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Conventional concentration-oriented approaches for nitrate risk diagnosis only provide overall risk levels without identifying risk values of individual sources or sources accountable for potential health risks. Therefore, a hybrid model combining the end-member mixing model tool on Excel™ (EMMTE) with human health risk assessment (HHRA) was developed to assess the source-oriented health risks for groundwater nitrate, particularly in the Poyang Lake Plain (PLP) region. The results indicated that the EMMTE and the Bayesian stable isotope mixing model (MixSIAR) exhibited remarkable consistency in source apportionment of groundwater nitrate. The source contribution of groundwater nitrate in PLP was related to land use types, hydrogeological conditions, and soil properties. Notably, manure and sewage sources, contributing up to 53.4 %, represented the largest nitrate pollution sources, with a significant contribution of soil nitrogen and nitrogen fertilizers. The non-carcinogenic risk for four potential sources was below the acceptable threshold of 1. Given the factors including rainfall dilution and economic development, attention should be directed towards mitigating the health risks posed by manure and sewage. This study can verify the efficacy of EMMTE in source apportionment and offer valuable insights for decision-makers to regulate the largest sources of nitrate contamination and enhance groundwater management efficiency., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The excessive presence of geogenic ammonium (NH 4 ) 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 NH + ) 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 NH 4 + 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 NH 4 in groundwater.+ produced through the direct mineralization of SOM may contribute more to the enrichment of NH 4 + 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 NH 4 + in groundwater., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Significant spatial variability of groundwater arsenic (As) concentrations in South/Southeast Asia is closely associated with sedimentogenesis and biogeochemical cycling processes. However, the role of fine-scale differences in biogeochemical processes under similar sedimentological environments in controlling the spatial heterogeneity of groundwater As concentrations is poorly understood. Within the central Yangtze Basin, dissolved organic matter (DOM) and microbial functional communities in the groundwater and solid-phase As-Fe speciation in Jianghan Plain (JHP) and Jiangbei Plain (JBP) were compared to reveal mechanisms related to the spatial heterogeneity of groundwater As concentration. The optical signatures of DOM showed that low molecular terrestrial fulvic-like with highly humified was predominant in the groundwater of JHP, while terrestrial humic-like and microbial humic-like with high molecular weight were predominant in the groundwater of JBP. The inorganic carbon isotope, microbial functional communities, and solid-phase As-Fe speciation suggest that the primary process controlling As accumulation in JHP groundwater system is the degradation of highly humified OM by methanogens, which drive the reductive dissolution of amorphous iron oxides. While in JBP groundwater systems, anaerobic methane-oxidizing microorganisms (AOM) coupled with fermentative bacteria, iron reduction bacteria (IRB), and sulfate reduction bacteria (SRB) utilize low molecular weight DOM degradation to drive biotic/abiotic reduction of Fe oxides, further facilitating the formation of carbonate associated Fe and crystalline Fe oxides, resulting in As release into groundwater. Different biogeochemical cycling processes determine the evolution of As-enriched aquifer systems, and the coupling of multiple processes involving organic matter transformation‑iron cycling‑sulfur cycling-methane cycling leads to heterogeneity in the spatial distribution of As concentrations in groundwater. These findings provide new perspectives to decipher the spatial variability of As concentrations in groundwater., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

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), CO 2 , and CH 4 , 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, δ 56 Fe, and δ 13 C-DIC, suggesting P accumulation due to the reductive dissolution of FeOOH under methanogenic conditions. The positive correlations of pmoA gene abundance versus δ 13 C-CH 4 and Fe concentrations versus δ 13 C-CH 4 , 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 δ 13 C-CH 4 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.

Identifying high-risk factors (heavy metals (HMs) and pollution sources) by coupling receptor models and health risk assessment model (HRA) is a novel approach within the field of risk assessment. However, this coupled model ignores the contribution of spatial differentiation to high-risk factors, resulting in the assessment being subjective. Taking Dongting Plain (DTP) as an example, a coupling framework by jointly using the positive matrix factorization model (PMF), HRA, Monte Carlo simulation, and geo-detector was developed, aiming to identify high-risk factors in groundwater, and further explore key environmental variables influencing the spatial heterogeneity of high-risk factors. The results showed that at least 82.86 % of non-carcinogenic risks and 97.41 % of carcinogenic risks were unacceptable for people of all ages, especially infants and children. According to the relationships among HMs, pollution sources, and health risks, As and natural sources were defined as high-risk HMs and sources, respectively. The interactions among Holocene thickness, oxidation-reduction potential, and dissolved organic carbon emerged as the primary drivers of spatial variability in high-risk factors, with their combined explanatory power reaching up to 74%. This proposed framework provides a scientific reference for future studies and a practical reference for environmental authorities in developing effective pollution management measures., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Organoiodine compounds (OICs) are the dominant iodine species in groundwater systems. However, molecular mechanisms underlying the geochemical formation of geogenic OICs-contaminated groundwater remain unclear. Based upon multitarget field monitoring in combination with ultrahigh-resolution molecular characterization of organic components for alluvial-lacustrine aquifers, we identified a total of 939 OICs in groundwater under reducing and circumneutral pH conditions. In comparison to those in water-soluble organic matter (WSOM) in sediments, the OICs in dissolved organic matter (DOM) in groundwater typically contain fewer polycyclic aromatics and polyphenol compounds but more highly unsaturated compounds. Consequently, there were two major sources of geogenic OICs in groundwater: the migration of the OICs from aquifer sediments and abiotic reduction of iodate coupled with DOM iodination under reducing conditions. DOM iodination occurs primarily through the incorporation of reactive iodine that is generated by iodate reduction into highly unsaturated compounds, preferably containing hydrophilic functional groups as binding sites. It leads to elevation of the concentration of the OICs up to 183 μg/L in groundwater. This research provides new insights into the constraints of DOM molecular composition on the mobilization and enrichment of OICs in alluvial-lacustrine aquifers and thus improves our understanding of the genesis of geogenic iodine-contaminated groundwater systems.

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 ( 2 H/ 18 O in water, 13 C in dissolved inorganic carbon, 13 C/ 2 H in methane, and 13 C 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 CO 2 -reduction and acetate-fermentation, and methane oxidation) were identified. CO 2 -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 CO 2 -reduction microorganisms (Methanobacterium and Methanoregula) was higher in samples with substantial CO 2 -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., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

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., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Long-term exposure to geogenic arsenic (As)-contaminated groundwater poses a severe threat to public health problems. Generally, elevated As concentrations have been observed with high amounts of ammonium in groundwater of floodplains. An extreme gradient boosting algorithm was conducted to develop a probability model based on hydrogeochemical data, which predicted the occurrence rates of groundwater As on a regional scale. Results showed that concentrations of NH 4 + , Eh, K, Cl - , SO 4 2- , suggesting that the mineralization of nitrogen-containing organic matter promoted the reduction of As-bearing iron-oxides. The predicted distribution of high-As groundwater showed high consistency with known spatial distribution of As contamination, and the model also accurately predicted As concentrations in Jiangbei Plain of China and typical As-affected floodplains of Southeast Asia. The model can serve as a low-cost and rapid virtual sensor for detecting As concentrations in private or newly drilled wells, thereby providing critical information for informed management decisions, environmental protection and public health safety.3 - were powerful predictive variables of As exposure. The model revealed the co-enrichment of As with NH 4 + , suggesting that the mineralization of nitrogen-containing organic matter promoted the reduction of As-bearing iron-oxides. The predicted distribution of high-As groundwater showed high consistency with known spatial distribution of As contamination, and the model also accurately predicted As concentrations in Jiangbei Plain of China and typical As-affected floodplains of Southeast Asia. The model can serve as a low-cost and rapid virtual sensor for detecting As concentrations in private or newly drilled wells, thereby providing critical information for informed management decisions, environmental protection and public health safety., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

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.

