Dissolved organic matter characterization in high and low ammonium groundwater of Dongting Plain, central China

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

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

Determination of high-risk factors and related spatially influencing variables of heavy metals in groundwater

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.

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

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

Predicting geogenic groundwater arsenic contamination risk in floodplains using interpretable machine-learning model

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 NH4+, Eh, K, Cl-, SO42-, and NO3- were powerful predictive variables of As exposure. The model revealed the co-enrichment of As with NH4+, 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.

The telltale fluorescence fingerprints of sewer flows for interpreting the low influent concentration in wastewater treatment plant

Low degradability of wastewater treatment plant (WWTP) influents negatively affects its ability to effectively remove pollutants through wastewater treatment processes. Proactive assessment of urban sewer system performance is highly valued in the selection of targeted countermeasures for this occurrence. In this study, a fluorescence spectrum interpretation approach was developed to identify the causes of low biodegradability of WWTP influent by using parallel factor analysis (PARAFAC) and fluorescence regional integration (FRI) of excitation-emission matrix spectroscopy. Statistical analysis was also used to further interpret the PARAFAC- and FRI-derived data. The urban sewer catchment served by a WWTP in Wuhan City, China, was used as the test site to demonstrate the effectiveness of this approach. The results showed that electronics manufacturing industrial wastewater and groundwater input into the urban sewer would significantly decrease the biodegradability of the WWTP influents, and these sources were characterized by much lower fluorescence peak intensities, especially for protein-like substances, including tryptophan-like T and tyrosine-like B1 and B2. The potential conversion of high freshness T into low freshness B2 within the sewer may also contribute to this undesirable scenario. The ratio of peak T to peak B2 and the ratio of the FRI fraction of region I to that of region II can be used together to determine the predominance of industrial wastewater and groundwater. T/B2 < 1.3 indicates the entry of industrial wastewater or groundwater into urban sewers, and I/II > 0.5 further confirms the input of industrial wastewater. Accordingly, the low biodegradability of the WWTP influents in our study site is mostly due to the inflow of industrial wastewater rather than groundwater infiltration into the urban sewers. Therefore, actions should be focused on the surveillance of industrial wastewater rather than widespread sewer inspection and repairs. In this way, this methodology is cost-effective in aiding targeted countermeasures to improve the urban sewer system performance.

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

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

Spatiotemporal distribution and controlling factors on ammonium in waters in the central Yangtze River Basin, China

High levels of ammonium in water can compromise the ecological environment and be harmful to human beings. It is of great significance to understand the source and controlling factors of ammonium in waters. However, the distribution and controlling factors on ammonium in the central Yangtze River Basin have been rarely reported. The results showed that 6.58% of the surface water (SW) exceeded the China national guideline of category III for NH4+-N (i.e., 1.0 mg/L) and 30.19% of the groundwater (GW) exceeded the China national guideline of category III for NH4+-N (i.e., 0.5 mg/L). Notably, the ammonium concentrations of the plain area in the middle were much higher, which reached to the highest value at the junction of the Yangtze River and Dongting Lake. Nitrogen in SW may originate from manure but more nitrogen sources in GW. The net anthropogenic nitrogen input (NANI) can provide enough organic nitrogen for the mineralization. NH4+-N in SW was more affected by fertilizer nitrogen and feed nitrogen input but more affected by agricultural nitrogen fixation in GW. Agricultural and industrial activities controlled NH4+-N in SW and GW by increasing nitrogen input and changing hydrological conditions. In general, this research exposed the controlling of different types of factors on ammonium in waters, providing a guidance for the water pollution prevention in study area.

