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Study Abstract In alluvial aquifers with near-neutral pH and high dissolved arsenic (As) concentrations, the presence and character of sedimentary organic matter (SOM) regulates As mobility by serving as an energetically variable source of electrons for redox reactions or forming As–Fe-OM complexes. Near tidally and seasonally fluctuating rivers, the hyporheic zone (HZ), which embodies the mixing zone between oxic river water and anoxic shallow groundwater, may precipitate (or dissolve) iron (Fe)-oxides which sequester (or mobilize) As. To understand what is driving the mobilization of As within a shallow aquifer and riverbank sands adjacent to the tidally fluctuating Meghna River, we characterized the chemical reactivity of SOM from the sands, and a silt and clay layer, underlying the HZ and aquifer, respectively. Dissolved As (50–500 μg/L) and Fe (1–40 mg/L) concentrations increase with depth within the shallow aquifer. Similar vertical As and Fe concentration gradients were observed within the riverbank sands where concentrations of the products of reductive dissolution of Fe-oxides increase with proximity to the silt layer. Compared to all other sediments, the SOM in the clay aquitard contains older, more recalcitrant, terrestrially-derived material with high proportions of aromatic carboxylate functional groups. The shallow silt layer contains fresher SOM with higher proportions of amides and more labile polysaccharide moieties. The SOM in both the riverbank and aquifer is terrestrially-derived and humic-like. The labile SOM from the silt layer drives the microbially mediated reductive dissolution of As-bearing Fe-oxides in the HZ. In contrast, the carboxylate-rich SOM from the clay aquitard maintains dissolved As concentrations at the base of the aquifer by complexing with soluble As and Fe. This highlights that SOM-rich fine (silt or clay) layers in the Bengal basin drive As and Fe mobility, however, the specific processes mobilizing As and Fe depend on the lability of the SOM. 

This repository contains all the measured inorganic and organic data obtained from the sediment samples used in this study, including the experimental data from a water-sediment extraction. Study Abstract Elevated dissolved arsenic (As) concentrations in the shallow aquifers of Bangladesh are primarily caused by microbially-mediated reduction of As-bearing iron (Fe) (oxy)hydroxides in organic matter (OM) rich, reducing environments. Along the Meghna River in Bangladesh, interactions between the river and groundwater within the hyporheic zone cause fluctuating redox conditions responsible for the formation of a Fe-rich natural reactive barrier (NRB) capable of sequestering As. To understand the NRB's impact on As mobility, the geochemistry of riverbank sediment (<3 m depth) and the underlying aquifer sediment (up to 37 m depth) was analyzed. A 24-hr sediment-water extraction experiment was performed to simulate interactions of these sediments with oxic river water. The sediment and the sediment-water extracts were analyzed for inorganic and organic chemical parameters. Results revealed no differences between the elemental composition of riverbank and aquifer sediments, which contained 40 ± 12 g/kg of Fe and 7 ± 2 mg/kg of As, respectively. Yet the amounts of inorganic and organic constituents extracted were substantially different between riverbank and aquifer sediments. The water extracted 6.4 ± 16.1 mg/kg of Fe and 0.03 ± 0.02 mg/kg of As from riverbank sediments, compared to 154.0 ± 98.1 mg/kg of Fe and 0.55 ± 0.40 mg/kg of As from aquifer sediments. The riverbank and aquifer sands contained similar amounts of sedimentary organic matter (SOM) (17,705.2 ± 5157.6 mg/kg). However, the water-extractable fraction of SOM varied substantially, i.e., 67.4 ± 72.3 mg/kg in riverbank sands, and 1330.3 ± 226.6 mg/kg in aquifer sands. Detailed characterization showed that the riverbank SOM was protein-like, fresh, low molecular weight, and labile, whereas SOM in aquifer sands was humic-like, older, high molecular weight, and recalcitrant. During the dry season, oxic conditions in the riverbank may promote aerobic metabolisms, limiting As mobility within the NRB. 

