Environmental status of Daya Bay surface sediments inferred from a sequential extraction technique

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Abstract

The chemical partitioning of 12 trace metals (Ba, Cd, Co, Cu, Mn, Mo, Ni, Pb, Sc, Sr, U, Zn) in 9 surface sediments from Daya Bay, northern South China Sea, was studied by a four-step sequential extraction technique to assess their environmental/pollution status. The studied metals take on different fraction composition patterns, and some of them show significant spatial variation especially for Cd. A majority of the studied metals can be considered immobile because of their high percentages in the residual fraction. It seems that the Daya Bay surface sediments have not yet been polluted by the studied heavy metals which are used as parameters to classify marine sediment quality, i.e., Cd, Cu, Ni, Pb and Zn. For all metals except for Pb, the extent of anthropogenic influences on their total concentrations in the Daya Bay surface sediments is generally imperceptible. The metals’ potential risks to the environment were assessed, and the results showed that they have generally no risk or low risk except for Cd, Mn and Sr.

Introduction

Many elements that occur in low concentration in the Earth's crust are now mined extensively for use. Large quantities of different kinds of elements are discharged into the environment as contaminants each year by human activities. Marine sediment serves as one of the major reservoirs for all kinds of contaminants, including metals. The properties of metals in soils and sediments depend on the physicochemical form in which they occur (Gleyzes et al., 2002). Metals associate with sediments through various ways, including adsorption, complexation, and so on (Tessier et al., 1979). Changes in environmental conditions, such as temperature, pH, redox potential and organic ligand concentrations, can cause metals to be released from solid to liquid phase and sometimes cause contamination of surrounding waters in aquatic systems (Sahuquillo et al., 2003). Therefore, it cannot provide sufficient information about mobility, bioavailability and toxicity of metals if their total contents are studied alone.

The ecotoxicity and mobility of metals in the environment depend strongly on their specific chemical forms or method of binding. Consequently, toxic effects and biogeochemical pathways can only be studied on the basis of the determination of these forms (Quevauviller, 1998). The sequential extraction technique is widely used now to provide information about the strength of metal binding to particulates, the phase associations of metals in solid matrix, etc. Many sequential extraction procedures have been put forward, among which the BCR (Community Bureau of Reference, now superseded by the Standards, Measurement and Testing Programme of the European Community) procedure is one of the most widely used procedures, and has been applied to soils, terrestrial/marine sediments and standard reference materials by a large number of researchers (e.g. Mossop and Davidson, 2003, Hjorth, 2004, Yuan et al., 2004).

Daya Bay is a busy region greatly affected by human activities. Its surrounding area is in the important economic development district in Guangdong, China. Petrochemical, plastic, printing and other industries as well as harbors are present. Two nuclear power stations, the Daya Bay and Ling'ao, which came into operation in 1994 and 2002 respectively, are situated at its western coast. Fish, shrimp and shellfish aquaculture are well developed in Daya Bay, and cage-cultured fishery is widespread in its inner waters. The main objectives of this study are to assess the environmental/pollution status of 12 trace metals in the surface sediments covering the whole Daya Bay area, including Ba, Cd, Co, Cu, Mn, Mo, Ni, Pb, Sc, Sr, U and Zn, by determining their concentrations and chemical forms in which they occur using the BCR sequential extraction procedure, and provide a background estimation of baseline data of these metals for future surveys on the responses of Daya Bay environment to anthropogenic activities.

Section snippets

Study area

Daya Bay is a subtropical drowned valley bay of northern South China Sea located in the eastern coast of Guangdong Province, southern China (Fig. 1). It is one of a series of large embayments along the southern coast of China and covers an area of ∼600 km2, with a width of about 20 km and a north–south length of about 30 km. The water depth in Daya Bay ranges from 6 to 16 m and the western part is deeper than the eastern part (Xu, 1989, Han, 1995). Although there is no large river discharging into

Sediment characteristics

The studied sediments are mainly composed of clay and silt, and the percentages of fine fraction (clay + silt) are >90% for most of the samples (Table 1). Sediments of the stations close to the mouth of the bay, however, have relatively less clay but more silt than those of the other stations. Spatially, the CaCO3 content is more variable than TOM. The TOM content varies from 7.1% to 12.6% of the dry sediment weight with an average of 10.1%; the CaCO3 content varies from 4.1% to 12.2% of the dry

Conclusions

The study reveals that the total concentrations of Ba, Co, Ni, Pb, Sc and Zn show a relatively slight spatial variation in the surface sediments of Daya Bay, indicating that they are mainly from non-point source inputs. There is a considerable spatial variation in the total concentrations of other metals, especially for Cd and Sr.

According to the National Standard of China, the total concentrations of Cd, Cu, Pb and Zn are fairly low at present and suitable for mariculture, nature reserves,

Acknowledgments

This study was co-supported by the Chinese Academy of Sciences (KZCX1-YW-06-02, KZCX2-YW-Q07-03) and the CAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology (MSGL08-08), CTA Chen wishes to acknowledge the financial support of the Science Foundation of Taiwan: NSC 97-2621-M110-002.

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