Hydrogeochemical modeling of the spatiotemporal variations in 87Sr/86Sr isotope ratios and Sr concentrations of spring waters in a headwater catchment (Strengbach CZO–France)

Abstract

The variations in strontium (Sr) isotope ratios in river waters at a regional scale are usually discussed in terms of lithological diversity constituting the watershed bedrock. To discuss the origin of the spatiotemporal variations in Sr concentrations and isotopic compositions of spring waters at the headwater watershed scale, we analyzed the 87Sr/86Sr and 143Nd/144Nd isotopic ratios of the main constitutive minerals of the granitic bedrock of the Strengbach Critical Zone Observatory (CZO) and developed coupled hydrogeochemical modeling integrating the Sr concentration and isotopic composition. The simulation results show that the long-term (1990–2010) decrease in Sr concentrations in the CS springs of the Strengbach catchment can be explained by the response of mineral weathering along the water pathway to the pH increase that affected the incoming solutions to the weathering system during the same period. This pH increase reduces the water path proportion marked by a high flux of apatite and biotite dissolution. Thus, it reduces the Sr concentration of the outgoing spring waters. The different responses of the various CS springs to the incoming solutions' pH increase can be explained by the more or less long residence time of the water in the bedrock for the different springs. The analytical data also confirm that the 87Sr/86Sr isotope ratios of the CS springs varied little or not at all on average from 1990 to 2010. In contrast, they varied significantly during a hydrological cycle and from one spring to another. The constancy of the average 87Sr/86Sr isotope ratios over several years is explained by a granitic bedrock water path sufficiently long for soil solution pH variations to no longer influence the emergence of the spring. The simulation results also show that the isotopic variations observed in the CS springs during a hydrological cycle can be explained by differences in the transit time of the water in the substratum between high and low waters and the modifications that they entail on the degree of saturation of the water with respect to biotite and apatite. These differences do not need to be explained by differences in the mineralogy of the rocks drained by waters between high and low discharge periods. Similarly, the different average isotope ratios from one spring to another can be explained by the more or less long transit times in the bedrock of the waters feeding these different springs. Again, it is not necessary to explain them by local lithological differences from one spring to another. These different results highlight that at the scale of an upstream basin such as the Strengbach, the Sr isotopic variations observed in the CS springs should not systematically indicate lithological differences in the rock. The development of coupled isotope modeling approaches should help to disentangle the part of this process relating to hydrological processes in the isotopic variability of the spring waters.