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

Spectral and molecular insights into the characteristics of dissolved organic matter in nitrate-contaminated groundwater

The characteristics of dissolved organic matter (DOM) serve as indicators of nitrate pollution in groundwater. However, the specific DOM components associated with nitrate in groundwater systems remain unclear. In this study, dual isotopes of nitrate, three-dimensional Excitation emission matrices (EEMs) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were utilized to uncover the sources of nitrate and their associations with DOM characteristics. The predominant nitrate in the targeted aquifer was derived from soil organic nitrogen (mean 46.0%) and manure &sewage (mean 34.3%). The DOM in nitrate-contaminated groundwater (nitrate-nitrogen >20 mg/L) exhibited evident exogenous characteristics, with a bioavailable content 2.58 times greater than that of uncontaminated groundwater. Regarding the molecular characteristics, DOM molecules characterized by CHO + 3N, featuring lower molecular weights and H/C ratios, indicated potential for mineralization, while CHONS formulas indicated the exogenous features, providing the potential for accurate traceability. These findings provided insights at the molecular level into the characterization of DOM in nitrate-contaminated groundwater and offer scientific guidance for decision-making regarding the remediation of groundwater nitrate pollution.

The effect of carbonized zeolitic imidazolate framework-67 (ZIF-67) support on the reactivity and selectivity of bimetal-catalytic aqueous NO3- reduction

A metallic catalyst, Cobalt N-doped Carbon (Co@NC), was obtained from Zeolitic-Imidazolate Framework-67 (ZIF-67) for efficient aqueous nitrate (NO3-) removal. This advanced catalyst indicated remarkable efficiency by generating valuable ammonium (NH3/NH4+) via an environmentally friendly production technique during the nitrate treatment. Among various metals (Cu, Pt, Pd, Sn, Ru, and Ni), 3.6%Pt-Co@NC exhibited an exceptional nitrate removal, demonstrating a complete removal of 60 mg/L NO3--N (265 mg/L NO3-) in 30 min with the fastest removal kinetics (11.4 × 10-2 min-1) and 99.5% NH4+ selectivity. The synergistic effect of bimetallic Pt-Co@NC led to 100% aqueous NO3- removal, outperforming the reactivity by bare ZIF-67 (3.67%). The XPS analysis illustrated Co's promotor role for NO3- reduction to less oxidized nitrogen species and Pt's hydrogenation role for further reduction to NH4+. The durability test revealed a slight decrease in NO3- removal, which started from the third cycle (95%) and slowly proceeded to the sixth cycle (80.2%), while NH4+ selectivity exceeded 82% with no notable Co or Pt leaching throughout seven consecutive cycles. This research shed light on the significance of the impregnated Pt metal and Co exposed on the Co@NC surface for the catalytic nitrate treatment, leading to a sustainable approach for the effective removal of nitrate and economical NH4+ production.

Impacts of vegetable processing and cheese making effluent on soil microbial functional diversity, community structure, and denitrification potential of land treatment systems

The cheese making and vegetable processing industries generate immense volumes of high-nitrogen wastewater that is often treated at rural facilities using land applications. Laboratory incubation results showed denitrification decreased with temperature in industry facility soils but remained high in soils from agricultural sites (75% at 2.1°C). 16S rRNA, phospholipid fatty acid (PLFA), and soil respiration analyses were conducted to investigate potential soil microbiome impacts. Biotic and abiotic system factor correlations showed no clear patterns explaining the divergent denitrification rates. In all three soil types at the phylum level, Actinobacteria, Proteobacteria, and Acidobacteria dominated, whereas at the class level, Nitrososphaeria and Alphaproteobacteria dominated, similar to denitrifying systems such as wetlands, wastewater resource recovery facilities, and wastewater-irrigated agricultural systems. Results show that potential denitrification drivers vary but lay the foundation to develop a better understanding of the key factors regulating denitrification in land application systems and protect local groundwater supplies. PRACTITIONER POINTS: Incubation study denitrification rates decreased as temperatures decreased, potentially leading to groundwater contamination issues during colder months. The three most dominant phyla for all systems are Actinobacteria, Proteobacteria, and Acidobacteria. The dominant class for all systems is Nitrosphaeria (phyla Crenarchaeota). No correlation patterns between denitrification rates and system biotic and abiotic factors were observed that explained system efficiency differences.

