Research On The Pollution Characteristics And Potential Ecological Risk Of Heavy Metals In Surface Sediments From The Xinan River And Its Tributary Dalangwo Stream

The main river pollution including internal pollution and external pollutions, and sediment pollution were investigated as the main representative in the internal pollution. As sources and sinks of heavy metals and other pollutants, sediment would cause secondary pollution. Xinan River was an important surface water bodies connecting Guangzhou and Foshan. In recent years, with the rapid development of industry and agriculture and the urbanization process in the Xinan River basin, and pollution emissions growth and the environmental protection facilities construction lags behind, resulting in serious pollution to Xinan River. In order to provide important scientific basis for pollution treatment and control planning of the Xinan River and its serious polluted branch Dalangwo Stream, the pollution characteristics and chemical speciation of heavy metals in surface sediments were carried out.Samples of the surface sediment were taken at11positions along the Xinan River basin and7positions from the key tributary Dalangwo Stream. Parts of the sediment samples were analysed by X-ray fluorescence spectrometer method for elemental analysis, while the contents of heavy metals of Cu, Zn, Pb, Cd, Cr, Mn and Fe in the surface sediments were carried out by microwave digestion-atomic absorption spectrophotometry, and the chemical speciations of heavy metals of Cu, Zn, Pb, Cd and Cr in surface sediment were determined by the Tessier sequential extraction method. Bassed on which, the source, pollution state, distribution rule and chemical speciation characteristics of heavy metals in the surface sediment from the Xinan River basin and its branch Dalangwo Stream were studied. The pollution level and potential ecological risk of the heavy metals in surface sediments were evaluated on the basis of analysis of the total amount and chemical speciations of heavy metals. The main findings were as follows:1. The heavy metal concentration (mg/kg) of Xinan River sediment ranged as flowings: Cu,58.53-4522.65; Zn,114.18-1322.43; Pb,54.53-550.00; Cd,1.01-5.81; Cr,10.54-2535.64; Mn,95.45-949.40; and Fe,22264.18-54044.36in the surface sediments. The average Cu, Zn, Pb, Cd and Cr were1.39,0.91,0.31,2.45and0.84times higher than Environmental Quality Standard for Soils â…¢, and the pollution degree of heavy metals was established as the following orders:Cd>Zn>Cu>Cr>Pb. The contents of heavy metal were1.81(Pb) to19.45(Cd) times higher than the Pearl River Delta soil background values, respectively. Obviously, the pollution of heavy metals in Xinan River sediment was serious, and the most serious pollution section was S2located in the industrial zone.Dalangwo Stream sediment heavy metal concentration (mg/kg) were:Cu,52.95-2168.95; Zn,415.00-3743.75; Pb,7.05-1884.60; Cd,0.97-8.72; Cr,82.30-762.40; Mn,73.85-240.05; and Fe,22544.64-43191.96in the surface sediments. Compared with Environmental Quality Standard for Soils â…¢, Cu, Zn and Cd were out of limits badly. The average contents of Cu, Zn, Cd and Cr were far beyond the Guangdong soil background values, were higher than8.52-75.14times, respectively. The sediment dredging out form Dalangwo River could not be directly used in farmland as fertilizer because the average concentration of Cu, Zn and Pb exceed the control standards for pollutants in sludges from agricultural use.2. The heavy metals Cu, Zn, Cd and Cr in the Xinan River sediment showed a very significant positive correlation(p<0.01), which indicated that the agricultural wastewater, domestic sewage and industrial wastewater along the river bank may be their common sources of pollution. The correlation of organic matter and heavy metals Cu, Zn, Cd and Cr reached an outstanding level(p<0.01), which showed that organic matter and clay minerals in sediment played an important role in the distribution and concentration of heavy metals.3. The speciation distribution trends of heavy metals in Xinan River and Dalangwo Stream sediments were similar. Cu mainly existed in the organic fraction; Zn mainly in carbonate adsorbed fraction; Pb mainly in residual fraction and Fe-Mn oxide fraction; Cd mainly existed in exchangeable fraction and organic fraction; Cr mainly existed in Fe-Mn oxide fraction.4. Seven kinds of evaluation methods based on total concentration and sequential extraction of heavy metals in sediment were employed to assess the degree of contamination and potential ecological risk of heavy metals in sedimens from Xinan River and its tributary Dalangwo Stream, The results of comprehensive assessment indicated that the potential ecological risk and bioavailability of Cd, Cu and Zn in Xinan River and its tributary Dalangwo Stream were very high, and had great impact on the local environment. Additionally, the more serious pollution sections of Xinan River were S1and S2in the upstream, S9, S10and S11in the downstream, but the more serious pollution sections of Dalangwo Stream were W1and W2in the upstream, W5and W6in the midstream.