly expressed podzols, as well as the subsoil Bh horizon of better-expressed ones, as the main sources for the transfer of Fe^III_OM to the rivers. The concentration of Fe^III_OM rises from soil sources to river colloids, suggesting drastic biogeochemical changes in more oxygenated black waters of the Negro River watershed. The contribution of soil organic matter to the transfer of Fe to rivers is likely at the origin of the peculiar Fe isotope pattern recently recognized in podzolic environments.     

High levels of uranium and radium in groundwaters at Canada's Underground Research Laboratory, Lac du Bonnet, Manitoba, Canada

High concentrations of U and²²⁶Ra, and elevated²³⁴U/²³⁸U activity ratios have been measured in groundwater samples collected from water supply wells and exploratory boreholes in the area surrounding the Underground Research Laboratory (URL) of Atomic Energy of Canada Limited, in southeastern Manitoba. All groundwaters come from the Lac du Bonnet granite batholith or sediments overlying the batholith.Uranium concentrations attain almost 1 mg/l in some shallow, low-salinity groundwaters, whereas²²⁶Ra tends to be high (up to 38 Bq/l) in deeper, saline waters. The U concentrations are some of the highest observed in global groundwaters, yet no significant ore body or mineralization is known in the area. Analyses of unaltered rock samples of the Lac du Bonnet granite show slight U enrichment over average Canadian Shield granites (6.5 μg/g vs 4 μg/g), and altered wall rock in fracture zones is enriched in U by up to an order of magnitude compared to adjacent bedrock. Low²³⁴U/²³⁸U activity ratios in this altered rock indicate active and recent leaching of U by groundwater.The key control on U concentration appears to be redox potential. Concentrations of U in rock, residence time and groundwater composition are of lesser importance. Geochemical modelling of the shallower, oxidized waters indicates that U speciation consists mainly of anionic carbonate complexes of the uranyl ion. This is supported by the remarkable efficiency of an anionic filter developed to remove high levels of U from drinking water in the area.In more reducing groundwaters, U concentrations are similar to those determined in recent experimental work on uraninite solubility in the pH range 7–8.5. Colloidal U is <10% of total U and organic complexation is unlikely to be significant because of low dissolved organic concentrations. The results emphasize the significance of redox potential in controlling U mobility in both oxidizing and reducing environments and indicate the usefulness of U concentration in estimating groundwater Eh.     

The mobility of radium-226 and trace metals in pre-oxidized subaqueous uranium mill tailings

The exchange of ²²⁶Ra and trace metals across the tailings-water interface and the mechanisms governing their mobility were assessed via sub-centimetre resolution profiling of dissolved constituents across the tailings–water interface in Cell 14 of the Quirke Waste Management Area at Rio Algom's Quirke Mine, near Elliot Lake, Ontario, Canada. Shallow zones (<1.5 m water depth) are characterized by sparse filamentous vegetation, well-mixed water columns and fully oxygenated bottom waters. Profiles of dissolved O₂, Fe and Mn indicate that the tailings deposits in these areas are sub-oxic below tailings depths of ∼3 cm. These zones exhibit minor remobilization of Ra in the upper 5 cm of the tailings deposit; ²²⁶Ra fluxes at these sites are relatively small, and contribute negligibly to the water column activity of ²²⁶Ra. The shallow areas also exhibit minor remobilization of Ni, As, Mo and U. The release of these elements to the water cover is, however, limited by scavenging mechanisms in the interfacial oxic horizons. The presence of thick vegetation (Chara sp.) in the deeper areas (>2 m water depth) fosters stagnant bottom waters and permits the development of anoxia above the benthic boundary. These anoxic tailings are characterized by substantial remobilization of ²²⁶Ra, resulting in a relatively large flux of ²²⁶Ra from the tailings to the water column. The strong correlation between the porewater profiles of ²²⁶Ra and Ba (r²=0.99), as well as solubility calculations, indicate that the mobility of Ra is controlled by saturation with respect to a poorly ordered and/or impure barite phase [(Ra,Ba)SO₄]. In the anoxic zones, severe undersaturation with respect to barite is sustained by microbial SO₄ reduction. Flux calculations suggest that the increase in ²²⁶Ra activity in the water cover since 1995 (from <0.5 to 2.5 Bq l⁻¹) can be attributed to an increase in the spatial distribution of anoxic bottom waters caused by increased density of benthic flora. The anoxic, vegetated areas also exhibit minor remobilization with respect to dissolved As, Ni and Zn. The removal of trace metals in the anoxic bottom waters appears to be limited by the availability of free sulphide. Collectively, the data demonstrate that while the water cover over the U mill tailings minimizes sulphide oxidation and metal mobility, anoxic conditions which have developed in deeper areas have led to increased mobility of ²²⁶Ra.     

