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Introduction: In the past few decades, nitrate contamination in water resources has become a global environmental problem often caused by urban and agricultural activities. In the past two decades, the capital of Afghanistan, Kabul has experienced rapid urbanization. This rapid urbanization has caused groundwater storage depletion and contamination of nitrate in the Kabul Plain. In the Kabul Plain, groundwater is the only source of drinking water for Kabul city residents. This study aims to determine the different sources of nitrate pollution, investigate the nitrogen transformation processes, and estimate the proportional contribution of various sources of nitrate pollution in groundwater of the Eastern Kabul Plain. Material and Methods: Eighteen groundwater samples were collected from the Eastern Kabul Plain aquifer in November 2020. In-situ parameters such as electrical conductivity (EC), pH, dissolved oxygen (DO), and temperature were measured in the sampling sites. The collected samples for analysis of nitrate and nitrate were transported to the Green Tech Laboratory in less than six hours, and the concentrations of mentioned ions were measured on the same day. The collected samples for analysis of major ions were transported to the Regional Water Organization Laboratory of Tehran, Iran. Nitrate samples for isotopic analysis were shipped to the UFZ laboratory in Germany, where isotopic measurements were carried out using the bacterial denitrifier method. The proportional contribution of nitrate sources was determined using the BSIMM model. Results and Discussion: Kabul city does not have a central sewage collection system, and sewage is disposed of mostly through septic and absorption wells. The groundwater nitrate concentration varied from 4 mg/L to 120.4 mg/L with an average value of 21 mg/L. The values ​​of δ15N-NO3̄ varied from 4.8 ‰ to 20.8 ‰ and the values ​​of δ18O-NO3̄ varied from 0.7 ‰ to 18.6 ‰. The composition diagram of δ18O-NO3̄ and δ15N-NO3̄ was employed to determine the main sources of nitrate in the aquifer. Considering that most of the samples are plotted on sewage and manure area, therefore, urban sewage plays a major role in increasing the concentration of nitrate in the groundwater of Eastern Kabul Plain. Since chloride is a conservative ion and is not affected by environmental factors, the diagram of NO3̄/Cl‾ molar ratio versus Cl‾ was used to identify potential sources of nitrate in the Kabul Plain aquifer. The plot indicated that urban sewage is the primary source of nitrate in the aquifer. The high concentration of dissolved oxygen and nitrate in the groundwater samples indicated the dominance of aerobic conditions in the aquifer and the absence of denitrification. Sewage plays a major role in nitrate pollution of groundwater. Therefore, the rapid urbanization and population growth in the Kabul Plain in the past two decades have caused groundwater nitrate pollution. The uncertainty in nitrate sources estimation has been quantitatively assessed using the uncertainty index (UI90). The results of this study display a relatively high uncertainty for the contribution of nitrate sources, particularly sewage and manure (0.49) and soil organic nitrogen (0.44). Conclusion: Sewage and manure, soil organic nitrogen, and chemical fertilizers are potential sources of nitrate in the Kabul Plain. The Bayesian stable isotope mixing model showed that sewage and manure (69.5%) are the main sources of nitrate pollution in the aquifer. The results indicated that nitrification is the main biogeochemical nitrogen transformation process in the Kabul Plain.

Nitrate pollution is a major environmental problem threatening rivers, and nitrogen and oxygen isotopes have proved to be an effective means of analyzing the sources and transformations of nitrate in rivers. However, a low monitoring frequency cannot accurately reflect the changes in nitrate. In this study, the sources and transformations of nitrate in the middle reaches of the Yellow River and its tributaries during the dry season and the wet season were analyzed based on water quality parameters and nitrate isotopes. Stable isotope analysis conducted using the R (SIAR) model was used to estimate the proportions of different nitrate sources. The results showed that the main nitrate sources in the main stream were soil nitrogen (40.95–45.83%) and domestic sewage and manure (30.93–32.60%), respectively, with little variation between the dry season and wet season because of the large flow of the Yellow River. During the dry season, the nitrate sources of the two tributaries were mainly domestic sewage and manure (45.23–47.40%), followed by soil nitrogen (31.35–34.00%). However, the primary nitrate source of T2 (Qin River) became soil nitrogen (40.05%) during the wet season, a phenomenon that was mainly caused by the significant increase in river discharge and in soil erosion in the basin. During the wet season, the concentrations of total nitrogen (TN) and nitrate (NO3−) significantly decreased in the main stream and tributaries, and nitrification and denitrification processes occurred in both the main stream and tributaries of the Yellow River. In addition, the T2 tributary (Qin River) was also significantly affected by mixed dilution. High-frequency sampling can reflect the isotopic information of nitrate in the river more comprehensively, which helps us to understand the conversion process of nitrate more accurately. [ABSTRACT FROM AUTHOR]

