Geochemistry of uranium in the Ganga (Hooghly) River estuary, India: The role of processes in the water column and below the sediment-water interface

Abstract

In this study, we discuss the cycling of uranium (U) in the Ganga (Hooghly) River estuary through investigation of the composition of the water samples of six periods of contrasting water discharge, the bulk and exchangeable phases of the coexisting suspended particulate matter (SPM) and the bed sediments. In addition, we evaluate data on urban and industrial effluent waters that drain into the estuary. Similar U concentrations of freshwaters and effluent waters do not indicate the anthropogenic source to be an important contributor of U to the estuary. The removal of dissolved U in the low-salinity zones, observed for five of six study periods, is primarily linked to the cycling of the FeMn oxyhydroxide phases. The compositions of exchangeable phases of SPM provide supportive evidence for the removal of dissolved U. Mass balance calculations show that the degree of removal of dissolved U can be sustained by the observed enhancement of exchangeable U in the SPM. Loss of U from the bed sediments is inferred from the distributions of exchangeable U concentrations as a function of salinity. Multiple observations based on compositions of the exchangeable and bulk phases of bed sediments indicate mobilization and loss of both U and Mn through suboxic diagenesis in the sediment columns followed by tidally-induced sediment resuspension.We estimate that 40–60% of riverine U is removed in the low-salinity zone during the monsoon and post-monsoon periods. More importantly, the salinity range over which U removal occurs varies between the periods of similar water discharge. The estimated U flux from the estuary is ∼70% higher in 2012 than in 2013. A significant difference between the annual U fluxes, driven mainly by considerable variation in the magnitude of U removal, suggests that estimates of meaningful annual U fluxes would require data at higher temporal resolution. This study highlights the importance of processes operating in the water column and below the sediment-water interface in the cycling of U in the Hooghly River estuary, where the solute-particle interaction driven by high SPM load is known to influence the cycling of trace metals.