Zwahlen François

Détermination des paramètres hydrauliques et hydrochimiques d'un aquifère alluvial dans une vallée alpine (Valais, Suisse)

Project Number: CH-4398
Project Type: Dissertation
Project Duration: 03/01/1997 - 09/30/2000 project completed
Funding Source: other ,
Project Leader: Prof. em. François Zwahlen
Head of institute
Hydrogéologie générale
Centre d'Hydrogéologie et de Géothermie
Université de Neuchâtel
Rue Emile-Argand 11
2000 Neuchâtel
Phone: +41 (0) 32 718 26 89 ; +41 (0) 32 718 26 00
FAX: +41 (0) 32 718 26 03
e-Mail: francois.zwahlen(at)unine.ch
http://www.unine.ch/chyn

related to this project.
for which the project has a relevance.


Research Areas:
Landscape

Disciplines:
environmental sciences
hydrology, limnology, glaciology

Keywords:
Hydrogéochimie, aquifère poreux, hydrogéologie, méthodes de prospection, sédiments quaternaires, eau sulfatée, relation eau souterraine – eau de surface, mélange d’eau, guide pratique, vallée du Rhône, Suisse

Abstract:
In the Rhône valley, upstream the town of Sierre (central Wallis), the Bois de Finges area is composed of various Quaternary geological environments: the Rhône alluvial sediments, the Illgraben alluvial fan deposits, the Sierre rockslide deposits and the debris deposits of the south valley side. The aquifer of the Rhône alluvial sediments is generally characterized by a hydraulic conductivity larger than 10-3 m/s corresponding to sandy gravel deposits with an important flow velocity of about 20 m/d and an average hydraulic gradient of 3.5 ‰. During high waters period, taking place between June and August, the Rhône River strongly recharges this aquifer with a weakly mineralized water of the type Ca, Mg-HCO3, SO4. Another component of the type Ca, Mg-SO4 is flowing from evaporitic rocks of the south valley side into the Rhône alluvial sediments. On the other hand, during low waters period (November till April), the aquifer of the Rhône alluvium is exclusively recharged by SO4-rich water from the south valley side, because the Rhône River bed is almost dry in winter. The Sierre rockslide deposits are essentially composed of sandy and silty Helvetic carbonates originating from the north valley side, showing a hydraulic conductivity between < 10-5 and 10-3 m/s. The chemistry of their groundwaters is characterized by an important spatial heterogeneity. Each water type represents more or less a local fluid quality, as well as physical and chemical processes depending on rock type and transit time.
The different techniques used, such as piezometric water levels, flow gauging, hydrogeological and geochemical modelling, three dimensional sampling in space and time, isotopes and physical and chemical borehole logging, allowed to determine the hydraulic and hydrochemical parameters of the complex porous aquifer of the Bois de Finges. They also allowed to set up the processes, which are controlled by the flow of the different groundwater components. A drilling campaign offered the opportunity to realize a network of five boreholes and to carry out rock sampling, pumping and tracer tests.
Each used technique measures one or several parameters in a well-defined environment of the aquifer. The piezometric water levels provide information about time and space distribution of flow lines, hydraulic gradients as well as recharge and discharge zones. On the other hand, pumping and tracer tests determine hydraulic conductivity, transport time of the tracers (uranine, bacteriophages), flow velocity and effective porosity. The origin of waters (precipitation, surface water and groundwater) was determined with the help of stable isotopes (oxygen-18, deuterium), whereas the transit time in the aquifer was estimated from tritium and radon concentrations. Applying different techniques simultaneously increased significantly their respective capacity to understand linked hydrochemical and hydraulic processes in a complex aquifer, and further gave the possibility to quantify and to compare them.
In collaboration with R. Monnet of the Geophysical Institute of the University of Lausanne, instructions for diagnosis and intervention on a porous aquifer were developed. These instructions represent a practical guidebook in the following cases: water prospecting, evaluation of water quality and origin, determination of hydraulic properties of an aquifer and, finally, determination of protection zones and risks of contamination of a porous aquifer. This guide explains the techniques and the application domain for each method, in order to allow the person in charge of such studies to choose the most efficient methods to reach the goals selected, as well as to spare time and money.

Leading questions:
  • Caractérisation de l'aquifère alluvial du secteur du Bois de Finges.

  • Détermination de la conductivité hydraulique, de la porosité et des débits qui transitent dans plusieurs sections de la vallée.

  • Évaluation des différents apports d'eau à la nappe et leurs mélanges respectifs (nappe en amont, Rhône, versant sud, galerie d'amenée Finges - Chippis, canaux, précipitations).

  • Quantification des paramètres du transport de matière dissoute dans l'eau (advection, dispersion, diffusion, retardement).

  • Comparaison des résultats des différentes méthodes utilisées (méthodes hydrogéologiques, hydrochimiques et géophysiques).

  • Élaboration d'une méthodologie de diagnostic et d'intervention sur un aquifère poreux, notamment pour une prospection d'eau, ainsi que pour déterminer les propriétés d'un aquifère graveleux, pour évaluer la qualité de l'eau et finalement pour délimiter les zones de protection.

    Publications:
    Schürch Marc. 2000. Détermination des paramètres hydrauliques et hydrochimiques d’un aquifère alluvial dans une vallée alpine (Valais, Suisse). Thèse de doctorat, Université de Neuchâtel, Institut de géologie.
    pdf Dissertation


    Last update: 7/18/17
    Source of data: ProClim- Research InfoSystem (1993-2024)
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