Research On The Distribution Characteristics And Environmental Risks Of Heavy Metals In Wuliangsuhai, Inner Mongolia

The Wuliangsuhai Lake is the most typical shallow grassland lake in the semi-arid region of the Inner Mongolia Plateau,and it is also the only drainage channel for farmland drainage,industrial and domestic sewage in the Hetao Irrigation District.In recent years,the pollution of heavy metals in the lake has become increasingly prominent.In this study,the Wuliangsuhai Lake was as the study area,the overlying water,surface sediments,plants and fish samples were collected,and the concentration levels and distribution characteristics of the heavy metals(As,Cd,Cr,Cu,Mn,Ni,Pb and Zn)were investigated.The modified BCR continuous extraction method was used to analyze the chemical fractions of heavy metals in surface sediments,and the difference of the accumulation capacity of heavy metals between in emergent plants and submerged plants was compared.Then,the ecological risk of heavy metal pollution and the health risk of human exposure was evaluated in the Wuliangsuhai Lake.The results not only have important theoretical and practical significance for elucidating the distribution characteristics of heavy metals in the Wuliangsu Lake and the bioavailability characteristics of heavy metals in sediment-plant system,but also provide reliable experimental data for better protection of water resources security in the Wuliangsuhai Lake and the middle and lower reaches of the Yellow River.The results showed:In the overlying water,the mean concentraations of As,Cr,Cu,Mn,Ni,Pb and Zn were 4.66,0.24,0.41,0.80,18.45,1.44 and 0.40?g/L,respectively,which did not exceed the Class ? water body standard of the Surface Water Environmental Quality Standard(GB3838-2002)and the Fishery Water Standard(GB11607-89),and Cd was not detected.However,in the sediments,the mean contents of As,Cd,Cr,Cu,Mn,Ni,Pb and Zn were 39.83,0.28,50.42,19.79,485.74,26.00,53.58 and 52.05mg/kg,respectively,and the mean contents were all higher than that of the background values of the Hetao Plain,in which As and Cd exceed 5.6 and 6.5 times the background value,respectively.The high value areas of heavy metals were located in the Xidatan,Dongdatan and estuarine waters,and the low value area was in the center of the lake.The results of heavy metal pollution assessment and source analysis showed that As and Cd pollution in surface sediments were relatively serious,which was related to the waste emissions from local sulfuric acid plants,smelters and chemical industries,and As also came from non-point source pollution in agriculture.However,Cr,Cu,Ni,Pb and Zn are highly correlated,and they have the same pollution sources.The results of heavy metal chemical fractions in surface sediments showed that the ratio of the effective fraction(the sum of the acid-soluble fraction and the reducible fraction)to the total content was in the order of Cd>Pb>Zn>Cu>Ni>As>Cr,and the reducible and oxidizable fractions of Cd,Cu,Pb and Zn were higher,while As and Cr mainly existed in residue fractions.The risk assessment results of heavy metal chemical frations showed that Cd and Ni were of the medium risk,while the other elements were of low risk.The activity coefficients of the 7 heavy metals were in the order of Cd>Ni>Pb>Zn>Cu>Cr>As.The analysis of the influencing factors of the heavy metals fractions shows that the total contents of Cu and Pb in the surface sediments were positively correlated with the residual fraction.The content of organic matter had the greatest influence on the oxidizable fraction ofheavy metals.The four fractions of heavy metals were affected by the grain size of surface sediments,but the degree of influence was smaller than that by organic matter,and there was no obvious rule.Because the pH in sediments varies little at each sampling point,it has little influence on the distribution of heavy metals chemical frations.The contents of As,Cd and Cr in submerged plant Potamogeton pectinatus L.were higher than the average contents of heavy metals in normal plants,and the highest content was more than 200 times.But,the average contents of Cd and Cr in the roots,stems and leave of the emergent plant Phragmites australi.s were higher than those in normal plants,as well as the contents of As,Cu,Mn and Pb in the roots,and the highest was more than 150 times.It presented that the order of contents was:roots>leave>stems.The enrichment capecity of Phragmites australis to heavy metals was higher than that of Potamogeton pectinatus L..In the surface sediment-Phragmites australis system,Cd had the strongest migration ability from sediment to root and from root to stem,while Cr had the strongest migration ability from stem to leaf.There was a positive correlation between the content of Cd in roots,stems and leaves of Phragmites australis(p<0.05),and there was a positive correlation between the content of As and Cr in Potamogeton pectinatus L.and the extractable fraction of weak acid in sediment(p<0.05).Both the content of heavy metal in Potamogeton pectinatus L and different parts of Phragmites australis was not correlated with the total content in sediment.The contents of As,Cr,Ni and Zn in fish of different diets(herbivores,omnivores and carnivores)in the Wuliangsuhai Lake all exceeded the standards of the United Nations Food and Agriculture Organization/the World Health Organization,the World Health Organization and Food Contamination Limits of China(GB2762-2012)in varying degrees.The results of human health risk assessment indicated that if the local residents ingest the fish in the Wuliangsuhai Lake for a long time,there will be potential non-carcinogenic and carcinogenic risks for them,which should be paid attention to.