Water Chemical Characteristics and Water Quality Evaluation of the River under the Ecological Water Replenishment: A Case Study in the Yongding River Basin in North China

Ecological water replenishment projects have been implemented in the Yongding River (YDR) Basin since 2019 to adjust the contradiction between regional ecology and social development. To clarify the water chemistry, sources of riverine solutes, and the water quality assessment of the YDR under the ecological water replenishment, concentrations of major ions (Na⁺, K⁺, Ca²⁺ Mg²⁺, SO₄^2–, HCO₃^–, Cl^–, and NO₃^–) of the YDR collected in August 2021 are determined. The dominant water chemical type of the YDR water was SO₄^2–·Cl^––Na⁺. Anthropogenic activities were the dominant factors affecting concentrations of riverine solutes, making up 71% of the total cations, and the main type was urban domestic sewage inputs. According to the evaluation of irrigation water quality, the YDR was facing a low-level sodium hazard and moderate salinity hazard, and the harm to soil aggregates was limited according to irrigation. The health threats to adult residents were limited in YDR, while potential health risks for children remained. Ecological water replenishment projects have been implemented to improve the ecological function of the YDR with significant effects observed near the replenishment outlets, leading to a significant improvement in the irrigation water quality. This study highlights the disturbance of ecological replenishment to water chemical characteristics and ecological restoration function to the dry-up river and provides a valuable example for further studies of water chemistry under ecological water replenishment.