Nitrate dynamics in the streamwater-groundwater interaction system: Sources, fate, and controls

Integrating stable isotopes and hydrological models to track nitrogen sources and transport pathways in plateau watersheds: a case study in Southwest China

Exogenous nitrogen inputs from agriculture and anthropogenic activities have dramatically altered the material cycling processes in the Plateau watershed, leading to a range of water pollution issues. Effective management of nitrogen pollution in water bodies is predicated on clarifying N export loads under different pathways in the watershed, as well as the contributions of different sources. Here, this study proposes an integrated framework that introduces multiple stable isotope techniques (δD-H2O, δ18O-H2O, δ15N-NO3- and δ18O-NO3-) based on the coupled Eckhardt's digital baseflow filter (ECK) and load estimation model (LOADEST). The integrated approach was applied for the first time in a typical plateau watershed in southwest China. Results showed that baseflow as the Fengyu River watershed (FRW) major hydrologic pathway, provides 69.6 % of the mean annual stream flow and 59.1 % of the mean annual NO3--N load. Furthermore, the FRW average annual TN and NO3--N export is 94.0 t and 55.1 t, respectively. The NO3--N was the primary form of N pollutant, with its average concentration in groundwater being 7 times that in river water. In groundwater, manure and sewage (M&S) and soil nitrogen (SN) contribution rates to NO3--N 53.6 %, and 37.2 %, respectively. While the river water shows low M&S (26.8 %) and high SN (61.5 %) characteristics. It can be seen that the baseflow is a key pathway for coupled water-nitrogen export from plateau agricultural watersheds. Blocking the migration of nitrogen-containing pollutants to groundwater is an important measure to control the degradation of the water environment in plateau watersheds from the root cause.

Biogeochemical mechanisms and biomarkers of groundwater salinization in Jinghuiqu Irrigation District, China

Groundwater salinization poses significant challenges to water resource management, agriculture, and ecosystem sustainability. However, the biogeochemical mechanisms and microbial responses underlying this process in irrigation districts are still poorly understood. This study integrated hydrochemical ratios (Cl--Cl-/Br-, Cl--NO3-/Cl-), stable isotopes (δ2H, δ18O, δ15N-NO3-, δ18O-NO3-), and the MixSIAR model to investigate the dominant factors contributing to salinization in the Jinghuiqu Irrigation District. The results showed that TDS concentrations in groundwater samples ranged from 688 to 5420 mg/L, with 82 % of the samples exceeding WHO drinking water standards. Groundwater salinization was predominantly driven by mineral dissolution and evaporation, compounded by agricultural and domestic inputs. 16S rRNA microbial sequencing identified Candidatus Omnitrophus from the phylum Verrucomicrobiota as a potential biomarker for saline groundwater. PICRUSt2 predictions revealed that the functional traits of microorganisms in saline groundwater tend to enhance adaptability, whereas those in fresh groundwater are more oriented toward growth and metabolism. Spearman correlation analysis showed strong correlations between carbon fixation and nitrification (r = 0.69) and thiosulfate oxidation (r = 0.60). Additionally, as groundwater salinization progressed, the abundance of nitrate- and sulfate-reducing bacteria increased, further impacting nitrogen, sulfur, and carbon cycles. This study deepens knowledge of the biogeochemical processes driving groundwater salinization in irrigation districts and provides new insights for research and management of groundwater salinization in these regions.

Surface water and groundwater interactions contributed the seasonal NO3- and NH4+ sources and transformations in a hilly region, China

Comprehensive understanding of nitrogen (N) sources and transformations under the interactions of variable water cycle processes were of great significance for formulating nitrogen mitigation strategies in aquatic systems. Multiple stable isotopes (δ15N-NO3-, δ18O-NO3-, δ15N-NH4+, δ2H-H2O, and δ18O-H2O) and hydrochemical characteristics in surface water (SW) and groundwater (GW) were to reveal seasonal NO3- and NH4+ sources and transformations in a hilly region with frequent vertical movements in this study. The δ2H-H2O, δ18O-H2O, and hydrochemical analysis highlighted surface water and groundwater interactions. The correlation between δ15N-NO3- and d-excess demonstrated soil leaching. Nitrification was the predominant nitrogen transformation process in the watershed during both seasons although denitrification occurred in certain points. Primary source of NO3- in SW was soil nitrogen (SN; 34.5 ± 14.9 % in the dry season and 76.4 ± 8.1 % in the wet season). The contributions of various sources to NH4+ in SW were relatively equal during the dry season, while GW (30.5 ± 19.3 %) and manure and sewage (M&S) (29.0 ± 15.2 %) were the primary sources during the wet season. This study proved that groundwater was a significant source for NO3- and NH4+ input when utilizing source-track models, especially in hilly regions. These findings improve input sources for mixture models and identify the sources and transformations of water cycling and N biogeochemical processes.

