Geochemistry and origin of inorganic contaminants in soil, river sediment and surface water in a heavily urbanized river basin

Understanding the geochemistry and contamination of rivers affected by anthropogenic activities is paramount to water resources management. The Asopos river basin in central Greece is facing environmental quality deterioration threats due to industrial, urban and agricultural activities. Here, the geochemistry of river sediments and adjacent soil in terms of major and trace elements (Al, Ca, Mg, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) and the geochemical composition of surface water in terms of major ions, trace elements and nutrients along the Asopos river basin were determined. In addition, this study characterized potential nitrate sources through the analysis of stable isotope composition of NO3- (δ15Ν-ΝΟ3- and δ18Ο-ΝΟ3-). Results indicated that specific chemical constituents including nutrients (NO2-, NH4+, PO43-) and major ions (Na+, Cl-) were highest in the urban, industrialized and downstream areas. On the other hand, nitrate (NO3-) concentration in river water (median 7.9 mg/L) showed a decreasing trend from the upstream agricultural sites to the urban area and even more in the downstream of the urban area sites. Ionic ratios (NO3-/Cl-) and δ15Ν-ΝΟ3- values (range from +10.2 ‰ to +15.7 ‰), complemented with a Bayesian isotope mixing model, clearly showed the influence of organic wastes from septic systems and industries operating in the urban area on river nitrate geochemistry. The interpretation of geochemical data of soil and river sediment samples demonstrated the strong influence of local geology on Cr, Fe, Mn and Ni content, with isolated samples showing elevated concentrations of Cd, Cu, Pb and Zn, mostly within the industrialized urban environment. The calculation of enrichment factors based on the national background concentrations provided limited insights into the origin of geogenic metals. Overall, this study highlighted the need for a more holistic approach to assess the impact of the geological background and anthropogenic activities on river waters and sediments.

Stable isotopes reveal organic nitrogen pollution and cycling from point and non-point sources in a heavily cultivated (agricultural) Mediterranean river basin

The Pinios River Basin (PRB) is the most intensively cultivated area in Greece, which hosts numerous industries and other anthropogenic activities. The analysis of water samples collected monthly for ∼1 ½ years in eight monitoring sites in the PRB revealed nitrate pollution of organic origin extending from upstream to downstream and occurring throughout the year, masking the signal from the application of synthetic fertilizers. Nitrate concentrations reached up to 3.6 mg/l as NO3--N, without exceeding the drinking water threshold of ∼11.0 mg/l (as NO3--N). However, the water quality status was "poor" or "bad" in ∼50 % of the samples based on a local index, which considers the potential impact of nitrate on aquatic biological communities. The δ15Ν-ΝΟ3- and δ18O-NO3- values ranged from +4.4 ‰ to +20.3 ‰ and from -0.5 ‰ to +14.4 ‰, respectively. The application of a Bayesian model showed that the proportional contribution of organic pollution from industries, animal breeding facilities and manure fertilizers exceeded 70 % in most river sites with an overall uncertainty of ∼0.3 (UI90 index). The δ18O-NO3- and its relationship with δ18O-H2O revealed N-cycling and mixing processes, which were difficult to identify apart from the uptake of nutrients by phytoplankton during the growing season and metabolic activities. The strong correlation of δ15Ν-ΝΟ3- values with a Land Use Index (LUI) and a Point Source Index (PSI) highlighted not only the role of non-point nitrate sources but also of point sources of nitrate pollution on water quality degradation, which are usually overlooked. The nitrification of organic wastes is the dominant nitrate source in most rivers in Europe. The systematic monitoring of rivers for nitrate isotopes will help improve the understanding of N-cycling and the impact of these pollutants on ecosystems and better inform policies for protection measures so to achieve good ecological status.

Identify nitrogen transport paths and sources contribution in karst valley depression area using isotopic approach

