Nitrogen Pollution In Groundwater Research Articles

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.

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.

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.

Field and simulation studies on yield, water and nitrogen dynamics and use efficiency in rice-wheat crops in sequence

ContextThe dominant rice-wheat cropping system in the Indo-Gangetic plain faces challenges of water depletion and nitrogen pollution of groundwater as well as declining productivity caused by over irrigation and nitrogen fertilization by the farmers. To appreciate the apt use of irrigation water and nitrogen fertilizer in rice and wheat crops it is essential to explore the water and nitrogen balances as well as dynamics in these crops, involving complex interaction between weather, soil, irrigation water and nitrogen fertilizer management. ObjectiveThe objective of the study is to quantify yield, field water balance components, water use efficiency, water productivity, water footprint, and water and nitrogen dynamics of rice and wheat crops in relation to season, varieties, irrigation regimes and nitrogen levels. MethodsField experimentations were conducted in double split plot design along with calibrated and validated HYDRUS 1D model use for two years comprising of treatments in rice i.e. two varieties (PR122-V1 and PR126-V2), two irrigation regimes (2-days drainage period-I1 and soil water suction 16 kPa-I2) and two nitrogen levels (120-N1 and 90-N2 kg N ha−1); and in wheat, two varieties (Unnat PBW343-V1 and Unnat PBW550-V2), two irrigation (1.2 IW/CPE-I1 and 0.9 IW/CPE-I2) regimes and two nitrogen levels (120-N1 and 90-N2 kg N ha−1). ResultsThe highest grain yield recorded was 7.6 t ha−1 in treatment combination of V1I2N1 in rice and 5.4 t ha−1 in V1I1N1 in wheat owing to better plant growth and yield attributes (biomass production, number of effective tillers, panicle length, and 1000 grain weight). Total water footprints were lower in the treatment combination of V1I1N1 in rice and V1I2N1 in wheat. The water budget revealed a higher percent contribution of the total water input to drainage (62 %) in rice ensuing restoration of water in the soil profile; and to evapotranspiration (ET) (100 %) in wheat ensuing water depletion. Modeling water and nitrogen dynamics showed higher root water uptake in I2 treatment in rice and with I1 treatment in wheat; and leaching losses of NO3-N under rice than wheat; and under I2 treatment than I1 as well as in N1 than N2. ConclusionsThe V1I2N1 treatment in rice and V1I1N1 treatment in wheat demonstrated superior grain yield, soil water storage, and root water uptake. Conversely, V1I1N1 treatment in rice and V1I2N1 treatment in wheat showed higher water use efficiency and lower water footprint. SignificanceIt helps to evaluate the effectiveness of irrigation and nitrogen management strategies in maximizing crop productivity while minimizing environmental impacts.

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.

Groundwater nitrogen pollution index in an urbanized area, Bandung Regency, Indonesia

Despite previous reports of contamination from domestic waste, agricultural runoff, and fertilizers, shallow groundwater (GW) within Bandung Regency remains a clean water source. Nitrogen is one of the most common contaminants identified in the region. This study aims to assess nitrogen concentrations and determine the groundwater pollution index (GWPI) caused by nitrogen. The GWPI categorizes water as excellent water (GWPI <0.5), good (0.5< GWPI <0.75), moderately polluted (0.75< GWPI <1), and highly polluted (GWPI >1). Twenty-seven GW samples were collected during the wet season in October 2022, and physicochemical parameters were analyzed, including pH, dissolved oxygen, GW temperature, electrical conductivity, total dissolved solids, oxidation-reduction potential, dissolved iron and manganese, ammonium, nitrate, and nitrite. According to the analysis, there was a significant variation in the dissolved inorganic nitrogen (DIN), with 12 samples dominated by ammonium and 15 samples dominated by nitrate. The GWPI calculation ranges from 0.96 to 3.52, indicating that 33% of the samples exhibit excellent water quality, 19% show good water quality, 22% demonstrate moderately polluted water, and 26% demonstrate highly polluted water. Based on the spatial distribution analysis, it was found that the settlement exhibited the most substantial levels of GW pollution due to DIN.