Using Geochemistry, Stable Isotopes and Statistical Tools to Estimate the Sources and Transformation of Nitrate in Groundwater in Jinan Spring Catchment, China

Nitrate (NO3-) pollution resulting from anthropogenic activities represents one of the most prevalent environmental issues in karst spring catchments of northern China. In June 2021, a comprehensive study was conducted in the Jinan Spring Catchment (JSC), where 30 groundwater and surface water samples were collected. The sources and spatial distribution of nitrate pollution were systematically investigated through hydrochemical analysis combined with dual-isotope tracing techniques (δ15NNO3 and δ18ONO3). Analytical results revealed that the predominant anion and cation sequences were HCO3- > SO42- > Cl- > NO3- and Ca2+ > Na+ > Mg2+ > K+, respectively, with HCO3·SO4-Ca identified as the primary hydrochemical type. Notably, the average NO3- concentration in groundwater (46.62 mg/L) significantly exceeded that in surface water (4.96 mg/L). Among the water samples, 11 locations exhibited substantial nitrate pollution, demonstrating an exceedance rate of 42%. Particularly, the NO3--N concentrations in both the upstream recharge area and downstream drainage area were markedly higher than those in the runoff area. The spatial distribution of NO3- concentrations was primarily influenced by mixing processes, with no significant evidence of denitrification observed. The isotopic compositions ranged from -1.42‱ to 12.79‱ for δ15NNO3 and 0.50‱ to 15.63‱ for δ18ONO3. Bayesian isotope mixing model (MixSIAR) analysis indicated that domestic sewage and manure constituted the principal nitrate sources, contributing 37.1% and 56.9% to groundwater and surface water, respectively. Secondary sources included soil organic nitrogen, rainfall and fertilizer NH4+, and chemical fertilizers, while atmospheric deposition showed the lowest contribution rate. Additionally, potential mixing of soil organic nitrogen with chemical fertilizer was identified.

Combustion-driven inorganic nitrogen in PM2.5 from a city in central China has the potential to enhance the nitrogen load of North China

Inorganic nitrogen (NH4+ and NO3-) is a significant component of PM2.5, influencing not only regional ecological systems but also on other regions through the migration of air masses. However, few studies have simultaneously investigated the sources of NH4+ and NO3-, and their potential transport pathways remain poorly understood. Here, daily PM2.5 samples were collected in Jiaozuo, a key city in the air pollution transmission channel to the north China, from 1 September to 5 December, 2017. Major water-soluble inorganic ions and the isotope compositions of NH4+ and NO3- in PM2.5 were analyzed. The results indicated substantial amounts of inorganic nitrogen in PM2.5, particularly at high PM2.5 concentrations. The Bayesian isotope mixing model (MixSIAR) results revealed that combustion sources contributed 79.5 % to NO3- and 51.6 % to NH4+. Moreover, the medium to high potential source regions for combustion-related NH3 is basically consistent with combustion-related NOx. Therefore, stringent regulation of combustion emissions has the potential to mitigate inorganic nitrogen pollution in PM2.5 in Jiaozuo. The results of the forward trajectory cluster and PSCF (potential source contribution function) analysis revealed that a significant amount of combustion-driven inorganic nitrogen in PM2.5 from Jiaozuo will transport to downwind area, particularly north China. Combustion-driven inorganic nitrogen levels carried by these air masses exceeded half the average value for cities in North China during the same period. Our study highlights that combustion emissions dominate the inorganic nitrogen sources in PM2.5 and that substantial amounts of combustion-driven inorganic nitrogen can be transported from Jiaozuo to North China, potentially enhancing the nitrogen load in those areas.

Nitrate sources and transformation in surface water and groundwater in Huazhou District, Shaanxi, China: Integrated research using hydrochemistry, isotopes and MixSIAR model

