Sources and hydrogeochemical processes of groundwater under multiple water source recharge condition

Ecological water replenishment (EWR) is an essential approach for improving the quantity and quality of regional water. The Chaobai River is a major river in Beijing that is replenished with water from multiple sources, including reclaimed water (RW), the South-North Water Transfer Project (SNTP), reservoir discharge (RD). The effects of multiple water source recharge (MWSR) on groundwater quality remain unclear. In this study, hydrochemical ions, isotopes (δ²H-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃^-, and δ¹⁸O-NO₃^-), mixing stable isotope analysis in R (MixSIAR), and hydrogeochemical modeling were used to quantify the contributions and impacts of different water sources on groundwater and to propose a conceptual model. The results showed that during the period before reservoir discharge, RW and SNTP accounted for 38 %-41 % and 54 % of the groundwater in their corresponding recharge areas, respectively. The groundwater in the RW recharge area contained high levels of Na⁺ and Cl^- leading to the precipitation of halite, and was the main factor for the spatial variation in groundwater hydrochemical components. The surface water changed from Na·K - Cl·SO₄ type to Ca·Mg - HCO₃ type which was similar to groundwater after reservoir discharge. RD accounted for 30 % of the groundwater; however, it did not change the hydrochemical type of groundwater. Dual nitrate stable isotopes and MixSIAR demonstrated that RW was the primary source of NO₃^- in groundwater, contributing up to 76-89 %, and reservoir discharge effectively reduced the contribution of RW. δ¹⁵N-NO₃^- or δ¹⁸O-NO₃^- in relation to NO₃-N suggests that denitrification is the main biogeochemical process of nitrogen in groundwater, whereas water recharge from the SNTP and RD reduces denitrification and dilutes NO₃^-. This study provides insights into the impact of anthropogenically controlled ecological water replenishment from different water sources on groundwater and guides the reasonable allocation of water resources.