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Iodate reduction by dissimilatory iodate-reducing microorganisms (DIRMs) plays a crucial role in the biogeochemical cycling of iodine on Earth. However, the occurrence and distribution of DIRMs in iodine-rich groundwater remain unclear. In this study, we isolated the dissimilatory iodate-reducing bacteriumAzonexus hydrophilusstrain NCP973 from a geogenic high-iodine groundwater of China for the first time. The analysis of genome, transcriptome, and heterologous expression revealed that strain NCP973 uses the dissimilatory iodate-reducing enzyme IdrABP1P2 to reduce dissolved or in situ sediment-bound iodate to iodide. The location of IdrABP1P2 in the conjugative plasmid of strain NCP973 implies that IdrABP1P2 could be spread by horizontal gene transfer and allow the recipient microorganisms to participate in the enrichment of iodide in aquifers. Based on the global iodine-rich groundwater metagenomes and genomes, the identification of idrAshowed that phylogenetically diverse DIRMs are widely distributed not only in geogenic high-iodine groundwater of China but also in radionuclide-contaminated groundwater of USA as well as in subsurface cavern waters in Germany and Italy. Moreover, the abundance of idrAwas found to be higher in groundwater with a relatively high iodine content. Collectively, these results suggest that terrestrial iodine-affected groundwater systems are another important habitat for DIRMs in addition to marine environments, and their activity in aquifers triggers the mobilization and enrichment of iodine in groundwater worldwide.

Siderite oxidation can be achieved with dissolved oxygen (DO), which would control the oxidation and the fate of As(iii), but the interaction mechanism between siderite and As(iii) with the presence of DO needs more investigation. Herein, As(iii) adsorption and oxidation on siderite with DO were investigated systematically through batch kinetics in combination with Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy depth profiling, and quenching experiments. Results showed that As(iii) adsorption increased with increasing DO concentrations from 0 to 4.6 mg L−1, but stabilized at a DO concentration of 9.0 mg L−1. Siderite was oxidized to form Fe (hydr)oxides with DO, which promoted As(iii) adsorption. However, with excess DO, siderite was intensively covered by newly formed Fe (hydr)oxides, which limited further oxidation of siderite and As adsorption. During the reaction of structural Fe(ii) with DO, ·OH was generated by the Fenton-like reaction, which oxidized the adsorbed As(iii). As(iii) adsorption was quicker than its oxidation because of the higher As(iii) proportion on the reacted siderite. These findings provide new insights into the interaction mechanism between As(iii) and structural Fe(ii) in the presence of DO.

While microbial reduction has gained widespread recognition for efficiently remediating environments polluted by toxic metavanadate [V(V)], the pool of identified V(V)-reducing strains remains rather limited, with the vast majority belonging to bacteria and fungi. This study is among the first to confirm the V(V) reduction capability of Streptomyces microflavus, a representative member of ubiquitous actinomycetes in environment. A V(V) removal efficiency of 91.0 ± 4.35% was achieved during 12 days of operation, with a maximum specific growth rate of 0.073 d–1. V(V) was bioreduced to insoluble V(IV) precipitates. V(V) reduction took place both intracellularly and extracellularly. Electron transfer was enhanced during V(V) bioreduction with increased electron transporters. The electron-transfer pathways were revealed through transcriptomic, proteomic, and metabolomic analyses. Electrons might flow either through the respiratory chain to reduce intracellular V(V) or to cytochrome c on the outer membrane for extracellular V(V) reduction. Soluble riboflavin and quinone also possibly mediated extracellular V(V) reduction. Glutathione might deliver electrons for intracellular V(V) reduction. Bioaugmentation of the aquifer sediment with S. microflavusaccelerated V(V) reduction. The strain could successfully colonize the sediment and foster positive correlations with indigenous microorganisms. This study offers new microbial resources for V(V) bioremediation and improve the understanding of the involved molecular mechanisms.

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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 (hzsABCand 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.

Metals are increasingly important risk factors for the evolution of antibiotic resistance in environments.