Effect and genesis of soil nitrogen loading and hydrogeological conditions on the distribution of shallow groundwater nitrogen pollution in the North China Plain.

The North China Plain (NCP) has experienced increasingly severe groundwater nitrogen (TN) pollution. However, the factors influencing TN distribution are still poorly understood. Previous studies have identified surface soil nitrogen (TSN) loading and intrinsic groundwater vulnerability (Inv) as the main factors controlling groundwater TN pollution. However, in this study, based on 3245 shallow groundwater samples in the NCP, the multiple regression analysis results(R2=0.105, p<0.001) revealed that the TN was not mainly controlled by TSN and Inv. The lower prediction accuracy indicated the large data dispersion of TN, which might be affected by nitrogen attenuation or accumulation. Thus, the NCP was divided into balance, attenuation, and accumulation zones according to the regression equation. The attenuation zone was mainly distributed in the inter-fan and fan edge parts of the pre-mountain alluvial floodplain, as well as the west and south of the runoff area, while the accumulation zone was mainly distributed in the top part of the pre-mountain alluvial floodplain and the east of discharge area. Multi-indicators comparative analysis showed that compared to the balance (Eh= 76.2mV) and accumulation (Eh=126.7mV) zones, the attenuation zone has a stronger reducing environment (Eh=30.8mV) favorable to denitrification, which can reduce the TN pollution (0.49mg/L) caused by surface nitrogen input and consume more electron donors. Conversely, the stronger oxidizing environment in the accumulation zone limited denitrification, resulting in higher TN concentrations (19.14mg/L) in the aquifers under the same TSN and Inv conditions as the other two zones. The standardized effects and significance on each path of the structural equation model (SEMs) fully confirmed the reliability of the above zonal analysis. Importantly, the feature importance (23.6%) of random forest and standardized effects (0.455, p<0.001) of SEMs showed that the Eh had the strongest influence on TN. Thus, the redox conditions of the aquifer, in addition to TSN and Inv, played a crucial role in controlling the TN pollution in the groundwater of a large region. The zoning work and the analysis of influencing factors are important to guide scientific prevention and control of groundwater nitrogen pollution.

Groundwater contamination apportionment in Beida-Bordj rural territory, northeast Algeria, using the nitrate pollution index (NPI) and groundwater pollution index (GPI)

Most of the time, too much use of artificial fertilizers in rural areas, especially nitrogen fertilizers used to make crops grow faster, causes nitrate levels in groundwater to rise. In the Beida-Bordj area in northeast Algeria, groundwater is the main source of water for agriculture and drinking. Because of this, the quality of groundwater is very important. Therefore, nitrogen pollution in groundwater, which is one of the most important environmental problems, needs to be looked into. Therefore, evaluating groundwater contamination to protect human health was the primary focus of this investigation. As a result, 111 samples were taken in May 2022 from wells in different parts of the study area for physical and chemical research. Therefore, the nitrogen pollution index (NPI) and the groundwater pollution index (GPI) have been used to measure how clean groundwater is. The results show that the groundwater in the area that was tested is alkaline. Based on their average values, the abundance of cations and anions is as follows: Ca2+ > Na+ > Mg2+ > K+; and SO42− > Cl− > HCO3− > NO3−, in that order. The estimated NPIs ranged from – 1 to 4.5, with 0.92 being the average. Overall, the NPI data showed that only 25% of groundwater samples were clean, and the other 75% were dirty. The GPI readings ranged from 6.3 to 0.4, with an average of 1.2. This means that only 54% of the samples of groundwater from the study area were safe to drink. According to analyses of water quality parameters like SAR, SSP, RSC, PI, MH, and KR, the majority of groundwater samples from the research region are suitable for irrigation, because they fall into the "good" or "suitable" quality classes. This study's findings offer some intriguing suggestions for lowering pollution levels and bolstering groundwater management strategies for the future.

