Stable isotopes as an effective tool for N nutrient source identification in a heavily urbanized and agriculturally intensive tropical lowland basin

© 2020, Springer Nature Switzerland AG. We present the application of dual stable isotope analyses of NO3 (δ15N-NO3 and δ18O-NO3) to provide a comprehensive assessment of the provenance, partitioning, and conversion of nitrate across the Day River Basin (DRB), Vietnam, which is heavily impacted by agriculture and urbanization. Stable isotope compositions of river water δ18O-H2O, in addition to their δ15N-NO3 and δ18O-NO3 signatures, were sampled at 12 locations in the DRB. Sample collection was conducted during three different periods to capture changes in regional weather and agricultural fertilization regimes; April (the dry season and key fertilization period), July (the rainy season and another key fertilization period) and October (the rainy season with no regional fertilization). Ranges of NO3 stable isotopes are − 7.1 to + 9.2‰ and − 3.9 to + 13.2‰ for δ18O and δ15N, respectively. Interpretation of the stable isotope data characterizes 4 main sources of NO3 in the DRB; (1) nitrified urea fertilizer derived from an intensive agricultural irrigation network, (2) soil and groundwater leaching from within the basin (3) manure and sewage inputs (which is more prevalent in downstream river sections) and (4) upstream inflow from the Red River which discharges into the Day River through the Dao River. We applied a mixing model for the DRB consisting of 4 variables, representing these 4 different sources. The partition calculation shows that during the fertilization and rainy period of July, more than 45% of river NO3 is derived from nitrified urea sources. During the other sampling periods (April and October), manure and sewage contribute more than 50% of river NO3 and are derived from the middle portion of the DRB, where the Day River receives domestic wastewater from the Vietnamese capital, Hanoi. Stable isotope data of O and N reveal that nitrification processes are more prevalent in the rainy season than in dry season and that this predominantly takes place in paddy field agricultural zones. In general, data demonstrate that nitrate loss in the DRB is due to denitrification which takes place in polluted stretches of the river and dominates in the dry season. This study highlights that (i) domestic waste should be treated prior to its discharge into the Day River and (ii) the need for better catchment agricultural fertilization practices as large portions of fertilizer currently discharge into the river, which greatly impacts regional water quality

Nitrate isotopes in catchment hydrology: insights, ideas and implications for models

Models that simulate water flow and quality, particularly related to nitrate ions, are commonly used on a catchment-scale. However, tracking nitrate ions is a challenging task due to the intricate processes that affect them, such as phase exchanges, transformations, and interactions with various environmental media. In general, models capable of carrying out all tasks required to simulate water flow and quality at the same time, are rare. Additionally, most available models focus only on specific compartments of the watershed, such as surface water, topsoil, unsaturated zone, or groundwater. Taken together, these two challenges can lead to oversimplified representations of a system’s hydrology, as catchment internal processes become neglected due to missing information (lack of informative measurements, or models not focusing on all watershed compartments). Attempting to combine these models or to couple different watershed compartments results in complex calculations, increased run times, and a large number of parameters to estimate. Artificial Intelligence (AI) models have been massively used in environmental studies but, so far, the majority of them have been tested theoretically and not under real conditions. To overcome these challenges, stable isotope data are often employed to calibrate and validate internal catchment processes of these models. While water stable isotopes (δ18O and δ2H of H2O) have been extensively used in many water flow models, the use of nitrate isotopes (δ15N and δ18O of NO3-) in water quality models remains poorly explored. Nitrate isotopes can help trace the origin of NO3- contamination and disentangle the complex reactions and dynamics that nitrate undergoes during transport. Hence, we propose that incorporating nitrate isotopes into catchment-scale water flow and quality models can substantially enhance the accuracy of these models. This review provides an overview of the current use of catchment hydrological models in predicting flow and fate of solutes. We discuss their limitations and highlight the potential of combining these models with nitrate isotopes. Ultimately, this approach may reduce prediction uncertainties and provide more effective guidance for water management decisions

Nitrate pollution in the Red River Delta

The Red River Delta is home to the capital of Vietnam, Hanoi. The large population relies on the catchment for the provision of primary water resources for its industry and agriculture. This critical reserve is rapidly becoming impacted by anthropogenic activities and we demonstrate how nitrate pollution, in particular, is influencing the health of the Red River Delta