Uranium geochemistry and dating of Pacific island apatite

Uranium-series disequilibrium dating of island phosphate deposits is evaluated in terms of known associated coral ages, uranium geochemistry, and stratigraphic sequences as well as the concordance between the geochronometers ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U and ²²⁶Ra/²³⁸U. U(VI) is the predominant oxidation state of uranium in island phosphorites and by analogy to the youngest surficial deposits, most of the uranium initially bound is in the form of U(VI) sorbed by surfaces from seawater. Insular deposits contain more organic matter than even very young ocean floor samples and this leads to a greater probability of reduction of available recoil uranium than occurs in marine deposits. As a consequence, R(VI) ? R(T) ? R(VI), where R represents the ²³⁴U/²³⁸U activity ratio. This situation is completely opposite from that observed for marine-origin phosphorites. We determined that a fraction of U(VI) in ancient insular phosphorites is very labile and lost to alkaline carbonate solutions with a uranium activity ratio even more depleted in ²³⁴U than the bulk R(VI).Most younger samples appear to have no more discordance between ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U and ²²⁶Ra/²³⁸U than marine phosphorites of similar apparent age. Young, surficial atoll-rim apatite cements and unconsolidated phosphorites date in the range of 1500–8000 years B.P., consistent with the concept of partial submergence of low-lying coral islands prior to 2000 years B.P. Sub-surficial samples in the same environment date older at about 20,000 years B.P. Violation of the closed system assumption occurs in at least 6 out of 13 ancient (> 800,000 year) samples. Uranium-series disequilibrium dating of insular apatite shows some promise as a recorder of climatic/sea level events, but the assumptions necessary for valid ages must be carefully evaluated for each occurrence.     

Implications from concentrations and isotopic data for Pb partitioning processes in soils

Lead concentrations and stable isotopic measurements were examined in the different chemical fractions of Czech forest soils to investigate the mechanisms of Pb partitioning. A method of selective sequential dissolution (SSD) was employed that distinguished between five different fractions: exchangeable, surface bound, organic matter, Fe-oxides, and silicates (non-labile).From an analysis of the concentrations and isotopic compositions associated with the different fractions, it is apparent that Pb in the deep Czech mineral soils is of predominantly natural origin and is primarily associated with silicates (69-81%) and Fe-oxides (11-19%). Natural Pb associated with surface bound and organic matter fractions in mineral soils accounts for only 7 to 15%. Anthropogenic Pb in the Czech soils is concentrated primarily in the organic horizons and is strongly associated with the surface-bound and organic matter fractions in which the proportion of total Pb is 33 to 50% and 23 to 47%, respectively.At high and low levels of contamination, Pb isotopic signatures within the labile fractions of the same soil samples are generally homogenous, although a degree of heterogeneity among these fractions is noted in samples of intermediate degrees of contamination. Such heterogeneity probably reflects different levels of natural and anthropogenic Pb mixing.Determination of the mass-normalized affinity of Pb to the primary components using solid-solution distribution coefficients suggests that in Czech forest soils, the order of affinity may be summarized as Fe-oxides > organic matter > silicates. A similar treatment of the data reported for semiarid Mediterranean soils indicates the prevailing order to be Fe-oxides > carbonates > organic matter > silicates. The general similarity of the behaviour of Pb with respect to the different soil components in both temperate and semiarid soils suggests that these orders of affinity may have wider significance for a variety of other soil types.     

234U238U and 230Th234U activity ratios in mineral phases of a lateritic weathered zone

A selective phase extraction procedure was developed for the identification of the significant phases of a typical deep soil profile sampled in the vicinity of the Ranger No. 1 uranium ore body, Alligator Rivers region, N.T., Australia. The significant phases were identified as amorphous iron oxide, crystalline iron oxide and a clay/quartz resistate. The distribution of ²³⁸U, ²³⁴U, ²³⁰Th and ²²⁶Ra between the phases was measured. The results indicated that the amorphous iron oxide phase is in adsorption/desorption equilibrium with the ground water. The crystalline iron oxide phase contains a chemical control, the kinetics of which are commensurate with or less than the half-life of ²³⁰Th (7.52 × 10⁴ y). The clay/quartz resistate is enriched in ²³⁸U descendants in such a way that they are not readily accessible to the ground water.     

