Excess 234U: An aging effect in confined waters

doi:10.1016/0012-821X(75)90046-1

Earth and Planetary Science Letters

Volume 27, Issue 2, September 1975, Pages 342-345

https://doi.org/10.1016/0012-821X(75)90046-1 Get rights and content

Abstract

Strong isotopic fractionation between²³⁴U and²³⁸U has been noted in deep oil-well brines. The waters are stratigraphically and structurally isolated from fresh-water inflow and have remained stagnant for more than five half-lifes of²³⁴U. Excess²³⁴U is explained by the²³⁴Th alpha-recoil nucleus event.

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Mixing volume calculations, sources and aging trends of Floridan aquifer water by uranium isotope methods
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Uranium disequilibrium in groundwater: an isotope dilution approach in hydrologic investigations
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Uranium-234 and uranium-238 in the waters and bottom sediments of Lake Balkah and the age of the lake
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Cited by (38)

Use of U-isotopes in exploring groundwater flow and inter-aquifer leakage in the south-western margin of the Great Artesian Basin and Arckaringa Basin, central Australia
2018, Applied Geochemistry
Citation Excerpt :
Where the uranium activity concentrations (or more commonly 234U/238U activity ratios) behave conservatively along groundwater flow paths variability can potentially identify inter-aquifer mixing, identify source waters and determine the relative proportions of source waters (Dabous and Osmond, 2001; Herczeg et al., 1996; Osmond et al., 1974; Paces et al., 2002; Paces and Wurster, 2014; Rosenthal and Kronfeld, 1982). Uranium isotope variations have also been used to determine groundwater flow rates by either the 234U excess decay model (Guttman and Kronfeld, 1982; Henderson et al., 1999; Kronfeld et al., 1975; Kronfeld and Rosenthal, 1981) or alternatively, by determining the progressive augmentation by aquifer processes (Ivanovich et al., 1991; Schaffhauser et al., 2014). Other groundwater studies found that U activities depended more upon the groundwater redox conditions (Andrews and Kay, 1983), radioelement distribution from diverse vadose zone inputs and weathering conditions (Kronfeld et al., 2004; Méjean et al., 2016; Post et al., 2017; Tricca et al., 2001), extent of water-rock interactions (Andrews et al., 1982) and stabilisation by the formation of uranium complexes (Herczeg et al., 1988; Jurgens et al., 2010).
The distribution of uranium isotopes (²³⁸U and ²³⁴U) in groundwaters of the south-western margin of the Great Artesian Basin (GAB), Australia, and underlying Arckaringa Basin were examined using groundwater samples and a sequential extraction of aquifer sediments. Rock weathering, the geochemical environment and α-recoil of daughter products control the ²³⁸U and ²³⁴U isotope distributions giving rise to large spatial variations. Generally, the shallowest aquifer (J aquifer) contains groundwater with higher ²³⁸U activity concentrations and ²³⁴U/²³⁸U activity ratios close to secular equilibrium. However, the source input of uranium is spatially variable as intermittent recharge from ephemeral rivers passes through rocks that have already undergone extensive weathering and contain low ²³⁸U activity concentrations. Other locations in the J aquifer that receive little or no recharge contain higher ²³⁸U activity concentrations because uranium from localised uranium-rich rocks have been leached into solution and the geochemical environment allows the uranium to be kept in solution. The geochemical conditions of the deeper aquifers generally result in lower ²³⁸U activity concentrations in the groundwater accompanied by higher ²³⁴U/²³⁸U activity ratios. The sequential extraction of aquifer sediments showed that α-recoil of ²³⁴U from the solid mineral phases into the groundwater, rather than dissolution of, or exchange with the groundwater accessible minerals in the aquifer, caused enrichment of groundwater ²³⁴U/²³⁸U activity ratios in the Boorthanna Formation. Decay of ²³⁸U in uranium-rich coatings on J aquifer sediments caused resistant phase ²³⁴U/²³⁸U activity ratio enrichment. The groundwater ²³⁴U/²³⁸U activity ratio is dependent on groundwater residence time or flow rate, depending on the flow path trajectory. Thus, uranium isotope variations confirmed earlier groundwater flow interpretations based on other tracers; however, spatial heterogeneity, and the lack of clear regional correlations, made it difficult to identify recharge and inter-aquifer leakage.
Processes controlling 234U and 238U isotope fractionation and helium in the groundwater of the St. Lawrence Lowlands, Quebec: The potential role of natural rock fracturing
2016, Applied Geochemistry
Citation Excerpt :
