Novel Insights into Dissolved Organic Matter Processing Pathways in a Coastal Confined Aquifer System with the Highest Known Concentration of Geogenic Ammonium

Decoding the role of organic matter in groundwater Feammox processes: Insights from the Yangtze River paleochannel

Groundwater nitrogen (N) contamination is becoming increasingly severe worldwide. Anaerobic ammonia oxidation coupled with iron-reduction processes (Feammox) has great potential as an effective method for N removal in groundwater systems. However, previous studies on nitrogen removal by Feammox have generally focused on surface sediment soils, and the quantification of this process in groundwater remains inadequate. Moreover, the impact of native organic matter (OM) within the groundwater system on the Feammox process remains uncertain. The paleochannel of the middle reaches of the Yangtze River was selected as a representative study area for this research. The occurrence of Feammox and other N cycle (non-Feammox) processes in regional groundwater was identified and differentiated through the analysis of δ15N/δ56Fe isotopes and 16S rRNA functional gene quantification, along with hydrochemical characteristics. These findings indicate that the groundwater in the study area is characterized by anoxic conditions and slight acidity. The occurrence of Feammox is substantiated by an increase in δ15NNH4, which coincides with the concurrent increase of Fe(II) concentrations and δ56Fe values in the groundwater, alongside the predominance of Acidimicrobiaceae bacterium A6. 15N isotope-labeled incubation experiments demonstrated that the potential rate of N removal via the Feammox process in the groundwater system ranged from 0.09 to 0.16 mg N kg-1d-1. Correlation results suggested that the functional microorganisms facilitating the Feammox process are closely linked to environmental factors associated with organic matter activity. Terrestrial humic substances present in groundwater, characterized by a high degree of unsaturation, aromaticity, humification, elevated biological activity, and nitrogen-rich composition, may act as pivotal drivers of the Feammox process.

Electrochemistry and Molecular Compositions Reflect Electron Shuttling of Dissolved Organic Matter in High Arsenic Groundwater

Little is known about the electron shuttle ability of dissolved organic matter (DOM) and its effects on arsenic (As) mobilization, which makes the underlying mechanism of groundwater As enrichment elusive. In this study, both the electrochemical properties and molecular compositions of DOM in high As groundwater were quantified in the Hetao Basin, China. We found that, along the flow path, the average electron-transferring capacity (ETC) of DOM, including the capacities of electron-accepting and electron-donating, continuously increased from 2.85 to 3.59 mmole-/gC along with As concentrations. The increasing ETC reflected an increase in electron shuttle ability of DOM. Furthermore, the increasing electron shuttle ability was mainly attributed to the recalcitrant compounds in DOM, especially CHOS and CHONS formulas in highly unsaturated structures with high oxygen (HUSHO) and CHO and CHON formulas in aromatic structures (AS). The significantly positive correlation between As concentration and ETC indicated that recalcitrant DOM promoted groundwater As enrichment through electron shuttling for inducing the reductive dissolution of As-containing Fe(III) oxide minerals, which was further supported by our culture experiments showing that goethite was more reduced [133 μM Fe(II)] in the presence of DOM with a higher ETC (3.35 mmole-/gC) as electron shuttling than that [65.2 μM Fe(II)] with a relatively lower ETC (2.41 mmole-/gC). Our study highlights that recalcitrant DOM compounds with unsaturated and AS have high electron shuttle ability, promoting As enrichment in groundwater.

Dissolved Inorganic Carbon Evolution of Sediment Porewater in the Huixian Wetland, Southwest China

Wetlands, as crucial terrestrial carbon reservoirs, have recently suffered severe degradation due to intense human activities. Lacustrine sediments serve as vital indicators for understanding wetland environmental changes. In the current paper, porewater samples were extracted from lacustrine sediment in three boreholes with a depth of ~75 cm in the Huixian karst wetland, southwest China, to study the chemical and dissolved inorganic carbon (DIC) evolution under anthropogenic influence. Two boreholes are situated beneath the Mudong Lake, while the other one is in the degraded wetland area. The results show that porewater in the central region of Mudong Lake is natural HCO3-Ca type water and recharged by karst groundwater as evidenced by depleted 2H -18O isotopes. Methanogenesis prevails in this area, suggested by positive δ13C values ranging from 4.29‰ to 7.05‰. However, shallow porewater at the western edge of Mudong Lake and porewater in the degraded wetland exhibit significantly higher concentrations of NO3 - and SO4 2-, resulting from the agricultural input and recharged groundwater influenced by oxidation of pyrite. These processes lead to a decrease in methane production and generate DIC through degradation of organic fertilizer and carbonate weathering by sulfuric acid, thereby significantly altering porewater δ13C values. Two types of DIC mixing processes were observed based on the increasing δ13C values with depth, which can be attributed to the unique karst groundwater subsystems. This work highlights the potential impact of human-induced porewater chemical variations on the fate of DIC, particularly in karst wetland environments.

