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  • Publication
    Métadonnées seulement
    Geochemical indicators to determine relations between spring behaviour and water losses of a divergence tunnel
    Schurch, Marc
    Hydraulic and geochemical relations between a subthermal, SO4-rich spring (Q6) and water losses of a divergence tunnel indicate that both features are hydraulically connected. The divergence tunnel, crossing the evaporite rocks of the Penninic Pontis-Nappe (anhydrite, gypsum, dolomitic limestones) in the Rhone River valley (Central Wallis, Switzerland), deviates water from the Rhone River at a rate of up to 62 m(3)/s. The recharge rate of weakly mineralized water exfiltrating from the divergence tunnel and flowing to the Q6 spring varied between 0.5 and 5 l/s, depending on seasonal variations of the discharge in the divergence tunnel. Methods used to determine the degree of connection included discharge measurements, continuous and monthly water quality measurements, and geochemical modelling. Hydrochemical indicators of particular significance include major inorganic ions, as well as temperature, pH, calcite and gypsum saturation indices. Following the divergence tunnel decommissioning in August 17 1998, simultaneous rises in spring temperature from 13.0 to 13.7 degreesC and electrical conductivity from 2430 to 2600 muS/cm were recorded. A decline in spring discharge from 10 to 6.7 l/s was also observed.
  • Publication
    Accès libre
    Use of Cl/Br Ratio to Decipher the Origin of Dissolved Mineral Components in Deep Fluids from the Alps Range and Neighbouring Areas
    Sonney, Romain
    Cattin, Stéphane
    Cl/Br ratios were studied in deep groundwaters to decipher the origin of dissolved mineral components from the Alps and neighbouring areas. Cl/Br molar ratio represents a good marker to define if the salinity comes from seawater or residual brines (655 and lower) or from dissolution of halite or halite-rich gypsum, often present in the Triassic formations (upper than the seawater ratio). It can be an interesting tool for projects dealing with exploration and production of geothermal fluids. Results of this study showed the presence of trapped seawater in formations of the large basins from the Quaternary to the basement, resulting from infiltration during different marine intrusion periods. This method also showed the presence of brines in crystalline aquifers. Some of these waters discharge along subvertical faults and are diluted to various degrees by different types of meteoric waters. In some cases, this method raises new questions about the true geological origin of deep circulations.