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

B.C. Reynolds  G.J. Wasserburg 《Geochimica et cosmochimica acta》2003,67(11):1955-1972

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.  相似文献   

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

《Applied Geochemistry》1993,8(5):447-471

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.  相似文献   

高分辨电感耦合等离子体质谱法测定铀矿石样品中234U/238U、230Th/232Th和228Ra/226Ra同位素比值    总被引：1，自引：0，他引：1  

郭冬发  张彦辉  武朝晖  朱明燕  王玉学  崔建勇  谭靖 《岩矿测试》2009,28(2):101-107

采用氢氟酸-硝酸-盐酸混合酸密闭消解含铀矿石样品,用阴离子交换树脂、阳离子交换树脂和锶特效树脂逐级分离富集铀、钍和镭。使用高分辨电感耦合等离子体质谱(HR-ICPMS)测定分离纯化液中234U/238U2、30Th/232Th和228Ra/226Ra同位素。比值的测量精密度取决于比值的大小和对应核素浓度的大小。对质量浓度为10 ng/mL天然铀测量液,234U/238U的测量精密度优于1.2%;对230Th质量浓度为0.6ng/mL且230Th和232Th质量浓度接近的测量液,230Th/232Th的测量精密度为1.2%;对228Ra质量浓度为0.48 pg/mL且228Ra和226Ra质量浓度接近的测量液,228Ra/226Ra的测量精密度为4.0%。  相似文献   

Ground water geochemistry of ²²⁸Ra, ²²⁶Ra and ²²²Rn     

Philip T. King  Jacqueline Michel  Willard S. Moore 《Geochimica et cosmochimica acta》1982,46(7):1173-1182

²²⁸Ra, ²²⁶Ra, and ²²²Rn activities were determined on over 150 ground water samples collected from drilled, public water supply wells throughout South Carolina. A wide range of aquifer lithologies were sampled including the crystalline rocks of the Piedmont and sedimentary deposits of the Coastal Plain. A significant linear relationship between log ²²⁸Ra and log ²²⁶Ra (n = 182, r = 0.83) was indistinguishable between Piedmont and Coastal Plain ground water. Median ${}^{228}\text{Ra}{}^{226}\text{Ra}$ activity ratios for the Piedmont, 1.2, and Coastal Plain, 1.3, ground water are close to estimated average crustal ${}^{232}\text{Th}{}^{238}\text{U}$ activity ratios of 1.2 to 1.5 corresponding to Th/U weight ratios of 3.5 to 4.5. A linear correlation was also found between log ²²²Rn and log ²²⁶Ra for Piedmont (n = 68, r = 0.62) and Coastal Plain (n = 89, r = 0.64) ground water. However, the median ${}^{222}\text{Rn}{}^{226}\text{Ra}$ activity ratio for Piedmont ground water, 6100, was much higher than for Coastal Plain ground water, 230. Higher excess ²²²Rn activities may be due to greater retention of ²²⁶Ra by the chemically active Piedmont aquifers compared to the more inert sand aquifers sampled in the Coastal Plain. The relationship between log ²²⁸Ra and log ²²⁶Ra was used to predict total Ra (${}^{228}\text{Ra +}{}^{226}\text{Ra}$) distributions in Appalachian and Atlantic and Gulf Coastal Plain ground water. Predictions estimate that 2.4% of Appalachian and 5.3% of Atlantic and Gulf Coastal Plain ground water supplies contain total Ra activities in excess of the 5 pCi/l limit established by the U.S. Environmental Protection Agency. These predictions also indicate that 40–50% of these ground water wells may be overlooked using the presently suggested screening activity of 3.0 pCi/l of ²²⁶Ra for ²²⁸Ra analysis.  相似文献   