Contrastive mechanisms of groundwater ammonium enrichment in different hydrogeologic settings

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

Sampling terrigenous diffuse sources in watercourse: Influence of land use and hydrological conditions on dissolved organic matter characteristics

Diffuse and point sources of dissolved organic matter (DOM) in streams influence its composition, interactions and fate in the aquatic ecosystem. These inputs can be very numerous at the scale of a watershed, and their identification remains a challenge, especially for diffuse sources related to land use. The complexity of the transfer mechanisms and the reactivity of DOM throughout the soil-water column continuum raise questions about the sampling of diffuse sources in watercourses. To answer this issue, we compared the characteristics of soil-extracted DOM influenced by a particular land use (homogenous sub-catchment of forest and vineyard) and DOM collected from the watercourse adjacent to the soil samples. A 28-day incubation experiment of soil extracts was designed to remove the labile fraction of DOM. During the first 3 days, between 40 and 70 % of the DOC mass was lost for both types of soils. A set of optical indicators (UV-Visible, EEM fluorescence and HPSEC/UV-fluorescence) showed that the labile fraction was mostly composed by low (<1 kDa) and high (>10 kDa) protein-like molecules. At the end of the incubation, soil-extracted DOM was mainly composed of medium molecules (1-10 kDa) associated to terrigenous humic-like compounds. Its optical and size molecular signature tended towards that in the adjacent watercourses and was specific to land use. However, the characteristics of DOM in watercourses was also influenced by the hydrological conditions, probably due to a transfer of top soil DOM during high water periods and both deep soil and autochthonous DOM during low water periods. These results were obtained by a set of indicators, including novel ones derived from HPSEC/UV-fluorescence. Finally, this study demonstrated that it is possible to sample the DOM representative of a land use directly in the river downstream of the homogeneous sub-basin by multiplying the samples during contrasting hydrological conditions.

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

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

Effects of Biochar on the Fluorescence Spectra of Water-Soluble Organic Matter in Black Soil Profile after Application for Six Years

At present, extracting water-soluble organic matter (WSOM) from agricultural organic waste is primarily used to evaluate soil organic matter content in farmland. However, only a few studies have focused on its vertical behavior in the soil profile. This study aims to clarify the three-dimensional fluorescence spectrum characteristics of the WSOM samples in 0-60 cm black soil profile before and after different chemical fertilizer treatments after six years of fertilization. Fluorescence spectroscopy combined with fluorescence and ultraviolet-visible (UV-Vis) spectroscopies are used to divide four different fertilization types: no fertilization (T0), nitrogen phosphorus potassium (NPK) (T1), biochar (T2), biochar + NPK (T3), and biochar + N (T4) in a typical black soil area. The vertical characteristics of WSOC are also analyzed. The results showed that after six years of nitrogen application, T2 had a significant effect on the fluorescence intensity of Zone II (decreasing by 9.6% in the 0-20 cm soil layer) and Zone V (increasing by 8.5% in the 0-20 cm soil layer). The fluorescent components identified in each treatment group include ultraviolet radiation A humic acid-like substances (C1), ultraviolet radiation C humic acid-like substances (C2), and tryptophan-like substance (C3). As compared with the land with T1, the content of C2 in the 20-60 cm soil layer with T2 was lower, while that of C2 in the surface and subsoil with T3 was higher. In addiiton, there were no significant differences in the contents of C1, C2, and C3 by comparing the soils applied with T3 and T4, respectively. The composition of soil WSOM was found to be significantly influenced by the addition of a mixture of biochar and chemical fertilizers. The addition of biochar alone exerted a positive effect on the humification process in the surface soil (0-10 cm). NPK treatment could stimulate biological activity by increasing biological index values in deeper soil layers (40-50 cm). Nitrogen is the sovereign factor that improves the synergism effect of chemical fertilizer and biochar during the humification process. According to the UV-Vis spectrum and optical index, soil WSOM originates from land and microorganisms. This study reveals the dynamics of WSOC in the 0-60 cm soil layer and the biogeochemical effect of BC fertilizer treatment on the agricultural soil ecosystem.

