Main Sources Of Nitrate Research Articles

Distribution and Source Analysis of Nitrate in Surface Waters of China

The nitrate pollution of surface water has attracted worldwide attention, and it is not optimistic in China. To identify the distribution, sources, and transformation mechanisms of nitrate in China's surface waters, the nitrate data of 71 major rivers from 7 regions were systematically collected. The spatial distribution of nitrate concentrations in surface water was analyzed, and the main nitrate pollution sources were revealed based on nitrogen and oxygen isotopes of nitrate. The results show that approximately 7.83% of rivers in China exceeded the Chinese drinking water standard for nitrate (45 mg·L-1). The concentrations of nitrate in Mudanjiang, Haihe River, and the estuary of Yangtze River was even more than 90 mg·L-1, which indicates a serious pollution phenomenon. The isotopic compositions of surface water in China ranged from -23.5‰ to 26.99‰ for δ15N-NO3 and from -12.7‰ to 83.5‰ for δ18O-NO3. The main sources of nitrate are domestic sewage in Northeast, Central, and East China, while those are inorganic fertilizers and domestic sewage in Southwest and South China. The nitrate sources of surface water in Northwest and North China are complex, mainly from domestic sewage, inorganic fertilizer, and nitration of soil organic matter. Through correlation analysis, it is found that the nitrate concentrations of surface water have a positive relationship with population, wastewater discharge, agricultural nitrogen fertilizer application, and GDP per capita in China. It is urgent to solve the problem of pollution and prevent the further pollution of China's surface waters. The new "10-Point Water Plan" issued by the Chinese government solved the previous problems, but it will take decades to control and repair the polluted surface waters. In Northwest, North China, Southwest, and South China, not only the scale of sewage treatment plants in cities and counties should be increased but also the use of agricultural fertilizers should be controlled and managed by the government. Northeast, Central, and East China need to further control point source pollution and reduce the discharge of industrial wastewater and domestic sewage into rivers.

Origin, implications and management strategies for nitrate pollution in surface and ground waters of Anthemountas basin based on a δ15N-NO3− and δ18O-NO3− isotope approach

Nitrate pollution of surface and groundwater resources is a major worldwide environmental problem. In this study nitrogen isotopes of water, soil, fertilizer and manure were analyzed to determine the pollution sources of nitrate in the groundwater and surface waters of Anthemountas basin. The SIAR model and multivariate statistical analysis were used to determine and quantify the contribution of different NO3̄ sources in groundwater and surface water. Additionally, a detailed literature overview was carried out to identify the origin of nitrate pollution in surface and ground waters based on ΝΟ3− isotopes. The Piper diagram identified the dominant water types as Mg-Ca-HCO3 and Ca-Mg-HCO3. Nitrate concentrations reached 162.0 mg/L in groundwater and 39.0 mg/L in surface waters. The main source of nitrate in groundwater was mainly nitrified ammonium-based synthetic urea and less nitrate-based synthetic fertilizers. The correlation of SIAR results with other trace elements revealed a negative correlation between hexavalent chromium and a) nitrate-based synthetic fertilizers, and b) nitrification of urea synthetic fertilizers. However, a positive correlation was observed between hexavalent chromium and anthropogenic organic matter. The literature overview provided the basis to design a novel management protocol for nitrate pollution that includes three steps: a) fundamental research, b) management tools, c) monitoring and preservation actions. However, an integrated management protocol for nitrate pollution requires a deeper understanding of the hydro-system and the full participation of local farmers and stakeholders.

The Importance of Incorporating Denitrification in the Assessment of Groundwater Vulnerability

