Distribution And Ecological Risk Assessment Of Heavy Metals In Surface Sediments Of Typical Inflow Rivers In Nansi Lake

Heavy metal contamination in aquatic environment has been one of the hotspots in environmental research. The surface sediment has the potential to be the source of secondary pollution, thus studies on heavy metal pollution in surface sediments are important in understanding the potential risk of heavy metal contamination in aquatic environment. Nansi Lake is on the east route of China’s South-to-North Water Transfer Project, therefore, the status of heavy metal contamination in typical inflow rivers of Nansi Lake, are vital to the security of water tranfer.In this study, the contents of7heavy metals (Cr、Cu、Ni、Zn、As、Pb、Cd) in the surface sediments of typical inflow rivers of Nansi Lake were determined, as well as sediments characteristics(eg. total phosphorus and pH). The potential ecological risk index and the geo-accumulation index were calculated to evaluate the risk and pollution level of heavy metals in the surface sediments of the study area. The main conclusions were given as follows:(1) Based on China’s envrironmental quality standards for soils of paddy fields, the contents of7heavy metals in this study were below the standard of Level Ⅲ. However, according to the strict sediment quality standards in Canada and Hong Kong, the contents of chromium (Cr), nickel (Ni) and arsenic (As) exceeded the limits of these standards, whereas the content of cadmium (Cd) was lower than the value ruled by the standard. The sampling sites, which exceeded the limits, were generally located in Guangfu River and Xinxue River. One of the reasons minght be that the rivers were significantly closed to urban areas and affected by human activities. The relatively high content of arsenic might be due to the overuse of fertilizers and pesticides around Nansi Lake drainage area. The contents of total phosphorus (TP) in the study area were relatively stable and higher in comparison with that in Nansi Lake. The mean value of total phosphorus in the upstream inflow rivers was greater than that in the downstream inflow rivers. (2) One-way ANOVA analysis showed significant differences in contents of heavy metals in the inflow rivers of Nansi Lake, in some instances. Higher contents of Cr, Cu, Ni and Zn occurred in Guangfu River and Jiangji River, whereas no significant differences were observed in six typical inflow rivers. The highest content of As occurred in Xinxue River and the lowest in Chengguo River, whereas significant differences were found in any two of the six typical inflow rivers. The content of Pb was the highest in Jiehe River and the lowest in Jiangji River, whereas, there were no significant differences between Xinxue River and Xuesha River, and there were significant differences between other four typical inflow rivers. The content of Cd was the highest in Jiehe River and the lowest in Chengguo River. No significant differences were found between Guangfu River, Xinxue River and Xuesha River, or between Chengguo River and Jiangji River, whereas there were significant differences between Jiehe River and any other five inflow rivers.(3) According to the single contamination factor, the values of single contamination factor from Cr, Cu, Ni, Zn, As, Pb and Cd were1.29,2.65,1.78,2.00,1.49,1.64and1.49, respectively, which were all moderate contamination factors. The highest contamination factors were found in Cu and Cd. The averages of potential ecological risk index in six typical inflow rivers were123.98(Guangfu River),140.11(Jiehe River),84.15(Chengguo River),131.04(Xinxue River),104.58(Xuesha River),93.42(Jingji River), thus, the rivers in order of ecological risk was Jiehe River> Xinxue River> Guangfu River> Xuesha River> Jiangji River> Chengguo River. This finding indicated that Guangfu River, Jiehe River and Xinxue River were in the state of moderate ecological risk because the averages were larger than110, and the other three inflow rivers were in the state of low ecological risk because the averages were smaller than110. In terms of the geo-accumulation index, the order of the contamination of7heavy metals was Cu> Zn> Ni> Cd> As> Pb> Cr, and the order of six typical inflow rivers was Jiehe River> Guangfu River> Xinxue River> Xuesha River> Jiangji River> Chengguo River. According to the three assessment methods, the orders were slightly different, whereas Cu pollution was the most serious and Cr pollution was lightest in7heavy metals. The pollution of Jiehe River was the most serious and the pollution of Chengguo River was the lightest in the studied inflow rivers of Nansi Lake.To sum up, water pollution was no longer regarded as the main ecological risk of Nansi Lake wetland following the completion of the east route of South-North Water Transfer Project and the increasingly strengthen for water pollution control. Howere, heavy metals storing in surface sediments might release back to water due to the disturbance by water transfer. The effect of Cu and the pollution of Jiehe River should be paid more attention.