The 3^rd correction of the Rhône (R3) is the largest flood protection project in Switzerland. Begun in early 2010, the main aim of the works is to widen the Rhône in flood-prone areas.

The town of Monthey has a drinking water well in the Iles d’en Bas area, close to the Rhône. This well guarantees the town’s supply of untreated drinking water at all times. At present, the bottom of the river bed is clogged. This layer of sediment protects the groundwater from contamination by surface water from the Rhône. In the long term, the town of Monthey has demonstrated the benefits of retaining a well within the Iles d’en Bas perimeter.

To meet the criteria set by the Rhône 3 project, the dykes need to be moved to give the river more freedom and space. The works will have an impact on the bed and may damage the layer of sediments that have been clogged up, at least temporarily. There is a significant risk to groundwater quality.

The town of Monthey and the Rhône 3 project group are working to find the best technical solution to meet the challenge. Reconnaissance drilling in the water table and the western embankment is planned to provide a better description of the nature of the subsoil and the hydrogeological conditions.

On the basis of all the existing data and the results of these new boreholes, the aim will be to draw up a development option for the Rhône and the installation of a new well offering a maximum guarantee of groundwater protection, at the best possible cost.

It should be noted that this problem affects many lowland municipalities.

At present, the Iles d’en Bas well has a potential capacity of 10,000 l/min. As part of the Rhône renaturation project and with the relocation of the dykes, without countermeasures and despite the choice of a new location for the well, its capacity could be reduced to 1,000 l/min.

Faced with this challenge, the town of Monthey would like to have a methodology, at the confluence of hydrology and hydrogeology, to help it make the right decisions.