Radium Isotopes as a Tracer of Water Sources and Mixing in the Vidourle Stream (South of France)

Abstract

This study reports analyses of Ra isotopes in a Mediterranean stream, the Vidourle river, whose upper course drains the granitic and metamorphic basement of the SE part of the French Massif Central (Cévennes) and then flows through the karstified carbonates of Jurassic and Cretaceous ages. In these low-Ra waters (²²⁶Ra activities range from 1.5 to 4.9?mBq/L), all four Ra isotopes were successfully analyzed through gamma spectrometry during a single analysis. ²²⁶Ra activities and (²²⁸Ra/²²⁶Ra) ratios are distinctly higher in waters draining the Variscan basement than in waters affected by dissolution of Mesozoic carbonates, in agreement with U contents and Th/U ratios of both rock types. This results in a general N-S decrease, which parallels the evolution of the ⁸⁷Sr/⁸⁶Sr ratios. (²²⁸Ra/²²⁶Ra) ratios reported vs 1/(²²⁶Ra) display linear relationships suggesting mixing of several water components related to the lithology. Ra might thus have a more conservative behavior than usually assumed, possibly because of the high water/rock ratio and flow rate in karst environment. Short-lived Ra isotopes (²²⁴Ra and ²²³Ra) are often in excess compared to their equilibrium values, due to their supply through alpha-recoil processes. ²²³Ra activities in a Vidourle tributary can be explained by mixing of two water components, with a negligible radioactive decay of ²²³Ra during underground water flow. The calculated minimum flow rates (40–60?m/h) are in agreement with those deduced from artificial tracer experiments.