Groundwater Nitrogen Research Articles

Differences in Nitrogen Sources and Contributions in Shallow Groundwater in Plateau Lake Area with Different Climate Types

Clarifying the concentration, major sources, and contribution differences of nitrogen in shallow groundwater in plateau lake areas with different climate types can provide a novel direction for the control of nitrate （NO3-） pollution in regional groundwater. Taking the shallow groundwater around Erhai Lake in the subtropical monsoon climate zone and Chenghai Lake in the dry-hot valley area of the Jinsha River as the research objects, using hydrochemical indexes and multi-isotope techniques （δ15N-NO3-, δ18O-NO3-, δ18O-H2O, and δ2H-H2O） combined with the stable isotope （SIAR） model； the differences in nitrogen concentration in shallow groundwater around Erhai Lake and Chenghai Lake were analyzed, the sources of NO3- were identified, and the contribution rates of each pollution source were calculated. The results showed that water quality of more than 33% and 5% of shallow groundwater sampling points around Erhai Lake and Chenghai Lake was worse than the groundwater Class Ⅲ quality requirements （GB/T 14848） of 20 mg·L-1 for nitrate nitrogen （NO3--N）, respectively. The δ18O-H2O and δ2H-H2O in shallow groundwater around Erhai Lake and Chenghai Lake were parallel to the global and Chinese atmospheric precipitation lines, and a large intercept was present, indicating that atmospheric precipitation was not the major recharge source of groundwater in the two regions. The contribution rate of different NO3- sources in shallow groundwater around Erhai Lake was the highest for soil organic nitrogen （53.77%）, followed by nitrogen fertilizer （21.75%） and manure and sewage （21.55%）, and atmospheric deposition nitrogen （2.93%） was the lowest. Denitrification occurred in the transformation process of nitrogen in groundwater. The contribution rate of different NO3- sources in shallow groundwater around Chenghai Lake was manure and sewage （44.88%） > soil organic nitrogen （37.03%） > nitrogen fertilizer （16.17%） > atmospheric deposition nitrogen （1.92%）, and nitrification occurred in the transformation process of nitrogen in groundwater. The climate type significantly affected the shallow groundwater level, altering the migration and transformation process of nitrogen, thereby affecting the nitrogen concentration in groundwater and the contribution of NO3- as the chief source. However, the major source of NO3- was not affected by the climate type； however was more affected by land use, agricultural activities, and manure treatment methods.

Simultaneous removal of groundwater manganese and ammonia nitrogen based on high-flux gravity driven ceramic membrane (HF-GDCM) coupled with aerated fluidized birnessite

High concentrations of manganese ion (Mn2+) and ammonia nitrogen (NH3-N) in groundwater are indicative of a critical environmental issue that necessitates immediate attention. The gravity-driven ceramic membrane (GDCM) technology has shown great potential for groundwater treatment in rural communities, owing to its low energy demand and user-friendly operation. Active manganese oxide (MnOx) is extensively used for the concurrent removal of Mn2+ and NH3-N, leveraging its large specific surface area and abundant adsorption sites. Our research group has developed a GDCM-MnOx coupled system to address this challenge. However, membrane fouling, manifested as a reduction in flux or an increase in transmembrane pressure, has been a significant barrier to its widespread adoption. To address this challenge, we have implemented a continuous aeration system in conjunction with GDCM to fluidize birnessite to achieve the higher membrane flux, which has also proven effective in mitigating fouling while maintaining high water purification performance. Over a period of 100 days or more, the high membrane flux in the high-flux GDCM system (HF-GDCM) enhanced with aerated fluidized birnessite has been consistently maintained at approximately 34 L/(m2·h) at a water head of 1 m. Moreover, the HF-GDCM system efficiently removed manganese and NH3-N from groundwater under a hydraulic retention time (HRT) of less than 2.5 h, while also improving membrane permeability. The involvement of manganese oxidizing bacteria (MnOB) and ammonium-oxidizing bacteria (AOB) of Hypomicrobium and Nocardioides in the removal processes within the HF-GDCM system was confirmed. Additionally, XPS analysis confirmed the predominant oxidation state of MnOx to be Mn(III). The MnOx, deposited on powdered activated carbon (PAC) particles in a flower-like configuration, progressively formed a birnessite-like functional layer as the manganese ion content increased over time. Consequently, the HF-GDCM coupled with aerated fluidized birnessite is deemed suitable for water purification in small-scale rural or reservoir settings.

