Éditeur(s) : HAL CCSD Elsevier
Résumé : Several nuclides of the ;238;U (;234;U, ;230;Th, ;226;Ra, ;210;Pb) and ;235;U (;231;Pa, ;227;Ac) decay series have been measured through gamma spectrometry in the U-bearing episyenites of the Bernardan mine (northwest Massif Central, France), in order to study recent mobilizations in the U-deposit and their time-scales. (;230;Th/;238;U) and (;231;Pa/;235;U) ratios show that U has been little affected during the last 350 ky, especially in the U-rich samples. In contrast, ;226;Ra was subjected to a general redistribution, with a clear tendency for Ra to be lost from the U-rich samples and added to U-poor samples. The highest Ra excesses are present in low-U samples, and are related to adsorption onto iron hydroxides. Ra appears to have been redistributed within the mineralised episyenitic bodies, which seem to remain in ;226;Ra–;238;U radioactive equilibrium, a result consistent with the earlier study of Leroy [1984. Episyénitisation dans le gisement d'uranium du Bernardan (Marche): Comparaison avec des gisements similaires du Nord-Ouest du Massif Central français. Miner. Depos. 19, 26–35]. As shown by the ;226;Ra and ;210;Pb data, the Ra redistribution was very active between 8 and 0.1 ky, and this might be a consequence of the hydrological changes in groundwater circulation accompanying the Holocene climate warming.
ISSN: 0009-2541

hal-00407600
https://hal.archives-ouvertes.fr/hal-00407600
DOI : 10.1016/j.chemgeo.2007.06.020

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present in this study the first data on Ra isotopes, 222Rn, and 210Pb in waters from the Lez karst system, one of the main ground-water resources in the South of France. The low Ra isotope activities of the Lez system waters (from 0.35 to 3.45 mBq/L of 226Ra) were measured by gamma spectrometry using the method developed in our previous work. Our data reveal the coherent behavior of 226Ra in Lez spring waters, well correlated to other major and trace elements, like Cl, Na, K, Mg, Li, Rb and Ba. The (228Ra/226Ra) ratios are distinct between the Lez spring (0.60 to 0.72) and two neighboring springs (0.52 to 0.65), showing the spatial variability in karst systems. In spite of their small variations in the Lez spring, it was possible to trace different water components and mixing processes from Lower-Cretaceous, Upper-Jurassic and Middle Jurassic end-members, in agreement with previous studies based on major and trace elements. Data obtained on waters derived from the Upper-Jurassic karst reveal that their (228Ra/226Ra) ratios are not related to the Th/U ratio of the Jurassic limestone, but are rather influenced by the soil and alterite cover of the recharge area.222Rn activities in the Lez spring are relatively low (0.5 to 3.7 Bq/L) compared to other karst regions. The highest 222Rn activities are found in diluted waters following heavy rainfall episodes, suggesting a subsurface origin of Rn. Soil and alterite samples that have much higher 226Ra activities (from 51 to 130 Bq/kg), compared to those from the main aquifer rocks (from 2.9 to 16 Bq/kg), are probably the main source of Rn, rather than the rocks of the karst itself. The study of several flood episodes reveals the correlation between Rn peak-values or total activity discharged at the spring with cumulative rainfall. Finally, 222Rn dissolved in water decays to 210Pb, and our data show that constraints on transfer times of underground waters can potentially be obtained from their coupled measurement.
ISSN: 0009-2541

hal-01622749
https://hal.archives-ouvertes.fr/hal-01622749
DOI : 10.1016/j.chemgeo.2017.06.022