Abstract
The distribution, controlling geochemical factors and contamination current status of heavy metals in bottom sediments near Cửa Ông Harbour, Hạ Long Bay have been investigated. 36 surface sediment samples were collected and analyzed for major elements (Al, Ca, Fe, K, Mg and S), heavy metals (As, Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn), organic matter, loss of ignition (LOI), grain size composition and pH. Spatial distribution patterns of heavy metals as well as their controlling factors have been elucidated based on geochemical mapping and statistical methods, such as the Pearson product-moment linear correlation and factor analysis. The results have been showing that the distribution patterns of As, Cd, Cr, Cu, Ni, Pb and Zn are mainly controlled by organic matter and clay minerals and determined by the distribution of the fine-grained fraction (Φ < 63 µm) in sediments. In contrast, Fe and Mn compounds seem to exert some control on the distribution of Co. Carbonates partly control the distribution of Mn, but are not important with respect to the other studied heavy metals. The contamination state by heavy metals has been assessed based on comparison with Canadian, Wisconsin (United States) and Flemish numerical Sediment Quality Guidelines, and calculation of geo-accumulation index (Igeo) and enrichment factor (EF). The results have been indicating that natural processes, such as weathering and erosion of bedrocks, are main supply sources of heavy metals in sediments in the Cửa Ông Harbour. Among the studied heavy metals, only As is of concern, whereas Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn seem to reflect their background concentration in sediments of the Hạ Long Bay.