The 234U//238U activity ratio in river water in southern Africa is generally ... For all of Africa, ..... 1 w.s. Moore, Amazon and Mississippi river concentrations.
Earth and Planetary Science Letters, 105 (1991) 191-195 Elsevier Science Publishers B.V., Amsterdam
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Uranium isotopes in surface waters from southern Africa J. Kronfeld
a
and J.C. Vogel b
a Department of Geophysics and Planetary Sciences, Tel A oW University, Ramat A viv, Tel A viv 69978, Israel b Quaternary Dating Laboratory, EMA-CSIR, P.O.Box 395, Pretoria, 0001, South Africa Received JuLy 14, 1989; revision accepted March 18, 1991
ABSTRACT
The 234U//238Uactivity ratio in river water in southern Africa is generally higher than that reported for rivers in other regions of the world. This is interpreted as due to the prevailing environmental conditions: in this warm dry region mechanical weathering predominates over chemical weathering, causing the isotope activity ratio of leached uranium to be, on average, 2.03 _+ 0.42 as compared to a ratio of 1.20 for river water in the more humid tropical and temperate regions. The isotopic composition of leachable uranium from river sediment is similar to that in the water. Rivers draining the Witwatersrand gold and uranium mining area clearly show pollution inputs characterised by high uranium content and low activity ratios.
1. Introduction Knowledge of the uranium content and the degree of 234U/238U disequilibrium in river water is not only useful in furthering our understanding of the mobilization of uranium during weathering, but also for providing information on the uranium isotope mass balances of the oceans. The number of rivers hitherto analyzed for both their uranium content and their 234U/238U ratio is small. Available data come mainly from the rivers of the United States [1-3], western Europe [4-5], Russia [6], India [7-9] and the Amazon [1]. For all of Africa, uranium isotopic data are available only for the Congo River [10]. The rivers that have hitherto been studied are mainly the larger ones, representing runoff from humid temperate or tropical environments. The present study was carried out to gain information about the isotopic composition of river waters and sediments in a typical semiarid region not unduly affected by contamination with phosphate fertilizers.
2. Sampling and preparation Water and associated riverbed sediment was collected from a number of the larger rivers of southern Africa as well as from some smaller tributaries (Fig. 1). The water samples were col0012-821X/91/$03.50
© 1991 - Elsevier Science Publishers B.V.
lected in 6- to 20-1itre plastic containers and shipped to the laboratory where they were filtered, acidified and spiked w i t h 232U/228Th. Uranium was c o n c e n t r a t e d by co-precipitation with Fe(OH)3, and purified by ion exchange and solvent extraction techniques prior to alpha-spectrometric analysis. Deciding on the method of chemical treatment of the river sediments was more difficult due to the variability in the grain size and sediment composition in the different rivers. Uranium in stream sediments may be present in essentially two different fractions: (1) within the crystal lattice of the grains, in which case the 234U/238U activity ratio is expected to be near equilibrium, at least in unweathered minerals; and (2) absorbed in ion-exchange positions onto clay particles, retained by ion and aluminium oxide scavenger coatings, or associated with organic matter. In this fraction the 234U/238U activity ratio may approximate closely the riverine values from which the uranium is basically derived. It was decided to use an acid leach technique described by Goldberg and Koide [11] in order to preferentially sample the more exchangeable uranium without removing the structurally bound uranium. To ensure a greater degree of uniformity among samples, the sediments were sieved and the less than 152 micron fraction was used. After leaching with 6 N HC1 a
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232U/228Th spike was added and the solution analysed in the same way as the water samples. The thorium isotopes, though not directly relevant to the present study, were also analysed where possible.
activity ratios (av. 1.19), similar to those obtained for the other great tropical rivers of the world. The second group comprises the rivers of South Africa excluding those draining the southern slopes of the Witwatersrand. These rivers collectively have a distinctly high 2 3 4 U / 2 3 8 U activity ratio. The weighted arithmetic average is 2.03 _+ 0.42. This figure excludes the anomalous Blyde River sample which exhibits extreme radioactive disequilibrium (234U/23SU = 6.97). The sample resembles the groundwater in the Transvaal Dolomite system (unpubl. data) and obviously represents spring discharge from this source. The three rivers that drain the southwest coast of the subcontinent, i.e., the Olifants, Doring and Berg rivers, have very low uranium contents (av. 0.07/zg/1) and as such
3. Results
The results of the uranium isotope analyses of both the water and associated sediment samples are listed in Table 1 and presented graphically in Fig. 2. The data can be divided into two groups. Firstly the water samples from the more humid tropical regions in the north, viz the Zambezi River and Lake Malawi, have low uranium concentrations (av. 0.16 /~g/1) and low 234U/238U
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