Pollution Of Urban Wetland Sediment By Heavy Metals In Wenzhou,Southeastern China

The total concentration,chemical speciation of heavy metals(Cu,Zn,Cr,Cd and Pb)in sediment samples collected from three representative wetlands in urban area of Wenzhou City were determined.The wetlands were Sanyang Wetland(SYW)located in a lowland of agricultural and residential land use,Weisipu River(WSR)in a industrial area and Wolong River(WLR)flowing by natural forests and fruit plantations in upper reaches and residential households in lower reaches.Distributions of total contents and speciation of metals affected by land uses were analyzed.For better understanding of the risks posed by toxic metals,this study employed different methods to assess pollution levels of river sediments investigated.An attempt using an algorithm combined chemical speciation with Geoaccumulation Index was made to assess ecological risk of heavy metals.WLR,representing forestry and residential land uses,contained lower metal contents in sediment compared to SYW(agricultural and residential)and WSR(industrial).Cadmium and Pb were highest in sediment of Sanyang Wetland,averaged 101.56mg Kg-1 and 228.73mg Kg-1 respectively,while Cr,Cu and Zn were as high as 353.33mg Kg-1,319.92mg Kg-1 and 1584.67mg Kg-1 in Weisipu River sediment.Land uses in Wenzhou had effects on distribution of heavy metals in wetland sediment.The order of land uses influencing metal contents in sediment was:industrial>agricultural and residential>agricultural>forestry and fruits.Principal Component Analysis results indicated that atmospheric sedimentation and transpotation were the primary sources for Pb and electroplating effluent was the prinmary sources for Cd in sediment while fruit industry possibly discharged more Zn,and waste water of electroplate and hardware were primary sources for Cu and Cr.The contents of heavy metals(Cu,Zn,Cr,Cd and Pb)in three representative wetland sediments of Wenzhou were at higher level than those in other urban wetlands of China(33 urban wetlands from 15 cities,including Hangzhou,Ningbo,Nanjing,Shenzhen and Dongguan,et al.)and also higher than those overseas.Metal contamination in sediment in Wenzhou is possibly because rapid industrial economic development since early 1980s and late governance.The percentages of four chemical forms(speciation)of five heavy metals varied in the sediment samples in Wenzhou wetlands.In SYW,Cu and Cr were significantly in the form of oxidizable fraction(F3),taking up 50.56%and 42.42%out of total contents respectively.Residual fraction(F4)of Pb occupied 41.63%,and Cd and Zn in reducible fraction(F2)were 33.44%and 37.92%respectively.It was found the most toxic and risky form,exchangeable fraction,of all the five metals were between 1.21%-25.91%.In WSR,The highest percentage of F1 was 32.13%for Cd,while other four metals existed mainly in the least toxic F4,the percentages of residual form were between 61.64%and 77.03%.The different algorithms of each evaluation system may result in different evaluation results of the same object。The different algorithms of each evaluation system may result in different evaluation results of the same object.The total amount of heavy metal was used to evaluate the results,The Index of Geoaccumulation showed that pollution was the most serious in three rivers,among which the most heavily polluted was Cd.The evaluation results of China’s Marine sediment quality standard,Pollution Load Index and potential ecological risk index showed that the pollution of sediments in Sanyang Wetlands was the most serious,of which the heaviest heavy metal was Cd.The result of the Ratio of secondary phase and primary phase(RSP)showed that three of the three river segments had the most pollution and the most serious heavy metal was Zn.Fuzzy mathematics is a method to model a large numeber of parameters among which relationships are not easy to formulate.The results were not identical when evaluating the potential ecological risk in the sediments by using different assessment methods such as the Index of Geoaccumulation(Igeo)and the Potential Ecological Risk Index(RI).The order of risks of metals assessed by Igeo was Cd>Pb>Cu>Zn>Cr,while RI Cd>Zn>Pb>Cu>Cr.The deterministic Igeo calculated Cd>Zn>Pb>Cu>Cr,and the deterministic RI Cd>Pb>Cu>Zn>Cr.It was mainly depended on weather chemical speciation was considered during the calculation.