Assessing groundwater-surface water interactions in an anthropized alluvial aquifer by combining hydrochemical and isotopic data with a statistical approach

To better understand the dynamics of groundwater in an anthropized alluvial aquifer in the Lower Rhône Valley, we applied multivariate statistical methods including hierarchical clustering analysis (HCA) to two datasets. The first dataset included conservative tracers (Cl − , Br − and stable water isotopes, δ 2 H and δ 18 O) and comprised 667 water samples. The second dataset included major and minor ions (Ca 2+ , Mg 2+ , K + , Na + , Cl − , SO 4 2 , alkalinity, NO 3− , Br − and U(VI)) and stable water isotopes (δ 2 H and δ 18 O), comprising 374 water samples. Cluster analysis and dimension reduction techniques applied to the conservative tracer dataset revealed three groups of groundwater distributed between two recharge sources, as shown by the resulting distribution maps of clusters and boxplots. 1) The first group was characterized by Rhône recharge and was located along the Rhône River boundary. This group was characterized by highly depleted δ 2 H (mean = - 66.7‰) and δ 18 O (mean = -9.5‰) signatures. 2) Rainfall Recharge: Groundwater located along the northern and northwestern parts of the site. This group was characterized by highly enriched δ 2 H (mean = -41.4‰) and δ 18 O (mean = -6.4‰) signatures. 3) The third group, Mixing Recharge, was predominantly distributed in the southern part of the site and was characterized by intermediate δ 2 H and δ 18 O signatures. Additionally, the HCA analysis revealed the influence of 2022 Saharan dust event on the signatures of rainwater, the Rhône River, and thus, groundwater, with high deuterium excess, calcium and chloride concentrations. Cluster analysis of the second dataset identified water samples with high uranium and chloride concentrations, consistent with historical pollution in the area. Conversely, the results revealed temporal variations in groundwater and surface water linked to rainfalls events. Changes in the HCA cluster index and chloride concentrations suggest vertical infiltration or lateral inflow. This approach provides a better understanding of local-scale groundwater dynamics in an anthropized alluvial aquifer and offers valuable insights for groundwater management and modeling.