[Distribution Characteristics and Formation Mechanism of Main Hydrochemical Ions in Qingshui River]

Qingshui River is a vital source for human life and industrial production in Zhangjiakou City. Determination of the formation mechanism of the main hydrochemical ions is important for the sustainable development and utilization of surface water resources in the Qingshui River. In view of this, 20 surface water samples were collected from the agricultural and urban reaches of the Qingshui River in July 2022. Based on the detection of hydrochemical ions and stable isotopes （δ²H-H₂O and δ¹⁸O-H₂O）, the hydrogeochemical methods, multivariate statistical analysis, and positive matrix factorization model （PMF） were used to comprehensively understand the chemical formation mechanism of surface water from qualitative to quantitative perspectives. The results showed that the average pH value in the Qingshui River was 8.66, indicating weakly alkaline water. The average concentrations of cations and anions decreased in the orders of Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ > NH₄⁺ and HCO₃^- > SO₄^2- > NO₃^- > Cl^- > F^- > NO₂^- in the river water, respectively. The main hydrochemical type was identified as HCO₃-Ca·Mg water. For the natural background, surface water was mainly controlled by rock weathering and evaporation crystallization and rock weathering was identified as the primary driving factor. The main hydrochemical compositions in the Qingshui River were derived from the dissolution of carbonate rocks and silicate rocks. Regarding anthropogenic activities, the portions of river water in the agricultural reaches were mainly affected by the excessive application of chemical fertilizers. Furthermore, the concentration of NO₃^- （1.88-47.4 mg·L^-1） exceeded the standard for drinking purposes （10 mg·L^-1）. The concentration of F^- （0.38-1.92 mg·L^-1） was above the acceptable limit for drinking purposes （1.0 mg·L^-1） in the southern urban reaches portion of the river water, which could have been attributed to the industrial sewage discharge. The obtained results from the PMF model showed that the hydrochemical compositions in the river water of agricultural reaches were mainly controlled by four factors, namely synthetic fertilizers （29.6%）, livestock manure （16.4%）, rock weathering （17.4%）, and natural geology （13.7%）, while hydrochemical compositions in the river water of urban reaches were mainly affected by the domestic pollutants （30.5%）, industrial discharges （20.1%）, natural geology （18.4%）, and rock weathering （16.7%）.