An improved method for Ra isotope (226Ra, 228Ra, 224Ra) measurements by gamma spectrometry in natural waters: application to CO2-rich thermal waters from the French Massif Central

An improved method was developed to measure 226Ra, 228Ra and 224Ra in freshwaters by gamma spectrometry. Radium was selectively extracted from acidified samples using specific filters (3M EMPORE™ Radium Rad disks). The latter was subsequently analysed by gamma spectrometry. Simultaneous and direct determination of the activities of the three isotopes was performed by comparison of gamma rays of the Radium Rad disks with those of a calibrated standard disk. This efficient and reliable method allowed a reduction of sample processing to a few hours. echnique was applied to analyse the Ra isotope compositions of several CO2-rich hydrothermal springs of the western border of the Limagne graben (French Massif Central). The studied springs emerge from a succession of granitic outcrops lined up along a major fault. Their chemical compositions evolve from calcic and magnesian chloro-bicarbonated to sodic bicarbonated. All the springs display high Ra activities, probably linked to high CO2 content and/or high cation content of these waters, with various Ra isotope ratios. 226Ra activity ranges from 588 to 2287 mBq/l and 228Ra activity from 260 to 1590 mBq/l, whereas 224Ra displays an activity between 245 and 1808 mBq/l. Four of the six analysed springs have (228Ra/226Ra) activity ratios lower than 0.7, thus, significantly lower than the ratio expected from an interaction with a calc-alkaline granitoid (typically having (232Th/238U) activity ratio between 1 and 2). Low (228Ra/226Ra) ratio (0.27) of the northern water (Montpensier) suggests the existence in this area of a zone of U concentration, possibly resulting from U mobilization and accumulation induced by previous hydrothermal events. The (224Ra/228Ra) ratios display smaller variations. They suggest short transit times from the zone of Ra leaching to the surface (a few days) or a very shallow addition of 224Ra (e.g., from a localised zone where 228Th could be preferentially adsorbed on the mineral surfaces). In some cases, these ratios might be used to infer differences in transit times of waters between neighboring springs.