With the intensification of human activities, nitrate pollutants in groundwater are receiving increasing attention worldwide. Especially in the arid Nile Delta of Egypt, groundwater is one of the most valuable water resources in the region. Identifying the source of nitrate in groundwater with strong human disturbances is important to effective water resource management. This paper examined the stable isotopes (δ15N/δ18O-NO3 and δ2H/δ18O-H2O) and the hydrogeochemical parameters of the shallow groundwaters in the arid southeast of the Nile Delta to assess the potential sources and transformation processes of nitrate under severe urban and agricultural activities. The results revealed that the groundwaters were recharged by the Nile River. Meanwhile, the infiltration of irrigation water occurred in the west, while the mixing with the deep groundwater occurred in the east regions of the study area. The TDS, SO42−, NO3−, and Mn2+ concentrations of groundwaters (n = 55) exceeded the WHO permissible limit with 34.6%, 23.6%, 23.6%, and 65.5%, respectively. The NO3− concentrations in the shallow groundwaters ranged from 0.42 mg/L to 652 mg/L, and the higher levels were observed in the middle region of the study area where the unconfined condition prevailed. It extended to the deep groundwater and eastward of the study area in the groundwater flow direction. The δ15N-NO3 and δ18O-NO3 values suggested that the groundwater NO3− in the west and east regions of semi-confined condition were largely from the nitrification of soil organic nitrogen (SON) and chemical fertilizer (CF). In contrast, wastewater input (e.g., domestic sewage and unlined drains) and prevalent denitrification were identified in the middle region. The denitrification might be tightly coupled with the biogeochemical cycling of manganese. This study provides the first report on the groundwater NO3− dynamics in the Nile Delta, which generated valuable clues for effective water resource management in the arid region. [ABSTRACT FROM AUTHOR]

Human activities and climate change input more reactive nitrogen into alpine lakes. Alpine saline lakes are usually located in endorheic watersheds at high-altitude areas, with no other drainage methods than evaporation, and are prone to accumulate nutrients. Meanwhile, alpine saline lakes are usually oligotrophic and sensitive to reactive nitrogen inputs, and even modest reactive nitrogen inputs may have significant effects on them, such as eutrophication. Nitrate is the main form of reactive nitrogen in lakes; therefore, clarifying the sources and transformations of nitrate in alpine saline lakes is important to prevent or mitigate eutrophication in alpine saline lakes. In this study, the sources and transformations of nitrate in Qixiangcuo Lake and its inflow rivers in the northern Tibetan Plateau were identified using dual nitrate isotopes and hydrochemistry. The results show that (1) the ranges of NO3− concentrations, δ15N − NO3−, and δ18O − NO3− values were 3.6 ~ 26.1 μg/L, − 10.5 to + 6.0‰, and − 10.4 to + 9.2‰ in Qixiangcuo Lake and 194.4 ~ 728.1 μg/L, + 5.8 ~ + 8.8‰, and − 1.9 to + 2.4‰ in its inflow rivers, respectively. The NO3− concentrations and δ15N − NO3− values were significantly lower in Qixiangcuo Lake than in its inflow rivers (P < 0.05). (2) The main sources of nitrate in both surface water and bottom water of Qixiangcuo Lake were ammonium in atmospheric deposition (mean probability estimate (MPE) 41.0% and 32.2%, respectively) and livestock manure (MPE 28.9% and 21.7%, respectively). The main sources of nitrate in the inflow rivers of Qixiangcuo Lake were domestic sewage (MPE 35.7%) and livestock manure (MPE 29.6%). (3) The main nitrogen transformation process in Qixiangcuo Lake was nitrification. The conservative mixing of multiple sources controlled the nitrate concentration and isotopic composition of Qixiangcuo Lake. Improvement in grazing area planning and the installation of sewage treatment facilities are effective measures to prevent eutrophication in Qixiangcuo Lake and its inflow rivers. [ABSTRACT FROM AUTHOR]