Nitrogen removal performance and bacterial flora analysis of a nitrification‑sulphur autotrophic denitrification coupled with permeable reaction wall (NSAD+PRW) treating rare earth mine groundwater

Rare earth elements (REEs) are strategic resources for the development of modern society. However, their mining can cause nitrogen pollution in groundwater. In this study, a nitrification-sulphur autotrophic denitrification-enhanced permeable reaction wall (NSAD+PRW) was constructed to treat groundwater in rare earth mines and the feasibility of its engineering application was demonstrated. The results show that a maximum removal rate of 100 % was achieved for NO3−-N. The average removal content of NH4+-N was 27.15 mg/L and the highest TN removal content of total nitrogen (TN) reached 145.49 mg/L. The removal rates of most REEs exceeded 99.88 %. The composite filler provides a good environment for microorganisms, which also produce extracellular polymeric substances (EPS) to protect themselves during nitrogen removal. In addition, some nitrifying bacteria (Rhodanobacter and Nitrospira) and denitrifying bacteria (Thiobacillus, Sulfurovum, and Ferritrophicum) with high adaptability to rare earth mine groundwater were found. In conclusion, the results of this study provide an important reference for nitrogen removal from groundwater in rare earth mines with high engineering application significance.

Nitrate Migration and Transformation in Low Permeability Sediments: Laboratory Experiments and Modeling

The excessive application of fertilizers and the uncontrolled discharge of untreated wastewater on land contribute to the nitrogen pollution of groundwater. Low permeability sediment plays an important role in slowing down the migration of nitrate in groundwater systems. This study focuses on the mechanisms that affect nitrate transport in the low permeability layer. A soil column experiment was conducted and an MT3D transport model (a modular three-dimensional multispecies transport model) of nitrate was proposed. The results show that a low permeation layer could act as a strong barrier to nitrate migration and transportation, as demonstrated by the column soil experiment. The denitrification rates were controlled by many other factors, including nitrate concentration, flow rate, pH values and sediment particle characteristics. This indicated that denitrification is the most important factor in the attenuation of nitrate in low permeability sediments. This study clarified the law of migration and transformation of nitrate in low permeability soils and the factors that influence these processes. The results not only provide a basis for large-scale nitrate simulations, but also provide a reference for nitrate treatment in low permeability layers.

Watershed-Scale Shallow Groundwater Anthropogenic Nitrate Source, Loading, and Contamination Assessment in a Typical Wheat Production Region: Case Study in Yiluo River Watershed, Middle of China

Nitrate pollution in groundwater has become a global concern for agriculture and regional ecology. However, tracing the spatiotemporal groundwater nitrate pollution sources, calculating the total nitrogen loading, and assessing contamination at the watershed scale have not been well documented. In this study, 20 groundwater samplings from 2020 to 2021 (in dry and wet seasons) on the Yiluo River watershed in middle China were collected. Tracing groundwater nitrate pollution sources, calculating total nitrogen loading, and assessing contamination using dual isotopes (18ONO3 and 15NNO3), conservation of mass, and the nitrate pollution index (NPI), respectively. The results indicated that there were three nitrate sources in groundwater: (1) manure and sewage waste input (MSWI), (2) sediment nitrogen input (SNI), and (3) agriculture chemical fertilizer input (ACFI) in the Yiluo River watershed. ACFI and SNI were the main groundwater nitrogen pollution sources. The average nitrogen loading percentages of ACFI, SNI, and MSWI in the whole watershed were 94.7%, 4.34%, and 0.96%, respectively. The total nitrogen loading in the Yiluo River watershed was 7,256,835.99 kg/year, 4,084,870.09 kg/year in downstream areas, 2,121,938.93 kg/year in midstream areas, and 1,050,026.95 kg/year in upstream areas. Sixty percent of groundwater in the Yiluo River watershed has been polluted by nitrate. Nitrate pollution in midstream areas is more severe. Nitrite pollution was more serious in the wet season than in the dry season. The results of this study can provide useful information for watershed-scale groundwater nitrogen pollution control and treatment.

