Distribution Characteristics And Pollution Risk Assessment For Heavy Metals In Sediments Of Duliujian River Basin

Heavy metal pollution has attracted the attention of many scientists and environmentalists.The main reason is that heavy metals are easy to be bio-accumulated,and then accumulate and amplify through the food chain,and eventually threaten human health.As the convergence point of heavy metals,the pollution risk of sediment is closely related to the source,concentration and biological behavior of heavy metals.Also,the sediment contains many microorganisms,some of which play an important role in the migration and transformation process of heavy metals.The channels in river basin are the main pathways of pollutant transport.Therefore,the study of heavy metal distribution and microbial driving effect in upstream catchment area(CA),estuarine area(EA)and offshore area(OA)at the river basin scale is important for understanding pollutant transport dynamics and pollution risk assessment.In this study,sediments from Duliujian River basin in Tianjin were selected as the research object,and the distribution characteristics of heavy metals in sediments were measured and analyzed at the river basin scale.Then,the microbial driving mechanism and environmental influencing factors of heavy metals in different spatial units of the basin were discussed based on the changes of bacterial flora and physicochemical properties in the sediments.The finding can provide data and theoretical basis for management departments to prevent the heavy metal pollutants and divide risk areas at the river basin scale.It is useful for constructing a reasonable pollution controlling system of"CA-EA-OA".The main results are as follows:(1)The measured values of As and Cr in the sediments of three successive spatial units in the river basin were higher than the local background values in Tianjin.The measured value of Cu in OA,Ni in EA and OA,Zn in the upstream CA is higher than the background value of Tianjin.Compared with different spatial units of the basin,the concentrations of Al,Cu,Fe,Mn and Sr in the sediments of the OA are significantly higher than those in the CA and the EA.The concentrations of As,Cr and Ni are higher in the EA,while the concentration of Zn are higher in the upstream CA.Redundancy analysis(RDA)showed that the physicochemical properties of moisture content(MC),total nitrogen(TN),p H,sediment organic matter(SOM),Salt and total phosphorus(TP)significantly affected the overall distribution of heavy metals in the sediments,and the explanations of MC,TN and p H are higher than 10%,which are the three most important environmental factors affecting the distribution of heavy metals in the Duliujian River Basin.(2)The trends of diversity and abundance of bacterial communities in sediments are similar,which decrease in an order of CA>EA>OA in Duliujian River Basin.Proteobacteria are the dominant bacteria in the sediments of EA and OA,while Actinobacteria are the dominant bacteria in the sediments of CA.RDA result showed that the physicochemical properties of Salt,TP,SOM,p H and TN significantly affected the overall distribution of bacterial phyla.Salt and TP accounted for more than 10%of the variation,suggesting they are the two most important environmental factors affecting the distribution of phyla.Heavy metal element of Fe significantly affected the diversity index of bacteria in sediments,while Mn,Fe,Al,Cu and Sr significantly affected the distribution of phyla in sediments.As,Cr,Ni and Zn had no direct relationship with the changes of bacterial flora,but Fe had a well linear fitting degree with As and Cr.These results indicated that bacteria could affect the accumulation of As and Cr by affecting Fe-As and Fe-Cr compounds.(3)In this study,enrichment factor(EF),geo-accumulation index(Igeo),and Nemero pollution index(P_N)were used for risk assessment of heavy metals.Results showed that the As,Cr,Ni,and Zn in different spatial units reached mild to severe degree of risk.Thereinto,the accumulation degree of As and Cr in the sediments of CA and EA are high,which belong to the status of moderate to severe pollution.The accumulation of Ni in the sediment of EA is high,and it belongs to the state of heavy pollution.The accumulation degree of Zn in the sediment of CA is relatively high,which belongs to the moderate pollution level.The accumulation degree of Al、Sr、Cu、Fe and Mn are low,showing no pollution.Overall,the pollution level of heavy metals in sediments of CA and OA area is moderate,and the pollution level of heavy metals in the sediments of EA is heavy.(4)According to the correlations between environmental factors and risk coefficient,MC in sediment is considered as an important environmental factor in the risk assessment of heavy metals pollution.The concentrations of Fe and Al are considered as important reference elements in the risk assessment of heavy metals pollution.According to the correlation between microorganisms and risk coefficient,Fusobacteria and Chloroflexi at phylum level are considered as important microbial indicators for risk assessment of heavy metals pollution.