Characterization of groundwater circulations in a headwater catchment from an analysis of chemical concentrations, Sr-Nd-U isotope ratios, and CFC, SF6 gas tracers (Strengbach CZO, France)

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In order to characterize the chemical composition and the age of the water circulating in the critical zone of the Strengbach catchment (Vosges mountains, France), water samples from springs, 10–15 m deep piezometers and 50–120 m deep boreholes were collected and analyzed in elementary concentrations, in Sr, Nd, and U isotopic ratios, and in chlorofluorocarbon (CFC) and sulfur hexafluoride (SF6) concentrations. The results evidence a clear distinction between surface water (<≈10–15 m) and deeper water. The latter has much higher conductivity and cationic loads and is marked by lower Sr isotopic ratios and higher U activity ratios. Such a water typology suggests that the spring and piezometer water flows within the same shallow subsurface aquifer while deep borehole water belongs to a different circulation system, which flow path is controlled by the bedrock fracture network. The CFC data show that these two circulation systems are marked by contrasted residence times with a short residence time for the surface water and a longer residence time (water ages > 50 years) for the deep water. These results confirm different circulation histories for surface and deep water in the Strengbach catchment. They also suggest that the higher degree of chemical saturation of the deep water compared to the surface water is caused more by longer water residence times in the deep circulation systems than by differences in the primary minerals involved in each of the water-rock interaction systems. Our results also point that in the Strengbach granitic catchment, the SF6 concentrations cannot be used for water dating due to their lithogenic production in granitic bedrocks.