Abstract: The chemical runoff materials of rivers usually originate from rock weathering, soil erosion and human activities in the basin. Hydrochemical characteristics comprehensively present the environmental information of the basin. The hydrochemical characteristics and influence factors of Danjiangkou Reservoir are investigated in this paper based on sampling data in October 2017, January 2018, April 2018, and July 2018. The pH value of water in Danjiangkou Reservoir is between 7.02 and 8.58, which is moderately alkalescent. The dissolved oxygen, ORP and conductivity of water change obviously with seasons. The average value of TDS is 176.14±15.98 mg/L, much higher than that of other rivers in the world. Among the main cations in the water, Ca²⁺ is the highest in content, ranging from 50% to 70%, and the content of Mg²⁺ is between 20% and 30%. Among the anions, the content of HCO₃^- in most of the samples is more than 70%; the content of (Cl^-+SO₄^2- ) is below 20%; and the content of DSi is below 10%. The statistical results show that the weathering of carbonate rocks controls the overall characteristics of the water. Some of the ions are affected by the weathering of silicate rocks. Human activity is also an important factor affecting the chemical characteristics of the water. Compared with previous monitoring results, the water quality in Danjiangkou Reservoir has been improved. The research results provide data support for the comprehensive management, development and utilization of water resources in the basin.

Key words: hydrochemical characteristics, chemical runoff, rock weathering, human activities, water environment, Danjiangkou Reservoir