Geogenic arsenic (As) contaminated groundwater has been widely accepted associating with dissolved organic matter (DOM) in aquifers, but the underlying enrichment mechanism at molecular-level from a thermodynamic perspective is poorly evidenced. To fill this gap, we contrasted the optical properties and molecular compositions of DOM coupled with hydrochemical and isotopic data in two floodplain aquifer systems with significant As variations along the middle reaches of Yangtze River. Optical properties of DOM indicate that groundwater As concentration is mainly associated with terrestrial humic-like components rather than protein-like components. Molecular signatures show that high As groundwater has lower H/C ratios, but greater DBE, AI mod , and NOSC values. With the increase of groundwater As concentration, the relative abundance of CHON3 formulas gradually decreased while that of CHON2 and CHON1 increased, indicating the importance of N-containing organics in As mobility, which is also evidenced by nitrogen isotope and groundwater chemistry. Thermodynamic calculation demonstrated that organic matter with higher NOSC values preferentially favored the reductive dissolution of As-bearing Fe(III) (hydro)oxides minerals and thus promoted As mobility. These findings could provide new insights to decipher organic matter bioavailability in As mobilization from a thermodynamical perspective and are applicable to similar geogenic As-affected floodplain aquifer systems., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

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.

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.

The prevalence of arsenic (As)-affected groundwater in the Late Pleistocene and Holocene aquifers leads to serious arsenicosis worldwide. However, the geogenic foundational processes underlying the high As aquifers remain elusive. Here we present joint lines of evidences from chronological, sediment geochemical and geomicrobial analysis that climate change since the Last Glacial Maximum (LGM) initiates the genesis of high As aquifers in the central Yangtze River Basin, which represents Quaternary alluvial-lacustrine floodplains affected by arsenicosis occurrence. Optically stimulated luminescence-based sediments dating and grain size characterization indicate that the LGM depositional boundary also separates the Late-Pleistocene/Holocene high arsenic aquifers from the underlying arsenic-depleted aquifers. Further examination of solid-phase As/Fe/S speciation and associated microbial communities function suggests that the pre-LGM depositional environments characteristic of S metabolism engender the fixation of As in pyrite, whereas during the post-LGM period climate change to warm and humid leads to As repartitioning to Fe/Mn oxides in response to strong chemical weathering. This may have contributed to a dynamic fate of As in the post-LGM depositional environments and thus a highly variable aqueous As concentrations over depth. Our results highlight the important roles of climate change has played in the genesis of high As aquifers, with implications for other LGM-affected regions worldwide as well as for the evolution of high arsenic aquifers under future climate change., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Groundwater nitrogen contamination is becoming increasingly serious worldwide, and natural nitrogen attenuation processes such as anaerobic ammonium oxidation coupled to iron reduction ("Feammox") play an important role in mitigating contamination. Although there has been intensive study of Feammox in soils and sediments, still lacks research on this process in groundwater. This study makes effort to demonstrate the occurrence of Feammox in groundwater by combining information from Fe/N isotope composition, the quantitative polymerase chain reaction (qPCR) assay, and 16S rRNA gene sequencing. Poyang Lake Plain of Yangtze River in central China was selected as the case study area. The critical evidences that indicate Feammox in groundwater include favorable hydrogeochemical conditions of the alluvia-lacustrine aquifer systems, the simultaneous enrichment of 15 N in ammonium and 56 Fe, the relative high abundance of Acidimicrobiaceae bacterium A6, and the joint elevation of the abundance of the Feammox bacteria and the concentration of Fe(III). Redundancy analysis (RDA) indicated that Geothrix and Rhodobacter may participate directly or cooperatively in the Feammox process. Ammonium-oxidizing archaea (AOA) involved in ammonium-oxidizing or Feammox process may be stimulated by Fe(III) under a low oxygen concentration and weakly acidic condition. Anammox may be indirectly enhanced by products of the nitrogen transformation processes involving Feammox bacteria and AOA. Fe(III) concentration is an important environmental factor affecting the abundance of functional microorganisms related to nitrogen cycling and the composition of ammonium-oxidizing and iron-reducing microbes. Specific geological background (such as the widespread red soils) and anthropogenic input of ammonium, iron, and acidic substances may jointly promote Feammox in groundwater., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

The biodegradation of organic matter triggers the reductive dissolution of iron oxides with the transformation among iodine species has been mostly accepted as the key iodine mobilization process in groundwater system. However, molecular characteristics of natural organic matter (NOM) and their interaction with iron oxides on geogenic iodine enrichment remain unclear. We used Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to characterize the molecular composition of both dissolved organic matter (DOM) in groundwater and water-soluble organic matter (WSOM) in aquifer sediments being depth-matched with groundwater from monitoring wells in typical iodine-affected aquifers within the central Yangtze River Basin. The results show that WSOM in high-iodine sediments contains more high molecular weight (HMW) organic compounds with higher aromaticity and nominal oxidation state of carbon (NOSC), including polycyclic aromatics, polyphenols and highly unsaturated compounds. These compounds are mostly positively associated with amorphous iron oxides (Fe ox1 ) in aquifer sediments. The association between iodine and WSOM is highly consistent with that between amorphous Fe ox1 and WSOM, but is contrary to that between crystalline iron oxides (Fe ox2 ) and WSOM. DOM in groundwater with higher iodine concentration contains more aliphatic compounds and less polyphenols. The complexation of HMW organic compounds of WSOM to iodine-bearing amorphous Fe ox1 plays an important role in iodine mobilization, which could inhibit the amorphous Fe ox1 transformation to crystalline Fe ox2 . These observations indicate the biodegradation of HMW organic matter (polycyclic aromatics, polyphenols and highly unsaturated compounds) in WSOM fueling the reductive dissolution of amorphous Fe ox1 predominantly promotes the release of iodine from aquifer sediments into groundwater. This research provides new insights into the mobilization mechanisms of iodine in alluvial-lacustrine groundwater system controlled by the Fe-OM complexation at the molecular level., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

It is challenging to dependably keep the native distribution of arsenic (As) species before sample analysis in the laboratory. The on-site separation method can avoid sample contamination and species change in the process of sample collection and transportation from field to laboratory. In this study, As species distribution and variation of the extracted groundwater was first analyzed by an on-site species separation method in Jianghan Plain, China. Our study illustrated that: 1) high-As groundwater generally existed under mildly reducing conditions (Eh < 200 mV), weak alkaline conditions (pH < 7.2), elevated concentrations of dissolved Fe(II) and S(-II), and high proportions of As (III); 2) As species in the groundwater changed dramatically at room temperature in 36 hours post extraction (HPE). Fe-sulfide and Fe oxides minerals, which adsorbed As (V), were the main reasons influencing the As species concentration; 3) Acidification and strong complexing agents cannot preserve As species effectively. The average proportion of As (III) in the wells, where groundwater samples from the depth of 25 m exceed 10 μg L -1 As, can be reduced by 61% and 63% after HCl and EDTA were added, respectively. Accurate assessment of concentrations and distribution variation of As species in groundwater can guide the removal of As and the safe use of water resources, especially in drought areas relying on drinking well water., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021. Published by Elsevier B.V.)

