Hydrogeochemical characterization of groundwater flow systems in the discharge area of a river basin

The lowest reaches of a large-scale basin could be the discharge area of local, intermediate and regional groundwater flow systems with different travel distances and travel times. However, little research has been devoted to identify the flow systems in such a site. In this study, 44 groundwater samples were collected from wells with different depths in the discharge area of the Dosit River Watershed in the Ordos Cretaceous Basin. Using the major ions and pH as the input, hierarchical cluster analysis was conducted, which leads to five clusters (from C1 to C5) with distinct geochemical compositions. The relationship between δ D and δ18O shows that clusters C1 and C2 were recharged during the cold time while clusters C3, C4 and C5 were recharged recently. According to the Gibbs scheme, groundwater samples in all clusters except for C5 were controlled by rock–water interaction. Both hydraulic and geochemical perspectives were considered to identify the mechanisms of groundwater evolution. Both hydrochemistry and isotopes versus depth show a three-part structure with boundaries around depths of 200m and 600–750m, which were assumed to be associated with the circulation depths of local and intermediate flow systems. After excluding C4 with highly variable water-types, three geochemical indicators (Cl−, SO42−, and the slope of (Ca2++Mg2+)−(SO42−+HCO3−) versus (Na+−Cl−) imply that the processes of water–rock interaction for clusters C1, C2 and C3 include dissolution of halite and gypsum, weathering of feldspar, and ion-exchange. Moreover, it was found that C3 from local flow system, C1 from intermediate flow system, and C2 from regional flow system has increasingly higher concentrations of Cl− and lower values of Chloro-Alkaline index, which indicate increasingly higher degrees of dissolution and ion-exchange. This study not only reveals the mechanisms of groundwater evolution in the study area, but also provides a method to identify nested flow systems using hydrochemistry and isotopes.