Dissolved organic matter (DOM) is widespread in uranium (U) hosting aquifer, a key substance affecting CO2+ O2in situ leaching (ISL) of U by consuming injected O2. However, molecular composition and molecular transformation of DOM are poorly understood in this aquifer during the U mining. To address this issue, Fourier transform ion cyclotron resonance mass spectrometry was applied to investigate DOM molecules in groundwater samples and rock samples collected from a U mining aquifer. The high proportion of labile tyrosine‐like component and BIX value in monitoring wells (BM) indicated the biological origin of DOM. CHOS and aliphatic were the major components of water‐soluble organic matter (WSOM) in U‐bearing rocks, but CHO and highly unsaturated structures with high oxygen (HUSHO) were the main components of DOM in BM. The ISL led to the release of WSOM and the oxidation of DOM, which accounted for 24.5%–57.4% and 39.1%–65.7% of expected DOM, respectively. Potential molecular transformations (PMTs) results showed that the intragroup transformations dominated, and remarkable transformations from CHON and CHONS to CHO occurred during ISL. The positive net PMTs of condensed aromatic structure (CAS), aromatic structures (AS), and aliphatic suggested that they were transformed from HUSLO and HUSHO. A gradual decrease in (DBE−O)/Cwaand a gradual increase in NOSCwaindicated that the saturated degree and oxidation state of DOM increased during mining. This work successfully identified molecular characteristics of DOM in U hosting aquifer, and proposed a paradigm of DOM transformation pathways during ISL. Dissolved organic matter (DOM) was a key substance widely distributed in Uranium (U) hosting aquifer, however, the characteristics and its potential molecular transformation during neutral in situ leaching (ISL) remained unclear. Here, 13 groundwater samples and 5 rock samples were collected from the Qianjiadian U deposit, a sandstone‐type U deposit by neutral ISL in China. Groundwater chemistry, 3D fluorescence spectroscopy, FT‐ICR MS, and carbon isotope approaches were employed to investigate the characteristics of DOM, identify and quantify the processes affecting the composition of DOM during ISL. Spectral and molecular characteristics of DOM revealed the biological origin of DOM and the biological activity in U‐hosting aquifers. CHOS and aliphatic were the major components of water‐soluble organic matter (WSOM) in U‐bearing rocks. The release of WSOM (24.5%–57.4%) and oxidative degradation of DOM (39.1%–65.7%) together controlled the composition of DOM during ISL. The gradual decrease of (DBE−O)/Cwaand gradual increase of NOSCwaindicated that the saturated degree and oxidation state of DOM increased as mining. This work improves our understanding of molecular characteristics of DOM in U hosting aquifer and molecular transformation pathways of DOM during ISL. The release of WSOM and oxidation of DOM controlled the composition and molecular transformations of DOM.

Lake Dongting is the largest freshwater lake of China. Because of eutrophication, it has become a hot topic of water environmental protection in China. Although the input of exogenous phosphorus (P) is controlled, the release of endogenous P can maintain eutrophication for an extended time. This study explored the relationship between different environmental factors and the release of endogenous P in sediments through a large-scale field investigation. The migration of endogenous P from sediment to the overlying water layer follows a three-phase cycle process, in which iron and aluminum phosphorus (Fe/Al–P), calcium phosphorus (Ca–P), and TP transformation will occur. Fe/Al–P is the most important source of phosphorus release. The content of total P in Lake Dongting shows the following gradient East Dongting Lake > West Dongting Lake > South Dongting Lake. The contents of Fe/Al–P in East Dongting Lake were highest, which may represent a risk of P release. Therefore, canonical variate analysis showed that the effect relationship of phosphorus release followed ORP > pH > T > depth > conductivity ≈ TDS. Among these, ORP was the most important driving factor, as it affects the conversion of Fe3+–P and Fe2+–P. Finally, a model to predict P release was developed and the P release rate (Ri) was Ri = 0.472 + 0.085[T] + 0.029[TDS] + 0.058 [Cond.] – 0.207[ORP] – 0.134[DO] – 0.047[Depth]. These results have important guiding significance for the selection and implementation of P control technologies.