Karst groundwater provides drinking water for a quarter of Earth's population. However, in intensive agricultural regions worldwide, karst water is commonly polluted by nitrate (NO₃^-), particularly in the valley depression areas with well hydrological connectivity. The valley depression aquifers are particularly vulnerable to anthropogenic pollution because their pipes and sinkholes respond quickly to rainfall events and anthropogenic inputs. Identifying nitrate sources and transport paths in the valley depression areas is key to understanding the nitrogen cycle and effectively preventing and controlling NO₃^- pollution. This study collected high-resolution samples at four sites including one surface stream-SS, two sinkholes-SH and reservoir-Re, during the wet season in the headwater sub-catchment. The chemical component concentrations and stable isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were analyzed. The stable isotope analysis model in R language (SIAR) was used to quantitatively analyze the contribution rate of NO₃^- sources. The results showed that the down section site (Re) has the highest [NO₃^--N], followed by SH and the lowest SS. The sources calculation of SIAR demonstrated that, during the non-rainfall period, soil organic nitrogen was the primary source of the down section site, followed by fertilizer and the upper reaches sinkholes. During the rainfall period, fertilizer was the primary source of the down section site, followed by soil organic nitrogen and from upper reaches sinkholes. Rainfall events accelerated fertilizer-leaching into the groundwater. Slight denitrification may have occurred at the sampling sites but the assimilation of Re and SH could not occur. In conclusion, agricultural activities were still the primary influencing factor of [NO₃^--N] in the study area. Therefore, the focus of NO₃^- prevention and control in the valley depression areas should consider the methods and timing of fertilization and the spatial distribution of sinkholes. To reduce nitrogen flux in the valley depression area, effective management policy should consider, e.g., prolongation of water residence time by wetland, and blocking nitrogen loss paths by sinkholes.

Assessment of natural and anthropogenic contamination sources in a Mediterranean aquifer by combining hydrochemical and stable isotope techniques

The Atalanti basin is an intensively cultivated area in central Greece, facing groundwater quality deterioration threats due to natural and anthropogenic-related contamination sources. A combination of statistical and hydrogeochemical techniques, and stable isotope compositions (δ²H-H₂O and δ¹⁸Ο-Η₂Ο, δ¹⁵Ν-ΝΟ₃^- and δ¹⁸Ο-ΝΟ₃^-, δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2-) were applied to elucidate the origin of salinity and nitrate contamination, and shed light on the potential associations between geogenic Cr(VI) and NO₃^- sources and transformations. Nitrate and Cr(VI) concentrations reached up to 337 mg L^-1 and 76.1 μg L^-1, respectively, exceeding WHO threshold values in places. The cluster of samples with the high salinity was mostly influenced by irrigation return flow and marine aerosols, and less by seawater intrusion, as evidenced by the ionic ratios (e.g., Na⁺/Cl^-) and the stable isotopes of oxygen and hydrogen in water, and sulphur and oxygen in sulphates. The δ¹⁵Ν-ΝΟ₃^- and δ¹⁸O-NO₃^- values ranged from +2.0 ‰ to +14.5 ‰ and + 0.3 ‰ to +11.0 ‰, respectively. We found that the dominant sources of NO₃^- in groundwater were fertilizers in the central part of the area and sewage waste in the northern part around the residential area of Livanates. The occurrence of denitrification was evident in the northern part of the basin, where the DO levels were lowest (≤ 2.2 mg L^-1), whereas nitrification of NH₄⁺-fertilizers prevailed in the central part. Elevated Cr(VI) values (≥ 20 μg/l) were associated with the lowest deviation of the measured from the theoretical nitrification δ¹⁸Ο-NO₃^- values, whereas the lowest Cr(VI) values were observed in the denitrified water samples. Our isotope findings revealed the strong influence of redox conditions on the biogeochemical transformations of N species and the mobilization of Cr(VI) that will help improve the understanding of the fate of these contaminants from the unsaturated zone to the groundwater in areas of agricultural and urban land use.

Determining nitrate sources in storm runoff in complex urban environments based on nitrogen and oxygen isotopes

Urban storm runoff, as the primary transport medium for nutrients entering urban rivers, contributes to urban water contamination. Accurate source identification is critical for controlling water pollution. Although some studies have used nitrate isotopic composition (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) to identify nitrate (NO₃^--N) in urban storm runoff, the relatively low frequency of collecting samples in surface runoff within a single functional area hinders the understanding of spatial variations and dynamic process of NO₃^--N sources over the runoff process. This study investigated the nitrogen (N) concentrations and analyzed dynamic changes of NO₃^--N sources in surface runoff in different urban functional areas, drainage pipeline runoff, and channels during the complete runoff process in Wuxi, east China. The results showed that N concentrations in pipeline runoff and channels were higher than those in surface runoff, indicating that high concentration of N pollutants were accumulated in drainage pipelines. Information of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- suggested that the main NO₃^--N source varied between runoff stages. NO₃^--N contribution from atmospheric deposition decreased in the order: surface runoff (57%) > residential pipeline runoff (25%) > channels (14%), while the opposite trend was observed for the contributions from sewage, increasing from 10%, 26% to 39%. In urban storm runoff, more sewage, fertilizers, and soil N were carried into the surface runoff after 30% of cumulative runoff ratio and carried into pipeline runoff in the initial 25% of cumulative runoff ratio in the residential area. As the first attempt to identify nitrate sources over the cumulative runoff in different urban functional areas, this work expands our understanding of the primary nitrate source in urban storm runoff. The findings provide important insights for developing strategies to mitigate non-point source water pollution.

