The fate of arsenic in groundwater discharged to the Meghna River, Bangladesh

Abstract

Environmental contextArsenic contamination of groundwater is a major environmental problem in many areas of the world. In south-east Asia, iron-rich reducing groundwater mixes with oxidising river water in hyporheic zones, precipitating iron oxides. These oxides can act as a natural reactive barrier capable of accumulating elevated solid-phase concentrations of arsenic. AbstractShallow, anoxic aquifers within the Ganges–Brahmaputra–Meghna Delta (GBMD) commonly contain elevated concentrations of arsenic (As), iron (Fe) and manganese (Mn). Highly enriched solid-phase concentrations of these elements have been observed within sediments lining the banks of the Meghna River. This zone has been described as a Natural Reactive Barrier (NRB). The impact of hydrological processes on NRB formation, such as transient river levels, which drive mixing between rivers and aquifers, is poorly understood. We evaluated the impact of groundwater flow dynamics on hydrobiogeochemical processes that led to the formation of an Fe- and Mn-rich NRB containing enriched As, within a riverbank aquifer along the Meghna River. The NRB dimensions were mapped using four complementary elemental analysis methods on sediment cores: X-ray fluorescence (XRF), aqua regia bulk extraction, and HCl and sodium phosphate leaching. It extended from 1.2 to 2.4 m in depth up to 15 m from the river’s edge. The accumulated As was advected to the NRB from offsite and released locally in response to mixing with aged river water. Nearly all of the As was subsequently deposited within the NRB before discharging to the Meghna. Significant FeII release to the aqueous phase was observed within the NRB. This indicates the NRB is a dynamic zone defined by the interplay between oxidative and reductive processes, causing the NRB to grow and recede in response to rapid and seasonal hydrologic processes. This implies that natural and artificially induced changes in river stages and groundwater-tables will impact where As accumulates and is released to aquifers.

Spatial Variability of Groundwater Arsenic Concentration as Controlled by Hydrogeology: Conceptual Analysis Using 2‐D Reactive Transport Modeling

“…When reduced groundwater, enriched in As(III) and Fe(II), discharges into the oxidizing riverine environment, reactive As‐containing Fe oxides precipitate near the sediment surfaces (Berube et al, ; Datta et al, ). While groundwater discharge mainly occurs at the sides of the river, currents will redistribute the high As‐reactive sediment throughout the river bed, resulting in the occurrence of high As‐reactive river sediment in major rivers (Jung et al, ; Postma et al, ; Stahl et al, ).…”

Influences on tidal channel and aquaculture shrimp pond water chemical composition in Southwest Bangladesh

“…This supports data from [ 6 ] where elevated As in SW Bangladesh surface waters was also reported. However, dissolved As is not expected to be as elevated in near-neutral pH, oxidizing conditions such as those seen in surface waters (e.g., [ 6 , 65 ]), and thus more research on As cycling at the groundwater-surface water interface in Bangladesh is needed (e.g., [ 12 ]).…”

Solute dynamics across the stream‐to‐riparian continuum under different flood waves

“…Stream–aquifer interaction caused by bank storage facilitates solute transport into the riparian zone (Boutt & Fleming, ; Findlay, ; Gu et al, ; McCallum et al, ; Welch et al, ). Similarly, the riparian zone can act as a source of solutes for the cycling of nitrogen (Bishop et al, ; Hornberger et al, ; Inamdar et al, ; Pellerin et al, ) and iron/arsenic or iron/chromium (e.g., Liu et al, ; Berube et al, ). Solute efflux (from aquifer to stream) is generally enhanced during flood wave compared with that under base flow (e.g., Hornberger et al, ; Sawyer et al, ; Wondzell & Swanson, ).…”