Human activities are deeply connected with groundwater reservoirs, so protecting them from pollution has become a priority in many regions of the world. Nitrate is considered the main groundwater pollutant since it is directly linked to many human activities. Agricultural activities and domestic wastewater have been identified as the main sources of nitrate in groundwater. Nevertheless, there are some natural processes that can mitigate nitrate pollution. Together with dilution processes, the degradation of nitrate through denitrification has been acknowledge as a process that can potentially reduce nitrogen loads, in both deep and shallow aquifers. Usually these processes were not properly quantified in vulnerability assessment methods, until the introduction of LOS indices. In this study, the application of the LOS indices on 4 agricultural areas is discussed, highlighting how the LOS indices can identify portions of the landscape with higher potential denitrification and how they directly enhance the groundwater vulnerability assessment. Previous studies have shown that LOS indices are a valuable tool for proper vulnerability assessment to nitrate, however they need to be coupled with other parameters that also describe nitrate behavior in groundwater. The SINTACS-SVN and DRASTIC-PA methods that include the LOS indices, were applied for the first time in the Epanomi coastal area to evaluate the reliably of the assessment and, despite the different classes range and the weights applied, similar groundwater vulnerability assessment maps were obtained. The nitrate vulnerability maps were comparable with the observed nitrate concentrations and were found to be highly comparable with original LOS maps. Nevertheless, it should be kept in mind that vulnerability methods are only screening tools and groundwater quality observations are pivotal information for environmental management.

Diffuse nitrogen pollution in a forest-dominated watershed: Source, transport and removal

Along with gradually controlled point source discharges, diffuse nitrogen pollution has become an important cause of eutrophication. Diffuse nitrogen export from forests is an inherent characteristic, and increasing human disturbance aggravates this process. To control diffuse nitrogen pollution effectively, it is of great significance to determine the source, transport path and removal method of diffuse nitrogen export in forest-dominated watersheds. Long-term variations in the nitrogen concentrations indicate that nitrate and dissolved organic nitrogen (DON) are the main diffuse nitrogen export forms in the Buerhatong River watershed (BRW). Thus, the Soil and Water Assessment Tool (SWAT) hydrological model is conducted to simulate diffuse nitrate and DON export. First, main source of diffuse nitrate and DON export is determined by the fitting results of nitrogen contribution and forest/farmland cover. Then, the relationship between nitrogen export from forest/farmland and main hydrologic streamflow is explored. Finally, the role of wetlands in the diffuse nitrogen load removal is analyzed, and sensitivity results by Monte Carlo simulation of nitrogen export from wetlands, forests and farmlands are compared. The average annual diffuse nitrate and DON export values are 1.19 and 0.43 kg ha−1 yr−1, respectively, from 2014 to 2017. When forest cover and farmland cover are higher than 70% and 5%, farmlands and forests are the main sources of diffuse nitrate and DON export, respectively. Lateral flow is the dominant transport path of DON export from forests and nitrate export from farmlands, with correlation coefficient of 0.73 and proportion of 98%, respectively. Wetlands have the ability to reduce the diffuse nitrogen load, and their efficiency is related to the relative geographic location. The in situ experimental results show that the nitrate and DON removal rates by wetlands are 24.46% and 11.71%, respectively. The sensitivity of forest and wetland for diffuse nitrate and DON load removal is 0.31, −0.12 and 0.2, −0.21, respectively. Returning forests to wetlands is a good method for diffuse nitrogen removal with food production taken into consideration.

Agricultural and urban delivered nitrate pollution input to Mediterranean temporary freshwaters

Nitrate dual stable isotopes (δ15NNO3 and δ18ONO3) have proven to be a powerful technique to trace nitrate sources and transformations in freshwater systems worldwide. However, most studies have focused on perennial systems, and less is known about intermittent ones. The impacts of intensive agricultural practices and wastewaters in Doñana (SW Spain), an iconic Mediterranean temporary wetland protected as a UNESCO World Heritage Site, were quantified using stable isotope mixing models in a Bayesian framework under different denitrification scenarios. We aimed to identify the main nitrate sources and transformation processes in surface waters of interconnected temporary streams, ponds and marshes, and link them with the main human pressures in the watershed (e.g. intensive fruticulture, urban wastewaters). We measured nitrate (NO3−) concentrations and stable isotopes (δ15NNO3 and δ18ONO3) in water samples collected during different periods over two years (2015–2016). Most sites showed coupled increases of nitrate isotopic values (δ15NNO3 and δ18ONO3), which were higher than reference values of any possible sources (e.g. synthetic/organic fertilizers and wastewaters), indicating fractionations typical of denitrification processes. The main nitrate sources to the watershed were linked to agricultural practices and the use of synthetic fertilizers, but further investigations in other transformation processes that occur simultaneously should be evaluated. These results highlight an important nitrate removal capacity (i.e. denitrification) of the system, which may positively contribute to natural resilience against eutrophication. However, given the high intra and interannual hydrological fluctuations of Mediterranean aquatic systems, future studies on the relative contribution of nitrate sources and processes should increase spatio-temporal resolution of water sampling, and include measurements of groundwater and interstitial water as well as surface water.

