Interpretation of 234U238U activity ratios in groundwaters

doi:10.1016/0168-9622(85)90028-4

Chemical Geology: Isotope Geoscience section

Volume 58, Issues 1–2, 20 December 1985, Pages 83-88

Chemical Geology: Is...

https://doi.org/10.1016/0168-9622(85)90028-4 Get rights and content

Abstract

The quasi-steady variation in $^{234} U^{238} U$ activity ratio has been determined along an idealised homogeneous one-dimensional aquifer in which the transfer of radionuclides from the aquifer material to solution occurs by both α-recoil and ion exchange. The results highlight the importance of considering these two effects in predicting the variation of ²³⁴ U/U activity ratios in a flowing system. In particular, it was shown that such ratios in old waters should achieve the moderate values indicated by measurements.

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Cited by (15)

Uranium isotopes as a tracer of sources of dissolved solutes in the Han River, South Korea
2009, Chemical Geology
Citation Excerpt :
In this situation, the inputs from bedrock and deep horizons of the weathering profiles, as well as from superficial soil horizons, have significant effects on the 234U/238U activity ratios of river water. Generally, an enhanced U activity ratio in groundwater can arise from the α-recoil process, which would require either very large U concentrations at the water–rock interface or very long residence times, or it can arise from a steady-state chemical etch/leach process within relatively short timescales (Dickson and Davidson, 1985; Bonotto and Andrews, 1993, 2000; Luo et al., 2000; Durand et al., 2005). To investigate possible impacts on the 234U/238U activity ratio in the Han River system, we collected samples from springs, drainage from an abandoned coalmine, and groundwater from the Han River basin (Table 2).
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.
Uranium series disequilibria in ground waters from a fractured bedrock aquifer (Morungaba Granitoids-Southern Brazil): Implications to the hydrochemical behavior of dissolved U and Ra
2008, Applied Radiation and Isotopes
Activity concentrations of dissolved ²³⁴U, ²³⁸U, ²²⁶Ra and ²²⁸Ra were determined in ground waters from two deep wells drilled in Morungaba Granitoids (Southern Brazil). Sampling was done monthly for little longer than 1 year. Significant disequilibrium between ²³⁸U, ²³⁴U and ²²⁶Ra were observed in all samples. The variation of ²³⁸U and ²³⁴U activity concentrations and ²³⁴U/²³⁸U activity ratios is related to seasonal changes. Although the distance between the two wells is short (about 900 m), systematic differences of activity concentrations of U isotopes, as well as of ²³⁴U/²³⁸U, ²²⁶Ra/²³⁴U and ²²⁸Ra/²²⁶Ra activity ratios were noticed, indicating distinct host rock–water interactions. Slightly acidic ground water percolation through heterogeneous host rock, associated with different recharge processes, may explain uranium and radium isotope behavior.
The transport of U- and Th-series nuclides in sandy confined aquifers
2003, Geochimica et Cosmochimica Acta
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.
Natural radioactive nuclides in some Tunisian thermo-mineral springs
2002, Journal of Environmental Radioactivity
Radioactivity in some Tunisian thermo-mineral springs (11 hot mineral springs and one cold spring) has been determined for the first time in Tunisia using radiochemical separation procedures. The obtained results show that $^{238} U$ activity concentrations vary between 1.5 and about 43 mBq/l. The measured activities of $^{234} U$ and $^{226} Ra$ range from 1.1 to about 82.2 mBq/l and 34–3900 mBq/l, respectively. The radioactive disequilibria in these waters are in excess of concentration of $^{234} U$ compared to that of $^{238} U$ . The $^{226} Ra$ / $^{234} U$ activity ratios are high and in the range of 9.0–691.0. U, Th and Ra activities are similar to those published for other non-polluted regions of the world. Radioactivity in the only cold mineral water from Aı̈n Oktor is very low, and thus health hazards due to the consumption of this water are not expected.
Determination and comparison of uranium and radium isotopes activities and activity ratios in samples from some natural water sources in Morocco