Seasonal dynamics of dissolved inorganic nitrogen in groundwater: Tracing environmental controls and land use impact

High levels of dissolved inorganic nitrogen (DIN) in groundwater pose challenges for regions like northern Anhui Province, China, where groundwater is a crucial domestic resource. This study utilized modern geostatistics to explore the spatial and temporal dynamics of DIN in groundwater. Significant seasonal influences on DIN concentrations were identified: ammonium peaks during wet season driven by agricultural activities, while nitrate peaks during the dry season primarily influenced by municipal inputs. This study established a Bayesian Maximum Entropy - Random Forest (BME-RF) model based on Land Use/Land Cover data to infer the spatio-temporal performance of DIN, achieving an interpretation rate above 90 %. It also highlighted the role of hydrogeological conditions and aquifer types in the evolution of DIN. By employing a DIN environmental interaction model, it further analyzed the eco-hydrological drivers and seasonal trends affecting DIN variability, enhancing the understanding of groundwater nitrogen dynamics and their link to environmental factors with low consumption. SynopsisThis study reveals seasonal shifts in groundwater DIN, links them to human activity, and uses the BME model to guide targeted nitrogen fluctuation.

Unveiling the unknown viral world in groundwater

Viruses as the prevailing biological entities are poorly understood in underground realms. Here, we establish the first metagenomic Groundwater Virome Catalogue (GWVC) comprising 280,420 viral species ( ≥ 5 kb) detected from 607 monitored wells in seven geo-environmental zones throughout China. In expanding ~10-fold the global portfolio of known groundwater viruses, we uncover over 99% novel viruses and about 95% novel viral clusters. By linking viruses to hosts from 119 prokaryotic phyla, we double the number of microbial phyla known to be virus-infected in groundwater. As keystone ultrasmall symbionts in aquifers, CPR bacteria and DPANN archaea are susceptible to virulent viruses. Certain complete CPR viruses even likely infect non-CPR bacteria, while partial CPR/DPANN viruses harbor cell-surface modification genes that assist symbiont cell adhesion to free-living microbes. This study reveals the unknown viral world and auxiliary metabolism associated with methane, nitrogen, sulfur, and phosphorus cycling in groundwater, and highlights the importance of subsurface virosphere in viral ecology.

Regional quality analysis of the hydrological environment with an improved random forest model based on the chimpanzee algorithm.

High-precision evaluations of water environment quality are highly important for improving the accuracy of early warning systems of regional water pollution risk and improving the regional water environment. This paper employs the chimp optimization algorithm (ChOA) to enhance the traditional random forest model, resulting in the chimp optimization algorithm-random forest (ChOA-RF) water quality assessment model for evaluating the Jiansanjiang area in Heilongjiang Province, China. The results show that the overall water environment in Jiansanjiang has the following characteristics: "The water quality of farms in the northwest is poor, and the quality of groundwater is better than that of surface water." Total nitrogen (TN) and total phosphorus (TP) in surface water and ammonium nitrogen (NH3-N), ferrum (Fe), and manganese (Mn) in groundwater are the main pollutants. The TP and TN in surface water and the NH3-N in groundwater exceeded the relevant standards, likely due to the excessive application of chemical fertilizers, especially nitrogen fertilizers. Additionally, Fe and Mn are harmful native substances. According to these findings, targeted improvement strategies, such as reducing nitrogen fertilizer application, plugging well, and increasing the surface water utilization rate, are proposed. Moreover, the ChOA-RF model is compared with the traditional empirical value model and the particle swarm optimization-random forest (PSO-RF) model. The results show that the ChOA-RF model can effectively reduce the root mean square error and mean absolute percentage error and improve the coefficient of determination. The running time and convergence ability are also better than those of the PSO-RF model, which is a more accurate and efficient machine learning model. The model can be used not only for high-precision evaluation of regional water environment quality but also for other machine learning fields.

