234Th238U radioactive disequilibrium in the surface layer of the ocean

²³⁴Th produced from ²³⁸U within sea water was found to be in radioactive disequilibrium with respect to its progenitor nuclide ²³⁸U in the surface layer of the ocean. The median value for ${}^{234}\text{Th}{}^{238}\text{U}$ activity ratio is 0.80 in the upper 200 m layer. A box-model calculation gives a removal residence time of thorium of about 0.38 yr. This suggests that ²³⁴Th is scavenged from the surface layer by the uptake of thorium by biota.     

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.     

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.     

TL and ESR dating of speleothems and radioactive disequilibrium in the uranium series

Dating of speleothems by TL and ESR is seriously complicated by radioactive disequilibria in the uranium series. This paper presents a simple method which takes into consideration the U-series disequilibria in the case where the annual dose is determined by gamma scintillation spectrometry. The method described is based on the concept of an appropriately defined ‘laboratory age’ and enables direct reading of the age of a sample from the curve (the GH plot) independently of the results of measurements.     

Direct measurement of U-238 and disequilibrium state in geological samples by low energy scintillation gamma spectroscopy

A method has been developed to determine the state of disequilibrium and uranium content in geological samples using low energy gamma scintillation spectrometry. Ore standards in radioactive equilibrium were prepared in different concentrations and their low energy spectra were compared with those of chemical uranium standards of the same concentrations. The standards were analysed carefully for their U contents using X-ray fluorescence and high resolution gamma spectrometers. Counts of the complex peak under which 63 keV and 93 keV photopeaks take place in ore spectra were compared with the counts in corresponding channels of chemical uranium spectra. For standards in equilibrium it was found that (26 ± 2)% of the counts under complex peak came from Th-234 and also there was a constant relation between Th-234 (63 keV and 93 keV) and Pb-210 (47 keV) counts. The technique was applied to the geological samples for which eU concentrations had been measured previously by means of high energy gamma spectroscopy in the same laboratory.     

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.     

Geochemistry and origin of saline groundwaters in the Fennoscandian Shield

Chemical and isotopic analyses of water from drill holes and mines throughout the Fennoscandian Shield show that distinct layers of groundwater are present. An upper layer of fresh groundwater is underlain by several sharply differentiated saline layers, which may differ in salinity, relative abundance of solutes, and O, H, Sr and S isotope signature. Saline groundwater can be classified into four major groups based on geochemistry and presumed origin. Brackish and saline waters from 50–200 m depth in coastal areas around the Baltic Sea exhibit distinct marine chemical and isotopic fingerprints, modified by reactions with host rocks. These waters represent relict Holocene seawater. Inland, three types of saline groundwater are observed: an uppermost layer of brackish and saline water from 300–900 m depth; saline water and brines from 1000–2000 m depth; and superdeep brines which have been observed to a depth of at least 11 km in the drill hole on the Kola Peninsula, U.S.S.R. Electrical and seismic studies in shield areas suggest that such brines are commonly present at even greater depths. The salinity of all inland groundwaters is attributed predominantly to water-rock interaction. The main solutes are Cl, Ca, Na and Mg in varying proportions, depending on the host rock lithology. The abundance of dissolved gases increases with depth but varies from site to site. The main gas components are N₂, CH₄ (up to 87 vol.%) and locally H₂. The δ¹³C value for methane is highly variable (−25 to −46%), and it is suggested that hydrothermal or metamorphic gases trapped within the surrounding rocks are the most obvious source of CH₄. The uppermost saline water has meteoric oxygen-hydrogen isotopic compositions, whereas values from deeper water plot above the meteoric water line, indicating considerably longer mean residence time and effective low temperature equilibration with host rocks. Geochemical and isotopic results from some localities demonstrate that the upper saline water cannot have been formed through simple mixing between fresh water and deep brines but rather is of independent origin. The source of water itself has not been satisfactorily verified although superdeep brines at least may contain a significant proportion of relict Precambrian hydrothermal or metamorphic fluids.     

