Nitrate Isotope Research Articles

Assessment of rapid low-cost isotope (δ15 N, δ18 O) analyses of nitrate in fruit extracts by Ti(III) reduction to differentiate organic from conventional production.

The isotopic composition (δ15 N, δ18 O) of nitrate in fruits and vegetables differentiates organic from conventional food production practices. Organic systems do not use synthetic nitrate fertilizers high in 18 O and low in 15 N and thereby help reveal producers' fertilization claims. Isotope analyses of nitrate extracted from fruits and vegetables are done by bacterial reduction which is costly and by specialized laboratories. Rapid, low-cost methods are needed to promulgate nitrate isotope analyses of food products to support organic food product certification and to verify the authenticity of production claims. Fresh strawberry samples were obtained from certified organic and conventional growers in Andalucía, Spain. We applied a new, rapid, one-step Ti(III) reduction method to convert the nitrate from strawberry extracts to N2 O gas for headspace isotope analyses using isotope-ratio mass spectrometry. Using the Ti(III) reduction method, 70 samples, controls and references were prepared and analyzed for NO3 - , δ15 N and δ18 O per 48 h. We also analyzed extracts and solids for anions and cations and for bulk δ15 N for multivariate chemometric evaluation. The Ti(III)-based isotope analyses of nitrate in strawberry extracts revealed clear differentiation between organic and conventional production with mean δ18 O and δ15 N values of +18.3 ± 1.2 ‰ and +17.6 ± 1.2 ‰ versus +28.2 ± 4.5 ‰ and +14.9 ± 3.0 ‰, respectively. The δ15 N of strawberry dry mass differed slightly (+3.0 ± 1.4 ‰ versus +4.0 ± 1.4 ‰) between organic and conventional samples, respectively. Chemometric analyses of nitrate isotopes and extract chemistry revealed that the δ18 O of nitrate along with δ15 N and Ca2+ fully differentiated organic from conventional strawberry production. Our results show the Ti(III) reduction method provides a new low-cost and rapid analytical method to facilitate compound-specific δ15 N and δ18 O isotope analyses of nitrate in selected fruit types, and likely other food products, for the purposes of assessing nitrate fertilization practices of organic versus conventional production claims and to support authenticity investigations.

Relationship between non-point source pollution and fluorescence fingerprint of riverine dissolved organic matter is season dependent

The source identification of water pollution and quantification of pollution sources are vital for water environment management. Dissolved organic matter (DOM) affects the form, solubility, and toxicity of pollutants, so the migration and transformation of DOM are crucial for water quality assessment. Therefore, the aim of this study was to identify pollution sources and quantify their contribution in water environments using fluorescence fingerprint of DOM. The Danjiangkou Reservoir and its main tributaries were selected as the study area. The DOM fluorescence components of pollution source samples were analyzed and the quantitative relationship between DOM spectral indices and nitrate isotopes (δ15N-NO3− and δ18O-NO3−) was established. It was found that humic-like substances were mainly derived from non-point source (NPS) pollution and protein-like substances were from point source (PS) pollution. The fluorescence index (FI) of DOM was positively correlated with PS pollution and negatively correlated with NPS pollution. The quantitative relationship between three-dimensional (3D) fluorescence spectra characteristics of DOM and water quality indicators or NPS pollution was established, which realized the convenient quantitative evaluation of pollution load and pollution source contribution using DOM fluorescence spectra characteristics. The strength of the correlation between DOM fluorescence spectra characteristics and water quality indicators (or NPS pollution) was affected by seasonal rainfall due to changes in the contribution of PS and NPS pollution. This study provides a new approach for source identification of water pollution and quantification of pollution sources using 3D fluorescence fingerprint of DOM.

