Uranium-series disequilibrium studies in phosphorite nodules from the west coast of South America

Sedimentary phosphorites occurring on the sea floor off Peru and Chile have been analyzed for U and Th isotopes, to establish their ages and hence obtain clues for their mode of formation. Fission-track distribution studies indicate that the U is primarily associated with the apatite fraction. Uranium-series disequilibrium methods, therefore, should be applicable, if the U incorporation is syngenetic with the apatite.The fractionation of U isotopes between oxidation states in the relatively young phosphorites from South America is low compared to that in older deposits. This supports the contention of Kolodny and Kaplan (1970) that the major mechanism of ${}^{234}\text{U}{}^{238}\text{U}$ fractionation is displacement of ²³⁴U atoms into sites where they are more ‘oxidizable’ than the ²³⁸U parent. Age estimates based on ²³⁴U(IV) and ²³⁰Th contents are internally consistent and range from late Pleistocene to Recent.The results indicate that marine phosphorites are currently forming in this area of intense oceanic upwelling. The age pattern during the last 150,000 yr suggests a correlation with eustatic high sea level stands and implies that conditions were more favorable for apatite genesis in this area during interglacials rather than during glacial times.     

Uranium in Phosphorites

The uranium concentration in phosphorites on continents and modern seafloor varies from 0.nto n· 10²ppm (average 75 ppm). The average uranium concentration is 4–48 ppm in Precambrian and Cambrian deposits, 20–90 ppm in Paleozoic and Jurassic deposits, 40–130 ppm in Late Cretaceous–Paleogene deposits, 30–130 ppm in Neogene deposits, and 30–110 ppm in Quaternary (including Holocene) deposits. On the whole, the variation range is almost similar for phosphorites of different ages. The U/P₂O₅ratio in phosphorites ranges from less than unity to 24 · 10^–4(average 3.2 · 10^–4). Major phosphorite deposits of the world with ore reserves of approximately 250 Gt (or 58 Gt P₂O₅) contain up to 19 Mt of uranium. Uranium is present in phosphorites in the tetra- and hexavalent, i.e., U(IV) and U(VI) forms, and their ratio is highly variable. At the early diagenetic stage of the formation of marine phosphorites in a reductive environment, U(VI) diffuses from the near-bottom water into sediments. It is consequently reduced and precipitated as submicroscopic segregations of uranium minerals (mainly uraninite) that are probably absorbed by phosphatic material. During the subsequent reaction between phosphorites and aerated water and the weathering in a subaerial environment, uranium is partly oxidized and lost. The uranium depletion also occurs during catagenesis owing to a more complete crystallization of calcium phosphate and replacement of nonphosphatic components.     

New information on radionuclides concentration in phosphorites originating from Tunisia and Algeria

In this study we investigate the radiological hazard of naturally occurring radioactive material in Tunisian and Algerian phosphorite deposits. Eight samples of phosphorite were collected from the phosphorite mines. The Tunisian and Algerian phosphorites occur in the Late Paleocene and Lower Eocene (Ypresian-Lutetian) in age (Béji Sassi 1984 and Zaïer 1999). Activity concentrations in all the samples were measured by alpha spectrometry and gamma spectrometry. Alpha spectrometry analyses show that the specific activity values of ²³⁸U, ²³⁴U and ²³⁵U in the samples of Tunisian phosphorite were 327?±?7 (321–327), 326?±?6 (325–331) and 14.50?±?0.72 (13.90–15.57) Bq kg^?1, respectively. Specific activity measured by gamma spectrometry in the samples of the Tunisian and Algerian phosphorite shows a small difference. Specific activity levels of ⁴⁰K, ²²⁶Ra, ²³²Th, ²³⁵U and ²³⁸U in the phosphorite samples from Tunisia were, respectively, 71.10?±?3.80, 391.54?±?9.39, 60.38?±?3.74, 12.72?±?0.54 and 527.42?±?49.57 Bq kg^?1 and Algeria were 15.72?±?1.73, 989.65?±?12.52, 12.08?±?1.20, 47.50?±?1.52 and 1,148.78?±?7.30 Bq kg^?1, respectively. The measured value of specific activity of ²³²Th and ⁴⁰K in the Tunisian phosphorite samples is relatively higher than that found in the samples of Algerian phosphorite. The measured activity of uranium (²³⁸U) in the Tunisian phosphorite (527?±?49) Bq kg^?1 is lower than in Algerian phosphorite. The measured activity of ²³⁸U in the Tunisian phosphorite samples was (527–1,315?±?65) ²³⁸U Bq kg^?1 which is higher than its maximum background value of 110 Bq kg^?1 in soils of the various countries of the world (Tufail et al. Radiat Meas 41:443–451, 2006). Different geological origins of phosphorites deposits are the main reason for the large spread in worldwide specific activities. The obtained results of uranium concentrations in phosphorites of different types (Algerian and Tunisian) demonstrate that the uranium concentrations are mainly governed by the phosphatic material. The present study reveals that phosphorite deposits contain natural radioactivity higher than background level.     