A detailed discussion on the helium isotopic systematics is beyond the scope of this paper and is reported in Vautour et al. (2015). Fig. 3 compares the measured [U] and (234U/238U)act in the current study area with those from other sedimentary aquifers characterized by similar lithologies and confinement conditions (except for confined oil brines; Kronfeld et al., 1975; Banner et al., 1990). Both measured [U] (Fig. 3a) and (234U/238U)act (Fig. 3b) from the study area are within the range of values observed in other unconfined and confined sedimentary aquifers (Banner et al., 1990; Bonotto and Andrews, 2000; Durand et al., 2005; Hubert et al., 2006; Reynolds et al., 2003; Riotte and Chabaux, 1999; Tricca et al., 2001), but are characterized by higher variability.
The goal of this study is to explain the origin of ²³⁴U–²³⁸U fractionation in groundwater from sedimentary aquifers of the St. Lawrence Lowlands (Quebec, Canada), and its relationship with ³He/⁴He ratios, to gain insight regarding the evolution of groundwater in the region. (²³⁴U/²³⁸U) activity ratios, or (²³⁴U/²³⁸U)_act, were measured in 23 groundwater samples from shallow Quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the Becancour River watershed. The lowest (²³⁴U/²³⁸U)_act, 1.14 ± 0.01, was measured in Ca–HCO₃-type freshwater from the Quaternary Shallower Aquifer, where bulk dissolution of the carbonate allows U to migrate into water with little ²³⁴U–²³⁸U isotopic fractionation. The (²³⁴U/²³⁸U)_act increases to 6.07 ± 0.14 in Na–HCO₃–Cl-type groundwater. Preferential migration of ²³⁴U into water by α-recoil is the underlying process responsible for this isotopic fractionation. An inverse relationship between (²³⁴U/²³⁸U)_act and ³He/⁴He ratios has been observed. This relationship reflects the mixing of newly recharged water, with (²³⁴U/²³⁸U)_act close to the secular equilibrium and containing atmospheric/tritiogenic helium, and mildly-mineralized older water (¹⁴C ages of 6.6 kyrs), with (²³⁴U/²³⁸U)_act of ≥6.07 and large amounts of radiogenic ⁴He, in excess of the steady-state amount produced in situ. The simultaneous fractionation of (²³⁴U/²³⁸U)_act and the addition of excess ⁴He could be locally controlled by stress-induced rock fracturing. This process increases the surface area of the aquifer matrix exposed to pore water, from which produced ⁴He and ²³⁴U can be released by α-recoil and diffusion. This process would also facilitate the release of radiogenic helium at rates greater than those supported by steady-state U–Th production in the rock. Consequently, sources internal to the aquifers could cause the radiogenic ⁴He excesses measured in groundwater.
Uranium isotopes in groundwater from the continental intercalaire aquifer in Algerian Tunisian Sahara (Northern Africa)
2009, Journal of Environmental Radioactivity
Citation Excerpt :
These mechanisms are favoured (Osmond and Cowart, 1992; Ivanovich et al., 1991) by long period of interaction between water of low dissolved uranium contents and hosting formations of relatively higher uranium concentrations. Several studies have reported activity ratios reaching 12 in large confined aquifers with steady long term system such as 9.0 for deeper and older shallow groundwater of the Transvaal Dolomite Aquifer in South Africa (Kronfeld et al., 1994), 10.1 for groundwater sampled from a deep well in Ramallah (Kronfeld et al., 1975), 10.5 in confined aquifers in the Cambrian and Ordovician bedrock in the tri-state region of Missouri, Kansas and Oklahoma (Cowart and Osmond, 1980), 11.8 at a redox front of the Milk River aquifer (Ivanovich et al., 1991) and 12.3 in a confined sandstone aquifer in Texas (Kronfeld, 1974). Activity ratios as high as 28 have also been observed in the brazilian part of the Guarani aquifer (Bonotto, 2006) and have been explained by the large extension of the system enhancing the three main factors responsible of uranium isotopes disequilibria (rock–water interactions, preferential chemical dissolution and alpha-recoil release of 234U).
The disequilibrium between ²³⁴U and ²³⁸U is commonly used as a tracer of groundwater flow. This paper aims to identify uranium contents and uranium isotopic disequilibria variation in groundwater sampled from deep Continental Intercalaire aquifer (southern Algeria and Tunisia). Large variations in both U contents (0.006–3.39 ppb) and ²³⁴U/²³⁸U activity ratios (0.4–15.38) are observed. We conduct a first assessment in order to verify whether the results of our investigation support and complete previous hydrogeological and isotopic studies. The dissolved U content and ²³⁴U/²³⁸U activity ratio data were plotted on a two-dimensional diagram that was successfully utilized on sharing the CI aquifer into different compartments submitted to different oxidising/reducing conditions and leads also to distinguished two preferential flow paths in the Nefzaoua/Chott Fejej discharge area. Uranium isotopes disequilibrium indicate that ranium chemistry is mainly controlled by water–rock interaction enhanced by long residence time recognised for this aquifer.
Chapter 10 Uranium- and Thorium-Series Radionuclides in Marine Groundwaters