Effects of valley reshaping and damming on surface and groundwater nitrate on the Chinese Loess Plateau

Remote rural areas, which are generally the source regions for drinking water in small towns in China, experience nitrate pollution due to intensive agricultural activities. However, little is known about the nitrate sources of these waterbodies and how human activities affect the water quality in these rural areas. Here, a pair of rural watersheds on the Chinese Loess Plateau (CLP) were selected for study. One of the watersheds, which provides drinking water for local villages and a nearby small town, has experienced valley reshaping, damming and agricultural land consolidation, whereas the other one was not affected by human activities. Water chemistry monitoring and environmental isotope analysis were combined to identify the nitrate sources of the surface and groundwater in the dammed and natural watersheds and to assess the effects of landform transformation and agricultural and living activities on water quality. The results demonstrated that nitrate concentrations in both watersheds were lower than 10 mg L−1. However, the seasonal nitrate contents in the surface (0.42–0.52 mg L−1) and groundwater (1.86–3.65 mg L−1) of the dammed watershed were significantly lower than those of the natural watershed (surface water: 0.92–2.07 mg L−1; groundwater: 2.73–8.21 mg L−1). Isotope analysis demonstrated that nitrate fertilizer, soil ammonium, and manure and septic waste were the main sources of nitrate in the surface water of the two watersheds, while the nitrate in the groundwater mainly originated from the nitrification of soil ammonium and manure and septic waste. Compared to the natural watershed, the dammed watershed had increased inputs of agricultural fertilizers in the surface water. However, there was no significant difference in the groundwater nitrate sources between the two watersheds. The results also demonstrated that denitrification and dissimilatory nitrate reduction to ammonium (DNRA) probably occurred in the groundwater during the wet season, while nitrification likely occurred during the dry and snowmelt seasons. Hierarchical cluster analysis (HCA) showed that there were strong interactions between the surface runoff and the reservoir water in the dammed watershed, while in the natural watershed, streamflow had a close relationship with the groundwater. We concluded that valley reshaping, damming and agricultural land consolidation increased the nitrate input from mineral fertilizers in the watershed, but decreased the nitrate concentrations in the surface water through changing the transformation and transportation processes of nitrate. However, these activities had little influence on the groundwater, while the time lag between land consolidation and the groundwater quality response should be considered in the future. This study aids in developing strategies for the comprehensive management of watershed and drinking water sources in rural regions.

Association of daily nitrate consumption with blood pressure and other risk factors for cardiovascular diseases in a representative UK population

AbstractDietary inorganic nitrate has been shown to lower blood pressure (BP) and improve endothelial function(1). The main sources of dietary nitrate are vegetables (root and green leafy varieties) as well as drinking water but data available on dietary analysis software on nitrate levels in vegetables and vegetable-based foods is very limited. To date, very few studies have investigated the relationship between the level of consumption of dietary nitrate on BP and other cardiovascular disease (CVD) risk factors in a representative UK population. The aim of the study was to address this knowledge gap using data from the National Diet and Nutrition Survey (NDNS) years 1–8, a cross-sectional study conducted in 3339 men and women aged 19–64 y between 2008/09–2011/12. A comprehensive database was first developed to evaluate the nitrate and nitrite levels in vegetables, cured meats and composite dishes to more accurately estimate the dietary nitrate intakes of the NDNS participants. The population was then classified into quartiles of daily nitrate intake, with quartile 1 (Q1: 26–106 mg/d) and quartile 4 (Q4: 183–559 mg/d) representing diets with the lowest and highest intakes, respectively. ANCOVA analysis was performed to determine the relationship between the level of daily nitrate intake with available data on biomarkers of CVD risk (including BP (systolic, diastolic and pulse pressure), lipid profile, (total, high-density lipoprotein and low-density lipoprotein (LDL-C) cholesterol), C-reactive protein, anthropometric measures (body mass index and waist to hip ratio) and glycaemic control (glucose and glycated haemoglobin). There were significant differences in systolic (P-trend = 0.008) and diastolic (P-trend = 0.025) BP across increasing quartiles of dietary nitrate intake, with BP significantly lower in Q3 than all other quartiles. Pulse pressure (calculated as systolic–diastolic BP) was also found to be significantly different across quartiles (P-trend = 0.001), with diets of participants in Q3 and Q4 being associated with significantly lower pulse pressures than those in Q1 (P-Q1 vs. Q3 = 0.005, P-Q1 vs. Q4 = 0.007). All of the other CVD risk markers were not different between quartile groups. Our preliminary results suggest that the level of dietary nitrate intake may be significantly associated with BP, a key independent CVD risk factor. There is an urgent need to more accurately estimate the dietary nitrate intake in the UK population and to determine whether the source of dietary nitrate (vegetables vs cured meats) impacts on the significant relationship with BP.

