Study On Water Chemical Characteristics And Spatiotemporal Evolution Of Heavy Metal Pollution In The Chu River In Southern Shaanxi Province

The Chu River is located in the southernmost part of Shaanxi Province,straddling the cities of Hanzhong and Ankang,and is the largest tributary of the Ren River,a tributary of the Han River,with the rapidly growing industrialization and urbanization in the Chu River Basin,heavy metal pollution of rivers from metal mineral-related industries,agroforestry and urban sewage is gradually coming to light.Based on the monitoring results of physicochemical parameters,anion and cation contents and heavy metal quality concentrations in 17 river sections of the Chu River in May,September and December 2021,this paper studies the water chemistry characteristics of the Chu River and its affecting factors and the changes of heavy metal ions in terms of spatial and temporal evolution patterns,determine the extent of heavy metal pollution in water bodies and the main pollution sources,estimate the human health risks caused by heavy metal pollution to local populations and the pollutants that need to be focused on prevention and control,providing a scientific basis for the prevention and control of heavy metal pollution in the Chu River.The main research results are as follows:（1）The overall water body of Chu River is weakly alkaline and its mineralization is higher than the average value of major rivers in the world.The temporal variation pattern of anion and cation concentration is obvious:wet season>dry season>level period,with high consistency of spatial distribution,cation:Ca²⁺>Mg²⁺>Na⁺>K⁺,anion:HCO₃^->SO₄^2->Cl^-.Combined with Shukarev classification,Gibbs diagram and Piper diagram,the water chemistry of the Broussonet River is HCO₃-Ca type,which is mainly affected by rock weathering,and is a carbonate rock weathering with calcite（CaCO₃）as the main mineral.The comprehensive hydrogen and oxygen isotope study judges that the Broussonet River water body is mainly the source of atmospheric precipitation affected by evaporation in the process of conversion to surface water.（2）Heavy metal levels in rivers are greater during level period than during wet season,with stable changes in the proportion of each element.The sum of the proportion of four elements,Fe,Cr,Zn and Mn,mainly Fe,exceeds 90%.Due to the variability of precipitation,high concentrations of heavy metals,mainly Fe,Cu and Zn,appear earlier in the same element concentration sequential peak wet season.Most metal element concentration maxima or regional peaks appear in rivers Upstream,midstream concentration drops to a low level,and downstream concentration values show more drastic fluctuations.The value of pH has less influence on the adsorption-desorption of solid-phase heavy metals in alkaline waters.The larger the TDS value,the stronger the desorption of heavy metals from the solid phase.（3）Based on the water quality index to evaluate the overall pollution level of the river is no pollution;single factor pollution index shows that there are individual sections of the river in different periods of low pollution;Nemero comprehensive pollution index evaluation shows that the comprehensive pollution level in the flat water period is low pollution,C3,C12 two sections of the comprehensive pollution index in different seasons are more than the low pollution limit,Fe is the largest contributing factor to the pollution index.（4）The non-carcinogenic risk values of heavy metal pollution in rivers are mainly caused by Fe,Co and As in the rich water period and Fe,Cr,Co,As,Pb and Sb in the flat water period,and the non-carcinogenic risk increases along the river direction in different periods,but the health risk values of the whole river section are within the acceptable range of human body;the carcinogenic risks of the whole river section in different periods are mainly caused by Cr and Pb,and the carcinogenic risk values exceed the acceptable range of human body,so the corresponding pollution control work should be carried out.（5）The analysis results of heavy metal sources show that most of the heavy metal sources in rivers are mainly industrial sources,supplemented by agricultural and natural sources.In view of the current situation of heavy metal pollution in the Chu River,we should start from three aspects:pollution sources,heavy pollution points and main pollution elements,make good control of pollution sources,intervention of transmission channels and clean-up of pollutants to systematically improvement of heavy metal pollution in the Chu River water and safeguard water quality.