Quantities And Distributions Of Sediment And Heavy Metals In Anurban River System Of Wenzhou,China

The present study developed a method that can create sediment surfaces of plain river systems and more accurately calculate the amount of sediment and pollutants stored in river deposits,by combining GIS technologies with chemical analyses of sediment samples.The method was applied to theSanyang Wetland which is a typical urban river system of the Wenruitanghe Plain River Network in Wenzhou City,southern China.An on-boat sonar detector was employed to acquire the dataset of more than26,000 elevation points of sediment surface.Digital bathymetric surface was then interpolated from the dataset of these elevation points.To find the best model for creation of the bathymetric surface,Triangulated Irregular Network(TIN)and four other interpolation tools were compared.It was validated that anisotropic ordinary Kriging(aniso OK)was the best algorithm for the urban river system to be interpolated into the riverbed surface.The interpolation was conducted on an Esri ArcGIS platform,and the total volume of the polluted sediment in the Sanyang Wetland was calculated with the “3D Analyst” tools of the software.The chemical analyses of sediment samples collected from representative river sections in the study wetland provided data for estimation of storage and distribution of heavy metals in sediment.Samples were collected for analyzing sediment properties and concentrations of six heavy metals(As,Cr,Cu,Ni,Pb and Zn).We also determined four chemical fractions(exchangeable,oxidable,reducible and residual according to the three-step BCR sequential extraction technique)of heavy metals in sediment samples.When compared with the residual fraction(F4),the fractions of exchangeable(F1),oxidable(F2)and reducible(F3)are less stable and thus they have a higher risk of migration to ambient waterbodies.We first calculated the total sediment mass and total amounts of heavy metals(total and four chemical fractions for each metal)in the Sanyang Wetland.The calculated results were used as parameters to estimate total sediment volumes,totalamounts of six metals and their fractions in the three districts of Wenzhou City,including Lucheng,Ouhai and Longwan.Total sediment volume in Lucheng,Ouhai and Longwan was estimated to be 6.53,18.17 and 9.38 million m3,respectively.Statistical analysis of heavy metals in the sediments of Sanyang Wetland indicated that Zn,Ni,Cr and Cu had a strong correlation in the sediment of the river section which is greatly affected by human activities,implying that these four heavy metals have the same pollution source.It was inferred that their source is from the electroplating industry.Moreover,the sum of F1,F2,and F3 fractions of heavy metal in the sediments of the three districts was 8017 tons for Zn,541 tons for Ni,483 tons for Cu,102 tons for Cr,45 tons for Pb,and 3.7 tons for As,respectively.This would raise worrying concerns that these heavy metal elements may potentially migrate into the water under some specific environmental conditions.