Reconstructing the historical pollution levels and ecological risks over the past sixty years in sediments of the Beijiang River, South China
Graphical abstract
Introduction
Trace metal contamination can seriously impact aquatic organisms and threaten human health via the food chain due to the toxicity, persistence and bioaccumulation problems associated with these metals (Horai et al., 2007; McCormick et al., 1994; Roman et al., 2007). Metals enter a river system via natural inputs (e.g., rock weathering) and anthropogenic inputs. Anthropogenic sources include mining activities, smelting industries, and waste incineration as well as biomass and fossil fuel combustion. Among these sources, smelting wastewater and/or acid mine drainage (AMD) have been the most concerning due to the serious damage they cause to the aquatic ecosystem (Sakan et al., 2015; Audry et al., 2004; Axtmann and Luoma, 1991; Equeenuddin et al., 2013; Galán et al., 2003; Rybicka et al., 2005; Schaider et al., 2007). As an important part of river ecosystems, sediment often acts as a major sink for trace metals (Kronvang et al., 2003; Liu et al., 2016; Ntekim et al., 1993; Sadeghi et al., 2012; Singh et al., 2003) and accounts for 30–98% of the total metal loads in the river (Gibbs, 1973). Therefore, sediment has become a valuable environmental archive for the investigation of anthropogenic contamination (Gao et al., 2017; Alves et al., 2014; Xiao et al., 2014; Swarzenski et al., 2008).
The impacts of anthropogenic activities on river ecosystems vary considerably over time. However, for most rivers globally, the historical impacts of anthropogenic activities on river ecosystems are hard to evaluate because of the lack of long-term monitoring and historical archives. Some studies have used core sediment profiles to reconstruct the historical impacts of anthropogenic activities in aquatic environments with stable depositional conditions, such as estuaries, lakes and reservoirs (Dhivert et al., 2015; Córdoba et al., 2016; Fernández et al., 2003; Müller et al., 2000; G. Zhang et al., 2016; Y. Zhang et al., 2016). However, there are still not enough well-documented studies using lead-210 (210Pb) dating technique to rebuild the historical ecological risk of trace metals in the rivers affected by mining and industrial activities (Conrad et al., 2007; Grousset et al., 1999; Wang et al., 2015).
The 210Pb dating technique is widely used to identify sedimentary chronologies on time scales of 100–150 years (Córdoba et al., 2016; Appleby, 2008). A high-precision dating result is dependent on reliable age models (Jones et al., 2009). Generally, the constant 210Pb flux and constant sedimentation rate model (CF-CS) describes a situation in which 210Pb activity decreases exponentially with depth (Guo and Yang, 2016). However, 210Pb activity profiles exhibit significant deviations from the simple exponential decline in most cases for river sediment, which limits the application of the CF-CS model (Mulsow et al., 2009; Chapron et al., 2007). On the other hand, the constant rate of excess 210 Pb supply (CRS) model can provide better calculated results for a situation with variable sedimentation rates, especially when the results are validated by other discrete sedimentary markers (such as pollution incidents) (Grousset et al., 1999; Delbono et al., 2016).
In terms of basin management, understanding the pollution and ecological risk of trace metals in both the past and present is a prerequisite for pollution remediation. Many methods (such as enrichment factor, anthropogenic metal flux, ratio of secondary and primary phases, bioavailable metal index and toxic risk index, etc.) have been developed to assess the anthropogenic inputs, pollution levels, bioavailability and toxicity of trace metals in sediment (Wang et al., 2014; Gómez-Álvarez et al., 2011; Rosado et al., 2016; Spencer et al., 2003). However, each of these methods is insufficient for obtaining a clear idea of the general status of an aquatic ecosystem. Instead, the integrated pollution and risk assessment methods, including a group of simple methods that complement each other, can improve the understanding of the overall environmental quality of the water body (Rosado et al., 2016).