Identifying the source and transformation of riverine nitrates in a karst watershed, North China: Comprehensive use of major ions, multiple isotopes and a Bayesian model

Nitrate (NO3−) contamination of surface water is a globally concern, especially in karstic regions affected by intensive agricultural activities. This study combines hydrochemistry, and environmental isotopes (δ2HH2O, δ18OH2O, δ15NNO3, and δ18ONO3) with a Bayesian isotope mixing model (Simmr) to reduce the uncertainty in estimating the contributions of different pollution sources. Samples were collected from 32 surface water sites in the Yufu River (YFR) watershed, North China, in September and December 2019. The results revealed that NO3−-N was the predominant form of inorganic nitrogen that caused the deterioration of water quality in the watershed, accounting for approximately 58% of the total nitrogen (TN). The hydrochemical compositions and nitrate isotopes indicated that NO3− mainly originated from soil nitrogen (SN), ammonium fertilizer (AF), but nitrate fertilizer (NF), manure and sewage (M&S) and atmospheric precipitation (AP) were limited. The isotopic composition of nitrate in the upper reaches of the watershed was mainly affected by microbial nitrification, while the mixture of multiple sources was the dominant nitrogen transformation process in the mid-lower reaches of the watershed. Simmr model outputs revealed that SN (56.5%) and AF (29.5%) were the primary contributor to riverine NO3− pollution, followed by NF (7.1%), MS (3.6%), and AP (3.4%) sources. Moreover, an uncertainty index (UI90) of the isotope mixing showed that SN (0.73) and AF (0.67) had the highest values, followed by NF (0.22), M&S (0.22) and AP (0.10). Chemical fertilizer and SN collectively contributed >50% of nitrate during the two sampling campaigns. These results indicated that reducing the application of nitrogen fertilizers and rational irrigation are the keys to alleviate of NO3− pollution. The study is helpful in understanding the source and transformation of riverine NO3− and effectively reducing NO3− pollution in karst agricultural rivers or watersheds.

Nitrate fate and decadal shift impacted by land use change in a rural karst basin as revealed by dual nitrate isotopes

Nitrate pollution in oxygenated karst aquifers is common due to nitrification and anthropogenic inputs. However, the shift of nitrogen sources influenced by enhanced rural tourism activities and land use changes are not well understood. In this study, hydrochemistry and dual nitrate isotopes of water samples from a rural karst basin in Chongqing, southwestern China were employed to investigate the nitrate fate and its decadal change during the periods from 2007–2008 and 2017–2019. The results showed that δ15N–NO3 and δ18O–NO3 values at the groundwater basin resurgence averaged 9 ± 3.4‰ and 2.5 ± 3.4‰, respectively, with a mean NO3− concentration of 19.7 ± 5.4 mg/L in 2017–2019, clearly exceeding natural background levels. The dual isotope results suggested that nitrification occurred at the sampled sites. From 2007–2008 to 2017–2019, the mean δ15N–NO3 values from the primary sink point and the resurgence of the underground river water samples increased from −0.2 ± 2.1 to 11.2 ± 4.8‰, 4.2 ± 0.9 to 9.0 ± 3.4‰, respectively. A Bayesian mixing model in R (MixSIAR) based on the isotopes revealed that soil organic nitrogen, and manure and sewage proportions for the groundwater increased by 34% and 23%, respectively, while chemical fertilizer and atmospheric precipitation proportions decreased by 32% and 25%, respectively. These decadal changes resulted from reforestation practices and enhanced rural tourism activities in the basin, which were evidenced by the change of land use patterns. The elevated nitrogen load from the rapid development of rural tourism is likely to increase this contamination in the near future if the infrastructure cannot meet the demands. The results from this study could contribute to minimizing environmental health risks in drinking water when rural tourism activities are increasing.

Developments in the investigation of nitrogen and oxygen stable isotopes in atmospheric nitrate

Under the influence of human activities and the rapid development of industry and agriculture, atmospheric nitrate (NO − 3 ) pollution is becoming increasingly serious and has become an environmental problem worldwide. The stable isotopic composition of atmospheric NO − 3 (δ 15 N, δ 18 O, and Δ 17 O) can provide a strong basis for understanding the atmospheric nitrogen cycle to effectively control atmospheric NO x pollution. In this work, the δ 15 N values associated with different sources of atmospheric NO − 3 , the seasonal variation characteristics of δ 15 N-NO − 3 , and the main influencing factors are reviewed. The δ 18 O and Δ 17 O values of different oxidants in the atmosphere and the spatiotemporal distribution characteristics and possible influencing factors of the δ 18 O and Δ 17 O values ​​of atmospheric NO − 3 are summarised. In addition, key advances in NO − 3 isotopic analysis technique is reviewed. Based on previous work, it is suggested that more attention should be given to the oxidative formation mechanism of NO − 3 (spatio-temporal differences in the isotopic compositions of different types of oxidants), the δ 15 N composition of different NO x sources, and the processes of formation, transport, depositional/chemical loss ofatmospheric NO − 3 with the help of observation and chemical models.

