Research On Water Quality Change And Influencing Factors In The Zuli River Basin

In recent years,due to social and economic development and human exploitation and utilization of natural resources,environmental problems such as water pollution had become increasingly prominent,seriously affecting human health.Zuli River Basin is the first tributary of the Yellow River Basin,the study of water quality change and its influencing factors are helpful to analyze the cause mechanism of water quality evolution,predict the future trend of water quality change,and provide reference for water environmental pollution control in Zuli River Basin.Ten indexes including salinity,total hardness,dissolved oxygen（DO）,permanganoate index(COD_Mn),chemical oxygen demand（COD）,five-day biochemical oxygen demand（BOD₅）,ammonia nitrogen（NH₃-N）,total phosphorus（TP）,volatile phenol and hexvalent chromium in the five water quality monitoring sections（Jingyuan,Dayangying,Chankou,Dingxi and Huining）were selected as the research object.Based on the actual monitoring data from 2001 to 2021,five sections were evaluated,ten indexes were analyzed through trend,mutation and wavelet,three models were used to predict the main pollution factors of five water quality monitoring sections and the main factors affecting water quality changes were determined by analysis of stepwise regression.The main conclusions were obtained as follows:（1）Although the water quality in the Zuli River Basin gradually improved during the study period,the water quality was generally poor.Through the principal component analysis,it was found that the main pollution factors of different sections were different,but they all had common factors:COD,COD_Mnand NH₃-N.Meanwhile,it can be seen that the water quality of the upstream section was better than that of the downstream section,and upstream pollutants contributed to the downstream section.It can be seen from the PCA score chart,organic pollution index contributed the most in PC1,and water environment index contributed the most in PC2.（2）From the perspective of annual variation,except Jingyuan section and Dayangying section,water quality indexes in other sections showed obvious seasonality,except DO content showed a downward trend in wet season,other indexes showed an upward trend.From the perspective of inter-annual variation,the hexvalent chromium,volatile phenol,NH₃-N,TP,salinity and total hardness of the five sections all showed a downward trend after 2010,except for the Chankou section,COD showed an obvious downward trend after2015,and COD_Mnshowed an obvious downward trend after 2016.（3）According to the mutation analysis,the abrupt change time of water quality index in each section was roughly the same,all of which was between 2016 and 2019.After mutation,other indexes except DO index showed a decreasing trend.According to the periodic analysis,the main periodic change of the five sections were basically the same.Since 2016,except DO,which was at the peak,the content of COD_Mn,BOD₅,COD,NH₃-N,TP,salinity,total hardness,hexavalent chromium and volatile phenol stayed in a low ebb;according to the change trend of each index,the water quality in the Zuli River Basin will continue to improve after 2021.（4）ARIMA model,GM（1,1）model,GM（1,1）and Markov combination model were used to predict 10 water quality indexes in five different sections.Through the accuracy verification of the model,it was found that GM（1,1）and Markov combination model had the highest accuracy.Therefore,this model was used to predict the next ten years,and it was found that the water quality of each section was gradually getting better,which fully confirmed the above trend analysis results.（5）Analysis of the correlation between the main water quality indexes and influencing factors in Jingyuan section showed that the content of COD_Mn,NH₃-N and total hardness decreased significantly with the increase of area of soil and water conservation measures and cultivated land,while the contents of COD increased significantly.The main factors affecting the change of water quality including dam land,sheep stock,rural household garbage,soil and water loss control area,annual runoff,shrub land,terraced land,closed land,pig stock and total fertilizer application.The regression model was established with water quality indexes and influencing factors,2021 was taken as the based year,combined with the 14th Five-Year Plan to predict the water quality in 2025,the analysis found that soil and water loss control area was related to the change of water quality,especially when the number of terracing and closure measures increased,the water quality improved significantly.Finally,combined with relevant national policies and the actual situation of Zuli River basin,the prevention and control measures were proposed.