Distribution and speciation of heavy metals in sediments from the mainstream, tributaries, and lakes of the Yangtze River catchment of Wuhan, China

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Abstract

A comparative study of the heavy metal (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) concentrations in sediments collected from the mainstream, tributaries, and lakes of the Yangtze River catchment of Wuhan, China, was conducted. Compared with the maximum background values in Wuhan Province soils, Cd was the metal with the highest contamination level, especially in the mainstream, followed by Zn and Cu. In a few samples from tributaries and lakes, the Hg concentration was very high compared to background levels. The concentrations of As and Ni in all sites fluctuated close to background levels. Partitioning of speciation of each heavy metal in sediments was similar for samples taken from the mainstream, tributaries, and lakes. Compared to the other metals studied, Cd, Cu, Zn, and Pb had higher bioavailability in the three zones, which means they pose a higher ecological risk. Significant correlations among group a (Zn, Cu, and Pb, r > 0.9) and group b (Cr and Ni, r = 0.978) in the mainstream; Hg, Cu, Cd, and Pb in lakes (r > 0.9); and Cu and Pb (r > 0.9) in tributaries were observed using Cluster and correlation analysis. However, a low correlation between As and the other elements in the three zones was shown. Overall, 63.6% of samples from the mainstream, 75.0% from tributaries, and 88.9% from lakes exhibited low and moderate ecological risk of heavy metals, and the potential ecological risks in the mainstream and tributaries were higher than those in lakes.

Introduction

The Yangtze River is the largest river in China and the third largest in the world. Many tributaries and lakes within the middle reaches of the mainstream of the Yangtze River form a complicated watershed within Wuhan City, which is the industrial and economic center of central China. Aquatic resources in this catchment constitute the drinking water supply for Wuhan and the downstream communities. In addition, because of south-to-north water diversion, the water quality of the Yangtze River catchment of Wuhan also affects the residents in northern China. Therefore, the Yangtze River catchment of Wuhan is a crucial resource for China; millions of inhabitants are impacted daily by water from this catchment, and it thus affects social, economic, and community health.

Unfortunately, overpopulation, local soil erosion, inadequate water use management, and intensive deforestation have severely reduced the river's water quality. Significantly, large uncontrolled contaminant inputs, including heavy metals from industrial and urban sources, have contributed to increased pollution of the mainstream, tributaries, and lakes and have accumulated in the sediments [1], [2], [3]. Heavy metals are serious pollutants because of their toxicity, persistence, and non-degradability in the environment [4], [5], [6]. When the environmental conditions of the water lying over the sediments change, heavy metals in the sediments can be released into the water, thereby deteriorating the quality of the water [7], [8]. A geochemical sediment survey of heavy metals in the mainstream, tributaries, and lakes of the Yangtze River catchment of Wuhan is crucial to assessing the ecological risks of heavy metals in this region.

The potential ecological risk of each heavy metal in sediments is determined by its total content and speciation [9], [10]. Depending on chemical and geological conditions, heavy metals can be partitioned into different chemical forms that are associated with a variety of organic and inorganic phases [11]. Thus, speciation analysis of heavy metals might provide much useful information regarding the chemical nature or potential mobility and bioavailability of a particular element, which consequently can offer a more realistic estimate of actual environmental impact [12], [13], [14].

To date, many methodologies have been developed to assess ecological risks of heavy metals [15], [16], [17]. However, most of them are suitable only for ecological risk assessment of a single contaminant (e.g., the geoaccumulation index method) [17], [18]. In reality, many kinds of heavy metals usually accumulate simultaneously and cause combined pollution. To address this, Hakanson [19] developed the potential ecological risk index, which introduced a toxic-response factor for a given substance and thus can be used to evaluate the combined pollution risk to an ecological system.

In this paper, comparative studies of concentration, speciation, and ecological risk of heavy metals in sediments from the mainstream, tributaries, and lakes in the Yangtze River catchment of Wuhan were systematically conducted. The results provided a comprehensive look at the current sediment contamination status of heavy metals and the potential origin of contamination in the three zones. These data can form the foundation for the protection of water quality of the Yangtze River catchment of Wuhan.

Section snippets

Study area and sample collection

The Yangtze River catchment of Wuhan lies in the middle reaches of the Yangtze River. It contains many lakes and tributaries, including the Hanjiang River, which is the largest tributary of the Yangtze River. Approximately 8 million people reside in this area. Industrial activities, such as chemical, electric plating, and refining industries in the Hanyang industrial zone, are growing fast. Therefore, urban sewage and industrial wastewater could be the source of the load of pollutants

Levels of heavy metals

Table 1 lists the concentration range, median, mean, S.D., skew, and CV of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn in the sediments from the mainstream, tributaries, and lakes of the Yangtze River catchment of Wuhan and the maximum background values of the soils in Wuhan. Two levels were considered: the threshold effect level (TEL), below which adverse biological effects are expected to occur rarely, and the probable effect level (PEL), above which adverse effects are expected to occur frequently.

Conclusions

The comparative assessment of heavy metal concentrations (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) in sediments from the mainstream, tributaries, and lakes of the Yangtze River catchment of Wuhan, China, indicated that Cd is the highest contaminating metal, especially in the mainstream. The accumulation of Zn and Cu was the second most serious problem. In some samples from tributaries and lakes, the concentration of Hg was very high compared to background levels; however, such accumulation was not

Acknowledgements

The research was supported by the National Basic Research Program of P.R. China (973 Project, 2003CB415204) and National Natural Science Foundation of China (No. 50708007).

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