TIMS测定铀矿石样品中^234U/^238U、^230Th/^232Th、^228Ra/^226Ra的方法研究   总被引：1，自引：0，他引：1  

朱明燕  郭冬发  崔建勇  刘宇昂  谭靖  张彦辉 《铀矿地质》2009,25(3):184-192

研究了TIMS测定铀矿石样品中234U/238U、230Th/232Th、228Ra/226Ra的方法。建立了铀矿石密闭混酸一次溶样的方法和采用阴离子、阳离子和Sr特效树脂逐级离子交换分离纯化U、Th和Ra的流程,满足了TIMS测量要求。测定结果表明：100～1000 ng的天然铀中234U/238U,其测量精密度从静态多接收的2.34%提高到动态多接收的0.47%;对230Th与232Th丰度接近、质量为1μg左右的钍,采用三带点样技术和法拉第多接收技术测定230Th/232Th,其内精度平均值为0.0048%,外精度为0.028%;采用单带加钽发射剂,ETP跳峰测定50～100 fg镭-228稀释剂中的228Ra/226Ra,其内精度小于0.10%,外精度小于0.20%。比较TIMS和HR-ICP-MS、α能谱法测定234U/238U、230Th/232Th、228Ra/226Ra结果,三者结果相吻合。TIMS测量法样品用量少、快速、准确、精密度高,是U、Th、Ra同位素比值测定方法的又一补充。  相似文献   

Redistribution of uranium and thorium series isotopes during isovolumetric weathering of granite     

Jacqueline Michel 《Geochimica et cosmochimica acta》1984,48(6):1249-1255

Previous studies of the distribution of U and Th in parent versus weathered granites have shown both depletion and enrichment of these elements during weathering. In this study, the distribution of U and Th decay series isotopes was determined in a weathering profile of a granitic saprolite, which showed textural preservation indicating isovolumetric weathering. Two types of dissolution methods were used: a whole-rock dissolution and a sodium-citrate dithionite leach to preferentially attack noncrystalline phases of weathering products. Using volume-based activities, 45–70 percent of the total ²³²Th was gradually removed during weathering. Although the whole-rock ${}^{228}\text{Th}{}^{232}\text{Th}$ activity ratios were in equilibrium, there were large excesses of ²²⁸Th in the leachable fraction of both parent rock (${}^{228}\text{Th}{}^{232}\text{Th}\text{= 2.06}$) and partially weathered saprolite (${}^{228}\text{Th}{}^{232}\text{Th}\text{= 3–6.5}$), due to alpha recoil and release of daughter ²²⁸Th to the weathering rind of the mineral grain. For the most weathered sample, 81 percent of the thorium was in the teachable fraction and ${}^{228}\text{Th}{}^{232}\text{Th}\text{= 1}$, indicating that even the more resistant minerals were attacked.The total U activities showed as much variation in the six parent rock samples as in the weathered profile, and ${}^{234}\text{U}{}^{238}\text{U}$ were in equilibrium in both the whole-rock and leachable fractions. ²³⁰Th was deficient relative to ²³⁴U and ²²⁶Ra in both fractions, suggesting recent addition of U and Ra to the entire profile. The large variation in U was not from absorption onto the intermediate weathering products, because only 11–23 percent of the U was in the leachable fraction.  相似文献   

厦门火烧屿裸露岩石的铀放射系不平衡   总被引：2，自引：1，他引：2  

刘广山  徐茂泉  黄奕普 《地球学报》2003,24(6):618-621

用HPGeγ谱方法测定了厦门火烧屿裸露岩石天然放射性核素^40K、^228Ra、^228Th、^238U、^226Ra和^210Pb含量，对其铀系不平衡关系进行了讨论，发现钍系核素^228Ra和^228Th基本上是平衡的，而大部分样品^226Ra相对于^228U、^210Pb相对于^226Ra亏损。由此推论，水体作用下岸边岩石中^226Ra直接进入水体，可以是海水中^226Ra的一个来源；岸边岩石中^222Rn逸出后，衰变^210Pb再进入水体，可以是海水中^210Pb的一个来源。  相似文献   

U-238 series radioactive disequilibrium in groundwaters: implications to the origin of excess U-234 and fate of reactive pollutants     