Nitrate sources and nitrogen dynamics in a karst aquifer with mixed nitrogen inputs (Southwest China): Revealed by multiple stable isotopic and hydro-chemical proxies

The nitrate (NO3−) contamination of karst aquifers as an important drinking water reservoir is increasing globally. Understanding the behavior of nitrogen (N) in karst aquifers is imperative for effective groundwater quality management. This study combined multiple stable isotopes (δ2H-H2O, δ18O-H2O, δ13C-DIC, δ15N-NO3, and δ18O-NO3), including hydro-chemical data, with a tracer test and a Bayesian isotope mixing (SIAR) model to elucidate the NO3− sources and N cycling within the Babu karst aquifer in Guizhou Province, Southwest China. Nitrate isotopes and SIAR model revealed that manure and sewage, nitrogen fertilizer, and soil organic nitrogen were the three dominant NO3− sources in winter, contributing to 37%, 32%, and 31% to spring NO3−, and 38%, 31%, and 31% to surface water NO3−, respectively. The δ18O-NO3 values of sampled waters ranging from 0.3‰ to 13.7‰ (mean of 7.7 ± 3.0‰; N = 63) and the significant negative correlations between δ15N-NO3 and δ13C-DIC in the spring waters (P < 0.01) revealed that nitrification was the primary N transformation process in the Babu watershed. Whereas, denitrification might still occur locally, confirmed by the enriched values of δ15N-NO3 (14.3 ± 7.6‰; N = 6) and high denitrification extent (46.6 ± 22.2%; N = 6) in the springs from residential areas, and by elevated δ13C-DIC (−11.2 ± 0.6‰; N = 26) and δ15N-NO3 values (18.9 ± 5.2‰; N = 26) in the boreholes. During the base flow period, point-inputs of the AMD-impacted stream and sewage waters, and short transit time (<5 days) were conducive to nitrification processes in the karst conduit, resulting in elevated NO3− concentration and NO3−/Cl− ratio at the watershed outlet. Approximately 50% of NO3− flux at the outlet was derived from nitrification, indicating that a significant extent of nitrification occurred in the NH4+-contaminated karst conduit, which may be a new NO3− source to receiving rivers and lakes. This study provided an integrated method for exploring the N dynamics in contaminated karst aquifers. Moreover, the study highlighted that the point N sources control required particular attention for groundwater protection and restoration.

Isotopic composition (δ15N, δ18O) of nitrate in high-frequency precipitation events differentiate atmospheric processes and anthropogenic NOx emissions

Reactive nitrogen (Nr) species from the atmosphere contribute to adverse impacts like acid deposition and eutrophication; thus, information about local or distal emission N sources towards wet deposition of nitrogen oxides (NOx) is important information. Given N emissions source regions and atmospheric chemistry processes can change on an hourly basis, a thorough assessment of Nr removal from the atmosphere requires high resolution precipitation sampling. We measured nitrate isotopes (δ15Ν, δ18O of NO3−) and δ18Ο-H2O of all precipitation events at high-frequency (5–30 min) for Vienna, Austria, in 2019. Average monthly δ15N of NO3− ranged from −7.0‰ (AIR) to −0.1‰ with lowest and higher values in summer and winter, respectively. Similarly, the average δ18O of NO3− was highest in winter (+75.0‰, VSMOW) and lowest in summer (+61.2‰). Nitrate in nighttime precipitation and intense rainouts had higher δ15N-NO3− values (−4.4‰ and − 4.6‰, respectively) compared to daytime and washout samples (−5.7‰ and − 5.5‰). Atmospheric oxidation and cloud scavenging on a diel basis were correlated to air temperature, with an opposite non-linear effect between the δ18O of NO3− and H2O. The δ15N of NO3− showed a negative correlation with rain intensity, but a positive relationship to the number of days without rain prior to a precipitation event. Overall, nitrate source partitioning with and without photochemical equilibrium isotope fractionation suggested strong year-round influences from vehicle emissions and biomass burning by day and night. Biogenic, fertilized soil, and coal combustion N sources (power plants) showed a lower contribution to precipitation NO3− and was linked to both harvest periods and long-range transport of air masses from North-Eastern and southern Europe. N emissions from local municipal waste incineration were significant in the autumn. Our study reveals that nitrate isotopes in wet deposition combined with δ18O-H2O contribute new information to inform NOx source management strategies aimed at improving environmental air and water quality.