Mechanisms controlling spatial variability of geogenic ammonium in coastal aquifers: Insights from Holocene sedimentary evolution

The contamination of groundwater with geogenic ammonium (NH4+) across various geological backgrounds has garnered significant attention, particularly in coastal aquifer systems. However, there remains a gap in our understanding of the mechanisms governing the spatial variability of NH4+ in coastal groundwater at a macroscopic scale. In this study, we collected the sediment samples from two boreholes corresponding to high-NH4+-N and low-NH4+-N groundwater. We analyzed the age, physicochemical properties, and soluble organic matter (SOM) characteristics of these sediments. The aim was to reconstruct the sedimentary evolutionary history of the Pearl River Delta and establish a link between sedimentary evolutionary processes and organic matter (OM) to further elucidate the mechanism underlying the formation of spatial heterogeneity of NH4+ in groundwater. The results suggested that the studied Quaternary shallow confined porous aquifer system was predominantly formed during the Holocene and comprised three depositional stages, including fluvial facies, estuarine-tidal flat facies, and deltaic plain facies. The depositional environment significantly controlled the physicochemical and SOM characteristics of sediments. In the paleo-channel area, the aquifer was covered by estuarine-tidal flat facies sediments abundant in OM and exhibited considerable SOM degradation. Consequently, a substantial amount of ion-exchange form NH4+-N (IEF) was liberated through compaction and diffused into the aquifer. In the paleo-interfluve area, the aquifer was covered by fluvial sediments characterized by extensive weathering and low OM content, resulting in the limited production of significant amounts of IEF that could infiltrate into the aquifer. This study provided an inaugural elucidation of the control mechanism of sedimentary evolution on the spatial variability of NH4+ in coastal groundwater at a macroscale, thereby enhancing the scientific substantiation for both the exploitation and protection of groundwater resources.

Seasonal hydrological variation impacts nitrogen speciation and enhances bioavailability in plateau lake sediments

Global warming has intensified the distinction between dry and wet seasons in monsoonal climates. The synergistic effect of high temperatures and rainfall during the wet season promotes the release of endogenous nitrogen (N) and eutrophication within lake ecosystems. However, the seasonal variations in sediments N speciation and bioavailability, and their intrinsic connection to release potential, remain unclear. This study employed sequential extraction method and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) to characterize extractable N (Ex-N) in Erhai Lake sediments during dry and wet seasons. The results indicated that ion-exchangeable organic form N (IEF-ON) serves as a substrate for microbial mineralization, with the highest proportion of protein-like substances (26.2%). The influx of N-containing polysaccharides and poly-N glycoproteins during the wet season further increased its bioavailability. Algal-derived N exists as the weak acid-extractable organic form N (WAEF-ON), which has the highest lipid proportion (11.7%) and the lowest double bond equivalent (DBE) values. Overall, elevated water temperatures and N input during the wet season accelerate both the mineralization rate of organic N (ON) and the content of labile N components. This potentially triggers a "priming effect" that could further activate the refractory N components in the sediments. Additionally, the wet season reduces sediment pH and redox potential, making WAEF-ON and strong alkali-extractable form N (SAEF-N) more labile and susceptible to release. This study reveals the adverse effects of seasonal variations on N sequestration in lake sediments, complicating the control of endogenous pollution release under the backdrop of climate change.

Marine Recalcitrant Dissolved Organic Matter Gained by Processing at Sandy Subterranean Estuaries

The sandy subterranean estuary (STE) connecting fresh groundwater to saline sea water is characterized by strong geochemical (salinity, redox, and pH) gradients, with evidence emerging for its role as a hot spot for consumption of labile substrates. This inspired us to conduct a study to evaluate whether this holds true for dissolved organic matter (DOM), especially given the still mysterious origin of marine recalcitrant DOM. Here, characterization of DOM of 21 groundwater samples (depth 1-13 m, salinity 3.9‰ to 32.4‰) across a 65 m transect of an STE located in coastal Guangdong, China, has found systematic biotransformation toward "recalcitrant" carboxyl-rich alicyclic molecules (CRAM). The fraction of CRAM (%CRAM) increases from 33.1% to 76.7% with an increasing degree of DOM degradation and increasing salinity. Further, processing of DOM, including the more "biolabile" DOM with lower %CRAM released from aquitard, is more active under oxic conditions than under reducing conditions. Given the large quantities of sea water that recirculates through the sandy STEs globally, the amount of "recalcitrant" DOM (RDOM) entering the ocean after processing is likely to be considerable. While more studies are needed, the ocean can gain "recalcitrant" CRAM-like compounds in this way.

Molecular characteristics of dissolved organic phosphorus in watershed runoff: Coupled influences of land use and precipitation

Land use and precipitation are two major factors affecting phosphorus (P) pollution of watershed runoff. However, molecular characterization of dissolved organic phosphorus (DOP) in runoff under the joint influences of land use and precipitation remains limited. This study used Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to study the molecular characteristics of DOP in a typical P-polluted watershed with spatially variable land use and precipitation. The results showed that low precipitation and intense human activity, including phosphate mining and associated industries, resulted in the accumulation of aliphatic DOP compounds in the upper reaches, characterized by low aromaticity and low biological stability. Higher precipitation and widespread agriculture in the middle and lower reaches resulted in highly unsaturated DOP compounds with high biological stability constituting a higher proportion, compared to in the upper reaches. While, under similar precipitation, more aliphatic DOP compounds characterized by lower aromaticity and higher saturation were enriched in the lower reaches due to more influence from urban runoff relative to the middle reaches. Photochemical and/or microbial processes did result in changes in the characteristics of DOP compounds during runoff processes due to the prevalence of low molecular weight and low O/C bioavailable aliphatic DOP molecules in the upper reaches, which were increasingly transformed into refractory compounds from the upper to middle reaches. The results of this study can increase the understanding of the joint impacts of land use and precipitation on DOP compounds in watershed runoff.