Recent advances in the source identification and remediation techniques of nitrate contaminated groundwater: A review

Researchers have long been committed to identify nitrate sources in groundwater and to develop an advanced technique for its remediation because better apply remediation solution and management of water quality is highly dependent on the identification of the NO₃^- sources contamination in water. In this review, we systematically introduce nitrate source tracking tools used over the past ten years including dual isotope and multi isotope techniques, water chemistry profile, Bayesian mixing model, microbial tracers and land use/cover data. These techniques can be combined and exploited to track the source of NO₃^- as mineral or organic fertilizer, sewage, or atmospheric deposition. These available data have significant implications for making an appropriate measures and decisions by water managers. A continuous remediation strategy of groundwater was among the main management strategies that need to be applied in the contaminated area. Nitrate removal from groundwater can be accomplished using either separation or reduction based process. The application of these processes to nitrate removal is discussed in this review and some novel methods were presented for the first time. Moreover, the advantages and limitations of each approach are critically summarized and based on our own understanding of the subject some solutions to overcomes their drawbacks are recommended. Advanced techniques are capable to attain significantly higher nitrate and other co-contaminants removal from groundwater. However, the challenges of by-products generation and high energy consumption need to be addressed in implementing these technologies for groundwater remediation for potable use.

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.

Hypoxia formation in the East China Sea by decomposed organic matter in the Kuroshio Subsurface Water

Although internal decomposition of organic matter (OM) in the Kuroshio Subsurface Water (KSSW) is a crucial factor for hypoxia formation in the East China Sea (ECS), the seasonal source and contributions of this OM remain debated. This study applied datasets collected in June and October 2015 to discuss these issues qualitatively and quantitatively. According to the variations in several parameters along the KSSW route, N₂ fixation signals related to decomposed OM were apparent in the southern ECS during June, while terrestrial input signals were revealed in the northern ECS during June and most of the ECS during October. The terrestrial input contributed 47% of the decomposed OM near the historic hypoxic area in June, indicating that the terrestrial and marine sources contributed almost equally to the development of ECS hypoxia. These results provide vital information for understanding the mechanism of hypoxia formation driven by eutrophication and oceanic circulation.

Determining nitrate and sulfate pollution sources and transformations in a coastal aquifer impacted by seawater intrusion-A multi-isotopic approach combined with self-organizing maps and a Bayesian mixing model

Over the past few decades, the La Paz aquifer system in Baja California Sur, Mexico, has been under severe pressure due to overexploitation for urban water supply and agriculture; this has caused seawater intrusion and deterioration in groundwater quality. Previous studies on the La Paz aquifer have focused mainly on seawater intrusion, resulting in limited information on nitrate and sulfate pollution. Therefore, pollution sources have not yet been identified sufficiently. In this study, an approach combining hydrochemical tools, multi-isotopes (δ²H_H2O, δ¹⁸O_H2O, δ¹⁵N_NO3, δ¹⁸O_NO3, δ³⁴S_SO4, δ¹⁸O_SO4), and a Bayesian isotope mixing model was used to estimate the contribution of different nitrate and sulfate sources to groundwater. Results from the MixSIAR model revealed that seawater intrusion and soil-derived sulfates were the predominant sources of groundwater sulfate, with contributions of ~43.0% (UI₉₀ = 0.29) and ~42.0% (UI₉₀ = 0.38), respectively. Similarly, soil organic nitrogen (~81.5%, UI₉₀ = 0.41) and urban sewage (~12.1%, UI₉₀ = 0.25) were the primary contributors of nitrate pollution in groundwater. The dominant biogeochemical transformation for NO₃^- was nitrification. Denitrification and sulfate reduction were discarded due to the aerobic conditions in the study area. These results indicate that dual-isotope sulfate analysis combined with MixSIAR models is a powerful tool for estimating the contributions of sulfate sources (including seawater-derived sulfate) in the groundwater of coastal aquifer systems affected by seawater intrusion.

