Element Transport and Partitioning Along Tidal Channels in Southwest Bangladesh

Studies of element partitioning between suspended sediment and water with increased seawater mixing are sparse, particularly in Bangladesh. However, these studies are important for understanding elemental cycling, pollutant transport, and impacts on aquaculture and sensitive ecosystems in estuaries and tidal deltas such as the Sundarbans mangrove forest in Southwest Bangladesh. Thus, water samples collected within the upper 1 m of the water column along a transect of well-mixed tidal channels in Southwest Bangladesh during the dry season were analyzed for dissolved and suspended sediment element concentrations and other geochemical parameters. While most elements in the suspended load were close to or depleted relative to upper continental crust (UCC), several trace elements such as Sb, As, Cd, and Se were slightly enriched. Additionally, most trace elements in the dissolved load were well above world average riverine concentrations, particularly Se and As. Dissolved load Ba and Se displayed mostly conservative mixing trends with seawater. Barium was likely originally sourced from sediment desorption and groundwater exfiltration, while Se may have been anthropogenically sourced from the city of Khulna or farther upstream. Dissolved As did not display conservative mixing trends, and may ultimately be geogenic in origin, possibly from groundwater. Ni and Co show trends consistent with desorption from competitive seawater cation exchange along the transect, similar to a study in the nearby Hooghly Estuary in West Bengal. Collectively, our results show that combined anthropogenic and natural influences on trace element distributions in coastal environments are important to quantify for continual protection of natural areas and better understanding of trace element discharge to global oceans.