Uranium and rare earth elements in CO2-rich waters from Vals-les-Bains (France)

Waters from springs at Vals-les-Bains result from the mixing of a CO2-rich, highly mineralized water with dilute, shallow subsurface water. The pH is controlled by CO2 outgassing, redox conditions are controlled by both the iron hydroxide-siderite buffer and the introduction of oxygen with shallow subsurface waters. Among the major dissolved elements, concentrations of Ca, Mg, Mn, Fe, are related to mixing, CO2 outgassing and carbonate precipitation. Uranium shows a complex behaviour controlled by carbonate complexing, redox conditions, mixing of waters and leaching from the rocks. The 234U/238 activity ratio is near secular equilibrium. In the more dilute waters, dissolved rare earth element (REE) patterns are almost flat with a slight negative Eu anomaly. In the concentrated waters, heavy rare earth elements (Gd-Yb, HREE) are strongly enriched relative to light rare earth elements (Ce-Eu, LREE). We relate the enrichment in HREE to water chemistry and to complexing with carbonate species. (author)