Carbon and methane cycling in arsenic-contaminated aquifers

Coupled transformation pathways of iron minerals and natural organic matter related to iodine mobilization in alluvial-lacustrine aquifer

The complex of natural organic matter (NOM) and iron minerals in sediment is the main host and source of groundwater iodine. However, the transformation pathways of the complex remain unclear. The groundwater and sediment from the Hetao Basin were collected in this study to analyze multi-isotopes, NOM molecular characteristics, and iron mineral phases. The results showed that high-iodine groundwater was mainly observed in the discharge area, where biodegradation of NOM, sulfate reduction and methanogenesis occurred. Compared to the shallow clayey sediments, the confined sandy sediments had lower iodine content, a lower fraction of crystalline iron oxides, and a higher fraction of carbonate associated Fe(II) minerals, suggesting that the release of sediment iodine in the aquifer is related to the transformation of sediment Fe(III) hydroxides/oxides. Moreover, the molecular features of high-iodine groundwater NOM and sandy sediment NOM were characterized by a higher proportion of refractory compounds, suggesting that the reductive transformation of sediment Fe(III) hydroxides/oxides is fueled by degradable organic compounds. The microbial Fe-reducing and/or sulfate-reducing processes cause the enrichment of groundwater iodine in the form of iodide via the transformation of iodine species. These findings provide new insights into the genesis of high-iodine groundwater.

Seasonal Dynamics of Methane Fluxes from Groundwater to Lakes:Hydrological and Biogeochemical Controls

Methane (CH4) inputs to lakes through lacustrine groundwater discharge (LGD-derived CH4) represent a potentially important but often overlooked source of lake methane emissions. Although great efforts have been made to quantify LGD-derived CH4 fluxes and their spatial variablity, the underlying mechanisms controlling seasonal LGD-derived CH4 fluxes and their influence on lake CH4 emissions remain poorly understood, particularly in humid inland areas. To address this gap, we applied the 222Rn mass balance model, as well as hydrological, isotopic and microbial methods to assess seasonal LGD-derived CH4 fluxes and their influence on the seasonal variability of lake methane emissions in a typical oxbow lake, central Yangtze River. The results revealed wide seasonal differences in LGD-derived CH4 fluxes, which were controlled by hydrological and biogeochemical processes. During the dry season, although more intense methane oxidation and weaker methanogenesis occurred in groundwater, the much higher LGD rate (51.71 mm/d) produced a higher LGD-derived CH4 flux (16.41 mmol/m2/d). During the wet season, methanogenesis was more active and methane oxidation was weaker, but a lower LGD rate (12.16 mm/d) led to a lower LGD-derived CH4 flux (5.33 mmol/m2/d). Furthermore, higher LGD-derived CH4 flux in the dry season resulted in higher CH4 emissions from the lake and diminished the extent of methane oxidation in the lake. In comparison to other regions, the differences in LGD-derived CH4 fluxes and their seasonal variations were found to be controlled by climatic conditions and lake types in different global regions. Higher LGD-derived CH4 fluxes and more pronounced seasonal variations could be associated with higher temperature, larger water depth and more intense water level fluctuations. This study provides a novel perspective and broader implications for the comprehension and evaluation of seasonal methane emissions and understanding the carbon cycle in global lake ecosystems in humid areas with intense water level fluctuations.

Using a combination of δ13CDIC-DOC-difference in dissolved inorganic and organic carbon, δ2H, and δ18O to localize leachate leaks at landfill sites in China