High spatial‐resolution monitoring to investigate nitrate export and its drivers in a mesoscale river catchment along an anthropogenic land‐cover gradient

AbstractNitrate monitoring is commonly conducted with low‐spatial resolution, only at the outlet or at a small number of selected locations. As a result, the information about spatial variations in nitrate export and its drivers is scarce. In this study, we present results of high‐spatial resolution monitoring conducted between 2012 and 2017 in 65 sub‐catchments in an Alpine mesoscale river catchment characterized by a land‐use gradient. We combined stable isotope techniques with Bayesian mixing models and geostatistical methods to investigate nitrate export and its main drivers, namely, microbial N turnover processes, land use and hydrological conditions. In the investigated sub‐catchments, mean values of NO3− concentrations and its isotope signatures (δ15NNO3 and δ18ONO3) varied from 2.6 to 26.7 mg L−1, from −1.3‰ to 13.1‰, and from −0.4‰ to 10.1‰, respectively. In this study, land use was an important driver for nitrate export. Very strong and strong positive correlations were found between percentages of agricultural land cover and δ15NNO3, and NO3− concentration, respectively. Mean proportional contributions of NO3− sources varied spatially and seasonally, and followed land‐use patterns. The mean contribution of manure and sewage was much higher in the catchments characterized by a high percentage of agricultural and urban land cover comparing to forested sub‐catchments. Specific NO3− loads were strongly correlated with specific discharge and moderately correlated with NO3− concentrations. The nitrate isotope and concentration analysis results suggest that nitrate from external sources is stored and accumulated in soil storage pools. Nitrification of reduced nitrogen species in those pools plays the most important role for the N‐dynamics in the Erlauf river catchment. Consequently, nitrification of reduced N sources was the main nitrate source except for a number of sub‐catchments dominated by agricultural land use. In the Erlauf catchment, denitrification plays only a minor role in controlling NO3− export on a regional scale.

Factor affecting nitrate in a mixed land-use watershed of southern China based on dual nitrate isotopes, sources or transformations?

River nitrate (NO3–) pollution is ubiquitous and has attracted worldwide attention. The continuous increase nitrate flux in the river system is mainly caused by inputs of exogenous nitrate sources or nitrogen transformations. In this study, coupled nitrogen and oxygen isotopes of nitrate were used to identify sources and transformations of nitrate in a mixed land use watershed of southern China (the Beijiang River basin), aimed at evaluating the impacts of sources or transformations on nitrate pollution and provide guidance for aquatic environment management. The results showed that the NO3–-N flux in the Beijiang River Basin was 9.37 × 104 tons in 2015; isotopic evidence indicated that the impacts of nitrogen transformations in the river on the removal and production of nitrate were not significant, and the increased NO3–-N concentration along the river was caused by inputs of exogenous nitrate sources. The isotope-based source apportionment revealed that approximately 52% of the influx NO3–-N was contributed by synthetic fertilizer, 27% by soil nitrogen and 21% by domestic sewage. There was no significant temporal variations in source apportionment; however, significant spatial variations were found across the study area. The spatial variations were mainly affected by the different land use patterns. With the increasing percentage of urbanized areas, the proportional contributions of domestic sewage have increased, which has resulted in an increased NO3–-N concentration. In addition, the relationships between the land use index and proportional contributions of different nitrate sources suggested that the effect of urbanized area might be more pronounced on nitrate pollution in a mixed land use watershed. Therefore, the results of this study suggested that land use should be considered in nitrate pollution management strategies and reducing inputs of nitrate sources to river water is crucial to reducing nitrate pollution in mixed land use watersheds.

