Relationship among water quality and hydrochemical indices reveals nutrient dynamics and sources in the most sediment-laden river across the continent

Deep-going analysis of the temporal and spatial variations of the major ions in rivers can reveal the sources of nutrients to a certain extent, which is of great value for the development of specialized governance strategies to control nutrient flows into rivers. The Yellow River stem and its reservoirs were selected as the study area, which is the most sediment-laden river in the world. A systematic survey of the Yellow River main steam was conducted from April to May and September to October 2019. The Yellow River was divided into three regions via cluster analysis of ion composition data, which represent the three states of natural and man-made influences. Based on the analysis of hydrochemical characteristics, the results indicated that: (1) the ion and nutrient concentrations in Region Ⅰ were generally low, the concentrations in Region Ⅱ were higher than those in Region I, and the concentrations in Region Ⅲ remained consistently high. (2) the transportation of phosphorus is mainly controlled by the erosion of rainfall runoff and the adsorption of sediment in the Yellow River Basin; nitrogen is mainly derived from point source pollution during spring, but there is combined impact of point and nonpoint sources during autumn; and (3) ultimately, six indicators (Na+, K+, Mg2+, Cl-, SO42- and HCO3-) were used to qualitatively analyze pollution sources. Of these, K+, Mg2+, and HCO3- were selected as nonpoint source pollution tracers, and Na+, Cl-, and SO42- were selected as point source pollution tracers. Our results improve the understanding of applicability of hydrochemical characteristics method, and are important for water resource management in the Yellow River Basin.