Abstract
We present in this study the first data on Ra isotopes, 222 Rn, and 210 Pb 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 226 Ra), were measured by gamma spectrometry using the method developed in our previous work. Our data reveal the coherent behavior of 226 Ra in Lez spring waters, well correlated to other major and trace elements, like Cl, Na, K, Mg, Li, Rb and Ba.
The ( 228 Ra/ 226 Ra) ratios are distinct between the Lez spring (0.63 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 ( 228 Ra/ 226 Ra) 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.
Rn activities in the Lez spring are relatively low (0.5 to 3.7 Bq/L) compared to other karst regions.
The highest 222 Rn activities are found in diluted waters following heavy rainfall episodes, suggesting a subsurface origin of Rn. Soil and alterite samples that have much higher 226 Ra 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, 222 Rn dissolved in water decays to 210 Pb, and our data show that constraints on transfer times of underground waters can potentially be obtained from their coupled measurement.