Although groundwater nitrogen pollution has been widely studied, the control of hydrogeological conditions on behavior of nitrogen compounds has been poorly understood. In this study, multiple stable isotopes (N/C/H/O), spectral characteristics of DOM coupled with water chemistry were used to reveal the sources and fate of nitrate and ammonium in three subareas with different hydrogeological conditions in the Central Yangtze River Basin. We identified three contrasting patterns of nitrogen sources and fate in groundwater controlled by different aquifer features. In a reducing porous aquifer mainly composed of carbonate minerals overlain by a thick low-permeability layer, the NH 4 -N concentration is high (mean 4.12 mg/L) but with quite low NO 3 -N concentration (mean 0.28 mg/L). The high ammonium is mainly from intense degradation of organic matter (OM), while denitrification at a higher rate results in nitrate removal. Feammox may be favored owing to abundant humics acting as the electron shuttle. In a weakly reducing to oxidizing porous aquifer mainly composed of aluminosilicate minerals overlain by a varying thickness of low-permeability layer, high ammonium occurs in a weakly reducing condition and is affected by both anthropogenic input and OM degradation, while high nitrate occurs in a more oxidizing condition and could be mainly from soil nitrogen, manure or sewage. Feammox may be also favored due to more acidic environment formed by weathering of aluminosilicate minerals, fluctuating redox condition and low abundance of labile organic carbon, while denitrification occurs at a slower rate coupled with concurrent re-oxidation of nitrite to nitrate. In an oxidizing porous - fissured aquifer system overlain by a thin low-permeability layer, the concentrations of ammonium and nitrate are both low, possibly due to strong hydrodynamic and flushing condition, although slightly higher concentration of nitrate exhibit similar sources and fate with the weakly reducing to oxidizing porous aquifer mentioned above., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

High levels of geogenic ammonium in groundwater is a highly neglected nitrogen pool in coastal aquatic systems. Although organic matter (OM) mineralization is known to significantly influence geogenic ammonium enrichment, the detailed mechanism underlying ammonium enrichment based on dissolved organic matter (DOM) characterization in coastal aquifer systems remains unclear. In this study, we characterized the optical and molecular signatures of DOM coupled with hydrogeochemistry and multiple isotopes (H/O/C/N) to elucidate in detail the mechanisms underlying the anomalously high ammonium in the coastal confined aquifer system of the Pearl River Delta, which exhibits the highest reported geogenic ammonium concentration in groundwater on the Earth. We identified three DOM fluorescent components, a marine humic-like component (C1) and two other humic-like components (C2 and C3). The autochthonous OM was first processed to the C1 component, which was further transformed to C2 and C3 components. In terms of molecular classes, the processing pathway from bacterial- or algal-derived OM to aliphatic compounds and highly unsaturated-low O compounds was identified, and highly unsaturated-low O compounds were accumulated as the main products. Compounds containing two or three N atoms were processed, and compounds with one N atom gradually accumulated, which was further degraded into CHO compounds. The ammonium (up to 179 mg/L as N) was gradually enriched due to the decomposition of CHO+3N to CHO+2N, CHO+1N, and CHO compounds. Owing to the longer residence time and less frequent fresh water flushing, the produced ammonium was retained in the aquifer as a "long-term result". The contrasting DOM characteristics, together with the differing depositional and hydrogeological conditions, give rise to the higher levels of geogenic ammonium in coastal confined aquifer systems compared with inland alluvial-lacustrine confined aquifer systems. To our knowledge, this is the first study to characterize DOM and its relationship with geogenic ammonium in coastal aquifer systems.

Understanding the biogeochemical processes controlling arsenic (As) mobilization under bacterial sulfate reduction (BSR) in aquifer sediments is essential for the remediation of high As groundwater. Here, we conducted microcosm experiments with shallow aquifer sediments from the Jianghan Plain (central Yangtze River Basin) under the stimulation of exogenous sulfate. Initially, co-increases of As(III) (from 0.0 to 88.5 μg/L), Fe(II) (from 0.5 to 6.0 mg/L), and S(-II) (from 0.0 to 90.0 μg/L) indicated the concurrent occurrence of sulfate, Fe(III), and arsenate reduction. The corresponding increase of the relative abundance of OTUs classified as sulfate-reducing bacteria, Desulfomicrobium (from 0.5 to 30.6%), and dsrB gene abundance indicated the strong occurrence of BSR during the incubation. The underlying mechanisms of As mobilization could be attributed to the biotic and abiotic reduction of As-bearing iron (hydro)oxides either through the iron-reducing bacteria or the bacterially generated sulfide, which were supported by the variations in solid speciation of Fe, S, and As. As the incubation progressed, we observed a transient attenuation followed by a re-increase of aqueous As, due to the limited abundance of newly-formed Fe-sulfide minerals with a weak ability of As sequestration. Moreover, the formation of thioarsenate (H 2 AsS 4 - ) during the mobilization of As from the sediments was observed, highlighting that BSR could facilitate As mobilization through multiple pathways. The present results provided new insights for the biogeochemical processes accounting for As mobilization from sediments under BSR conditions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Lacustrine groundwater discharge (LGD) can play an important role in water and contaminant mass balance of lakes. Dongting Lake is the second largest fresh lake in China which is connected to Yangtze River and has quite prominent ecological status and function within Yangtze River basin. However, the effect of groundwater discharge on the balance of water and contaminant in Dongting Lake has long been overlooked. This study estimated the groundwater discharge and associated contaminants input into Dongting Lake during the dry season using multiple tracers ( 222 Rn, 18 O, Cl - ). After sensitivity analysis of different models, it is found that the result of 222 Rn mass balance model is the most reliable. Based on the 222 Rn mass balance model, the groundwater discharge rate is estimated to be 73.94 mm/d and the contribution of LGD to water balance is 10.94%. As the main nutrient components, NH 3 -N, P and Si from groundwater input account for 23.65%, 5.12% and 30.15%  % of the total input, respectively. As the main heavy metal components, Fe, Mn and As from groundwater input all account for more than 50% of the total input. Although the LGD rate is relatively small, the contaminant input from LGD is significant enough, which may be a potential threat to the ecological stability of Dongting Lake. In this study, the mass balance models of multiple tracers were integrated to understand the role of groundwater in maintaining the water balance and pollution status of Dongting Lake, which has certain reference significance for the LGD study in plain lakes or reservoirs with complex water systems in humid regions.

