| The total concentration, speciation of elements (Fe, Mn, Cr, Co, Ni, Cu, Pb, Zn), AVS (acid volatile sulfide) and SEM (simultaneously extracted metals) were determined in sediment cores collected from the Pearl River Estuary (PRE) and adjacent shelf. Based on which, the spatial and temporal distributions, speciation, sources, transport and accumulation were investigated. Several ecological assessment methods, which based on three different assessment systems (total concentration, sequential extraction and AVS/SEM), were applied to assess the heavy metals pollution and potential biotoxicity in sediments from the PRE and adjacent shelf. The interrelationship and correspondence relationship of different evaluation methods were also discussed.The background values of elements in sediment of the PRE and adjacent shelf were determined with the samples collected from sites E4 and C5, where were slightly influenced by human activities. The result showed that the background values were: Fe 20240.4 mg/kg, Mn 306.8 mg/kg, Cr 41 mg/kg, Co 12.3 mg/kg, Ni 23.1 mg/kg, Cu 9.8 mg/kg, Zn 45 mg/kg, Pb29mg/kg, Cd 0.2mg/kg, P 203 mg/kg.The total heavy metal concentrations indicated that there was an obvious difference between the heavy metal contents in the PRE and in the adjacent shelf, especially for Cu, Zn, Cd, with the average contents of heavy metals in the PRE being trinary to quintuple of those in the adjacent shelf. Further, differences in the distribution pattern of heavy metals between the PRE and the adjacent shelf were also found: (1) The Cu content is higher than that of Ni and Co in sediment cores from the PRE, but is lower in the adjacent shelf; (2) the content of Zn is significantly higher than that of Cr and Pb in the PRE, but is slightly higher in the adjacent shelf. These observations may reflect the difference in the sources of heavy metals between the PRE and the adjacent shelf, and reflect the characteristics of the sources. The concentrations of heavy metals in the west shoal and at site 4 were higher than the site 1 and 3. This difference was an integrated manifestation of terrestrial inputs, hydrodynamic conditions and sedimentary environment. According to the vertical distribution, the heavy metal contents decrease with the depth in sediment cores at sites 2, 4 and 7, which reflect that these sites received more land source pollutants and the heavy metal pollution became more and more serious in the past years. Obvious vertical variation trend of heavy metal contents was not observed at other sites in the PRE.The modified BCR-sequential extraction technique was used to determine the speciation (acid-soluble, reducible, oxidisable and residual) of metals (Cr, Co, Ni, Cu, Pb, Zn, Fe, Mn) in sediment samples from the PRE and the adjacent shelf. The results showed that most Cr, Ni, Fe and Co in the residual fraction were more than 60% of their respective content. There was a positive correlation between the recoverable (acid-soluble+reducible+oxidisable) fractions and the total content of heavy metals. This may reflect that heavy metals, which came from human activities, were mainly in the recoverable fractions, not in the residual fraction.The spatial and seasonal distribution of AVS and SEM in sediment cores from the PRE, showed that the AVS content varies in a large range (0～27.47μmol·g-1) and usually increases with depth. On the whole, the AVS content was higher in spring and summer than in winter, and was higher in bottom sediments than in surface sediments. The activity of sulfate-reducing bacteria and content of dissolved oxygen in sediments were the main influencing factor. The SEM concentration ranges from 0.82 to 4.66μmol·g-1 and displays a slightly decreasing trend with depth. The seasonal variations of AVS and SEM may lead to variations in metal bioavailability during the year.Several methods, which based on total concentration, sequential extraction and AVS/SEM, were applied to assess the pollution level and potential ecological toxicity of heavy metals in sediments from the PRE and adjacent shelf. A comprehensive assessment of the PRE indicated that the potential ecological risk of heavy metal was higher at sites 4, 6 and 7 than at sites 1,2 and 3.Generally, the SEM is close to the sum of acid soluble and reducible fractions, and is obviously lower than the total contents of heavy metals. Because the risk assessing methods that are based on total metal content and sequential extractions ignore the immobilization of heavy metals by sulfides, these methods would most probably over-estimate the ecological risk of heavy metals, especially in reducing environments. AVS/SEM analysis is proposed to combine with other methods to better assess the ecological risk of heavy metals in sediments.
|
PDF Full Text Request