To unravel the crucial components of natural organic matter that respond to the process of anthropogenic groundwater recharge (AGR) from different recharge water sources, dissolved organic matter (DOM) and base-extractable particulate organic matter (POM) in groundwater and surface water were analyzed using excitation–emission matrix spectroscopy coupled with parallel factor analysis (EEM–PARAFAC). The EEM and traditional spectral indices of samples show that the fluorescent intensity, molecular weight, and humification degree of the DOM were relatively higher than those of the POM, and the groundwater in the reclaimed water recharge area (RWRA) was more contaminated than in the south-to-north water recharge area (SNWRA). PARAFAC analysis indicates that the DOM was dominated by an allochthonous humic-like substance (C1), whereas the POM was dominated by tryptophan-like substances associated with microbial activity (C2). Partitioning of PARAFAC components between DOM and POM showed that the humic-like substances (C1 and C4) were more likely to be distributed into a dissolved phase compared to the protein-like substances (C2 and C3), which suggested the potential use of C1 and C4 as a tracking indicator. In particular, the clear gradient distributions along both the hydrogeological profile and different aquifer systems in terms of the concentration and composition of C1 also discriminated between the RWRA and SNWRA with regard to the effects of various AGRs on the groundwater. The association between C1 and water-quality indicators revealed by principal component analysis further indicated that refractory humic-like substances would track the environmental impacts of intentional AGR processes.

Hexavalent chromium (Cr(VI)) is known to occur naturally in shallow oxic groundwater, typically from aquifers associated with mafic and ultramafic formations, but information on the occurrence of Cr(VI) in deep groundwater from large sedimentary basins is limited. This study shows that groundwater from the Baiyangdian Lake Basin (BYB), home to the future second capital city of China, had high Cr concentration (>10 μg/L, up to 86 μg/L) in the deep aquifer (>150 m), while shallow groundwater had lower Cr concentration (<10 μg/L). Chromium occurred predominantly as Cr(VI) (>95%). Shallow groundwater was characterized by higher Mn and Fe concentrations relative to deep groundwater, likely indicating more reducing conditions. Sequential extraction experiments from aquifer sediments suggest that Cr(VI) may derive from silicate weathering and that Mn oxides in the aquifer play a major role in the formation of Cr(VI) in groundwater. Inverse correlations between Mn and Cr(VI) suggest that reductive dissolution of Mn oxides constrains Cr(VI) mobilization in the shallow groundwater, while oxic–suboxic conditions in the deep aquifer limit Mn solubility, which enhances oxidation of Cr(III) to Cr(VI) and promotes desorption of Cr(VI) under alkaline conditions. This study demonstrates the potential geogenic occurrence of high Cr(VI) concentration in deep groundwater from a nonmafic, large sedimentary basin containing Mn oxides in the aquifer sediments.

Molecular characteristics of natural organic matter (NOM) and their potential connections to arsenic enrichment processes remain poorly understood. Here, we examine dissolved organic matter (DOM) in groundwater and water-soluble organic matter (WSOM) in aquifer sediments being depth-matched with groundwater samples from a typical arid–semiarid basin (Hetao Basin, China) hosting high arsenic groundwater. We used Fourier transform ion cyclotron resonance mass spectrometry to determine molecular characteristics of DOM and WSOM and evaluate potential roles of biodegradable compounds in microbially mediated arsenic mobility at the molecular level. High-arsenic groundwater DOM was generally enriched in recalcitrant molecules (including lignins and aromatic structures). Although potential contribution of recalcitrant compounds to arsenic enrichment cannot be ruled out, preferential degradation of the labile molecules coupled with reduction of Fe(III) (oxyhydr)oxides seemed to dominate arsenic mobilization. Both the number and the intensity of biodegradable compounds (including aliphatic/proteins and carbohydrates) were higher in WSOM than those in DOM in depth-matched high-arsenic groundwater (arsenic >0.67 μmol/L or 50 μg/L). Groundwater arsenic concentration generally increased with the increase in the number and the intensity of unique biodegradable compounds (especially N-containing compounds) in WSOM at matched depths. Anoxic incubations of sediments and deionized water show that more arsenic and Fe(II) were released from aquifer sediments with greater numbers and intensities of consumed biodegradable compounds in WSOM (especially N-containing compounds), with a higher proportion of microbially derived compounds produced. These observations indicate that the biodegradation of aliphatic/proteins and carbohydrates (especially CHON formulas) in WSOM fueling the reductive dissolution of Fe(III) (oxyhydr)oxides predominantly promotes arsenic release from aquifer solids. Our unique data present a better understanding of arsenic mobilization shaped by microbial degradation of labile organic compounds in anoxic aquifers at the molecular level.