Elucidating biogeochemical characterization of nitrogen in the vadose zone integrating geochemistry, microorganism, and numerical simulation

A thorough comprehension of nitrogen biogeochemical processes in the vadose zone is crucial for the effective prevention and remediation of soil-groundwater system contamination. Despite the growing research on this subject, the full scope of nitrogen biogeochemical characterization in different geological environments remains poorly understood. This study addresses this knowledge gap by integrating geochemical, microbiological and numerical simulation approaches to gain a deeper insight into nitrogen biogeochemistry in agriculture. Our findings indicate the biogeochemical behavior of nitrogen in the vadose zone is mediated by microorganisms, driven by hydraulics, influenced by geological conditions and environmental factors. Along the groundwater flow, NH4+-N was found to be heavily accumulated in the topsoil of 0–40 cm, while NO3−-N was transported and driven by hydrodynamics from both vertical and horizontal directions. Microbial diversity, species composition and functional microorganisms were significantly influenced by soil depth, rather than geomorphological types. Oxidation-reduction potential (ORP), total organic carbon (TOC), soil moisture (MOI), bicarbonate (HCO3−), and ferrous (Fe2+) were identified as the principal environmental factors that regulate nitrogen metabolism and the dominant biochemical processes, encompassing nitrogen fixation, nitrification, and denitrification. Driven by hydrodynamics, NH4+-N, NO2−-N and NO3−-N tend to form distinct biochemical reaction zones in the vertical vadose zone. These areas are dynamic and subject to geomorphologies. It should be noted that NO3−-N can migrate towards groundwater from the clayey sand in the Alluvial Plain, which presents a potential risk of groundwater contamination. The fissure structure of loess may serve as the major transport pathway for groundwater nitrogen contamination in the Loess Tableland. This finding highlights the importance of integrating microbiology, geochemistry and hydraulics to elucidate the biogeochemical processes of nitrogen in the vadose zone with a dynamic mindset.

Assessment of soil-groundwater nitrogen cycling processes in the agricultural region through flux model, stable isotope

The nitrogen cycle in the soil-groundwater system of agricultural land is a crucial process within the global nitrogen cycle. Human activities have significantly intensified this cycling process in agricultural fields, consequently leading to substantial accumulation of nitrogen in the soil-groundwater system and giving rise to numerous ecological health issues. Quantitative assessment of soil-groundwater nitrogen cycling processes can facilitate the optimization of nitrogen management strategies in agricultural fields and the prevention and management of groundwater nitrogen pollution. However, accurately quantifying the intricate soil-groundwater nitrogen cycling dynamics in agro-irrigation areas characterized by diverse nitrogen sources and complex hydrogeological conditions poses a significant challenge. The Songhua River Basin in the Sanjiang Plain was selected as the study area for this investigation. We utilized the INCA-N model to simulate annual nitrogen fluxes in the soil-groundwater system of an agricultural watershed, and employed stable isotope and water chemistry methods to identify sources of groundwater nitrogen contamination and transformation processes. Ultimately, we conducted a comprehensive assessment of nitrogen cycling within the soil-groundwater system of the agricultural watershed. The findings revealed that atmospheric deposition, nitrogen fixation, and fertilization constituted the primary mechanisms of soil nitrogen input. Plant uptake, riverine nitrogen transport, and denitrification were identified as the three principal processes responsible for soil-groundwater nitrogen export. The results obtained from the MIXSIAR model demonstrate a substantial contribution of nitrogen fertiliser and soil nitrogen to groundwater nitrate, followed by faeces and sewage. Additionally, the annual input fluxes of nitrogen simulated by INCA-N reveal that fertiliser application is the primary contributor, which aligns to some extent with the findings of the MIXSIAR model. Soil nitrogen can serve as a relatively stable source of groundwater nitrate, while anthropogenic activities such as fertilizer application, manure deposition, and sewage discharge are likely to be the primary drivers of groundwater nitrate pollution. By quantifying the N input and output fluxes, it was determined that approximately 58% of the total annual nitrogen input has the potential to accumulate within the soil-groundwater system. The effective utilization of legacy nitrogen can contribute to the reduction of soil-groundwater nitrogen fluxes, while maintaining crop yields and mitigating greenhouse gas emissions. This study aims to optimize nitrogen management practices in agricultural areas and provide valuable insights for water conservation strategies.