N. Hussain  S. Krishnaswami 《Geochimica et cosmochimica acta》1980,44(9):1287-1291

The concentrations of ²³⁸U, ²³⁴Th, ²²⁶Ra, ²²²Rn and ²¹⁰Pb and ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios have been measured in several groundwater samples from Gujarat, India. In the aqueous phase the abundances of ²³⁴Th and ²¹⁰Pb are grossly deficient relative to their parents, ²³⁸U and ²²²Rn respectively. The deficiency is ascribable to the impact adsorption of ²³⁴Th and ²¹⁰Pb atoms onto particle surfaces which are very abundant in the groundwater regimes. The scavenging residence times for both these nuclides is about a day, suggesting that irreversible removal of ‘reactive’ metals and pollutants in groundwaters can occur on very short time scales. The fast removal of ²³⁴Th onto particles necessitates that in these groundwaters ²³⁴U ‘excess’ has to originate through leaching of soil grains rather than through in situ decay of dissolved ²³⁴Th in the water.  相似文献   

Ra and Rn isotopes as natural tracers of submarine groundwater discharge in the patagonian coastal zone (Argentina): an initial assessment     

Américo I. Torres  Carlos F. Andrade  Willard S. Moore  Mauricio Faleschini  José L. Esteves  Luis F. H. Niencheski  Pedro J. Depetris 《Environmental Earth Sciences》2018,77(4):145

Submarine groundwater discharge (SGD) is herein recognized as a significant pathway of material transport from land to the coastal SW Atlantic Ocean and thus, it can be a relevant factor affecting the marine biogeochemical cycles in the region. This paper focuses on the initial measurements of ²²⁶Ra, ²²⁸Ra and ²²²Rn made in Patagonia’s coastal zone of Chubut and Santa Cruz provinces (42°S–48°S, Argentina). ²²⁶Ra activity ranged from 2.9 to 73.5 dpm 100 L^?1, and ²²⁸Ra activity ranged from 11.9 to 311.0 dpm 100 L^?1 in groundwater wells. The radium activities found in Patagonia’s marine coastal regions and adjacent shelf indicate significant enrichment throughout the coastal waters. Groundwater samples presented the largest ²²²Rn activity and ranged from 2.66 to 1083 dpm L^?1. Conversely, in the coastal marine environment, the ²²²Rn activity ranged from 1.03 to 6.23 dpm L^?1. The Patagonian coastal aquifer showed a larger enrichment in ²²⁸Ra than in ²²⁶Ra, which is a typical feature for sites where SGD is dominant, probably playing a significant role in the biogeochemistry of these coastal waters.  相似文献   

Relation between "terra rossa" from the Apulia aquifer of Italy and the radon content of groundwater: Experimental results and their applicability to radon occurrence in the aquifer   总被引：1，自引：1，他引：0  

T. Tadolini  M. Spizzico 《Hydrogeology Journal》1998,6(3):450-454

 The radon-222 (²²²Rn) activity in groundwater of the Apulian karstic aquifer in southern Italy is as great as 500 Becquerel per liter (Bq/L) locally. Normal radium-226 (²²⁶Ra) activity in the limestone and calcareous dolomites of the aquifer is not enough to explain such a high level. Laboratory investigations identified high ²²⁶Ra activity in the "terra rossa," the residuum occupying fissures and cavities in the bedrock, and also the relation between (1) ²²⁶Ra-bearing bedrock and "terra rossa" and (2) ²²²Rn in water. The "terra rossa" is the primary source of the radon in the groundwater. The experimental results show the need to characterize the "terra rossa" of Apulia on the basis of ²²⁶Ra activity and also to study the distribution and variations in ²²²Rn activity over time in the aquifer. Received, January 1996 · Revised, March 1997, September 1997, June 1998 · Accepted, July 1998  相似文献   

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

Sophie Rihs  Damien Lemarchand  Marie-Claire Pierret  François Chabaux 《Geochimica et cosmochimica acta》2011,75(23):7707-7724

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.  相似文献   

The relative importance of terrestrial versus marine sediment sources to the Nueces-Corpus Christi Estuary, Texas: An isotopic approach     

