Analysis And Simulation Of Water Environment Changes And Pollution Sources In The Hengkou Section Of The Yuehe River

Yuehe River is a first-class tributary of the Han River and an important water source protection zone for the South-North Water Diversion Project and the"Diversion from Han River to Wei River"Project in Shaanxi Province.In recent years,the quality of the water environment has been seriously affected by the rapid development of migration projects,economy and agricultural production.Therefore,it is important to clarify the trend of changes in water and surface sediments and the main sources of pollutants,not only to understand the characteristics of changes in the water environment,but also to provide a scientific and theoretical basis for the prevention and control of water pollution in rivers,as well as to protect the life and health of rivers and promote the improvement of urban and rural habitat.This paper takes Hengkou section of the Yuehe River as the study area,and explores the change characteristics and correlation of water and surface sediment indicators by sampling and analyzing the water and surface sediment in the study area,and evaluates the pollution status of water and sediment in the study area,and principal component analysis-absolute principal component sore-multivariate linear regression（PCA-APCS-MLR）was established to analyze the main pollution sources of water bodies and surface sediments,and the SWAT model was established to analyze the main source areas of nitrogen and phosphorus pollution loads for non-point source pollution in the study area,and to propose management measures for the prevention and control of water pollution in the study area.The main results are as follows:（1）Through the analysis of the water environment indicators can be seen:COD has the largest variance in each sampling period,indicating that it is subject to greater human influence.TN in the study area is the worst water quality pollution factor,indicating that the problem of nitrogen exceedance in the study area is more serious.According to Pearson correlation analysis,TN and NH₃-N,COD and COD_Mn showed highly significant positive correlation.The values of surface sediment TN and TP were 1088 and 604.15 mg/kg,respectively,and the average contents of Cu,Zn,Cd,Ni and Hg were higher than the background values of Shaanxi soil.The mean value of C/N in the surface sediments of the study area was 11.29,indicating that organic matter in the surface sediments was mainly exogenous organic matter.According to Pearson correlation analysis,TOC was significantly and positively correlated with Cu,Pb and Hg,indicating that the accumulation of TOC in surface sediments has an important influence on the enrichment of heavy metals Cu,Pb and Hg.（2）According to the Nemerow pollution index method,the water quality condition in the study area was poor,with 36.5%of Class V water in each sampling site.The fuzzy comprehensive evaluation results show that the main indicators affecting the water quality in the study area are t DO,TN and NH₃-N,and the overall pollution status of the water quality is as follows:August 15,2021>February 25,2022>May 15,2022>August 15,2022.Among them,the comprehensive water quality in the study area was in Class III-V on August 15,2021 and February 25,2022,and in Class I-III on May 15 and August 15,2022.（3）The average value of organic nitrogen pollution index of surface sediment in the study area is 0.10,and the overall state is still clean.The average value of organic pollution index is0.152,and the whole is in a relatively clean state.The pollution load index of heavy metal elements at each sampling point ranged from 0.88 to 1.42,and the heavy metal pollution status of the study area was no pollution-light pollution.The mean value of the accumulation index of heavy metals is from large to small:Cd（1.36）>Zn（-0.11）>Cu（-0.20）>Ni（-0.46）>Cr（-0.71）>Pb（-0.82）>As（-0.88）>Hg（-1.39）,among which Cd is light pollution-heavy pollution,except for Cd,the mean value of the accumulation index of all heavy metals is less than 0,and the pollution level is clean.According to the results of potential ecological risk evaluation,Cd and Hg have the highest risk level,and the mean value of potential ecological risk index RI in the study area is 174.49,which is in moderate ecological risk overall.（4）The PCA-APCS-MLR receptor model of the water environment in the study area shows that DO,BOD₅,TN and NH₃-N in the water column mainly originate from F1（influenced by domestic sewage and agricultural surface pollution）,COD and COD_Mn mainly originate from F2（influenced by urban domestic and industrial pollution）,TP,NO₃^--N and NO₂^--N are mainly influenced by F3（influenced by livestock breeding and agricultural wastewater discharge）;Cu,Hg,Zn and Pb in surface sediments are mainly from F1（industrial production activities and human activities such as transportation）,Cd and As are mainly from F2（agricultural production activities and natural sources）,and Cr and Ni are mainly influenced by F3（combustion emissions and natural sources）.（5）By constructing the SWAT model,the rate determination and validation of runoff,total nitrogen and total phosphorus,the coefficient of determination（R²）and Nash coefficient(E_NS)of evaluation indexes meet the requirements of model accuracy,indicating that the SWAT model has good applicability in the study area.According to the model output,the rainfall in 2020 is the lowest,and the average annual load output of total nitrogen and total phosphorus is also the lowest,44.06 t and 3.35 t,respectively.The intra-annual pollution load of total nitrogen and total phosphorus is the highest during the abundant water period from June to September,indicating that the abundant water period is a critical period for generating non-point source pollution of total nitrogen and total phosphorus.Spatially,the average annual load of total nitrogen output showed a distribution characteristic of low in the north south and high in the middle.Under different land use types,total nitrogen and total phosphorus pollution loads from arable land accounted for the largest proportion,87.09%and 82.83%,respectively.The quantitative nitrogen and phosphorus loadings of sub-basins 11,20,27 and 31 were simulated by three scenarios,and the results showed that the adoption of fallowing of arable land above20°slope was better for pollutant reduction,with the average reduction of total nitrogen reaching 26.85%and total phosphorus reaching 23.88%.