Identifying the controls on nitrate and metabolic state within the Red River delta (Vietnam) with the use of stable isotopes

In many places around the world, anthropogenic activities have resulted in nitrate (NO3−) pollution and changes in the metabolic state of aquatic ecosystems. Here we combined stable isotope and physico-chemical monitoring to assess the sources of NO3− and the overall metabolic state within the Red River delta, Vietnam. River water stable isotope compositions (δ18O-H2O) ranged between −11.2 and −2.7 ‰, δ18O-NO3− between −7.1 and + 29.7 ‰ and δ15N-NO3− between −3.9 and + 14.0 ‰. We identified the dominant NO3− sources as: 1) soil leachate, 2) domestic waste flushed from urban areas, and 3) NH4+ fertilizers washed from paddy fields. The relative impact of each source depends on geographical location within the delta and the time of year, due to dilution and concentration effects during wet and dry seasons. The primary NO3−source upstream is natural soil leachates, predominantly from tributaries connected to the Red River’s main stream. Within the middle-lower section of Red River delta, urban pollution from manure and septic waste reaches as high as 50 % of the total NO3− load during dry season. NO3− leached from fertilizers is also high at sites in the middle of the delta, related to agricultural activities. Dissolved oxygen isotope (δ18O-O2) values calculated from δ18O-H2O and δ18O-NO3− values indicate that the aquatic metabolism is net autotrophic (oxygen from primary production exceeds consumption by respiration), but high inputs of biodegradable organic matter from untreated domestic waste and high rates of sediment oxygen demand (SOD) and chemical oxygen demand (COD) have resulted in the whole river system becoming undersaturated in oxygen. High NO3− loads and low DO saturation are of critical concern and require mitigation practices to improve water quality for millions of people

Global patterns of nitrate isotope composition in rivers and adjacent aquifers reveal reactive nitrogen cascading

Remediation of nitrate pollution of Earth’s rivers and aquifers is hampered by cumulative biogeochemical processes and nitrogen sources. Isotopes (δ15N, δ18O) help unravel spatiotemporal nitrogen(N)-cycling of aquatic nitrate (NO3−). We synthesized nitrate isotope data (n = ~5200) for global rivers and shallow aquifers for common patterns and processes. Rivers had lower median NO3− (0.3 ± 0.2 mg L−1, n = 2902) compared to aquifers (5.5 ± 5.1 mg L−1, n = 2291) and slightly lower δ15N values (+7.1 ± 3.8‰, n = 2902 vs +7.7 ± 4.5‰, n = 2291), but were indistinguishable in δ18O (+2.3 ± 6.2‰, n = 2790 vs +2.3 ± 5.4‰, n = 2235). The isotope composition of NO3− was correlated with water temperature revealing enhanced N-cascading in warmer climates. Seasonal analyses revealed higher δ15N and δ18O values in wintertime, suggesting waste-related N-source signals are better preserved in the cold seasons. Isotopic assays of nitrate biogeochemical transformations are key to understanding nitrate pollution and to inform beneficial agricultural and land management strategies

