Environ Monit Assess (2014) 186:3321–3336 DOI 10.1007/s10661-014-3620-1
Mercury in the sediments of Vembanad Lake, western coast of India Mahesh Mohan & M S Shylesh Chandran & K K Jayasooryan & E V Ramasamy
Received: 13 August 2013 / Accepted: 9 January 2014 / Published online: 23 January 2014 # Springer International Publishing Switzerland 2014
Abstract Mercury, a global pollutant, is popping up in places where it was never expected before and it burdens in sediments and other non-biological materials. It is estimated to have increased up to five times the prehuman level due to anthropogenic activities. Vembanad backwaters, one of the largest Ramsar site in India, which have extraordinary importance for its hydrological function, are now considered as one of the mercury hot spots in India. In this study, surface sediment samples of Vembanad Lake and nearshore areas have been seasonally analysed for total mercury and methyl mercury concentrations while the core sediment samples were analysed for total mercury. The results showed that the northern part of the lake was more contaminated with mercury than the southern part. The mercury concentration was relatively high in the subsurface sediment samples, indicating the possibility of historic industrial mercury deposition. A decreasing trend in the mercury level towards the surface in the core sediment was also observed. The geochemical parameters were also analysed to understand the sediment mercury chemistry. Anoxic conditions, pH and organic carbon, sulphur and Fe determined the presence of various species of mercury in the sediments of Vembanad Lake. The prevailing physical and geochemical conditions in Vembanad Lake have indicated the chances of chemical M. Mohan : M. S. Shylesh Chandran : K. K. Jayasooryan : E. V. Ramasamy (*) School of Environmental Sciences, Mahatma Gandhi University, Kottayam, Kerala, India e-mail:
[email protected]
transformation of mercury and the potential hazard if the deposited mercury fractions are remobilised. Keywords Core sediment . Methyl mercury . Marine pollution
Introduction Mercury is one of the most toxic heavy metals present in the environment. In the global biogeochemistry of Hg, estuaries and coastal waters represent an important link between the terrestrial environment and open waters. Once mercury enters into the aquatic systems, it begins to react with various components in the water and a portion of it precipitates into the sediments (Ruiz et al. 2005). Sediments are considered as the major sink and source of mercury (Shi et al. 2005; Bravo et al. 2008). Distribution of mercury in the sediments of estuaries and coastal areas was studied worldwide (Kannan et al. 1998; Horvat 1996; Choe and Gill 2003; Leonardo et al. 2006; Lewis and Chancy 2007; Marco et al. 2006; Ritchie et al. 2006; Roy et al. 2004; Ruiz et al. 2005; Shi et al. 2007; Ramasamy et al. 2012). Many studies have been conducted all over the world on the historical reconstruction of mercury in sedimentary deposits of lakes and reservoirs (Bloom et al. 1999; Sunderland et al. 2004; Loizeau et al. 2004; Castelle et al. 2007; Bravo et al. 2008). Mercury in sediments can undergo many chemical transformations, leading to the formation of more toxic organic forms (Boszke et al. 2003). These toxic forms may remain; besides, it can be
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accumulated in the sediments or can be remobilised in to the water column by further biogeochemical transformations. The rate of these processes depends on the prevailing environmental conditions. Earlier studies have also indicated that not only the highly contaminated industrial effluents but also even the less contaminated ones can lead to a high accumulation profile of mercury in the sediments if the discharge continues for a long period (Talbot 1990). Earlier studies on the vertical profile of mercury in sediments have strongly related their findings with the possibility of historical release of mercury in the area (Gobeil and Cossa 1993; Gagnon et al. 1996; Pereira et al. 1998). Out of the total global anthropogenic release of mercury into the atmosphere, two third appears to come from Asian sources, with China as the largest contributor (Pacyna et al. 2011). The USA and India are the second and third largest emitters, but their combined total emissions accounts to only one third of that of China. Mercury deposition has significantly increased to 26 and 8 % during the period 1995–2005 in East and South Asia, respectively (Ryzhkov et al. 2011). However, very limited studies are reported from India on mercury pollution in the sedimentary environments of aquatic systems (Subramanian et al. 2003; Mohan and Omana 2004; Roy et al. 2004; Karunasagar et al. 2006; Agarwal et al. 2007; Sinha et al. 2007; Omana and Mohan 2008; Koshle et al. 2009; Ram et al. 2009). Hence, a study on the geochemical interactions of mercury and environmental conditions of methylation in the aquatic ecosystems of India is highly relevant. Studies on mercury pollution in Kerala are meagre (Ouseph 1992; Nair 1994; Mohan and Omana 2004; Ramasamy et al. 2012). Hence, the present study was undertaken with an aim of analysing various forms of mercury and its relation with geochemistry in the sediments of Vembanad Lake and also to assess the mercury pollution profile of the region.
Materials and methods Study area Vembanad Lake, part of the Vembanad-Kol Wetland Ramsar site of Kerala, having an area of 151,250 ha, is the largest brackish, humid tropical wetland ecosystem in the peninsular India. This wetland is of extraordinary importance for its hydrological function, its rich
Environ Monit Assess (2014) 186:3321–3336
biodiversity, its support for huge fish population and its link with the Arabian Sea through Cochin estuary. The major source of mercury in Vembanad Lake can be attributed to the effluents from a chlor-alkali industry, which has been using mercury cell process for the production of caustic soda until the year 2004; thereafter, the Hg-cell process was replaced with membrane technology. Sample collection and preservation Sediment samples from 30 locations from Vembanad Lake, nearshore areas and adjoining river systems (Fig. 1) were collected using a grab sampler during all seasons. Each sediment sample is a combination of three to four subsamples, collected from the same location, pooled and homogenised, and a single sample was drawn for further analysis. About 1 kg of the sample from each site was transported to the lab in a sealed cover placed in an icebox. Sediment core samples of 50 cm were also collected from 10 sites (Fig. 1) using a Kajak sediment corer (KC Denmark), and the sites were determined based on the Hg content of surface sediments. Core sediment samples were cut into 2- and 5cm segments and stored in cold until analysis. Sample analysis Physico-chemical analysis Samples were dried and ground using an agate mortar. A
