[Analysis and Evaluation of Hydrochemical Characteristics and Its Driving Force in Miyun Reservoir Basin].

Miyun Reservoir is the most important surface drinking water source of the capital city, which is of great importance to guarantee the safety of the water supply in the capital city. A total of 39 surface water samples and 16 groundwater samples were collected in the field during the flood season in the Miyun Reservoir Basin to analyze the hydrochemical characteristics and to carry out water quality analysis and evaluation using the single-factor evaluation method and the Nemero Pollution Index and applying geo-detectors to analyze the spatial variability of the water quality indicators and spatial data, such as land use, meteorology, and socio-economics. The results showed that： ① Surface water and groundwater had similar hydrochemical characteristics, including close hydraulic connection, homology of component sources, and uneven spatial distribution of ion concentration, and three principal component factors could be extracted from surface water and groundwater hydrochemical ions, respectively, which were mainly affected by the combination of human activities and rock weathering. ② Water quality evaluation results showed that there were 15 water quality sampling points to meet the surface water class II standards and 14 water quality sampling points to meet the surface water class III standards, and there were seven water quality sampling points for the surface water class Ⅳ standards, two for class V, and one for inferior class V. According to the Nemero integrated pollution index classification of water quality categories, class I had 23, class II had 14, and class III and class V each had 1. The permanganate index was the main exceeding index, revealing that the surface water faced the risk of organic pollution. ③ Human activities were the main factors affecting the river water quality differentiation, with the explanatory power factor <i>q</i>-value of 0.204 9, and rainfall and elevation also had a significant impact on the water quality changes. Two-factor interaction significantly increased the explanatory power of the spatial differentiation of water quality in the river, which manifested itself as an enhancement or nonlinear enhancement, reflecting that the water quality of the basin level was affected by several factors together.