Hydrogeological and isotopic investigations in regions of Argolis peninsula

The present doctoral thesis aimed at the combination of the isotopic characteristics of groundwater aquifers in Argolis peninsula with the existent hydrogeological conditions and vice-versa. The study area is located in the southern part below the imaginary line of Nea Epidavros – Tolo. It is characterized by a complex geological structure, a rapid tourist development, a lack of significant groundwater resources and the fact that the majority of the coastal aquifers (e.g. Iria, Tolo, Kranidi, Ermioni, Galatas etc.) are brackish to a large extent. This part of the peninsula which covers approximately 1300Km2 is mainly occupied by Mt. Arachnaio (1200m), Mt. Dhidhyma (1110m) and Adheres mountain ridge (726m) which host the most important aquifers in the region. The hydrogeological approach included the detection and the investigation of the recharge mechanism of numerous individual springs that are found in the peninsula. It should be noted that the majority of the spring water sites have not been registered officially, thus, from a hydrogeological point of view, the specific regions have not been studied thoroughly or at all by previous researchers. Initially, the research concerned a geological mapping at scale 1/5000 of the sites of hydrogeological interest which explained the appearance and the function mechanism of the springs. Afterwards, the hydrochemical characteristics of the groundwater sampling points were systematically monitored in order to interpret the subsurface water movement and the geochemical reactions that take place between the water and the rock formations. The second part of the present study concerned the implementation of isotopic methods so as to confront specific hydrogeological issues which are difficult to solve with the conventional ways of research. Therefore, this doctoral thesis aims to shed much light on several stable and radioactive isotope techniques which are then practiced as a case study in Argolis peninsula. Although in a global scale “Isotope Hydrology” has been recognized as an important scientific field since the late 1950’s, in Greece it has not been acknowledged as much as it should be. For this reason, our goal was to approach the isotopic characteristics of the study area in a systematic and not superficial way and to point out the potentials and the prospects of these methods in our country. The importance of the stable and radioactive isotope techniques in hydrogeology has been expressed by several pioneer scientists such as H. Craig, A.O. Nier, W. Dansgaard, S. Epstein, E. Eriksson, I. Friedman, W.F. Libby, K.O. Munnich, E. Tongiorgi and J.C. Vogel. At the same time the International Atomic Energy Agency has praised this prospect by creating an individual Isotope Hydrology Section in Vienna in 1958. It should be noted that isotopes were featured in less than 100 scientific papers on hydrological research and application in the period 1960 to 1965. During 1995-2000, however, more than 7000 such papers were published recognizing “Isotope hydrology” today as an important discipline of hydrogeological sciences. .............................................................Η παρούσα διδακτορική διατριβή είχε ως βασικό στόχο τη σύζευξη των αποτελεσμάτων της ισοτοπικής σύστασης των υδάτων της χερσονήσου της Αργολίδας με τις υδρογεωλογικές συνθήκες που διαμορφώνονται στην περιοχή αυτή και αντιστρόφως. Συγκεκριμένα, εξετάστηκε το τμήμα που εκτείνεται νότια της νοητής γραμμής Νέα Επίδαυρος – Τολό το οποίο χαρακτηρίζεται από πολύπλοκη γεωλογική δομή, από αλματώδη τουριστική – παραθεριστική ανάπτυξη, από έλλειψη υδατικών πόρων και από το ότι οι παράκτιοι υδροφορείς (Ίρια, Τολό, Κρανίδι, Ερμιόνη, Γαλατάς κ.α.) έχουν υποστεί έντονη υφαλμύρινση. Η έκταση της ερευνηθείσας περιοχής ξεπερνάει τα 1300Km2 και οι σημαντικότεροι ορεινοί όγκοι τους οποίους φιλοξενεί και στους οποίους αναπτύσσεται υδροφορία είναι το όρος «Αραχναίο» (1200m), η οροσειρά των «Αδερών» (726m) και το όρος «Δίδυμα» (1110m). Η μελέτη των υδρογεωλογικών συνθηκών αρχικά περιελάμβανε τον εντοπισμό και την απογραφή των θέσεων εκφόρτισης του υπόγειου νερού στη χερσόνησο με στόχο την εξέταση του μηχανισμού λειτουργίας αυτών. Αξίζει να σημειωθεί ότι οι περισσότερες από τις θέσεις πηγαίων εκφορτίσεων δεν είχαν απογραφεί επίσημα και ως εκ τούτου τα διαθέσιμα υδρογεωλογικά δεδομένα από προηγούμενους μελετητές ήταν ελάχιστα έως ανύπαρκτα. Η προσέγγιση που έγινε αφορούσε στη γεωλογική χαρτογράφηση των περιοχών σε κλίμακα 1/5000 με τη βοήθεια της οποίας δόθηκαν απαντήσεις σχετικά με την παρουσία και τον τρόπο λειτουργίας των πηγαίων εκφορτίσεων. Στη συνέχεια, εξεταστήκαν επισταμένως τα υδροχημικά χαρακτηριστικά του υπόγειου νερού στις θέσεις ενδιαφέροντος τα οποία έδωσαν τη δυνατότητα εξαγωγής συμπερασμάτων που συνδέονται με την κίνηση του υπόγειου νερού και τις αντιδράσεις που λαμβάνουν χώρα από την επαφή του υπόγειου νερού με τα πετρώματα μέχρι τη θέση της πηγαίας εκδήλωσής του. Το δεύτερο μέρος της ερευνητικής δραστηριότητας εστιάστηκε στην εφαρμογή των ισοτοπικών μεθόδων στην επίλυση επιμέρους υδρογεωλογικών ζητημάτων στα οποία δύσκολα δίνεται απάντηση μέσα από τις κλασσικές μεθόδους έρευνας. Έτσι, η συμβολή της παρούσας εργασίας έγκειται στην εμβάθυνση σε θέματα Ισοτοπικής Υδρολογίας σταθερών και ραδιενεργών ισοτόπων και στην προσπάθεια εφαρμογής των μεθόδων αυτών σε μια περιοχή του ελληνικού χώρου, όπως είναι η χερσόνησος της Αργολίδας. Αν και το επιστημονικό πεδίο της Ισοτοπικής Υδρολογίας είναι γνωστό σε παγκόσμια κλίμακα από τις τα τέλη της δεκαετίας του 1950, στην Ελλάδα δεν βρήκε την ανταπόκριση που του αναλογεί. Η συγκεκριμένη διδακτορική διατριβή προσπαθεί να προσεγγίσει με συστηματικό τρόπο και όχι επιδερμικά τα ισοτοπικά χαρακτηριστικά της περιοχής μελέτης δίνοντας έτσι την ευκαιρία ανάδειξης των δυνατοτήτων αλλά και της προοπτικής που μπορούν να έχουν οι εν λόγω μέθοδοι και στον ελληνικό χώρο. Η σημασία της εφαρμογής των σταθερών και ραδιενεργών ισοτόπων στην υδρογεωλογία εκφράστηκε από πρωτοπόρους ερευνητές όπως οι: H. Craig, A.O. Nier, W. Dansgaard, S. Epstein, E. Eriksson, I. Friedman, W.F. Libby, K.O. Munnich, E. Tongiorgi και J.C. Vogel, ενώ ο Διεθνής Οργανισμός Ατομικής Ενέργειας συνέβαλλε προς την κατεύθυνση αυτή με τη δημιουργία ενός ανεξάρτητου τομέα Ισοτοπικής Υδρολογίας στη Βιέννη το 1958. ....................................