Nitrate pollution in groundwater is a global environmental concern. As a result, accurate identification of potential sources for such pollution is of critical significance to the effective control of groundwater quality. In this study, forty-nine shallow groundwater samples were collected from the Huaibei mining area. Hydro-chemical characterization, geospatial analysis technique, dual nitrate isotopes (δ15N-NO3- and δ18O-NO3-), Bayesian model and health risk assessment model were adopted for exploring the conditions, sources, proportion, and potential health risks of nitrate pollution for the first time in the study area. The results showed that the nitrate concentration ranged from 0.00 to 293.21 mg/L, and that 18.37% groundwater samples exceeded the standard of drinking water in China (GB 5749–2006). Based on the dual isotopic values of nitrate, it could be concluded that nitrification was dominated migration and transformation process of nitrogen. The results of Bayesian model showed that the proportional contributions of the potential nitrate pollution sources in shallow groundwater were manure and sewage (M&S) (39.54 %), NH4+ in fertilizer and precipitation (NHF&P) (34.93 %), soil nitrogen (SN) (14.89 %), and NO3- in atmospheric deposition (NAD) (10.64 %). The health risk assessment indicated that non-carcinogenic risks posed by NO3--N was higher for children than adults. The primary exposure pathway was oral ingestion. Monte Carlo simulation were applied to evaluate model uncertainty. The probabilities of non-carcinogenic risks were up to 12.54 % for children and 5.22 % for adults. In order to protect water quality and drinking water safety, it was suggested that effective nitrate reduction strategies and better management practices can be implemented.

Abstract 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-NO 3 − and δ18O-NO 3 − 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. Graphical abstract Unlabelled Image Highlights • The dual isotope technology was first used to identify the source of nitrate in rivers, lakes and wetlands in Tibet. • The nitrification of soil organic nitrogen was the main sources of nitrate in the Yalu Tsangpo River. • The nitrate of rivers in Ngari Prefecture is mainly derived from desert deposits manure and sewage, and chemical fertilisers. • Biological nitrification and denitrification could be the main processes of nitrogen transformation in wetlands. [ABSTRACT FROM AUTHOR]

Abstract Monthly analytical results for more than two years long of δ15N and δ18O of nitrate in wet deposition are reported for a tropical city of Zhanjiang in the southernmost mainland China, in an effort to elucidate NO x sources and its oxidation pathways to nitrate. The results showed that monthly variations of δ18O NO 3 - responded well to changes in sunshine hours, with lower δ18O NO 3 - values corresponding to longer sunshine hours. This pattern suggests that NO x oxidation via the OH radical was the predominant pathway, which, by estimate, accounted for 87% in winter and 94% in summer, for nitrate formation. Remarkably, we found that the δ18O NO 3 - here are prominently low relative to previous studies, likely due to that annual sunshine hours in this tropical city is relatively long. Moreover, available data of reported δ18O NO 3 - in wet deposition showed an increasing trend with latitude, reflecting a shift of the predominant NO x oxidation pathways from those via the OH radical in low latitudes to those via O 3 in high latitudes, corresponding to the decrease of sunshine hours. A temporarily co-variation of δ15N NO 3 - and δ18O NO 3 - was observed, exhibiting higher values in dry winter and lower values in wet summer, which was attributed to the washout effect on the dual isotopes. During wet seasons, complete nitrate washout suggests that δ15N NO 3 - in precipitation should be similar to the nitrate formed in the atmosphere, and thereby can be used for source apportionment. A Bayesian model showed that the source of atmospheric nitrate deposition is mainly natural (61%) in Zhanjiang, with less anthropogenic contribution. Highlights • OH radical is the predominant pathway for nitrate formation in this tropical city. • Rainfall washout exerts an effect on δ15N NO 3 - and δ18O NO 3 - values. • The source of atmospheric nitrate deposition is mainly natural in this tropical city. [ABSTRACT FROM AUTHOR]