The investigation of leachate leakage at numerous landfill sites is urgently needed. This study presents an exploration of environmental tracing methods using δ2H and δ13C-difference in dissolved carbon (δ13CDIC-DOC) to localize leachate leak points at landfill sites. δ2H, δ13CDIC, δ13CDOC, δ18O, and an array of physicochemical indices (e.g., total dissolved solids, temperature, and oxidation reduction potential) were monitored in both leachate and groundwater from different zones of a landfill site in China during the year of 2021-2023. Moreover, data for these parameters (i.e., the isotopic composition and physicochemical indices) from twelve published landfill cases were also collected, and these groundwater/leachate data points were located within 1 km away from the landfill boundary. Then statistical analyses, such as Pearson correlation analysis and redundancy analysis (RDA), were performed using both the detected and collected parameters at landfill sites. Consequently, the intensity of interaction between leachate and background groundwater was found to significantly control the isotopic fractionation features of hydrogen and carbon, and both the content of major contamination indicators (total dissolved solids, chemical oxygen demand, and ammoniacal nitrogen) and the oxidation reduction potential were the key impact factors. Accordingly, the water type used to indicate leachate leakage points was determined to be leachate that significantly interacted with the background groundwater or precipitation (LBGP). δ2H showed a perfect linear correlation (0.81 ≤ r2 < 1.0) with δ13CDIC-DOC in leachate under highly anaerobic landfill conditions, and the δ2H & δ13CDIC-DOC combinations in the LBGP were significantly different from those in the other water types. For groundwater with total dissolved solids lower than 1400 mg/L at landfill sites, a strong positive linear correlation (r = 0.83) was revealed between δ13CDIC and δ13CDOC. Based on these insights, δ2H versus δ13CDIC-DOC plots and RDA using δ2H and δ13CDIC-DOC as response variables were proposed to localize leak points at both lined landfills and leachate facilities. These findings further understanding of the isotopic fractionation features of hydrogen, carbon, and oxygen and provide novel environmental tracer methods for investigating leachate leak points at MSW landfill sites.

Deciphering the spatial heterogeneity of groundwater arsenic in Quaternary aquifers of the Central Yangtze River Basin

Significant spatial variability of groundwater arsenic (As) concentrations in South/Southeast Asia is closely associated with sedimentogenesis and biogeochemical cycling processes. However, the role of fine-scale differences in biogeochemical processes under similar sedimentological environments in controlling the spatial heterogeneity of groundwater As concentrations is poorly understood. Within the central Yangtze Basin, dissolved organic matter (DOM) and microbial functional communities in the groundwater and solid-phase As-Fe speciation in Jianghan Plain (JHP) and Jiangbei Plain (JBP) were compared to reveal mechanisms related to the spatial heterogeneity of groundwater As concentration. The optical signatures of DOM showed that low molecular terrestrial fulvic-like with highly humified was predominant in the groundwater of JHP, while terrestrial humic-like and microbial humic-like with high molecular weight were predominant in the groundwater of JBP. The inorganic carbon isotope, microbial functional communities, and solid-phase As-Fe speciation suggest that the primary process controlling As accumulation in JHP groundwater system is the degradation of highly humified OM by methanogens, which drive the reductive dissolution of amorphous iron oxides. While in JBP groundwater systems, anaerobic methane-oxidizing microorganisms (AOM) coupled with fermentative bacteria, iron reduction bacteria (IRB), and sulfate reduction bacteria (SRB) utilize low molecular weight DOM degradation to drive biotic/abiotic reduction of Fe oxides, further facilitating the formation of carbonate associated Fe and crystalline Fe oxides, resulting in As release into groundwater. Different biogeochemical cycling processes determine the evolution of As-enriched aquifer systems, and the coupling of multiple processes involving organic matter transformation‑iron cycling‑sulfur cycling-methane cycling leads to heterogeneity in the spatial distribution of As concentrations in groundwater. These findings provide new perspectives to decipher the spatial variability of As concentrations in groundwater.

Geogenic Phosphorus Enrichment in Groundwater due to Anaerobic Methane Oxidation-Coupled Fe(III) Oxide Reduction

Accumulation of geogenic phosphorus (P) in groundwater is an emerging environmental concern, which is closely linked to coupled processes involving FeOOH and organic matter under methanogenic conditions. However, it remains unclear how P enrichment is associated with methane cycling, particularly the anaerobic methane oxidation (AMO). This study conducted a comprehensive investigation of carbon isotopes in dissolved inorganic carbon (DIC), CO2, and CH4, alongside Fe isotopes, microbial communities, and functions in quaternary aquifers of the central Yangtze River plain. The study found that P concentrations tended to increase with Fe(II) concentrations, δ56Fe, and δ13C-DIC, suggesting P accumulation due to the reductive dissolution of FeOOH under methanogenic conditions. The positive correlations of pmoA gene abundance versus δ13C-CH4 and Fe concentrations versus δ13C-CH4, and the prevalent presence of Candidatus_Methanoperedens, jointly demonstrated the potential significance of Fe(III)-mediated AMO process (Fe-AMO) alongside traditional methanogenesis. The increase of P concentration with δ13C-CH4 value, pmoA gene abundance, and Fe concentration suggested that the Fe-AMO process facilitated P enrichment in groundwater. Redundancy analysis confirmed this assertion, identifying P concentration as the primary determinant and the cooperative influence of Fe-AMO microorganisms such as Candidatus_Methanoperedens and Geobacter on P enrichment. Our work provided new insights into P dynamics in subsurface environments.