High levels of ammonium in groundwater is a potential threat to drinking water security and ecological status. The role of dissolved organic matter (DOM) in mobilization of natural ammonium in groundwater is crucial but the intrinsic link between them has still been poorly understood. This study used high-pressure size exclusion chromatography (HPSEC) and fluorescence excitataion-emission-matrix spectra (EEMs) with parallel factor analysis (PARAFAC) to elucidate the influence of DOM characteristics in groundwater systems having contrastive ammonium levels in Dongting Plain, central Yangtze River. The results indicate that NH 4 -N concentration in groundwater of western plain (0-16.75 mg/L) are much higher compared with southern plain (0-1.5 mg/L). The groundwater in western plain is in a more reductive environment and characterized by larger molecular weight (MW) of DOM and lower polydispersity (ρ), whereas DOM with relatively small molecular weight and high polydispersity is detected in the south with a more oxidative condition. The groundwater in western plain is characterized by lower fluorescence index (f + ) and biological index (BIX), and dominated by the high molecular weight terrestrial humic-like component and larger amounts of microbial humic-like components. Protein-like is the main component in groundwater of southern plain with higher f 450/500 and BIX. The ammonium concentration in groundwater correlates well with molecular weight and increases significantly with the content of high molecular weight terrestrial humic-like component, indicating that mobilization of ammonium is more closely associated with the terrestrial organic matter of high molecular weight. This study further enriches the theory on mobilization of ammonium in Quaternary alluvial-lacustrine aquifer systems and provides theoretical basis for the local water supply security.450/500 and BIX. The ammonium concentration in groundwater correlates well with molecular weight and increases significantly with the content of high molecular weight terrestrial humic-like component, indicating that mobilization of ammonium is more closely associated with the terrestrial organic matter of high molecular weight. This study further enriches the theory on mobilization of ammonium in Quaternary alluvial-lacustrine aquifer systems and provides theoretical basis for the local water supply security., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Intake of groundwater with arsenic (As) concentrations exceeding the World Health Organization standard of 10 μg L -1 adversely impacts over 100 million people worldwide. Geogenic As contaminated groundwater within central Yangtze River Basin has recently been reported, but the variations within different depths of aquifers are not commonly observed and the processes controlling As variations have yet to be resolved. Here we report the significant As variations within two different depths (10 m and 25 m) of shallow multi-level alluvial aquifers at Jianghan Plain, a floodplain in the central Yangtze River Basin, which is also a recently discovered geogenic As affected area with cases of waterborne arsenicosis. The multi-year monitoring of aquifer chemistry results show that the As concentrations increase with the Fe(II) concentrations when As contents are relatively lower (<200 μg L -1 ) in upper phreatic aquitard (at 10 m depth), while decrease with Fe(II) concentrations when As contents are much greater in lower confined aquifer (at 25 m depth), and the highest is up to 1070 μg L -1 . Iron isotope analysis were conducive to characterize Fe cycling in the aquifers and thus illustrate geochemical processes controlling As mobilization of shallow groundwaters. Results showed that groundwater is generally enriched in isotopically light Fe with δ 56 Fe values between - 1.60‰ and + 0.06‰ (median - 0.55‰). In the upper phreatic aquitard, microbial reductive dissolution of As-associated Fe(III) oxides, hydroxides and oxyhydroxides is the major process controlling As concentrations lower than 200 μg L -1 . The reduction process could lead to the increasing As concentrations with the gradually increasing δ 56 Fe values, and a positive correlation between Fe and δ 56 Fe, and between dissolved As and δ 56 Fe values is observed, respectively. In strongly reducing conditions as the lower confined aquifer, jointly microbial reduction of sulfate promotes the As mobilization through HS - abiotic reduction of Fe(III) minerals, resulting in As concentrations greater than 200 μg L -1 . These findings could provide new insights for differentiating the major factors controlling As mobilization at different depths of aquifers, and provide better water managements for similar geogenic As-affected shallow alluvial aquifers., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Bacterial sulfate reduction (BSR) plays a vital but complex role in regulating groundwater arsenic concentration. A quarterly hydro-biogeochemical investigation was conducted to clarify how BSR participated in arsenic dynamics in the geogenic As-contaminated alluvial aquifers of the Jianghan Plain, central Yangtze River Basin. Anthropogenic input of sulfate was identified in the transitional season with higher Cl concentrations and Cl/Br molar ratios compared to the monsoon season. Seasonal increase of S(-II) and Fe(II) concentrations in monsoon season suggests the co-occurrence of iron and sulfate reduction. Quantitative analysis of dsrB gene abundance revealed the corresponding variations between dsrB gene abundance (up to 1.2 × 10 7 copies L -1 ) and Fe(II) in groundwater. High-throughput sequencing of the dsrB gene identified a considerable proportion of sequences in the sulfate-reducing bacterial community was affiliated with Desulfobulbus (22.7 ± 20.8%) and Desulfocapsa (11.5 ± 11.9%). Moreover, the relative abundance of Desulfocapsa increased with the Fe(II) in the groundwater (R = 0.78, P < 0.01). These results suggest that microbially-mediated sulfate reduction facilitated the abiotic reduction of As-bearing Fe-oxides in the monsoon season after anthropogenic input of sulfate in the transitional season under oscillating redox conditions in the groundwater systems. The present research provides new insights into the critical role of BSR in the seasonal redox cycling of iron and variation of As in the aquifer systems, which are not only applicable in the central Yangtze River basin, but also to other similar As-rich alluvial aquifers worldwide., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Geogenic ammonium in groundwater owing to mineralization of natural organic matter (NOM) has been reported in different geologic settings, but detailed mechanisms responsible for high ammonium concentration levels are poorly understood. To this end, we chose Quaternary high ammonium aquifer systems in central Yangtze River basins and used carbon isotopes in both dissolved organic carbon and inorganic carbon together with characterization of dissolved organic matter (DOM) and groundwater chemistry to reveal mechanisms related to the genesis of ammonium. The results indicate that high levels of geogenic ammonium (up to 33.50 mg/L as N) occur due to long-term water-rock interactions in a relatively sluggish hydrogeological environment with abundant organic matter that is rich in both C and N. The stable carbon isotope data suggest that ammonium in the groundwater is released from intensive degradation of organic matter with higher contents of ammonium associated with methanogenesis. The optical signatures of DOM indicate ammonium in the groundwater is mostly associated with terrestrial humic-like components rather than protein-like components. Molecular characterization of DOM by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) shows that, compared to low ammonium groundwater, high ammonium groundwater has larger mass weights, greater abundance of CHO+N compounds, higher percentages of lignin- and condensed-hydrocarbon-like components, lower H/C ratios, higher nominal oxidation state of carbon (NOSC) values and more double bonds, rings, and aromatic structures. Strong degradation of NOM and preferential utilization of energetically more favorable, terrestrial humic-like components (lignin-like as the main class) with high NOSC values facilitates the formation of high ammonium groundwater. To the best of our knowledge, this is the first effort to use carbon isotopes and DOM characteristics to identify enrichment mechanisms for geogenic ammonium in alluvial-lacustrine aquifer systems.

It becomes increasingly important and challenging for nitrogen pollution prevention to identify key controls for spatial variability of nitrogen in groundwater that could be affected by multiple factors, including anthropogenic input, groundwater flow, and local geochemistry. This study characterized spatial variability of both nitrate and ammonium in the Pleistocene aquifer of central Yangtze River Basin and assessed the effect of various factors in controlling nitrate and ammonium levels based on multiple statistical approaches (correlation, geostatistics, multiple liner regression). The results indicate that nitrate is mostly influenced by Cl - that represents anthropogenic input, while Eh representing local redox state is a secondary variable influencing nitrate concentrations. The groundwater with elevated nitrate concentrations are estimated to occur mainly in areas with higher-permeability near-surface sediments which can facilitate more anthropogenic nitrate transport and less nitrate removal owing to more oxidized state. Ammonium is mostly correlated to Eh, followed by dissolved organic carbon (DOC), but only DOC improves significantly the accuracy of co-kriging prediction model. The groundwater with elevated ammonium concentrations are estimated to occur mainly in areas with more organic-rich sediments within or around the aquifer which can facilitate more ammonium release owing to natural organic matter consumption accompanying strong reducing conditions. The regional groundwater flow is not a factor significantly controlling nitrate or ammonium levels owing to flat topography and sluggish lateral flow., (© 2019, National Ground Water Association.)

Determining the temporal variation of microbial communities in groundwater systems is essential to improve our understanding of hydrochemical dynamics in aquifers, particularly as it relates to the fate of redox-sensitive contaminants like arsenic (As). Therefore, a high-resolution hydrobiogeochemical investigation was conducted in the As-affected alluvial aquifer systems of the Jianghan Plain. In two 25 m-deep monitoring wells, the seasonal variation in the composition of groundwater microbial communities was positively correlated with the change in groundwater level (R = 0.47 and 0.39 in NH03B and NH05B, respectively, P <  0.01), implying that the latter could be a primary driver of the seasonal microbial dynamics. In response to the fluctuating groundwater level, iron (Fe) reducers within the Desulfuromonadales were dominant (9.9 ± 4.7% among different sampling sites) in groundwater microbial communities during the monsoon season and associated with high concentrations of Fe(II) and As, while the predominance (16.7 ± 15.2% among different sampling sites) of iron-oxidizers the Gallionellaceae was accompanied by low Fe(II) and As in the non-monsoon season. These results suggest that microbially-mediated iron reduction/oxidation may have governed the seasonal mobilization/scavenging of As in groundwater. Our results provide new insights into mechanisms responsible for seasonal variations in groundwater As concentrations in similar aquifer systems., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Significant seasonal variation of groundwater arsenic (As) concentrations in shallow aquifers of the Jianghan Plain, central Yangtze River Basin has been reported recently, but the underlying mechanisms remain not well understood. To elaborate biogeochemical processes responsible for the observed As concentration variation, 42-day incubation experiments were done using sediment samples collected respectively from the depth of 26, 36 and 60m of the As-affected aquifer which were labeled respectively as JH26, JH36, JH60. Where JH denotes Jianghan Plain, and the number indicates the depth of the sediment sample. The results indicated that As could be mobilized from the sediments of 26m and 36m depth under the stimulation of exogenous organic carbon, with the maximum As release amount of 1.60 and 1.03mgkg -1 , respectively, while the sediments at 60m depth did not show As mobilization. The microbially mediated reductive dissolution of amorphous iron oxides and reduction of As(V) to As(III) could account for the observed As mobilization. The 16S rRNA high-throughput sequencing results indicated that the variation of microbial community correlated with the released As concentration (R=0.7, P<0.05) and the iron-reducing bacteria, including Pseudomonas, Clostridium and Geobacter, were the main drivers for the As mobilization from the sediments at 26m and 36m depth. The increase of arsC gene abundance (up to 1.4×10 5 copies g -1 ) during As release suggested that As reduction was mediated by the resistant reduction mechanism. By contrast, in the 60m sediments where the Fe and As release was absent, the iron-reducing bacteria accounted for a very minor proportion and sulfate-reducing bacteria were predominant in the microbial community. In addition, after 30days of incubation, the released As in the 26m sediments was immobilized via co-precipitation with or adsorption onto the Fe-sulfide mineral newly-formed by the bacterial sulfate reduction. These results are consistent with the results of our previous field monitoring, indicating that the bacterial sulfate reduction could lead to the temporal decrease in groundwater As concentrations. This study provides insights into the mechanism for As mobilization and seasonal As concentration variation in the Pleistocene aquifers from alluvial plains., (Copyright © 2017 Elsevier B.V. All rights reserved.)