Nitrate pollution in water is a common environmental problem worldwide. The Qinhe Basin (QHB) faces with the risk of eutrophication. To clarify nitrate pollution of river water, water chemical data, water isotope values (δD and δ18O-H2O), and dual nitrate isotope values (δ15N-NO3− and δ18O-NO3−) were used to discern sources and transformation mechanisms of nitrogen in the QHB. The nitrate concentrations of river water ranged from 0.71 to 20.81 mg L−1. The δD and δ18O-H2O values of river water varied from − 74 to −52‰ and from − 10.8 to − 7.2‰, with an average value of − 60‰ and − 8.2‰, respectively. The δ15N-NO3− and δ18O-NO3− values of nitrate ranged from − 6.7 to + 14.8‰ and from − 6.0 to + 5.6‰, with a mean value of + 4.6‰ and − 0.6‰, respectively. Assimilation by algae and the mixing of soil nitrogen, chemical fertilizer, sewage, and industrial wastewater could account for increasing δ15N-NO3− values. There was neither significantly positive nor negative correlation between δ15N-NO3− and δ18O-NO3− in river water, indicating that no obvious denitrification shifted isotopic values of nitrate in the QHB. Based on the dual isotopic values of nitrate and land use change in the watershed, it could be concluded that intensive nitrification dominated in the QHB, and dissolved nitrate was mainly derived from nitrification of ammonium in fertilizer, soil nitrogen, and domestic sewage. As the primary nitrate sources identified in the QHB, effective fertilization and afforestation can be taken to protect water resource from nitrate pollution. [ABSTRACT FROM AUTHOR]

Abstract Recent analytical and conceptual advances related to the nitrate (NO 3 −) 17O anomaly (Δ17O) have opened the door to a new method that probes soil nitrification and NO 3 − consumption using Δ17O of soil NO 3 −. Because biological NO 3 − production and consumption processes in soil obey the mass-dependent fractionation law, Δ17O of soil NO 3 −, an index of excess 17O over that expected from 18O, can be used to trace gross nitrification and NO 3 − consumption in a way analogous to the 15NO 3 − tracer typically employed in studies of soil NO 3 − cycling. Moreover, coupling Δ17O with the dual NO 3 − isotopes (δ15N and δ18O) at natural abundances offers additional valuable insights into mechanisms that underlie soil NO 3 − dynamics. In this study, we conducted both laboratory and field experiments to assess the use of Δ17O-NO 3 - for tracing soil nitrification and NO 3 − consumption. Soil samples spanning a wide range of physical and chemical properties were sampled from four sites for batch incubations and amendments with a Δ17O-enriched NO 3 − fertilizer. After amendments, the triple isotopes (δ15N, δ18O, and Δ17O) of soil NO 3 − were measured periodically and used in a developed Δ17O-based numerical model to simultaneously derive gross rates and isotope effects of soil nitrification and NO 3 − consumption. The measured Δ17O-NO 3 - was also used in the classical isotope dilution model to estimate gross NO 3 − turnover rates. In situ field soil sampling was conducted in a temperate upland meadow following snowmelt input of Δ17O-enriched atmospheric NO 3 − to assess the robustness of Δ17O-NO 3 - as a natural tracer. The results show that the temporal dynamics of Δ17O-NO 3 - can provide quantitative information on soil nitrification and NO 3 − consumption. In the laboratory incubations, a wide range of gross nitrification and NO 3 − consumption rates were estimated for the four soils using the Δ17O-based models. The estimated rates are well within the range reported in previous 15N tracer-based studies and not sensitive to oxygen isotopic fractionations during nitrification and NO 3 − consumption. Coupling Δ17O-NO 3 - with the dual NO 3 − isotopes using the numerical model placed strong constraints on the δ15N and δ18O endmembers of nitrification-produced NO 3 − and revealed soil-specific N isotope effects for nitrification and NO 3 − consumption, consistent with the inferred differences in soil microbial community structure among these soils. Non-zero Δ17O-NO 3 - values, up to 4.7‰, were measured in the meadow soil following the snowmelt event. Although soil heterogeneity in the field prevents quantitative rate estimation using Δ17O-NO 3 -, active NO 3 − cycling via co-occurring nitrification and denitrification was revealed by the covariations in the triple NO 3 − isotopes. Integrating the field observations with the incubation results uncovered isotopic overprinting of nitrification on denitrification in the surface soil following the snowmelt, which has important implications for explaining the discrepancies between field- and laboratory-derived isotope systematics of denitrification. We conclude that Δ17O-NO 3 - is a conservative and powerful tracer of soil nitrification and NO 3 − consumption and future applications are expected to help disentangle soil NO 3 − cycling complexity at various scales. Highlights • Δ17O is a conservative tracer of gross nitrification and NO 3 − consumption. • Δ17O helps constrain isotope effects during nitrification and NO 3 − consumption. • Δ17O helps disentangle isotopic overprinting from nitrification and denitrification. [ABSTRACT FROM AUTHOR]

