How estuaries modulate the riverine fluxes of copper (Cu) and cadmium (Cd) to seawater

The export of dissolved copper (Cu) and cadmium (Cd) to coastal ocean is important for sustaining marine biological productivity. Riverine input is the dominant source of Cu and Cd to seawater, which supplies about 830–1040 million moles/yr and 10–30 million moles/yr respectively. We demonstrate that estuarine processes may augment the riverine export fluxes by a maximum 20 % for Cu and 271 % for Cd. To understand the underlying mechanism of estuarine cycling of Cu and Cd, we studied the Ganga (Hooghly) River estuary, a monsoon-dominated tropical river characterized by a high sediment-to-water ratio, serving as a natural laboratory to investigate large-scale global estuarine processes. The remobilization of metals from exchangeable (Cd) and/or amorphous oxide (Cu) phases of the riverine suspended sediments are identified to be the primary processes contributing to their estuarine production. In addition, during the lean flow season, Cu input from shallow porewater discharge modulates dissolved Cu concentration in the high salinity zone. Steady state mass balance models reveal that the variation in dissolved Cu and Cd concentrations in the estuary is a function of dissolved inorganic complexation, which in turn is modulated by changes in pH, bicarbonate-ion concentrations and salinity (chlorinity). The key finding is that the dissolved Cu and Cd in the mid to high salinity range (4–13 ‰) exhibit a concentration “excess” relative to conservative river water-seawater mixing which is corroborated by a proportionate decrease in reactive solid phase concentration of the metals. The “excess” concentrations result in ∼46 % and ∼ 306 % rise in Cu and Cd export, relative to the riverine supply, to the Bay of Bengal. Overall, this study underscores the importance of estuarine processes in modulating net riverine metal fluxes to seawater.