Identification of methane cycling pathways in Quaternary alluvial-lacustrine aquifers using multiple isotope and microbial indicators

Groundwater rich in dissolved methane is often overlooked in the global or regional carbon cycle. Considering the knowledge gap in understanding the biogeochemical behavior of methane in shallow aquifers, particularly those in humid alluvial-lacustrine plains with high organic carbon content, we investigated methane sources and cycling pathways in groundwater systems at the central Yangtze River basins. Composition of multiple stable isotopes (2H/18O in water, 13C in dissolved inorganic carbon, 13C/2H in methane, and 13C in carbon dioxide) was combined with the characteristics of microbes and dissolved organic matter (DOM) in the study. The results revealed significant concentrations of biogenic methane reaching up to 13.05 mg/L in anaerobic groundwater environments with abundant organic matter. Different pathways for methane cycling (methanogenic CO2-reduction and acetate-fermentation, and methane oxidation) were identified. CO2-reduction dominated acetate-fermentation in the two methanogenic pathways primarily associated with humic DOM, while methane oxidation was more closely associated with microbially derived DOM. The abundance of obligate CO2-reduction microorganisms (Methanobacterium and Methanoregula) was higher in samples with substantial CO2-reduction, as indicated by isotopic composition. The obligate acetate-fermentation microorganism (Methanosaeta) was more abundant in samples exhibiting evident acetate-fermentation. Additionally, a high abundance of Candidatus Methanoperedens was identified in samples with apparent methane oxidation. Comparing our findings with those in other areas, we found that various factors, such as groundwater temperature, DOM abundance and types, and hydrogeological conditions, may lead to differences in groundwater methane cycling. This study offered a new perspective and understanding of methane cycling in worldwide shallow alluvial-lacustrine aquifer systems without geothermal disturbance.

Release of arsenic during riverbank filtration under anoxic conditions linked to grain size of riverbed sediments

Geogenic arsenic contamination of groundwater poses a health threat to millions of people worldwide, particularly in Asia. Riverbank filtration (RBF) is a pre-treatment technique that aims to improve surface water quality through natural processes during water infiltration before abstraction. A study in Hanoi, Vietnam is presented, where the water quality of 48 RBF wells from 5 large well fields located in the Pleistocene aquifer along the Red River was analyzed. >80 % of the wells had arsenic concentrations above the WHO limit of 10 μg/l. The riverbed sediment and riverbed pore-water from 23 sites along a stretch of 30 km of the Red River near the well fields was also analyzed. Muddy riverbeds were found to be a hotspot for arsenic release. Already at a 30 cm depth from the riverbed sediment surface, the pore-water at many sites had high concentrations of arsenic (>100 μg/l). Arsenic concentrations in the pore-water of sites where mud lenses were present in the riverbed were significantly higher compared to sites with sandy riverbeds. At well fields along stretches of the Red River where riverbed was mostly muddy, higher arsenic concentrations were found than at well fields where the riverbed was mostly sandy. This indicates that river muds deposition and river morphology can influence arsenic concentrations in the aquifer in Hanoi and potentially other RBF sites in regions with geogenic arsenic contamination. At the end, recommendations regarding site selection of new potential RBF wells in affected regions is given.