Hydrologic changes induced by groundwater abstraction lead to arsenic mobilization in shallow aquifers

Intensive groundwater abstraction leads to hydrologic changes of groundwater. Nevertheless, the effects of hydrologic change on groundwater arsenic (As) mobilization remain controversial. Here, we investigated fluctuations in water levels and their effects on As mobilization in the shallow aquifer of the Hetao Basin. Results showed that large groundwater level fluctuations and high horizontal hydraulic gradients occurred in irrigation seasons. In the groundwater near the wetland with higher surface water levels than groundwater levels, biological index values of dissolved organic matter (DOM) ranged from 0.54 to 0.72, and a positive correlation between δ18O values and dissolved organic carbon (DOC) was observed, indicating that groundwater DOM was mainly sourced from surface water. The degradation of allochthone labile DOM drove the reductive dissolution of As-bearing Fe(III) oxides to Fe(II). Both DOC and humification indices of DOM exhibited positive correlations with horizontal hydraulic gradients downstream of the study area, implying that the humified organic matter flushed from aquifer sediments contributed to groundwater DOM. The humified DOM controlled by hydraulic conditions participated in the redox reactions mainly by shuttling electrons to As-bearing Fe(III) oxides. These findings highlight distinct roles of hydrologic changes induced by groundwater abstraction in As mobilization.

Exploration of nitrogen sources and transformation processes in eutrophic estuarine zones based on DOM and stable isotope compositions

Our study examines nitrogen sources and transformations in Xiamen Bay, where eutrophication has increased due to higher nitrogen levels. By analyzing dissolved organic matter (DOM) and nitrate stable isotopes (δ15N-NO3-and δ18O-NO3-), the study finds that nitrate in low salinity areas is influenced by freshwater-seawater mixing and biogeochemical processes, while in high salinity areas, it is mainly affected by physical mixing. Bayesian mixing model (MixSIAR) results show that the primary nitrate sources are fecal matter and sewage, followed by atmospheric deposition. During the high flow period, DOM may facilitate nitrogen transformation and release through processes such as degradation or mineralization. In contrast, during the low flow period, the system is mainly influenced by the physical mixing of saline and freshwater. Studies have shown that DOM can indicate the biogeochemical intensity in water bodies, further identifying the main factors influencing the distribution and transformation processes of nitrate content, providing a basis for mitigating eutrophication in estuarine areas.

Hydrogeochemical dynamics under saltwater-freshwater mixing in a mangrove wetland over tidal cycles

Seawater and groundwater interactions shape the hydrogeochemical profile of mangrove aquifers, revealing how biogeochemical processes adapt to saline-freshwater mixing via the fluctuating patterns of key hydrochemical indicators and primary biogenic elements. This study, utilizing a multi-level monitoring profile spanning the entire submerged aquifer within a mangrove wetland, analyzed the spatiotemporal dynamics of DO, ORP, pH, alkalinity and biogenic elements (C, N, S). The results revealed that among the basic hydrochemical parameters, total alkalinity showed the most stable spatiotemporal distribution and was positively correlated with salinity. pH demonstrated a significant negative correlation with salinity, whereas the correlations of ORP and DO with salinity were not substantial. The discharge of terrestrial freshwater into the mangrove wetland is marked by hydrogeochemical reactions favoring the input of Mg2+ and DIC, with potential iron mineral precipitation within the aquifer. Spatial distribution of biogenic elements in the groundwater showed no apparent pattern across sampling periods. DOC concentrations ranged from 0.3 to 1.3 mmol/L. Three components of dissolved organic matter were identified using three-dimensional fluorescence spectroscopy, with high molecular weight components (C1 + C2) accounting for an average of 47 to 73 %. Both elevated DOC concentrations and high molecular weight component ratios were primarily found in shallow layers of dense mangrove areas, decreasing with depth. Concentrations of ammonia, nitrite, and nitrate varied dynamically, reflecting active biochemical processes in the shallow to mid-layers of the aquifer. Furthermore, sulfate and sulfide concentrations, ranging from 0 to 26 mmol/L and 0.4 to 576.8 μmol/L, respectively, underscore the interplay of biogeochemical reactions, especially sulfate reduction. These findings highlight valuable insights into the complex biogeochemical processes within mangrove aquifers and provide theoretical guidance for protecting the ecological health of mangrove wetlands.

Constraints on vertical variability of geogenic ammonium in multi-layered aquifer systems

The elevated levels of geogenic (natural) ammonium in groundwater have been frequently documented in recent years. Although improving insights have been achieved in understanding the genesis of ammonium in the subsurface environment, the vertical variability of the geogenic ammonium in groundwater remains poorly understood. Here, we selected typical multi-layered aquifer systems in the central Yangtze River plain and characterized the vertical heterogeneity of geogenic ammonium through the hydrogeochemical analysis. Subsequently, the controlling factors were identified by examining the molecular composition of dissolved organic matter (DOM) and aquifer sediment features. The results indicated that the ammonium concentration in groundwater increased from the deep to shallow aquifers (2.13 to 9.88 mg/L as N), accompanied by a transition in organic matter (OM) degradation towards the methanogenic stage (δ13C-DIC: -23.07 to -0.34‰). Compared to the deeper aquifers, the DOM in the shallow aquifer was characterized by a higher abundance of the N-containing OM (15.1% > 13.13% > 12.76%) with a lower molecular lability index, corresponding to more thorough degradation extent. The characteristics of the soluble OM in depth-matched sediments were similar to those of the DOM in groundwater, suggesting the persistent water-rock interactions. Besides, the pumping tests revealed that the hydraulic conductivity decreased from deep to shallow aquifers (2.28 to 0.62 m/d), which further facilitated the more retention of geogenic ammonium in the shallow aquifer. That is, the combined effects of the abundant N-containing OM in sediments, strong degradation of the bioactive DOM, and long retention governed by hydrodynamics contributed to the increased ammonium enrichment in the shallow aquifer, thereby generating the vertical variability. The findings underscore the significance of the complex coupled factors in controlling the vertical distribution of geogenic ammonium in multi-layered aquifer systems, which was crucial for understanding the spatial heterogeneity of geogenic contaminated groundwater.

