| Heavy metals are non-degradable in the environment and most of them have toxic effects on living organism. Therefore, heavy metal pollution in river sediment has drawn attention worldwide. The content of heavy metal is an important index to evaluate the quality of river. The Red River is the second largest river in Vietnam, but there has been a lack of source-to-sink study on heavy metal pollution in sediments from the Red River. A total of 50 surface sediment samples were obtained from the river bank along the main channel from upstream down to the coast and subaqueous delta. The samples were subjected to mineral magnetic, particle size and geochemical analyses. The purpose of this research is to assess heavy metals pollution status in the Red River sediments and its spatial variations. The factors influencing heavy metal contents are discussed. Furthermore, the feasibility of using environmental magnetic method in heavy metal pollution study is explored.The main results are as follows:1. From upstream down to the river mouth, sediments become coarser first and then finer. The prodelta sediments have a higher clay fraction than the floodplain sediments. In the subaqueous delta, the sediments from the shallow water depth (<5 m) are dominated by sand fraction, sediments from deeper water depth are dominated by silts and clays. The particle size composition reflects the spatial variations of hydrodynamics in different sedimentary units of the Red River.2. Two groups of heavy metals (Cr, V, Cu, Cd, Pb, Ni and Zn) can be distinguished in the Red River sediments. (1) Cr and V show strong correlations with Al, Fe, Mn and TOC concentrations as well as particle size, indicating the enrichments of these metals in the fine fraction of sediments with abundant Fe/Mn oxides, clay minerals and organic carbon. It suggests that particle size is one of the major factors influencing heavy metal concentrations. It means that these two metals are derived primarily from natural sources such as rock weathering processes. (2) Cu, Cd, Pb, Ni and Zn concentrations do not show significant correlations with the particle size. They show no obvious trends along the river. At some upstream sites, Cu, Cd, Pb, Ni and Zn show extreme high concentrations, suggesting that these heavy metals may have anthropogenic source input.3. According to the enrichment factor (EF) of Cu, Cd, Pb, Ni and Zn, it is found that heavy metals pollution is present along the Red River from upstream down to coast and is more serious in the upstream. The study indicates that Cd, Cu and Pb are the dominant pollutants in the Red River with their concentrations reaching moderate to high degree of pollution.4. Magnetic susceptibility (χ) and SIRM show similar trends along the river, suggesting that ferrimagnetic minerals dominate magnetic properties of the sediments. Magnetic mineralogy mainly consists of magnetite and hematite in the Red River sediments. Particle size is one of the most important factors influencing magnetic properties of the sediments. Magnetic parameters χfd%, χARM, χARM/χ and χARM/SIRM exhibit positive correlations with the <4 μm fraction, while S-ratios are negatively correlated with this fraction, suggesting that fine SP/SD magnetite grain and hematite are preferentially associated with the clay fraction. From upstream down to the river mouth and subaqueous delta, grain-size of ferrimagnetic minerals becomes finer due to the increase in the clay fraction. The variations in magnetic properties response to particle-size compositions, and magnetic parameters can be used as proxies for clay content.5. In this study, no significant correlations are found between magnetic susceptibility (x) and heavy metals contents. It is quite possible that magnetic minerals and heavy metals are derived from different sources.χfa% shows significantly correlations with Cr and V, with Mn, Ni, Zn, and Pb to a less extent; but no correlations with Cu and Cd. This is due to the fact that superparamagnetic (SP) grains are commonly associated with clay fractions, Al and Fe. Since Cr and V are primarily derived from weathering source, and their contents are controlled by the particle size effect, with fine-grained sediments contain higher clay mineralogy and iron oxides having high capacity for binding heavy metals. Therefore, χfd% is significantly correlated with Cr and V. Since Cu and Cd are heavily polluted, and their contents are largely not due to particle size effect, therefore they show poor relationships with χfd%. Ni, Zn and Pb may comprise both natural and anthropogenic sources, and therefore show intermediate relationships with Xfd%. Therefore,χfd% can be used as a proxy for clay fraction, and be used to normalize heavy metal concentrations for particle size effects in the Red River sediments. |
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