New insight into the response and transport of nitrate in karst groundwater to rainfall events

Although numerous studies focused on nitrate source, transformation and transport of river water in karst area have been reported, it's still unclear in understanding nitrate main source and transformation in karst groundwater system and how nitrate transport from soil to water during rainfall events in karst critical zone. In order to explore the response and transport of nitrate in karst groundwater to rainfall events, different depths of well water before, during and after rainfall event were sampled, and hillslope runoff, surface runoff of different land-use types during rainfall event were sampled synchronously at a typical karst agricultural catchment in Southwest China. Results showed that fluctuations of EC, pH and DO in deep borehole well (W1) and artesian well (W2) were small, on the contrary, variations of EC and DO in shallow well (W3) were large during sampling period. The nitrate concentrations and isotopic values indicated that nitrate in karst groundwater mainly originated from chemical fertilizer (CF), and influenced by denitrification process. High intensity of denitrification was observed in deep groundwater (87%) and artesian well water (almost 100%). Extremely high dual nitrate isotope values up to 46.8 ± 1.5‰ and 24.7 ± 0.5‰ were found in the deep artesian well. The small variation of water chemistry (EC, DO and pH), nitrate concentration and dual nitrate isotope values in deep wells during sampling period suggested that newly supplied nitrogen in deep groundwater during rainfall events also comes from deep groundwater. Low nitrogen concentrations in hillslope subsurface flow and surface runoff suggests that nitrogen transport process leading to increase of water nitrogen content mainly occur in depression. Nitrogen in depression soil is mainly transported to groundwater through fissures, fractures and conduits, rather than through vertical migration processes in the soil during rainfall events.

Terrestrial Nitrogen Inputs Affect the Export of Unprocessed Atmospheric Nitrate to Surface Waters: Insights from Triple Oxygen Isotopes of Nitrate

Terrestrial Nitrogen Inputs Affect the Export of Unprocessed Atmospheric Nitrate to Surface Waters: Insights from Triple Oxygen Isotopes of Nitrate

Seasonal Sources and Cycling of Nitrogen Revealed by Stable Isotopes in the Northeastern Beibu Gulf, China

Isotope measurements were performed on dissolved nitrate (NO3−) and ammonium (NH4+) in the coastal waters of the northeastern Beibu Gulf, China, to investigate the seasonal nitrate sources and their biogeochemical processes, which are due to the rapid development of local industrialisation and urbanisation. The high N/P ratio observed in the coastal bay during both fall and spring suggests that P is a limiting nutrient, which in turn indicates that increasing P causes conditions favourable for algal blooms. Higher nutrient concentrations and δ15N-NO3− and δ15N-NH4+ values were found in the nearshore area in the fall, suggesting that nutrients originated mainly from land-based pollution. A Bayesian isotope mixing model was used to calculate the contribution of potential NO3− sources and the results showed that in the nearshore area, NO3− originated mainly from manure and sewage (58%). In the spring, however, in addition to the impact of urban sewage effluents, the exchange of sediment and water was another important factor causing higher nutrient concentrations and positive NO3− isotopes in the nearshore area. There were lower concentrations of nutrients and an increase in δ15N-NO3− and δ15N-NH4+ values in the offshore area in the fall, and the NO3− loss in the surface water was mainly caused by the process of assimilation. However, the exchange of sediment and water was the dominant factor causing higher nutrient concentrations (except for NO3−) and positive dual nitrate isotopes but lower NO3− concentration in the offshore area during the spring. Overall, isotope analysis of NO3− and NH4+ helps to illustrate the major sources of the former and their biological transformation in the northeastern Beibu Gulf.