Aquifer pumping tests represent a standard method for estimating hydraulic characteristics, with practitioners often focusing on late period drawdown data because these are less affected by within‐ and near‐borehole effects (e.g., borehole‐storage and skin effects). Alternatively, groundwater responses to natural forcing (e.g., barometric pressure and earth tides) provide a passive method for estimating aquifer parameters at a low cost. However, to the best of our knowledge, no studies have compared parameters calculated from different periods within a pumping test with those from passive methods. Herein, we compare the aquifer transmissivity estimated using both active and passive methods in two wells located in the Beetaloo Region of Northern Australia. The active method estimates aquifer transmissivity during three periods (i.e., the early, middle, and late periods) of an aquifer pumping test, while the passive method employs groundwater responses to barometric‐pressure and earth‐tide fluctuations. We find that the range of best‐fit aquifer transmissivity is 1.18 × 10−5–1.79 × 10−5m2/s and 1.73 × 10−5–2.14 × 10−5m2/s for OW1 and OW2, respectively. The transmissivity estimated from the barometric pressure response method is the largest. The aquifer transmissivity using barometric pressure responses are consistent with early‐ and middle‐period estimates. This suggests that barometric pressure responses are more sensitive to within‐ and near‐borehole effects. The scales of the tidal response method are smaller than those of the pumping test method. The accurate estimation of the hydraulic properties of aquifers is important for effective groundwater management. Both active (aquifer pumping tests) and passive (natural forcing such as barometric‐pressure and earth‐tide fluctuations) methods are used to estimate aquifer hydraulic properties. However, the aquifer parameters estimated from aquifer pumping tests are variable as borehole effects (e.g., borehole storage and skin effects) dissipate as the cone‐of‐depression expands over time. This leads to dynamic changes in parameters during the early, middle, and late periods of an active pumping test. By comparing the aquifer transmissivity from these different periods with those estimated from passive (tidal/barometric pressure) responses and combining with scale effect, we find that barometric pressure responses are consistent with early and middle periods (corresponding to within‐ and near‐borehole effects). The scales of the tidal response method are smaller than those of the pumping test method. We estimate aquifer transmissivity using groundwater responses to natural disturbances (earth tides, barometric pressure fluctuations)From early to late period pumping, transmissivity changes slightly and decreases while storage coefficient changes in magnitude and increaseThe result of atmospheric response located in early to middle‐period pumping while AQTESOLV method is within late‐period pumping We estimate aquifer transmissivity using groundwater responses to natural disturbances (earth tides, barometric pressure fluctuations) From early to late period pumping, transmissivity changes slightly and decreases while storage coefficient changes in magnitude and increase The result of atmospheric response located in early to middle‐period pumping while AQTESOLV method is within late‐period pumping

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 δ34SSO4and δ18OSO4values 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.

Ammonium-related pathways are important for groundwater arsenic (As) enrichment, especially via microbial Fe(III) reduction coupled with anaerobic ammonium oxidation; however, the key pathways (and microorganisms) underpinning ammonium-induced Fe(III) reduction and their contributions to As mobilization in groundwater are still unknown. To address this gap, aquifer sediments hosting high As groundwater from the western Hetao Basin were incubated with 15N-labeled ammonium and external organic carbon sources (including glucose, lactate, and lactate/acetate). Decreases in ammonium concentrations were positively correlated with increases in the total produced Fe(II) (Fe(II)tot) and released As. The molar ratios of Fe(II)totto oxidized ammonium ranged from 3.1 to 3.7 for all incubations, and the δ15N values of N2from the headspace increased in 15N-labeled ammonium-treated series, suggesting N2as the key end product of ammonium oxidation. The addition of ammonium increased the As release by 16.1% to 49.6%, which was more pronounced when copresented with organic electron donors. Genome-resolved metagenomic analyses (326 good-quality MAGs) suggested that ammonium-induced Fe(III) reduction in this system required syntrophic metabolic interactions between bacterial Fe(III) reduction and archaeal ammonium oxidation. The current results highlight the significance of syntrophic ammonium-stimulated Fe(III) reduction in driving As mobilization, which is underestimated in high As groundwater.