Elucidating heterogeneous nitrate contamination in a small basement aquifer. A multidisciplinary approach: NO3 isotopes, CFCs-SF6, microbiological activity, geophysics and hydrogeology

Nitrate contamination of groundwater remains a major concern despite all the measures and efforts undertaken over the last decades to protect water resources. We focused on a small catchment in Brittany (France) facing nitrate pollution with concentrations over the European drinking water standard of 50 mg.L^-1. This is a common situation in catchments where - supposedly effective - measures were applied for reducing the transfer of N to groundwater. At the scale of this small (~100 ha) basement aquifer, nitrate concentrations are very heterogeneous in the groundwater, sampled up to 15-20 m below the soil surface in several observation wells (hereafter referred as piezometers) and up to 110 m deep in a borehole drilled through a faulted area near the Spring (outlet of the catchment). We used complementary and robust approaches for exploring and constraining the driving parameters of nitrate transfer and distribution in groundwater. Detailed geological work and a geophysical electrical resistivity tomography survey identified the lithologies, tectonic structures and weathering layers. This highlighted a complex geological structure with several compartments delimited by faults, as well as the highly variable thickness of the weathered layer. It also illustrated the heterogeneity of the hydrosystem, some compartments appearing to be disconnected from the general groundwater flow. This was confirmed by geochemical analyses and by the mean apparent groundwater residence time based on CFCs-SF6 and noble-gas analyses, locally revealing old and nitrate-free groundwater, and very old water with a recharge temperature below than the current average temperature in the area, reflecting water dating back to the last period of glaciation (-19 to -17 ky). Nitrate isotopes clearly showed denitrification processes in a few piezometers, which was generally supported by microbiology and molecular biology results. This highlighted the presence of functional genes involved in denitrification as well as a capacity of the groundwater microbial community to denitrify when in situ conditions are favourable. This type of combined approach - covering chemistry, isotopic methods, dissolved gases, microbiological activity, geophysics and hydrogeology - appears to be indispensable for implementing the most relevant programme of measures and for accurately assessing their effectiveness, notably by considering the timeframe between implementation of the measures and their impact on groundwater quality.

Comparative evaluation of urban versus agricultural nitrate sources and sinks in an unconfined aquifer by isotopic and multivariate analyses

Nitrate (NO₃^-) is one of the most widespread contaminants in groundwater primarily due to agricultural activities utilizing N-containing fertilizers and the presence of animal wastes. Hydrochemical and nitrate isotope data (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) from the unconfined aquifer in the urban area of Del Campillo city and its surrounding rural area with different land-use types, i.e. individual sanitation systems, agricultural areas and livestock breeding facilities, were generated to investigate the impact of nitrogen pollution sources and to assess N-biogeochemical processes. The Principal Component Analysis of hydrochemical and isotopic data were used to compare the factors that control the groundwater quality and particularly the nitrate concentrations in the urban and the rural area. The results showed that nitrate pollution in the urban area of Del Campillo city originated mainly from the on-site sanitation systems and/or animal domestic wastes, whereas in the rural area nitrate pollution was mostly attributed to a combination of urea-based fertilizers and manure from livestock breeding activities. The aquifer is under oxic to suboxic conditions in the rural area and becomes suboxic in the urban area where the higher supply of organic matter consumes oxygen. As a result, denitrification was more significant in the urban area compared to the rural area, as evidenced by the higher N and O isotope enrichment factor (ε). This work will be used to benchmark the current nitrate contamination status in the region and evaluate effective planning of environmental measures and remediation strategies.

Nitrate sources and biogeochemical processes in karst underground rivers impacted by different anthropogenic input characteristics

Nitrate is one of the most common pollution sources in groundwater, particularly in highly vulnerable karst aquifers. The potential for nitrification and denitrification within karst aquifers varies in different settings depending on the extent of anthropogenic inputs, so that accurate identification of nitrate sources can be difficult. Geochemical data and dual nitrate isotopes were measured in this study, incorporating a Bayesian isotopic mixing model, and used to identify nitrate sources, nitrification and denitrification, and quantitatively determine nitrate sources under different extents of anthropogenic inputs in three karst catchments within Chongqing Municipality, SW China: Laolongdong (an urbanized area), Qingmuguan (a suburban village), and Shuifang Spring (a protected natural area). At the Laolongdong catchment, the groundwater was in a reducing condition and enriched in δ¹⁵N_NO3 (averaging 18.9 ± 6.9‰) and δ¹⁸O_NO3 (averaging 8.5 ± 4.6‰). Manure and sewage waste were the main contributing nitrate sources. A slope of 1.8: 1 of the dual isotopes suggested a denitrification process occurring in anaerobic conduit flow. Within the Qingmuguan catchment, groundwater had average δ¹⁵N_NO3 and δ¹⁸O_NO3 values of 9.7 ± 3.5‰, and 1.9 ± 3.4‰, respectively. The data showed evidence for nitrification, and the contribution of soil organic nitrogen was 52.1%, followed by a contribution of 44.8% from manure and wastewater. At the Shuifang Spring catchment, the mean δ¹⁵N_NO3 and δ¹⁸O_NO3 values in groundwater were 8.8 ± 2.9‰, 2.3 ± 4.6‰, respectively. Nitrification was the dominant process and most of the nitrate was derived from soil organic nitrogen. This study suggests that karst underground rivers overlain by urban land use undergo denitrification, while the suburban and relatively pristine karst aquifers are dominated by nitrification, allowing development of a conceptual model for nitrate sources and transformations in karst aquifers from the categories of land use (i.e., urban, suburban, and pristine areas). MAIN FINDING: Anthropogenic activities can change biogeochemical nitrogen dynamics of vulnerable karst aquifers, such that the groundwater overlain by an urban settlement has undergone denitrification, while suburban and pristine areas have been dominated by nitrification.