Sustaining aquifers hydrologically, economically, and institutionally: Policy analysis of the Ogallala in New Mexico

Groundwater discharge exceeding recharge threatens sustainable aquifer water use internationally. Interest remains high in discovering more hydrologically sustainable and economically affordable measures to protect these aquifers. Previous research has conducted various aquifer assessments. Some work has investigated costs and benefits of various plans that would limit aquifer pumping. Despite notable advances in this kind of analysis, little published work to date has unified these elements into a science-based integrated framework to inform more sustainable aquifer policy design. This work's novel contribution is to integrate analysis of hydrology, economics, institutions, and policy into a unified scientific framework to inform choices on more sustainable pumping strategies while protecting economic activity for agricultural and urban water-using sectors. It does so by conceptualizing, formulating, designing, and applying a mathematical programming framework to replicate historically observed pumping patterns in parts of the Southern and Central High Plains Ogallala Aquifer region in New Mexico, USA. We first calibrated the optimization framework to replicate the historically observed data. We then go on to identify least cost pumping caps that would have partly restored the aquifer to its 2014 level by 2020, while comparing the performance of four other partial aquifer protection policy measures. Findings indicate a surprisingly low cost that could have been incurred to partially protect the aquifer over that period. However, these low costs are complicated by (1) decreasing water quality outside of the irrigated regions and (2) focusing of lateral inputs to a narrower zone of depression around the irrigated regions. These findings carry important implications for identifying more sustainable aquifer management plans internationally. The work's importance comes from its capacity to inform policy debates over a range of water shortage sharing plans, while respecting institutional constraints governing equitable burden sharing.

Performance and microbial response to nitrate nitrogen removal from simulated groundwater by electrode biofilm reactor with Ti/CNT/Cu5-Pd5 catalytic cathode

To enhance the removal of nitrate nitrogen (NO3 - -N) in groundwater with a low C/N ratio, electrocatalytic reduction of NO3 - -N has received extensive attention since its electrons can be directly produced in situ while simultaneously providing a clean electronic donor of hydrogen for denitrifying bacteria. In this study, Ti/CNT/CuPd bimetallic catalytic electrodes with different copper-palladium (CuPd) ratios were prepared by electrodeposition onto carbon nanotube (CNT) using titanium (Ti) plates. The results showed that the NO3 - -N conversion rate by Ti/CNT/Cu5-Pd5 electrode was the highest (53.60%) compared with other CuPd electrode ratios because of the combined role of the copper's high NO3 - -N catalytic activity and the palladium's high N2 selectivity. A new type of electrode biofilm reactor (EBR) with Ti/CNT/Cu5-Pd5 cathode, biochar substrate was constructed to explore the removal ability of NO3 - -N in simulated low C/N groundwater. When the influent NO3 - -N concentration was 30 mg/L, under the condition of a 30 mA electronic current and hydraulic retention time (HRT) of 12 h, the removal rate of NO3 - -N could reach as high as 78.1 ± 1.2%, and the N2 conversion rate was 99.7%. The horizontal distribution of microbial communities in EBR showed that the denitrification capacity was significantly improved through the electrochemical catalytic reduction of the Ti/CNT/Cu5-Pd5 cathode and the supply of the hydrogen electron donor to autotrophic denitrogenerating microbes such as Anaerobacillus, Thauera, and Hydrophaga. This study provides a new bimetallic catalytic cathode to enhance the removal of NO3 - -N in groundwater with a low C/N ratio. PRACTITIONER POINTS: The Cu5Pd5/CNTs/Ti electrode is beneficial to the adsorption and reduction of NO3 - -N to N2 . The production of hydrogen electron donors by cathode promoted nitrogen degradation. Activated electrodes together with denitrifying microorganisms contributed to the improved N removal rate.