Hypoxia exerts greater impacts on shallow groundwater nitrogen cycling than seawater mixture in coastal zone.

There is no doubt that hypoxia and seawater mixture are profoundly affecting the global nitrogen (N) cycle. However, their mechanisms for altering N cycling patterns in shallow coastal groundwater are largely unknown. Here, we examined shallow groundwater N transformation characteristics (dissolved inorganic N and related chemical properties) in the coastal area of east and west Shenzhen City. Results showed that common hypoxic conditions exist in this study area. Ions/Cl- ratios indicated varying levels of saltwater mixture and sulfide formation across this study area. Dissolved oxygen (DO) affects the N cycle process by controlling the conditions of nitrification and the formation of sulfides. Salinity affects nitrification and denitrification processes by physiological effects, while sulfide impacts nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) processes through its own toxicity mechanism and the provision of electron donors for DNRA organisms. Redundancy analysis (RDA) results indicate that the influence magnitude is in the following order: DO > sulfide > salinity. Seawater mixture weakened the nitrification and denitrification of groundwater by changing salinity, while hypoxia and its controlled sulfide formation not only weaken nitrification and denitrification but also stimulated the DNRA process and promotes N regeneration. In this study area, hypoxia is considered to exert greater impacts on N cycling in the coastal shallow groundwater than seawater mixture. These findings greatly improve our understanding of the consequences of hypoxia and seawater mixture on coastal groundwater N cycling.

Impact of groundwater nitrogen legacy on water quality

The loss of agricultural nitrogen (N) is a leading cause of global eutrophication and freshwater and coastal hypoxia. Despite regulatory efforts, such as the European Union’s Nitrogen Directive, high concentrations of N persist in freshwaters. Excessive N leaching and accumulation in groundwater has created a substantial N reservoir as groundwater travel times are orders-of-magnitude slower than those of surface waters. In this study we reconstructed past and projected future N dynamics in groundwater for four major river basins, the Rhine, Mississippi, Yangtze and Pearl, showcasing different N trajectories. The Rhine and Mississippi river basins have accumulated N since the 1950s and although strategies to reduce excess agricultural N have worked well in the Rhine, groundwater legacy N persists in the Mississippi. The Yangtze and Pearl river basins entered the N accumulation phase in the 1970s and the accumulation is expected to continue until 2050. Policies to reduce N pollution from fertilizers have not halted N accumulation, highlighting the importance of accounting for the N legacy in groundwater. Restoring groundwater N storage to 1970 levels by diminishing N leaching will therefore take longer in the Yangtze and Pearl (>35 years) than in the Rhine (9 years) and Mississippi (15 years). Sustainable watershed management requires long-term strategies that address the impacts of legacy N and promote sustainable agricultural practices aligned with the Sustainable Development Goals to balance agricultural productivity with water conservation.

Effect of Fallow on Nitrogen Accumulation in Soil Profile and Shallow Groundwater in Cropland Around Fuxian Lake