A multi-approach assessment of land use effects on groundwater quality in a karstic aquifer

Groundwater represents almost half of the drinking water worldwide and more than one third of water used for irrigation. Agro-industrial activities affect water resources in several manners; one of the most important is leaching of agrochemical residues. This research identifies the major contributors of changes in groundwater quality comparing two contrasting land uses in a karstic area of the Yucatan peninsula as case study. Using a multiple approach, we assess the impact of land use with physicochemical data, multivariate analyses, hydrogeochemistry and nitrate isotopic composition. We confirmed that agricultural land use has a greater impact on groundwater quality, observed in higher concentration of nitrates, ammonium, potassium and electrical conductivity. Seasonality has an influence on phosphates and the chemical composition of the groundwater, increasing the concentration of dissolved substances in the rainy season. There was a clear effect of manure application in the agricultural zone and the nitrate isotopic composition of groundwater points toward recharge in certain areas. We consider that seasonality and land use effects are intertwined and sometimes difficult to separate, likely because of land use intensity and hydrogeochemical process at a local scale. Finally, we observed poor groundwater quality in the agricultural area during the wet season; thus, it is desirable to maintain non-agricultural areas that provide groundwater of appropriate quality.

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.

Tracing nitrate sources with a combined isotope approach (δ15NNO3, δ18ONO3 and δ11B) in a large mixed-use watershed in southern Alberta, Canada

Rapid population growth and land-use intensification over the last century have resulted in a substantial increase in nutrient loads degrading marine and freshwater ecosystems worldwide. In mixed-use watersheds, elevated nitrogen loads from wastewater treatment plant (WWTP) effluent or agricultural runoff often drive the eutrophication of waterways. Accordingly, the objective of this research was to identify sources of riverine nitrate (NO₃), a deleterious dissolved species of nitrogen, with a combined isotopic tracing technique in the Bow River and the Oldman River in Alberta, Canada. Riverine NO₃ and boron (B) concentrations, mean daily flux and δ¹⁵N_NO3, δ¹⁸O_NO3, and δ¹¹B values were determined at 17 mainstem sites during high and low discharge periods in 2014 and 2015. The data for mainstem sites were then compared to results for effluent from seven WWTPs, eight synthetic fertilizers, cow manure, and three predominantly agricultural tributary sites to estimate point and non-point NO₃ sources. The NO₃ flux, δ¹⁵N_NO3 and δ¹⁸O_NO3 values indicated the city of Calgary's Bonnybrook WWTP effluent accounts for the majority of the NO₃ flux in the Bow River downstream of Calgary. δ¹⁵N_NO3 and δ¹¹B values in the Bow River highlighted an increase in agricultural NO₃ loading downstream of irrigation return-flows. A three-fold decrease in the NO₃:B flux ratio indicated NO₃-removal processes are active in the lower reaches of the Bow River. For the Oldman River, δ¹¹B values revealed elevated nutrient loading from the Lethbridge WWTP effluent (10% of downstream B flux). Furthermore, the agricultural tributaries contributed 25% of the local B flux to the Oldman River. Overall, δ¹¹B was proven to be an effective co-tracer for discriminating between urban and agricultural sources of NO₃ in these large mixed-use watersheds. This combined isotope tracing approach has significant potential to identify point and non-point NO₃ sources driving eutrophication around the world.

