Vertical Distribution Characteristics Of Heavy Metals In Urban Wetland Sediments

Urban wetlands serve as the sink of heavy metals and other pollutants because there are large number of residential houses and manufacturing facilities in the vicinity of rivers and lakes in urban areas.There is great significance to understand vertical distribution of heavy metals on the profile of sediment.Sanyang Wetland,located in urban area of Wenzhou City,is a typical wetland in Southeastern China.This study analyzed sediment cores collected from Sanyang Wetland and investigated the fates of five toxic metals in sediment.Eight sediment cores(labeled 1 through 8 from top to bottom)of 60cm in height were collected from Sanyang Wetland rivers affected by three land uses.Cores number 1 through 3 made sample 1(S1)for land use of residential area,number 4,5 and 6 for residential and agricultural land use(S2),and 7 and 8 were agricultural(S3).Cores were sliced into 8 sub-layers in a anaerobic chamber full of nitrogen.Those sub-layers were grouped into three layers for furture analysis according to cluster analysis of heavy metals content.The upper layer was 0-15 cm,middle layer 15-40 and bottom 40-60 cm.Eight cores and.8 sub-layers produced 64 sub-samples for laboratory analyses of the total.content and chenical forms of heavy metals(Zn,Cu,Pb,Cr,Cd),as well as total phosphorus(TP)and total organic carbon(TOC).The following results were reached by data analysis.(1)The average content of Cr,Zn,Cu,Pb and Cd in Sanyang Wetland sediments was 237.48 mg/kg,616.17 mg/kg,180.86 mg/kg,113.20 mg/kg and 28.50 mg/kg respectively,all greatly exceed the background value of soil in Wenzhou and also higher than that in seven rivers(Yangtze River,Yellow River,Songhua River,Songhua River,Huaihe River,Pearl River)and five lakes(Chaohu,Dongting Lake,Poyang Lake,Taihu,Hongze Lake)in China.According to the chemical forms(exchangeable,oxidized,reducible and residual marked with F1,F2,F3 and F4)of metals in Sanyang Wetland sediments,Cr exists in the form of oxidized,accounts for 31.47%of total forms,Zn exists in form of exchangeable whose proportion is over 50%,while Cu mainly exists in oxidized and exchangeable forms,accounted for 41.19%and 39.16%respectively.F3 of Pb was 35.05%which exists in form of reducible.Cd's mainly existence form is exchangeable as well,which accounts for 73.64,and oxidized Cd cannot be found in most samples.The ratio method of secondary and primary phases(RSP)was hired to evaluate the heavy metal forms in sediments,the results indicated that the second constituent have higher proportion,which means the source of heavy metals is closely related with human being activities.(2)The heavy metals in the sediments of Sanyang Wetland has been affected by land use.Which causes the amounts of Zn,Cu,Pb,Cd in the living area increase.The content is 877.29 mg/kg,190.06 mg/kg,152.99 mg/kg,34.47 mg/kg respectively.Cr has the highest content in section S2 of 263.55 mg/kg,The living area makes the greatest contribution to the heavy metal pollution in the Sanyang Wetland sediments.(3)There are different forms and types of heavy metals in the vertical distribution of the sediments in the Sanyang Wetland:In terms of morphology,the exchangeable content of heavy metal in surface.sediments is higher,which is between 2.43%to 88.42%,Among them,the exchangable of Zn and Cd accounts for more than 50%,and is easy to transfer for the heavy metals.In terms of total volume,Zn,Cu,Cr and Cd in sediments has lower amount on upper layer and higher amount on middle and lower layer in the vertical distribution.However,Pb has a small change in the vertical distribution,which can be considered as the result of the reduction of heavy metal emissions in recent years,but there is still some pollution.(4)There are significant effects on benthic organisms in the sediments of Sanyang Wetland,which showed highly biotoxic detected by mean sediment quality guideline quotient,the toxicity sequence of heavy metal are:Cd>Zn>Cu>Cr>Pb,The ratio method of secondary and primary phases shows that the ecological threat of heavy metal is serious in both horizontal and vertical distribution,among which the biggest threat is Cd pollution.The sediments of Sanyang Wetland is not suitable for agricultural use?reclamation or utilization ways without pretreatment.(5)Correlation analysis found several pairs of strong relationship.Zn and Pb were strongly correlative each other in total content and speciation of F1,F2 and F4,implying same sources of this two metal species deposited in sediment.Cd and Cu had strong correlation in total content and reducible form possibly because there were sane sources im the form of oxide-bound metals.Cd and Cr were strong only in reducible form,which indicated sufficient sources of oxides to bound bivalent metals.TOC was closely related with F3 of the metals,except Cd in which oxidizible form was not detected in majority of the samples,showing the reliability of our sample analysis.