Organic Carbon transport model of abandoned river channels- a motif for floodplain geomorphology influencing biogeochemical swaying of arsenic

2020 
Abstract Meandering-river geomorphology, forming abandoned channels/lakes with organic carbon-burial and microbial reductive dissolution, play many crucial roles in controlling arsenic (As) fluxes in sinks such as contaminated aquifers of riverine alluvial plains across the world. Suhiya oxbow-lake in the middle alluvial plain of the River Ganga, was selected as the natural laboratory. A top-down multidisciplinary approach was chosen employing satellite imagery to analyse the annual oxbow-lake surface vegetation dynamics (Eichhornia and Hydrilla). Side-scan sonar profiles across two oxbow lakes along with River Ganga core data and vintage topographical maps, estimated the lake-sedimentation rate of 9.6 cm/yr. Organic carbon [amino acids (LCFD), aromatics (SUVA), lingo-phenols and lipids hydrocarbons (GSMS)] infiltration-based on hydrophobicity and molecular-mass was detected at different depths along the water and sedimentary column. ICPMS analysis showed lake surface to groundwater the As conc. varied from (0.37 to 185 μg/l). Microbial diversity based on 16S rRNA gene showed photoautotrophs Nostoc , Anabaena at surface, replaced by Fe-oxido-reducing As-metabolizing bacteria e.g. Acidovorax, Dechloromonas and enteric organisms e.g. Enterobacter, Salmonella at the lake bottom, indicating anthropogenic inputs. Finally, a conceptual organic carbon transport model was constructed to understand the preferential preservation and microbial diagenesis resulting in mobilization of As and other geogenic elements.
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