High Fe and Mn groundwater in the Nanchang, Poyang Lake Basin of China: hydrochemical characteristics and genesis mechanisms

Based on the water quality test data of 257 groups of phreatic groundwater and 165 groups of confined groundwater in the Nanchang area and the redox conditions, acid-base conditions and the organic matter content in groundwater, we identified hydrochemical characteristics and genesis of groundwater with high Fe and Mn contents in Nanchang. The results showed that Fe and Mn exceeded the standard in both phreatic and confined groundwater. The over-standard rates of Fe and Mn in groundwater were 8.56-11.52% and 33.07-36.36%, respectively. The degree of pollution Fe and Mn in the confined groundwater is higher than that in the phreatic groundwater, and the degree of pollution caused by Mn is higher than that caused by Fe. The high Fe and Mn contents in groundwater were caused by the release of Fe and Mn minerals from the native environment due to changes in the groundwater environment of the study area. A mild redox environment (Eh < 100) and low pH value are favorable for Fe and Mn enrichment in groundwater. The presence of organic matter accelerates microbial activity and promotes the release of Fe and Mn from aquifer sediments. Therefore, the change in the native environment played an important role in the increase in Fe and Mn content in the study area.

Spatial and temporal distribution and affecting factors of iron and manganese in the groundwater in the middle area of the Yangtze River Basin, China

Iron (Fe) and manganese (Mn) are heavy metals ubiquitous in groundwater. High levels of Fe and Mn in groundwater can compromise water quality and pose a risk to human health if the groundwater is used for drinking or irrigation. In the middle region of the Yangtze River Basin, groundwater has been extensively used for domestic and agricultural purposes. However, little is known about the distribution of Fe and Mn in the groundwater in this area. It was found that the 74.4% and 48.9% of the groundwater exceed the China national guideline for Fe (i.e., 0.3 mg/L) and Mn (i.e., 0.1 mg/L), respectively. And 6.38% and 2.13% of the wells had Fe and Mn health risks, respectively. Spatial heterogeneity of Fe and Mn was observed. Notably, the concentrations of Fe and Mn in a plain region located between two major rivers (i.e., the Yangtze River and the Han River) were significantly higher than those in other regions. Modeling using PHREEQC revealed that the Fe-bearing minerals in the plain region were more saturated compared to those in the other regions. Besides, temporal change of Fe and Mn was observed in the plain region, significantly affected by rainfalls and groundwater levels. In addition, the distribution of Fe and Mn was significantly affected by various physicochemical factors. Particularly, Fe was more sensitive to redox potential compared to Mn. Under a reducing condition, organic matter concentration and water residence time also affect the release of Fe from Fe-bearing minerals. Overall, a comprehensive understanding of distribution characteristics of Fe and Mn and affecting factors in the middle area of the Yangtze River Basin can provide guidance for the distribution of industrial water, agricultural water, and drinking water in different regions of the study area. Especially in the plain area between the Yangtze River and the Han River, direct drinking of groundwater shall be reduced since the higher health risk value of Fe and Mn.

Light-absorption and fluorescence fingerprinting characteristics of water and methanol soluble organic compounds in PM2.5 in cold regions of Northeast China

High-performance liquid chromatography-size exclusion chromatography and excitation-emission matrix (EEM) fluorescence spectroscopy were used to analyze the seasonal variations and potential sources of molecular weight (MW) separated light-absorbing chromophores and fluorophores of water-soluble organic compounds (WSOC) and methanol-soluble organic compounds (MSOC) in PM_2.5 in cold areas of northern China. The results showed that the light-absorbing organics in MSOC had larger weight-average MW (M_w) (3.19 kDa) and number-average MW (M_n) (1.13 kDa) compared with WSOC (M_w: 1.41 kDa, M_n: 0.692 kDa). The light-absorption of organics showed a trend of winter>spring>autumn>summer and increased on air pollution days. Three fluorescent components including humic-like, protein-like, and terrestrial humic-like components in WSOC were extracted by parallel factor analysis (PARAFAC). Fluorophores in WSOC were dominated by humic-like and terrestrial humic-like components (67.7%). Three fluorescent components extracted from MSOC were low oxidation humic-like, polycyclic aromatic hydrocarbon (PAH)-like, and protein-like components respectively. It is worth noting that compared with WSOC, MSOC may have a higher human health risk due to the presence of PAH-like components. The combination of PARAFAC and self-organizing map had the potential to identify potential sources of fluorophores. It provided a new perspective for comprehensively exploring the characteristics of fluorophores in aerosols. This study provided a reference for further understanding the chemical composition and optical properties of organic aerosols in the cold regions of northern China.