K. M. Yeager  P. H. Santschi  K. J. Schindler  M. J. Andres  E. A. Weaver 《Estuaries and Coasts》2006,29(3):443-454

Determining sources of sediment to coastal systems is an important and complex problem that figures prominently in a myriad of geological, geomorphological, geochemical, and biological processes. Lithogenic (²²⁶Ra,²²⁸Ra,²²⁸Th,²³⁰Th,²³²Th) and fallout (¹³⁷Cs,²¹⁰Pb) isotopes were employed in conjunction with sedimentological methods to determine rates of sedimentation in the Nueces Delta and Nueces-Corpus Christi Estuary and to assess the relative importance of marine versus terrestrial sediment sources to the estuary. Similarity of lithogenic isotope ratios in surface sediments throughout the system precluded a numeric approach to discerning the importance of each of the two large scale sediment sources (terrestrial and marine). A stepwise, graphical examination of discrete lithogenic isotope activity concentrations shows more promise. Terrestrial, marine, and bay sediment means for²²⁶Ra versus²³²Th,²²⁶Ra versus²³⁰Th, and²²⁸Ra versus²³²Th show that terrestrial and marine sediment sources have different signatures, despite having similar grain size distributions (sands), and that sediment deposited in Nueces and Corpus Christi Bays are indistinguishable from the terrestrial component. Supporting evidence is provided by thorium isotopes,²³⁰Th versus²³²Th,²²⁸Th versus²³²Th, and²²⁸Th versus²³⁰Th. Nueces Delta sedimentation (0.09–0.53 g cm⁻² yr⁻¹) shows a subtle gradient, with rates generally lower in the west and progressively higher moving east, likely reflecting contrasts in land use and topography. Nueces Bay cores differ from those in Corpus Christi Bay in that sands comprise a larger percentage of their composition, and they are mixed over greater depth, most likely due to geographic and physiographic effects. Sediment accumulation rates consistently decrease over the first 20 km from the Nueces River and become constant after that, implying that the river is the most significant source of sediment to the estuary. The interpretation of sediment supply to this estuary as dominated by terrestrial inputs is based on three complimentary sets of data: sediment grain size distributions, discrete lithogenic isotope data (Ra versus Th and Th versus Th), and sediment accumulation rates for both Nueces and Corpus Christi Bays.  相似文献   

Relationships between radium and radon occurrence and hydrochemistry in fresh groundwater from fractured crystalline rocks, North Carolina (USA)     

David S. Vinson  Avner Vengosh  Daniella Hirschfeld  Gary S. Dwyer 《Chemical Geology》2009,260(3-4):159-171

Naturally-occurring radionuclides (uranium, radium, and radon), major dissolved constituents, and trace elements were investigated in fresh groundwater in 117 wells in fractured crystalline rocks from the Piedmont region (North Carolina, USA). Chemical variations show a general transition between two water types: (1) slightly acidic (pH 5.0–6.0), oxic, low-total dissolved solids (TDS) waters, and (2) near neutral, oxic to anoxic, higher-TDS waters. The uranium, radium, and radon levels in groundwater associated with granite (Rolesville Granite) are systematically higher than other rock types (gneiss, metasedimentary, and metavolcanic rocks). Water chemistry plays a secondary role on radium and radon distributions as the ²²²Rn/²²⁶Ra activity ratio is correlated with redox-sensitive solutes such as dissolved oxygen and Mn concentrations, as well as overall dissolved solids content including major divalent cations and Ba. Since ²²⁴Ra/²²⁸Ra activity ratios in groundwater are close to 1, we suggest that mobilization of Ra and Rn is controlled by alpha recoil processes from parent nuclides on fracture surfaces, ruling out Ra sources from mineral dissolution or significant long-distance Ra transport. Alpha recoil is balanced by Ra adsorption that is influenced by redox conditions and/or ion concentrations, resulting in an approximately one order of magnitude decrease (~ 20,000 to ~ 2000) in the apparent Ra distribution coefficient between oxygen-saturated and anoxic conditions and also across the range of dissolved ion concentrations (up to ~ 7 mM). Thus, the U and Th content of rocks is the primary control on observed Ra and Rn activities in groundwater in fractured crystalline rocks, and in addition, linked dissolved solids concentrations and redox conditions impart a secondary control.  相似文献   

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

Matti Asikainen 《Geochimica et cosmochimica acta》1981,45(2):201-206

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.  相似文献   

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     

《Chemical Geology》2002,182(2-4):409-421

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.  相似文献   

Naturally occurring radionuclides and their geochemical interactions at a geothermal site in the North German Basin   总被引：1，自引：1，他引：0  

Simona Regenspurg  Jörg Dilling  Jürgen Mielcarek  Frank Korte  Uwe-Karsten Schkade 《Environmental Earth Sciences》2014,72(10):4131-4140

