Spatial Distribution And Environmental Risk Assessment Of Heavy Metals In Sediment Of The Upstream Yellow River

The water quality of the Yellow River is directly related to the economic development and ecological security of China,especially the western region.However,with the rapid development of industry and agriculture and the continuous exploitation of mineral resources on both sides of the Yellow River,the impact and pollution on the water quality and environment of the Yellow River have been intensified.Heavy metals,which are highly toxic,difficult to degrade and accumulate along the food chain,have always been concern pollutants in aqueous environment.Sediment is the source and sink of heavy metal pollutants in water and an indicator of water environment.In this context,the heavy metals in the sediment of the upstream Yellow River were studied by using spatial interpolation,multivariate statistics,pollution and ecological risk assessment methods and geographic detector model.The distribution characteristics,pollution and potential risk degree,influence factors and sources of heavy metals in the sediment were studied.The results of this study have important practical significance for environmental protection,prevention and control of heavy metal pollution and construction of heavy metal influence model in the upstream Yellow River.The main results are as follows:In sediment of the upstream Yellow River,except for Zn,the average contents of other heavy metals(Fe,Mn,Cu,Ni,Cr,Pb and Cd)were more than those of their bank soil and background value(world shale content),especially Cr and Cd were 2.62 and 1.03 times of soil content,showing that human activities had certain influence on all heavy metals in sediment,especially on the influence of Cr and Cd.The distributions of Fe,Mn,Cu,Zn and Pb in the sediment were more concentrated,and the distributions of Ni,Cr and Cd were more dispersed.In space,most of the heavy metals had high values in Jianzha country of Qinghai Province.However,Fe,Mn,Cu,Ni and Zn showed low values in Maqu County,Gansu Province.The distributions of Fe,Mn,Cr and Pb were relatively stable and similar and demonstrated a decreasing trend along the river.The distributions of Cu,Ni and Zn along the river were relatively random and not regular.The distribution of Cd along the river had the largest difference,and its distribution was relatively stable in Qinghai section and then increased slightly with obvious oscillation.The enrichment levels of sediment heavy metals were not high and unpolluted,but there were differences among the heavy metals.Fe,Mn,Cu and Pb in the sediment of the upstream Yellow River were all mean EF<1 for non-enrichment,I_geo<0 for non-pollution and ERI<40for low risk levels.However,mean EF of Zn,Cr and Cd were 1.36,1.56 and 1.25,respectively,showing that Zn,Cr and Cd were slightly enriched levels.The I_geoof sample sites of Zn for 22.95%,Cr for 14.75%and Cd for 6.56%was between 0 and 1,which was slightly polluted level.In terms of potential ecological risks,Zn and Cr were low risks,but the ERI of sample sites of Cd for 27.05%was between 40 and 80,which belonged to medium risks.Therefore,Cr and Cd were the main pollution factors.The concentrations of sediment heavy metals with different land use types were different in the upstream Yellow River.The concentrations of all heavy metals in the sediment were relatively high in industrial and cropland areas for slight enrichment and slight pollution levels and low to medium potential ecological risk.In the water area,the concentrations of 6heavy metals were the smallest with no enrichment,no pollution and low risk.Among the six heavy metals,Cd was the most affected by land use types.The change order of Cd EF in different land use types was industrial area(1.93)>forest area(1.92)>cropland area(1.65)>urban area(1.04)>Water area(0.27),indicating that all except the water area was the micro-enrichment level.The I_geo of Cd in forest area,cropland area and industrial area was0.32,1.07 and 1.07,respectively,indicating that Cd was slightly polluted in forest area and moderately polluted in industrial area and cropland area.The ERI of Cd in cropland,forest,urban area,industry and water area were 64.50,58.00,20.00,71.00 and 4.50,respectively,indicating that the risk level of Cd in cropland,forest and industrial area was medium,while the risk level of Cd in urban area and water area was low.The selected factors affected the spatial distribution of sediment heavy metals,and the soil type and land use type had little influence on the spatial distribution of heavy metals in sediments.However,the key influencing factors varied with different environments.The key influencing factors for the spatial differentiation of sediment heavy metals in Ningxia section were road network density(RD)(Q_D,H=0.78)and per capita income(PI)(Q_D,H=0.68),while vegetation coverage(CV)had little influence(Q_D,H=0.08).The key influencing factors in Inner Mongolia were vegetation coverage(VC)(Q_D,H=0.76)and DEM(Q_D,H=0.74),and the influence of road network density(RD)was very small(Q_D,H=0.17).Based on multivariate statistical analysis,it was concluded that the sources of Fe,Mn,Zn and Cr in the sediment of the upper reaches of the Yellow River were mainly natural sources,Ni and Pb were from different anthropogenic sources,and Cu and Cd were from both natural and anthropogenic sources.Based on the geodetector model,it was concluded that Fe,Mn,Cu and Pb were mainly from natural sources,Cd was from anthropogenic sources,and Ni,Zn and Cr were from natural and anthropogenic sources in Ningxia section.In the Inner Mongolia section,Fe,Mn,Zn and Cr were mainly from natural sources,Cu and Pb were from artificial sources,and Ni and Cd were from both natural and artificial sources.The sediment heavy metals were all affected by human activities.The specific anthropogenic sources of each heavy metal were as follows:In Ningxia section,Cu and Pb from transportation,human economic activities and sediment migration;Ni and Zn mainly by atmospheric deposition and human illegal discharge;Cr from industrial emissions;Cd from transportation and agricultural production.In the Inner Mongolia section,Cu from agricultural production;Ni mainly by agricultural production and a small amount by industrial production;Zn and Cr mainly from industrial discharge and sediment migration;Pb and Cd mainly from atmospheric deposition and industrial emission.