Elements of the water - food - environment nexus for integrated sustainability analysis

Water and food security are constantly on the sustainable development agenda since they are interrelated with anthropogenic and ecosystemic issues present in the economic, environmental, and social spheres. The non-integrative management of these issues points to unsustainable futures. In this context, nexus approaches deserve considerable attention in the search for integrative management solutions capable of contributing to leveraging synergies that increase agricultural productivity, while simultaneously reducing environmental impacts, including water resources. This systematic literature review article aims to analyze the integration factors from the perspective of the water - food - environment nexus in the context of water and agricultural sustainability. The systematic methodology, including a content analysis, allowed the identification of analytical categories composed of the most present integrating factors and discussed in the scientific scope and how they are correlated from the perspective of the nexus. Among the extensive number of factors, the systemic management inserted in the integrated management of watersheds, the sustainable intensification from the perspective of food security and the demand for water resources, categorized in water security, presented higher frequency of occurrence when compared with the other factors. It is proposed that these factors can be considered as indicators of sustainability in the context of integrated water resources management and agricultural food production, since their synergistic effects have consequences in the water, agricultural and environmental management sectors. Thus, this study stands out for identifying trends and gaps in the water - food - environment nexus that can contribute to the development of innovative decision-making tools, in order to assist in the management of the watershed, where agriculture plays a key role in socio-environmental issues.

Nitrate sources and transformation processes in groundwater of a coastal area experiencing various environmental stressors

In coastal salinized groundwater systems, contamination from various nitrate (NO3) inputs combined with complex hydrogeochemical processes make it difficult to distinguish NO3 sources and identify potential NO3 transformtation processes. Effective field-based NO3 studies in coastal areas are needed to improve the understanding of NO3 contamination dynamics in groundwater of such complex coastal systems. This study focuses on a typical Mediterranean coastal agricultural area, located in Tunisia, experiencing substantial NO3 contamination from multiple anthropogenic sources. Here, multiple isotopic tracers (δ18OH2O, δ2HH2O, δ15NNO3, δ18ONO3, and δ11B) combined with a Bayesian isotope MixSIAR model are used (i) to identify the major NO3 sources and their contributions, and (ii) to describe the potential NO3 transformation processes. The measured NO3 concentrations in groundwater are above the natural baseline threshold, suggesting anthropogenic influence. The measured isotopic composition of NO3 indicates that manure, soil organic matter, and sewage are the potential sources of NO3, while δ11B values constrain the NO3 contamination to manure; a finding that is supported by the results of MixSIAR model revealing that manure-derived NO3 dominates over other likely sources. Nitrate derived from manure in the study area is attributed to organic fertilizers used to promote crop growth, and livestock that deposit manure directly on the ground surface. Evidence for ongoing denitrification in groundwaters of the study area is supported by an enrichment in both 15N and 18O in the remaining NO3, although isotopic mass balances between the measured and the theoretical δ18ONO3 values also suggest the occurrence of nitrification. The simultaneous occurrence of these biogeochemical processes with heterogeneous distribution across the study area reflect the complexity of interactions within the investigated coastal aquifer. The multiple isotopic tracer approach used here can identify the effect of multiple NO3 anthropogenic activities in coastal environments, which is fundamental for sustainable groundwater resources management.

