Data for "Diverse sedimentary organic matter within the river-aquifer interface drives arsenic mobility along the Meghna River Corridor in Bangladesh"

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.

研究摘要
在近中性pH、高溶解砷（As）浓度的冲积含水层中，沉积有机质（sedimentary organic matter, SOM）的存在与特性可通过两种方式调控砷的迁移性：一是作为氧化还原反应的可变电子供体，二是形成砷-铁-有机质络合物。在受潮汐与季节波动影响的河流附近，潜流带（hyporheic zone, HZ）作为含氧河水与缺氧浅层地下水的混合区域，可沉淀（或溶解）铁氧化物（Fe-oxides），进而固持（或活化）砷。
为明确潮汐波动的梅格纳河相邻浅层含水层与河岸砂体中砷活化的驱动因素，我们分别对潜流带及含水层下方的砂体、粉砂与黏土层中的沉积有机质的化学反应活性进行了表征。
浅层含水层内，溶解态砷（50~500 μg/L）与铁（1~40 mg/L）的浓度随深度增加而升高。河岸砂体中也观测到类似的垂直砷、铁浓度梯度，且铁氧化物还原溶解产物的浓度随与粉砂层的距离减小而升高。
与其余所有沉积物相比，黏土层隔水层（clay aquitard）中的沉积有机质含有更古老、更难降解的陆源物质，且带有高比例的芳香族羧酸官能团。浅层粉砂层则含有更新鲜的沉积有机质，酰胺占比更高，且含有更多易降解的多糖基团。河岸砂体与含水层中的沉积有机质均为陆源腐殖质类物质。
粉砂层中的易降解沉积有机质驱动了潜流带内含砷铁氧化物的微生物介导还原溶解过程。与之相反，黏土层隔水层中富含羧酸的沉积有机质可通过与可溶性砷、铁络合，维持含水层底部的溶解砷浓度。
本研究结果表明，孟加拉盆地内富含沉积有机质的细粒（粉砂或黏土）层驱动了砷与铁的迁移性，但具体活化砷、铁的过程取决于沉积有机质的易降解性。