Dynamics of mercury in a tropical estuarine system during dry season

Understanding mercury (Hg) dynamics in an estuarine system is vital because of its potential toxicity to the ecosystem. India is the second largest producer of atmospheric Hg, and the dynamics of Hg are poorly understood in tropical estuarine systems. This study describes the distribution, speciation and mobility of Hg in a monsoon-fed tropical estuarine system in India (Mandovi estuary, East coast of India) during dry period (November-March) when the pollutants stay for a long time within the estuary due to negligible river discharge.This study presents the impact of changing physicochemical parameters of water column (such as pH, dissolved oxygen, salinity) and sediment [such as texture, sedimentary organic matter (SOM), molar carbon to nitrogen ratio and chemical characterisation of SOM] on distribution and speciation of Hg in the surface sediments,  porewater and water column along the length of the estuary (from upstream to downstream). There was a significant increase in concentration of sedimentary methylmercury (MeHg)(a neurotoxin) with an increase in total sedimentary Hg concentration in the estuary. However, concentration of sedimentary MeHg was found to decrease with increasing concentration of SOM depend on the nature of SOM. Results from an equilibrium-based model (Visual MinteQ) showed that a significant concentration of Hg was associated with type-II fulvic acid, and reduction of Hg(II) was more in presence of type-II humic acid.This study showes that the reduction and methylation process of Hg control Hg distribution in the study area, and estuarine sediment is one of the primary sources of dissolved Hg in the water column.