Tracing Nitrate Source and Transformation in Glacier Runoffs on the Tibetan Plateau Using Triple Nitrate Isotopes

Glaciers are the largest reservoir of fresh water on earth. Global warming accelerates glacier retreat and enhances glacier runoff, which heavily impacts the geochemical cycling in downstream ecosystems. Nitrate is the main form of nitrogen in glacier runoff, while little is known about its source in high-altitude regions. Here, we measured triple nitrate isotopes (δ 15 N, δ 18 O, and Δ 17 O) in ten geographically separated glacier runoffs on the Tibetan Plateau with the altitudes ranged from 2,409 to 5,564 m. The isotope results were combined with hydrogeochemical indicators to assess the primary sources and transformation processes of nitrate. Results showed that all the glacier runoffs exhibited low nitrate concentration, which ranged from 0.3 ± 0.0 to 1.5 ± 0.8 mg·L -1 . The SIAR model results indicated that soil organic nitrogen and atmospherically oxidized nitrate accounted for 67% of nitrate in the runoffs, whereas human-related source (i.e., chemical fertilizers) contributed 33% on average. Significant geospatial differences in the contribution of chemical fertilizers were identified, with their contributions being significantly higher in the monsoon domain (43% ± 3%) than that in the westerlies domain (22% ± 8%). Furthermore, δ 18 O of nitrate ranged from -15‰ to 15‰, this suggested that the nitrification process is responsible for the nitrogen transformation in Tibetan glacier ecosystems. Our study reveals the source of nitrogen in Tibetan glaciers and highlights the substantial impact of human activity on glacier nutrient cycling. Our work may shed light on the understanding of nitrogen cycling in downstream ecosystems.

The effects of clean energy production and urbanization on sources and transformation processes of nitrate in a subtropical river system: Insights from the dual isotopes of nitrate and Bayesian model

Source appointment and control of water pollution caused by nitrate in river are the key to water quality management as well as the basis for achieving aquatic environment sustainability. However, the effects of clean energy production (hydropower generation) and urbanization on the nitrate accumulation and transformation remain unclear. For that, this study quantified and identified the nitrate sources and key processes in a subtropical river, which has been impounded and urbanized in varying degree. The variations of dual isotopes and water chemistry indicated that NO3− in agriculture and forest land mainly from the chemical fertilizer and soil N, while manure and sewage were the main riverine NO3− sources. Nitrification was the dominate process while influenced greatly by inputs of manure and sewage, especially in urban area. Progressive denitrification has been found in the upper reaches of the Beixi River where the river impounded by dams for hydropower generation, with an estimated contribution of 36% (1%–68%). Results from Bayesian model showed that fertilizer and soil N contributed more than half of the NO3− in the wet season as a result of rainfall and surface runoff; while manure and sewage were the dominate NO3− sources in the dry season due to the urbanization. Land use including agriculture, dam construction, and urbanization have characterized the nitrate sources and pollution in the Jiulong River, and cascade dam impoundment could enlarge nitrate retention. We concluded that the combined use of dual isotopes of NO3−, hydrochemicals and the SIAR model offers an efficient approach to understand the nitrogen transformation and quantify the contributions of nitrogen source in river waters.

Using Dual Isotopes of Nitrate to Identify the Source of Nitrogen Pollution in the Songhua River Basin, Northeast China

Nitrogen pollution is a severe problem in the Songhua River Basin (SHR) in China. Samples were collected from 36 sections of the SHR during the high, low, and flat seasons of the river, and the main sources of nitrogen in the water were qualitatively analyzed with isotope data for nitrogen and oxygen of nitrate. The contribution rates of each major pollution source were quantitatively analyzed using the Iso Source mass balance model. The results from these experiments indicate that the values for δ15N-NO3 and δ18O-NO3 in the flat flow season range from 1.52‰ to 14.55‰ and −14.26‰ to 2.03‰, respectively. The values for δ15N-NO3 and δ18O-NO3 in the low flow season range from 6.66‰ to 15.46‰ and −5.82‰ to 65.70‰, respectively. In the low flow season, nitrogen comes from the input of domestic and manure sewage (53%) and soil organic N (45%). The values of δ15N-NO3 and δ18O-NO3 in the high flow season range from 2.07‰ to 14.24‰ and −3.99‰ to 8.03‰, respectively. In the high flow season, nitrogen comes from soil organic nitrogen (41%), domestic and manure sewage (32%), and nitrogen fertilizer (27%), which are the main sources of nitrogen pollution in the SHR. The conclusions from the qualitative and quantitative analysis of nitrogen sources in the SHR can provide a scientific basis for the source control and treatment of nitrogen pollution.

