Radium isotopes, radon and 210Pb in karstic waters: Example of the Lez system (South of France)

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