Previous studies have documented a weathering-limited regime in the upper reaches of the Ganges River Basin. Chemical weathering and element mobility at six sites in the lower reaches of the Ganges in the tidal floodplain of Southwest Bangladesh were investigated by comparing compositions of rice paddy soils, precursor tidal channel sediments, surface waters, and extract solutions, which represent the soluble fraction of solids. Little spatial variation in water and solid compositions is observed in each season, indicating similar processes are acting to transport elements across this region. Roughly one to several decades after deposition, rice paddy soils are not significantly different in mineralogy or composition from precursor tidal channel sediments, and both are similar to the composition of average upper continental crust. There is no detectable change in composition of tidal channel water between upstream and downstream sites. Rice paddy and tidal channel waters are saturated in the dominant minerals present in the silt-sized soils and sediments, including quartz and clay minerals. Together, these observations indicate the dominance of weathered material and weak chemical weathering in the tidal floodplain, consistent with a transport-limited regime. Multiple lines of evidence indicate a lack of exchange equilibrium between surface waters and coexisting solids, which may be a common feature in tidal river deltas where transport-limited regimes likely dominate.

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.

Soil samples (n = 45) and water samples (n = 111) were collected in the coastal zone of SW Bangladesh in wet (November) and dry (May) seasons in 2016 to identify the factors influencing soil arsenic concentrations. Soils are entisols formed from recently deposited, predominantly silt-sized sediments with low carbon concentrations typical of the local mangrove forests. Arsenic concentrations in bulk soil are higher in November than in May and vary little between sites. Arsenic concentrations in deionized H2O extracts are ~2 orders of magnitude lower, indicating only ~1% of As is soluble. Water samples show that As concentrations are highest in groundwater from tubewells. Bulk soil As is positively correlated with As concentration in irrigation water, suggesting that As from irrigation water is added to the soil. Unlike other water types, As in rice paddy water is much higher in the wet season, consistent with some fields being irrigated with tubewell water. Arsenic in rice paddy water increases by soil sulfide dissolution, and decreases by dilution during the monsoon. Water soluble As in rice paddy soils is positively correlated with S and DOC concentrations in rice paddy soil extracts due to sulfide dissolution and complexation with DOC. Thus, waterlogging of rice paddy soils leads to reducing conditions, the absence of ferric oxyhydroxides that could sorb As, and the presence of sulfides that incorporate As. As soil pH increases from 7 to 8, KD(soil/extract) increases, consistent with the observed positive correlation between irrigation water pH and bulk soil As concentration. Arsenic bioavailability could be decreased through soil aeration (draining and tilling) and by avoiding the use of groundwater for rice paddy irrigation.