Analysis On The Reasons For Typical Pollutants Exceeded The Water Quality Standards In Luanhe River Basin(Chengde)

In recent years,the problems of permanganate index(COD_Mn),chemical oxygen demand（COD）,and total phosphorus（TP）in the Luanhe River Basin（Chengde）frequently pose a potential threat to the water ecology of the basin and the water safety of Chengde and downstream residents.Screening natural or anthropogenic factors of the reasons for exceeding the water quality standards is of great manage support significance.Screening these reasons is the basis of optimizing the methods of assessment and evaluation and helps developing pollution control measures.Historical data on state-controlled sections,sampling data,land use data,and contamination load were analyzed in this work.Considering the characteristics of large sediment content in Luanhe River,this work designed a comparative experiment to analyze the impact of sediment on water quality.Screening the reasons of permanganate index,chemical oxygen demand,and total phosphorus exceeding the standard in the Luanhe River Basin（in Chengde City）was implemented,and obtained the following conclusion:（1）Through the analysis of the historical water quality data of the state-controlled sections in the past 10 years,seriously polluted rivers are mainly in the mainstream of the Luanhe River and Yixunhe River.COD,COD_Mn,and TP are the main factors that exceed the standard.In terms of spatial distribution,sections between Gonghou and Shangbanchengdaqiao on the mainstream of Luanhe River and sections between Weichangshangyou and Litai on Yixunhe River are contaminated.In terms of time distribution,rivers are most contaminated from April to August.And the indicators during 2017 to 2020 are much more variable than 2011 to 2015.（2）Through the analysis of 42 sections’indicators,most of the water quality indicators of Luanhe River meet the surface water class III during the ice period（2020.12）,only 3%-6%sections were contaminated by TP.COD,COD_Mn,TP were exceeded during the spring flood season（2021.3）in Luanhe River basin.And the excessive rate was 60%-90%.COD,COD_Mn,TP were also exceeded during the wet season（2021.9）,the excessive rate was 19%-71%.With Excitation-Emission Matrix spectroscopy was used in water samples,the main source of DOM in Luanhe River is humus and is greatly affected by land sources.（3）Soil erosion and human influence were significantly increased in the last 10years through the interpretation of satellite images.Soil erosion rate was increased from32.5 t/（km²a）in 2015 to 635.4 t/（km²a）in 2018,increased by nearly 20 times.Artificial land and bare land area increased by 0.7-1.4 times,while grassland,woodland,cultivated land,wetland,shrubland,and water area were decreased.Different pretreatments on water samples,such as standing and filtering,have a great influence on the determination of TP and COD_Mn.Sediment particles were removed in different degrees after pretreatment.TP and COD_Mnwere significantly positively associated with turbidity decreased significantly.（4）Contamination load mainly includes urban life and non-point source in rural,accounting for 55.2%,41.3%respectively.Rural life contributed the largest part COD（88.4%）,and planting contributed the largest part NH₃-N（51.2%）and TP（54.3%）of non-point source pollution.While point source pollution is mainly caused by urban lifestyle（94.5%）.Combined with the analysis of spatial and temporal change of water quality in the basin,compared to the steady entry of urban point source,non-point source pollution intensively enters into the river mostly in wet season（April to August）,causing a great impact on water quality.This intensively flows into the river is the main reason for the excessive COD,COD_Mn,TP of the Luanhe River（Chengde City）.