Distribution, risk and bioavailability of metals in sediments of Lake Yamdrok Basin on the Tibetan Plateau, China
Graphical abstract
Introduction
With the rapid development of global industrialization, much attention has been paid to metals because of their toxicity and persistence in the environment (Wang et al., 2015). Sediments are often metal-contaminated because of natural or anthropogenic influences. Metals can be provisionally immobilized in sediments as a result of adsorption, coprecipitation, and hydrolysis processes, making sediments an important sink for metal pollutants (Guo et al., 2018; Singh et al., 2005). Meanwhile, sedimentary metals can also be released into water in response to certain environmental changes and then enter the food chain, causing harm to the ecological environment and human health (Yi et al., 2011). Therefore, comprehensively understanding the accumulation, mobility and bioavailability of metals in sediments is important for evaluating regional sediment pollution status and proposing effective measures.
The geochemical baseline of metals is defined as the level of metals in the surficial environment that are not under direct anthropogenic influence at present or within a certain period (de Paula Filho et al., 2015; Teng et al., 2009). It can be used to distinguish the natural and anthropogenic origins of metals in the environment, thereby acting as an important reference for evaluating metal contamination in regional sediments (Teng et al., 2009; Wang et al., 2019). The comparison of measured total content and baseline value is usually used to determine the metal contamination degree in sediments. However, this is insufficient to predict potential toxicity of metals in sediments. In fact, total metal content embraces large fractions that are unavailable to aquatic organisms because of the repletion of binging ligands in sediments, and only a proportion of metals can be remobilized and released into the water phase, potentially leading to an ecotoxicological risk (Costello et al., 2012; Ren et al., 2015). Therefore, characterizing metal mobility and bioavailability is important to evaluate the potential toxicity risk of the metals.
The mobility and bioavailability of metals in sediments are highly dependent upon the metal's chemical form (Xu et al., 2018). Therefore, a range of ex-situ single or sequential extraction methods have been proposed to determine the chemical partitioning of metals in solid phases. BCR is a sequential extraction procedure that has been widely used to estimate the readily bioavailable metal forms (Liang et al., 2017; Peng et al., 2009; Pueyo et al., 2008). However, this procedure has limitation because of the probable readsorption and redistribution of metals during the extraction and consequent changes for the chemical forms (Prica et al., 2010). The diffusive gradient in thin-films (DGT) technique has recently been developed as an alternative method for rapidly measuring bioavailable metal in water, soil and sediment (Fan et al., 2019; Schintu et al., 2008; Xu et al., 2018). Some previous studies have applied conventional chemical extraction (such as BCR) and DGT synthetically to evaluate metal bioavailability in soil or sediment and explore the associations and differences between the two methods (Liu et al., 2015; Ren et al., 2015; Roulier et al., 2010; Xu et al., 2018; Yin et al., 2014).
The Tibetan Plateau contains numerous inland glaciers and lakes. It is the source of major rivers in Asia and the main water source for approximately one-third of the global population (Guo et al., 2018). Paying attention to metal accumulation and toxicity in the aquatic environment of the Tibetan Plateau is highly important for the stability of the vulnerable highland ecosystem and human health (Guo et al., 2018). Because it is a remote area with limited industrial and agricultural activities, the distribution and enrichment of metals in the soil and sediment of the Tibetan Plateau are generally affected by natural processes, such as water-rock interactions, weathering and geothermal fluid. Li et al.(2011) identified that the parent rocks of the Yarlung Tsangbo river basin as the fundamental sources of metals in the river sediments. However, it is notable that the Tibetan Plateau is adjacent to Eastern China, Nepal and India, where population is booming and industries are developed; meanwhile, tourism-related activities have also increased on the Tibetan Plateau (Sheng et al., 2012). Guo et al. (2018) proposed that metals in lake sediments on the Tibetan Plateau mainly originated from natural and traffic sources, and atmospheric transport of metal contaminants from adjacent areas. Accordingly, the contamination degree and ecological risk of metals in soil and sediment on the Tibetan Plateau have been previously studied (Guo et al., 2018; Li et al., 2011; Sheng et al., 2012; Zhang et al., 2011), however, these researches only studied the total contents, possibly overestimating or underestimating the degree of risk in metals.
Lake Yamdrok Basin located in southern Tibet contains the largest natural inland lake in the northern foothills of the Himalayas, and is a typical representative of Tibetan Plateau lakes. Meanwhile, the lake is one of the three sacred lakes in Tibet. Due to increasing tourism-related activities in recent years, the lake may have been affected by anthropogenic disturbance. In this study, Lake Yamdrok Basin was selected to investigate the geochemical characteristics of metals in sediments via synthetic utilization of the geochemical baseline, BCR sequential extraction and DGT technique. The primary aims of this study were to (1) determine the distribution and fraction characteristics of sediment metals, (2) assess the enrichment degree and potential risk of metals in surface sediments, and (3) evaluate the mobility and bioavailability of metals in surface sediments. The results could provide a better understanding of metal contamination information and benefit effective evaluation of metal ecological hazard in sediments on the Tibetan Plateau.
Section snippets
Study area
Lake Yamdrok Basin is located in the middle reaches of the Yarlung Zangbo River, containing four lake zones (Yamdrok Tso, Chen Co, Kongmo Co, Bajiu Co) and several inflowing rivers. The lake basin was formed by local subsidence and water accumulation during the uplift of the Himalayas. It was originally an outflow lake flowing into the Yarlung Zangbo River, and then about a million years ago, it turned into an inflow lake as the river became blocked and was gradually separated into several lake
General properties of the surface sediments
The general properties of the surface sediments in Lake Yamdrok Basin are presented in Appendix A Table S2. The pH values ranged from 7.0 to 8.1 with the average of 7.4, indicating a near-neutral pH environment. The ORP values varied between -231.0 mV and 162.7 mV, with an average of -34.9 mV, which suggests an anoxic-suboxic environment. The MC of the sediments was in the range of 11.0%-59.0%, and relatively low MC usually occurred in the sites near the inflowing rivers. Inhomogeneous TOC
Conclusion
The distribution, potential risk and bioavailability of metals in sediments of Lake Yamdrok Basin in Tibet were studied. The spatial distribution of Cr, Ni, Cu, Zn, As, Cd and Pb in surface sediments showed similar tendencies, with relatively high concentrations generally observed near the inflowing rivers. Furthermore, metals in surface sediments may be influenced by natural processes and anthropogenic disturbances, resulting in higher average concentrations than those in the core sediment.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (No. 42202018027).
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2022, Science of the Total EnvironmentCitation Excerpt :In fact, DGT only captured metals in the pore water and on the sediment particle surface, showing a dynamic diffusion process of unstable metal flux over the period of DGT deployment (Yin et al., 2014). In contrast, chemical sequential extractions were static and measured metals within sediment particles, including the surface and interior, and reflected the potential for the maximum amount of metals possibly being released from sediment (Che et al., 2020). In addition, no significant relationship between DGT-labile metals and sediment characteristics (pH and organic matter) was observed, possibly due to the narrow pH range and limited organic-associated metals captured by DGT during deployment.