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.  相似文献   

Ra-226 and Rn-222 in saline water compartments of the Aral Sea region     

《Applied Geochemistry》2015

The Aral Sea has been shrinking since 1963 due to extensive irrigation and the corresponding decline in the river water inflow. Understanding of the current hydrological situation demands an improved understanding of the surface water/groundwater dynamics in the region. ²²²Rn and ²²⁶Ra measurements can be used to trace groundwater discharge into surface waters. Data of these radiometric parameters were not previously available for the study region. We determined ²²²Rn activities after liquid phase extraction using Liquid Scintillation Counting (LSC) with peak-length discrimination and analyzed ²²⁶Ra concentrations in different water compartments of the Amu Darya Delta (surface waters, unconfined groundwater, artesian water, and water profiles from the closed Large Aral Sea (western basin).The water samples comprise a salinity range between 1 and 263 g/l. The seasonal dynamics of solid/water interaction under an arid climate regime force the hydrochemical evolution of the unconfined groundwater in the Amu Darya Delta to high-salinity Na(Mg)Cl(SO₄) water types. The dissolved radium concentrations in the waters were mostly very low due to mineral over-saturation, extensive co-precipitation of radium and adsorption of radium on coexisting solid substrates.The analysis of very low ²²⁶Ra concentrations (<10 ppq) at remote study sites is a challenge. We used the water samples to test and improve different analytical methods. In particular, we modified a procedure developed for the α-spectrometric determination of ²²⁶Ra after solid phase extraction of radium using 3M Empore™ High Performance Extraction Disks (Purkl, 2002) for the analysis of the radionuclide using an ICP sector field mass spectrometer. The ²²⁶Ra concentration of 17 unconfined groundwater samples ranged between 0.2 and 5 ppq, and that of 28 artesian waters between <0.2 and 13 ppq. The ICP-MS results conformed satisfactorily to analytical results based on γ-measurements of the ²²²Rn ingrowth after purging and trapping on super-cooled charcoal. The ²²⁶Ra concentrations were positively correlated with the salinity and the dissolved NaCl concentrations. The occurrence of unusually high ²²⁶Ra activities is explained by radium release from adsorption sites with increasing salinity. The inferred spatial variability of ²²²Rn in the Aral Sea and of ²²²Rn and ²²⁶Ra in the groundwater of the Amu Darya Delta is discussed in the context of our own previous hydrochemical studies in the study sites. Relatively low ²²²Rn activities in the unconfined GW (1–9.5 Bq/l) indicate the alluvial sediments hosting the GW to be a low-²³⁸U(²²⁶Ra) substrate. Positive correlations between U and ²²⁶Ra, and U and ²²²Rn are likely related to locally deposited Fe(Mn)OOH precipitates. The ²²²Rn activity of the GW, however, distinctly exceeds the ²²²Rn concentration in the Aral Sea (10 mBq/l), in principle, making ²²²Rn a sensitive tracer for the inflow of GW. The high water volume of the Large Aral Sea and wind induced mixing of its water body, however, hamper the detection of local groundwater inflow.  相似文献   

Dependence of radon exhalation on grain size of sedimentary waste     

Eman M. Ibrahim  Ibrahim E. El Aassy  Hayam Ahmed Abdel Ghany  S. H. Gamil 《Environmental Earth Sciences》2018,77(14):534

Tailings resulted from sulphuric acid leaching process of uranium from sedimentary rocks contain high concentrations of ²²⁶Ra and its daughters, the most important of which is ²²²Rn. Movement of radon gas out of the tailings is strongly influenced by the physicochemical characteristics of these tailings especially their radium content and the grain size. So, the tailing samples were size fractionated into four sizes (>?250, 250–125, 125–74 and <?74 µm). The natural radioactivity was investigated using hyper-pure germanium detector and solid-state nuclear track detectors (CR-39) for bulk size and after size fractionation. The activity concentrations of different radionuclides in size-fractionated tailing samples have been shown to be strongly dependent on the size of the particles. In the range of >?250 and <?74 µm, the activity concentrations of ²³⁰Th, ²²⁶Ra, ²¹⁴Pb, ²¹⁴Bi, ²¹⁰Pb, ²³²Th and ⁴⁰K increased throughout with decreasing particle size, while that of ²³⁸U, ²³⁴U and ²³⁵U have an opposite effect. The results revealed an inverse relationship between the radon exhalation rate and size fractionation. Also, the results showed a good correlation between radium activity concentration and radon mass exhalation rate.  相似文献   