Soil nitrogen accumulation in cropland and groundwater nitrogen pollution can be effectively alleviated by reducing exogenous nitrogen input, and fallow is an important measure for reducing exogenous nitrogen input. To explore the effects of fallow on nitrogen accumulation in the soil profile and shallow groundwater, the soil profile and shallow groundwater in cropland around Fuxian Lake were selected as research objects. The changes in nitrogen accumulation in the 0-100 cm soil profile and nitrogen concentration in shallow groundwater before (December 2017) and after (August 2020 and April 2021) fallow and their relationships were analyzed. The results showed that the content and storage of nitrogen in soil profiles were significantly reduced by fallow, and the contents of TN, ON, DTN, NO3--N, and NH4+-N in 0-30, 30-60, and 60-100 cm soil profiles after fallow decreased by 18.4 %-36.5 %, 16.1 %-26.8 %, 54.0 %-130.2 %, 59.5 %-90.8 %, and 60.1 %-110.6 %, respectively. The storages of TN, ON, DTN, NO3--N, and NH4+-N in 0-100 cm soil profiles before fallow were (17.20 ±0.97) t·hm-2, (15.50 ±1.23) t·hm-2, (0.68 ±0.06) t·hm-2, (266.8 ±31.17) kg·hm-2, and (18.7 ±3.04) kg·hm-2, respectively. However, their storages after fallow decreased by 25.5 %, 23.3 %, 44.7 %, 80.1 %, and 59.9 %, respectively. Fallow also changed the concentration and composition of different forms of nitrogen in shallow groundwater. The concentrations of TN, ON, NO3--N, and NH4+-N in groundwater after fallow decreased by 88.4 %, 82.7 %, 92.1 %, and 65.8 %, respectively, and ON/TN and NH4+-N/TN increased from 26 % and 6 % before fallow to 39 % and 17 % after fallow, respectively, whereas NO3--N/TN decreased from 61 % before fallow to 41 % after fallow. Changes in nitrogen concentrations and their forms in groundwater were closely related to DTN, NO3--N, and NH4+-N in the soil profile and pH, ORP, and DO in groundwater before and after fallow. Our study highlights that fallow effectively reduced nitrogen accumulation in cropland soil profiles, further alleviating nitrogen pollution in shallow groundwater, and was conducive to preventing the deterioration of water quality in plateau lakes.

Co-occurring characteristics and mechanisms of geogenic ammonium and phosphorus in Quaternary alluvial-lacustrine aquifer systems

High loads of nitrogen (N) and phosphorus (P) in groundwater are a pressing global environmental concern. Geogenic ammonium (NH4+) and P anomalies in groundwater as unique forms of contamination, have been extensively reported. However, despite significant efforts on the enrichment of geogenic NH4+ 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-NH4+ & low-P, low-NH4+ & high-P, high-NH4+ & high-P, and high-NH4+ & low-P. In organic matter (OM) degradation process, when dissolved oxygen and nitrate acted as electron acceptors, the low-NH4+ & low-P groundwater dominated. The low-NH4+ & 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-NH4+ & high-P groundwater dominated when SO42-, crystalline Fe(III) (oxyhydr)oxides, and CO2 served as the electron acceptors. During this phase, OM mineralization resulted in the enrichment of both NH4+ 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 HCO3– and Fe(II) in groundwater. During this phase, large amounts of HCO3– 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-NH4+ & low-P groundwater. This study offers novel insights into the simultaneous cycling of N and P in Quaternary alluvial-lacustrine aquifer systems.

Characterizing the impact of reservoir storage and discharge on nitrogen dynamics in an upstream wetland using a δ15N and δ18O dual-isotope approach

The identification of nitrate sources in reservoir water is important for watershed-scale surface pollution management. Significant fluctuations in river water levels arising from reservoir storage and discharge influence nitrate sources and transport processes. The Sanmenxia Reservoir, in the middle reaches of the Yellow River in China, undergoes significant water level changes (290–316 m), altering the composition of the nitrogen sources. This study employed a δ15N and δ18O dual-isotope method and MixSIAR modeling to quantify the contributions of nitrate sources. This reveals the impact of reservoir water impoundment and discharge on nitrogen dynamics in the upstream region of the wetland and the model sensitivity for each nitrate source. The results showed that the average concentrations of nitrate‑nitrogen (NO- 3-N) were elevated during the impoundment period compared to the discharge period. Nitrogen sources exhibited varying proportions in surface water, groundwater, and soil water during both the impoundment and discharge periods. The predominant sources include manure and sewage (MS), with a maximum proportion of 57.4 % in surface water. Soil nitrogen (SN) accounted for 25.8 % of groundwater nitrogen and 32.1 % of soil water nitrogen during the impoundment period, whereas, during the discharge period, soil nitrogen made up 41.4 % of surface water nitrogen, manure and sewage contributed 44.8 % of groundwater nitrogen, and manure and sewage dominated with 56.7 % of soil water nitrogen. Sensitivity analysis of the MixSIAR model revealed that the isotopic composition of the manure and sewage primary source most significantly influenced the apportionment results of the riverine nitrate source. Reservoir discharge facilitates the dissimilatory nitrate reduction to ammonium (DNRA). The migration of NO- 3 from surface water to soil water and groundwater occurred from the impoundment period to the discharge period.

