Hoehn Eduard

Untersuchungen im Pfynwald (Wallis) : Zu Wechselwirkungen zwischen Rhône und Grundwasser mittels Tracern und Modellrechnungen 
Interactions between River Rhône and ground water of the Valais valley aquifer, Pfynwald, ct. Wallis/Valais

Project Number: CH-4847

Project Type:	Master
Project Duration:	01/01/2002 - 12/31/2004 project completed
Funding Source:	other ,
Leading Institution:	EAWAG Dübendorf
Project Leader:	Dr. Eduard Hoehn emeritus EAWAG Buchserstr. 44 8157 Dielsdorf Phone: ; +41 (0) 79 509 8032

related to this project.

for which the project has a relevance.

Research Areas:

Biodiversity

Disciplines:

ecology

Keywords:
river restoration, ground water, river-groundwater interactions

Abstract:
The Pfynwald is situated between Sierre and Susten in the Valais (Switzerland). It is virtually the last section of the river Rhône that is in a relatively natural state from an ecological point of view. Therefore, it is a unique place to study the natural interactions between the Rhône and the adjacent groundwater in the Pfynwald. There is a project running to revitalise the Upper-Rhône to which this study contributes additional knowledge.

The Samples were taken in the groundwater mostly, but also in standing waters. Geochemistry-, Radon-, SF6-, Freon- and 18O-analysis were conducted on the water samples. The groundwater table was also measured. The sampling occured in the beginning of Mai, that is towards the end of the low water level in the Rhône. At the same time dataloggers were installed in two piezometers and recorded the temperature as well as the ground water table continuously for 2.5 months. The study by Schürch (2000) provided the basis for the development of a groundwater
model for the Pfynwald, which was created using Modflow. The research on the groundwater fauna in the previous year (Walther, 2002) was compared and contrasted with the new findings in this study. This led to an improved understanding of the faunal distribution.

The chemical analysis revealed that the groundwater was highly mineralised in vicinity of the mountainside in the south (Gorwetschgrat). Calcium and sulphate (gypsum, CaSO4) dominate in the south-east, which stems from the eastern part of the Gorwetschgrat. Calcium and magnesium were prevailing in the south-wester groundwater. This is due to the dolomite (CaMg(CO3)2) found in the western part of the Gorwetschgrat. As expected, a low mineral content was observed in the river Rhône.

The composition and the concentration of the different compounds in the water depended, as a consequence, on the origin of the water. Therefore, it was possible to observe pronounced mixing effects among three chemically different types of water. Especially the sulphate-rich water from the south-east showed a strong and far-reaching effect. The mixing effects were confirmed by the results of the SF6- and CFC-analysis. Due to a strong over-saturation, especially for CFC, it was not feasible to determine sound water-ages.

The model of the groundwater was an additional source of information that led to a better interpretation of the mixing effects. Furthermore, it was an appropriate tool to guess quantitatively the influence of the Rhône on the groundwater. It was shown that in the alluvial area the influence of the water from the Gorwetschgrat dominates over the river water (hence, the pronounced impact of sulphate).

In the upper half-section of the study area, between Susten and the farm, infiltration was observed only (the Rhône looses water to the groundwater). The infiltrating amount of water is low up to may. There are clear indications for the existence of an unsaturated zone between the riverbed and the groundwater, which might be the reason for the low infiltration. Furthermore, some evidence was found that groundwater in this area flows under the riverbed from the southern riverside to the North. Below the farm starts the zone that ist dominated by exfiltration. On closer inspection it may be revealed that this exfiltration zone consists in fact of a mosaic of many small zones of in- and exfiltration. The interaction between the Rhône and the groundwater is particularly intense here. As a consequence, an extensive groundwater fauna was found in the exfiltration area.

Leading questions:
What are the hydraulic constraints to river-groundwater interactions in pristine sections of alpine rivers (effects of floods and droughts on riverbed material: clogging; effects of variable saturation between river bed and water table).
This question is important in the context of the 3rd correction of River Rhône in Canton Valais/Kanton Wallis

URL: http://www.rhone-thur.eawag.ch

Publications:
Fette, M., R. Kipfer, C.J. Schubert, E. Hoehn & B. Wehrli, 2005, Assessing river-groundwater exchange in the regulated Rhône River (Switzerland) using stable isotopes and geochemical tracers, Appl. Geochem. 20, 701-712 (http://dx.doi.org/101016/j.apgeochem.2004.11.006)


Fette, M., E. Hoehn, & B. Wehrli, 2004, Infiltration von Flusswasser ins Grundwasser, Wasser-Energie-Luft 96(11/12), 301-304.


Last update: 12/29/16
Source of data: ProClim- Research InfoSystem (1993-2024)
Update the data of project: CH-4847

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