Identification of nitrogen pollution sources and transport transformation processes in groundwater of different landforms using C, H, N, and O isotope techniques: an example from the lower Weihe River.

Nitrogen pollution in groundwater is an environmental issue of global concern. Identifying nitrogen pollution sources and determining migration and transformation processes are the major ways to prevent and control nitrogen pollution in the groundwater on a regional scale. In this study, groundwater in the lower Wei River was investigated by combining multi-isotope tracing techniques with the SIAR hybrid model (source resolution) to trace the nitrate sources and their contribution rate to nitrogen pollution in groundwater of different geomorphological units, considering types of geomorphology as the units. The multi-isotope tracing technique allows dynamic analysis of nitrate sources, and the combination of this technology can improve the accuracy of nitrogen source traceability. The results indicated that the pH of the water bodies in the study area ranged from 6.83 to 8.01, which is neutral and weakly alkaline. The nitrogen pollution was mainly due to nitrates. The significant factors affecting nitrogen migration in groundwater are the geomorphological type, the chemical characteristics of the groundwater, and the age of the groundwater. Nitrogen migration and transformation processes in the study area were dominated by nitrification, and sources of nitrate pollution were mainly animal manure and domestic sewage (32.6%), followed by atmospheric deposition (26.8%), soil nitrogen (20.9%), and chemical fertilizer (19.7%). The main sources of nitrate in groundwater from river flats, alluvial plains, and loess tableland were animal manure and domestic sewage (43.7%), animal manure and domestic sewage (59.1%), and atmospheric deposition (55.5%), respectively. The result is mainly related to the different structural characteristics of various geomorphic units and the intensity of human activities. This study can provide a theoretical basis for the relevant agencies to develop plans to combat groundwater pollution.

Community characteristics of autotrophic CO2-fixing bacteria in karst wetland groundwaters with different nitrogen levels.

Karst wetlands are important in the global carbon and nitrogen cycles as well as in security of water resources. Huixian wetland (Guilin) is the largest natural karst wetland in China. In recent years, groundwater nitrogen pollution has increasingly affected the wetland ecosystem integrity due to anthropogenic activities. In this study, it was hypothesized that autotrophic microbial diversity is impacted with the advent of pollution, adversely affecting autotrophs in the carbon and nitrogen cycles. Autotrophic microbes have important roles in abating groundwater nitrogen pollution. Thus, it is of great significance to study the characteristics of autotrophic bacterial communities and their responses to environmental parameters in nitrogen-polluted karst groundwaters. The abundances of the Calvin–Benson cycle functional genes cbbL and cbbM as well as the autotrophic CO2-fixing bacterial communities were characterized in the karst groundwater samples with different levels of nitrogen pollution. The cbbM gene was generally more abundant than the cbbL gene in the groundwater samples. The cbbL gene abundance was significantly positively correlated with dissolved inorganic nitrogen (DIN) concentration (P < 0.01). In the autotrophic CO2-fixing bacterial communities, Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria of the phylum Proteobacteria were predominant. At the genus level, Rubrivivax and Methylibium were the dominant cbbL gene containing genera, while Halothiobacillus and Endothiovibrio were the dominant genera for the cbbM gene. The abundance of autotrophic CO2-fixing bacterial communities increased but their diversity decreased with the inflow of nitrogen into the karst groundwater system. The community structure of autotrophic CO2-fixing bacteria in the groundwaters was also significantly affected by environmental factors such as the carbonic anhydrase (CA) activity, dissolved inorganic carbon (DIC) concentration, temperature, and oxidation-reduction potential (ORP). Nitrogen inflow significantly changed the characteristics of autotrophic CO2-fixing bacterial communities in the karst groundwaters. Some key genera such as Nitrosospira and Thiobacillus were clearly abundant in the karst groundwaters with high nitrogen levels. Their respective roles in nitrification and denitrification impact nitrogen removal in this ecosystem. The findings in this study provide an important reference for biological abatement of nitrogen pollution in the karst groundwater system.