Hydrochemical and microbial community characteristics and the sources of inorganic nitrogen in groundwater from different aquifers in Zhanjiang, Guangdong Province, China

Groundwater from different aquifers in the Zhanjiang area suffers from different degrees of nitrogen pollution, which poses a serious threat to the health of urban and rural residents as well as the surrounding aquatic ecological environment. However, neither the water chemistry and microbial community characteristics in different aquifer media nor the sources of inorganic nitrogen pollution have been extensively studied. This study integrated water quality parameters, dual isotopes (δ15N–NO3− and δ18O–NO3−), and 16S rRNA data to clarify the hydrochemical and microbial characteristics of loose rock pore water (LRPW), layered bedrock fissure water (LBFW), and volcanic rock pore fissure water (VRPFW) in the Zhanjiang area and to determine inorganic nitrogen pollution and sources. The results show that the hydrochemistry of groundwater in different aquifers is complex and diverse, which is mainly affected by rock weathering and atmospheric precipitation, and the cation exchange is strong. High NO3− concentration reduces the richness of the microbial community (VRPFW). There are a large number of bacteria related to nitrogen (N) cycle in groundwater and nitrification dominated the N transformation. A quarter of the samples exceeded the relevant inorganic nitrogen index limits specified in the drinking water standard for China. The NO3− content is highest in VRPFW and the NH4+ content is highest in shallow loose rock pore water (SLRPW). In general, NO3−/Cl−, dual isotope (δ15N–NO3− and δ18O–NO3−) data and MixSIAR quantitative results indicate manure and sewage (M&S) and soil organic nitrogen (SON) are the main sources of NO3−. In LRPW, as the depth increases, the contribution rate of M&S gradually decreases, and the contribution rate of SON gradually increases. The results of uncertainty analysis show that the UI90 values of SON and M&S are higher. This study provides a scientific basis for local relevant departments to address inorganic nitrogen pollution in groundwater.

Riparian Groundwater Nitrogen (N) Isotopes Reveal Human Imprints of Dams and Road Salt Salinization

AbstractGroundwater nitrate‐N isotopes (δ15N‐) have been used to infer the effects of natural and anthropogenic change on N cycle processes in the environment. Here we report unexpected changes in groundwater δ15N‐ for riparian zones affected by relict milldams and road salt salinization. Contrary to natural, undammed conditions, groundwater δ15N‐ values declined from the upland edge through the riparian zone and were lowest near the stream. Groundwater δ15N‐ values increased for low electron donor (dissolved organic carbon) to acceptor ratios but decreased beyond a change point in ratios. Groundwater δ15N‐ values were particularly low for the riparian milldam site subjected to road‐salt salinization. We attributed these N isotopic trends to suppression of denitrification, occurrence of dissimilatory nitrate reduction to ammonium (DNRA), and/or effects of road salt salinization. Groundwater δ15N‐ can provide valuable insights into process mechanisms and can serve as “imprints” of anthropogenic activities and legacies.