Spatial and temporal variability of 234U/238U activity ratios in the Shu River,Central Asia

This study presents the temporal and spatial variability of ²³⁴U/²³⁸U activity ratios in the Shu River and provides interpretation to explain the downstream changes of uranium and the ²³⁴U/²³⁸U activity ratios in the study area. The positive linear correlation (R ² = 0.98, p < 0.001) between uranium concentration and specific electrical conductance is consistent with rock weathering and leaching as the major contributor of dissolved uranium in the studied area of the river. The ²³⁴U/²³⁸U activity ratio ranged between ~1.6 in the upper reaches of the river to ~1.15 furthest downstream. Activity ratios at specific sampling points do not show significant seasonal variability.     

U-Pb systems and U isotopic composition of the sandstone-hosted paleovalley Dybryn uranium deposit, Vitim uranium district, Russia

The isotopic (U-Pb, ²³⁸U-²³⁵U, ²³⁴U-²³⁸U) and chemical study of whole-rock samples and finegrained fractions of rocks in a vertical section of the terrigenous sequence at the Dybryn uranium deposit in the Khiagda ore field shows that a wide U-Pb isotopic age range (26.9-6.5 Ma) is caused by oxidation and disturbance of the U-Pb isotopic system in combination with protracted uranium ore deposition. The oxidation of rocks resulted in the loss of uranium relative to lead and eventually to an overestimated ²⁰⁶Pb/²³⁸U age at sites with a low U content. The ²³⁸U/²³⁵U ratios in the studied samples are within the range of 137.74–137.88. Samples with a high uranium content are characterized by a decreasing ²³⁸U/²³⁵U ratio with a decrease in ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²³⁸U ages. A nonequilibrium ²³⁴U/²³⁸U ratio in most studied samples furnishes evidence for young (<1.5 Ma) transformation of the Miocene uranium ore, which is responsible for uranium migration and its redeposition.     

Variability in the uranium isotopic composition of the oceans over glacial-interglacial timescales

Uranium-series mass spectrometric analyses of corals from the uplifted last glacial terraces at Huon Peninsula, Papua New Guinea, that grew from 50,000 years ago to 30,000 years ago show systematically low values of ²³⁴U/²³⁸U, at the time of coral growth, compared with modern corals. When combined with coral data from other studies a systematic trend emerges indicating shifts in the ²³⁴U/²³⁸U ratio at times of major glacial-interglacial transitions that involve large variations in sea-levels. From last glacial to Holocene, the rate of change in δ²³⁴U is approximately 1‰ per thousand years. The variations in the U budget of the oceans appear to be due to accumulation of excess ²³⁴U in near shore areas in anoxic and suboxic sediments, in salt marshes and mangroves, in estuaries, and in continental margins during periods of warm climate and high sea-levels. These near-shore areas are exposed during periods of low sea level resulting in rapid oxidation of U into highly soluble phases. The subsequent release of ²³⁴U-enriched uranium into the oceans occurs over a sustained period, in step with rising sea-levels.     

