Biogeochemistry of mercury and methylmercury in sediment cores from Sundarban mangrove wetland, India - A UNESCO World Heritage Site

This study was performed to elucidate the distribution, concentration trend and possible sources of total mercury (Hg _T) and methylmercury (MeHg) in sediment cores (<63 μm particle size; n=75) of Sundarban mangrove wetland, northeastern part of the Bay of Bengal, India. Total mercury was determined by atomic absorption spectrometry (AAS) in a Leco AMA 254 instrument and MeHg by gas chromatography- atomic fluorescence spectrometry (GC-AFS). A wide range of variation in Hg _T (0.032-0.196 μg g ^-1 dry wt.) as well as MeHg (0.04-0.13 ng g ^-1 dry wt.) concentrations revealed a slight local contamination. The prevalent low Hg _T levels in sediments could be explained by sediment transport by the tidal Hugli (Ganges) River that would dilute the Hg _T values via sediment mixing processes. A broader variation of MeHg proportions (%) were also observed in samples suggesting that other environmental variables such as organic carbon and microbial activity may play a major role in the methylation process. An overall elevated concentration of Hg _T in surface layers (0-4 cm) of the core is due to remobilization of mercury from deeper sediments. Based on the index of geoaccumulation (I _geo) and low effects-range (ER-L) values, it is considered that the sediment is less polluted by Hg _T and there is less ecotoxicological risk. The paper provides the first information of MeHg in sediments from this wetland environment and the authors strongly recommend further examination of Hg _T fluxes for the development of a detailed coastal MeHg model. This could provide more refine estimates of a total flux into the water column.