Arsenic (As) release and mobility in groundwater is coupled to the iron (Fe) cycling and the associated transformation of Fe-oxides present in sediments. Recent in situ experiments have provided observations on arsenic mobilization and co-occurring reductive mineral transformation when placing As-loaded ferrihydrite-coated sand for 80 days in wells of an As-contaminated aquifer of Northern China. However, the complex temporal change in solid-associated arsenic and the multiple geochemical processes occurring when the flowing groundwater contacts the As-loaded ferrihydrite-coated sand hamper a detailed evaluation of the experimental data set. In this study, we develop a modeling approach that allows a quantitative interpretation of arsenic release and ferrihydrite transformation observed during the in situ experiments. The model accounts for the interplay of abiotic and biotic geochemical processes (i.e., surface complexation, reductive dissolution, formation of secondary iron minerals, and arsenic sequestration into the newly formed minerals) involved in the transformation of Fe-oxides and controlling arsenic mobility. The results show the capability of the proposed approach to reproduce the temporal trends of solid arsenic and ferrihydrite concentrations, as well as the spatial variability of mineral transformation, observed in different wells using a common set of surface complexation parameters and kinetic rate constants. The simulation outcomes allowed us to disentangle the specific contribution of the different mechanisms controlling the release of arsenic. It was possible to identify an initial rapid but minor release of As (13–23% of the initial surface concentration) due to desorption from ferrihydrite, as well as the reduction of adsorbed As(V) to As(III) upon contact with the flowing anoxic groundwater. Successively, reductive dissolution of ferrihydrite caused the decrease of the amount of the Fe mineral phase and led to a major depletion of solid-associated arsenic. The produced Fe(II) catalyzed the ferrihydrite conversion into more crystalline Fe(III) oxides (i.e., lepidocrocite and goethite) through Ostwald ripening, and resulted in the formation of siderite and mackinawite upon reaction with carbonates and sulfides naturally present in the groundwater. The model results also showed that, whereas the decrease in surface sites during reductive dissolution of ferrihydrite promoted arsenic mobilization, the mineral transformation limited As release through its sequestration into the newly formed secondary mineral phases.

Although neutral in situ leaching through CO2+ O2is employed to extract uranium (U) in sandstone by in situ leaching (ISL), mechanisms of U mobilization and O2consumption remained unclear. To address this gap, 18 groundwater samples were taken from the Qianjiadian sandstone U ore field, including seven samples from production wells in mining area M1 (mining for 5 years), six samples from production wells in mining area M2 (mining for 4 years), and five samples from monitoring wells (GC), to quantify U‐mobilizing processes in the mining aquifer by employing hydrogeochemical compositions and multi‐isotopes. The introduction of O2and CO2efficiently stimulated U mobilization in the mining aquifer. The injected CO2critically promoted the dissolution of carbonate minerals, which enhanced the formation of uranyl carbonate (predominantly CaUO2(CO3)22−and Ca2UO2(CO3)3(aq)) and thus facilitated U mobility. Generally, δ34SSO4and δ18OSO4in M2 and M1 were significantly lower than those in GC (p< 0.01). A Bayesian isotope mixing model of δ34SSO4and δ18OSO4showed that the contribution of pyrite oxidation to SO42−concentration increased from 1.7% in GC to 13.6% in M2 and to 15.0% in M1. During ISL, pyrite, ammonium, and dissolved organic carbon were major compounds competing with U(IV) for introduced O2in the ore‐bearing aquifer. Most of the consumed O2was used for pyrite oxidation (56.2%) and U(IV) oxidation (39.3%), following the thermodynamic sequence of those redox reactions. The current results highlighted the significance of increasing O2utilization efficiency in improving the performance of ISL operations.