The occurrence of 14 antibiotics (fluoroquinolones, tetracyclines, macrolides and sulfonamides) in groundwater and surface water at Jianghan Plain was investigated during three seasons. The total concentrations of target compounds in the water samples were higher in spring than those in summer and winter. Erythromycin was the predominant antibiotic in surface water samples with an average value of 1.60μg/L, 0.772μg/L and 0.546μg/L respectively in spring, summer and winter. In groundwater samples, fluoroquinolones and tetracyclines accounted for the dominant proportion of total antibiotic residues. The vertical distributions of total antibiotics in groundwater samples from three different depths boreholes (10m, 25m, and 50m) exhibited irregular fluctuations. Consistently decreasing of antibiotic residues with increasing of depth was observed in four (G01, G02, G03 and G05) groundwater sampling sites over three seasons. However, at the sampling sites G07 and G08, the pronounced high concentrations of total antibiotic residues were detected in water samples from 50m deep boreholes instead of those at upper aquifer in winter sampling campaign, with the total concentrations of 0.201μg/L and 0.100μg/L respectively. The environmental risks posed by the 14 antibiotics were assessed by using the methods of risk quotient and mixture risk quotient for algae, daphnids and fish in surface water and groundwater. The results suggested that algae might be the aquatic organism most sensitive to the antibiotics, with the highest risk levels posed by erythromycin in surface water and by ciprofloxacin in groundwater among the 14 antibiotics. In addition, the comparison between detected antibiotics in groundwater samples and the reported effective concentrations of antibiotics on denitrification by denitrifying bacteria, indicating this biogeochemical process driven by microorganisms won't be inhibitory influenced by the antibiotic residues in groundwater., (Copyright © 2016. Published by Elsevier Inc.)

Lacustrine groundwater discharge (LGD) has a profound impact on the status of lake ecosystems. The distinct spatial variability of LGD has been acknowledged, but corresponding research has rarely been conducted, especially in urban lakes with changing land use. In this study, the spatial variability of LGD in Tangxun Lake, the largest urban lake in Asia, was quantitatively estimated using the 222Rn mass balance model. Catchments with different land use and hydrogeological conditions within the study area are divided into five regions. The results revealed three ranges of the LGD rate in five regions: the first range of 25 mm/day (region II: 27.25 ± 8.38 mm/day and region IV: 23.04 ± 7.51 mm/day), the second range of 15 mm/day (region III: 14.16 ± 5.64 mm/day), and the third range of 10 mm/day (region I: 10.61 ± 4.31 mm/day, region V: 9.05 ± 3.60 mm/day). Here, we found that the LGD rate of urban lakes was largely controlled by land use patterns, although it was also inherently affected by hydrogeological conditions, which is consistent with a significant positive correlation between the proportion of impermeable/total surfaces within 1 km of the lakeshore and the LGD rate of the corresponding region. The proposed reason is that the head difference between groundwater and lake water decreases with increasing proportion of impermeable surfaces around the urban lake because of reduced infiltration, leading to the decrease of LGD rate. This study provides new insights into the spatial variability of LGD for lakes in a changing environment. [ABSTRACT FROM AUTHOR]

Lacustrine groundwater discharge (LGD) is of great importance to the hydrological cycle and the eco‐environment of lakes. LGD is often characterized by large spatial variability, but the spatial patterns of LGD have rarely been quantified, particularly at large whole‐lake scales. This study presents quantitative estimation of the spatial patterns of LGD in an oxbow lake in the central Yangtze River Basins by a segmented application of 222Rn mass balance model. The results show that the LGD rates in all points ranged from 10.05 to 197.25 mm/d, with an average of 61.23 mm/d. Larger LGD rates were not only occurring in the lakeshore area, but also in the lake center. The high LGD rate in the lake center area here was likely a result of a direct connection between the water‐rich confined aquifer and the lake. Sensitivity analysis of the 222Rn mass balance show that lake water 222Rn concentration, groundwater concentration, wind speed and water depth were sensitive parameters, which were important parameters in controlling the quantitative results of LGD. The average LGD rate uncertainty and percentage were 28.45 mm/d and 49.20%. This study thus provides a solution in quantifying the spatial patterns of LGD lakes. Plain Language Summary: Lacustrine groundwater discharge (LGD) is of great importance to the hydrological cycle and the eco‐environment of lakes. Lacustrine groundwater discharge is often characterized by large spatial variability, particularly at large whole‐lake scales. This study presents quantitative estimation of the spatial patterns of LGD in an oxbow lake in the central Yangtze River Basins by a segmented application of 222Rn mass balance model. The results show that the LGD rates in all points ranged from 10.05 to 197.25 mm/d, with an average of 61.23 mm/d. Larger LGD rates were not only occurring in the lakeshore area, but also in the lake center. Sensitivity analysis of the 222Rn mass balance show that lake water 222Rn concentration, groundwater concentration, wind speed and water depth were sensitive parameters, which were important parameters in controlling the quantitative results of LGD. The average LGD rate uncertainty and percentage were 28.45 mm/d and 49.20% Key Points: Spatial pattern of Lacustrine groundwater discharge (LGD) was quantified using a segmented application of 222Rn modelLarger LGD were not only occurring in the lakeshore area, but also in the lake centerThe sources of uncertainty in LGD rate were identified and the average uncertainty was 49.20% [ABSTRACT FROM AUTHOR]

Understanding the Ca isotope variability of groundwaters in carbonate- or silicate-dominated catchments could provide new insights into processes controlling the Ca isotope fluxes in the global Ca budget. Two geographically close but lithologically different sites were investigated along the middle reaches of the Yangtze River, where Xiangxi River Basin drains carbonate rocks and Jianghan Plain drains mostly silicates. Although the Ca isotope variations are small, there is a significant correlation between δ44/40Ca and Mg/Ca, suggesting that calcite precipitation has affected the Ca isotope signatures, which tends to narrow the differences so that average groundwater δ44/40Ca values are close to Bulk Silicate Earth values. Besides, carbon isotopes indicated that buried organic matter particularly in the meanders, palaeo-channels, and oxbow lakes along the middle reaches of the Yangtze River provided a unique favorable environment for the formation of elevated Ca groundwater in the basin. These findings should be applicable to similar fluvial-lacustrine sedimentary aquifers, including floodplains and river delta regions worldwide. • Groundwater δ44/40Ca varied between two catchments in the central Yangtze River Basin. • Microbial-mediated degradation of organic matter drives groundwater Ca enrichment. • Secondary carbonate precipitation governs groundwater δ44/40Ca signatures. [ABSTRACT FROM AUTHOR]

Chronic exposure to geogenic contaminated groundwaters (GCGs) has caused serious health problems among billions of people worldwide. The hydrogeological features, main sources of geogenic hazardous substances, dominant hydrogeochemical processes and key factors controlling the occurrence of high arsenic (As ≥ 10 μg/L), high fluoride (F ≥ 1 mg/L), and high iodine (I ≥ 100 μg/L) groundwater are discussed in this review. Although the global and/or regional occurrence, and the environmental and geochemical behaviors of groundwater As, F and I have been well documented in the past three decades, there is still a lack of holistic theoretical analysis to demonstrate the hydrogeological patterns of their (co)occurrence. We here propose four basic genetic types of GCGs by summarizing the characteristics of the distribution and major hydrogeochemical processes: leaching-enrichment type, burial-dissolution type, compaction-release type, and evaporation-concentration type. The complex genetic mechanisms of GCGs are thus integrated into a new theoretical framework to improve our understanding of their genesis and our capability of predicting their spatial and temporal distribution. Moreover, new paradigms of interdisciplinary convergent research are required to find best solutions to ensure the sustainable safe supply of groundwater resources and the well-being of the groundwater-dependent ecosystems and the human society. [ABSTRACT FROM AUTHOR]