A biological and nitrate isotopic assessment framework to understand eutrophication in aquatic ecosystems

Eutrophication is a globally significant challenge facing aquatic ecosystems, mostly associated with human induced enrichment of these ecosystems with nitrogen and phosphorus. Given the complexity of assigning eutrophication issues to local primary N sources in field-based studies, this paper proposes a multi-stable isotope and biological framework to track nitrogen biogeochemical transformations, inputs and fate of nitrate in groundwater-dependent shallow lakes. Three representative freshwater ecosystems from the Pampa Plain (Argentina), with different land uses and topographic features were selected. Groundwater (N = 24), lake (N = 29) and stream (N = 20) samples were collected for isotope (δ15N-NO3− and δ18O-NO3−, δ18O-H2O) and hydrogeochemical (major ions and nutrients) determinations, and in the case of surface water, also for biological determinations (chlorophyll-a, fecal coliforms and nitrifying bacteria abundance). Both chemical and isotopic characteristics clearly indicated that denitrification was limited in lakes and streams, while evidence of assimilation in shallow lakes was confirmed. The results suggested that groundwater denitrification plays a role in the nitrate concentration pattern observed in the Pampeano Aquifer. The proportional contribution of nitrate sources to the inflow streams for all years were estimated by using Bayesian isotope mixing models, being ammonium nitrified in the system from soil and fertilizers ~50 - 75 %, sewage/manure ~20 - 40 % and atmospheric deposition ~5 - 15 %. In this sense, agricultural practices seem to have a relevant role in the eutrophication and water quality deterioration for these watersheds. However, limnological, bacterial and algal variables, assessed simultaneously with isotopic tracers, indicated spatio-temporal differences within and between these aquatic ecosystems. In the case of Nahuel Rucá Lake, animal manure was a significant source of nitrogen pollution, in contrast to La Brava Lake. In Los Padres Lake, agricultural practices were considered the main sources of nitrate input to the ecosystem