2008, Radioactivity in the Environment
Citation Excerpt :
This application is based on the fact that 234U, unlike 238U, can be mobilized to sediment pore water via recoil of 234Th and subsequent decay of the 234Th. This process accounts for greater-than-unity 234U/238U activity ratios in terrestrial groundwaters (e.g. Hussain and Krishnaswami, 1980; Kronfeld et al., 1975; Hussain and Lal, 1986; Osmond and Cowart, 1992; Porcelli, this volume) and 234U/238U activity ratios in marine sediment pore waters that are elevated relative to the seawater value of 1.149 (Cochran and Krishnaswami, 1980; Maher et al., 2004). Maher et al. (2004, 2006) have used high-resolution mass-spectrometric measurements of U concentrations and 234U/238U ratios in sediment pore water to determine low-temperature bulk and mineral dissolution rates of sediment over time-scales of ∼500,000 ky.
This chapter summarizes the systematics of U- and Th-series nuclides in “marine groundwaters (MGW).” In this chapter, the discussion is confined to hydrothermal systems and to the pore waters of deep-sea and nearshore/estuarine sediments. Radionuclide distributions have been extensively modeled in terrestrial groundwater systems and the same approach can be applied to MGW. The supply rate of U- and Th-series radionuclides to MGW comprises several processes: recoil associated with production from radioactive decay of a parent in the solid phase, in situ decay of a dissolved parent, and other reactions between the fluid and solid phases. The ratios of ²²⁸Ra/²²⁶Ra and ²¹⁰Pb/Pb in vent fluids have been used to constrain the residence time of the circulating hydrothermal fluid in the crust from the onset of high temperature seawater–basalt interaction to venting at the seafloor. Both low- and high-temperature hydrothermal fluids are greatly enriched in ²²²Rn over ambient seawater. Temporal variability of the radon content has been used to gain insight into sub-sea floor processes. The marine sediments serve as both important sources and sinks for U- and Th-series radionuclides. The flux of ²²⁶Ra and ²²⁸Ra out of sediments constitutes the dominant source of these radionuclides to the oceans, and sediments are locally important as sources for ²²²Rn and the short-lived Ra isotopes ²²³Ra and ²²⁴Ra. Marine sediments also constitute an important sink for U supplied to the oceans. There have been relatively few measurements of the activities of thorium isotopes in sediment pore waters. The presence of elevated ²³⁴Th only in the upper few centimeters of the sediments suggests that it is instead related to the presence of excess ²³⁴Th scavenged from the overlying water column.
Rates of silicate dissolution in deep-sea sediment: In situ measurement using 234U/238U of pore fluids
2004, Geochimica et Cosmochimica Acta
Citation Excerpt :
Rocks and minerals maintain the equilibrium activity ratio of unity unless they have been disturbed recently by physical or chemical processes such as comminution or leaching; thus unlike other radiogenic isotope systems used in geochemistry, rocks are nearly uniform in 234U/238U. Fine-grained solids, however, do not quantitatively retain intermediate decay products of uranium because of α-recoil effects (Bonotto and Andrews, 1993; Fleischer, 1980, 1982a, 1988; Kigoshi, 1971; Kronfeld et al., 1975; Ku, 1965). The α-recoil effect causes the 234U/238U ratio of fine-grained solids (<∼250 μm in diameter) to depart from the equilibrium value resulting in excess 234U in the interstitial fluid phase.
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 series isotopes in the Avon Valley, Nova Scotia
2004, Journal of Environmental Radioactivity
An U-series isotopic study was carried out in the waters of the Avon Valley, Nova Scotia. The fresh and acidic recharge waters flow rapidly through the watershed composed of a granitic highland and a sedimentary, largely carbonate, lowland plain, before draining to the sea. There is no significant anthropogenic pollution; but, naturally elevated U levels can be encountered within the bedrock. Nonetheless, the U concentrations of the surface and groundwater are low (generally within the range of several hundredths to several tenths of a μg l⁻¹), except in the proximity to weathering of U mineralization. The dissolved U in the surface waters appears to be stabilized by organic rather than inorganic complexes. Both the groundwaters and surface waters have similar ²³⁴U/²³⁸U activity ratios that rarely deviate from secular equilibrium by more than 20% throughout the watershed. The magnitude of the ²³⁴U/²³⁸U activity ratio is not determined by lithology but rather by the weathering mechanism, the high rate of flushing, and the leaching of local U mineralization. Dissolved Ra is consistently absent. The dissolved Rn concentrations, though variable, are measurable even in surface waters. This may be due to a continual degassing from the U-enriched bedrock or release from local sites of U mineralization underlying the surface water sources.