Geogenic arsenic (As) contaminated groundwater poses a threat to drinking water safety and ecological health. Dissolved organic matter (DOM) plays a crucial role in As mobilization in Quaternary groundwater systems. Significant spatial variations in As concentrations in groundwater inside and outside the oxbow lakes were observed, but the controlling mechanisms remains unknown. In this study, carbon stable isotopes and fluorescence excitation-emission matrices (EEMs) combined with parallel factor analysis (PARAFAC) were used to elucidate the influence of DOM on contrasting occurrence of As in both inside and outside groundwater of the oxbow lakes along the middle reaches of the Yangtze River. The results showed that groundwater As concentrations outside the oxbow lakes (0.37–485 μg/L) were significantly higher than those in the groundwater inside (0.75–204 μg/L). Stable carbon isotopes suggest that methanogenesis promotes the reductive dissolution of As-bearing Fe(III) oxides/hydroxides, which is the primary process responsible for As enrichment in groundwater outside of oxbow lakes. In contrast, microbial fermentation dominated As mobilization in the groundwater inside of the oxbow lakes. The DOM in the groundwater outside of the oxbow lakes is characterized by a more reductive, predominantly more highly humified and less biologically active humus, while DOM in the groundwater inside consists mainly of microbial humic-like components that is less humified but more biologically active. This study provides new insight to decipher the controlling mechanism of As mobilization in the alluvial-lacustrine aquifers around typical oxbow lakes and is applicable to similar geogenic As-affected floodplain aquifer systems. [Display omitted] • As concentrations in groundwater outside the oxbow lakes are higher than inside the oxbow lakes. • Controls on contrastive As occurrence in groundwater outside and inside the oxbow lakes were identified. • Organic matter degradation and composition dictate As heterogeneity in groundwater inside and outside the oxbow lakes. [ABSTRACT FROM AUTHOR]

Lacustrine groundwater discharge (LGD) can play an important role in water and contaminant mass balance of lakes. Dongting Lake is the second largest fresh lake in China which is connected to Yangtze River and has quite prominent ecological status and function within Yangtze River basin. However, the effect of groundwater discharge on the balance of water and contaminant in Dongting Lake has long been overlooked. This study estimated the groundwater discharge and associated contaminants input into Dongting Lake during the dry season using multiple tracers (222Rn, 18O, Cl−). After sensitivity analysis of different models, it is found that the result of 222Rn mass balance model is the most reliable. Based on the 222Rn mass balance model, the groundwater discharge rate is estimated to be 73.94 mm/d and the contribution of LGD to water balance is 10.94%. As the main nutrient components, NH3-N, P and Si from groundwater input account for 23.65%, 5.12% and 30.15% % of the total input, respectively. As the main heavy metal components, Fe, Mn and As from groundwater input all account for more than 50% of the total input. Although the LGD rate is relatively small, the contaminant input from LGD is significant enough, which may be a potential threat to the ecological stability of Dongting Lake. In this study, the mass balance models of multiple tracers were integrated to understand the role of groundwater in maintaining the water balance and pollution status of Dongting Lake, which has certain reference significance for the LGD study in plain lakes or reservoirs with complex water systems in humid regions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Elevated Mn groundwater occurs spatial distribution along the lower Han River. • Mn-enriched sediment in the aquifer matrix releases Mn into groundwater. • Major hydrogeochemical process of Mn mobilization is burial-dissolution type. Geogenic contaminants in groundwater limit the drinking water safety and sustainable water supply for domestic purposes throughout the world. Elevated concentrations of manganese (Mn) in groundwater have been one of the major concerns since its adverse health effects to humans, whereas the underlying enrichment mechanism within aquifers is poorly understood. This research aims to connect the spatial distributions of groundwater Mn in different units of the aquifer and the variations of sediment geochemistry to provide comprehensive understanding of Mn provenance and its mobilization through sediment pore-networks into groundwater. Along the lower reaches of Han River within the central Yangtze River Basin, significant heterogenous Mn distribution has been observed with over 60 % of groundwater exceeding the World Health Organization drinking water guideline. Groundwater chemistry and sequential extraction results indicated that the oxidation of organic matter and reduction of Mn oxides and Fe (hydr)oxides in sediments could drive groundwater Mn enrichment. Herein, we propose a burial-dissolution type as the major hydrogeochemical process of Mn mobilization in the region. These findings are of great importance in understanding the geogenic sources of Mn and the controlling processes for its spatial heterogeneity in alluvial-lacustrine aquifer system, which might be applicable to similar Mn-affected geological settings. [ABSTRACT FROM AUTHOR]

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High arsenic (As) groundwater is widely distributed in northwestern Hetao Plain, an arid region with sluggish groundwater flow. Observed As concentration in groundwater from wells ranges from 76 to 1,093 μg/l. Most water samples have high total dissolved solids, with Cl and HCO3 as the dominant anions and Na as the dominant cation. The major hydrochemical types of most saline groundwaters are Na–Mg–Cl–HCO3 and Na–Mg–Cl. By contrast, fresh groundwaters generally belong to the Na–Mg–HCO3 type. High concentrations of arsenic in shallow aquifers are associated with strongly reducing conditions, as evidenced by high concentrations of dissolved organic carbon, ammonium, as well as dissolved sulfide and Fe, dominance of arsenite, relatively low concentrations of nitrate and sulfate, and occasionally high content of dissolved methane (CH4). High As groundwaters from different places at Hetao Plain experienced different redox processes. Fluoride is also present in high As groundwater, ranging between 0.40 and 3.36 mg/l. Although fluorosis poses an additional health problem in the region, it does not correlate well with As in spatial distribution. Geochemical analysis indicates that evapotranspiration is an important process controlling the enrichment of Na and Cl, as well as trace elements such as As, B, and Br in groundwater. [ABSTRACT FROM AUTHOR]

In addition to the anthropogenic sources for elevated concentrations of phosphorus (P) in groundwater systems, the importance of geogenic enrichment of P warrants attention. To assess factors controlling the sources and enrichment processes of P in Quaternary aquifers, 355 groundwater samples were collected in the Jianghan Plain of central Yangtze River Basin. In the phreatic aquifer, the total dissolved phosphorus (TDP) concentrations range from below detection limit (BDL) to 2.56 mg/L, with 6.25% of samples exceeding 1 mg/L; in the confined aquifer, TDP concentrations range from BDL to 4.31 mg/L, with 22.18% of samples exceeding 1 mg/L. Factor analysis and correlation were used to determine major factors controlling P enrichment in the groundwater. Elevated levels of P in the confined aquifer are related to reductive dissolution of P-rich Fe(III)-(hydr)oxides (FeOOH) as well as organic phosphorus (OP) mineralization. The SEDEX sequential extraction procedure was applied to core samples from two boreholes with high and low P levels in groundwater, respectively, to characterize phosphorus speciation in aquifer sediments. Bioavailable P, particularly exchangeable P, in sediments with high P groundwater are significantly higher than those with low P groundwater. The content of Fe-bound P is higher than that of residual organic P (Res-OP) in sediments from both boreholes, indicating the greater contribution of reductive dissolution of P-rich FeOOH to geogenic P enrichment in groundwater than OP mineralization. Using the Redfield ratio, groundwater samples collected from the confined aquifer can be divided into three groups, with 65% of the samples falling into the group closely related to reduction of FeOOH. The present research provides new insights into the enrichment of geogenic P in groundwater systems, which are not only applicable in the Jianghan Plain, but also to other similar alluvial aquifers in floodplains and delta regions worldwide. Unlabelled Image • High P concentrations up to 4.31 mg/L were detected in groundwater of JHP. • Bioavailable P in sediments control geogenic P release to groundwater. • Geogenic P originates from reductive dissolution of FeOOH and OP mineralization. • Reductive dissolution of FeOOH dominates over OP mineralization in aquifers. [ABSTRACT FROM AUTHOR]