U- and Th-decay series constraints on the timescales of crystal fractionation to produce the phonolite erupted in 2004 near Tristan da Cunha, South Atlantic Ocean     

Mark K. Reagan  Simon Turner  Kenneth W.W. Sims 《Geochimica et cosmochimica acta》2008,72(17):4367-4378

Phonolite pumice found floating offshore of Tristan da Cunha following intense seismic activity southeast of the island July 29-30, 2004 was analyzed for ²³⁸U- and ²³²Th-series nuclides to determine initial ²³⁰Th, ²²⁶Ra, ²¹⁰Pb, ²¹⁰Po, ²²⁸Ra, and ²²⁸Th activities. The initial (²¹⁰Po/²¹⁰Pb) value of 0.15 for the phonolite shows that, like most subaerial lavas, this subaqueous tephra degassed most of its ²¹⁰Po upon eruption. The (²³⁰Th/²³²Th) and (²³⁸U/²³²Th) values for the phonolite are similar to those of the trachyandesites erupted in 1961 from Tristan da Cunha. However, the relative activities of ²¹⁰Pb, ²²⁶Ra, and ²³⁰Th in the phonolite contrast with those of the trachyandesites, in that ²¹⁰Pb and ²³⁰Th are both strongly enriched with respect to ²²⁶Ra. In addition, the phonolite had a small deficit in ²²⁸Ra with respect to ²³²Th. The Ra deficits likely resulted from partitioning into feldspars and hornblende in a time frame that extended over several decades to a century. These disequilibria can be explained by crystal fractionation at a decreasing rate through time at an average of 3-5 × 10⁻³ year⁻¹. The calculated crystallization rate is about an order of magnitude faster than has been calculated for most other phonolites and trachytes, and about half that calculated for crystallization of the Makaopuhi lava lake. These data imply that the 2004 magma was not the differentiated cap of a much larger body that remained at depth. Instead, it was likely the residue of a relatively small body of more mafic magma that was injected into the crust southeast Tristan and underwent extensive and rapid crystal fractionation before it erupted.  相似文献   

Weathering rates from top to bottom in a carbonate environment     

Bernard Bourdon  Sarah Bureau  Morten B. Andersen  Eric Pili  Amélie Hubert 《Chemical Geology》2009,258(3-4):275-287

This study investigates U-series, Sr isotopes, major and trace elements in a chalk aquifer system located in Eastern France. Soil and rock samples were collected along depth profiles down to 45 m in four localities as an attempt to investigate the weathering processes in the soil, the unsaturated zone and the saturated zone of the aquifer. Interstitial water was extracted from soils and rocks by a centrifugation technique. U-series offer a powerful tool to calculate weathering rates because the relative mobility of the U- and Th-isotopes can be precisely measured and it does not require the determination of a reference state as in other approaches. As expected, the data show very large mobile element depletion in the soil with large ²³⁰Th excess relative to ²³⁸U, while the rocks show more limited but not insignificant mobile element depletion. The U-series data have been used to constrain weathering rates based on a 1-D reactive transport model. Weathering rates in the near surface are about 10–100 times faster than at depth. However, when integrated over the depth of the cores, including the unsaturated and the saturated zones, this underground weathering represents more than 30% of the total weathering flux, assuming congruent dissolution of carbonates. The (²³⁴U/²³⁸U) ratios in interstitial water are consistent with solid samples showing ²³⁴U depletion near the surface and an excess ²³⁴U at depth. A leaching experiment performed on chalk shows that the excess ²³⁴U in natural waters percolating through carbonate rocks results both from preferential ²³⁴U leaching and direct recoil in the interstitial water. A new approach was used to derive the recoil ejection factor based on BET measurements and the fractal dimension of chalk surface. Consideration of preferential leaching and recoil allows a more accurate modeling of weathering rates.  相似文献