Abstract:

[Objective] The Hanzhong section of Hanjiang River is an important water source for the “South-to-North Water Diversion” and “Hanjiang-to-Weihe Diversion” projects. Its water quality directly affects the ecological environment and residents’ production and livelihoods along these water transfer systems. This study aims to analyze the current pollution status of heavy metals in Hanzhong section, explore their spatiotemporal distribution characteristics, and identify their primary sources. [Methods] Based on 240 sets of measured data of seven heavy metals (Hg, Pb, Cu, Zn, As, Cd, Cr) and four conventional monitoring indicators (pH, DO, NH₃-N, COD_Cr) collected from 20 sampling sites in Hanzhong section from January to December 2022, we analyzed the temporal differences of heavy metal pollution during wet season (July-September) and dry season (December-February) using an improved heavy metal pollution index (HPI). The Nemerow comprehensive pollution index was used to evaluate the current pollution status of heavy metals in the water bodies, with reference to the Class I surface water quality standard. Pearson correlation analysis and principal component analysis were jointly applied to investigate the correlations among heavy metals and between heavy metals and conventional indicators, and to identify their main pollution sources. [Results] The annual average concentrations of seven heavy metals in this river section followed the order: Zn>Cr>Cu>Pb>As>Cd>Hg, all within the Class II standard limits of GB 3838—2002. Spatially, high-concentration cross-sections mainly distributed around mining and metal smelting enterprises. Temporally, the HPI index during wet season was slightly higher than that during dry season, but neither exceeded the critical threshold. At some urban traffic arteries, due to traffic pollution and atmospheric deposition, the pollution index during dry season was relatively higher. The single-factor pollution index evaluation results indicated that Cu and Cr were primary pollution factors, followed by Hg and Zn. Correlation analysis results showed that Cd had highly significant positive correlations with Pb, Zn, and As. As was highly significantly positively correlated with Pb and Zn, and Zn was highly significantly positively correlated with Pb. Hg, Zn, Pb, Cu, As, and Cd had significant positive correlations with NH₃-N, suggesting that they had the same sources. Principal component analysis results revealed that the first principal component, including Pb, Zn, As, Cd, and Hg, was mainly affected by industrial sources, transportation, and domestic sewage discharge. The second principal component, including Cr, Hg, and Cu, was mainly affected by industrial and agricultural production activities. [Conclusion] The prevention and control of heavy metals in the water bodies of the Hanzhong section of the Hanjiang River should focus on Cu pollution, monitor the concentration changes of heavy metals such as Cr and Pb, optimize the layout of mining and smelting enterprises along the river, improve farmland soil to reduce the migration of heavy metals, and protect the water quality safety in this section of the river.

Key words: heavy metal, spatio-temporal distribution, pollution characteristics, pollution source, Hanzhong section of Hanjiang River, water pollution treatment, water quality protection