A multi-isotopic evaluation of groundwater in a rapidly developing area and implications for water management in hyper-arid regions

Management of water resources in hyper-arid areas faces vital challenges in a global climate change context. Consequently, understanding the effects on groundwater sources can help mitigating the problem of water scarcity and the negative impact of human intervention on the environment. A case study area in the hyper-arid climate of the United Arab Emirates, was tackled here with the focus on applying stable isotopes as tools for evaluating groundwater sources and quality assessment. The results of major ions indicate variable increase in groundwater salinity moving away from Al Hajar Mountains recharge areas to the discharge areas (Arabian Gulf coast). The data of stable isotopes (δ18OH2O, δ2HH2O, δ18ONO3, δ15NNO3, δ18OSO4, δ34SSO4, δ11B) suggest impact of paleo-groundwater in the abstractions of the wells nearest to the coast. Nitrate isotopes indicate farming activities sources that can be masked due to the contribution from the nitrate-poor paleo-groundwater. Nitrate reduction processes are expected near to the recharge front. Sulphate and boron isotopes further suggest that influence of ancient evaporite dissolution in salinization. Management efforts should be focused on the diffuse sources of quality mitigations that can be vital in fingerprinting local and regional (transboundary) effects.

Stable isotopic characterization of nitrate wet deposition in the tropical urban atmosphere of Costa Rica

Increasing energy consumption and food production worldwide results in anthropogenic emissions of reactive nitrogen into the atmosphere. To date, however, little information is available on tropical urban environments where inorganic nitrogen is vastly transported and deposited through precipitation on terrestrial and aquatic ecosystems. To fill this gap, we present compositions of water stable isotopes in precipitation and atmospheric nitrate (δ18O-H2O, δ2H-H2O, δ15N-NO3-, and δ18O-NO3-) collected daily between August 2018 and November 2019 in a tropical urban atmosphere of central Costa Rica. Rainfall generation processes (convective and stratiform rainfall fractions) were identified using stable isotopes in precipitation coupled with air mass back trajectory analysis. A Bayesian isotope mixing model using δ15N-NO3- compositions and corrected for potential 15N fractionation effects revealed the contribution of lightning (25.9 ± 7.1%), biomass burning (21.8 ± 6.6%), gasoline (19.1 ± 6.4%), diesel (18.4 ± 6.0%), and soil biogenic emissions (15.0 ± 2.6%) to nitrate wet deposition. δ18O-NO3- values reflect the oxidation of NOx sources via the ·OH + RO2 pathways. These findings provide necessary baseline information about the combination of water and nitrogen stable isotopes with atmospheric chemistry and hydrometeorological techniques to better understand wet deposition processes and to characterize the origin and magnitude of inorganic nitrogen loadings in tropical regions.

Characteristics and traceability analysis of nitrate pollution in the Yellow River Delta, China

The Yellow River Delta is one of the regions with the highest nitrogen application rate in China. Because of the high-intensity development and utilization of the Yellow River Delta, a large number of nitrate inorganic nitrogen pollutants have been transported offshore through rivers, which poses a threat to the ecological environment security of the region. The distribution characteristics of nitrate pollution, the source and transport characteristics of nitrate in surface water are unclear. In this study, we collected soil and surface water samples, determined stable isotopes of water (δ2H-H2O and δ18O-H2O) and nitrate (δ15N-NO3 and δ18O-NO3), and used the Isosource model to quantitatively analyze the distribution characteristics of soil nitrate-nitrogen and source characteristics of surface water nitrate. The results showed that: (1) The average content of NO3 --N in cultivated soil was 3.99 times higher than that in non-cultivated soil, in which cornfield > cotton field > soybean field > paddy field; (2) The nitrate flux of surface water increased from upstream to downstream, which was positively correlated with basin area; (3) In the middle and upper reaches of the study area, the surface water is mainly replenished from the Yellow River water and groundwater, while the lower reaches are affected by seawater intrusion; (4) The primary source of nitrate in the study area was dominated by human activities of agricultural production and residents’ lives, with a total contribution of 60.8%. This study will provide a scientific basis for the treatment of nitrate pollution in coastal areas.