Effect of depositional evolution on phosphorus enrichment in aquifer sediments of alluvial-lacustrine plain

Groundwater with high geogenic phosphorus (P) is increasingly concerned as a potential risk to surface water eutrophication. Although hydrogeochemical processes responsible for P mobilization in groundwater systems have been studied, the burial characteristics of P and the effect of depositional evolution on P enrichment in aquifer sediments remain unclear. In this study, aquifer sediments were collected from the Dongting Lake Plain (DTP) within the central Yangtze River Basin, a high P groundwater area, and the effect of depositional evolution on P enrichment was elucidated by comprehensively analyzing the lithology, grain size, geochronology, and geochemistry of the sediments, coupled with groundwater chemistry and sediment incubation experiments. The results showed that the contents of total organic carbon (TOC), iron (Fe), and P (the relative content of bioavailable phosphorus (BAP)) were higher in lacustrine sediments deposited under a warm-wet climate, but lower in fluvial sediments deposited under a cold-dry climate. During depositional evolution, the sedimentary facies mainly controlled the content of organic phosphorus (OP), while the paleo-climate controlled the content of both OP and Fe-bound inorganic P (FeP), which jointly affected total P content in aquifer sediments. Under the interaction of groundwater and sediment, the reductive dissolution of P-rich Fe (oxyhydr)oxides and the mineralization of OP in sediment continuously release P into groundwater. Notably, the rapid accumulation of alluvial sediments after the Last Glacial Maximum in the DTP and rapid evolution of Dongting Lake during the Holocene led to a large amount of organic matter (OM) and P buried in sediments, providing materials for P release in aquifers, which seriously threatens groundwater quality. This exploration can provide a new understanding of the enrichment of geogenic P in groundwater from the perspective of depositional evolution.

An Initial Proteomic Analysis of Biogas-Related Metabolism of Euryarchaeota Consortia in Sediments from the Santiago River, México

In this paper, sediments from the Santiago River were characterized to look for an alternative source of inoculum for biogas production. A proteomic analysis of methane-processing archaea present in these sediments was carried out. The Euryarchaeota superkingdom of archaea is responsible for methane production and methane assimilation in the environment. The Santiago River is a major river in México with great pollution and exceeded recovery capacity. Its sediments could contain nutrients and the anaerobic conditions for optimal growth of Euryarchaeota consortia. Batch bioreactor experiments were performed, and a proteomic analysis was conducted with current database information. The maximum biogas production was 266 NmL·L^-1·g VS^-1, with 33.34% of methane, and for proteomics, 3206 proteins were detected from 303 species of 69 genera. Most of them are metabolically versatile members of the genera Methanosarcina and Methanosarcinales, both with 934 and 260 proteins, respectively. These results showed a diverse euryarcheotic species with high potential to methane production. Although related proteins were found and could be feeding this metabolism through the methanol and acetyl-CoA pathways, the quality obtained from the biogas suggests that this metabolism is not the main one in carbon use, possibly the sum of several conditions including growth conditions and the pollution present in these sediments.

Organic matter degradation and arsenic enrichment in different floodplain aquifer systems along the middle reaches of Yangtze River: A thermodynamic analysis

Geogenic arsenic (As) contaminated groundwater has been widely accepted associating with dissolved organic matter (DOM) in aquifers, but the underlying enrichment mechanism at molecular-level from a thermodynamic perspective is poorly evidenced. To fill this gap, we contrasted the optical properties and molecular compositions of DOM coupled with hydrochemical and isotopic data in two floodplain aquifer systems with significant As variations along the middle reaches of Yangtze River. Optical properties of DOM indicate that groundwater As concentration is mainly associated with terrestrial humic-like components rather than protein-like components. Molecular signatures show that high As groundwater has lower H/C ratios, but greater DBE, AI_mod, and NOSC values. With the increase of groundwater As concentration, the relative abundance of CHON3 formulas gradually decreased while that of CHON2 and CHON1 increased, indicating the importance of N-containing organics in As mobility, which is also evidenced by nitrogen isotope and groundwater chemistry. Thermodynamic calculation demonstrated that organic matter with higher NOSC values preferentially favored the reductive dissolution of As-bearing Fe(III) (hydro)oxides minerals and thus promoted As mobility. These findings could provide new insights to decipher organic matter bioavailability in As mobilization from a thermodynamical perspective and are applicable to similar geogenic As-affected floodplain aquifer systems.