[Display omitted] • We firstly study geogenic P enrichment based on DOM characterization in groundwater. • P-containing DOM predominantly consisted of 1P and 2P compounds. • Considerable molecular differences were observed between 1P and 2P compounds. • P-containing DOM degradation results in an increase in H/C and a decrease in O/C. • During P-containing DOM degradation, a gradient is observed from 2P to 1P compounds. Although degradation of phosphorus-containing natural organic matter (NOM) is an important process associated with geogenic phosphorus (P) enrichment, the detailed mechanism underlying the association remains unclear. Herein, we used molecular characteristics of P-containing dissolved organic matter (DOM) coupled with hydrogeochemistry and carbon isotopes to unravel this controlling mechanism. Results indicate that P-containing DOM is indeed an important factor controlling groundwater P enrichment. Further, one-P-atom (1P) and two-P-atom (2P) compounds were widely detected. Compared to 1P compounds, 2P compounds have greater numbers of N/S-containing compounds; smaller proportions of highly unsaturated and aliphatic compounds; larger proportions of polyphenols and polycyclic aromatics; lower H/C and nominal oxidation state of carbon (NOSC) values; and higher m / z , O/C, P/C, N/C, double bond equivalents (DBE), and modified aromaticity index (AI mod) values. At the molecular level, P-containing DOM degradation overall results in an increase in H/C and a decrease in O/C, and a processing gradient is observed from 2P to 1P compounds, during which dissolved inorganic phosphorus (DIP) was gradually accumulated and retained, eventually becoming the predominating P pool in groundwater. To our knowledge, this is the first study to reveal geogenic P enrichment from a molecular perspective in groundwater systems worldwide. [ABSTRACT FROM AUTHOR]

• LGD rates in the west and east of the lake were 71.47 and 34.76 mm/d, respectively. • Groundwater flow field and water-richness of aquifer were key factors for LGD difference. • Nutrients concentrations in western porous aquifer were higher than eastern fissured aquifer. • Nutrients loads with LGD in the west was one order of magnitude higher than that in the east. The spatial patterns of lacustrine groundwater discharge (LGD) and associated nutrient inputs are crucial for effective management and protection of lakes. Multiple factors have been found to influence the spatial differences in LGD rates and associated nutrients loads, but the influence of geological conditions on the differences have not been well understood. In this study, we quantified LGD rates and associated nutrients loads on the west and east of the lake with contrasting geological conditions of East Dongting Lake (EDL) within central Yangtze catchment and discuss the influence of geology on the spatial differences, through 222Rn mass-balance model, water chemistry coupled with existing geological data. The results showed that LGD rates were 34.76 ± 23.36 mm/d in the east EDL which is characterized by hilly geomorphy, deep/fast/narrow flowing, coarse-grained lakebed and large hydraulic gradients (0.004–0.006). Surprisingly, LGD rates were higher (71.47 ± 52.16 mm/d) in the west EDL which is characterized by alluvial-lacustrine plain geomorphology, shallow/sluggish flowing, clayey or silty lakebed and low hydraulic gradients (0.0002–0.0015). The remaining factor determining the higher LGD rates in the west EDL is the permeability of the porous aquifer connected with the lake. The groundwater around the east EDL existed in a less confined environment, and frequent flushing led to low concentrations of nutrients. By contrast, rapid burial of sediments and deposition of paleo-lake sediments since Last Deglaciation formed an organic-rich and reducing environment, which facilitated the enrichment of geogenic nutrients. As a result, the loads of LGD-derived nutrients in the west generally exceeded that in the east by one order of magnitude. The study provides new understanding of the spatial differences in LGD and associated nutrients input to the large lakes resulting from variations in geological conditions, thus serving as a reference for ecological protection of lakes. [ABSTRACT FROM AUTHOR]

• Co-occurrence of geogenic NH 4 + and P in groundwater was firstly studied. • Four distinct groundwater types regarding NH 4 + and P emerge sequentially. • In low-NH 4 + & high-P type, amorphous Fe(III) reductive dissolution enriches P. • In high-NH 4 + & high-P type, OM degradation and Fe(III) dissolution prevail. • In high-NH 4 + & low-P type, secondary siderite precipitation adsorbs P. High loads of nitrogen (N) and phosphorus (P) in groundwater are a pressing global environmental concern. Geogenic ammonium (NH 4 +) and P anomalies in groundwater as unique forms of contamination, have been extensively reported. However, despite significant efforts on the enrichment of geogenic NH 4 + or P, little is known about their co-occurring characteristics and mechanisms in groundwater. Therefore, focusing on the Dongting Plain (DTP) within the central Yangtze River basin, a typical alluvial-lacustrine plain, this study analyzed hydrogeochemistry, stable carbon isotopes, and dissolved organic matter (DOM) fluorescence. The results revealed a clear correlation between the groundwater DOM humification degree and the redox processes. In different redox processes, four distinct groundwater types emerged sequentially: low-NH 4 + & low-P, low-NH 4 + & high-P, high-NH 4 + & high-P, and high-NH 4 + & low-P. In organic matter (OM) degradation process, when dissolved oxygen and nitrate acted as electron acceptors, the low-NH 4 + & low-P groundwater dominated. The low-NH 4 + & high-P groundwater dominated when Mn(IV) and amorphous Fe(III) (oxyhydr)oxides served as the electron acceptors, where P enrichment was primarily attributed to the reductive dissolution of amorphous Fe(III) (oxyhydr)oxides. The high-NH 4 + & high-P groundwater dominated when SO 4 2-, crystalline Fe(III) (oxyhydr)oxides, and CO 2 served as the electron acceptors. During this phase, OM mineralization resulted in the enrichment of both NH 4 + and P. Furthermore, the reductive dissolution of crystalline Fe(III) (oxyhydr)oxides further increased P concentrations in groundwater. During the strong degradation of OM to the methanogenic stage, two processes occurred simultaneously: anaerobic oxidation of methane and reductive dissolution of Fe(III) (oxyhydr)oxides. These processes led to an increase in the concentration of HCO 3 – and Fe(II) in groundwater. During this phase, large amounts of HCO 3 – and Fe(II) combine to form siderite precipitates. These precipitates adsorb P from the groundwater, eventually leading to a decrease in P concentration and the formation of high-NH 4 + & low-P groundwater. This study offers novel insights into the simultaneous cycling of N and P in Quaternary alluvial-lacustrine aquifer systems. [ABSTRACT FROM AUTHOR]

• Diverse hydrochemical and geostatistical methods were used to identify P sources. • Groundwater P was mainly geogenic and little affected by anthropogenic activities. • Ca2+, Fe2+ and NH 4 -N were key factors for groundwater P enrichment. • Ca-P minerals, OP and Fe-P minerals controlled P enrichment in different areas. • Determinants of geogenic P sources in groundwater were linked to regional geology. The contribution of geogenic phosphorus (P) in groundwater to surface water eutrophication has become a major concern for aquatic environment protection, but the differences in groundwater P sources in different geological settings are less concerned. In this study, three areas in the central Yangtze River basin under various geological conditions were selected for the identification of groundwater P sources and key factors affecting P enrichment, using hydrogeochemical and geostatistical methods. It was found that P in groundwater of middle and upper Pleistocene aquifers is mainly geogenic and is little affected by anthropogenic activities. In the lower reaches of the Han River (an area with phosphate mining upstream), the dissolution of phosphate minerals such as apatite from P-rich strata is the main source of groundwater P (feature importance of Ca2+ 39%). Along the Jingjiang River (an area with widespread peat deposition in paleo-channels and ancient lakes), the mineralization of organic P (OP) is the dominant groundwater P source (feature importance of NH 4 -N 46%), with the reductive dissolution of iron (oxyhydr)oxides as a secondary source (feature importance of Fe2+ 26%). In the western part of the Dongting Plain (an area with iron-rich sedimentary rocks and granites in the surrounding mountains), the reductive dissolution of iron (oxyhydr)oxides is the main source of P in groundwater (feature importance of Fe2+ 34%), with mineralization of organic P as a secondary but important source (feature importance of NH 4 -N 25%). The differences in groundwater P sources in different subareas are clearly linked to specific geological conditions. This study provides new insights into the dependence of P source and enrichment in groundwater systems on regional geology and highlights the importance of using a wide range of research methods for understanding the extremely complex processes in the subsurface environment. [ABSTRACT FROM AUTHOR]