State of disequilibrium between 238U, 234U, 226Ra and 222Rn in groundwater from bedrock

Approximately one thousand drilled wells were investigated for their natural radioactivity. The determinations of ²³⁸U, ²³⁴U, ²²⁶Ra and ²²²Rn from 310 samples showed a high state of radioactive disequilibrium between the members of the uranium series present in water. The ${}^{238}\text{U}{}^{226}\text{Ra}$ activity ratio usually fell in the range 1–20 and the ${}^{238}\text{U}{}^{222}\text{Rn}$ activity ratio in the range 1–20 × 10^?4, the highest activity ratios being from samples with an elevated uranium content. The ${}^{234}\text{U}{}^{238}\text{U}$ activity ratio varied between 0.76 and 4.67, the most frequent values showing a 60% excess of ²³⁴U in the samples. Most of the ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios near unity were found in samples with a high uranium content. Several drilled wells with anomalously high uranium contents were found in southern Finland. The average ²²⁶Ra and ²²²Rn contents of these wells were not exceptionally high, which suggests high mobility of uranium in groundwater from the areas involved.     

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.     

Use of 234U/238U disequilibrium in measuring chemical weathering rate of rocks

The rate of chemical weathering of rocks has been determined by using uranium as a natural isotopic tracer. The concentration of uranium and ²³⁴U/²³⁸U ratio in natural waters, rocks, and soils of the Preto river basin (Bahia State, Brazil) was measured by alpha-ray spectroscopy.The activity ratio U234/U238 measured in the various samples indicates the uranium fraction which is dissolved from rocks during the weathering process. The results obtained show that 1 m of rock needs 25,000 yr to be weathered in this region under present climatic conditions.     

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.     

The transport of U- and Th-series nuclides in sandy confined aquifers

Abundances of ²³⁸U, ²³⁴U, ²³²Th, ²²⁶Ra, ²²⁸Ra, ²²⁴Ra, and ²²²Rn were measured in groundwaters of the Ojo Alamo aquifer in northwest New Mexico. This is an arid area with annual precipitation of ∼22 cm. The purpose was to investigate the transport of U-Th series nuclides and their daughter products in an old, slow-moving groundwater mass as a means of understanding water-rock interactions and to compare the results with a temperate zone aquifer. It was found that ²³²Th is approximately at saturation and supports the view of Tricca et al. (2001) that Th is precipitated irreversibly upon weathering, leaving surface coatings of ²³²Th and ²³⁰Th on aquifer grains. Uranium in the aquifer waters has very high [²³⁴U/²³⁸U] ∼ 9 and low ²³⁸U concentrations. These levels can be explained by low weathering rates in the aquifer (w_238U ∼ 2 × 10⁻¹⁸ to 2 × 10⁻¹⁷s⁻¹) using a continuous flow, water-rock interaction model. The Ra isotopes are roughly in secular equilibrium despite their very different mean lifetimes. The ²²²Rn and ²²⁸Ra isotopes in the aquifer correspond to ∼10% of the net production rate of the bulk rock. This is interpreted to reflect an earlier formed irreversible surface coating of Th that provides Ra and Rn to the aquifer waters. The surface waters that appear to be feeding the aquifer have low [²³⁴U/²³⁸U] and high ²³⁸U concentrations. The flow model shows that it is not possible to obtain the high [²³⁴U/²³⁸U] and low [²³⁸U] values in the aquifer from a source like the present vadose zone input. It follows that the old aquifer waters studied cannot be fed by the present vadose zone input unless they are greatly diluted with waters with very low U concentrations. If the present sampling of vadose zone sources is representative of the present input, then this requires that there was a major change in water input with much larger rainfall some several thousand years ago. This may represent a climatic change in the Southwest.     

Uranium in supergene phosphorites

The distribution of uranium was studied in supergene phosphorites from the zones of the weathering of sedimentary and endogenous rocks, as well as in nonmarine coprolitic phosphorites and, to a lesser extent, phosphorites from ocean islands. These phosphorites show a diversity of the composition of their carbonate-apatite and structural characteristics. The uranium content ranges mostly from 5 to 100 ppm, with minimum and maximum values of 0.5 and 790 ppm. There is no correlation between the uranium content of a phosphorite and the type of rock with which it is connected. Lacustrine coprolitic phosphorites show elevated uranium contents (about 200 ppm). The maximum uranium content was detected in finely laminated phosphorite encrustations. The correlation analysis of the whole data set (63 samples) showed that uranium content is not correlated with any other component of phosphorites at a confidence level of 0.95. In contrast, there is a correlation between U and P₂O₅, CaO, and F for the combined set of samples from southern Siberian deposits. The significant correlation of U with Na₂O and CO₂ is variable both for southern Siberia on the whole and for particular deposits from this region.     