Unveiling microbial community and function involved in anammox in paddy vadose under groundwater irrigation

The extensive application of nitrogen fertilizer in intensive irrigation areas poses a potential threat to groundwater. Given that the vadose zone acts as a buffer zone for the underground entry of surface pollutants, an in-depth understanding of its microbial community structure and function was crucial for controlling groundwater nitrogen pollution. In this study, soil samples from paddy vadose under groundwater irrigation with different depths (G1: 6.8 m, G2: 13.7 m, G3: 15.6 m, and G4: 17.8 m) were collected to unravel the differences in microbial community structure and function at different vadose depths (0–250 cm), as well as their relationship with soil properties. Results showed some differences among soil physicochemical factors under groundwater irrigation with different depths and that some electron acceptors were more abundant than others under deep groundwater irrigation (G2–G4). Remarkable differences in microbial communities under shallow- and deep-groundwater irrigation were found. The high abundances of anammox bacteria Candidatus_Brocadia in G2 and G3 indicated that deep groundwater irrigation was beneficial to its enrichment. Iron-reducing bacteria Anaeromyxobacter and sulfate-reducing bacteria Desulfovibrio were widely distributed in vadose zone and possessed the potential for anammox coupled with Fe(III)/sulfate reduction. Norank_f_Gemmatimonadaceae had nitrate- and vanadium-reducing abilities and could participate in anammox in vadose zone. Dissimilatory nitrate reduction to ammonia (DNRA) bacteria Geobacter facilitated Fe(II)-driven DNRA and thus provided electron donors and acceptors to anammox bacteria. Soil nutrients and electron donors/acceptors played important roles in shaping microbial community structure at phylum and genus levels. Microorganisms in vadose zone under groundwater irrigation showed good material/energy metabolism levels. Deep groundwater irrigation was conducive to the occurrence of anammox coupled with multi-electron acceptors. Our findings highlight the importance of understanding the structure and function of microbial communities in paddy vadose under groundwater irrigation and reveal the potential role of indigenous microorganisms in in-situ nitrogen removal.

Assessment of Ammonium–N and Nitrate–N Contamination of Shallow Groundwater in a Complex Agricultural Region, Central Western Taiwan

The characteristics of nitrogen contamination of shallow groundwater were evaluated through current status analysis and trend detection of ammonium–N and nitrate–N concentrations under various cropping patterns to assess the effectiveness of rational fertilization in the Choushui River alluvial fan, central Western Taiwan. The influence of cropping patterns on both ammonium–N and nitrate–N contamination associated with redox conditions/dissolved oxygen (DO) in shallow groundwater was also discussed in this study. The analysis revealed that shallow groundwater beneath double rice cropping and rotational cropping regions is still characterized by high ammonium–N concentration despite rational fertilization promotion. However, very few monitoring wells showed an upward trend of ammonium–N/nitrate–N concentrations, indicating that shallow groundwater is not further deteriorated by nitrogen pollution in most parts of the study area. Therefore, the remediation of nitrogen contaminated groundwater will be a long-term process and more effort must be invested. Moreover, the strict redox conditions defined by a single DO threshold value may not account for groundwater nitrogen pollution in the study area. It is difficult to determine the redox conditions and predominant nitrogen pollution patterns of shallow groundwater purely from cropping patterns. Instead, contamination may have resulted from an integrated process governed by several other factors. Tracing the potential sources of nitrogen pollution and establishing a more integral monitoring network should be implemented to formulate a more comprehensive nitrogen pollution control strategy in this area.