Revealing degradation pathways of soluble and dissolved organic matter in alluvial-lacustrine aquifer systems impacted by high levels of geogenic ammonium

The excessive presence of geogenic ammonium (NH4+) in groundwater poses a global environmental concern, commonly linked to the degradation of nitrogen-containing dissolved organic matter (DOM). However, there is a gap in systematic studies on the combination of soluble organic matter (SOM) in sediments and DOM in groundwater, with few indoor incubation experiments to validate their degradation pathways. This study utilized ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry to analyze the molecular characteristics of DOM and SOM in aquifer systems affected by geogenic NH4+. Subsequently, indoor incubation experiments spanning up to 140 d were conducted to verify the degradation pathways. The experimental results revealed a two-phase degradation process for both the DOM and SOM. The initial stage was characterized by the degradation of aliphatic compounds (ALC) with the production of polyphenols (PPE) and highly unsaturated compounds (HUC). The second stage was dominated by the degradation of PPE and HUC, accompanied by the re-consumption of some ALC, while more recalcitrant HUC persisted. Notably, the first stage of SOM degradation exceeded that of DOM degradation, indicating that SOM exhibited greater resistance to aging. This phenomenon may be attributed to a wider range of active enzymes in sediments, the rapid replenishment of SOM by organic matter in sediments, or the accelerated degradation of DOM. The experimental results aligned with the molecular characterization of DOM and SOM in actual aquifer systems. It is hypothesized that NH4+ produced through the direct mineralization of SOM may contribute more to the enrichment of NH4+ in groundwater than that produced through the mineralization of DOM. This study is the first to analyze DOM and SOM together in aquifer systems and validate their degradation pathways through incubation experiments, thereby providing novel insights into the enrichment of geogenic NH4+ in groundwater.

Photoinduced Evolutions of Permafrost-Derived Carbon in Subarctic Thermokarst Pond Surface Waters

In subarctic regions, rising temperature and permafrost thaw lead to the formation of thermokarst ponds, where organics from eroding permafrost accumulate. Despite its environmental significance, limited knowledge exists regarding the photosensitivity of permafrost-derived carbon in these ponds. In this study, laboratory experiments were conducted to explore the photochemical transformations of organic matter in surface water samples from thermokarst ponds from different environments in northern Quebec, Canada. One pond near Kuujjuarapik is characterized by the presence of a collapsing palsa and is therefore organically rich, while the other pond near Umiujaq is adjacent to a collapsing lithalsa and thus contains fewer organic matters. Photobleaching occurred in the Umiujaq sample upon irradiation, whereas the Kuujjuarapik sample exhibited an increase in light absorbance at wavelength related to aromatic functionalities, indicating different photochemical aging processes. Ultrahigh-resolution mass spectrometry analysis reveals that the Kuujjuarapik sample preferentially photoproduced highly unsaturated CHO compounds with great aromaticity, while the irradiated Umiujaq sample produced a higher proportion of CHON aromatics with reduced nitrogen functionalities. Overall, this study illustrates that the photochemical reactivity of thermokarst pond water varies with the source of organic matter. The observed differences in reactivity contribute to an improved understanding of the photochemical emission of volatile organic compounds discovered earlier. Further insights into the photoinduced evolutions in thermokarst ponds may require the classification of permafrost-derived carbon therein.

Metagenomic and FT-ICR MS insights into the mechanism for the arsenic biogeochemical cycling in groundwater

Arsenic (As) is a groundwater contaminant of global concern. The degradation of dissolved organic matter (DOM) can provide a reducing environment for As release. However, the interaction of DOM with local microbial communities and how different sources and types of DOM influence the biotransformation of As in aquifers is uncertain. This study used optical spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), metagenomics, and structural equation modeling (SEM) to demonstrate the how the biotransformation of As in aquifers is promoted. The results indicated that the DOM in high-As groundwater is dominated by highly unsaturated low-oxygen(O) compounds that are quite humic and stable. Metagenomics analysis indicated Acinetobacter, Pseudoxanthomonas, and Pseudomonas predominate in high-As environments; these genera all contain As detoxification genes and are members of the same phylum (Proteobacteria). SEM analyses indicated the presence of Proteobacteria is positively related to highly unsaturated low-O compounds in the groundwater and conditions that promote arsenite release. The results illustrate how the biogeochemical transformation of As in groundwater systems is affected by DOM from different sources and with different characteristics.

Spectral and molecular insights into the characteristics of dissolved organic matter in nitrate-contaminated groundwater

The characteristics of dissolved organic matter (DOM) serve as indicators of nitrate pollution in groundwater. However, the specific DOM components associated with nitrate in groundwater systems remain unclear. In this study, dual isotopes of nitrate, three-dimensional Excitation emission matrices (EEMs) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were utilized to uncover the sources of nitrate and their associations with DOM characteristics. The predominant nitrate in the targeted aquifer was derived from soil organic nitrogen (mean 46.0%) and manure &sewage (mean 34.3%). The DOM in nitrate-contaminated groundwater (nitrate-nitrogen >20 mg/L) exhibited evident exogenous characteristics, with a bioavailable content 2.58 times greater than that of uncontaminated groundwater. Regarding the molecular characteristics, DOM molecules characterized by CHO + 3N, featuring lower molecular weights and H/C ratios, indicated potential for mineralization, while CHONS formulas indicated the exogenous features, providing the potential for accurate traceability. These findings provided insights at the molecular level into the characterization of DOM in nitrate-contaminated groundwater and offer scientific guidance for decision-making regarding the remediation of groundwater nitrate pollution.

