Hydrochemistry and 87Sr/86Sr ratios as tracer for groundwater origin and discharge in crystalline basement

The construction of tunnels through mountain ranges requires a detailed understanding of the hydrogeological situation. Especially, the prognosis of groundwater discharge is of significance. Groundwater discharge is affected by the geological, tectonic and lithological conditions in the catchment area. Highly soluble rocks e.g. carbonate rocks may be intercalated locally in the crystalline sequences resulting in karst formation that may not be recognizable on the surface. Various tracer methods including conventional and isotope tracers are generally applied to analyze the water balance, characterize flow regimes, and determine groundwater residence times in catchments. Investigations of the hydrochemical composition of the water in combination with natural isotope tracers like the 87Sr/86Sr ratio of dissolved strontium can provide important information on the origin of groundwater, and identify flow paths and mixing processes. Here we present preliminary data from a tunnel project in Austria. The investigated area consists mainly of crystalline rocks with subordinate occurrences of carbonate rocks. To investigate the genesis of the water with focus on the influence of weathering carbonate rocks in the catchments, various springs were sampled and analyzed for major solute concentrations, environmental isotopes, and 87Sr/86Sr ratios. Based on saturation indices for calcite and 87Sr/86Sr ratios the influence of dissolving calcium carbonate and mixtures between crystalline and carbonate waters were identified. 87Sr/86Sr ratios vary from 0.7098 to 0.7200 with lowest values reflecting carbonate dissolution and highest values associated with the weathering of silicate minerals. 87Sr/86Sr ratios support the results from other hydrochemical parameters analyzed in this study.