Tracing nitrate sources with dual isotopes and hydrochemical characteristics during wet season in Lake Caohai, Guizhou Province

为明确草海湖水及其入湖河流硝酸盐污染的主要来源，定量分析各来源的贡献率，对草海湖水与入湖河流水化学特征和水体硝酸盐的氮氧同位素组成进行了系统研究.通过对草海湖水、河水、井水丰水期水体理化参数和同位素分析发现：湖水的NO₃^-/Cl^-比值和Cl^-浓度表明其主要受牲畜粪便和城镇污水输入的影响，而河水与井水则受农业活动和城镇污水的共同影响.δD-water与δ¹⁸O-water显示草海水体主要源于大气降水，并有较强的蒸发作用.湖水δ¹⁵N-NO₃^-和δ¹⁸O-NO₃^-值分别为-5.56‰~11.30‰和0.02‰~25.40‰，较河水偏负而较井水偏正.稳定同位素混合模型（SIAR）计算结果表明草海湖水及其入湖河流硝酸盐主要源于化肥、土壤有机氮、牲畜粪便相关的农业活动，其贡献率在50%以上；城镇污水贡献率在22%左右；大气降水的贡献主要体现在湖水中.;The nitrogen and oxygen isotopic composition and hydrochemical characteristics of water were studied systematically to determine the main sources of nitrate in Lake Caohai and its rivers, and the contributions of each source were quantitatively analyzed. The ratio of NO₃^-/Cl^- to Cl^- in the lake water showed that the lake was mainly affected by livestock manure and urban sewage in wet season, while the ratio in rivers and wells indicated that rivers were affected by agricultural activities and urban sewage. The analysis of δD-water and δ¹⁸O-water showed that atmospheric precipitation was the main source of water in Lake Caohai and its rivers, with a strong evaporation. The value of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- in lake water were -5.56‰-11.30‰ and 0.02‰-25.40‰ respectively, which were more negative than the rivers and more positive than the wells. The results of SIAR model showed that nitrate in Lake Caohai and its rivers mainly came from agricultural activities related to fertilizer, soil organic N and livestock manure, and its contributions was more than 60%. The contribution rate of urban sewage was about 25%. The contribution of atmospheric precipitation was mainly reflected in the lake water.

Rainfall driven nitrate transport in agricultural karst surface river system: Insight from high resolution hydrochemistry and nitrate isotopes

High discharge associated with rainfall events causes rapid transport of nutrients to drainage systems in karst terrain. The objective of this study was to understand nitrate dynamics and transport mechanisms during rainfall events in a typical mixed land-use karst catchment in Southwest China. High frequency (hourly) water sampling, discharge and hydrochemistry sensor data at a 15-minute interval were collected from a karst agricultural surface river over four rainfall events, spanning 40 days at the commencement of the wet season. The results showed that high [NH4+–N] (up to 3.12 mg L−1) and [PO43−– P] (up to 1037 μg L−1) were observed in the first rainfall event of the wet season, after a long preceding dry period. Increased [NO3−–N] also occurred during the high discharge associated with early event flows, but the highest [NO3−–N] concentrations (up to 14.9 mg L−1) were observed later in the season during the highest discharge events. All minima for dual nitrate isotopes were found during high discharge conditions and increased through the event time series. This indicates that nitrogen accumulated in the catchment prior to the commencement of the wet season and was flushed by the early rainfall events into the drainage system. Analysis of [NO3−–N], δ15N and δ18O-NO3− values indicate that denitrification, not assimilation, resulted in the high nitrate isotope values before and after rainfall events. According to source apportionment, chemical fertilizer, soil organic nitrogen, and manure and sewage, which varied with discharge, were the main sources of nitrate during rainfall events. The high proportion of nitrate from soil organic nitrogen suggested that soil loss from peak clusters and lowland farmland during heavy rainfall events should be paid more attention in karst area.