Due to the intensive and complicated human activities, the identification of nitrate pollution source of coastal aquifer is usually a challenge. This study firstly adopted stable isotope technique and stable isotope analysis in R (SIAR) model to identify the nitrate sources and contribution proportions of different sources in typical coastal groundwater of northern China. The results showed that about 91.5% of the groundwater samples illustrated significantly high nitrate concentrations exceeding the maximum WHO drinking water standard (50 mg/l), reflecting the high risk of groundwater nitrate pollution in the coastal area. A total of 57 sampling sites were classified into three groups according to hierarchical cluster analysis (HCA). The δ15N-NO3− and δ18O-NO3− values of groundwater samples from Group C (including nine samples) were much higher than those from Group A (including 40 samples) and Group B (including 8 samples). SIAR results showed that NH4+ fertilizer was the dominant nitrate source for groundwater samples of Groups A and B while manure and sewage (M&S) served as dominant source for Group C. This study provided essential information on the high risk and pollution sources of coastal groundwater nitrate of northern China. [ABSTRACT FROM AUTHOR]

Nitrate pollution in groundwater is a global environmental concern. As a result, accurate identification of potential sources for such pollution is of critical significance to the effective control of groundwater quality. In this study, forty-nine shallow groundwater samples were collected from the Huaibei mining area. Hydro-chemical characterization, geospatial analysis technique, dual nitrate isotopes (δ15N-NO 3 - and δ18O-NO 3 -), Bayesian model and health risk assessment model were adopted for exploring the conditions, sources, proportion, and potential health risks of nitrate pollution for the first time in the study area. The results showed that the nitrate concentration ranged from 0.00 to 293.21 mg/L, and that 18.37% groundwater samples exceeded the standard of drinking water in China (GB 5749–2006). Based on the dual isotopic values of nitrate, it could be concluded that nitrification was dominated migration and transformation process of nitrogen. The results of Bayesian model showed that the proportional contributions of the potential nitrate pollution sources in shallow groundwater were manure and sewage (M&S) (39.54 %), NH 4 + in fertilizer and precipitation (NHF&P) (34.93 %), soil nitrogen (SN) (14.89 %), and NO 3 - in atmospheric deposition (NAD) (10.64 %). The health risk assessment indicated that non-carcinogenic risks posed by NO 3 --N was higher for children than adults. The primary exposure pathway was oral ingestion. Monte Carlo simulation were applied to evaluate model uncertainty. The probabilities of non-carcinogenic risks were up to 12.54 % for children and 5.22 % for adults. In order to protect water quality and drinking water safety, it was suggested that effective nitrate reduction strategies and better management practices can be implemented. [Display omitted] • Nitrogen concentrations showed significant spatial variations. • Nitrate isotopes indicate that manure & sewage was the main source of NO 3 -. • Bayesian isotope mixing model reveal highest NO 3 - contribution by manure & sewage. • Children are more vulnerable to non-carcinogenic risks associated with NO 3 -. • Two models reveals that primary exposure pathway for non-carcinogenic risk is oral. [ABSTRACT FROM AUTHOR]