Effects of recharge process on groundwater nitrate concentration in an oasis of Tengger Desert hinterland, China

Groundwater nitrate concentrations cannot be effectively diluted in an oasis of desert hinterland without direct recharge from external rivers. Therefore, there is an urgent need to understand the relationship between groundwater nitrate concentration and the groundwater recharge process. Using hydrochemicals, stable isotopes, LUCC, and combining these with the MixSIAR model, the distributions of groundwater nitrate concentration in the Dengmaying Basin (DMYB) in 2006 and 2020 were obtained. The contributions of groundwater recharge and nitrate sources were also quantified. With the development of agriculture in the DMYB, groundwater irrigation leakage has gradually become a crucial recharge source of groundwater, with a recharge proportion reaching 30.3%. From 2006 to 2020, under the influence of well irrigation and agricultural fertilization, the groundwater nitrate concentration in the DMYB increased significantly, with an increased range of 1.3 ~ 14.3 mg L-1. Moreover, the δ15N-NO3- and δ18O-NO3- values of nitrate in cultivated soil water were similar to those in groundwater, which also proves the process of carrying nitrate from the vadose zone into groundwater by irrigation water. The contribution of anthropogenic sources (54.9%) to groundwater nitrate exceeded that of natural sources (45.1%) to groundwater nitrate in the DMYB. These results indicate that the potential for nitrate pollution in groundwater must be considered, even in desert oases.

Assessment of land use and monsoon impact on high nitrate groundwater and health risk in the hard rock aquifer, South India

Groundwater sustainability in hard rock aquifers is compromised largely due to nitrate contamination from anthropogenic sources resulting in diminishing potable resources and attendant health issues. A purpose-driven study through an integrated approach was undertaken in the area of interest (hard rock aquifer) to assess the variations in nitrate concentration and resultant health impacts in response to variations in monsoon and land use patterns. Groundwater samples (n = 284) were collected for a period of three years (2017-2019) and analysed. From the analytical data, it is inferred that 27% and 9% of groundwater samples in the study area have high NO3- values of > 45 mg/l and > 100 mg/l, respectively. NO3- contamination zones mapping illustrates that NO3-contaminated area (> 45 mg/l) varied seasonally 1164 km2 (2017), 1086 km2 (2018) and 1640 km2 (2019)) and high-risk area (NO3- > 100 mg/l) has reduced drastically during 2018 due to dilution by monsoon (277 km2 (2017), 41 km2 (2018), 634 km2 (2019)). The lowest NO3- and Cl-concentrations are recorded during 2018 which coincides with high rainfall (2061 mm). NO3- concentrations in response to land use pattern indicate that the hot spots (NO3- > 45 mg/l and > 100 mg/l) are observed in groundwater samples of residential areas which are vulnerable to contamination from domestic wastewater, septic tanks and other pollutants. Further, wastewater infiltration facilitated the dissolution of certain minerals in the unsaturated zone which enhanced the accumulation of NO3- and other ions in this aquifer. Mineral weathering, denitrification and evaporation processes also affected the groundwater chemistry. The health risk model (HQoral) indicates that groundwater in 1261 km2 (2017), 1232 km2 (2018) and 1669 km2 (2019) is unsuitable for drinking (HQ > 1) and causes adverse health risks to the local inhabitants. The study has identified areas from the central and southeastern regions significantly affected by nitrate pollution underpinning the necessity of using treated groundwater for drinking purposes.

A multi-model study for understanding the contamination mechanisms, toxicity and health risks of hardness, sulfate, and nitrate in natural water resources

