Sources and cycling of vanadium in a tropical coastal lagoon system and the Bay of Bengal

Abstract

The aim of this study is to constrain coastal behavior and chemical budget of vanadium (V), a bio-essential element, in a tropical lagoon system (Chilika lagoon, India) and the Bay of Bengal. The Chilika lagoon, the largest brackish-water lagoon in Asia, is located on the east coast of India within the delta of the Mahanadi River. In this study, dissolved vanadium along the salinity gradient of the lagoon and its possible sources (river, ground water and Bay of Bengal) were measured using a Quadrupole Inductively Coupled Plasma Mass Spectrometer (Q ICP MS) instrument. The V concentrations and salinity of the Chilika samples showed a wide range of variations during four distinct seasons, pre-monsoon (April-May, 2017; 34.9-69.4 nmol/L), onset of monsoon (June 2016; 26.8-46.5 nmol/L), monsoon (August 2017; 32-62.4 nmol/L), and post-monsoon (January 2018; 32.1-84.5 nmol/L) seasons. These concentrations are intermediate to that of the riverine input (16.9 ± 16 nmol/L), and groundwater (86.05 ± 126 nmol/L) samples. The average vanadium concentration for the BoB samples in this study is 45 ± 8 nmol/L (n = 45), for an average salinity of 34 ± 1 psu (n = 45). Surface seawater V concentrations of the west of the Bay of Bengal varies from 25.3-51.1 nmol/L with a mean of 41.2 ± 5.9 nmol/L. Considering the general trend, the variations in surface waters compared to depths is minimal with a few prominent deviations from conservative behavior. The vanadium salinity data for the Chilika during the pre-monsoon and post-monsoon periods do not fall on the theoretical mixing between rivers and seawater. It depicts a non-conservative mixing behavior of vanadium in the Chilika lagoon, pointing to additional source of V to the lagoon. Although we hypothesize V supply through submarine groundwater discharge and/or desorption, our preliminary data are not sufficient to constrain the exact source for this system.