Health risk assessment of groundwater nitrogen pollution in Yinchuan plain

High nitrogen concentration of groundwater poses a threat to human health. This study evaluated the potential health risk of nitrogen pollution in Yinchuan plain by geostatistical analysis and triangular stochastic model considering different land use types, and identified the uncertainties of the parameters. 163 samples were collected from groundwater wells in different land use types. The results show that the concentration of NO3−-N ranges from 0.059 to 450 mg/L, with an average of 22.439 mg/L. Approximately 32% of the samples exceed Grade III threshold (20 mg/L of N). The concentration of NH4+-N ranges from 0.011 to 11 mg/L, with an average of 0.456 mg/L. The concentration of NO2−-N ranges from 0.003 to 9.09 mg/L The NO3−-N and NH4+-N concentration in the groundwater of the unutilized land use is significantly lowest among all the land types. The concentration of nitrogen is highest in farmland use. The ranking of non-carcinogenic risk under different land types for infants, children, adult males and females is: farmland use > residential land use> unutilized land use. The non-carcinogenic risk value of farmland use is three times as much as that of the residential land use. Drinking groundwater can be potentially harmful to human health, and nitrogen pollutants pose an even greater threat to infant. At the same time, considering the impact of different land use types on groundwater would avoid overestimating or underestimating regional risk value. Triangular stochastic model is more sensitive to data changes and can reduce uncertainty. The contribution rate of nitrate concentration to risk is more than 83%, indicating that random sampling is needed to improve the reliability of evaluation results. The research results of this study will provide a new way to solve the uncertainty in groundwater security management.

Temporal and Spatial Variation Characteristics and Driving Factors of Nitrogen of Shallow Groundwater in Hetao Irrigation District

As an important food production area in the north of China, the Hetao plain is extremely vulnerable to nitrate pollution caused by agricultural production activities and additional factors. Thus, it is of great significance for the environmental protection and rational use of groundwater to detect the current situation of groundwater nitrate pollution, temporal and spatial evolution characteristics, and main influencing factors in the Hetao irrigation district. We selected the Wualte irrigation area as the study area, and the characteristics of temporal and spatial changes in groundwater nitrate concentration and the main influencing factors in this area were explored. We used statistical analysis to analyze the nitrogen content of groundwater in the study area, and the Piper three-line diagram was used to explore the characteristics of chemical composition and evolution; furthermore, we used ion ratio and correlation analysis methods to explore the source of NO3--N in groundwater. The results showed that NO3--N was the main existing form of nitrogen in the Wulate irrigation area, and its concentration varied from 0.01 to 60.00 mg·L-1, with an exceeding standard rate of 10.50%. In terms of time, the characteristic of time change was that the NO3--N concentration in August of groundwater was the highest (average 6.61 mg·L-1), followed by that in October (6.22 mg·L-1) and November (6.25 mg·L-1), and that in March (average value of 1.77 mg·L-1) was the lowest. With the influence of rainfall and irrigation, the NO3--N in the soil was infiltrated into the groundwater, showing the characteristic that wet season and concentrated irrigation periods were higher than those in other periods. Spatially, it appeared as southwest (8.87 mg·L-1)>northwest (4.25 mg·L-1)>east (0.89 mg·L-1), mainly due to the original geological conditions, land use, and domestic waste stacking. In addition, the concentration of NO3--N of groundwater in the study area was closely related to the depth of groundwater and redox conditions but was relatively less affected by the concentration of water chemical ions. Therefore, identifying the temporal and spatial distribution characteristics and main sources of groundwater nitrogen pollution can provide a scientific basis for scientific fertilization, groundwater nitrate pollution control, and water safety.

Analysis of urea nitrogen leaching under high-intensity rainfall using HYDRUS-1D

The leaching of nitrogen in farmland could lead to groundwater nitrogen pollution, while precipitation is the driving factor. Nevertheless, the influences of extreme precipitation on soil organic nitrogen leaching, such as urea, is not clear. In this study, we used soil column experimental data to construct a nitrogen migration and transformation model. The constructed model was applied to unsaturated zones in farmlands in the Dagu River Basin of China. Three rainfall scenarios, namely rainstorms, heavy rainstorms, and extreme rainstorms, were simulated to analyze the variation in urea nitrogen leaching to groundwater. As the rainfall intensity increased, the quantity of urea nitrogen leaching increased from 0.04% to 18.09%. The cumulative urea nitrogen leaching flux is related to the initial soil moisture content. The time interval between fertilization inversely influenced the urea nitrogen leaching flux. Applying urea-based fertilizers shortly before high-intensity rainfall significantly increases the soil-crop system urea leaching loss and decreases crop nitrogen utilization. These findings imply that urea leaching under extreme rainstorms should not be ignored, and based on weather forecasting, fertilizer application in farmland should be avoided before high-intensity rainfall.

