Abstract
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 1m 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.