Nitrate sources and processes in the surface water of a tropical reservoir by stable isotopes and mixing model

Nitrate is one of the primary nutrients associated with sedimentation and fuels eutrophication in reservoir systems. In this study, water samples from Bukit Merah Reservoir (BMR) were analysed using a combination of water chemistry, water stable isotopes (δ²H-H₂O and δ¹⁸O-H₂O) and nitrate stable isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-). The objective was to evaluate nitrate sources and processes in BMR, the oldest man-made reservoir in Malaysia. The δ¹⁵N-NO₃^- values in the river and reservoir water samples were in the range +0.4 to +14.9‰ while the values of δ¹⁸O-NO₃^- were between -0.01 and +39.4‰, respectively. The dual plots of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- reflected mixing sources from atmospheric deposition (AD) input, ammonium in fertilizer/rain, soil nitrogen, and manure and sewage (MS) as the sources of nitrate in the surface water of BMR. Nitrate stable isotopes suggested that BMR undergoes processes such as nitrification and mixing. Denitrification and assimilation were not prevalent in the system. The Bayesian mixing model highlighted the dominance of MS sources in the system while AD contributed more proportion in the reservoir during both seasons than in the river. The use of δ¹³C, δ¹⁵N, and C:N ratios enabled the identification of terrestrial sources of the organic matter in the sediment, enhancing the understanding of sedimentation associated with nutrients previously reported in BMR. Overall, the nitrate sources and processes should be considered in decision-making in the management of the reservoir for irrigation, Arowana fish culture and domestic water supply.

Identification of sources and transformations of nitrate in the Xijiang River using nitrate isotopes and Bayesian model

Coupled nitrogen and oxygen isotopes of nitrate have proven useful in identifying nitrate sources and transformation in rivers. However, isotopic fractionation and low-resolution monitoring limit the accurate estimation of nitrate dynamics. In the present study, the spatio-temporal variations of nitrate isotopes (¹⁵N and ¹⁸O) and hydrochemical compositions (NO₃^- and Cl^-) of river water were examined to understand nitrate sources in the Xijiang River, China. High-frequency sampling campaigns and isotopic analysis were performed at the mouth of the Xijiang River to capture temporal nitrate variabilities. The overall values of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- ranged from +4.4‰ to +14.1‰ and from -0.3‰ to +6.8‰, respectively. The results of nitrate isotopes indicated that NO₃^- mainly originated from soil organic nitrogen (SON), chemical fertilizer (CF), and manure and sewage wastes (M&S). The negative correlation of nitrate isotopic values with NO₃^-/Cl^- ratios suggested the importance of denitrification in NO₃^- loss. The results of Bayesian model with incorporation of isotopic fractionation during the denitrification showed that SON and CF contributed to the most (72-73%) nitrate in the wet season; whereas approximately 58% of nitrate was derived from anthropogenic inputs (M&S and CF) in the dry season. The nitrate flux was 2.08 × 10⁵ tons N yr^-1 during one hydrologic year between 2013 and 2014, with 86% occurring in the wet season. Long-term fluctuations in nitrate flux indicated that nitrate export increased significantly over the past 35 years, and was significantly correlated with nitrate concentrations. The seasonal pattern of nitrate dynamics indicated the mixing of nitrified NO₃^- and denitrified NO₃^- between surface flow and groundwater flow under different hydrological conditions. Overall, the present study quantitatively evaluates the spatio-temporal variations in nitrate sources in a subtropical watershed, and the high-frequency monitoring gives a better estimate of nitrate exports and proportional contributions of nitrate sources.

An overview of nitrate sources and operating processes in arid and semiarid aquifer systems

Nitrate concentration in most aquifers in arid and semi-arid areas has increased in the past several decades as a result of human activities. Under the predominantly oxic conditions of these aquifers, denitrification is inhibited, allowing nitrate, a soluble and stable form of nitrogen (N), to accumulate. Because of its close association with municipal and agricultural wastes, nitrate is commonly used as an indicator of anthropogenic contamination. Aquifers affected by agricultural waste may contain salts from irrigation returns and herbicides in addition to nitrates. Preventing leakage from soil to deeper parts of the aquifer is thus a priority in the sustainable management of aquifers in arid and semiarid areas. Studies report a wide range of nitrate concentrations distributed non-uniformly within the aquifer, with roughly 40% and 20% of sampled wells exceeding 50mg/L nitrate in shallow and deep parts of the aquifer respectively. In aquifers at risk of becoming contaminated, nitrate isotopes (δ¹⁵N, δ¹⁸O, Δ¹⁷O) can be used to identify the source of nitrogen as mineral or organic fertilizer, sewage, or atmospheric deposition. A variety of mathematical models (crop, hydrological, geochemical, or a combination of them) have been successful in identifying best practices that minimize N leakage without negatively affecting crop yield. In addition, field research in crop management, e.g., conservation agriculture, has yielded promising results in determining the adequate dosage and time of application of fertilizers to reduce N losses. Examples of key dryland aquifers impacted by nitrate are discussed, and some of the most pressing challenges to achieve sustainability are presented.