Assessment of sources and transformation of nitrate in the alluvial-pluvial fan region of north China using a multi-isotope approach

A multi-isotope approach and mixing model were combined to identify spatial and seasonal variations of sources, and their proportional contribution to nitrate in the Hutuo River alluvial-pluvial fan region. The results showed that the NO3− concentration was significantly higher in the Hutuo River valley plain (178.7 mg/L) region than that in the upper and central pluvial fans of the Hutuo River (82.1 mg/L and 71.0 mg/L, respectively) and in the river (17.0 mg/L). Different land use types had no significant effect on the groundwater nitrate concentration. Based on a multi-isotope approach, we confirmed that the main sources of groundwater nitrate in different land use areas were domestic sewage and manure, followed by soil nitrogen, ammonia fertilizer, nitrate fertilizer and rainwater, and there were no significant spatial or seasonal variations. Combining δ15N–NO3-, δ18O–NO3- and δ37Cl results can increase the accuracy of traceability. Nitrification could be the most important nitrogen migration and transformation process, and denitrification did not significantly affected the isotopic composition of the nitrate. The SIAR model outputs revealed that the main nitrate pollution sources in groundwater and river water were domestic sewage and manure, accounting for 55.9%–61.0% and 22.6% (dry season), 50.3%–60.4% and 34.1% (transition season), 42.7%–47.6% and 35.6% (wet season 2016) and 45.9%–46.7% and 38.4% (wet season 2017), respectively. This work suggests that the random discharge and disposal of domestic sewage and manure should be the first target for control in order to prevent further nitrate contamination of the water environment.

Quantitative identification of nitrate sources in the surface runoff of three dominant forest types in subtropical China based on Bayesian model

Nitrate pollution is a global environmental issue. Forests play an important role in altering hydrological processes and purifying water pollutants in rainfall and runoff. The quantitative identification of nitrate concentration and sources in surface runoff is of great significance for watershed management and water environment improvement. In this study, water quality of surface runoff was monitored in three typical forest types in subtropical eastern China: Phyllostachys pubescens, Cunninghamia lanceolate, and Cyclobalanopsis glauca. Combined with hydrochemical analysis, we adopted the dual isotope approach (δ15N-NO3– and δ18O-NO3–) and Bayesian model (SIAR) to identify nitrate sources in forests that are subject to low anthropogenic disturbance. Results showed that the temporal variability of NO3-N concentrations was similar for all forest types, with higher values in periods of low rainfall and lower values in heavy rainfall periods. The NO3–-N concentration in runoff was much higher in C. glauca forests relative to P. pubescens and C. lanceolata. Both the Cl− concentrations and NO3–/Cl− molar ratio suggested the fertilizer inputs was the dominant source of nitrate in surface runoff. In agreement, δ15N-NO3– and δ18O-NO3– values inferred atmospheric deposition and chemical fertilizers to be the main sources of nitrate in all forest types. The Bayesian model outputs demonstrated that atmospheric deposition was the main source in the runoff in P. pubescens and C. lanceolate forests, contributing 28.83% and 35.08% to the total nitrate, respectively. In contrast, chemical fertilizers were identified as the main source in C. glauca forests, with NH4+ fertilizers and NO3– fertilizers accounting for 27.07% and 24.83%, respectively. Both chemical and isotopic analysis indicated that nitrification had little contribution to nitrate concentrations in runoff. Our results suggest that, even in forests with low anthropogenic disturbance, the application of fertilizer in surrounding agricultural regions should be effectively managed to minimize watershed nitrogen contamination.

