The distributions of particulate heavy metals and its indication to the transfer of sediments in the Changjiang Estuary and Hangzhou Bay, China
Introduction
The fate of heavy metals in estuarine environment has been discussed extensively (Comans and van Dijk, 1988; Chiffoleau et al., 1999; Griscom et al., 2000; Angelidis and Aloupi, 2000; Turner, 2000). In the estuarine environment river water encounters the marine salt water, and geochemical and biogeochemical processes include sedimentation, precipitation and flocculation of particulate substances. Evolution of the composition of the particle in the mixing zone of the estuary indicates that the heavy metals do not behave conservatively and that they are also affected by the changing physico-chemical conditions, such as salinity, pH and redox (Comans and van Dijk, 1988; Calmano and Hong, 1993; Regnier and Wollast, 1993). However, the relationship between particulate heavy metals and salinity is not definite in high turbidity estuaries (Zhang, 1999). A number of laboratory studies and field studies have addressed the influence of dissolved oxygen on the chemistry of heavy metals under estuarine conditions (Luoma, 1990). The change of the redox conditions as a direct effect on the geochemical behavior of heavy metals in the water column is discussed under oxic conditions. Heavy metals may be bound to various forms of organic matter: living organisms, detritus and coatings on mineral particle, etc. (Tessier et al., 1979; Presley et al., 1980; Daviaon, 1985). Heavy metals are mainly concentrated in the fine-grained size fraction of the solid phase (Zhu et al., 1990; Breslin and Sanudo-Wilhelmy, 1999). The clay minerals are the main carriers of heavy metals during mobilizing and diffusing (Horowitz, 1991). Spatial and temporal distributions of metal concentrations in sediments are influenced by so many factors that it is difficult to find the main one, but some studies have shown that in high turbidity estuaries, biogeochemical exchange processes did not significantly modify the composition of the fine-grained detrital fraction of the suspended matter in the mixing zone and sedimentary dynamics dominate the fate of heavy metals (Zhang et al., 1990a; Zhang and Ying, 1996; Zhang, 1999).
The goal of the research reported here is to improve understanding of the relations between sedimentary dynamics and the pattern of particulate heavy metal distributions in the Changjiang Estuary and Hangzhou Bay and generally an high turbidity estuaries. Objectives of the results presented here are (1) to show the pattern of particulate heavy metal distributions in high turbidity estuaries; (2) to indicate that sedimentary dynamics dominate the fate of heavy metals in high turbidity estuaries; (3) to highlight that the distributions of particulate heavy metals can indicate the transfer of sediments in this area.
Section snippets
Main characteristics of the study area
The Changjiang River, the largest in China, empties into the East China Sea. The Changjiang Estuary is a funnel-shaped estuary, while Hangzhou Bay is the outer reach of Qiantang Estuary and it is a typical funnel-shaped bay (Fig. 1). The average water and sediment discharges from the Changjiang River are 925×109 m3/y and 486×106 ton/y, respectively (Shen et al., 1983), both being one of the largest in the world. Because of its overwhelming water and sediment discharges, the Changjiang River has
Distributions of suspended sediments
Fig. 2 shows that a part of the sediments discharging into the sea diffuse in the direction of east by south, and that it is an important source of the fine-grained sediments of Hangzhou Bay. In Hangzhou Bay, the turbidity maximum is always located in the southwest part and Nan Hui Zui area, while the concentrations of suspended sediments in the northwest and southeast of the bay are low whether in June or in October. A persistent NE–SW front exists inside Hangzhou Bay year-round (Su and Wang,
Conclusions
In the Changjiang Estuary and Hangzhou Bay, particulate Cu and Pb show a similar level, which is higher than the concentrations of particulate Cd. Cu, Pb and Cd concentrations in suspended sediments are higher than those in bed loads and distributions of heavy metals in suspended sediments are similar to those in bed load. Pb and Cd concentrations in suspended sediments are high in June, while Cu concentrations in suspended sediments are high in October. Generally, the concentrations of heavy
Acknowledgements
Funding for this research was provided by the Shanghai Priority Academic Discipline and SKLEC no. 9808.
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