Distribution Of Biogenic Elements, Heavy Metals And Their Potential Ecological Risk Assessment In Sediments Form Typical Maricultural Areas Of Guangdong Province

Surface sediments and sediment cores were collected from Zhelin Bay and Guishan Island area of Guangdong province, in October 2008 and January 2009. Biogenic elements and heavy metals were analyzed and their potential ecological risks were evaluated. Biogenic elements included biogenic silica (BSi), total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP), and heavy metals included Hg, Pb, Cu, Zn, As, Cr, Co, Ni, Fe, and Mn. The purpose of this study is to understand the resent pollution level of biogenic elements and heavy metals in these two important cultural sea areas, and to elucidate the history and progress of pollution in sediment and sea water. The results indicated the Pollution of heavy metals in surface sediments from Zhelin Bay and Guishan Island areaThe concentrations of biogenic elements were relatively high compared to other sea areas in China, which suggested high eutrophic levels and primary production in these two sea areas. Concentrations of BSi, TOC, TN and TP were in the range of 1.83%-2.98%,0.38%-1.28%,0.04%-0.25%, and 0.02%-0.06% in surface sediments from Zhelin Bay respectively, and 1.26-2.16%,1.17%-1.85%,0.13%-0.17%, and 0.03%-0.19% in surface sediments from Guishan Island area respectively. The higher BSi concentrations in Zhelin Bay matched the diatom dominant phytoplankton community, while the dike in Guishan Island area resulted in the high TOC, TN and TP levels. Based on the TOC/TN ratios, TOC in Zhelin Bay was mainly originated from aquatic organic carbon, while mixture sources of aquatic and terrestrial organic carbons in Guishan Island area. The pollution of TOC and TN in Zhelin Bay belonged to Class II sediments, and TP belonged to Class I sediments; however, the pollution of TOC, TN and TP in Guishan Island area belonged to Classâ…¡sediments.Biogenic elements in sediment core from Zhelin Bay increased from bottom to surface, and the highest concentrations occurred in the uppermost layer of the core for all elements. A obvious inflexion was observed in 50 cm. Based on TOC/TN ratios, the origins of TOC in the upper 50 cm sediments were mainly form aquatic organisms, and mixture origins in the deeper bottom layers. The results suggested the pollution of Zhelin Bay began at the early 1980s, consistent with the population increase and mariculture development. Concentrations of biogenic elements in core from Guishan Island area fluctuated greatly, but were high in upper and lower in bottom as the same as those displayed in Zhelin Bay. TOC and TP were obviously higher in the upper sediments and TN increased significantly in the upper 20 cm. This result also showed the accumulation of biogenic elements inside the dike recently.The levels of most of heavy metals from the surface sediments from both sea areas were below the standard levels of Class I marine sediment (Marine sediment quality of People Republic of China, GB 18668-2002), and only Pb in Zhelin Bay and Cu in both areas were over the standard levels. The average levels of Hg, Cd, Pb, Cu, Zn, As, Cr, Co, Ni, Fe, and Mn were 0.046 mg/kg,0.051 mg/kg,41.73 mg/kg,17.88 mg/kg,67.83 mg/kg,5.47 mg/kg,20.25 mg/kg,7.76 mg/kg,11.42 mg/kg,2.89%, and 0.80%, respectively in Zhelin Bay, and 0.067 mg/kg,0.043mg/kg,24.7mg/kg, 35.17mg/kg,82.96mg/kg,5.47mg/kg,22.59mg/kg,8.49mg/kg,14.06mg/kg,2.91%, and 0.443% respectively in Guishan Island area. Results form correlation and cluster analysis suggested the similar resource and geochemistry behavior of Cr, Co, Ni, Pb, Mn, and Fe.Based on results of five pollution evaluation methods, most of heavy metals has not been contaminated in Zhelin Bay, only parts of the area has been polluted by Mn. While the contaminations of Cu were common in Guishan Island area., next pollution element was Hg. The average value of potential ecological risk index (RI) was 106.33 in Zhelin Bay, indicating no to trace contamination of heavy metals, while average RI value was high up to 499.83 in Guishan Island sea area belonging to extremely high contamination.Levels of three metals (Hg, Cd, Cu) increased from bottom to surface in the core from Zhelin Bay, while no visible patterns in vertical distribution of other metals. All metals increased significantly in the upper sediments in the core from Guishan Island area, and most of them picked at 20-30 cm layer, which indicated serious contamination of heavy metals in this area in 1990s.