Several water quality contaminants have attracted the attention of numerous researchers globally, in recent times. Although the toxicity and health risk assessments of sulfate and water hardness have not received obvious attention, nitrate contamination has gained peculiar research interest globally. In the present paper, multiple data-driven indexical, graphical, and soft computational models were integrated for a detailed assessment and predictive modeling of the contamination mechanisms, toxicity, and human health risks of natural waters in Southeast Nigeria. Majority of the tested physicochemical parameters were within their satisfactory limits for drinking and other purposes. However, total hardness (TH), SO₄, and NO₃ were above stipulated limits in some locations. A nitrate health risk assessment revealed that certain areas present a chronic health risk to children, females, and males due to water intake. However, the dermal absorption route was found to have negligible health risks. SO₄ in some locations was above the 100 mg/L Nigerian limit; thus, heightening the potential health effects due to intake of the contaminated water resources. Most samples had low TH values, which exposes users to health defects. There are mixed contamination mechanisms in the area, according to graphical plots, R-mode hierarchical dendrogram, factor analysis, and stoichiometry. However, geogenic mechanisms predominate over human-related mechanisms. Based on the results, a composite diagrammatic model was developed. Furthermore, predictive radial basis function (RBF) and multiple linear regression (MLR) models accurately predicted the TH, SO₄, and NO₃, with the RBF outperforming the MLR models. Insights from the RBF and MLR models were useful in validating the results of the hierarchical dendrogram, factor, stoichiometric, and graphical analyses.

Impact of land use/land cover on groundwater resources in tropical unconfined aquifers of south-western India

Groundwater quality assessment is essential to understand land use impacts and implement water management plans. The present study aims to assess the impact of land use/land cover (LULC) on the groundwater table, and its quality in the tropical unconfined aquifers. Two hundred groundwater samples were collected from 100 sampling wells during monsoon and post-monsoon seasons. The drinking water quality index and irrigation quality indices were estimated based on the various parameters obtained from the laboratory analysis. Human health risk concerning nitrate contamination was evaluated based on the USEPA method. The land-use/land-cover map prepared using ArcGIS showed that the study area consists dominantly of croplands. Drinking water quality index results suggested that the groundwater samples were excellent to moderately suitable for drinking purposes. Only one sample was unsuitable for drinking. The different irrigation quality indices revealed various degrees of groundwater suitability for irrigation purposes. The spatial distribution of the corrosivity ratio suggests avoiding the metal pipe, for transportation of groundwater supply in the northern part of the study area. Fertilizers used in agriculture and soak pit leakages have contributed to high nitrate concentration in a few parts of the study area. Human health risk assessment showed that infants are vulnerable to non-carcinogenic health risks. The impact of the LULC assessment revealed that groundwater quality was moderately suitable for drinking in urban land. The study suggests implementing proper sewage treatment measures to avoid groundwater contamination. Overall, the findings are important in adopting site-specific, groundwater management strategies in the study area. Polluted and unpolluted areas demarcated in the study are beneficial for decision-makers to develop suitable groundwater management plans. The study recommends informed LULC development in the study area to improve groundwater quality and reduce human health risks.

Biogenic ferrihydrite-humin coprecipitate as an electron donor for the enhancement of microbial denitrification by Pseudomonas stutzeri

Nitrate pollution of groundwater has become an increasingly serious environmental problem that poses a great threat to aquatic ecosystems and to human health. Previous studies have shown that solid-phase humin (HM) can act as an additional electron donor to support microbial denitrification in the bioremediation of nitrate-contaminated groundwater where electron donor is deficient. However, the electron-donating capacities of HMs vary widely. In this study, we introduced ferrihydrite and prepared ferrihydrite-humin (Fh-HM) coprecipitates via biotic means to strengthen their electron-donating capacities. The spectroscopic results showed that the crystal phase of Fh did not change after coprecipitation with HM in the presence of Shewanella oneidensis MR-1, and iron may have complexed with the organic groups of HM. The Fh-HM coprecipitate prepared with an optimal initial Fh-HM mass ratio of 14:1 enhanced the microbial denitrification of Pseudomonas stutzeri with an electron-donating capacity 2.4-fold higher than that of HM alone, and the enhancement was not caused by greater bacterial growth. The alginate bead embedding assay indicated that the oxidation pathway of Fh-HM coprecipitate was mainly through direct contact between P. stutzeri and the coprecipitate. Further analyses suggested that quinone and organic-complexed Fe were the main electron-donating fractions of the coprecipitate. The results of the column experiments demonstrated that the column filled with Fh-HM-coated quartz sand exhibited a higher denitrification rate than the one filled with quartz sand, indicating its potential for practical applications.