Global water resources affected by excessive nitrate (NO3-) have caused a series of human health and ecological problems. Therefore, identification of NO3- sources and transformations is of pivotal significance in the strategic governance of widespread NO3- contaminant. In this investigation, a combination of statistical analysis, chemical indicators, isotopes, and MixSIAR model approaches was adopted to reveal the hydrochemical factors affecting NO3- concentrations and quantify the contribution of each source to NO3- concentrations in surface water and groundwater. The findings revealed that high groundwater NO3- concentration is concentrated in the southwestern region, peaking at 271 mg/L. NO3- concentration in the Wei River and Yuxian River exhibited an increase from upstream to downstream, but in the Shidi River and Luowen River, its concentration was highest in the upstream. Groundwater NO3- has noticeable correlation with Na+, Ca2+, Mg2+, Cl-, HCO3-, TDS, EC, and ORP. In surface water, NO3- level is significantly correlated with NH4+ and ORP. Major sources of NO3- in surface and groundwater comprise manure & sewage and soil nitrogen. Source contribution for surface water was calculated by MixSIAR model to obtain soil nitrogen (57.7%), manure & sewage (23.8%), chemical fertilizer (12%), and atmospheric deposition (6.4%). In groundwater, soil nitrogen and manure & sewage accounted for 19% and 63.8% of nitrate sources, respectively. Both surface water and groundwater exhibited strong oxidation, with nitrification the primary process. It is expected that this study will provide insights into the dynamics of NO3- and contribute to the development of effective strategies for mitigating NO3- contaminant, leading to sustainable management of water resources.

Using surface runoff to reveal the mechanisms of landscape patterns driving on various forms of nitrogen in non-point source pollution

Non-point source (NPS) pollution directly threatens river water quality, constrains sustainable economic development, and poses hazards to human health. Comprehension of the impact factors on NPS pollution is essential for scientific river water quality management. Despite the landscape pattern being considered to have a significant impact on NPS pollution, the driving mechanism of landscape patterns on NPS pollution remains unclear. Therefore, this study coupled multi-models including the Soil and Water Assessment Tool (SWAT), Random Forest, and Partial Least Squares Structural Equation Modeling (PLS-SEM) to construct the connection between landscape patterns, NPS pollution, and surface runoff. The results suggested that increased runoff during the wet season enhances the link between landscape patterns and NPS pollution, and the explained NPS pollution variation by landscape pattern increased from 59.6 % (dry season) to 84.9 % (wet season). Furthermore, from the impact pathways, we find that the sink landscape pattern can significantly and indirectly influence NPS pollution by regulating surface runoff during the wet season (0.301*). Meanwhile, the sink and source landscape patterns significantly and directly impact NPS pollution during different seasons. Moreover, we further find that the percentage of paddy land use (Pad_PLAND) and grassland patch density (Gra_PD) metrics can significantly predict the dissolved total nitrogen (DTN) and nitrate nitrogen (NO3--N) variation. Thus, controlling the runoff migration process by guiding the rational evolution of watershed landscape patterns is an important development direction for watershed NPS pollution management.

Mechanisms of evolution and pollution source identification in groundwater quality of the Fen River Basin driven by precipitation

Groundwater pollution has attracted widespread attention as a threat to human health and aquatic ecosystems. However, the mechanisms of pollutant enrichment and migration are unclear, and the spatiotemporal distributions of human health risks are poorly understood, indicating insufficient groundwater management and monitoring. This study assessed groundwater quality, human health risks, and pollutant sources in the Fen River Basin(FRB). Groundwater quality in the FRB is good, with approximately 87 % of groundwater samples rated as "excellent" or "good" in both the dry and rainy seasons. Significant precipitation elevates groundwater levels, making it more susceptible to human activities during the rainy season, slightly deteriorating water quality. Some sampling points in the southern of Taiyuan Basin are severely contaminated by mine drainage, with water quality index values up to 533.80, over twice the limit. Human health risks are mainly from As, F, NO3-, and Cr. Drinking water is the primary pathway of risk. From 2019 to 2020, the average non-carcinogenic risk of As, F, and NO3- increased by approximately 28 %, 170 % and 8.5 %, respectively. The average carcinogenic risk of As and Cr increased by 28 % and 786 %, the overall trend of human health risks is increasing. Source tracing indicates As and F mainly originate from geological factors, while NO3- and Cr are significantly influenced by human activities. Various natural factors, such as hydrogeochemical conditions and aquifer environments, and processes like evaporation, cation exchange, and nitrification/denitrification, affect pollutant concentrations. A multi-tracer approach, integrating hydrochemical and isotopic tracers, was employed to identify the groundwater pollution in the FRB, and the response of groundwater environment to pollutant enrichment. This study provides a scientific basis for the effective control of groundwater pollution at the watershed scale, which is very important in the Loess Plateau.