Interactions between arsenic migration and CH4 emission in a soil bioelectrochemical system under the effect of zero-valent iron

Dissimilatory soil arsenic (As) reduction and release are driven by microbial extracellular electron transfer (EET), while reverse EET mediates soil methane (CH₄) emission. Nevertheless, the detailed biogeochemical mechanisms underlying the tight links between soil As migration and methanogenesis are unclear. This study used a bioelectrochemical-based system (BES) to explore the potential effects of zero-valent iron (ZVI) addition on "As migration-CH₄ emission" interactions from chemical and microbiological perspectives. Voltage and ZVI amendment experiments showed that dissolved As was efficiently immobilized with increased CH₄ production in the soil BES, As release and CH₄ production exhibited a high negative exponential correlation, and reductive As dissolution could be entirely inhibited in the methanogenic stage. Gene quantification and bacterial community analysis showed that in contrast to applied voltage, ZVI changed the spatial heterogeneity of the distribution of electroactive microorganisms in the BES, significantly decreasing the relative abundance of arrA and dissimilatory As/Fe-reducing bacteria (e.g., Geobacter) while increasing the abundance of aceticlastic methanogens (Methanosaeta), which then dominated CH₄ production and As immobilization after ZVI incorporation. In addition to biogeochemical activities, coprecipitation with ferric (iron) contributed 77-93% dissolved As removal under ZVI addition. This study will enhance our knowledge of the processes and microorganisms controlling soil As migration and CH₄ emission.

Linking DOM characteristics to microbial community: The potential role of DOM mineralization for arsenic release in shallow groundwater

Dissolved organic matter (DOM) play critical roles in arsenic (As) biotransformation in groundwater, but its compositional characteristics and interactions with indigenous microbial communities remain unclear. In this study, DOM signatures coupled with taxonomy and functions of microbial community were characterized in As-enriched groundwater by excitation-emission matrix, Fourier transform ion cyclotron resonance mass spectrometry and metagenomic sequencing. Results showed that As concentrations were significantly positively correlated with DOM humification (r = 0.707, p < 0.01) and the most dominant humic acid-like DOM components (r = 0.789, p < 0.01). Molecular characterization further demonstrated high DOM oxidation degree, with the prevalence of unsaturated oxygen-low aromatics, nitrogen (N₁/N₂)-containing compounds and unique CHO molecules in high As groundwater. These DOM properties were consistent with microbial composition and functional potentials. Both taxonomy and binning analyses demonstrated the dominance of Pseudomonas stutzeri, Microbacterium and Sphingobium xenophagum in As-enriched groundwater which possessed abundant As-reducing gene, with organic carbon degrading genes capable of labile to recalcitrant compounds degradation and high potentials of organic nitrogen mineralization to generate ammonium. Besides, most assembled bins in high As groundwater presented strong fermentation potentials which could facilitate carbon utilization by heterotrophic microbes. This study provides better insight into the potential role of DOM mineralization for As release in groundwater system.

Revisiting the biological pathway for methanogenesis in landfill from metagenomic perspective-A case study of county-level sanitary landfill of domestic waste in North China plain

Landfill is the third highest contributor to anthropogenic methane (CH₄) emissions, produced primarily by the anaerobic decomposition of organic matter by microbes. However, how various microbial metabolic processes contribute to CH₄ production in domestic waste landfill remains elusive. We addressed this problem by investigating the methanogenic communities, methanogenic functional genes, KEGG modules and KEGG pathways in a county-level MSW sanitary landfill in North China Plain, China. Results showed that Methanomicrobiales, Methanobacteriales, Methanosarcinales, Micrococcales, Corynebacteriales and Bacillales were the dominant methanogens. M00357, M00346, M00567 and M00563 were the four major methane metabolic modules. The most abundant genes were ACSS, ackA and fwd with the relative abundance of 19.26-54.54%, 6.14-25.78% and 6.76-16.51%, respectively. The two essential genes of methanogenesis were detected with the relative abundance of 2.66-9.58% (mtr) and 1.63-9.14% (mcr). These findings indicated that acetotrophic and hydrogenotrophic methanogenesis were the major pathways. Methanomicrobiales, Methanosarcinales and Clostridiales were the key microbes to these pathways identified by co-occurrence network. Analysis of relative contribution of species to function further showed that Micrococcales, Corynebacteriales and Bacillales were special contributors to acetotrophic methanogenesis pathway. Redundancy analysis revealed that above functional genes and microbes were mainly controlled by NH₄⁺ and pH. Our results can help to provide develop the fine management strategies for methane utilization and emission reduction in landfill.