Use of 234U and 238U isotopes to identify fertilizer-derived uranium in the Florida Everglades

Surface water and peat in the northern Everglades have very low natural concentrations of U and are therefore sensitive to the addition of small amounts of U from anthropogenic sources such as fertilizer. Peat samples collected along a nutrient gradient in the northern Everglades have unusually high concentrations of U (>1 μg/g, dry basis) and also have a distinctive ²³⁴U/²³⁸U activity ratio (AR). AR values for U-enriched peat fall in the narrow range of AR values for commercial phosphate fertilizer (1.00±0.05). In contrast, AR values for low-U peat from background sites exceed 1.05. The spatial distribution of anomalous U concentration, and of fertilizer-like AR values in peat, parallel a previously documented pattern of P enrichment. These results strongly suggest that some of the U in nutrient-impacted peatlands is fertilizer-derived. Agricultural drainage water sampled in the northern Everglades has high concentrations of dissolved U (0.3–2.4 μg/l) compared to surface water from background sites (<0.1 μg/l). Measured AR values in drainage water (0.949–0.990) are also permissive of a fertilizer origin for the U and are different from AR values in surface water or peat at background sites (AR>1.05). Synoptic sampling of surface water along drainage canals indicate that Lake Okeechobee, and some drainage from agricultural fields, are sources of dissolved U, whereas wetlands farther downstream act as sinks for U. Historically cultivated agricultural soil has only a marginally elevated (+0.2 μg/g) average concentration of U compared to nearby uncultivated soil and incorporates only 20% of the U from an aqueous solution that was slurried with the soil. In contrast, a similar experiment with fresh Everglades peat indicated uptake of 90% of the added U. These experiments support the proposed removal of U from agricultural fields and concentration of U in downstream peatlands. The methodology of this study can be used to describe the behavior of fertilizer-derived U in other low-U environments.     

Age of Saurashtra miliolites by U-Th decay series methods: possible implications to their origin

The miliolite deposits of Saurashtra have been dated by²³⁴U,²³⁰Th,²³¹Pa and¹⁴C methods. Concordant ages of 10⁵ years using the U decay series isotopes are obtained which agree with the ages of the coral reefs of Okha-Dwaraka coast suggesting a contemporaneous origin for both. The lower¹⁴C ages (≤40,000 years) may be due to a recent influx of seawater or ground water. Quartz and clay minerals together constitute only ≤10% by weight, as such the aeolian characteristics of quartz grains may not be relevant to the origin of the miliolites.     

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.     

The implications of laboratoryRn flux measurements to the radioactivity in groundwaters: the case of a karstic limestone aquifer

Laboratory time-scale experiments were conducted on Carboniferous Limestone gravels from the Mendip Hills area, England, with the purpose of evaluating the release of²²²Rn to the water phase. The specific surface areas of the samples were 4.14 and 1.69 cm² g⁻¹ , which provided, respectively, values of 50.6 and 12.7 pCi for the released Rn. These results allowed the calculation of the emanation coefficient of this rock matrix with respect to the release of Rn, where completely different values corresponding to 23% and 6% were found, suggesting that the extent to which grain boundaries or imperfections in aggregates of micro-crystals of calcite intersect the particle surface certainly affects the Rn release. They also permitted the evaluation of models for the generation of Rn in rocks and transfer to water, in order to interpret the radioactivity due to this gas in groundwaters from the karstic aquifer of the Mendip Hills area, where the calculated activities in groundwater based on the values of 23% and 6% for the emanation coefficient were about 51 and 15 times higher than actually measured in groundwater. Therefore, the emanation coefficient in nature is considerably smaller than in the lab experiment, and another factork (0 < k < 1) may be introduced into the equations related to the modelling, with the aim of adjusting the theoretical-practical results.     

Dissolved Uranium and 234U/238U in the Yamuna and the Chambal Rivers, India

Dissolved uranium concentration and ²³⁴U/²³⁸U activity ratio have been measured in two distinctly different Indian drainage systems: the Yamuna headwaters in the Himalaya and the Chambal river system in the plains to study the weathering and mobility of uranium in these watersheds. The dissolved uranium in the Chambal river system ranges from 0.2 to 1.74 μg L⁻¹ during September (tail end of monsoon), whereas in the Yamuna river system, its concentration varies from 0.1 to 3.18 μg L⁻¹ during October (post-monsoon) and from 0.09 to 3.61 μg L⁻¹ in June (summer). In the Yamuna main stream, uranium is highest at its source and decreases steadily along its course, from 3.18 μg L⁻¹ at Hanuman Chatti to 0.67 μg L⁻¹ at Batamandi, at the base of the Himalaya. This decrease results mainly from mixing of the Yamuna mainstream with its tributaries, which are lower in uranium. The high concentration of uranium at Hanuman Chatti is derived from weathering of the Higher Himalayan Crystalline series (HHC) and associated accessary minerals, which may include uranium-mineralised zones. The ²³⁴U/²³⁸U activity ratios in the samples from the Chambal watershed are in the range of 1.15±0.05 to 1.67±0.04; whereas in the Yamuna the ratios vary from 0.95±0.03 to 1.56±0.07, during post-monsoon and from 0.98±0.01 to 1.30±0.03, during summer. The relatively high ²³⁴U/²³⁸U activity ratios in the Yamuna system are in its tributaries from the lower reaches viz., the Amlawa, Aglar, Bata, Tons and the Giri. It is estimated that ~9×10³ and ~12 × 10³ kg of dissolved uranium are transported annually from the Yamuna at Batamandi and the Chambal at Udi, respectively. This corresponds to uranium weathering rates of 0.9 and 0.09 kg U km⁻² y⁻¹ in the basins of the Yamuna and the Chambal headwaters. This study confirms that uranium weathering rate in the Himalaya is far in excess (by about an order of magnitude) of the global average value of ~0.08 kg U km⁻² y⁻¹.     