• Nitrate removal was positively linearly related to hydraulic residence time • Raised underdrains created immobile zones but enhanced mixing during flow events • Dual isotopes in nitrate distinguished dentification for mobile zones for all columns • The bottom underdrain performed best in nitrogen removal for all tested conditions • Multiple decision criteria such as nitrate and DOC may influence underdrain height Internal water storage (IWS), a below-grade saturated layer, is a bioretention design component created by adjusting the underdrain outlet elevation. Anaerobic conditions and the presence of a carbon source in IWS facilitates denitrification. Yet it remains unclear how underdrain height within the IWS impacts nitrate (NO 3 −) removal. This study applied synthetic stormwater with NO 3 − to three laboratory columns with underdrains located at the bottom, middle, or top of a 32 cm thick gravel-woodchip IWS. Under steady state conditions, underdrain nitrogen removal demonstrated a positive linear relationship with increasing hydraulic residence time (HRT). For a 1 cm/h hydraulic loading rate (HLR), nitrogen removal efficiency increased from 52 to 99% as underdrain height moved from the top to the bottom. Despite identical IWS thickness across columns, immobilize zones below the middle and top underdrains limited the steady state nitrogen removal. Dual isotopes in NO 3 − also indicated denitrification occurred in mobile zones and showed little or no denitrification in immobile zones due to limited mass transport. Transient flow conditions were applied, to mimic storms, followed by dry conditions. Lower effluent nitrogen concentrations and mass fluxes were observed from the bottom underdrain across the range of HLRs tested (1 to 5 cm/h) but performance of all three underdrains converged after the application of one pore volume. The top underdrain enhanced mixing between new incoming low-DOC stormwater and old IWS water with high-DOC which minimized effluent DOC concentrations. NO 3 − isotope enrichment factors indicated denitrification during transient flow for all three underdrain heights and enrichment increased for the 5 cm/h HLR. For sites with narrow IWS geometries (width to depth ratio < 1), optimal underdrain height is likely located between the bottom and top of the IWS to promote mixing with old IWS water high in DOC and sustain denitrification during storms. Graphical Abstract [Display omitted]. [ABSTRACT FROM AUTHOR]

Groundwater nitrate (NO 3 −) pollution is a worldwide environmental problem. Therefore, identification and partitioning of its potential sources are of great importance for effective control of groundwater quality. The current study was carried out to identify the potential sources of groundwater NO 3 − pollution and determine their apportionment in different land use/land cover (LULC) types in a traditional agricultural area, Weining Plain, in Northwest China. Multiple hydrochemical indices, as well as dual NO 3 − isotopes (δ15N–NO 3 and δ18O–NO 3), were used to investigate the groundwater quality and its influencing factors. LULC patterns of the study area were first determined by interpreting remote sensing image data collected from the Sentinel-2 satellite, then the Bayesian stable isotope mixing model (MixSIAR) was used to estimate proportional contributions of the potential sources to groundwater NO 3 − concentrations. Groundwater quality in the study area was influenced by both natural and anthropogenic factors, with anthropological impact being more important. The results of LULC revealed that the irrigated land is the dominant LULC type in the plain, covering an area of 576.6 km2 (57.18% of the total surface study area of the plain). On the other hand, the results of the NO 3 − isotopes suggested that manure and sewage (M&S), as well as soil nitrogen (SN), were the major contributors to groundwater NO 3 −. Moreover, the results obtained from the MixSIAR model showed that the mean proportional contributions of M&S to groundwater NO 3 − were 55.5, 43.4, 21.4, and 78.7% in the forest, irrigated, paddy, and urban lands, respectively. While SN showed mean proportional contributions of 29.9, 43.4, 61.5, and 12.7% in the forest, irrigated, paddy, and urban lands, respectively. The current study provides valuable information for local authorities to support sustainable groundwater management in the study region. [Display omitted] • Nitrate sources were identified by hydrochemical indices and dual nitrate isotopes. • Land use/land cover patterns of the study area were interpreted. • MixSIAR model was used to estimate nitrate source contributions. • Manure & sewage and soil nitrogen were the most two contributive nitrate sources. • Historical land use/land cover patterns may also contribute to groundwater nitrate. [ABSTRACT FROM AUTHOR]