Hotspots of geogenic arsenic and manganese contamination in groundwater of the floodplains in lowland Amazonia (South America)

Arsenic enrichment in groundwater resources in deltas and floodplains of large sediment-rich rivers is a worldwide natural hazard to human health. High spatial variability of arsenic concentrations in affected river basins limits cost-effective mitigation strategies. Linking the chemical composition of groundwater with the topography and fluvial geomorphology is a promising approach for predicting arsenic pollution on a regional scale. Here we correlate the distribution of arsenic contaminated wells with the fluvial dynamics in the Amazon basin. Groundwater was sampled from tube wells along the Amazon River and its main tributaries in three distinct regions in Peru and Brazil. For each sample, the major and trace element concentrations were analyzed, and the position of the well within the sedimentary structure was determined. The results show that aquifers in poorly weathered sediments deposited by sediment-rich rivers are prone to mobilization and accumulation of aqueous arsenic and manganese, both in sub-Andean foreland basins, and in floodplains downstream. Two zones at risk are distinguished: aquifers in the channel-dominated part of the floodplain (CDF) and aquifers in the overbank deposits on the less-dynamic part of the floodplain (LDF). Some 70 % of the wells located on the CDF and 20 % on the LDF tap groundwater at concentrations exceeding the WHO guideline of 10 μg/L arsenic (max. 430 μg/L), and 70 % (CDF) and 50 % (LDF) exceeded 0.4 mg/L manganese (max. 6.6 mg/L). None of the water samples located outside the actual floodplain of sediment-rich rivers, or on riverbanks of sediment-poor rivers exceed 5 μg/L As, and only 4 % exceeded 0.4 mg/L Mn. The areas of highest risk can be delineated using satellite imagery. We observe similar patterns as in affected river basins in South and Southeast Asia indicating a key role of sedimentation processes and fluvial geomorphology in priming arsenic and manganese contamination in aquifers.

Contrasting behaviors of groundwater arsenic and fluoride in the lower reaches of the Yellow River basin, China: Geochemical and modeling evidences

Genesis of the contrasting distributions of high arsenic (>10 μg/L) and fluoride (>1 mg/L) groundwater and their negative correlations remain poorly understood. We investigated spatial distributions of groundwater arsenic and fluoride concentrations in the lower reaches of the Yellow River basin, Henan Province, China, using bivariate statistical analyses and geochemical simulations. Results suggest that high arsenic and fluoride groundwater showed contrasting distributions with few overlapped area. Groundwater arsenic concentrations were significantly negatively correlated with oxidation-reduction potential (ORP) values and positively with NH₄⁺ and Fe(II) concentrations, while the opposites were true for groundwater fluoride concentrations. These may suggest that high arsenic groundwater is related to stronger organic matter degradation and Fe(III) oxide reduction, while groundwater fluoride enrichment occurs with less extent of organic matter degradation. Geochemical calculations supported that groundwater fluoride enrichment was governed by extent of fluorite dissolution, which was constrained by varied saturation indices of fluorite in groundwater. However, groundwater arsenic mobility may be explained by different solubility of Fe(III) oxides. Higher Fe(III) oxide solubility corresponding to goethite and lepidocrocite was related to higher arsenic concentrations, while hematite was too low in solubility to produce high arsenic groundwater. The study presented both geochemical and modeling evidences for the contrasting behaviors of groundwater arsenic and fluoride concentrations in anoxic aquifers.