Geogenic phosphorus (P) in groundwater has been found in different regions, posing a risk for surface water eutrophication. However, the prediction of groundwater P distribution is less studied. In this study, three machine learning-based regression models including random forest regression (RFR), support vector regression (SVR), and multiple linear regression (MLR) were applied to predict the spatial distribution of geogenic P in alluvial-lacustrine sedimentary aquifers of the central Yangtze River basin, with readily accessible field parameters, such as pH, Eh, EC, depth, NH 4 +-N and Fe2+. The results indicate that the RFR model achieves the best fit by three times 10-fold cross-validation with the highest R2 (0.49) and explanatory variance (0.52), the lowest root mean square error (0.48) and mean absolute error (0.30), producing a groundwater P distribution that is highly consistent with the observed results. According to the prediction results, the areas with high P (>0.4 mg/L) groundwater and abnormally high P (>1 mg/L) groundwater account for 55% and 11% of the whole study area in JH-DT-P, respectively. Meanwhile, NH 4 +-N and Fe2+ are the two most prominent indicating factors of P enrichment in groundwater, and NH 4 +-N has higher relative importance than Fe2+. The wider validity of the model was suggested by its successful application to two regions in Bangladesh with similar hydrogeological conditions. • P concentration in groundwater was predicted for the first time. • Geogenic P in groundwater was predicted by three machine learning models. • Indicative factors of geogenic P in groundwater was identified. • Predictive model was successfully applied to P-affected aquifers in Bangladesh. [ABSTRACT FROM AUTHOR]

[Display omitted] • We firstly use C and Fe isotopes to study P mobilization in groundwater. • We introduce two different processes between Fe and OM on P mobilization. • Prior to CH 4 production, labile OM and amorphous Fe(III) mobilize P. • During methanogenesis, recalcitrant OM and crystalline Fe(III) mobilize P. • The latter P-mobilizing redox processes prevail over the former. Iron (Fe) (oxyhydr)oxides and organic matter (OM) are important hosts of geogenic phosphorus (P) in groundwater systems. However, the coupled influence of Fe (oxyhydr)oxides and OM on the occurrence and mobility of P in groundwater remains unclear. In this study, we shed light on the underlying mechanisms of the control of Fe (oxyhydr)oxides and OM on P mobilization in the alluvial-lacustrine aquifers at the Dongting Plain (DTP), central China, using tracing techniques of isotopes (δ 13C-DIC and δ 56Fe), characterization of dissolved organic matter (DOM), and groundwater geochemistry. The results suggest that high concentrations of geogenic P up to 3.58 mg/L as total dissolved phosphorus (TDP) tend to occur in relatively reducing and sluggish hydrogeological environments experiencing longer time of water–rock interactions, and are closely related to the reductive dissolution of P-rich Fe(III) (oxyhydr)oxides as well as the mineralization of organic P. Our observations suggest that two different processes between Fe(III) (oxyhydr)oxides and OM contribute to the high concentrations of TDP in groundwater. Firstly, under moderately reducing conditions, labile OM is oxidized, with amorphous Fe(III) (oxyhydr)oxides acting as electron acceptors. Subsequently, the adsorbed and/or occluded P on/into OM or amorphous Fe(III) (oxhydr)oxides is concomitantly released into groundwater. Secondly, under strongly reducing conditions, the degradation of recalcitrant OM depends on the concurrent roles of crystalline Fe(III) (oxyhydr)oxides as electron acceptors and conduits, which further influence the mobility of P. The latter P-mobilizing redox processes were identified to prevail over the former in the groundwater of DTP. This study provides new insights into the anomalous concentration of P in groundwater of alluvial-lacustrine sandy aquifer systems. [ABSTRACT FROM AUTHOR]

Dissolved organic matter in groundwater (GDOM) plays an important role in many biogeochemical processes. The molecular composition of GDOM varies with the source and processing pathways, and also affects its reactivity. Sedimentary organic matter (SOM) is a key carbon source in groundwater, but its association with GDOM remains unclear. In this study, Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) and comprehensive data analysis were used to identify the molecular characteristics of SOM and the fate of DOM in groundwater by comparing the differences between the GDOM with depth-matched water-extractable aquifer SOM (WEOM; the most bioavailable and dynamic pool of organic carbon in SOM) in a Quaternary alluvial-lacustrine aquifer with abundant SOM in Jianghan Plain, central China. The results show that WEOM is a key carbon source in groundwater as reflected by the strong correlation (r s = 0.76, P = 0.028) of the DOC content between GDOM and WEOM, and the large proportion (70.64 ± 14.09%) of common compounds present both in GDOM and WEOM. However, the molecular compositions of GDOM and WEOM are significantly different even for common compounds when considering the relative abundance. WEOM contains relatively high abundance of low molecular weight and thermodynamically more favorable components for microbial oxidation (aminosugars and carbohydrates), making WEOM an energetic carbon source for groundwater that can fuel reduction half-reactions such as iron oxide reduction. Although the hydrochemical compositions and the DOC content of groundwater are quite different, the molecular composition of GDOM is relatively similar and enriched in oxygen-poor highly unsaturated and phenolic compounds compared with WEOM. This indicates that organic matter is gradually processed into stable components with relatively medium aromaticity, low average nominal carbon oxidation state, low O/C and medium H/C; these transformations occur through adsorption to minerals and biodegradation after being mobilized from aquifer sediment into groundwater. We identified the processing pathways of GDOM based on comprehensive comparisons of molecular compositions between aquifer WEOM and GDOM, and highlight the combined influence of aquifer SOM and processing pathways on the GDOM molecular compositions in the alluvial-lacustrine aquifer with abundant aquifer SOM. [Display omitted] • WEOM is a key carbon source for GDOM containing energetically favorable compounds. • GDOM molecular compositions are similar among themselves but different from WEOM. • The adsorption and biodegradation both affect the molecular compositions of GDOM. • High input of WEOM weakens the stable compounds signal of GDOM. [ABSTRACT FROM AUTHOR]

• Multiple tracers are used collectively to trace the LGD. • LGD and related nutrient fluxes are higher during wet season compared to dry season. • Lake depth and land use types contribute to the spatial variability of LGD. Lacustrine groundwater discharge (LGD) as a hidden hydrologic process has been widely found to affect water and nutrient balance in various lakes, but a comprehensive research on both spatial and temporal variations of LGD and associated nutrient fluxes was poorly conducted. Herein, this study elucidated their spatio-temporal variations and underlying factors of a typical lake in front of hillocks with spatially variable topography and hydrogeology, and intense seasonal groundwater-lake exchange, within central Yangtze River basin. The 222Rn mass balance model was used in conjunction with hydrochemistry, water isotope, and nutrient data. The results showed that the overall LGD rate and related N/P fluxes were clearly higher in the wet season than in the dry season, for which the stronger evaporation relative to precipitation, more groundwater recharge from agricultural irrigation or aquacultural pond infiltration, and more N/P input from anthropogenic or aquacultural activities could be responsible. In the wet season, the area with more recharge from irrigating water or fish pond water infiltration and associated extensive agricultural or aquacultural activities, had the significantly highest LGD rate and related N/P fluxes. In the dry season, the area with the deepest water depth and largest topographic slope, exhibited the significantly highest LGD rate and slightly higher LGD-related N/P fluxes due to relatively low groundwater N/P concentrations. This study enriched the understanding on LGD and related nutrient fluxes in the perspective of both spatial and temporal variations, taking a new type of lakes (i.e., lakes in the transitional zone of hills and plains in temperate climate) as a typical case. [ABSTRACT FROM AUTHOR]