The multiple and complex N sources in coastal upwelling areas make it difficult to understand the behaviour of the N cycle. Stable N and O isotopes (δ15N and δ18O) of nitrate (NO 3 −) from coastal upwelling off eastern Hainan Island in the South China Sea (SCS) identified the main NO 3 − sources and the processes that control NO 3 − distribution in the area. Our results revealed that upwelling occurred in the nearshore area of transects Q2 and Q3, and that the sources of NO 3 − in the surface water were mainly upwelling from the bottom water (50%), followed by nitrification (34%) and atmospheric deposition (AD) (16%). In the nearshore surface water of Transect Q1, the sources of NO 3 − were mostly manure and sewage (31%), followed by AD (24%), soil N (24%), and N in fertiliser (21%). Complete assimilation was an important process in the surface water. For the offshore area, NO 3 − assimilation and AD were the primary processes that affected N concentrations, and the proportion of AD was 30% in the upper water of the eastern coast of Hainan Island in the northern SCS. In the bottom water, coupled nitrification-denitrification could occur in sediment porewater and/or sediment-water interface, the isotope signal of which could then be diffused into the bottom water. The degree of denitrification for NO 3 − in the bottom water of the eastern Hainan coast ranged from 7.2% to 29.8% with an average of 22.5%. Our results suggested that upwelling plays an important role in the N cycle on the eastern coast of Hainan Island. • Upwelling was the important NO 3 − source in coastal surface water. • The NO 3 − assimilation and atmospheric deposition dominated in upper water. • Nitrification coupled with denitrification was dominant process in the bottom water and/or sediment-water interface. [ABSTRACT FROM AUTHOR]

Karst aquifers are extremely vulnerable to pollution, including from nitrate. This research advances a systematic evaluation of water quality dynamics and processes in a relatively pristine karst flow system impacted by seasonal tourism, in particular migration and transformation of nitrate. Water samples from the Shuifang Spring basin (Jinfoshan Karst World Heritage Site, Chongqing, China) were collected for analysis of ion concentrations and dual nitrate isotopes. The principal sampling sites included untreated tap water from the Jinfoshan Holiday Hotel (JHH), JHH septic system effluent, discharged effluent at Sinkhole #1, and groundwater at Shuifang Spring (SFS, the resurgence of the basin). Sampling from April 2017 through April 2018 showed that higher nitrate concentrations were observed at SFS during busy tourist seasons that occurred in the summer drought and winter dry seasons. Between the septic system and SFS, nitrate was diluted by rainwater and infiltration from the matrix regions of the karst aquifer in the wet season. The MixSIAR model quantitatively revealed that nitrate from manure and sewage (M&S), soil organic nitrogen (SON) and atmospheric precipitation (AP) contributed roughly 55%, 41% and 4% on average, respectively. The mixture of effluent and oxygen-rich tap water from the JHH underwent nitrification at the septic system and Sinkhole #1. After the nitrification and decomposition of organic matter with oxygen consumption in the effluent, denitrification was the dominant response to nitrate attenuation in the effluent of Sinkhole #1 during peak tourism periods. The septic effluent sinking into the aquifer deteriorated the groundwater quality, which was controlled by nitrification in the summer drought and winter dry seasons. In contrast, microorganisms in groundwater assimilated nitrate from effluent in the wet season. Because of the relatively pristine ambient background conditions and seasonal tourism, these results can inform interpretation of background and impacted nitrogen processes in a range of karst settings. Unlabelled Image • Nitrate migration and transformations were studied in a protected karst area. • Hydrochemistry, dual nitrate isotopes, and the MixSIAR model were used. • The increasing intensity of tourism activities poses a threat to groundwater. • Results provide guidance to control nitrate flux in karst areas worldwide. [ABSTRACT FROM AUTHOR]