Nitrate release from waste rock dumps in the Elk Valley, British Columbia, Canada

The origin, distribution and leaching of nitrate (NO₃^-) from coal waste rock dumps in the Elk Valley, British Columbia, Canada were defined using chemical and NO₃^- isotope analyses (δ¹⁵N- and δ¹⁸O-NO₃^-) of solids samples of pre- and post-blast waste rock and from thick (up to 180m) unsaturated waste rock dump profiles constructed between 1982 and 2012 as well as water samples collected from a rock drain located at the base of one dump and effluent from humidity cell (HC) and leach pad (LP) tests on waste rock. δ¹⁵N- and δ¹⁸O-NO₃^- values and NO₃^- concentrations of waste rock and rock drain waters confirmed the source of NO₃^- in the waste rock to be explosives and that limited to no denitrification occurs in the dump. The average mass of N released during blasting was estimated to be about 3-6% of the N in the explosives. NO₃^- concentrations in the fresh-blast waste rock and recently placed waste rock used for the HC and LP experiments were highly variable, ranging from below detection to 241mg/kg. The mean and median concentrations of these samples ranged from 10-30mg/kg. In this range of concentrations, the initial aqueous concentration of fresh-blasted waste rock could range from approximately 200-600mg NO₃^--N/L. Flushing of NO₃^- from the HCs, LPs and a deep field profile was simulated using a scale dependent leaching efficiency (f) where f ranged from 5-15% for HCs, to 35-80% for the LPs, to 80-90% for the field profile. Our findings show aqueous phase NO₃^- from blasting residuals is present at highly variable initial concentrations in waste rock and the majority of this NO₃^- (>75%) should be flushed by recharging water during displacement of the first stored water volume.

Determination of dominant sources of nitrate contamination in transboundary (Russian Federation/Ukraine) catchment with heterogeneous land use

Nitrate contamination of surface water and shallow groundwater was studied in transboundary (Russia/Ukraine) catchment with heterogeneous land use. Dominant sources of nitrate contamination were determined by applying a dual δ ¹⁵N-NO₃ and δ ¹⁸O-NO₃ isotope approach, multivariate statistics, and land use analysis. Nitrate concentration was highly variable from 0.25 to 22 mg L^-1 in surface water and from 0.5 to 100 mg L^-1 in groundwater. The applied method indicated that sewage to surface water and sewage and manure to groundwater were dominant sources of nitrate contamination. Nitrate/chloride molar ratio was added to support the dual isotope signature and indicated the contribution of fertilizers to the nitrate content in groundwater. Groundwater temperature was found to be an additional indicator of manure and sewerage leaks in the shallow aquifer which has limited protection and is vulnerable to groundwater pollution.

Isotopic constraints on water source mixing, network leakage and contamination in an urban groundwater system

Water supply in developing countries is prone to large water losses due to leaky distribution networks and defective sewers, which may affect groundwater quality and quantity in urban areas and result in complex subsurface mixing dynamics. In this study, a multi-stable isotope approach was used to investigate spatiotemporal fluctuations of surface and sub-surface water source partitioning and mixing, and to assess nitrogen (N) contamination in the urban water cycle of As-Salt, Jordan. Water import from the King Abdullah Canal (KAC), mains waters from the network, and wastewater are characterized by distinct isotopic signatures, which allowed us to quantify city effluents into the groundwater. Temporal variations in isotopic signatures of polluted groundwater are explained by seasonally fluctuating inflow, and dilution by water that originates from Lake Tiberias and enters the urban water cycle via the KAC. Isotopic analysis (N and O) and comparison between groundwater nitrate and nitrate from mains water, water imports and wastewater confirmed that septic waste from leaky sewers is the main contributor of nitrate contamination. The nitrate of strongly contaminated groundwater was characterized by highest δ¹⁵N_NO3 values (13.3±1.8‰), whereas lowest δ¹⁵N_NO3 values were measured in unpolluted groundwater (6.9‰). Analogously, nitrate concentration and isotopic ratios were used for source partitioning and qualitatively confirmed δD_H2O and δ¹⁸O_H2O-based estimates. Dual water isotope endmember mixing calculations suggest that city effluents from leaky networks and sewers contribute 30-64% to the heavily polluted groundwater. Ternary mixing calculations including also chloride revealed that 5-18% of the polluted groundwater is wastewater. Up to two thirds of the groundwater originates from mains, indicating excessive water loss from the network, and calling for improved water supply management.