Identification of nitrogen sources and cycling along freshwater river to estuarine water continuum using multiple stable isotopes

Nitrogen (N) transport from terrene to river water is a major source of N in estuarine water, contributing to eutrophication, harmful algal blooms and hypoxia. However, there is a lack of holistic and systematic research on N sources and transformation in the freshwater river-estuarine water continuum. In this study, multiple stable isotope signatures of nitrate (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-), ammonium (δ¹⁵N-NH₄⁺), and suspended particulate nitrogen (δ¹⁵N-PN) were employed to differentiate the sources and transformations of N and calculate the proportional contribution of NO₃^- sources by Bayesian model in Qiantang River (QTR)-Hangzhou Bay (HZB) during the dry season. The results showed that: (1) Evidences from isotopic signatures suggested the occurrence of N transformation instead of conservation mixing. (2) Negative correlations between the δ¹⁵N-NO₃^- and δ¹⁵N-NH₄⁺, the relationships between δ¹⁵N-NO₃^- and NO₃^--N concentrations, and smaller δ¹⁸O-NO₃^- values were found in almost all surface water, indicating that nitrification was the dominant N transformation. (3) In addition to the nitrification evidence, significant correlations between δ¹⁵N-PN and δ¹⁵N-NH₄⁺ revealed that assimilation and nitrification jointly affected the N transformation in the QTR's upstream, midstream and lower tributaries, which are unaffected or less affected by tides. (4) The lack of a relationship between δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- or ln(NO₃^-) indicated that denitrification was weakened in all surface waters. (5) Qualitative identification of N pollution sources and quantitative calculation of NO₃^--N potential sources revealed that sewage was the dominant source of N in the QTR and the HZB, while the internal nitrification was also important factor in determining N levels. This study provided evidence to further understand the sources, transport, and transformation of N in the river-estuary continuum, which deepens the understanding of the land-ocean integrated management of N contaminant.

Deciphering natural and anthropogenic nitrate and recharge sources in arid region groundwater

Recently, the subsoils of ephemeral stream (arroyos) floodplains in the northern Chihuahuan Desert were discovered to contain large naturally occurring NO₃^- reservoirs (floodplain: ~38,000 kg NO₃-N/ha; background: ~60 kg NO₃-N/ha). These reservoirs may be mobilized through land use change or natural stream channel migration which makes differentiating between anthropogenic and natural groundwater NO₃^- sources challenging. In this study, the fate and sources of NO₃^- were investigated in an area with multiple NO₃^- sources such as accidental sewer line releases and sewage lagoons as well as natural reservoirs of subsoil NO₃^-. To differentiate sources, this study used a large suite of geochemical tools including δ¹⁵N[NO₃], δ¹⁸O[NO₃], δ¹⁵N[N₂], δ¹³C[DIC], ¹⁴C, tritium (³H), dissolved gas concentrations, major ion chemistry, and contaminants of emerging concern (CEC) including artificial sweeteners. NO₃^- at sites with the highest concentrations (25 to 229 mg/L NO₃-N) were determined to be largely sourced from naturally occurring subsoil NO₃^- based on δ¹⁵N[NO₃] (<8 ‰) and mass ratios of Cl^-/Br^- (〈100) and NO₃^-/Cl^- (>1.5). Anthropogenic NO₃^- was deciphered using mass ratios of Cl^-/Br^- (>120) and NO₃^-/Cl^- (<1), δ¹⁵N[NO₃] (>8 ‰), and CEC detections. Nitrogen isotope analyses indicated that denitrification is fairly limited in the field area. CEC were detected at 67 % of sites including ³H dead sites (<1 pCi/L) with low percent modern carbon-14 (PMC; <30 %). Local supply wells are ³H dead with low PMC; as ³H does not re-equilibrate and ¹⁴C is very slow to re-equilibrate during recirculation through infrastructure, sites with low PMC, ³H < 1 pCi/L, and CEC detections were interpreted as locations with substantial anthropogenic groundwater recharge. Neotame was used to identify locations of very recent (<15 years before present) or ongoing wastewater influxes to the aquifer. This work shows the important influence of naturally occurring subsoil NO₃^- reservoirs on groundwater in arid regions and the major contribution of artificial recharge.