In alluvial aquifers with near-neutral pH and high dissolved arsenic (As) concentrations, the presence and character of sedimentary organic matter (SOM) regulates As mobility by serving as an energetically variable source of electrons for redox reactions or forming As–Fe-OM complexes. Near tidally and seasonally fluctuating rivers, the hyporheic zone (HZ), which embodies the mixing zone between oxic river water and anoxic shallow groundwater, may precipitate (or dissolve) iron (Fe)-oxides which sequester (or mobilize) As. To understand what is driving the mobilization of As within a shallow aquifer and riverbank sands adjacent to the tidally fluctuating Meghna River, we characterized the chemical reactivity of SOM from the sands, and a silt and clay layer, underlying the HZ and aquifer, respectively. Dissolved As (50–500 μg/L) and Fe (1–40 mg/L) concentrations increase with depth within the shallow aquifer. Similar vertical As and Fe concentration gradients were observed within the riverbank sands where concentrations of the products of reductive dissolution of Fe-oxides increase with proximity to the silt layer. Compared to all other sediments, the SOM in the clay aquitard contains older, more recalcitrant, terrestrially-derived material with high proportions of aromatic carboxylate functional groups. The shallow silt layer contains fresher SOM with higher proportions of amides and more labile polysaccharide moieties. The SOM in both the riverbank and aquifer is terrestrially-derived and humic-like. The labile SOM from the silt layer drives the microbially mediated reductive dissolution of As-bearing Fe-oxides in the HZ. In contrast, the carboxylate-rich SOM from the clay aquitard maintains dissolved As concentrations at the base of the aquifer by complexing with soluble As and Fe. This highlights that SOM-rich fine (silt or clay) layers in the Bengal basin drive As and Fe mobility, however, the specific processes mobilizing As and Fe depend on the lability of the SOM. 

This repository contains all the water chemistry data, sediment borehole lithology observations, and handheld XRF observations of elemental concentrations in sediments used in this study. Study Abstract Groundwater containing high concentrations of dissolved arsenic (As) and iron (Fe(II)) discharges to rivers across the Ganges-Brahmaputra-Meghna delta. Observed Fe(III)-oxyhydroxide (FeOOH)-As deposits lining the riverbanks of the Meghna River may have been created by bidirectional mixing in the hyporheic zone (HZ) from ocean tides. This process has been named the Natural Reactive Barrier (NRB). Sedimentary organic carbon (SOC) is deposited annually on floodplains. Floodwaters that infiltrate through this layer may chemically transform the groundwater prior to discharging through the HZ in ways that influence the capture and retention of As in the NRB. The goal of this study is to understand how the interaction of these two scales of river-groundwater mixing influence the fate of As trapped within an NRB. Monitoring wells were installed to 1-17 m depth, up to 100 m distance from the river’s edge during the dry season on the East (Site 1) and West (Site 2) sides of the river. They were sampled during the dry season (January) under gaining river conditions. The physical properties and elemental composition of the sediment was described by hand observation and hand-held X-Ray Fluorescence (XRF), respectively. Mixing with river water was quantified using the sum of charge of major cations (TC). Site 1 has a sloping bank that is only partially inundated during the wet season. The aquifer is composed of homogeneous sand. Site 2 is flat and therefore fully inundated in the wet season. The aquifer is composed of sand with thin (1-20 cm thick) clay layers. Both sites generate the dissolved products of FeOOH-reduction coupled to organic carbon oxidation, and silicate weathering beneath the floodplain. These products are dissolved Fe, As, silica, bicarbonate, calcium and phosphate. This chemistry is conducive to the formation of crystalline iron oxide minerals such as goethite which may co-precipitate with As, trapping it long-term. 

This is the data used to create the study that is currently under review in a peer-reviewed journal. This dataset contains groundwater chemistry data and groundwater level data across the Meghna riverbank field site near town called Nayapara in Bangladesh. Across the 131 m-wide transect oriented orthogonally to the river shoreline, three types of wells were installed: i) Drive-point piezometers (DP) (“DPa” wells (~0.5 m), “DPb” wells (~1.5 m), “DPc” wells (3 to 4.5 m)); ii) Fully screened shallow piezometers (PZ); iii) Monitoring wells (MW) wAll wells were numbered in descending order away from the river. For example, the DP well that is furthest from the river and has the shallowest depth is referred to as “DP1a”.