Study on the Contribution of Groundwater Nitrogen Pollution Sources in a Typical Section of the Wei River in China

Aiming to assess the groundwater nitrogen pollution problem in a typical section of the Wei River in China, the contribution of groundwater nitrogen pollution sources in the region was studied. Using Hydrus-1D to implement the simulation process of substituting points for surfaces, we calculated the volume and nitrogen concentration of the water leaching out from the bottom of the encompassing aeration zone. The results of the Hydrus-1D simulation were input as initial values into the nitrogen migration and transformation numerical model constructed using Visual MODFLOW to integrate the simulation calculations between the surface, the aeration zone, and the aquifer system. In addition, the contribution rates of different groundwater nitrogen pollution sources were calculated using the equilibrium formula combined with the groundwater nitrogen test results. The simulation results showed that the groundwater nitrogen in the southern part of the Wei River comes from two main sources: vertical infiltration and river recharge. Specifically, ammonia nitrogen vertical infiltration and river recharge contribute 95.82% and 4.18%, respectively; nitrite nitrogen vertical infiltration and river recharge contribute 92.41% and 7.59%; and nitrate nitrogen vertical infiltration and river recharge contribute 94.26% and 5.74%. According to the simulation results, an increase in the intensity of surface water pollution control is required in the study area. It is also necessary to control the use of nitrogen fertiliser on farmland in the study area and improve the utilisation rate of nitrogen fertiliser to reduce the nitrogen pollution loads from these sources.

Disentangling sources and transformation mechanisms of nitrogen, sulfate, and carbon in water of a Karst Critical Zone

In the Karst Critical Zone (KCZ), mining and urbanization activities produce multiple pollutants, posing a threat to the vital groundwater and surface water resources essential for drinking and irrigation. Despite their importance, the interactions between these pollutants in the intricate hydrology and land use of the KCZ remain poorly understood. In this study, we unraveled the transformation mechanisms and sources of nitrogen, sulfate, and carbon using multiple isotopes and the MixSIAR model, following hydrology and surface analyses conducted in spatial modelling with ArcGIS. Our results revealed frequent exchange between groundwater and surface water, as evidenced by the analysis of δD-H2O and δ18O-H2O. Nitrification predominantly occurred in surface water, although denitrification also made a minor contribution. Inorganic nitrogen in both groundwater and surface water primarily originated from soil nitrogen (48 % and 49 %, respectively). Sewage and manure were secondary sources of inorganic nitrogen in surface water, accounting for 41 % in urban and 38 % in mining areas. Notably, inorganic sulfur oxidation displayed significant spatial disparities between urban and mining areas, rendering groundwater more susceptible to sulfur pollution compared to surface water. The frequent interchange between groundwater and surface water posed a higher pollution risk to groundwater. Furthermore, the primary sources of CO2 and HCO3− in both groundwater and surface water were water‑carbonate reactions and soil respiration. Sulfide oxidation was found to enhance carbonate dissolution, leading to increased CO2 release from carbonate dissolution in the KCZ. These findings enhance our understanding of the transformation mechanisms and interactions of nitrogen, sulfur, and carbon in groundwater and surface water. This knowledge is invaluable for accurately controlling and treating water pollution in the KCZ.

Fluorescence-Enhanced Tb-MOF for Highly Sensitive and Selective Detection of Ammonia Nitrogen in Groundwater

Fluorescence-Enhanced Tb-MOF for Highly Sensitive and Selective Detection of Ammonia Nitrogen in Groundwater

Linking Transpiration to Reef Nitrogen Supply on a Tropical Coral Island

AbstractCoral reef islands are biodiversity hotspots with high conservation value, but we have a poor understanding of how island vegetation, through transpiration, influences groundwater nutrient supply to adjacent reef systems. Here we combine stable isotope tracing, geophysical surveys, and satellite analysis to unravel the links between transpiration and the discharge of groundwater nitrate to the waters surrounding a coral reef island (Lady Elliot Island in the Great Barrier Reef). Over a 2‐year study period (2020–2021) there was a net loss of freshwater from the island, that is, evapotranspiration (ET) exceeded rainfall, largely caused by high rates of transpiration (73% of ET). Transpiration was higher in forested areas, and groundwater salinity appeared to be higher there. By tracing the nitrogen stable isotope signature of groundwater nitrate into reef organism tissue we were able to map groundwater nitrogen discharge spatially and temporally. Groundwater nitrogen discharge was focussed on one side of the island and did not vary seasonally as expected, despite reduced rainfall and seabird guano inputs over the austral winter/spring. Based on our results, we propose that transpiration by island vegetation slows groundwater flow and concentrates nitrate in the groundwater being released to the surrounding reef system. High concentrations of nitrate in groundwater (up to 27 mmol L−) were observed on Lady Elliot Island and these seem to have increased since 2014, but further work is required to understand if this is a normal scenario for tropical coral islands or the result of the islands' revegetation program.

