ARSENIC AND IRON ASSOCIATION IN SEDIMENT COMPRISING THE HYPORHEIC ZONE OF THE MEGHNA RIVER IN BANGLADESH

The sediment in river beds and banks and of the shallow aquifers in the Bengal basin is important for the cycling and fate of Arsenic (As) because they are rich in As-bearing iron (Fe)-oxides and oxyhydroxides. The river-groundwater interface, or the hyporheic zone can serve as both source and sink for dissolved Fe and As depending on whether oxidizing or reducing conditions are present. The redox conditions within the hyporheic zone can be dynamic due to changing groundwater flow and mixing conditions, driven by river-stage fluctuations (tidal and seasonal) and by inputs and presence of suitable electron donors. For example, the presence of labile sedimentary organic matter serves as an electron donor to metal-reducing bacteria that mobilize sedimentary Fe and As in the groundwater under reducing conditions. This study investigates the association of Fe and As in Meghna River hyporheic zone sediment (0-50 cm depth). Sediment samples were subjected to a six-step sequential extraction procedure to quantify As (and to semi-quantify Fe) associated with the surface adsorbed (outer and inner sphere), organic matter, amorphous oxides, crystalline oxides, and residual phase, followed by ICP-MS analysis of the eluent. Optically stimulated luminescence (OSL) dating of the sediment revealed an average depositional age of 1.5 ka. Most of the Fe in the sediment (19.2 g/kg), of which ~58% was Fe(III), was associated with the refractory (i.e., silicate) phases. On the other hand, the sedimentary As (7.7 mg/kg) was associated with amorphous Fe-oxides (32%), crystalline Fe-oxides (23%) and particulate organic matter (12%). These findings provide valuable insights on the mobility and sequestration of Fe and As in river corridors of large and dynamic rivers such as the Meghna.