A dual isotopic framework for identifying nitrate sources in surface runoff in a small agricultural watershed, northeast China

The northeast black soil region of China, an important area of grain production, currently faces serious diffuse pollution issues. In this study, a dual isotopic (δ 15 N and δ 18 O) framework was used to determine nitrate sources in runoff of a small agricultural watershed of this region. The results indicated that the ranges of δ 15 N and δ 18 O values of NO 3 -sources were significantly affected by natural geographic features and human activities. Nitrate in cropland runoff was mainly derived from soil, fertilizer and precipitation. Meanwhile, natural inputs, including those of soil and precipitation, represented the main nitrate sources in forest runoff. Domestic sewage, manure and precipitation contributed to most of the nitrate in runoff from villages. In addition, based on runoff export from different land uses and results from a stable isotope mixing model, soil and chemical fertilizer were identified as the main NO 3 -source in the whole watershed; suggesting that soil erosion could lead to serious diffuse pollution in the study area. Over all, this study not only provided suggestions to the optimal selection of Best Management Practices (BMPs) in Northeast China, but also could contribute to the identification and quantification of NO 3 -sources in agricultural watersheds of the black soil regions worldwide. • Five nitrate sources were identified in surface runoff using a dual isotopic framework. • Nitrate isotopic signatures of surface runoff were significantly different among three land use. • Both natural and anthropogenic sources contributed to nitrate pollution. • Soil and fertilizer were identified as the main nitrate source in the whole watershed.

Analysis of Nitrate Pollution Sources in the Rainy Season of the Lower Fenhe River

Through the application of multi-isotope tracing, IsoSource model calculation, and microbial detection, the source of nitrate pollution in the lower reaches of the Fenhe River was screened, and the contribution rate of each source was calculated. The results showed that the main forms of nitrogen-containing substances in the lower reaches of the Fenhe River are NO3--N and NH4+-N and that the NO3--N content in 77.8% of the samples exceed the national drinking water standard. The abundance of denitrifying microorganisms in Jishan, Hejin, and the Fenhe River into the Yellow River is high, and many dominant bacteria participate in denitrification, resulting in nitrogen fractionation. The contents of δ15N-NO3- and δ18O-NO3- in the isotopes ranged from 5.30‰ to 12.90‰ and from 1.3‰ to 1.8‰, respectively. Manure and sewage were the main sources of nitrate in the Linfen section, accounting for 68% of the total nitrate source. In the Xiangfen section, the main sources of nitrate in the river were manure and sewage, at 37.5%, and agricultural fertilizer, at 37%. Agrochemicals are the main sources of nitrate in the Hejin section of the river, with a contribution ratio of 49.3%.

Archaeal nitrification is a key driver of high nitrous oxide emissions from arctic peatlands

Bare peat surfaces created by frost action and wind erosion in permafrost peatlands have been shown to emit high amounts of nitrous oxide (N2O). With global warming, emissions of this highly potent greenhouse gas are expected to increase in Arctic permafrost peatlands. In natural unmanaged soils with low nitrogen deposition, such as Arctic soils, nitrification is the main source of nitrite and nitrate, and thus a key driver of N2O emissions. Here, we investigated nitrification, ammonia oxidizer populations and N2O production in vegetated and bare peat soils from four distant Arctic geographic locations. Through a combination of molecular analyses and group-specific inhibitor assays, we show that ammonia oxidation, the first step in nitrification, is mainly performed by ammonia-oxidizing archaea (AOA). All soils from different locations, including bare peat soils with high N2O emissions, harbored only two AOA phylotypes, including an organism closely related to Ca. Nitrosocosmicus spp. This indicates that high N2O emissions from these ecosystems are primarily fueled by nitrification mediated by very few archaeal species. To our knowledge, arctic peat soils in this study are the first natural environments where high N2O emissions have been linked to AOA. Any changes in archaeal nitrification induced by global warming will therefore impact on N2O emissions from the permafrost peatlands.

Groundwater nitrate contamination in an area using urban wastewaters for agricultural irrigation under arid climate condition, southeast of Tehran, Iran

Nitrate contamination of groundwater has been identified as a major environmental problem of the last few decades. In this study, stable isotope ratios (δ15N-NO3−, δ18O-NO3−, δ18O-H2O, and δD-H2O) of groundwater samples from the Varamin aquifer located southeast of Tehran, Iran, were analyzed in two periods, in order to determine the sources of nitrate pollution and nitrogen transformation processes where treated wastewater from the capital city is used for irrigation purpose. The approach in this study was based on isotopic fingerprinting of nitrate contaminant sources of groundwater using δ15N-NO3− and δ18O-NO3−. In addition, a hierarchical cluster analysis was applied to classify groundwater samples into three different hydrochemical facieses (groups 1–3). Furthermore, the stable isotope ratios of molecular water showed that most of the groundwater samples were plotted along the local meteoric line and distinguishable isotopic compositions from those of river water and treated wastewater. Stable isotopic analysis together with these hydrochemical classifications suggested that the main sources of nitrate in groundwater are chemical fertilizers and treated wastewater in the irrigated area (group 1) in addition to domestic wastes in the residential area (group 2) and chemical fertilizers and sewage water recharged by the Shour River (group 3). Denitrification is a major transformation process occurring at the western and southwestern parts of the aquifer. Seasonal variations in the nitrogen and oxygen isotopic compositions of nitrate were more obvious in the wet season compared to the dry season. This study is unique in that it describes how groundwater nitrate contamination is enhanced in the area using treated wastewater for irrigation purposes. The results of the current study are applicable for other areas with similar settings, since treated wastewaters are important water resources for water users in arid to semi-arid regions all over the world.

