Geochemical behaviour and cycling of redox sensitive elements (U, Mo and V) in the Ganga (Hooghly) River estuary, India

Tiwari, Rakesh Kumar (2023) Geochemical behaviour and cycling of redox sensitive elements (U, Mo and V) in the Ganga (Hooghly) River estuary, India. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Rakesh Kumar Tiwari (16RS052)) 16RS052.pdf - Submitted Version Restricted to Repository staff only Download (8MB)

Abstract

The Ganga (Hooghly) River is a part of the Ganga River system and marks the western boundary of the Ganga-Brahmaputra (G-B) delta. This study, based on water and sediment samples of six contrasting water discharges over two consecutive years, is an attempt to investigate the processes regulating the cycling and fluxes of three redox-sensitive elements, i.e., Uranium (U), Molybdenum (Mo) and Vanadium (V) in the river and estuarine regions of the Hooghly River estuary (HRE). Specifically, we seek to determine if the estuary acts as a source or sink for these elements or if they behave perfectly conservatively. This thesis forms the most comprehensive study on U, Mo and V in an Indian estuary through investigation of chemical compositions of the (i) river/estuary water samples, (ii) coexisting suspended particulate matter (SPM), (iii) bed sediments, (iv) exchangeable phases of the sediments, (v) estuarine core sediments, (vi) urban and industrial effluent waters and (vii) groundwaters. Investigation of the coexisting dissolved and solid phases, allows us to evaluate the nature and intensity of solid-solution interaction processes in modifying the dissolved distributions of U, Mo and V. Furthermore, the role of tidally-induced resuspended sediments and suboxic diagenesis in the sediment column are brought out through data and observations of this study. Our results also point to the need for understanding organic cycling in the estuary. The anthropogenic contribution does not seem significant for U levels in the HRE, whereas it may be significant for Mo only in the freshwaters. The compositions of dissolved and particulate matter are suggestive of removal of U in the low-salinity zones of the HRE via its scavenging onto Fe-Mn oxyhydroxides. Additional evidence for such a process come from the compositions of exchangeable phases of SPM. The degree of removal of dissolved U is consistent with the enhancement of exchangeable U in the SPM. An important observation of this study is that the salinity range over which U removal occurs varies between the periods of similar water discharge. Multiple observations based on compositions of bed sediments and their exchangeable phases indicate loss of both U and Mn through suboxic diagenesis in the sediment column 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. The annual U fluxes from Hooghly constitute 0.4-1.4% of the global riverine U flux. The estuarine U flux is ~70% higher in 2012 than in 2013. A significant difference of annual U fluxes of two consecutive years, driven mainly by variation of the magnitude of U removal, suggests that estimates of meaningful annual U fluxes would require data at higher temporal resolution. Dissolved Mo distributions show behaviour opposite to U, it is produced in most of the study periods. The estuarine production is the highest in the monsoon periods and is clearly driven by the solid-solution interactions. However, the amount of Mo released from the SPM and bed sediments is insufficient to account for the observed maximum excess dissolved Mo. Thus, additional source of Mo is inferred. The data on sediment cores and multiple indicators help identify this source. In the sediment columns, Mo is lost via its mobilization driven by suboxic diagenesis. Calculations indicate that the loss of Mo from the sediment column can account for excess dissolved Mo in the waters overlying the core sediments. Our estimates show that annual Mo fluxes from the HRE are (4742-4824)×10³ mol, out of which ~80% of Mo is generated within the estuary. The annual Mo flux from the Hooghly River accounts for 1.5% of the global riverine Mo flux to the oceans. Such Mo flux from Hooghly is ~7-fold higher than its water contribution (~0.2%) to the global river water flux. The data on dissolved V concentrations, although suggestive of V removal, illustrate more complex behaviour. In the post-monsoon (PoM) period, both removal and release of V are inferred. In contrast, in the pre-monsoon (PrM) and monsoon (M) periods dissolved V is generally removed. The compositions of the SPM and its exchangeable phase clearly provide support for its adsorption. However, the exchangeable phases of bed sediments do not provide unambiguous evidence for removal of dissolved V and call for studies on cycling of organic matter of the HRE. In the PrM period, 12% of dissolved V is removed in the estuary. The estimated annual V flux from the estuary is 6856×10³ mol which constitutes 1.4% of the global V flux from rivers to the oceans. The comparison of the average U/Mo, V/Mo and V/U ratios in the dissolved, SPM, bed sediments and exchangeable phases of the sediments in the freshwaters provide understanding of relative mobilities of U, Mo and V. Higher U/Mo and V/Mo ratios in the sediments and exchangeable phases of the bed sediments compared to those in the dissolved phase indicate that Mo is relatively more mobile compared to U and V. The V/U ratios are observed to be higher in the solid phases than in the dissolved phase, indicating that the relative mobility of U is higher compared to V. Thus, the results of this study show that Mo has the highest relative mobility, whereas V is the least mobile. Such an inference is in line with known geochemical behaviour of U, Mo and V. The average U/Mo, V/Mo and V/U ratios in the Hooghly freshwaters are observed to be significantly higher than those in the seawaters. The U/Mo and V/Mo ratios in the estuarine region are lower than those predicted by mixing. The V/U ratios are observed to show scattered distributions with salinity in the PoM and PrM periods, but a decreasing trend with salinity in the M period. Based on the measured elemental ratios-salinity regressions, it is calculated that the U/Mo and V/Mo ratios at salinity 35‰ are comparable to the respective ratios reported in the seawater. However, additional data at higher spatial resolution is required for meaningful assessment of V/U ratios in the dissolved fluxes from the HRE. Our study highlights the importance of the estuarine processes in modifying the freshwater U/Mo, V/Mo and V/U ratios in the HRE dissolved fluxes to the ocean.

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