Distribution Characteristics Of Heavy Metals In Surface Sediments Of Hadashan-Songhua Lake Section Of The Songhua River

In recent years,rapid industrial and agricultural development has led to an increase in heavy metal contamination,eapecially the heavy metal contamination in sediments,which has attracted much attention.Songhua River is one of the seven major rivers in China,and it plays an important role in water environment treatment of this river basin through the investigation of the spatial-temporal distribution of heavy metals in sediments.Taking surface sediments of the Songhua River（Hadashan-Songhuahu Lake section）collected in 2006,and 2016 as example,we determine the mineral composition and chemical composition of the sediments,and chemical composition of the clays extracted from some sediment samples,including contents of As,Hg,Cr,Cd,Pb,Cu,Ni,Zn and so on.Then,we clarify the the spatial-temporal distribution characteristics of heavy metals in surface sediments,and research the status of heavy metal pollution and pollution source.Results of the fourier transform infrared（FTIR）spectroscopy and the X-ray diffraction（XRD）analyses show that the surface sediment samples are mainly consists of quartz,feldspar,kaolinite,illite,chlorite,and mixed-layer illite/smectite.Chemical composition analysis results reveal that clays are relatively enriched in elements As,Hg,Cr,Cd,Pb,Cu,Ni,and Zn,which indicates that the content of heavy metals in surface sediment is significantly controlled by the mineral composition.Compared to some major rivers and lakes in China,the contents of Hg,Cd,Pb,and Zn of the surface sediments collected in 2006 are quite high,and the contents of As,Cr,Cu and Ni are moderate.However,Hg concentrations in surface sediments collected in 2016 are significantly higer than those of the major rivers and lakes in China,and concentrations of As,Cr,Cd,Pb,Cu,Ni,and Zn are at moderate levels.The spatial distribution characteristics of heavy metals in surface sediments of 2006 is very similar to those of 2016,i.e.,heavy metal conctents in surface sediments of the Jilin urban area are higher than those of the non-urban area,and almost all the heavy metal contents reach the maxium near the Jiuzhan Bridge of Jilin city,and decrease gradually in both the upper and lower reaches.Moreover,the changing trend of heavy metal contents over time is very remarkable,i.e.,compared with heavy metal conctents in surface sediments of 2006,there is a certain degree of decline in those of 2016.A standardized method is applied to eliminate the mineral composition influence,and the spatial-temporal distribution of heavy metals in surface sediments changes,but the change is not significant.According to the index of geo-accumulation(I_geo)and the potential ecological risk index（PEPI）,the spatial distribution characteristics of pollution levels of heavy metals in surface sediments of 2006 is very similar to that of 2016,i.e.,heavy metal pollution levels in surface sediments of the Jilin urban area are much higher than those of the non-urban area,and almost all the heavy metal pollution levels reach the maxium near the Jiuzhan Bridge of Jilin city,and decrease gradually in both the upper and lower reaches.The surface sediments in 2006 are contaminated mainly with Hg,Cd,and Zn,and almost all their pollution levels are higher than levelⅡin the urban area of Jilin city.In the study river basin,the PEPI of Hg and Cd reach serious and much higer levels,respectively,and the PEPI of the total heavy metals reaches serious level.Compared with heavy metal pollution levels in surface sediments of 2006,there is a great decrease in those of the 2016,and the PEPI of the total heavy metals decreases to much higher level.However,the Hg pollution is still very serious.Moreover,the contamination level of Hg is extremely high（levelⅥ）near the Jiuzhan Bridge of Jilin city.There is a serious potential ecological risk of Hg in the study area.According to the spatial-temporal distribution of heavy metal polluitons,element associations,and the important heavy metal and industrial wastewater sources in Jilin city,we conclude that the industrial activities have contributed to the heavy metal pollution in the surface sediments,especially the industrial wastewater discharge.Compared with ten years ago,except Hg,pollutions of Cd,Zn and other elements have been greatly reduced after environmental treatment.It is noteworthy that,the sediments in the upper reach of the study section collected in 2016 are slightly contaminated with Hg.Therefore,a small proportion of the Hg may originate from the upper reach of the Songhua River.