The island of Barthelasse is the largest river island in Europe. Groundwater from this island provides water supply for 180000 inhabitant consumption through the use of two pumping wells systems on the Barthelasse part and on the nearby Motte part of the island. Backwaters few hundred meter-wide separate the two parts of the island. Because of proximity of the river and backwater, the aquifer is highly vulnerable to river pollution. To evaluate this risk, a study has been done on the dynamic of water exchange between river, backwater and groundwater in a context of increasing water demands. A large set of sensors was deployed in the two pumping fields and the Rhône river, to do a high frequency monitoring of the physicochemical parameters, temperature, electrical conductivity and groundwater levels. Several sampling campaigns have been conducted each 15 days since January 2018, to measure
\delta^2 H and
^3H. A pumping test of 1000 m
^-1 during 24 h was conducted on 2 wells (P7 and P8) in November 2018 in the Motte pumping field. An isotopic and physicochemical monitoring was performed, including time lapse isotopic monitoring of
\delta^2 H and Rn in an observation piezometer (PRD3) located 35 meters away from the pumping wells, 60 meters from the backwater and 300 meters from the Rhône river. In addition, discrete samples were collected in several observation piezometers, in the pumping wells, in backwater and in Rhône river to analyse major ions,
^3H. The results enabled us to build a conceptual model of the hydrogeological system and suggest that the pumped water resulted from two mixing end members whose contributions changed according to the seasons. Three different behaviours were observed between P7, P8 and PRD3 involving a complex distribution of flow paths and time transfer. Even if the backwater and the river are close to the wells and observation piezometer, the results didn’t show any direct contribution of surface waters to the groundwater during the pumping test. Conversely, the pumping field in the Barthelasse part of the island proves to be strongly influenced by the backwater and river with a transfer time of few weeks. This will therefore make it possible to improve the current groundwater flow model, particularly regarding boundary conditions and the heterogeneity of parameters. An in-depth study is under development to elucidate the water transfers between groundwater of different origins and surface water systems in the island of Barthelasse in order to compare with the modelling results.