Natural Attenuation of Groundwater Uranium in Post-Neutral-Mining Sites Evidenced from Multiple Isotopes and Dissolved Organic Matter

Although natural attenuation is an economic remediation strategy for uranium (U) contamination, the role of organic molecules in driving U natural attenuation in postmining aquifers is not well-understood. Groundwaters were sampled to investigate the chemical, isotopic, and dissolved organic matter (DOM) compositions and their relationships to U natural attenuation from production wells and postmining wells in a typical U deposit (the Qianjiadian U deposit) mined by neutral in situ leaching. Results showed that Fe(II) concentrations and δ34SSO4 and δ18OSO4 values increased, but U concentrations decreased significantly from production wells to postmining wells, indicating that Fe(III) reduction and sulfate reduction were the predominant processes contributing to U natural attenuation. Microbial humic-like and protein-like components mediated the reduction of Fe(III) and sulfate, respectively. Organic molecules with H/C > 1.5 were conducive to microbe-mediated reduction of Fe(III) and sulfate and facilitated the natural attenuation of dissolved U. The average U attenuation rate was -1.07 mg/L/yr, with which the U-contaminated groundwater would be naturally attenuated in approximately 11.2 years. The study highlights the specific organic molecules regulating the natural attenuation of groundwater U via the reduction of Fe(III) and sulfate.

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.

Insights into biogeochemistry and hot spots distribution characteristics of redox-sensitive elements in the hyporheic zone: Transformation mechanisms and contributing factors

Biogeochemical hot spots play a crucial role in the cycling and transport of redox-sensitive elements (RSEs) in the hyporheic zone (HZ). However, the transformation mechanisms of RSEs and patterns of RSEs hot spots in the HZ remain poorly understood. In this study, hydrochemistry and multi-isotope (N/C/S/O) datasets were collected to investigate the transformation mechanisms of RSEs, and explore the distribution characteristics of RSEs transformation hot spots. The results showed that spatial variability in key drivers was evident, while temporal change in RSEs concentration was not significant, except for dissolved organic carbon. Bacterial sulfate reduction (BSR) was the primary biogeochemical process for sulfate and occurred throughout the area. Ammonium enrichment was mainly caused by the mineralization of nitrogenous organic matter and anthropogenic inputs, with adsorption serving as the primary attenuation mechanism. Carbon dynamics were influenced by various biogeochemical processes, with dissolved organic carbon mainly derived from C3 plants and dissolved inorganic carbon from weathering of carbonate rocks and decomposition of organic matter. The peak contribution of dissolved organic carbon decomposition to the DIC pool was 46.44 %. The concentration thresholds for the ammonium enrichment and BSR hot spots were identified as 1.5 mg/L and 8.84 mg/L, respectively. The distribution pattern of RSEs hot spots was closely related to the hydrogeological conditions. Our findings reveal the complex evolution mechanisms and hot spots distribution characteristics of RSEs in the HZ, providing a basis for the safe utilization and protection of groundwater resources.

Arsenic biogeochemical cycling association with basin-scale dynamics of microbial functionality and organic matter molecular composition

Geogenic arsenic (As)-contaminated groundwater is a sustaining global health concern that is tightly constrained by multiple interrelated biogeochemical processes. However, a complete spectrum of the biogeochemical network of high-As groundwater remains to be established, concurrently neglecting systematic zonation of groundwater biogeochemistry on the regional scale. We uncovered the geomicrobial interaction network governing As biogeochemical pathways by merging in-field hydrogeochemical monitoring, metagenomic analyses, and ultrahigh resolution mass spectrometry (FT-ICR MS) characterization of dissolved organic matter. In oxidizing to weakly reducing environments, the nitrate-reduction and sulfate-reduction encoding genes (narGHI, sat) inhibited the dissolution of As-bearing iron minerals, leading to lower As levels in groundwater. In settings from weakly to moderately reducing, high abundances of sulfate-reduction and iron-transport encoding genes boosted iron mineral dissolution and consequent As release. As it evolved to strongly reducing stage, elevated abundance of methane cycle-related genes (fae, fwd, fmd) further enhanced As mobilization in part by triggering the formation of gaseous methylarsenic. During redox cycling of N, S, Fe, C and As in groundwater, As migration to groundwater and immobilization in mineral particles are geochemically constrained by basin-scale dynamics of microbial functionality and DOM molecular composition. The study constructs a theoretical model to summarize new perspectives on the biogeochemical network of As cycling.

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.

Characteristics of dissolved organic matter contribute to Geogenic ammonium enrichment in coastal versus alluvial-lacustrine aquifers

Elevated concentration levels of geogenic ammonium in groundwater arise from the mineralization of nitrogen-containing natural organic matter in various geological settings worldwide, especially in alluvial-lacustrine and coastal environments. However, the difference in enrichment mechanisms of geogenic ammonium between these two types of aquifers remains poorly understood. To address this knowledge gap, we investigated two representative aquifer systems in central Yangtze (Dongting Lake Plain, DTP) and southern China (Pearl River Delta, PRD) with contrasting geogenic ammonium contents. The use of optical and molecular characterization of DOM combined with hydrochemistry and stable carbon isotopes has revealed differences in DOM between the two types of aquifer systems and revealed contrasting controls of DOM on ammonium enrichment. The results indicated higher humification and degradation of DOM in DTP groundwater, characterized by abundant highly unsaturated compounds. The degradation of DOM and nitrogen-containing DOM was dominated by highly unsaturated compounds and CHO+N molecular formulas in highly unsaturated compounds, respectively. In contrast, the DOM in PRD groundwater was more biogenic, less degraded, and contained more aliphatic compounds in addition to highly unsaturated compounds. The degradation of DOM and nitrogen-containing DOM was dominated by aliphatic compounds and polyphenols and CHO+N molecular formulas in highly unsaturated compounds and polyphenols, respectively. As DOM degraded, the ammonium production efficiency of DOM decreased, contributing to lower ammonium concentrations in DTP groundwater. In addition, the CHO+N(SP) molecular formulas were mainly of microbial-derived and gradually accumulated with DOM degradation. In this study, we conducted the first comprehensive investigation into the patterns of groundwater ammonium enrichment based on DOM differences in various geological settings.