Sources of dissolved organic matter (DOM) triggering arsenic enrichment in deep confined groundwater is a highly debated issue. Along these lines, the spectroscopic and molecular characteristics of DOM were monitored in both shallow unconfined and deep confined groundwater from the Songnen Basin, China. Results demonstrated that deep groundwater with higher dissolved arsenic concentrations was characterized by more abundant labile organic compounds than shallow groundwater. The three‐end‐member mixing model estimated that shallow groundwater had more fraction of surface water, while deep groundwater was primarily recharged by lateral groundwater flow. Groundwater DOM sources were estimated from the fluorescence index (FI) values, which suggests that shallow groundwater DOM was primarily sourced from surface water with stronger terrestrial signatures. However, deep groundwater DOM was mainly sourced from in situ sedimentary organic matter release or by‐products from microbial activity with stronger microbial signatures. Higher values of degradation index (I_deg) indicate that deep groundwater DOM had undergone greater degradation extents than the shallow one. The microbial DOM degradation in anoxic environments mainly occurred via a pathway where more saturated and low‐molecular‐mass compounds were preferentially degraded to produce more mid‐molecular‐mass compounds with unsaturated, aromatic, and microbial signatures. This explained the significant positive correlations between groundwater arsenic concentrations and FI and I_deg values. A stronger DOM degradation state in deep groundwater was conducive to more arsenic enrichment than that in the shallow one. This work successfully identified distinct sources of shallow and deep groundwater DOM, and proposed a paradigm of DOM processing pathways during arsenic enrichment under anoxic environments. Sources of dissolved organic matter (DOM) in high arsenic groundwater has long been a highly debated issue, especially in a deep‐confined aquifer being capped by overlying clay layers. To distinguish the sources of DOM in shallow and deep groundwater, both the spectral and molecular characteristics of groundwater DOM were monitored. A three‐end‐member mixing model was established to estimate the fractions of surface water, polluted water, and fresh lateral recharge in groundwater samples. The acquired results clearly demonstrated that shallow groundwater DOM had a higher proportion of surface water‐derived organic matter with a stronger terrestrial signature, while deep groundwater DOM was primarily supplied by the release of in situ sedimentary organic matter and/or by‐products of microbial activity. Moreover, microbes appear to preferentially utilize low‐molecular‐mass organic compounds with relatively high H/C ratios as substrates under anoxic groundwater environments, during which mid‐molecular‐mass aromatic compounds with microbial signatures were produced. The calculated DOM degradation indices indicate that deep groundwater DOM underwent a stronger extent of degradation than a shallow one, which resulted in a stronger Fe(III) oxide reduction and higher arsenic concentrations in deep groundwater. Shallow groundwater dissolved organic matter (DOM) showed more terrigenous fraction from surface water, while that in deep groundwater was mostly in situ suppliedSaturated and low‐molecular‐mass DOM compounds were preferentially consumed, producing more unsaturated and mid‐molecular‐mass compoundsDeep groundwater DOM was more degraded than the shallow one, which may be responsible for higher arsenic levels in deep groundwater Shallow groundwater dissolved organic matter (DOM) showed more terrigenous fraction from surface water, while that in deep groundwater was mostly in situ supplied Saturated and low‐molecular‐mass DOM compounds were preferentially consumed, producing more unsaturated and mid‐molecular‐mass compounds Deep groundwater DOM was more degraded than the shallow one, which may be responsible for higher arsenic levels in deep groundwater

The groundwater quality impacts associated with anthropogenic groundwater recharge (AGR) are of great concern for water management. However, the impacts of AGR on the molecular properties of dissolved organic matter (DOM) in aquifers are poorly understood. Herein, Fourier transform ion cyclotron resonance mass spectrometry was used to unravel the molecular characteristics of DOM in groundwaters from recharge areas by reclaimed water (RWRA) and natural water from South-to-North Water Diversion Project (SNWRA). Compared with RWRA groundwater, significantly fewer nitrogenous compounds, more sulfur-containing compounds, higher concentrations of NO3–N, and lower pH were observed in SNWRA groundwater, indicating the occurrence of deamination, sulfurization, and nitrification. The occurrence of these processes was further supported by transformations of more molecules related to nitrogen and sulfur in SNWRA groundwater relative to RWRA groundwater. The intensities of most common molecules in all samples were significantly correlated with the water quality indicators (e.g., Cl–and NO3–N) and fluorescent indicators (e.g., humic-like components (C1%)), indicating that those common molecules may have the potential to track the environmental impact of AGR on groundwater, especially these specific molecules having great mobility and being significantly correlated with other inert tracers like C1% and Cl–. This study is helpful to understand the environmental risks and regional applicability of AGR.