Seasonality of nitrogen sources, cycling, and loading in a New England river discerned from nitrate isotope ratios

Abstract. Coastal waters globally are increasingly impacted due to the anthropogenic loading of nitrogen (N) from the watershed. To assess dominant sources contributing to the eutrophication of the Little Narragansett Bay estuary in New England, we carried out an annual study of N loading from the Pawcatuck River. We conducted weekly monitoring of nutrients and nitrate (NO3-) isotope ratios (15N / 14N, 18O / 16O, and 17O / 16O) at the mouth of the river and from the larger of two wastewater treatment facilities (WWTFs) along the estuary, as well as seasonal along-river surveys. Our observations reveal a direct relationship between N loading and the magnitude of river discharge and a consequent seasonality to N loading into the estuary – rendering loading from the WWTFs and from an industrial site more important at lower river flows during warmer months, comprising ∼ 23 % and ∼ 18 % of N loading, respectively. Riverine nutrients derived predominantly from deeper groundwater and the industrial point source upriver in summer and from shallower groundwater and surface flow during colder months – wherein NO3- associated with deeper groundwater had higher 15N / 14N ratios than shallower groundwater. Corresponding NO3- 18O / 16O ratios were lower during the warm season, due to increased biological cycling in-river. Uncycled atmospheric NO3-, detected from its unique mass-independent NO3- 17O / 16O vs. 18O / 16O fractionation, accounted for &lt; 3 % of riverine NO3-, even at elevated discharge. Along-river, NO3- 15N / 14N ratios showed a correspondence to regional land use, increasing from agricultural and forested catchments to the more urbanized watershed downriver. The evolution of 18O / 16O isotope ratios along-river conformed to the notion of nutrient spiraling, reflecting the input of NO3- from the catchment and from in-river nitrification and its coincident removal by biological consumption. These findings stress the importance of considering seasonality of riverine N sources and loading to mitigate eutrophication in receiving estuaries. Our study further advances a conceptual framework that reconciles with the current theory of riverine nutrient cycling, from which to robustly interpret NO3- isotope ratios to constrain cycling and source partitioning in river systems.

Seasonal variations in stable nitrate isotopes combined with stable water isotopes in a wastewater treatment plant: Implications for nitrogen sources and transformation

A multi-isotopes approach involving the use of stable nitrate isotopes (δ15NNO3 and δ18ONO3) combined with stable water isotopes (δ18OH2O and δ2HH2O) as tracers can help identify the nitrogen source and understand the transformation process in a river water system. In this study, we identify the potential impact of the N source in the effluent discharged from a wastewater treatment plant (WWTP) on the surrounding stream and clarified the seasonal variations in the isotope values of the effluent and stream water from 2019 to 2020 in South Korea. In addition, we investigate the factors that led to the seasonal variations of the stable isotopes and evaluated the relationship between the uncertainty of the contribution ratio of the N sources and isotopic variations in the river water system. To examine the potential impact of the inputs of N from the WWTP, samples were obtained from the stream (up and down with respect to the WWTP) and WWTP (influent and treated effluent) once a month. For the monthly effluent samples, δ18OH2O and δ2HH2O ranged from –9.37‰ to –8.68‰ and from –65.29‰ to –59.37‰, respectively, exhibiting isotopic depletion in wet season and enrichment in dry season. Moreover, in the effluent, δ15NNO3 and δ18ONO3 ranged from 10.56‰ to 16.60‰ and –4.86‰ to –0.35‰, respectively. To clarify the seasonal variation in the effluent, high-resolution samples of the influent and treated effluent were obtained every day in September 2019. It is noted that δ15NNO3 in the effluent was influenced by the N source instead of the seasonal variation, although δ18ONO3 exhibited a high dependence on the seasonal effects, influenced by δ18OH2O. When estimating the contribution of the effluent using isotope values to the water system, uncertainty could arise due to these seasonal variations. Overall, a multi-isotope approach involving the combined use of stable nitrate isotopes (δ15NNO3 and δ18ONO3) and stable water isotopes (δ18OH2O and δ2HH2O) as tracers can help evaluate the potential impact of N sources on water systems.