Fractionation factor of 238U and 235U isotopes in the process of hydrothermal pitchblende formation: A numerical estimate

Owing to the rapid increase in available data on the natural variations of the ²³⁸U/²³⁵U ratio, new isotopic geochemical mark of redox processes are beginning to emerge. In this connection, numerical estimates of the ²³⁸U and ²³⁵U fractionation factor (α(U^IV?U^VI)) accompanying the reduction U^VI → U^IV are needed. Such an estimate has been obtained for hydrothermal pitchblende formation based on results of high-precision (±0.06‰) measurements of the ²³⁸U/²³⁵U ratio in local microsamples of coarse spherulitic pitchblende from carbonate-pitchblende veins at the Oktyabr’sky deposit (Strel’tsovsky uranium ore field, eastern Transbaikal region). For this purpose, we used the formation temperature of hydrothermal pitchblende and a maximum estimate of the fractionation factor for ²³⁸U and ²³⁵U isotopes in the solution-solid phase system under normal (25°C) conditions (Murphy et al., 2014). The most probable isotopic fractionation factor accompanying pitchblende crystallization from hydrothermal solution at T = 320?250°C falls into the interval α(U^IV?U^VI) = 1.00020?1.00023.     

U-series in Fe-U-rich fracture fillings from the oxidised cap of the “Mina Fe” uranium deposit (Spain): implications for processes in a radwaste repository

Within the framework of the ENRESA (Spain) natural analogue programme, the U-ore deposit of “Mina Fe” is being studied as a natural analogue of radioactive spent fuel behaviour. In this context, the knowledge of the role played by fracture minerals as scavengers of certain analogue elements, mainly U, and the establishment of the time scale of the rock-water interaction processes controlling the uptakes or losses of U in the system are two relevant objectives. Fracture-infill materials from the site have first been mineralogically characterised, then the upper part of the U-series determined in both bulk samples and U-rich leachates obtained by sequential leaching. Uranium-series of the bulk samples indicate that most of the fractures remained as closed systems in the last 1.6 Ma, while in other fractures water/rock interaction processes affecting the upper part of the U-series have been identified. These processes indicate recent or rapid U accumulation or losses (<10² ka), old ²³⁴U accumulation (> 10² ka) or ²³⁴U+²³⁰Th recoil gain. The apparently random distribution in depth of fractures where these processes occurred corroborates the different hydraulic behaviour of fractures, as a result of their varying degree of sealing. Uranium concentrations and ²³⁴U/²³⁸U ARs of the leachates obtained with Morgan's solution and 6N HCl indicate that minerals dissolved with these reagents (U(IV/VI) oxyhydroxides and goethite+clays, respectively) are responsible for the retention of almost all of the U in the bulk samples. Furthermore, the ²³⁴U lost by the U minerals dissolved with Morgan's solution seems to be fixed onto goethite–clay mixtures, the intersticial water being the vehicle for the isotopic transfer, which in turn is a recent or recent-past process.     

Behavior of isotope (<Superscript>18</Superscript>O/<Superscript>16</Superscript>O, <Superscript>234</Superscript>U/<Superscript>238</Superscript>U) systems during the formation of uranium deposits of the “sandstone” type

The uneven character of the distribution of ¹⁸O/¹⁶O and ²³⁴U/²³⁸U values was established in the vertical cross section of the productive sequence of the Dybryn uranium deposit (Vitim uranium-ore region, Buryatia). Both a deficiency and an excess of ²³⁴U in relation to the equilibrium ²³⁴U/²³⁸U ratio in the vertical sequence may provide evidence for the extremely low rate of the infiltration water flow. The behavior of oxygen isotope characteristics for different size fractions of terrigenous rocks provides evidence for active uranium redistribution and openness of the isotope system of this element during interaction of terrigenous–sedimentary rocks with infiltration waters.     