New insights into the controlling factors of nitrate spatiotemporal characteristics in groundwater of Dagu aquifer in Qingdao, China

Identifying spatiotemporal variation of groundwater NO3-N and its primary controlling factors are vital for groundwater protection. This study, under the data scarce conditions and based on time series monitoring data in Dagu aquifer, applied methods including hydrochemical ion ratio, multiple linear regression, support vector regression and grey relational analysis and dedicated to revealing primary controlling factors of temporal variation patterns of groundwater NO3-N. The results showed that agricultural and manure fertilizer are the main sources of NO3-N in north and central area (vegetable farming area), and that domestic sewage discharge and manure fertilizer are the main sources of NO3-N in south area (residential and grain planting area). In addition, results identified the dominant influencing factors of variation of NO3-N in different regions, with human wastewater discharge, nitrogen load amount and water-table depth being the dominant factors of variations of NO3-N in north area, human wastewater discharge being the main factor of variations of NO3-N in central area, and irrigation water and human wastewater being the leading factors of variations of NO3-N in south area. Moreover, types of controlling factors can influence the seasonal variations of NO3-N. NO3-N in vegetable farming area that dominantly affected by fertilization generally shows higher concentration and larger variation range of concentration during summer and autumn than that during spring. NO3-N which mainly affected by human wastewater discharge and manure inputs shows minimal seasonal variation of mean concentration. NO3-N in grain area influenced by irrigation could show more significant variations during spring and autumn than that during summer. The conclusions can enhance understandings of major influencing factors on NO3-N variation in local aquifer. Importantly, the dominant roles of water-table depth and irrigation in NO3-N variation of N2 site (vegetable planting area) and S5 site (grain planting area), respectively, were highlighted.

Assessing catchment-scale groundwater discharge: Optimal tracers and factors analysis

Identifying streamwater-groundwater interactions (SGI) is crucial for effective water resource management, especially in arid and semi-arid regions. Despite the effectiveness of tracers in detecting these interactions, their large-scale application is challenged by the variability in tracer characteristics and natural conditions. This study addresses these challenges through extensive research across seven watersheds (7636-60,916 km2) in China's Loess Plateau (CLP). We utilized multiple physicochemical and stable isotope tracers (δ2H and δ18O) to elucidate the spatiotemporal variations and controlling factors of SGI, and to estimate uncertainties in quantifying SGI using various indicators during unidirectional water exchange periods. Our findings indicated that groundwater discharge into streamwater dominates SGI in the CLP, with mean discharge ratios (the percentage of river flow that originates from groundwater discharge) varying from 10% to 57%. Significant spatial variability was observed both across and within watersheds. The central watersheds exhibited lower discharge ratios (23 ± 11%) compared to the northern (29 ± 12%) and southern (25 ± 13%) watersheds. The upper reaches showed higher discharge ratios (28 ± 12%) compared to the middle and lower reaches (22 ± 8%). Loess thickness and vegetation primarily limit groundwater discharge by affecting groundwater storage and water flow velocity. The utilization of individual isotopic or hydrochemical indicators introduces large uncertainties in quantifying groundwater discharge ratios due to isotope fractionation or water-rock interaction, while the combination of these two indicators can reduce uncertainties in quantifying SGI. This study provides valuable insights for selecting environmental tracers to quantify SGI, contributing to sustainable water resource management in arid and semi-arid regions.

Combined effects of precipitation anomalies and dams on streamwater-groundwater interaction in the Fen River basin, China

Both water management measures like damming and changes in precipitation as a result of anthropogenic induced climate change have exerted profound effects on the dynamics of streamwater-groundwater interaction (SGI). However, their compound effects on SGI have not been investigated so far. Taking the Fen River of China as an example, this study aims to examine the synergistic impacts of damming and precipitation anomalies on SGI dynamics. The sampling considered the seasonal and interannual variability of precipitation (May and September in 2019 representing a dry year; May and August in 2021 representing a wet year), and long-term daily observational data, including water levels and water discharge, were combined to elucidate the compound effects. Precipitation anomalies and damming exert significant individual and combined influences on SGI. Separately, dams and reservoirs reversed the SGI dynamics, significantly increasing the contributions of streamwater to groundwater from 0 to 29 % to 78 % in the dam-affected areas. Further, the groundwater discharge ratios behind the dam (about 60 %) were three times higher than those in front of the dam. Precipitation anomalies significantly amplified interannual variability in SGI patterns, and groundwater discharge ratios increased by 47 % during the dry period (2019) compared to flood period (2021). The combined influence of precipitation anomalies and dam regulation remarkably changed the lateral, vertical, and longitudinal water exchange dynamics. Precipitation anomalies affected the SGI dynamics at the whole watershed scale, whereas dam regulation regimes exhibited a stronger control at the local scale. The compound effects of dam regulation and precipitation anomalies can result in different SGI patterns under various climate scenarios. More attention should be paid to the interrelated feedback mechanisms between damming, extreme precipitation events, and their impact on the watershed-scale hydrological cycle.