Non-conservative behaviour of molybdenum in the Ganga (Hooghly) River estuary, India: Role of solute-particle interaction and sediment diagenesis

Abstract

This study is focussed on understanding of molybdenum (Mo) cycling in the mesotidal to macrotidal estuary of the Ganga (Hooghly) River in India. Our investigation encompasses the composition of water and suspended particulate matter (SPM) samples collected in three separate periods (pre-monsoon: PrM, monsoon: M and post-monsoon: PoM) over two consecutive years, surface sediments, exchangeable phases of surface and suspended sediments, one sediment core, urban and industrial effluent waters and ground waters. The observed dissolved Mo concentrations in the estuary are in excess of those expected from conservative mixing of seawater and river water. The “excess” Mo values, observed in the mid-to-high salinity regions (4–27 ‰) for four out of the six study periods, are the highest in the monsoon samples. The bulk and exchangeable phase compositions are supportive of release of Mo from the Fe-Mn oxyhydroxide phases of the SPM and surface sediments in the estuary. Mass balance calculations indicate that Mo release from the SPM and surface sediments can account for up to 40 % of the median value of the “excess” Mo in the estuary. The compositions of the sediment core provide evidence for the mobilization of Mo and its release to porewater due to reductive dissolution of Fe-Mn oxyhydroxides. The tidally induced sediment resuspension likely facilitates the transport of porewater Mo to the overlying water. Mass balance calculations indicate that Mo loss from the sediment column can account for the bulk of the “excess” Mo in the estuary waters. This study further demonstrates that industrial effluents, urban waste waters and groundwaters do not constitute significant sources of dissolved Mo in the estuary.

The average annual Mo flux from the estuary is (∼2–3) × 10⁶ mol, of which ∼30–50 % of Mo is due to estuarine addition. More significantly, Mo contribution from the Hooghly River estuary, accounting for ∼0.7–1.0 % of the global riverine Mo flux, is ∼3–4 folds higher than its water contribution (∼0.25 %) to the global river water flux. Our study highlights the role of interactions between oxyhydroxide phases and estuarine water, diagenetic mobilization of elements in the sediment column and porewater transport in the cycling of Mo in a monsoon-dominated tropical estuary with high sediment loads. We also demonstrate that the mobilization of Mo through chemical weathering is driven by physical weathering in the catchments of the South Asian Rivers.