Analyzing geochemical fractions of trace metals is very useful for pollution and ecological risk assessment, because only a specific fraction of a metal may have adverse effects on the environment (Sundaray et al., 2011). Generally, a high concentration of metals in the exchangeable and carbonate fractions will lead to more bioavailable metals in the sediment, thereby resulting in more severe toxic effects on aquatic organisms (Rosado et al., 2016), whereas metal in the residual fraction is not bioavailable and presents a low ecological risk. Chemical speciation of trace metals in sediment can be significantly affected by anthropogenic activities. For example, anthropogenic activities noticeably affected the non-residual fraction of trace metals in the Shima River (Gao et al., 2018). Xie et al. (2018) reported that approximately 82.1–91.2% of trace metals in sediment were associated with the amorphous (e.g., Schwertmannite and ferrihydrite) and crystalline iron oxides (e.g., Goethite and hematite) in a mining-impacted stream, indicating the impact of the mining activities. A similar study was conducted by Schaider et al. (2014), who confirmed that iron oxides were responsible for the transportation of >70% of the Pb and 40% of the Zn in the Tar Creek stream.
In recent decades, mining and smelting activities were rapidly developed in the upper stream of the Beijiang River, including the polymetallic mines in Fankou, Lechang and Dabaoshan, which produce iron, copper, lead, zinc and molybdenum (Gao et al., 2012), and the smelter in Shaoguan, which is the third largest metal producer in China. The excessive wastewater discharge brought large amount of trace metals to the river and caused aquatic environments to deteriorate (Song et al., 2011). Specially, a serious Cd pollution incident occurred in 2005 when untreated smelting wastewater was discharged into the river (Zhang and Chao, 2009). To evaluate the impact of anthropogenic activities on the Beijiang River in recent decades, undisturbed sediment core samples were collected to 1) analyze the contents of trace metals and their geochemical fractions; 2) reconstruct the pollution history of trace metals in the Beijiang River; 3) evaluate the anthropogenic inputs, bioavailability, pollution degree and ecological risk of trace metals based on the excess flux (EMex), enrichment factor (EF), ratio of secondary and primary phases (RSP), bioavailable metal index (BMI), and toxic risk index (TRI); and 4) identify the sources of trace metals.
Section snippets
Study area and sampling
The Beijiang River presents 1528.4 m3 s−1 of runoff and is the second large tributary of the Pearl River in southern China (Chen et al., 2009). This river is also an important source of drinking water and a navigation channel for Guangdong Province (He et al., 2014; Gao et al., 2012). However, due to the intense mining and industrial activities, the drinking water safety of the local residents has been seriously threatened (Song et al., 2011). In addition, flood disasters often occur in the
Physicochemical parameters in the sediment cores
The average pH in the sediment at each sampling site was near-neutral and presented values of 6.8 for S1, 6.9 for S2, and 7.1 for S3 (Table 2). The average TN and TP contents in the sediment were similar at each sampling site in the Beijiang River. However, the texture of the sediment along the river changed from sand silt (S1 and S2) to clay silt (S3), suggesting relatively strong hydrodynamic conditions at the upstream sites (S1 and S2). The average silt percentages in S1, S2 and S3 were
History profiles of excess trace metals flux (EMex)
Excess metal flux is considered to be the proportion of total metal derived from anthropogenic activities, and it can be calculated as follows (Fernández et al., 2003):where MFex(i) (g m−2 y−1), Mex(i) (mg kg−1) and St(i) (g cm−2 y−1) are the excess metal flux, excess metal content and mass sedimentation rate for the i-th depth interval, respectively. In this study, the value of Mex(i) (mg kg−1) was calculated by the difference between the analytical value of each metal in
Conclusions
The aquatic ecosystem of the Beijiang River has been significantly affected by human activities in recent decades. Anthropogenic enrichments of Cu, Zn, Pb and Cd were noticeable in the sediment. Pollution intensities in all sediment cores decreased in the same order of Cd > Zn > Pb/Cu > Fe > Ni > Cr, indicating that the sediment was strongly polluted by Cd. The historical evolution of the trace metal inputs was revealed by 210Pb dating of the sediment core S3. Consistent with the mining history
Acknowledgments
We would like to acknowledge the Administration Office of the Feilaixia Water Conservancy Project of Guangdong Province for providing support for our sampling campaign. We would also like to thank the anonymous reviewers for their valuable comments and suggestions. This project was supported by the Special Foundation Project for Water Conservation and Protection in Guangdong Province (2015), National Natural Science Foundation of China (no. 41471020) and Science Research Programs of Guangzhou
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