Factors controlling the groundwater transport of U, Th, Ra, and Rn

A model for the groundwater transport of naturally occurring U, Th, Ra, and Rn nuclides in the²³⁸U and²³²Th decay series is discussed. The model developed here takes into account transport by advection and the physico-chemical processes of weathering, decay, α-recoil, and sorption at the water-rock interface. It describes the evolution along a flowline of the activities of the²³⁸U and²³²Th decay series nuclides in groundwater. Simple sets of relationships governing the activities of the various species in solution are derived, and these can be used both to calculate effective retardation factors and to interpret groundwater data. For the activities of each nuclide, a general solution to the transport equation has been obtained, which shows that the activities reach a constant value after a distance ϰ_i, characteristic of each nuclide. Where ϰ_i is much longer than the aquifer length, (for²³⁸U,²³⁴U, and²³²Th), the activities grow linearly with distance. Where gKⁱ is short compared to the aquifer length, (for²³⁴Th,²³⁰Th,²²⁸Th,²²⁸Ra, and²²⁴Ra), the activities rapidly reach a constant or quasi-constant activity value. For²²⁶Ra and²²²Rn, the limiting activity is reached after 1 km. High δ²³⁴U values (proportional to the ratio^ɛ234Th/^W238U) can be obtained through high recoil fraction and/or low weathering rates. The activity ratios²³⁰Th/²³²Th,²²⁸Ra/²²⁶Ra and²²⁴Ra/²²⁶Ra have been considered in the cases where either weathering or recoil is the predominant process of input from the mineral grain. Typical values for weathering rates and recoil fractions for a sandy aquifer indicate that recoil is the dominant process for Th isotopic ratios in the water. Measured data for Ra isotope activity ratios indicate that recoil is the process generally controlling the Ra isotopic composition in water. Higher isotopic ratios can be explained by different desorption kinetics of Ra. However, the model does not provide an explanation for²²⁸Ra/²²⁶Ra and²²⁴Ra/²²⁶Ra activity ratios less than unity. From the model, the highest²²²Rn emanation equals 2ɛ. This is in agreement with the hypothesis that²²²Rn activity can be used as a first approximation for input by recoil (Krishnaswamiet al 1982). However, high²²²Rn emanation cannot be explained by production from the surface layer as formulated in the model. Other possibilities involve models including surface precipitation, where the surface layer is not in steady-state.     

Study on the natural radioactivity level of stone coal-bearing strata in East China

Based on γ-radiation dose rate and radon concentration measurements and ²³⁸U, ²³²Th, ²²⁶Ra, and ⁴⁰K radionuclide testing, this study identifies the radioactive anomalies of stone coal-bearing strata in East China and evaluates the natural radioactivity levels in the air, solid, water and plant media in the typical area of the regional stone coal-bearing layers. The stone coal-bearing strata in East China occur in the lower Cambrian system along the margin of the Yangtze block; additionally, the radioactive anomaly area is sporadically distributed in the stone coal-bearing layers. The background values of ²³⁸U, ²³²Th, ²²⁶Ra, and ⁴⁰K are higher in the stone coal-bearing areas, and the spatial distribution of these natural radionuclides shows significant variability. ²³⁸U and ²²⁶Ra clearly accumulate in the coal, coal gangue and soil and are the main sources of the environmental radiation in coal mines. The γ-radiation shows a higher background value in the stone coal-bearing area, and this radioactive pollution cannot be ignored. Typically, the effective dose of γ-radiation exceeds the limit value of 5 mSv/a, and the total α and total β concentrations of the groundwater are 10–30 times the limit value at some points. The residents near the mining area are subjected to a higher radiation dose, and the groundwater, building materials, and plants have been contaminated by the radioactive pollution sporadically through time. It is necessary to strengthen the monitoring work of radioactive environments and to take appropriate control measures.     

Radium isotopic measurements in the search for uranium in palaeodrainage channels

Analyses have been made of many groundwater samples, some of which were collected from the vicinity of uranium deposits and others from unmineralized areas, for dissolved uranium and for the four naturally occurring isotopes of radium: ²²⁶Ra (²³⁸U decay series, y), ²²⁸Ra and ²²⁴Ra (²³²Th decay series, y and 3.8 d) and ²²³Ra (²³⁵U decay series, d). The radium isotopes ²²⁶Ra, ²²⁴Ra and ²²³Ra, are measured by alpha-spectrometry after extraction from a water sample soon after collection and ²²⁸Ra at a later time by determining the amount of ingrown ²¹²Po.     