2001, Journal of Environmental Radioactivity
Radiochemical results (²³⁸U, ²²⁶Ra and ²²⁸Ra activities; ²³⁴U/²³⁸U, ²²⁸Ra/²²⁶Ra and ²²⁶Ra/²³⁸U activity ratios) are reported for 42 natural water samples collected from wells, hot mineral springs, rivers, tap water, lakes and irrigation water in 15 Moroccan locations. Results show that ²³⁸U activity varies between 4.5 and about 309 mBq l⁻¹ in wells, 0.6 and 8.5 mBq l⁻¹ in hot springs, 9.7 and 28 mBq l⁻¹ in rivers, 2.5 and 16 mBq l⁻¹ in tap waters and between 6 and 24 mBq l⁻¹ in lakes. The ²³⁴U/²³⁸U activity ratio varies in the range 0.87–3.35 in all analyzed water samples except for hot springs where it reaches values higher than 7. Unlike well water, mineral water samples present low ²³⁸U activities and high ²³⁴U/²³⁸U activity ratios and ²²⁶Ra activities. The highest activity of radium in mineral water is 150 times higher than the highest activity of ²²⁶Ra found in well water. ²²⁶Ra/²³⁸U activity ratios are in the ranges 0.07–1.14 in wells, 0.04–0.38 in rivers, 0.04–2.48 in lakes, and 1.79–2115 in springs. The calculated equivalent doses to all the measured activities are inferior to the maximum contaminant levels recommended by the International Commission of Radioprotection and they do not present any risk for public health in Morocco.
In-situ radionuclide transport and preferential groundwater flows at INEEL (Idaho): Decay-series disequilibrium studies
2000, Geochimica et Cosmochimica Acta
Uranium and thorium-decay series disequilibria in groundwater occur as a result of water-rock interactions, and they provide site-specific, natural analog information for assessment of in-situ, long-term migration of radionuclides in the far field of a nuclear waste disposal site. In this study, a mass balance model was used to relate the decay-series radionuclide distributions among solution, sorbed and solid phases in an aquifer system to processes of water transport, sorption-desorption, dissolution-precipitation, radioactive ingrowth-decay, and α recoil. Isotopes of U (²³⁸U, ²³⁴U), Th (²³²Th, ²³⁰Th, ²²⁸Th, ²³⁴Th), Ra (²²⁶Ra, ²²⁸Ra, ²²⁴Ra), and Rn (²²²Rn) were measured in 23 groundwater samples collected from a basaltic aquifer at the Idaho National Engineering and Environmental Laboratory (INEEL), Idaho. The results show that groundwater activities of Th and Ra isotopes are 2–4 orders lower than those of their U progenitors which average 1.35 ± 0.40 dpm ²³⁸U/L, with ²³⁴U/²³⁸U ratios of ∼1.6–3.0. ²²²Rn activities range from 20 to 500 dpm/L. Modeling of the observed disequilibria places the following constraints on the time scale of radionuclide migration and water-rock interaction at INEEL: (1) Time for sorption is minutes for Ra and Th; time for desorption is days for Ra and years for Th; and time for precipitation is days for Th, years for Ra, and centuries for U. (2) Retardation factors due to sorption average >10⁶ for ²³²Th, ∼10⁴ for ²²⁶Ra, and ∼10³ for ²³⁸U. (3) Dissolution rates of rocks are ∼70 to 800 mg/L/y. (4) Ages of groundwater range from <10 to 100 years. Contours of groundwater age, as well as spatial patterns of radionuclide disequilibria, delineate two north-south preferential flow pathways and two stagnated locales. Relatively high rates of dissolution and precipitation and α-recoil of ²²²Rn occur near the groundwater recharging sites as well as in the major flow pathways. Decay of the sorbed parent radionuclides (e.g., ²²⁶Ra and ²²⁸Ra) on micro-fracture surfaces constitutes an important source of their daughter (²²²Rn and ²²⁸Th) activities in groundwater.

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Research articleInterpretation of 234U238U activity ratios in groundwaters

Abstract

Earth Planet. Sci. Lett.

Isot. Geosci.

Geochim. Cosmochim. Acta

J. Hydrol.

J. Geochem. Explor.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

J. Hydrol.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Radionuclide migration around uranium ore bodies — analogue of radioactive waste repositories

U.S. Nucl. Reg. Comm. Contract NRC-04-81-172, 2nd Annu. Rep., AAEC (Aust. At. Energy Comm.), Rep. C40

Research article
Interpretation of $^{234} U^{238} U$ activity ratios in groundwaters