Evolution of dissolved organic nitrogen chemistry during transportation to the marginal sea: Insights from nitrogen isotope and molecular composition analyses

Estuaries are hotspots where terrestrially originated dissolved organic matter (DOM) is modified in molecular composition before entering marine environments. However, very few research has considered nitrogen (N) modifications of DOM molecules in estuaries, limiting our understanding of dissolved organic nitrogen (DON) cycling and the associated carbon cycling in estuaries. This study integrated optical, stable isotopes (δ15N and δ13C) and molecular composition (FT-ICR MS) to characterize the transformation of DOM in the Yangtze River Estuary. Both concentration of dissolved organic carbon (DOC) and DON decreased with increasing salinity, while their δ13C and δ15N increased with the increasing salinity. A significant positive correlation was found between δ15N and δ13C during the transportation of DOM to marginal seas, indicating that the behavior of both DOC and DON are primarily controlled by the mixing of freshwater and the seawater in the YRE. During the mixing process, the DON addition was observed using the conservative mixing curves. In the view of molecular composition, DOM molecules became more aromatic as the number of N atoms increased. Spearman correlations reveal that DOM molecules with fewer N atoms exhibited a higher enrichment in protein-like components, while those with more N atoms were more enriched in humic-like components. In addition, the δ15N and δ13C tended to increase as the N content of DOM decreased. Therefore, DON molecules with fewer N atoms were likely to be transformed into those with more N atoms based on the isotopic fractionation theory. This study establishes a linkage between the molecular composition and the δ15N of DOM, and discovers the N transformation pattern within DOM molecules during the transportation to marginal seas.

Coupled Processes Involving Organic Matter and Fe Oxyhydroxides Control Geogenic Phosphorus Enrichment in Groundwater Systems: New Evidence from FT-ICR-MS and XANES

The enrichment of geogenic phosphorus (P) in groundwater systems threatens environmental and public health worldwide. Two significant factors affecting geogenic P enrichment include organic matter (OM) and Fe (oxyhydr)oxide (FeOOH). However, due to variable reactivities of OM and FeOOH, variable strategies of their coupled influence controlling P enrichment in groundwater systems remain elusive. This research reveals that when the depositional environment is enriched in more labile aliphatic OM, its fermentation is coupled with the reductive dissolution of both amorphous and crystalline FeOOHs. When the depositional environment is enriched in more recalcitrant aromatic OM, it largely relies on crystalline FeOOH acting concurrently as electron acceptors while serving as "conduits" to help itself stimulate degradation and methanogenesis. The main source of geogenic P enriched by these two different coupled processes is different: the former is P-containing OM, which mainly contained unsaturated aliphatic compounds and highly unsaturated-low O compounds, and the latter is P associated with crystalline FeOOH. In addition, geological setting affects the deposition rate of sediments, which can alter OM degradation/preservation, and subsequently affects geochemical conditions of geogenic P occurrence. These findings provide new evidence and perspectives for understanding the hydro(bio)geochemical processes controlling geogenic P enrichment in alluvial-lacustrine aquifer systems.

Effects of reservoir construction on optical and molecular characteristics of dissolved organic matter in a typical P-contaminated river

The source and composition characteristics of dissolved organic matter (DOM) are crucial to identify and evaluate the sources of pollution in the watershed. The construction of reservoirs changes the hydrological condition and pollutant fate of the river. However, the effects of reservoirs' construction on DOM in the watershed and the underlying mechanisms are still unclear. This study aims to examine and compare the characteristics of DOM in reservoirs and streams in the Huangbai River, a typical reservoir-affected and P-contaminated river within the Yangtze River catchment. The results showed that DOM in reservoirs was characterized by more contribution from autochthonous source, under the influence of reservoirs' construction; while, DOM in rivers was mainly originated from terrestrial input. Reservoirs had more lipid-like and protein-like compounds, while rivers contained more oxy-aromatic-like compounds. The percentage of CHOP molecules in reservoirs was significantly higher than that in rivers. The underlying mechanism is that more suitable conditions were created for plankton to grow after constructing reservoirs, which converted inorganic orthophosphate into organic phosphorus, and over time, organic phosphorus was gradually enriched in reservoirs, which exacerbated the risk of eutrophication in the reservoir water body. This study can provide theoretical support for monitoring and evaluation of water quality in reservoir-affected rivers.

Dissolved organic nitrogen cycling revealed at the molecular level in the Bohai and Yellow Sea

Marginal seas play a crucial role in the cycling of dissolved organic nitrogen (DON) between the terrestrial and marine environments. However, very few studies have considered the molecular transformation of DON in marginal seas, leaving the DON molecular modifications in its cycling largely unknown. Therefore, this study examined DON cycling in the Bohai Sea and Yellow Sea, two semi-closed marginal seas in northern China, using stable isotopes (δ15N and δ13C), optical characteristics, and molecular compositions. Compared to the Yellow Sea, the Bohai Sea had a weaker exchange with the open ocean, resulting in higher concentrations, lower δ15N, and more recalcitrant properties in DON. The DON cycling showed significant differences inside and outside the Yellow Sea Cold Water (YSCW). Degradation was the major sink of DON in the YSCW, during which more highly unsaturated compounds and carboxyl-rich alicyclic molecules were produced. Nitrogen atoms were found to be removed from the molecules with more N atoms to those with fewer ones during the DON degradation. This study discovered the molecular modifications in DON cycling and highlighted the intrinsic mechanisms in the cycling of DON in marginal seas.

