Impacts of Reclaimed River-Water Recharge on Groundwater of a Multi-Layered Aquifer System: Combining Hydrochemical Analysis and End-Member Mixing Approaches

A managed aquifer recharge (MAR) project utilizing reclaimed water has been operated for over 10 years in northeastern Beijing, China, with the goal of restoring the long-dried Chaobai River and replenishing the region’s depleted groundwater resources. To ensure the safe implementation of the project, we quantitatively assessed the impact of river water recharge on the multi-layered groundwater system by investigating the hydrochemical compositions of the reclaimed water, river water, and groundwater. Results show that river water is characterized by higher concentrations of Na+, Cl−, and SO42− than found in groundwater, and that river water recharge has altered the groundwater types in the 30 m-depth unconfined layer, changing them from Ca-Mg-HCO3 and Ca-HCO3 types to Na-Ca-HCO3-Cl and Ca-Mg-Na-HCO3 types. End-member mixing analyses of river water samples indicate that three end-members are needed to represent the seasonal and spatial variations in river water. A five-end-member mixing model is then developed to quantify fractions of river water (fR) in different aquifer layers. The estimated fR values vary from 18.4% to 100%, with an average of 67.6% in the 30 m-depth layer, while fR values in the 80 m-depth confined layer are mainly below 30%, with an average of 13.3%, which corresponds well to the known site geology. Overall, combining hydrochemical analysis with the end-member mixing approach is useful for assessing the impact of river recharge on groundwater. This study also highlights the need for high-resolution characterization of subsurface heterogeneity in MAR sites.