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Although biodegradation of organic matter is well-known to trigger enrichment of arsenic (As) in groundwater, the effects of DOM sources and biodegradability on As enrichment remain elusive. In this study, groundwater samples were collected from the Hetao basin to identify DOM source and evaluate biodegradability by using spectral and molecular techniques. Results showed that in the alluvial fan, DOM was mainly sourced from terrestrially derived OM, while DOM in the flat plain was more originated from microbially derived OM. Compared to terrestrially derived DOMs, microbially derived DOMs in groundwater, which had relatively higher H/C wa ratios, NOSC values and more biodegradable molecules, exhibited higher biodegradability. In the flat plain, microbially derived DOMs with higher biodegradability encountered stronger biodegradation, facilitating the reductive dissolution of Fe(III)/Mn oxides and As enrichment in groundwater. Moreover, the enrichment of As depended on the biodegradable molecules that was preferentially utilized for primary biodegradation. Our study highlights that the enrichment of dissolved As in the aquifers was closely associated with microbially derived DOM with high biodegradability and high ability for primary biodegradation., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Various functional groups of microorganisms and related biogeochemical processes are likely to control arsenic (As) mobilization in groundwater systems. However, spatially-dependent correlations between microbial community composition and geochemical zonation along groundwater flow paths are not fully understood, especially with respect to arsenic mobility. The western Hetao Basin was selected as the study area to address this limitation, where groundwater flows from a proximal fan (geochemical-group I: low As, oxidizing), through a transition area (geochemical-group II: moderate As, moderately-reducing) and then to a flat plain (geochemical-group III: high As, reducing). High-throughput Illumina 16S rRNA gene sequencing showed that the microbial community structure in the proximal fan included bacteria affiliated with organic carbon degradation and nitrate-reduction or even nitrate-dependant Fe(II)-oxidation, mainly resulting in As immobilization. In contrast, for the flat plain, high As groundwater contained Fe(III)- and As(V)-reducing bacteria, consistent with current models on As mobilization driven via reductive dissolution of Fe(III)/As(V) mineral assemblages. However, Spearman correlations between hydrogeochemical data and microbial community compositions indicated that ammonium as a possible electron donor induced reduction of Fe oxide minerals, suggesting a wider range of metabolic pathways (including ammonium oxidation coupled with Fe(III) reduction) driving As mobilization in high As groundwater systems., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The special hydrogeological conditions of wetland easily lead to the formation of high-arsenic (As) groundwater. However, it is unclear relative contribution of wetland water (surface water) dissolved organic matter (DOM) and sedimentary organic matter (SOM) to porewater DOM and its relations with dissolved As under the wetland. Porewater, groundwater and sediment samples were taken to monitor spatial-temporal variations in dissolved As affected by fluctuation of surface water levels near a typical wetland with high As groundwater in the arid-semiarid Hetao Basin, China. Hydrochemical data showed that As concentrations of sediment porewater (average: 17.1 μg/L) lay between As concentrations of surface water (average: 6.48 μg/L) and groundwater (average: 184 μg/L). A long-term observation showed concentrations of dissolved As, Fe, and dissolved organic carbon (DOC) of porewater obviously increased when levels of surface water rose during waterlogging period. There were positive correlations between dissolved As and Fe (r = 0.69, p < 0.01) and dissolved As and DOC (r = 0.38, p < 0.01). The sequential extraction showed that the main As form in sediment was amorphous Fe(III) oxide-coprecipitated form (averagely 45.0% of total As). It indicates that the dissolved As in porewater mainly originated from the Fe(III) oxide-bound forms in the sediment. Fluorescence indices and parallel factor analysis of the three-dimensional fluorescence spectra showed that porewater DOM mainly derived mainly from SOM during the drought period and from both surface water DOM and SOM during the waterlogging period. Higher proportions of As(III) in porewaters during waterlogging period were observed relative to those during drought period, indicating that surface water infiltration led to relatively reducing conditions. Surface water infiltration introduced DOM and stimulated the release of SOM into the porewater. DOM, acting as electron donor and electron shuttle, triggered the reductive dissolution of Fe(III) oxides, leading to the As mobilization under the wetland. This study suggests that drinking water supply near wetlands should be concerned, since high concentrations of dissolved As normally occurred in shallow aquifers near the wetland. • Porewater As, Fe, and DOC increased when wetland water level rose. • Arsenic-bearing Fe(III) oxides in sediments were the main source of porewater As. • Infiltration of surface water input DOM and mobilized sediment OM into porewater. • DOM triggered reductive dissolution of Fe(III) oxides, leading to As mobilization. [ABSTRACT FROM AUTHOR]

• High As sediments in the Hetao Basin were from the Yellow River sediments from Northeastern Tibetan Plateau. • Long transportation, recycling of sediments, and warm climate enhanced the silicate weathering in shallow aquifer. • The enhanced silicate weathering increased contents of Fe/Mn oxides and As in shallow aquifer. • Aquatic environmentof sedimentation provided organic matter that drove As enrichment in shallow groundwater. • Geochemical processes and geochemical conditions jointly contributed to As enrichment in groundwater. High arsenic (As) groundwater has been widely observed in the world, but the provenance, chemical weathering, and sedimentary environment of aquifer sediments hosting high As groundwater remain poorly understood. The geochemical and isotopic characteristics of high-resolution aquifer sediments were investigated to unravel the roles of provenance, chemical weathering, and sedimentary environment of the aquifer sediments. The average values of rare earth elements (REEs) showed the enrichment of light REEs and flat heavy REEs in the aquifer and the Yellow River sediments. The chemical index of alteration reflected incipient and moderate weathering (41 to 71). The δ13C SOM (−27.4‰ ± 4.3‰) and C/N values (8.1 ± 5.8) indicated aquatic environment of sedimentation. Shallow aquifer sediments mostly had relatively high ΣREEs contents and negative Eu anomalies, demonstrating that the aquifer sediments were mainly derived from the Yellow River (up to 94.3% ± 5%). Deep aquifer sediments had relatively low ΣREEs contents and positive Eu anomalies, which originated from the Yinshan Mountains (up to 72.2% ± 20.4%) principally with igneous and metamorphic rocks. Moreover, recycling of the Yellow River sediments from Northeastern Tibetan Plateau enhanced the silicate weathering and increased contents of Fe/Mn oxides and As. The δ13C SOM and C/N values evidenced that organisms of paleo-lake or paleo-Yellow River were responsible for abundant sedimentary organic matter. Concentrations of Fe, Mn, and As were high in shallow groundwater from the plain. The As enrichment in the shallow groundwater was jointly controlled by the provenance and chemical weathering of the aquifer sediments (14%), sedimentary environment (10%), reductive dissolution (35%) of Fe/Mn oxides, As desorption under elevated pH (20%), and competitive adsorption of HCO 3 - (21%). This study systematically reveals the roles of provenance, chemical weathering, and sedimentary environment of the aquifer sediments and highlights joint contributions of geochemical processes and geochemical conditions to As enrichment in groundwater. [ABSTRACT FROM AUTHOR]