Characteristics of nitrogen and phosphorus contents in soil and water in an agricultural catchment of the three Gorges reservoir area

To explore the impact of different land use modes on the contents of nitrogen and phosphorus in soil and water in the agricultural basin of the Three Gorges Reservoir, the differences in the nitrogen and phosphorus contents in soil and shallow groundwater under different land use modes were studied by using sample data collected in the field. The typical agricultural small watershed at the heart of the reservoir area was selected as the research object. The differences in the nitrogen and phosphorus loss concentrations during the rainfall process and in the daily surface runoff of the two subcatchments with different land use compositions and spatial layouts were compared. The results show that under the five land use modes, the average total nitrogen (TN) content of the paddy soil (1.51 g/kg) was the highest and was significantly higher than that of the other four land use modes (p < 0.05); the average nitrate nitrogen (NO3−-N) content of the terraced soil in dry land (30.05 mg/kg) was the highest, and the dispersion degree was the greatest; and the total phosphorus (TP) content of the three types of sloping farmland was higher than that of terraced farmland, among which the total phosphorus content of the dryland sloping farmland (1.37 g/kg) was the highest and was significantly greater than that of the other types (p < 0.05); moreover, the available phosphorus (AP) content in the soil of the closely planted mulberry garden was the highest, with an average of 36.85 mg/kg. Under the different land use modes, the concentrations of TN and NO3−-N in the shallow groundwater varied greatly, while there were no obvious differences in the TP concentrations. Influenced by fertilization, the concentrations of TN and NO3−-N in the shallow groundwater clearly increased after fertilization in spring and autumn. The concentration of TP increased slightly, and the concentration was the highest when the rainfall was concentrated in summer. A comparison of the two subcatchments revealed that the interplanted mulberry and paddy fields at the bottom of the basin effectively reduced TN and TP outputs of surface runoff in the subcatchment.

Spatiotemporal successions of N, S, C, Fe, and As cycling genes in groundwater of a wetland ecosystem: Enhanced heterogeneity in wet season

Microorganisms in wetland groundwater play an essential role in driving global biogeochemical cycles. However, largely due to the dynamics of spatiotemporal surface water-groundwater interaction, the spatiotemporal successions of biogeochemical cycling in wetland groundwater remain poorly delineated. Herein, we investigated the seasonal coevolution of hydrogeochemical variables and microbial functional genes involved in nitrogen, carbon, sulfur, iron, and arsenic cycling in groundwater within a typical wetland, located in Poyang Lake Plain, China. During the dry season, the microbial potentials for dissimilatory nitrate reduction to ammonium and ammonification were dominant, whereas the higher potentials for nitrogen fixation, denitrification, methane metabolism, and carbon fixation were identified in the wet season. A likely biogeochemical hotspot was identified in the area located in the low permeable aquifer near the lake, characterized by reducing conditions and elevated levels of Fe2+ (6.65–17.1 mg/L), NH4+ (0.57–3.98 mg/L), total organic carbon (1.02–1.99 mg/L), and functional genes. In contrast to dry season, higher dissimilarities of functional gene distribution were observed in the wet season. Multivariable statistics further indicated that the connection between the functional gene compositions and hydrogeochemical variables becomes less pronounced as the seasons transition from dry to wet. Despite this transition, Fe2+ remained the dominant driving force on gene distribution during both seasons. Gene-based co-occurrence network displayed reduced interconnectivity among coupled C-N-Fe-S cycles from the dry to the wet season, underpinning a less complex and more destabilizing occurrence pattern. The rising groundwater level may have contributed to a reduction in the stability of functional microbial communities, consequently impacting ecological functions. Our findings shed light on microbial-driven seasonal biogeochemical cycling in wetland groundwater.