Contrastive mechanisms of groundwater ammonium enrichment in different hydrogeologic settings

Although high levels of geogenic ammonium in groundwater have been widely reported, the mechanisms controlling its heterogeneous distribution are not yet well understood. In this study, a comprehensive investigation of hydrogeology, sediments, and groundwater chemistry was coupled with a set of incubation experiments to reveal the contrasting mechanisms of groundwater ammonium enrichment at two adjacent monitoring sites with different hydrogeologic settings in the central Yangtze River basin. Significant differences were found in the ammonium concentrations of groundwater at two monitoring sites, with the ammonium concentrations in the Maozui (MZ) section (0.30-5.88 mg/L; average of 2.93 mg/L) being much higher than those in the Shenjiang (SJ) section (0.12-2.43 mg/L; average of 0.90 mg/L). For the SJ section, the aquifer medium had a low organic matter (OM) content and a weak mineralization capability, leading to a limited potential for geogenic ammonium release. Moreover, due to the presence of alternating silt and continuous fine sand layers (with coarse grains) above the underlying confined aquifer, the groundwater was in a relatively open environment with oxidizing conditions, which may have promoted the removal of ammonium. For the MZ section, the aquifer medium had a high OM content and a strong mineralization capability, leading to a much higher potential for geogenic ammonium release. Furthermore, due to the presence of a thick and continuous muddy clay layer (aquitard) above the underlying confined aquifer, the groundwater was in a closed environment with strong reducing conditions, which was conductive to the storage of ammonium. Larger sources of ammonium in the MZ section and greater consumption of ammonium in the SJ section contributed collectively to the significant differences in groundwater ammonium concentrations. This study identified contrasting mechanism of groundwater ammonium enrichment in different hydrogeologic settings, which can help explain the heterogeneous distribution of ammonium levels in groundwater.

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.

Molecular characteristics of natural organic matter in the groundwater system with geogenic iodine contamination in the Datong Basin, Northern China

Natural organic matter (NOM) plays an important role in the iodine mobilization in the groundwater system. In this study, the groundwater and sediments from iodine affected aquifers in the Datong Basin were collected to perform chemistry analysis and molecular characteristics of NOM by Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Total iodine concentrations in groundwater and sediments ranged from 1.97 to 926.1 μg/L and 0.001-2.86 μg/g, respectively. A positive correlation was observed between DOC/NOM and groundwater/sediment iodine. FT-ICR-MS results showed that the DOM in the high-iodine groundwater system is characterized by less aliphatic and more aromatic compounds with higher NOSC, indicating the features of more unsaturated larger molecule structures and more bioavailability. Aromatic compounds could be the main carriers of sediment iodine and were easily absorbed on amorphous iron oxides to form the NOM-Fe-I complex. More aliphatic compounds, especially those containing N/S, experienced a higher degree of biodegradation, which further mediated the reductive dissolution of amorphous iron oxides and the transformation of iodine species, thereby causing the release of iodine into groundwater. The findings of this study provide some new insights into the mechanisms of high-iodine groundwater.

Enrichment of geogenic phosphorus in a coastal groundwater system: New insights from dissolved organic matter characterization

High concentrations of geogenic phosphorus (P) in coastal aquifer systems pose a serious and continuous threat to the health of marine ecosystems. A major source for geogenic P enrichment in aquifer systems is the mineralization of P-containing organic matter. However, the mechanisms that drive the enrichment remain unclear. Therefore, our study sought to characterize the occurrence, sources, and enrichment mechanisms of geogenic P in a coastal confined aquifer system of the Pearl River Delta, southern China. To achieve this, we conducted Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and fluorescence excitation-emission-matrix spectra (EEMs) as well as hydrochemistry and stable carbon isotope analyses. Our findings indicated that intense degradation of P-containing organic matter produced up to 8.07 mg/L of geogenic P in a reducing environment with abundant organic matter. The dissolved organic matter (DOM) of high-P groundwater (P > 1 mg/L) contained more humic-like fluorophores and exhibited higher humification. Groundwater with high P concentrations contained more aliphatic compounds and highly unsaturated-low O compounds, and the enrichment of P was mostly associated with CHOP compounds in the region of aliphatic compounds and CHON2P compounds in the region of highly unsaturated-low O compounds. Different types of dissolved organic phosphorus (DOP) can be mineralized into P, and even the mineralization of phosphonates takes precedence over the more unstable phosphate esters. P produced by the metabolism of different types of DOP was assimilated by marine microorganisms (e.g., heterotrophic bacteria and archaea), and the newly synthesized organic P compounds by chemosynthesis were subsequently released into the groundwater. Over time, P continues to be enriched in the aquifer system. This study provides new insights into subsurface P cycling in coastal aquatic systems.