Microbially mediated arsenic mobilization in the clay layer and underlying aquifer in the Hetao Basin, Inner Mongolia, China

The clay layer is a source to facilitate arsenic (As) enrichment in the aquifer. However, little is known about microbial processes in the clay layer and their roles in As mobilization in the underlying aquifer. In this study, high-throughput sequencing of full-length 16S rRNA gene and metagenomics were used to characterize the microbial composition and functional potential in a sediment borehole across the clay and sand layers in Hetao Basin, Inner Mongolia, China. Results showed the significant differences between the clay layer and underlying sand layer in the geochemistry, microbial composition and functional potential. Fermentation, Fe(III) reduction, As(V) reduction, sulfate reduction, thiosulfate disproportionation, reductive tricarboxylic acid and Wood-Ljungdahl pathway identified in sediments from the clay layer were positively correlated to the observed high levels of TOC, soluble ammonium, H₃PO₄-extractable As(III) and As(V) and HCl-extractable Fe(II). Although the microbial compositions of the clay and sand layers were different, the microbial functional potential at the interface between the clay and sand layers was similar with the characteristics of fermentation, ammonification and As(V) reduction. The similarity of microbial functional potential at the interface may be attributable to the interaction between the sand and adjacent clay layer with the fluctuation of groundwater level. These metabolic products from the microbial processes in the clay layers and interface would migrate into the underlying groundwater during groundwater overpumping, which facilitates As enrichment in groundwater.

Genome-Resolved Metagenomic Analysis of Groundwater: Insights into Arsenic Mobilization in Biogeochemical Interaction Networks

High-arsenic (As) groundwaters, a worldwide issue, are critically controlled by multiple interconnected biogeochemical processes. However, there is limited information on the complex biogeochemical interaction networks that cause groundwater As enrichment in aquifer systems. The western Hetao basin was selected as a study area to address this knowledge gap, offering an aquifer system where groundwater flows from an oxidizing proximal fan (low dissolved As) to a reducing flat plain (high dissolved As). The key microbial interaction networks underpinning the biogeochemical pathways responsible for As mobilization along the groundwater flow path were characterized by genome-resolved metagenomic analysis. Genes associated with microbial Fe(II) oxidation and dissimilatory nitrate reduction were noted in the proximal fan, suggesting the importance of nitrate-dependent Fe(II) oxidation in immobilizing As. However, genes catalyzing microbial Fe(III) reduction (omcS) and As(V) detoxification (arsC) were highlighted in groundwater samples downgradient flow path, inferring that reductive dissolution of As-bearing Fe(III) (oxyhydr)oxides mobilized As(V), followed by enzymatic reduction to As(III). Genes associated with ammonium oxidation (hzsABC and hdh) were also positively correlated with Fe(III) reduction (omcS), suggesting a role for the Feammox process in driving As mobilization. The current study illustrates how genomic sequencing tools can help dissect complex biogeochemical systems, and strengthen biogeochemical models that capture key aspects of groundwater As enrichment.

Noble gas constraints on the fate of arsenic in groundwater

Groundwater contamination of geogenic arsenic (As) remains a global health threat, particularly in south-east Asia. The prominent correlation often observed between high As concentrations and methane (CH4) stimulated the analysis of the gas dynamics in an As contaminated aquifer, whereby noble and reactive gases were analysed. Results show a progressive depletion of atmospheric gases (Ar, Kr and N2) alongside highly increasing CH4, implying that a free gas phase comprised mainly of CH4 is formed within the aquifer. In contrast, Helium (He) concentrations are high within the CH4 (gas) producing zone, suggesting longer (groundwater) residence times. We hypothesized that the observed free (CH4) gas phase severely detracts local groundwater (flow) and significantly reduces water renewal within the gas producing zone. Results are in-line with this hypothesis, however, a second hypothesis has been developed, which focuses on the potential transport of He from an adjacent aquitard into the (CH4) gas producing zone. This second hypothesis was formulated as it resolves the particularly high He concentrations observed, and since external solute input from the overlying heterogeneous aquitard cannot be excluded. The proposed feedback between the gas phase and hydraulics provides a plausible explanation of the anti-intuitive correlation between high As and CH4, and the spatially highly patchy distribution of dissolved As concentrations in contaminated aquifers. Furthermore, the increased groundwater residence time would allow for the dissolution of more crystalline As-hosting iron(Fe)-oxide phases in conjunction with the formation of more stable secondary Fe minerals in the hydraulically-slowed (i.e., gas producing) zone; a subject which calls for further investigation.