Plume-lithosphere interaction beneath Mt. Cameroon volcano, West Africa: Constraints from U-Th-Ra and Sr-Nd-Pb isotope systematics

Precise measurements of ²³⁸U-²³⁰Th-²²⁶Ra disequilibria in lavas erupted within the last 100 yr on Mt. Cameroon are presented, together with major and trace elements, and Sr-Nd-Pb isotope ratios, to unravel the source and processes of basaltic magmatism at intraplate tectonic settings. All samples possess ²³⁸U-²³⁰Th-²²⁶Ra disequilibria with ²³⁰Th (18-24%) and ²²⁶Ra (9-21%) excesses, and there exists a positive correlation in a (²²⁶Ra/²³⁰Th)-(²³⁰Th/²³⁸U) diagram. The extent of ²³⁸U-²³⁰Th-²²⁶Ra disequilibria is markedly different in lavas of individual eruption ages, although the (²³⁰Th/²³²Th) ratio is constant irrespective of eruption age. When U-series results are combined with Pb isotope ratios, negative correlations are observed in the (²³⁰Th/²³⁸U)-(²⁰⁶Pb/²⁰⁴Pb) and (²²⁶Ra/²³⁰Th)-(²⁰⁶Pb/²⁰⁴Pb) diagrams. Shallow magma chamber processes like magma mixing, fractional crystallization and wall rock assimilation do not account for the correlations. Crustal contamination is not the cause of the observed isotopic variations because continental crust is considered to have extremely different Pb isotope compositions and U/Th ratios. Melting of a chemically heterogeneous mantle might explain the Mt. Cameroon data, but dynamic melting under conditions of high D_U and D_U/D_Th, long magma ascent time, or disequilibrium mineral/melt partitioning, is required. The most plausible scenario to produce the geochemical characteristics of Mt. Cameroon samples is the interaction of melt derived from the asthenospheric mantle with overlying sub-continental lithospheric mantle which has elevated U/Pb (>0.75) and Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb > 20.47) due to late Mesozoic metasomatism.     

An improved method for Ra isotope (226Ra, 228Ra, 224Ra) measurements by gamma spectrometry in natural waters: application to CO2-rich thermal waters from the French Massif Central

An improved method was developed to measure ²²⁶Ra, ²²⁸Ra and ²²⁴Ra in freshwaters by gamma spectrometry. Radium was selectively extracted from acidified samples using specific filters (3M EMPORE™ Radium Rad disks). The latter was subsequently analysed by gamma spectrometry. Simultaneous and direct determination of the activities of the three isotopes was performed by comparison of gamma rays of the Radium Rad disks with those of a calibrated standard disk. This efficient and reliable method allowed a reduction of sample processing to a few hours.This technique was applied to analyse the Ra isotope compositions of several CO₂-rich hydrothermal springs of the western border of the Limagne graben (French Massif Central). The studied springs emerge from a succession of granitic outcrops lined up along a major fault. Their chemical compositions evolve from calcic and magnesian chloro-bicarbonated to sodic bicarbonated. All the springs display high Ra activities, probably linked to high CO₂ content and/or high cation content of these waters, with various Ra isotope ratios. ²²⁶Ra activity ranges from 588 to 2287 mBq/l and ²²⁸Ra activity from 260 to 1590 mBq/l, whereas ²²⁴Ra displays an activity between 245 and 1808 mBq/l. Four of the six analysed springs have (²²⁸Ra/²²⁶Ra) activity ratios lower than 0.7, thus, significantly lower than the ratio expected from an interaction with a calc-alkaline granitoid (typically having (²³²Th/²³⁸U) activity ratio between 1 and 2). Low (²²⁸Ra/²²⁶Ra) ratio (0.27) of the northern water (Montpensier) suggests the existence in this area of a zone of U concentration, possibly resulting from U mobilization and accumulation induced by previous hydrothermal events. The (²²⁴Ra/²²⁸Ra) ratios display smaller variations. They suggest short transit times from the zone of Ra leaching to the surface (a few days) or a very shallow addition of ²²⁴Ra (e.g., from a localised zone where ²²⁸Th could be preferentially adsorbed on the mineral surfaces). In some cases, these ratios might be used to infer differences in transit times of waters between neighboring springs.     

Preferential solution of234U from recoil tracks and234U/238U radioactive disequilibrium in natural waters

A mathematical model to calculate the²³⁴U/²³⁸U activity ratio (AR) in an aqueous phase in contact with rock/soil is presented. The model relies on the supply of²³⁸U by dissolution and that of²³⁴U by dissolution and preferential release from radiation damaged regions (recoil tracks). The model predicts that values of²³⁴U/²³⁸U AR>1 in the aqueous phase can be obtained only from weathering “virgin” surfaces. Thus, to account for the observed steady-state supply of²³⁴U excess to the oceans by the preferential leaching model, ‘virgin’ rock/soil surfaces would have to be continually exposed and weathered. The²³⁸U concentration and²³⁴U/²³⁸U AR in continental waters allow us to estimate the exposure rates of “virgin” rock/soil surfaces.