Quantifying nitrate sources in a large reservoir for drinking water by using stable isotopes and a Bayesian isotope mixing model.

Drinking water reservoirs are threatened globally by anthropogenic nitrogen pollution. Hydrochemistry and isotopes were analyzed to identify spatial and temporal varieties of main nitrate sources in a large drinking water reservoir in East China. The results showed that NO3- was the main nitrogen form in both the dry and wet seasons, but dissolved organic nitrogen (DON) was increased in the wet season. The δ15N-NO3- values (+ 1.3‰ to + 11.8‰) and δ18O-NO3- values (+ 2.5‰ to + 13.5‰), combined with principal component analysis (PCA), indicated that chemical fertilizer was the main nitrate source during the dry season, while chemical fertilizer, soil N, and sewage/manure were the main nitrate sources during the wet season in the Qiandao Lake area. And, the nitrate isotopes showed the significant nitrification and assimilation in the Qiandao Lake area. A Bayesian isotopic mixing model (Stable Isotope Analysis in R) was applied to the spatial and seasonal trends in the proportional contribution of four NO3- sources (chemical fertilizer (CF), soil nitrogen (SN), sewage and manure (SM), and atmospheric deposition (AD)) in the Qiandao Lake area. It was revealed that CF was the most important nitrate source in the dry season, accounting for 53.4% with 19.2% of SM and 18.9% of SN, while the contribution of SN increased in the wet season, accounting for 31.6%, followed by CF (30.8%) and then SM (24.2%). The main nitrate sources in the urban area, rural area, and central lake area were CF and SN, accounting for 66.1% in the urban area, 71.7% in the rural area, and 68.2% in the central lake area. Measures should be made to improve chemical fertilizer use efficiency and to reduce nitrogen loss in the Qiandao Lake area. Graphical abstract .

Spatial and temporal variability in hydrochemistry of a small-scale dolomite karst environment

Dolomite karst systems are less common and less exploited than limestone systems, but still they can significantly contribute to drinking water supply in many countries. The knowledge about spatial and temporal variations of hydrochemical parameters of such systems is much more limited than for limestone karst aquifers. In the present research, 40-year-long observations of hydrochemical parameters gathered from five springs and a pumping well in Waidhofen an der Ybbs (Austria) were used to show the variability of chemical water composition in a small-scale ( ~ 10 km2) dolomite karst system. Integration of classic geochemical methods and multivariate statistical analysis revealed that the hydrofacies correlate directly with the lithofacies (dolomite) in the study area. At the same time, measured concentrations of Ca2+, Mg2+, Na+, Cl−, and HCO3− allow for a classification of the springs in three groups based on their ionic ratios. This classification highlights the spatial and temporal variability that can be encountered in dolomite karst systems, even on small spatial scales, that are relevant for water suppliers. Moreover, temporal observations of hydrochemical parameters show increasing trends of nitrate concentrations in all sampling points, though with different rates. The analysis of the stable isotopes δ15N–NO3 and δ18O–NO3 revealed that nitrification processes in forest floors and mineral soils are the main source of nitrate in most locations investigated. The findings of the present study highlight the necessity of a detailed temporal and spatial distributed monitoring to support water resources management in dolomitic karst aquifers.