Characterizing Natural Organic Matter Transformations by Microbial Communities in Terrestrial Subsurface Ecosystems: A Critical Review of Analytical Techniques and Challenges

Determining the mechanisms, traits, and pathways that regulate microbial transformation of natural organic matter (NOM) is critical to informing our understanding of the microbial impacts on the global carbon cycle. The capillary fringe of subsurface soils is a highly dynamic environment that remains poorly understood. Characterization of organo-mineral chemistry combined with a nuanced understanding of microbial community composition and function is necessary to understand microbial impacts on NOM speciation in the capillary fringe. We present a critical review of the popular analytical and omics techniques used for characterizing complex carbon transformation by microbial communities and focus on how complementary information obtained from the different techniques enable us to connect chemical signatures with microbial genes and pathways. This holistic approach offers a way forward for the comprehensive characterization of the formation, transformation, and mineralization of terrestrial NOM as influenced by microbial communities.

Removing ammonium from contaminated water using Purolite C100E: batch, column, and household filter studies

Ammonium removal from drinking water to protect human and environmental health is one of the major global concerns. This study evaluates the performance of Purolite C100E, a commercial cation exchange resin, in eliminating ammonium in synthetic and real contaminated groundwater. The results demonstrate that the pH operation range of the resin for better ammonium removal is 3 to 8. Lower ammonium removal at low and high pH occurred due to competition from H⁺ and loss of ammonium as ammonia gas, respectively. Equilibrium data of ammonium removal fitted both the Langmuir and Freundlich isotherm models with the maximum Langmuir ion exchange capacities for initial ammonium concentrations of 10-200 mg/L and 50-2000 mg/L, reaching 18.37 mg/g and 40.16 mg/g, respectively. The presence of co-ions in the water reduced the ammonium removal efficiencies slightly (< 12%) in the order Mg²⁺ > Ca²⁺ > K⁺. The higher affinity of ammonium to adsorbent is due to its lower hydrated ionic radius and H-bonding. The maximum exchange capacity in the fluidized bed studies of the original Purolite C100E (bed height 27 cm, resin weight 75 g, initial ammonium concentration 17.4 mg/L, filtration velocity 0.5 m/h) was 10.48 mg/g. It progressively reduced slightly after three regeneration cycles to 8.79 mg/g. The column breakthrough data satisfactorily fitted the Thomas model. A household filter cartridge packed with 4 kg Purolite C100E (80 cm height) and operated at a filtration velocity of 1.9 m/h in Vietnam successfully reduced the initial 6 mg NH₄⁺/L in groundwater (after sand filter pre-treatment) to well below the Vietnam drinking water standard (3 mg/L-QCVN 01:2009/BYT) continuously for 1 week, suggesting that such a filter can be adopted in rural areas to successfully remove ammonium from groundwater.

Contrasting sources and fate of nitrogen compounds in different groundwater systems in the Central Yangtze River Basin

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 NH₄-N concentration is high (mean 4.12 mg/L) but with quite low NO₃-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.

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

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.

Microplastics retention by reeds in freshwater environment

Microplastic pollution has recently gained increasing attention. The accumulation of microplastics in plants has been confirmed in the marine environment. However, the extent of the microplastic retention in freshwater plants is still unknown. In this study, sediment and plant samples from six reed farms in the wetland of East Dongting Lake were collected and analyzed. The abundance of microplastics in the sediment of reed farms varied from 125.7 to 1219.5 items/kg dry weight (dw), with an average of 511.2 ± 295.0 items/kg. Moreover, different levels of microplastic abundance were found in reeds from 0 to 14 items/individual. The abundance of microplastics in sediment samples was moderate compared to that worldwide and higher than that in other regions of Dongting Lake. The microplastic pollution level was significantly higher in the reed vegetation belt than that in other sampling positions. On the basis of the distribution and characteristics of the collected microplastics, lake water and fishery are suggested as important sources of microplastics. Furthermore, the factors influencing microplastic retention in the reeds are discussed. This study, as the first direct evidence demonstrating that freshwater reeds tend to accumulate microplastics, constitutes valuable reference for future research.