A routine method for the determination of the 234U238U ratio in natural water

A simple method to determine the ${}^{234}\text{U}{}^{238}\text{U}$ ratio in water by α-spectrometry is presented. The thiocyanate complex of uranium is extracted from the water by anion exchange. After elution of the U from the resin it is further purified by extraction from nitrate medium into ether. The source for α-spectrometry is prepared by oxalate electrodeposition. The overall yield is 70%.     

我国西北地区盐湖中的铀234U/238U比值示踪研究

内蒙古二连盆地、鄂尔多斯盆地典型含铀碳酸型盐湖水、岩两相234U/238U综合分析表明,碳酸型盐湖铀来源于大气降水和潜水对盐湖盆地周围中生代到现代富铀沉积物的溶滤、浸出,具有快速、近源物质来源特点。盐湖卤水和对应沉积物234U/238U值一般为0.8～1.2,湖卤水和潜卤水（晶间卤水）-岩两相中的铀处于沉积平衡状态。早白垩世～上新世含膏盐地层对比研究证实了富铀岩层234U/238U值随铀含量增大而减小,并趋近于1。室内盐湖水蒸发模拟实验发现残余卤水、沉积物234U/238U具有随蒸发程度增大逐渐减小的变化特征。盐湖现代沉积物物相研究发现铀主要以碳酸铀酰和吸附形式赋存在富含有机物和碎屑成分的含盐粘土沉积中,铀在盐类晶体中含量极少,仅存在于封闭水和结晶水中。卤水和沉积物ARu值是盐湖铀源及铀含量水平的指示标志之一。     

Porosity of the melting zone and variations in the solid mantle upwelling rate beneath Hawaii: inferences from 238U-230Th-226Ra and 235U-231Pa disequilibria

Measurements of ²³⁸U-²³⁰Th-²²⁶Ra and ²³⁵U-²³¹Pa disequilibria in a suite of tholeiitic-to-basanitic lavas provide estimates of porosity, solid mantle upwelling rate and melt transport times beneath Hawaii. The observation that (²³⁰Th/²³⁸U) > 1 indicates that garnet is required as a residual phase in the magma sources for all of the lavas. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Hawaiian basalts. For chromatographic porous flow, the calculated maximum porosity in the melting zone ranges from 0.3–3% for tholeiites and 0.1–1% for alkali basalts and basanites, and solid mantle upwelling rates range from 40 to 100 cm yr⁻¹ for tholeiites and from 1 to 3 cm yr⁻¹ for basanites. For dynamic melting, the escape or threshold porosity is 0.5–2% for tholeiites and 0.1–0.8% for alkali basalts and basanites, and solid mantle upwelling rates range from 10 to 30 cm yr⁻¹ for tholeiites and from 0.1 to 1 cm yr⁻¹ for basanites. Assuming a constant melt productivity, calculated total melt fractions range from 15% for the tholeiitic basalts to 3% for alkali basalts and basanites.     

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.     

Radioisotope dynamics - the origin and fate of nuclides in groundwater

The use of radionuclides as clocks for groundwater dating and as probes to investigate the geometry and spatial extent of the contact area between rocks and water is reviewed. Subsurface production rates for²²²Rn³⁷Ar,⁸⁵Kr,³⁹Ar,³⁶CI,³He,⁴He and⁴⁰Ar in various rock types are listed. Measured Rn fluxes from the surface of sandstone grains and from pieces of granite point to scale-dependent diffusion coefficients. The temporal evolution of subsurface-produced²²²Rn-,³⁷Ar-,⁸⁵Kr- and³⁹Ar-activities in groundwaters yields radionuclide escape factors between 0.1 % and 9% for the Stripa granite (Sweden) and between 1% and 4% for the Milk River sandstone (Canada). The combination of³H,⁸⁵Kr,³⁹Ar,¹⁴C,³⁶CI,⁴He in the UK Triassic sandstone aquifer allows groundwater dating up to 40 000 a. Very old groundwaters can be studied using Cl,³⁶Cl and⁴He evolution as demonstrated in the Milk River aquifer in Canada.     

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.