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

PreprintIncreasing 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 (δ 18 O-H2O, δ 2 H-H2O, δ 15 N-NO3-, and δ 18 O-NO3-) collected daily between August 2018 and November 2019 in a tropical urban atmosphere of central Costa Rica. Rainfall generation processes (convective and stratiform) were identified using stable isotopes in precipitation combined with air mass back trajectory analysis. A Bayesian isotope mixing model forced with δ 15 N-NO3-values corrected for potential 15 N fractionation effects reveal the predominant contribution of biomass burning and lightning to nitrate wet deposition. δ 18 O-28 NO3-values in Caribbean convective rainfall reflect the oxidation chemistry of NOx sources whereas δ 15 N-NO3-values in Pacific stratiform rainfall indicate the transport of nitrogen sources contributing to nitrate in atmospheric deposition. 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 of inorganic nitrogen loadings in tropical regions.El aumento del consumo de energía y la producción de alimentos en todo el mundo da como resultado emisiones antropogénicas de nitrógeno reactivo a la atmósfera. Sin embargo, hasta la fecha, se dispone de poca información sobre los entornos urbanos tropicales donde el nitrógeno inorgánico se transporta y se deposita en gran medida a través de la precipitación en los ecosistemas terrestres y acuáticos. Para llenar este vacío, presentamos composiciones de isótopos estables en agua en precipitación y nitrato atmosférico (δ 18 O-H2O, δ 2 H-H2O, δ 15 N-NO3-, y δ 18 O-NO3-) recolectados diariamente entre agosto de 2018. y noviembre de 2019 en un ambiente urbano tropical del centro de Costa Rica. Los procesos de generación de lluvia (convectivos y estratiformes) se identificaron utilizando isótopos estables en la precipitación combinados con el análisis de la trayectoria inversa de la masa de aire. Un modelo de mezcla de isótopos bayesianos forzado con valores de δ 15 N-NO3 corregidos para efectos potenciales de fraccionamiento de 15 N revela la contribución predominante de la quema de biomasa y los rayos a la deposición húmeda de nitrato. Los valores de δ 18 O-28 NO3 en la lluvia convectiva del Caribe reflejan la química de oxidación de las fuentes de NOx, mientras que los valores de δ 15 N-NO3 en la lluvia estratiforme del Pacífico indican el transporte de fuentes de nitrógeno que contribuyen al nitrato en la deposición atmosférica. Estos hallazgos proporcionan la información básica necesaria sobre la combinación de isótopos estables de nitrógeno y agua con la química atmosférica y las técnicas hidrometeorológicas para comprender mejor los procesos de deposición húmeda y caracterizar el origen de las cargas de nitrógeno inorgánico en las regiones tropicales.Universidad Nacional, Costa RicaEscuela de Químic

Stable isotopic characterizacion 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.El aumento del consumo de energía y la producción de alimentos en todo el mundo da como resultado emisiones antropogénicas de nitrógeno reactivo a la atmósfera. Sin embargo, hasta la fecha, se dispone de poca información sobre los entornos urbanos tropicales donde el nitrógeno inorgánico se transporta y se deposita en gran medida a través de la precipitación en los ecosistemas terrestres y acuáticos. Para llenar este vacío, presentamos composiciones de isótopos estables en agua en precipitación y nitrato atmosférico (δ18O-H2O, δ2H-H2O, δ15N-NO3- y δ18O-NO3-) recolectados diariamente entre agosto de 2018 y noviembre de 2019 en una atmósfera urbana tropical. del centro de Costa Rica. Los procesos de generación de lluvia (fracciones de lluvia convectiva y estratiforme) se identificaron utilizando isótopos estables en la precipitación junto con el análisis de la trayectoria inversa de la masa de aire. Un modelo de mezcla de isótopos bayesianos utilizando composiciones de δ15N-NO3- y corregido por los posibles efectos de fraccionamiento de 15N reveló la contribución de los rayos (25,9 ± 7,1%), la quema de biomasa (21,8 ± 6,6%), la gasolina (19,1 ± 6,4%), el diésel (18,4%). ± 6,0%) y emisiones biogénicas del suelo (15,0 ± 2,6%) a la deposición húmeda de nitrato. Los valores de δ18O-NO3- reflejan la oxidación de las fuentes de NOx a través de las vías · OH + RO2. Estos hallazgos proporcionan la información de referencia necesaria sobre la combinación de isótopos estables de nitrógeno y agua con la química atmosférica y las técnicas hidrometeorológicas para comprender mejor los procesos de deposición húmeda y caracterizar el origen y la magnitud de las cargas de nitrógeno inorgánico en las regiones tropicales.Universidad Nacional, Costa RicaEscuela de Químic