The influence of bedrock hydrogeology on catchment-scale nitrate fate and transport in fractured aquifers

Characterising catchment scale biogeochemical processes controlling nitrate fate in groundwater constitutes a fundamental consideration when applying programmes of measures to reduce risks posed by diffuse agricultural pollutants to water quality. Combining hydrochemical analyses with nitrate isotopic data and physical hydrogeological measurements permitted characterisation of biogeochemical processes influencing nitrogen fate and transport in the groundwater in two fractured bedrock aquifers with contrasting hydrogeology but comparable nutrient loads. Hydrochemical and isotopic analyses of groundwater samples collected from moderately fractured, diffusely karstified limestone indicated nitrification controlled dissolved nitrogen fate and delivery to aquatic receptors. By contrast nitrate concentrations in groundwater were considerably lower in a low transmissivity highly lithified sandstone and pyrite-bearing shale unit with patchy subsoil cover. Geophysical and hydrochemical investigations showed shallower intervals contained hydraulically active fractures where denitrification was reflected through lower nitrogen levels and an isotopic enrichment ratio of 1.7 between δ(15)N and δ(18)O. Study findings highlight the influence of bedrock hydrogeological conditions on aqueous nitrogen mobility. Investigation results demonstrate that bedrock conditions need to be considered when implementing catchment management plans to reduce the impact of agricultural practices on the quality of groundwater and baseflow in receiving rivers. Nitrate isotopic signatures in the groundwater of a freely draining catchment underlain by a karstified aquifer and a poorly draining aquifer with a low transmissivity aquifer.

Effects of land-use patterns on in-stream nitrogen in a highly-polluted river basin in Northeast China

This study investigated the effects of land-use patterns on nitrogen pollution in the Haicheng River basin in Northeast China during 2010 by conducting statistical and spatial analyses and by analyzing the isotopic composition of nitrate. Correlation and stepwise regressions indicated that land-use types and landscape metrics were correlated well with most river nitrogen variables and significantly predicted them during different sampling seasons. Built-up land use and shape metrics dominated in predicting nitrogen variables over seasons. According to the isotopic compositions of river nitrate in different zones, the nitrogen sources of the river principally originated from synthetic fertilizer, domestic sewage/manure, soil organic matter, and atmospheric deposition. Isotope mixing models indicated that source contributions of river nitrogen significantly varied from forested headwaters to densely populated towns of the river basin. Domestic sewage/manure was a major contributor to river nitrogen with the proportions of 76.4 ± 6.0% and 62.8 ± 2.1% in residence and farmland-residence zones, respectively. This research suggested that regulating built-up land uses and reducing discharges of domestic sewage and industrial wastewater would be effective methods for river nitrogen control.

Nitrate source identification in groundwater of multiple land-use areas by combining isotopes and multivariate statistical analysis: A case study of Asopos basin (Central Greece)

Nitrate (NO3) is one of the most common contaminants in aquatic environments and groundwater. Nitrate concentrations and environmental isotope data (δ(15)N-NO3 and δ(18)O-NO3) from groundwater of Asopos basin, which has different land-use types, i.e., a large number of industries (e.g., textile, metal processing, food, fertilizers, paint), urban and agricultural areas and livestock breeding facilities, were analyzed to identify the nitrate sources of water contamination and N-biogeochemical transformations. A Bayesian isotope mixing model (SIAR) and multivariate statistical analysis of hydrochemical data were used to estimate the proportional contribution of different NO3 sources and to identify the dominant factors controlling the nitrate content of the groundwater in the region. The comparison of SIAR and Principal Component Analysis showed that wastes originating from urban and industrial zones of the basin are mainly responsible for nitrate contamination of groundwater in these areas. Agricultural fertilizers and manure likely contribute to groundwater contamination away from urban fabric and industrial land-use areas. Soil contribution to nitrate contamination due to organic matter is higher in the south-western part of the area far from the industries and the urban settlements. The present study aims to highlight the use of environmental isotopes combined with multivariate statistical analysis in locating sources of nitrate contamination in groundwater leading to a more effective planning of environmental measures and remediation strategies in river basins and water bodies as defined by the European Water Frame Directive (Directive 2000/60/EC).