Uranium isotopes as a tracer of sources of dissolved solutes in the Han River,South Korea

The uranium (U) content and ²³⁴U/²³⁸U activity ratio were determined for water samples collected from Korea's Han River in spring, summer, and winter 2006 to provide data that might constrain the origin of U isotope fractionation in river water and the link between U isotope systematics in river waters and the lithological nature of the corresponding bedrock. The large difference in the major dissolved loads between the two major branches of the Han River, the North Han River (NHR) and South Han River (SHR), is reflected in the contrasting U content and ²³⁴U/²³⁸U activity ratio between the tributaries: low U content (0.08–0.75 nM; average, 0.34 nM) and small ²³⁴U/²³⁸U activity ratio (1.03–1.22; average, 1.09) in the NHR; and high U content (0.65–1.98 nM; average, 1.44 nM) and large ²³⁴U/²³⁸U activity ratio (1.05–1.45; average, 1.24) in the SHR. The large spatial differences in U content and ²³⁴U/²³⁸U activity ratio are closely related to both lithological differences between the two tributaries and groundwater input. The low U content and small ²³⁴U/²³⁸U activity ratio in the NHR arise mainly from a combination of surface and meteoric weathering of the dominant silicate rocks in this branch and congruent dissolution of already weathered (secular equilibrium) materials. In contrast, the high U content and large ²³⁴U/²³⁸U activity ratio in the SHR are ascribed to the dissolution of carbonates and black shales along with significant inputs of deep groundwater.     

Fixation of radionuclides in the 238U decay series in the vicinity of mineralized zones: 1. The Austatom Uranium Prospect,Northern Territory,Australia

The minimum age of a zone of secondary uranium mineralization, located at the Austatom Prospect in the Alligator Rivers region of Australia, is estimated to be 3.6 × 10⁵y. This is derived from a geochronological model based on retarded leaching of ²³⁴U with respect to ²³⁸U and on ratios within the ore of these members of the ²³⁸U decay series. Although kaolinite is a dominant mineral in the weathered schist-host-rocks, retarded dissolution of ²³⁴U occurs only in the presence of the clay minerals illite and montmorillonite. In their absence the reverse occurs. A model is proposed to explain the results. Ratios of ²³⁰Th to ²³⁸U indicate that the mineralization has probably remained stationary within the weathered schist for at least 1 to 2 × 10⁵y. Future use of clay minerals as buffers in radioactive waste repositories is supported by the excellent long-term retention obtained for oxidized uranium, probably due in part to isomorphic substitution into the clay crystal lattice.     

Rates of silicate dissolution in deep-sea sediment: In situ measurement using U/U of pore fluids

Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the ²³⁴U/²³⁸U activity ratios of pore water, bulk sediment, and leachates. Site 984A is one of only several sites where closely spaced pore water samples were obtained from the upper 60 meters of the core; the sedimentation rate is high (11-15 cm/ka), hence the sediments in the upper 60 meters are less than 500 ka old. The sediment is clayey silt and composed mostly of detritus derived from Iceland with a significant component of biogenic carbonate (up to 30%).The pore water ²³⁴U/²³⁸U activity ratios are higher than seawater values, in the range of 1.2 to 1.6, while the bulk sediment ²³⁴U/²³⁸U activity ratios are close to 1.0. The ²³⁴U/²³⁸U of the pore water reflects a balance between the mineral dissolution rate and the supply rate of excess ²³⁴U to the pore fluid by α-recoil injection of ²³⁴Th. The fraction of ²³⁸U decays that result in α-recoil injection of ²³⁴U to pore fluid is estimated to be 0.10 to 0.20 based on the ²³⁴U/²³⁸U of insoluble residue fractions. The calculated bulk dissolution rates, in units of g/g/yr are in the range of 4 × 10⁻⁷ to 2 × 10⁻⁶ yr⁻¹. There is significant down-hole variability in pore water ²³⁴U/²³⁸U activity ratios (and hence dissolution rates) on a scale of ca. 10 m. The inferred bulk dissolution rate constants are 100 to 10⁴ times slower than laboratory-determined rates, 100 times faster than rates inferred for older sediments based on Sr isotopes, and similar to weathering rates determined for terrestrial soils of similar age. The results of this study suggest that U isotopes can be used to measure in situ dissolution rates in fine-grained clastic materials.The rate estimates for sediments from ODP Site 984 confirm the strong dependence of reactivity on the age of the solid material: the bulk dissolution rate (R_d) of soils and deep-sea sediments can be approximately described by the expression R_d ≈ 0.1 Age⁻¹ for ages spanning 1000 to 5 × 10⁸ yr. The age of the material, which encompasses the grain size, surface area, and other chemical factors that contribute to the rate of dissolution, appears to be a much stronger determinant of dissolution rate than any single physical or chemical property of the system.     

