Spatial risk assessment and sources identification of heavy metals in surface sediments from the Dongting Lake, Middle China
Introduction
Heavy metal contamination in aquatic environment has drawn particular attentions due to their toxicity, persistence and biological accumulation (Jiang et al., 2012, Varol, 2011). Direct hazardous effects or potential risks to human health and ecosystem stability can be caused by heavy metal residues through multi-exposure pathways owing to their transport and transformation among multi-media such as ambient air, soil, surface water, sediments etc. (Cooke et al., 1990, Mackay, 2001, Zeng et al., 2009).
Heavy metals have low solubility and primarily get absorbed and accumulated on bottom sediments. Bottom lake sediments are sensitive indicators for monitoring pollutants as they act as a sink and a carrier for contaminations in aquatic environment (Bai et al., 2011, Caeiro et al., 2005, Suresh et al., 2012). The only measurements of pollutants in water are not conclusive owing to water discharge fluctuations and short resident time. The same remains true for the suspended matter. The study of bottom lake sediments plays an important role for their longer residence time, and the role is called “the record of history” (Mackay, 2001). Spatial distribution and concentrations of heavy metals in the bottom lake sediments are effected by both natural environment factors and anthropogenic factors (Lalah et al., 2008). Natural environment factors are including benthic agitation, flow changes and natural erosion etc. Anthropogenic factors are including sewage discharge, industrial wastewater discharge, agricultural fertilizer leaching etc. For this reason, bottom lake sediments are not only the sink of heavy metals, but also potential secondary sources of heavy metals in aquatic system (Segura et al., 2006, Yu et al., 2008). Therefore, it is necessary to investigate spatial distributions of heavy metals in sediments from the Dongting Lake and assess risks caused by these heavy metals in order to protect corresponding aquatic ecosystem.
The Dongting Lake, which includes three national wetland nature reserves, is one of the most important inland freshwater lakes in China. Some reports have been published on the heavy metal levels in water and water-area variations in this aquatic system (Ding and Li, 2011, Du et al., 2001), the geochemistry of trace and rare earth elements in red soils from the Dongting Lake area (Mao et al., 2009), the effect of the Three Gorges Dam Project on flood control in the Dongting Lake area (Hayashi et al., 2008), and the numerous ecological studies (Fang et al., 2007, Li et al., 2000). In this paper, we report the first comprehensive study on spatial distributions, source identification and potential ecological risks of heavy metals in sediments from the Dongting Lake.
The objectives of this study were (i) to determine spatial distributions of heavy metals in surface sediments from the Dongting Lake using contour maps based on geostatistics method of the inverse distance weighted (IDW) interpolation, (ii) to assess potential environmental risk of these heavy metals by comparison with sediment quality guidelines (SQGs), (iii) to further study potential ecological risk of these heavy metals employing Hakanson's method with consideration of the model uncertainty, and (iv) to define the natural and/or anthropogenic sources of these heavy metals using combined multivariate statistical techniques.
Section snippets
Study area
The Dongting Lake is the second largest freshwater lake in China, with an extensive catchment area from 28°30′N to 29°38′N and 112°18′E to 113°15′E (Ding and Li, 2011) (Fig. 1). The Lake spans the Hunan and the Hubei province, and has the Xiangjiang River, the Zi River, the Yuan River and the Lishui River as its south and west input, the Miluo River and the Xinqiang River as east input, the Songzi estuary, the Taiping estuary and the Ouchi estuary as three bleeder of Yangtze River water flow.
Mean heavy metal concentrations in surface sediments
The basic statistics for all heavy metal parameters measured during the sampling period at 12 typical sites as well as background values of the Dongting Lake (Li et al., 1986), and corresponding values based on SQGs (MacDonald et al., 2000, Smith et al., 1996) were summarized in Table 2. According to the background values of heavy metals in the Dongting Lake, it indicated that heavy metal concentrations in surface sediments of this study were all higher than their corresponding background
Conclusions
In this work, 7 heavy metals in surface sediments from the Dongting Lake were investigated. Then spatial risk assessment at the screening level and source apportionment of these heavy metals were studied based on geostatistics methods, SQGs, Hakanson's method and integrated multivariate statistical methods (Pearson's correlation analysis, FA and CA included). The mean concentrations of studied heavy metals all exceeded the geochemical background values of the Dongting Lake. According to the
Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (51178172, 50978088, 51039001, 50808071, 51009063).
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