Implication of the CFCs and 4He Dating Approaches for Characterizing Water Circulation Flow in the Strengbach Mountain Catchment (Vosges Mountain, France)
Résumé
Characterizing water dynamics in the critical zone (CZO), i.e. the diversity of water pathways and residence times within the watershed is an issue of high scientific and societal relevance, especially in the sustainability management of the freshwater resources of the planet. In this study, it is proposed to highlight the interest of combining geochemical and isotopic tracing approaches (Sr, U) with CFC and 4He dating methods to characterize the diversity of water circulation patterns in the Strengbach watershed. For this purpose, water samples from springs, piezometers (15m deep) and boreholes (down to a 120m depth) were regularly collected at different hydrological periods. These samples were analyzed in major and trace element concentrations, in 87Sr/86Sr isotope and (234U/238U) activity ratios, and in CFCs and 4He concentration.
The results point to a systematic difference between the geochemical and Sr and U isotopic signatures of “surface waters”, i.e. waters from the upper part of the boreholes, piezometers and springs and “deep waters”, i.e. waters collected in front of the fractures in the deeper parts of boreholes. Surface waters are characterized by lower TDS concentrations than deep waters, as well as lower pH, alkalinity and conductivity. This distinction is also observed with the CFCs and 4He concentrations. Surface waters are marked by high CFCs concentrations, which indicates recharge of a few months to a few years, depending on the hydrological conditions. The CFCs concentrations in deep waters suggest water mixings between recent waters with characteristics closed to those of spring waters, and older waters, older than 70 years which is the maximum age given by the CFCs method. Concentrations of 4He in these deep waters suggest the occurrence of a potentially very old water end-member (several thousand years). All together these data confirm the occurrence of two contrasted water circulation pathways within the Strengbach watershed: i) a subsurface water pathway in the highly altered regolith, with short transit time of a few years or less, and ii) a deep water circulation in fracture networks of the bedrock with much slower transit time, and probably the mobilization of very old waters.