Submarine groundwater discharge (SGD) has been widely recognized as an important source of dissolved nutrients in coastal waters and affects nutrient biogeochemistry. In contrast, little information is available on SGD impacts on coastal carbon budgets. Here, we assessed the SGD and associated carbon (dissolved inorganic carbon [DIC] and total alkalinity [TA]) fluxes in Liaodong Bay (the largest bay of the Bohai Sea, China) and discussed their border implications for coastal DIC budget and buffering capacity. Based on 223Ra and 228Ra mass balance models, the SGD flux was estimated to be (0.92–1.43) × 109m3d−1. SGD was the largest contributor of DIC, accounting for 55%–77% of the total DIC sources. The low ratio (<1) of SGD‐derived TA to DIC fluxes and negative correlation between radium isotopes and pH in seawater implied that SGD would potentially reduce seawater pH in Liaodong Bay. Combining the groundwater carbon data in Liaodong Bay with literature data, we found that the SGD‐derived DIC flux off China was 4–9 times greater than those from rivers. By analyzing the TA/DIC ratios in groundwater along the Chinese coast and related carbon fluxes, SGD was thought to partially reduce the CO2buffer capacity in receiving seawater. These results obtained at the bay scale and national scale suggest that SGD is a significant component of carbon budget and may play a critical role in modulating coastal buffering capacity and atmospheric CO2sequestration. Submarine groundwater discharge (SGD) is an important process of land‐sea interaction, but its significance on coastal carbon budgets is still poorly understood. In this study, the SGD and associated carbon flux were quantified based on high‐resolution radium and carbon data in an urbanized bay (Liaodong Bay, China). By reviewing the coastal groundwater carbon data in the existing literature, this study provided a first approximation of SGD‐derived carbon fluxes at the shelf‐scale along the Chinese coastlines. The results suggested that SGD is an important component of dissolved inorganic carbon (DIC) budgets at the bay scale and shelf‐scale. SGD with low total alkalinity/DIC ratio would potentially reduce seawater pH and CO2buffering capacity in receiving coastal waters. This study highlights the important but easily overlooked implications of SGD for coastal carbon budget and carbonate system. Submarine groundwater discharge and associated carbon fluxes were estimated in the largest bay of the Bohai Sea, ChinaSubmarine groundwater discharge was an important component of coastal carbon budgets at the bay scale and shelf‐scaleGroundwater carbon inputs to ocean would modify local seawater pH and partially influence CO2buffering capacity in receiving coastal waters Submarine groundwater discharge and associated carbon fluxes were estimated in the largest bay of the Bohai Sea, China Submarine groundwater discharge was an important component of coastal carbon budgets at the bay scale and shelf‐scale Groundwater carbon inputs to ocean would modify local seawater pH and partially influence CO2buffering capacity in receiving coastal waters

Enrichment mechanisms of high‐arsenic (>10 μg/L) groundwater in confined aquifers with sand‐gravel sediments were rarely documented. Both groundwater and sediment were sampled to investigate mechanisms of groundwater arsenic mobility in sand‐gravel confined aquifers from the Songnen Basin, China. The results show that, although high arsenic groundwater was present in both shallow sandy (<60 m) and deep sand‐gravel aquifers (60–100 m), relatively higher arsenic groundwater was hosted in the deep confined aquifers. The significantly higher proportion of biologically degradable organic carbon (BDOC) in deep confined groundwater than those in shallow unconfined ones (p= 0.03) suggests that higher arsenic concentrations were associated with higher DOM biodegradability. High‐arsenic groundwater was the result of Fe(III) oxide reduction facilitated by BDOC degradation. Arsenic‐rich pyrite framboids were observed in shallow aquifers, which would lower groundwater arsenic concentrations. Sand‐gravel sediments in deep aquifers had remarkably higher contents of extracted Fe(III) oxides and the bound arsenic, HCl‐extractable Fe(II) minerals, and adsorbed arsenic, but considerably lower contents of Mn oxides and Mn(II) minerals than the shallow sandy sediments. Siderite, which is not as efficient as pyrite in sequestering arsenic, was the dominant Fe(II) mineral in the deep aquifer. In the deep aquifer, Mn oxide reduction was almost completed and Fe(III) oxide reduction was predominated, which resulted in high concentrations of groundwater arsenic due to high arsenic/iron molar ratio in the Fe(III) oxides. This study proposed that geogenic high‐arsenic groundwater can occur in sand‐gravel aquifers being high in Fe(III) oxide‐bound arsenic but poor in Mn oxides under reducing conditions. Deep aquifers being mainly composed of sand‐gravel sediments were generally believed to host “safe” drinking water resources. Recently, high arsenic groundwater has frequently been observed in those deep aquifers isolated from shallow groundwater by clay aquitard. However, mechanisms of groundwater arsenic enrichment in those deep aquifers are poorly understood. This comprehensive study shows that arsenic was in situ mobilized from microbially driven reduction of arsenic‐bearing Fe(III) oxides in the deep aquifer with higher proportion of degradable organic carbon to DOC, higher contents of Fe(III) oxide‐bound arsenic and adsorbed arsenic, and lower contents of Mn oxides in the deep aquifer relative to those in the shallow one. The Fe(III) oxide‐bound and adsorbed arsenic were the most reactive mobile phases in deep aquifer sediments with low contents of Mn oxides under reducing environments. Our findings emphasize that the contents of Fe(III) oxide‐bound arsenic and Mn oxides in aquifer sediments were key factors controlling groundwater arsenic enrichment. Elevated groundwater arsenic (As) concentration in both shallow sandy and deep sand‐gravel aquifers were caused by Fe(III) oxide reductionPyrite precipitation lowered shallow groundwater As level, while higher As level occurred in deep aquifers with siderite precipitateHigh‐As groundwater occurred in sand‐gravel aquifers with high contents of Fe(III)‐oxide bound As and negligible Mn oxides Elevated groundwater arsenic (As) concentration in both shallow sandy and deep sand‐gravel aquifers were caused by Fe(III) oxide reduction Pyrite precipitation lowered shallow groundwater As level, while higher As level occurred in deep aquifers with siderite precipitate High‐As groundwater occurred in sand‐gravel aquifers with high contents of Fe(III)‐oxide bound As and negligible Mn oxides

