(234U/238U) activity ratios in freshwaters as tracers of hydrological processes:: The Strengbach watershed (Vosges, France)

(U-234/U-238) and Sr-87/Sr-86 isotopic ratios, as well as major and trace (U, Ba, Sr, Rb) element concentrations were analyzed in dissolved loads of the Strengbach stream (Vosges, France) in order to constrain the sources of U isotopes (U-234-U-238) to river waters. The variations of the (U-234/U-238) activity ratios along the stream indicate a clear dependence between the (U-234/U-238) activity ratios of the surface waters and the different types of rocks forming the watershed, with near equilibrium values for the waters draining granites and high (U-234/U-238) disequilibria (1.4) for those flowing over carbonates. The high (U-234/U-238) disequilibria are related to a supply of U-234-enriched groundwaters located within the carbonate rocks. The (U-234/U-238) activity ratios of the waters at the outlet of the catchment collecting the sources of the stream on the granitic lithology, decrease from 1.02 to 0.96 when the discharge of the stream increases. Such a tendency requires mixing between a water body enriched in U-234 which weathered the granitic bed rock at secular equilibrium, and a water with a (U-234/U-238) activity ratio below unity representing a mobilization of U from material that has already been weathered. Comparison of the geochemical characteristics of waters collected during the year and those collected during a flood event, reveals the involvement of two different "weathered" end-members, depending on the hydrological conditions: during the year, the dissolved U transported by the river originates from bed-rock and deep horizons of the weathering profile, whereas a significant part of U, during the flood event, is supplied by superficial horizons of soils, probably complexed by organic colloids. These results outline the potential of (U-234/U-238) activity ratios to investigate hydrological processes and emphasize that their use as tracers of rock weathering should be made cautiously. Copyright (C) 1999 Elsevier Science Ltd.