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  • Author or Editor: F. Fernex x
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Abstract  

When a river flow rate is known on one point, it is possible to determine the one of upstream tributaries. This can be done by measuring the concentrations of a dissolved element (for example Ca2+), provided that no chemical precipitations have occurred in mixture waters. In this later case, the determination of flow rate is however possible by isotopic geochemistry, by knowing isotopic ratios of an element, for ex. 234U/238U, in each river considered and the ratio of the concentrations in dissolved 238U in two of them. A study of the Var and its tributary the Vésubie is performed: in January 31st, 2001, the flow rate D3, downstream the confluence was 35 m3/s. The ratio of the 238U concentrations in the Vésubie river and in the Var upstream (238U1/238U2) was 5.5. The isotopic ratios 234U/238U measured in the Var, downstream the confluence were R3 = 1.51, upstream R2 = 1.75, and in the Vésubie R1 = 1.08. We have calculated the Vésubie flow rate as D1 = 3.2 m3/s.

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Abstract  

Four cores were collected in weathered rocks and soils in the Boréon forest area (1765 m, Mercantour Massif, France). The samples were analyzed for the isotopes 230Th, 232Th, 234U and 238U. The activity and isotopic ratio profiles suggest that uranium was mobilized (leaching and precipitation) during the weathering process, as well as thorium but in a much less proportion. A model was drawn up to evaluate the U leaching rate and the time that some levels of the weathered rocks have been subjected to weathering. It utilizes LATHAM and SCHWARCZ’s two equations,15 expressed as 234U/238U and 230Th/238U activity ratios, which assume that the alpha recoil effect allows easier leaching for 234U than 238U and no Th mobility. But this last assumption does not correspond to the observations made in the Boréon area, since it appears that in some soil deeper layers 230Th and 228Th are in radioactive deficit relatively to their parents. As there are four unknown quantities (the time, the leaching rates of 238U, 234U, 230Th), the problem to be solved requires two more equations; these can be obtained utilizing the U activity ratio in water, and taking into account the 232Th behavior. In some sites the 238U leaching rate is high in deeper soil levels (near the fresh rocks); this would correspond to a loss of half the U amount in less than 24 000 years.

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