Data for "Contribution of Sedimentary Organic Matter to Arsenic Mobilization Along a Potential Natural Reactive Barrier (NRB) Near a River: The Meghna River, Bangladesh"

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.

本仓库收录了本研究中所用沉积物样品的全部实测无机与有机数据，其中涵盖水-沉积物萃取实验的相关实验结果。

研究摘要
孟加拉国浅层含水层中溶解态砷（As）浓度升高，主要由富有机质（OM）还原环境中微生物介导的含砷铁（氧）氢氧化物还原作用导致。在孟加拉国梅格纳河沿岸，河流与潜流带内地下水的交互作用会引发氧化还原条件波动，进而形成可固持砷的富铁天然反应屏障（NRB）。为明确该天然反应屏障对砷迁移性的影响，研究人员对河岸沉积物（深度<3 m）及下伏含水层沉积物（深度可达37 m）的地球化学特征展开分析。通过开展24小时沉积物-水萃取实验，模拟上述沉积物与含氧河水的相互作用过程，并对沉积物及沉积物-水萃取液的无机与有机化学参数进行检测。

结果显示，河岸沉积物与含水层沉积物的元素组成并无显著差异，二者铁（Fe）含量均为40±12 g/kg，砷（As）含量均为7±2 mg/kg。但二者的可萃取无机与有机组分含量差异显著：从河岸沉积物中萃取得到的铁为6.4±16.1 mg/kg、砷为0.03±0.02 mg/kg，而从含水层沉积物中萃取得到的铁为154.0±98.1 mg/kg、砷为0.55±0.40 mg/kg。

河岸砂与含水层砂的沉积有机质（SOM）含量相近，分别为17705.2±5157.6 mg/kg，但二者的水可萃取沉积有机质组分差异悬殊：河岸砂的水可萃取SOM为67.4±72.3 mg/kg，含水层砂则为1330.3±226.6 mg/kg。详细表征结果表明，河岸沉积物的沉积有机质为类蛋白、新鲜、低分子量且易降解；而含水层砂中的沉积有机质则为类腐殖质、老化、高分子量且难降解。旱季时，河岸区域的有氧环境可能促进有氧代谢，进而限制天然反应屏障内的砷迁移性。