Groundwater hydrogeochemical characterization and quality assessment based on integrated weight matter-element extension analysis in Ningxia, upper Yellow River, northwest China

• Groundwater chemistry and quality were analyzed in the Ningxia region. • NO 3 − , NH 4 + and NO 2 − are related to human pollution. • Water quality was assessed using an integrated weight matter-element extension model. • High health risks of F − , NO 3 − , NH 4 + , and NO 2 − were found, especially for children. Assessing groundwater quality and identifying pollutant risks are essential for managing groundwater resources in arid and semi-arid regions. An analysis of the hydrochemical characteristics, controlling factors, and pollutant characteristics of the groundwater in the upper Yellow River region is presented to understand the overall groundwater quality and identify the risk to human health in this region. Traditional hydrochemical analysis methods, integrated weight matter-element extension analysis, and health risk models were used in this study. A total of 210 groundwater samples were collected from the wells for physical and chemical analyses. Results showed that groundwater in the study area was weakly alkaline, with the predominant hydrochemical types of groundwater being SO 4 ·Cl-Ca·Mg, SO 4 ·Cl-Na, and HCO 3 -Ca·Mg. The hydrochemical composition of the groundwater is controlled by rock dominance, evaporation dominance, and cation exchange. Nitrogen pollution of groundwater is caused by industrial activities and fertilizers; fluorine pollution is caused by fluorite dissolution. To reasonably evaluate the groundwater quality, the groundwater evaluation method based on an integrated weight matter-element extension analysis was optimized. The method effectively avoids defects of G1 and entropy weight methods in determining weights and thoroughly considers the fuzziness and uncertainty of the water quality grade. The overall quality groundwater assessment results showed that 96.19% of the overall water quality in the study area was good and could be used in many ways. Land irrigated with groundwater is mainly exposed to salinity hazards. Health risk assessment results showed that 57% and 21% of the samples posed unacceptable health risks to children and adults, respectively. Therefore, more attention should be paid to real-time, continual monitoring of pollutants in groundwater management. The results help decision makers make wise decisions on identifying groundwater purposes to avoid further water quality deterioration and increase in pollutants, ensuring the cleanliness of groundwater resources in the upper Yellow River..

Spatial and Temporal Variations of Nitrogen and Phosphorus in Surface Water and Groundwater of Mudong River Watershed in Huixian Karst Wetland, Southwest China

When considering the factors affecting the spatial and temporal variation of nitrogen and phosphorus in karst watersheds, the unique karst hydrogeology as an internal influencing factor cannot be ignored, as well as natural factors such as meteorological hydrology and external factors such as human activities. A watershed-scale field investigation was completed to statistically analyze spatial and temporal dynamics of nitrogen and phosphorus through the regular monitoring and collection of surface water and shallow groundwater in the agricultural-dominated Mudong River watershed in the Huixian Karst Wetland over one year (May 2020 to April 2021). Our research found that non-point source pollution of nitrogen (84.5% of 239 samples TN &gt; 1.0 mg/L) was more serious than phosphorus (7.5% of 239 samples TP &gt; 0.2 mg/L) in the study area, and shallow groundwater nitrogen pollution (98.3% of 118 samples TN &gt; 1.0 mg/L) was more serious than surface water (68.6% of 121 samples TN &gt; 1.0 mg/L). In the three regions with different hydrodynamic features, the TN concentration was higher and dominated by NO3−-N in the river in the northern recharge area, while the concentrations of TN and TP were the highest in shallow groundwater wells in the central wetland core area and increased along the surface water flow direction in the western discharge area. This research will help improve the knowledge about the influence of karst hydrodynamic features on the spatial patterns of nitrogen and phosphorus in water, paying attention to the quality protection and security of water in karst areas with a fragile water ecological environment.