Distribution Characteristics And Risk Assessment Of Nutrient Elements And Heavy Metals In Surface Sediment Of Xiaoqing River

Xiaoqing River,located in the middle and south of the northwest plain of Shandong Province,belongs to the Bohai Sea system.It has functions of flood control,waterlogging control and irrigation,and its geographical position is very advantageous.All cities along the Xiaoqing River Basin have developed industries,rapid urbanization,dense population,and unreasonable discharge of production wastewater and urban sewage,which lead to water pollution in the basin.Therefore,the water ecological environment of Xiaoqing River has been greatly affected.Since the reform and opening up,as an important polluted river channel in Shandong Province,the nutrients and heavy metal elements chromium,lead,copper,zinc,cadmium,nickel,arsenic and mercury in surface sediments of Xiaoqing River have increased too rapidly,which leads to the continuous deterioration of the water ecological environment in Xiaoqing River Basin and the increase of risks and toxicities.In order to reflect the spatial distribution characteristics and risk level of nutrients and heavy metals in Xiaoqing River,this paper analyzed the distribution characteristics of carbon,nitrogen and phosphorus in surface sediments of Xiaoqing River and the content of eight heavy metal elements,discussed their correlation,evaluated the pollution level of surface sediments by various methods,and analyzed the relationship between economic development and pollution changes.The main contents of this paper are as follows:（1）Concentrations of total organic carbon（TOC）,total nitrogen（TN）and total phosphorus（TP）were not uniformly distributed in space.The mean TOC content was 23.6 mg/g,the mean TN content was 2272 mg/kg,and the mean TP content was 3218 mg/kg.All of them reached their maximum values at S2 sampling point.The spatial distribution of TOC,TN and TP showed a decreasing trend from upstream to downstream（from West to east）.TOC,TN and TP may have very similar sources,and nutrient elements are closely related to water environmental factors such as p H,chemical oxygen demand(COD_Mn)and total soluble solids（TDS）.（2）Organic index and organic nitrogen evaluation methods show that there is almost no organic nitrogen pollution in surface sediments of Xiaoqing River.The organic index evaluation results show that there is no serious pollution at six points,and there is slight pollution in the upstream and middle reaches,which generally conforms to the level of cleanliness.The results of comprehensive pollution index indicate that surface sediments of Xiaoqing River are heavily polluted at all sampling points and the water body is seriously eutrophic.The input of nutrient elements mainly comes from the production of agriculture,animal husbandry and the discharge of urban sewage along the river basin.（3）The spatial distribution of heavy metals in surface sediments is irregular,and the mean contents of Cr,Pb,Cu,Zn,Ni,As,Hg and CD exceed the background values.Heavy metal content in surface sediments of Jinan section of Xiaoqing River showed a downward trend from2004 to 2016.In correlation analysis,there may be the same input source between Cr and Cu,Ni,As and Hg elements,while heavy metals Ni and As show the same source characteristics.Heavy metals have close correlation with oxides such as Si O₂,Al₂O₃ and Fe₂O₃,Cd has significant negative correlation with the other seven heavy metals,and has high homology with TP.（4）The result of Nemero index method shows that Cd index is the largest and the risk of pollution is the highest.The evaluation results of heavy metals in surface sediments by geoaccumulative index method and potential ecological risk index method show that the pollution degree of single element is Hg,Cd,Zn,Cr,Cu,Pb,As,Ni from large to small,and the order of single potential ecological risk degree is Cd,Hg,As,Cu,Pb,Zn,Ni and Cr from strong to weak.The RI value of potential ecological risk index in Jinan section of Xiaoqing River decreases in recent years.（5）The nutrient elements and heavy metal elements in surface sediments show the same input sources.The trend of social and economic changes in Xiaoqing River basin over the years is analyzed.The results show that the proportion of heavy industry and agriculture and animal husbandry in the economy is decreasing.It is inferred that the water ecological environment of Xiaoqing River will gradually improve.The ecological risks of nutrients and heavy metals in surface sediments of Xiaoqing River Basin are analyzed and suggestions on pollution control are made from the perspectives of industry,agriculture,urban life,management measures and environmental protection awareness.