Isotopic evidence for the shift of nitrate sources and active biological transformation on the western coast of Guangdong Province, South China

Dual isotope nitrate (NO3−) analysis was performed on the western coast of Guangdong Province to investigate seasonal changes in the main nitrate sources and their biogeochemical processing, which are due to the rapid development of the local economy. In the nearshore area, significant seasonal variations of nitrate sources occurred. The dominant nitrate sources, originating from manure and sewage, suggested that the nitrate along the west coast of Guangdong Province was mainly influenced by local cities despite the westward flow of diluted Pearl River water. In the offshore area, the nitrate loss in the upper and mid water during both two seasons mainly caused by phytoplankton assimilation, whereas coupled nitrification-denitrification could be responsible for the nitrate loss in the bottom waters during summer. Our results suggest that, with the rapid development of local economy, the nitrate sources in the coastal area have shifted to manure and sewage from the local cities.

Tracing the sources of nitrate in the rivers and lakes of the southern areas of the Tibetan Plateau using dual nitrate isotopes

Based on a quantitative analysis of nitrate concentrations, the nitrate sources and temporal variability of the rivers, lakes, and wetlands of Tibet were assessed for the first time using dual isotope technology. Water samples were collected once in July 2017 for analysis of nitrate concentration and isotopic composition. The overall values of δ15N-NO3− and δ18O-NO3− ranged from +1.8‰ to +23.0‰, and from −6.3‰ to +22.2‰ respectively. Duel isotopic composition suggested that nitrification of soil organic nitrogen was the main source of nitrate in the Yalu Tsangpo River. Furthermore, anthropogenic nitrogen inputs become more important in downstream than upstream because of intensive agricultural activities and urban input. In the rivers of the Ngari District, nitrate is mainly derived from desert deposits, manure and sewage, and chemical fertilisers. Different rivers show different characteristics of nitrate sources depending on the location, topography, landform, and climate of the river basins. Animal manure, nitrification of soil organic matter, and desert deposits are mainly responsible for the shifting of nitrate isotopic signatures in lakes, which are minimally affected by human activities. In wetlands, biological nitrification and denitrification could be the main processes of nitrogen migration and transformation. These results provide useful information in revealing the fate of nitrate in different aquatic ecosystems and different areas of Tibet.

The influence of storage condition on nitrite, nitrate and vitamin C levels in vegetables

Vegetables are the main sources of nitrate and nitrite in food. The presence of nitrate and nitrite at a high level may cause a negative impact on health, because nitrite and nitrate when reduced to nitrite, may react with alkylamine to form carcinogenic nitrosamine. The influence of temperature and time of storage on nitrite, nitrate, and vitamin C contents in vegetables were investigated in this study. The vegetables were sweet mustard, bokchoy, spinach and lettuce obtained from a local market. Samples were stored at ±25oC and ±5oC. Analysis of nitrite, nitrate, and vitamin C was conducted in fresh samples, after storage for 24 and 48 hours. Nitrite was analyzed by spectrophotometry at 540 nm. Nitrate reduced into nitrite with Zn in acidic conditions and then analyzed as nitrite. Vitamin C was analyzed by titration with 2.6-dichlorophenolindophenol. During storage, nitrite and nitrate increased, while vitamin C decreased. Nitrite and nitrate content in fresh samples were 15.22 and 22.46 mg/kg (sweet mustard), 12.57 and 6.55 mg/kg (bokchoy), 20.26 and 90.60 mg/kg (spinach), 18.77 and 32.68 mg/kg (lettuce), respectively. Vitamin C content in fresh samples was 101.15 mg/100g (mustard), 92.17 mg/100g (bokchoy), 88.95 mg/100g (spinach), 40.03 mg/100g (lettuce). After storage for 48 hours at ±25oC, nitrite and nitrate increased 44.97% and 53.19% (mustard), 46.18% and 62.59% (bokchoy), 43.86% and 16.48% (spinach), and 41.05% and 47.09% (lettuce), respectively. Vitamin C decreased 67.57% (mustard), 24.68% (bokchoy), 81.25% (spinach), and 79.74% (lettuce). Storage at ±5oC, showed that nitrite and nitrate increased 27.54% and 35.08% (mustard), 13.75% and 43.51% (bokchoy), 19.59% and 10.60% (spinach), 19.85% and 25.16% (lettuce), respectively. Vitamin C decreased 30.88% (mustard), 6.05% (bokchoy), 60.92% (spinach), and 74.94% (lettuce). During storage, nitrite and nitrate increased more significantly at ±25oC than ±5oC while vitamin levels C decreased and were more effective at 25oC than 5oC.