Identification of ammonium source for groundwater in the piedmont zone with strong runoff of the Hohhot Basin based on nitrogen isotope

Groundwater with high ammonium concentration (HANC groundwater), mostly caused by anthropogenic pollution, is widely distributed in China, which could also result from natural geological genesis. Groundwater in the piedmont zone with strong runoff in the central Hohhot Basin has featured its excessive ammonium concentration since the 1970s. Currently, chemical factories also serve as potential pollution sources. In this study, based on the nitrogen isotopic technique and combined with hydrochemical methods, the sources of high concentration ammonium in the groundwater was identified. The HANC groundwater is mainly distributed in the alluvial-proluvial fan and the interfan depression in the western and central parts of the study area, and a maximum ammonium concentration of 529.32 mg/L was observed in the groundwater in the mid-fan of the Baishitou Gully (BSTG) alluvial-proluvial fan. Although the BSTG mid-fan is part of the piedmont zone with strong runoff, some of the HANC groundwater in this area still presents the typical hydrochemical characteristics in the discharge area. Moreover, an extremely high concentration of volatile organic compounds was observed in groundwater in the BSTG alluvial-proluvial fan, which indicated significant anthropogenic pollution. Besides, ¹⁵N-NH₄⁺ is enriched in groundwater in the BSTG root-fan and the interfan depression, which is consistent with the situation of organic nitrogen and exchangeable ammonium in natural sediments, as well as the natural HANC groundwater in other regions of China. These δ¹⁵N-NH₄⁺ values indicate that the ammonium of the groundwater in the BSTG root-fan and the interfan depression is derived from natural sediments. The ¹⁵N-NH₄⁺ in groundwater is depleted in the BSTG mid-fan, and the δ¹⁵N-NH₄⁺ values are similar with those of the pollution sources from the chemical factories in the mid-fan. Both hydrochemical and nitrogen isotopic characteristics indicate significant pollution in the mid-fan, but the ammonium pollution is limited to the area near the chemical factories.

Paleo-Geomorphology Determines Spatial Variability of Geogenic Ammonium Concentration in Quaternary Aquifers

Naturally occurring (i.e., geogenic) ammonium in groundwater has been widely detected globally, but the major controls on its regional distribution have been poorly characterized. Here, we identified the dominant role of paleo-geomorphology driven by paleo-climate in controlling the spatial variability of geogenic ammonium in groundwater using random forest algorithm and revealed the underlying mechanisms based on borehole sediment analysis of data obtained from the Dongting Lake Plain of the central Yangtze River basins in China. In the paleo-channel (PC) area, the aquifer depth-matched sediments were deposited during the last deglaciation when warm climate resulted in rapid filling into incised valleys, and terrestrial organic matter (OM) mainly as lignin experienced less degradation prior to sedimentation and had lower humification, higher N abundance, and nominal oxidation state of carbon (NOSC). In the paleo-interfluve (PI) area, the depth-matched sediments were deposited during the last glaciation, followed by intensive erosion in the surface during the last glacial maximum, and terrestrial OM mainly as lignin had been partly degraded into aliphatics prior to sedimentation and had higher humification, lower N abundance, and NOSC. As a result, under the present anaerobic conditions, less-humic and N-rich OM with more oxidized C tends to be more intensively mineralized into ammonium in the PC area than those in the PI area. These findings highlight the importance of paleo-geomorphology with paleo-climate in controlling the enrichment of geogenic ammonium in groundwater, which has a universal significance for understanding the genesis and distribution of high N loads in the aquatic environment worldwide.

Groundwater Quality and Health: Making the Invisible Visible

Linking groundwater quality to health will make the invisible groundwater visible, but there are knowledge gaps to understand the linkage which requires cross-disciplinary convergent research. The substances in groundwater that are critical to health can be classified into five types according to the sources and characteristics: geogenic substances, biogenic elements, anthropogenic contaminants, emerging contaminants, and pathogens. The most intriguing questions are related to quantitative assessment of human health and ecological risks of exposure to the critical substances via natural or induced artificial groundwater discharge: What is the list of critical substances released from discharging groundwater, and what are the pathways of the receptors' exposure to the critical substances? How to quantify the flux of critical substances during groundwater discharge? What procedures can we follow to assess human health and ecological risks of groundwater discharge? Answering these questions is fundamental for humans to deal with the challenges of water security and health risks related to groundwater quality. This perspective provides recent progresses, knowledge gaps, and future trends in understanding the linkage between groundwater quality and health.

Transition of source/sink processes and fate of ammonium in groundwater along with redox gradients

Ammonium (NH₄⁺) retention/removal processes in groundwater are of great interest because of the continuous increase in nitrogenous compound loading due to anthropogenic activities. However, the transition of multiple co-occurring transformation processes that determine the fate of NH₄⁺ in groundwater along a redox gradient remains underexplored. We selected a high nitrogen (N) groundwater system in the western Hetao Basin, China, to identify and quantify NH₄⁺ source and sink processes, including mineralization, dissimilatory nitrate reduction to ammonium (DNRA), nitrification, and anammox, to better understand the dynamics of NH₄⁺. Based on redox-sensitive parameters, that is, the oxidation-reduction potential (ORP) and NH₄⁺ and nitrate (NO₃^-) contents, etc., the groundwater system was classified into three zones from upstream to downstream: zone I (oxidizing), zone II (moderately reducing), and zone III (strongly reducing). Using the ¹⁵N tracing technique, we found that NH₄⁺ was mainly produced by mineralization while < 2% was produced by DNRA throughout the study area. Mineralization increased downstream because the supply of biodegradable N-containing compounds was augmented, which created a strong redox gradient to host a serial reaction chain. In zone I, NH₄⁺ was mainly transferred to NO₃^- via nitrification, whereas in zones II and III, NH₄⁺ was mainly transferred to N₂ via anammox. The average NH₄⁺ production/consumption ratios (P/C) in zones I, II, and III were 0.7, 6.9, and 51.1, respectively. Obviously, the NH₄⁺ purification ability can only exceed the supply under aerobic conditions, thus suggesting that NH₄⁺ will accumulate without limitation and be retained in strongly reducing groundwater. The situation of NH₄⁺ accumulation would deteriorate over space and time in groundwater as human activities increase without an additional artificial supply of oxidants. The results provide mechanistic insights for quantitatively comprehending the dynamics and fate of NH₄⁺ in groundwater, shedding light on groundwater NH₄⁺ mitigation techniques.

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.