Biogeochemical processes critically control the groundwater arsenic (As) enrichment; however, the key active As-mobilizing biogeochemical processes and associated microbes in high dissolved As and sulfate aquifers are poorly understood. To address this issue, the groundwater-sediment geochemistry, total and active microbial communities, and their potential functions in the groundwater-sediment microbiota from the western Hetao basin were determined using 16S rRNA gene (rDNA) and associated 16S rRNA (rRNA) sequencing. The relative abundances of either sediment or groundwater total and active microbial communities were positively correlated. Interestingly, groundwater active microbial communities were mainly associated with ammonium and sulfide, while sediment active communities were highly related to water-extractable nitrate. Both sediment-sourced and groundwater-sourced active microorganisms (rRNA/rDNA ratios > 1) noted Fe(III)-reducers (induced by ammonium oxidation) and As(V)-reducers, emphasizing the As mobilization via Fe(III) and/or As(V) reduction. Moreover, active cryptic sulfur cycling between groundwater and sediments was implicated in affecting As mobilization. Sediment-sourced active microorganisms were potentially involved in anaerobic pyrite oxidation (driven by denitrification), while groundwater-sourced organisms were associated with sulfur disproportionation and sulfate reduction. This study provides an extended whole-picture concept model of active As–N–S–Fe biogeochemical processes affecting As mobilization in high dissolved As and sulfate aquifers.

Agricultural soils are important reservoirs for antibiotic resistance genes (ARGs), which have close linkage to human health via crop production. Metal stress in environments may function as a selection pressure for antibiotic resistomes. However, there is still a lack of field studies focusing on the effect of historical mercury (Hg) contamination on antibiotic resistomes in agricultural soils. Here, we explored the ARG profile in soils with different cropping systems (paddy and upland) and linked them to legacy Hg exposure. We found that ARG profiles were significantly different between paddy and upland soils. However, both paddy and upland soils with long-term field Hg contamination harbored higher diversity and abundance of ARGs than non-polluted soils. The co-occurrence network reveals significant associations among Hg, Hg resistance genes, mobile genetic elements (MGEs), and ARGs. Together with path analysis showing legacy Hg possibly affecting soil resistomes through the shifts of soil microbiota, Hg resistance genes, and MGEs, we suggest that legacy Hg-induced potential co-selection might elevate the ARG level. Redundancy analysis further supports that legacy Hg pollution had a significant association with ARG variations in the paddy and upland soils (P< 0.01). Collectively, our results highlight the underappreciated role of legacy Hg as a potential persistent selecting agent in contributing to soil ARGs in agroecosystems.

The interference design between the mating surface of shell and retainer in new energy vehicle scroll compressor were important parts of the design of the shell assembly. The finite element analysis of static structure was used to analyze the strength of shell and the deformation value of the mating surface under the working pressure, and minimum surplus were used to analyze whether the matching surface surplus design meet the design requirements. An analytical method was provided for the design of new energy automobile scroll compressor shell by static structure analysis of shell and the interference of mating surface.