Multi frequency isotopes survey to improve transit time estimation in the case of river-aquifer relation

The Barthelasse alluvial aquifer has been used for decades to supply water to 180,000 inhabitants. The pumping field is located around 200 m from the Rhône river which makes it particularly vulnerable to any pollution from the Rhône. Between the Rhône and the pumping field is a Girardon unit whose role is to stabilise the banks and to facilitate river navigation. A fortnightly monitoring over a full hydrological year as well as hourly sampling during a flood event in winter 2019 were carried out to study the transfer processes between the Rhône and the pumping field. Samples were collected in the Girardon unit and in the pumping field.

The Rhône showed cyclicality in its isotopic signature with enrichment in heavy isotopes during the winter period, particularly during floods and a depletion during the summer period. This variation was also found within the alluvial aquifer. The application of LPMs models showed that the average transfer time between the Rhône and the Girardon unit was 20 days and 50 days between the Rhône and the Barthelasse pumping field. For the Girardon unit, this result was confirmed using a high frequency monitoring of electric conductivity.

This study highlighted the importance of using a sampling strategy including several frequencies to consider the diversity of hydrological situations. For the Rhône, event-based monitoring proved relevant to account for isotopic variability throughout the year. Models calibration procedure was also discussed. In LPM models, the RMSE criterion is the most commonly used. It is recommended to use standardised criteria instead and to adapt the choice of the criterion to the observed isotopic variability. This work also highlighted the impact of the disruption of hydraulic exchanges between the river and the water table caused by the presence of the Girardon unit in terms of the propagation of contaminants.