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¹: Present Address: Department of Geophysics and Planetary Sciences, Tel-Aviv University.

²: Permanent Address: Max-Planck-Institut für Kernphysik, Heidelberg.

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Earth and Planetary Science Letters

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References (11)

²³⁴U/²³⁸U disequilibrium as an aid to hydrologic study of the Floridan aquifer

J. Hydrol.

Hydrologic investigations of the groundwaters of central Texas using²³⁴U/²³⁸U disequilibrium

J. Hydrol.

Mixing volume calculations, sources and aging trends of Floridan aquifer water by uranium isotope methods

Geochim. Cosmochim. Acta

Uranium disequilibrium in groundwater: an isotope dilution approach in hydrologic investigations

Science

Uranium-234 and uranium-238 in the waters and bottom sediments of Lake Balkah and the age of the lake

Geochem. Int.

Cited by (38)

Use of U-isotopes in exploring groundwater flow and inter-aquifer leakage in the south-western margin of the Great Artesian Basin and Arckaringa Basin, central Australia

Processes controlling <sup>234</sup>U and <sup>238</sup>U isotope fractionation and helium in the groundwater of the St. Lawrence Lowlands, Quebec: The potential role of natural rock fracturing

Uranium isotopes in groundwater from the continental intercalaire aquifer in Algerian Tunisian Sahara (Northern Africa)

Chapter 10 Uranium- and Thorium-Series Radionuclides in Marine Groundwaters

Rates of silicate dissolution in deep-sea sediment: In situ measurement using <sup>234</sup>U/<sup>238</sup>U of pore fluids

Uranium series isotopes in the Avon Valley, Nova Scotia