The Assessment Of Present Situation And Potential Ecological Risk Of Heavy Metal Pollution In Wetland Sediment Of Xianghai

Wetland is an ecological landscape rich in biodiversities and one of the most important habitats for man, which has irreplaceable functions in flood resisting,runoff regulation,flood and drought control and pollution control.However,it has been found that the decrease of wetland area has greatly weakened its' function of runoff regulation and water storage,which lead to the reduction of plants & animals species and self-purification capacity of wetland.As a result,the extent of lateral erosion is strengthened,which increased the sediment concentration of river and water nutrients contents.Therefore,much importance has been attached to the research on heavy metals distribution characteristics in wetland.In this paper,the status and reason of heavy metal pollution in surface sediment of Xianghai wetland were studied.The total content of heavy metals was extracted through digesting the samples by HCl,HClO₄ and HNO₃,then each specie of heavy metal was extracted by Tessier's method of sequential extracting process and measured by AAS.The distribution and accumulation characteristics,pollution source and species distribution of heavy metals in surface sediment of Xianghai wetland were analyzed.The status of heavy metal pollution in the sediment of coastal wetland of Zhujiangkou was also assessed by Hakanson's potential ecological risk index method.The results showed that:(1) The sediment has been subjected to different extents of Cd,Zn,Cu and Pb contamination in some regions of Xianghai wetland.The average concentrations of Cd,Zn,Cu and Pb in the surface sediment of Xianghai wetland were 0.33mg/kg,55.01mg/kg,35.19mg/kg and 28.55mg/kg,and were 4.17,1.49,3.20 and 2.38 times as much as the mean background value,respectively,among which the concentration of Cd seriously exceeded the standard.(2) The order for the mean values of Cd,Zn,Cu and Pb concentrations in the surface sediment of Xianghai wetland followed the sequence of E section>F section>G section>W section.(3) There was no significant seasonal change of Cd,Zn,Cu and Pb concentrations in the surface sediment of Xianghai wetland between different sampling sites.Generally,the measurement results of heavy metal concentration during different period were consistent,though it was a little higher for August. (4) The correlation analysis of the four kinds of heavy metals indicated that the correlations amang different heavy metals were similar,which implied that the heavy metals in the sediment has almost the same source.(5) Heavy metals in the sediment of Xianghai wetland with different combined forms.The proportions of different heavy metal combined forms were different in the surface sediment.Pb and Zn mainly existed as organic and residual forms,while Cu mainly existed as organic form.The contents for organic forms of Cu and Pb were high then residual forms.The composition of Cd is very special,and the contents for exchangeable fraction and Fe-Mn oxide fractions reached 24%and 36.12%,respectively,while the content for residual fraction was very low.(6) According to the results of Cu,Pb,Zn and Cd in the surface sediment of Xianghai wetland,Xianghai wetland has been subjected to different extents of heavy metal pollution.The pollution caused by Cd was the most serious,followed by Zn and Cu.The pollution index of section E was 2.51,and the pollution degree was medium.The pollution index of section F was 1.74,and the pollution was at alert level.Section G and W were safe region,with their pollution index lower than 1.(7) The heavy metal assessment of surface sediment in Xianghai wetland by Hakanson's potential ecological risk index method indicated that:according to the potential ecological damage parameters of certain heavy metal,the order of different heavy metal pollutions follow the sequence of:Cd＞Cu＞Pb＞Zn, among which the pollution of Cd was particularly serious and others not.According to the integrated potential damage index of different heavy metals,the RI of section E and F belonged to 150-300,which was medium risk level.And the RI of section G and W were lower than 150,belonged to lower risk level.The order for soil heavy metal pollution of different sections followed the sequence of E＞F＞G＞W.