Uranium isotopes in rivers,estuaries and adjacent coastal sediments of western India: their weathering,transport and oceanic budget

The two major river systems on the west coast of India, Narbada and Tapti, their estuaries and the coastal Arabian sea sediments have been extensively studied for their uranium concentrations and ${}^{238}\text{U}{}^{238}\text{U}$ activity ratios.The ²³⁸U concentrations in the aqueous phase of these river systems exhibit a strong positive correlation with the sum of the major cations, σ Na + K + Mg + Ca, and with the HCO₃^? ion contents. The abundance ratio of dissolved U to the sum of the major cations in these waters is similar to their ratio in typical crustal rocks. These findings lead us to conclude that ²³⁸U is brought into the aqueous phase along with major cations and bicarbonate. The strong positive correlation between ²³⁸U and total dissolved salts for selected rivers of the world yield an annual dissolved ²³⁸U flux of 0.88 × 10¹⁰g/yr to the oceans, a value very similar to its removal rate from the oceans, 1.05 × 10¹⁰g/yr, estimated based on its correlation with HCO₃^? contents of rivers.In the estuaries, both ²³⁸U and its great-grand daughter ²³⁴U behave conservatively beyond chlorosities 0.14 g/l. These data confirm our earlier findings in other Indian estuaries. The behavior of uranium isotopes in the chlorosity zone 0.02–0.14 g/l, was studied in the Narbada estuary in some detail. The results, though not conclusive, seem to indicate a minor removal of these isotopes in this region. Reexamination of the results for the Gironde and Zaire estuaries (Martin et al., 1978a and b) also appear to confirm the conservative behavior of U isotopes in unpolluted estuaries. It is borne out from all the available data that estuaries beyond 0.14 g/l chlorosities act neither as a sink nor as a source for uranium isotopes, the behavior in the low chlorosity zones warrants further detailed investigation.A review of the uranium isotope measurements in river waters yield a discharge weighted-average ²³⁸U concentration of 0.22 μg/l with a ${}^{234}\text{U}{}^{238}\text{U}$ activity ratio of 1.20 ± 0.06ismissing. The residence time of uranium isotopes in the oceans estimated from the ²³⁸U concentration and the ${}^{234}\text{U}{}^{238}\text{U}$ A. R. of the rivers yield conflicting results; the material balance of uranium isotopes in the marine environment still remains a paradox. If the disparity between the results is real, then an additional ²³⁴U flux of about 0.25 dpm/cm₂·10³ yr into the oceans (about 20% of its river supply) is necessitated.     

A study of234U/238U ratios in groundwaters of Taiyuan area,Shanxi Province

Uranium contents and²³⁴U/²³⁸U ratios have been determined on 29 water samples from the Taiyuan area, Shanxi Province. The results show that the same artesian aquifer has similar uranium contents and²³⁴U/²³⁸U activity ratios, and the deeper aquifers have higher A. R. values but lower uranium contents. The A. R. values increase slightly towards groundwater flow in the artesian aquifers dominated by oxidizing ground waters. The Lancun Spring and the famous Jinci Spring belong to two different karst groundwater systems, i.e., the east and west karst groundwater systems. The recharge area of the Lancun Spring should cover the wide limestone outcrops of middle Ordovician in the northeast. The Ordovician fissure-karst ground water to the Jinci Spring is extensively mixed with fissure water in Carboniferous-Jurassic formations and seepage water from the Fenhe River.