Fraction Distribution and Risk Assessment of Heavy Metals in Stream Sediments from a Typical Nonferrous Metals Mining City
典型有色金属矿山城市小河流沉积物重金属形态分布及风险评估

A modified Tessier's sequential extraction procedure was used to investigate the fraction of seven types of heavy metals (Cd, Cr, Cu, Zn, Ni, Pb, As) in the surface sediments from Huixi Stream in Tongling City, a typical nonferrous metals mining city, China. Based on speciation distribution analysis of these metals, contamination degree and ecological risk assessment of heavy metals were conducted by means of risk assessment code (RAC) and mean sediment quality guideline quotient (SQG-Q). The results show that: 1 Cr and As are major composed with residual fractions, Zn, Ni and Pb are mainly constituted of residual and bound to iron and manganese oxides fractions, and Cu is dominated by bounding to organic matter, while Cd exists in approximate mass fractions of exchangeable, bound to carbonates, bound to iron and manganese oxides, and residue. 2 Carbonate and exchangeable mass fractions of Cd, Cr, Cu, Zn, Ni, Pb and As reach 46.48%, 4.62%, 4.05%, 4.12%, 9.17%, 0.97% and 0.03%, respectively. According to the RAC, Cd is of high risk to the environment, Cr, Cu, Zn and Ni are of low risk to the environment, while Pb and As pose extreme low risk to the environment. 3 The SQG index, calculated with SQG-Q, is 10.42, which is far higher than the threshold value 1.0, indicating that the sediment in Huixi Stream has a very high potential for biological toxicity effect. The PEL-Q indexes corresponding to Cd, Cr, Cu, Zn, Ni, Pb and As approach 4.23, 1.14, 20.75, 6.04, 2.33, 4.58 and 41.71, respectively, suggesting that all these metals have great potentials for biological toxicity and the adverse effects will frequently occur.