U isotope ratios as tracers of groundwater inputs into surface waters: Example of the Upper Rhine hydrosystem

Major elements, trace elements and Sr and U isotopes were analyzed in the dissolved load of water samples collected in the main reservoirs of the Rhine graben, i.e. the Rhine River, its main tributaries and the shallow and deep ground waters in several different aquifers. Our results confirm that the variations of major elements, trace elements (especially Sr and Rb) and Sr isotope ratios of the water samples are mainly correlated to the lithological variations of the watershed. The atmospheric inputs can be considered as a minor cause for the chemical variability of the surface waters in the Rhine hydrosystem, except for a few chemical elements in the mountain streams. Human activity, in particular the pollution by the exploitation of the Alsace potash mines, mainly affects the Na and Cl concentrations of the Rhine and the south alluvial groundwaters but remains negligible for the other water samples. The specific study of the Lauter stream located in the lower Triassic sandstones (Buntsandstein sequences) demonstrates the importance of groundwater influences onto the chemical composition of river waters. Up to 50% of the total dissolved load (TDS) of this stream is supplied by deep waters; this contribution significantly modifies the chemical composition of the stream, especially the 238U-234U radioactive disequilibrium, which, in turn, can be used as a specific tracer of deep water inputs into surface waters and could be useful to distinguish in the dissolved load of rivers, surface weathering fluxes from deeper ones.