Mercury contamination at Vembanad Lake and near-shore regions in the southwest coast of India
Introduction
Mercury (Hg), an extremely toxic global pollutant (Li et al., 2009, Yin et al., 2015), is naturally present in the air, water and soil (Lin et al., 2007, Gopikrishna et al., 2020a). On at a regional and global scale, the cycling of Hg is immensely significant to the aquatic environment and human health (Sizmur et al., 2018). Over the past few decades, noticeable efforts have been paid to the inventories on the environmental fate of Hg, which is evident from the improved number of scientific publications and international conferences on the same and its subsequent impact on the environmental and human health risks (Bonzongo and Donkor, 2003, Chakraborty et al., 2019). The major sources of anthropogenic mercury in the aquatic environment are industrial, agricultural, and surface runoff (UNEP Chemical Branch, 2008, Wang and Fisher, 1999). The impact of Hg is not limited to the point source and extends to regions far away from the emission source.
Once Hg is deposited in aquatic systems, it undergoes diverse chemical and biological transformations (Gopikrishna et al., 2020b). The process of transformation of inorganic Hg to organic Hg is similar to methylation, which yields methyl mercury (MHg) (Boszke et al., 2008, Garcia-Ordiales et al., 2018, Stein et al., 1996, Tomiyasu et al., 2000, Ullrich et al., 2001), and poses severe toxicity to aquatic organisms (Boening, 2000, WHO, 1989). Mercury methylation is a key step in Hg cycling in aquatic systems (Fitzgerald and Mason, 1996). The major factors affecting Hg methylation is the availability of inorganic Hg, pH, temperature, organic carbon, oxidation potential, salinity, sulphur compounds and methylating microbial communities (Compeau and Bartha, 1987, Ullrich et al., 2001). The other processes controlling the distribution of mercury in aquatic systems are adsorption/ desorption, redox, precipitation/ dissolution, etc. (Stein et al., 1996). All these factors together decide the chemical transformations, transport, bioavailability, and toxic impacts (Araújo et al., 2019, Lamborg et al., 2019, Nriagu, 1980); these processes vary with respect to aquatic ecosystems. It is essential to conduct detailed studies on Hg concentration in various matrices to study their chemical transformations and fate in the system. However, studies on chemical transformations of Hg and their cycling in tropical ecosystems, especially estuarine systems, in comparison to those on temperate regions, are limited (Bowles et al., 2001, Chakraborty et al., 2016). Therefore, it is necessary to study Hg pollution in tropical regions, especially India, which is one of the topmost emitters of Hg (Bhave and Shrestha, 2020). Most of them are focused on the monitoring aspects, while research studies on the fate and transport of Hg are limited. For a better insight on the process and factors regulating the fate and transport of Hg, the current study focuses on:
- (i)
assessing the interrelation between Hg in water, interstitial water and sediment
- (ii)
colligating the concentration of Hg in water and sediment with that in biota
- (iii)
understanding transformations and transport of Hg.
- (iv)
creating a conceptual model of Hg processes in the Vembanad Lake and near-shore environment
Section snippets
Study area
The Vembanad Lake system (Lat. 9° 30–10° 10 N and Long. 76° 10–76° 25 E) is the largest mixohaline system (Chandran and Ajaykumar, 2018) enjoying international importance as a Ramsar site. It is one of the most crucial wetland systems on the southwest coast of India, which protects the densely populated coastal stretch from floods and profoundly influences groundwater recharging (ShyleshChandran et al., 2019). To its North, the system accommodates nearly 247 chemical manufacturing units and
Physico-chemical parameters of water environment
The physico-chemical parameters of surface and bottom water samples were analysed, as shown in Supplementary Table 3. The water column’s pH ranged within 5.47–8.47, with an average of 7.27 ± 0.64, indicating the prevalence of the alkaline condition. The mean concentration of chloride and sulphate was 4383.89mg/L and 129.45 mg/L, respectively. The hardness of the water from Vembanad Lake varied within 2 mg/L-8600 mg/L. The maximum and minimum salinity observed in the study area was 26.58 ppt and
Conclusion
The study elucidates the potential for Hg methylation in the sediments of Vembanad Lake and its adjoining near-shore regions. The high concentration of Hg in the fish tissues was indicative of the inefficiency of the bio-geochemical control over MHg mobility and bioavailability through the food web. The relation between Hg in tissues and other environmental compartments showed that its accumulation potential subsequently increases along the food chain. The accumulation of Hg in biota was mainly
CRediT authorship contribution statement
Mahesh Mohan: Problem identification, Sampling and analysis, Interpretation, Writing, editing and proof reading, Identified the research problem, Funding acquisition, Done the review and editing. M.S. Shylesh Chandran: Problem identification, Sampling and analysis, Interpretation, Writing, editing and proof reading, Funding acquisition. E.V. Ramasamy: Problem identification, Sampling and analysis, Interpretation, Writing, editing and proof reading, Supervision, Done the review and editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The financial support from the Ministry of Earth Sciences (MoES), India, Government of India through their project is gratefully acknowledged. We also acknowledge the financial support from UGC, India and Department of Science and Technology, India (SERB, FIST & PURSE I & II), Government of India, and KSCSTE, India , Government of Kerala, for setting up the mercury and other metal analytical facility.
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