Abstract
The Barthelasse alluvial aquifer is used to supply water to 180,000 inhabitants. The pumping field is located less than 200 m from the Rhône and is 100% fed by water from the Rhône, which makes it particularly vulnerable to any pollution from the Rhône. Between the Rhône and the pumping field is a Girardon unit, an arrangement that can be found regularly along the banks of the lower and middle reaches of the Rhône, and whose role is to stabilise the banks (alluvial deposits) and to facilitate river navigation. In order to know the transfer times between the Rhône and the pumping field, fortnightly monitoring was carried out over a hydrological year, as well as hourly monitoring during a flood in the winter of 2019. The Rhône shows a 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 is found well within the associated 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. This study highlighted the importance of using several sampling frequencies to consider the diversity of hydrological situations. For the Rhône, event-based monitoring (flooding) proved to be relevant to account for isotopic variability throughout the year. 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.
Highlights
In France, alluvial aquifers provide 45% of the volume of water used for drinking water and for agricultural and industrial uses and play an important ecological role through their hydraulic link with wetlands [1]
The stable isotopes of the water molecule (2 H, 18 O) have many applications, allowing a better understanding of hydrosystems. They are used to estimate the recharge, to know the origin of water, the mixing processes and the transit times [2,3,4,5,6,7,8,9,10,11]. The use of this tracer is relevant in the context of short transit times within the aquifer, as is the case for exchanges between alluvial aquifers and rivers
Based on a 2-year isotopic survey of the various water masses and high-frequency physico-chemical monitoring in the Rhône and the groundwater, the main objective of this study is to propose, through a multi-method approach, a solid estimate of transit time between the Rhône and the pumping field
Summary
In France, alluvial aquifers provide 45% of the volume of water used for drinking water and for agricultural and industrial uses and play an important ecological role through their hydraulic link with wetlands [1] This strong contribution to the water supply is explained by a shallow depth and favourable hydraulic properties, which allow high flow rates to be obtained at low operating costs. The stable isotopes of the water molecule (2 H, 18 O) have many applications, allowing a better understanding of hydrosystems They are used to estimate the recharge, to know the origin of water, the mixing processes and the transit times [2,3,4,5,6,7,8,9,10,11]. The use of this tracer is relevant in the context of short transit times within the aquifer, as is the case for exchanges between alluvial aquifers and rivers
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