Contamination characteristics, source identification, and source-specific health risks of heavy metal(loid)s in groundwater of an arid oasis region in Northwest China

Heavy metal(loid)s accumulation in groundwater has posed serious ecological and health concerns worldwide. Source-specific risk apportionment is crucial to prevent and control potential heavy metal(loid)s pollution in groundwater. However, there is very limited comprehensive information on the health risk apportionment for groundwater heavy metal(loid)s in arid regions. Thus, the Zhangye Basin, a typical arid oasis region in Northwest China, was selected to investigate the contamination characteristics, possible pollution sources, and source-specific health risks of groundwater heavy metal(loid)s. The heavy metal pollution index (HPI), the Nemerow index (NI), and the contamination degree (CD) were adopted to assess the pollution level of heavy metal(loid)s; then source-specific health risk was apportioned integrating the absolute principal component scores-multiple linear regression (APCS-MLR) with health risk assessment. Noticeable accumulation of Mn, Fe, and As was observed in this region with especially Fe/As in 12.68%/2.11% of the samples revealing significant enrichment. Approximately 3.5% of the groundwater samples caused moderate or higher pollution level based on the HPI. The APCS-MLR model was more physically applicable for the current research than the positive matrix factorization (PMF) model. Industrial-agricultural activity factor (12.56%) was the major source of non-cancer (infants: 59.15%, children: 64.87%, teens: 64.06%, adults: 64.02%) and cancer risks (infants: 77.36%, children: 77.35%, teens: 77.40%, adults: 77.41%). Industrial-agricultural activities should be given priority to control health risks of heavy metal(loid)s in groundwater. These findings provide fundamental and significant information for mitigating health risks caused by heavy metal(loid)s in groundwater of typical arid oasis regions by controlling priority sources.

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.

APCS-MLR model: A convenient and fast method for quantitative identification of nitrate pollution sources in groundwater

Nitrate (NO₃^-) contamination in groundwater has diverse sources and complicated transformation processes. To effectively control NO₃^- pollution in groundwater systems, quantitative and accurate identification of NO₃^- sources is critical. In this work, we applied hydrochemical characteristics and isotope analysis to determine NO₃^- source apportionment. For the first time, the NO₃^- source contributions were calculated using hydrochemical indicators combined with multivariate statistical model (PCA-APCS-MLR). The results interpret that chemical fertilizers (58.11%) and natural sources (22.69%) were the primary NO₃^- sources in the vegetable cultivation area (VCA) which were rather close to the estimation by Bayesian isotope mixing model (SIAR). In particular, the contributions of chemical fertilizers in the VCA differed by only 3.79% between the two methods. Compared with previous approaches e.g. SIAR, the key advantage of the proposed PCA-APCS-MLR model is that it only requires the hydrochemical indicators which can be easily measured. A series of complicated experiments including measurement of isotope data of NO₃^- in groundwater, monitoring of in-situ pollution source information and calculation of isotopic enrichment factor can be simply avoided. The PCA-APCS-MLR model offers a much more convenient and faster method to determine the contribution rates of NO₃^- pollution sources in groundwater.

Development and Application of a Hydrogeochemical Model for the Groundwater Treatment Process in Waterworks

Drinking water quality is one of the most important factors affecting human health. The task of the waterworks is to purify raw water into drinking water. The quality of drinking water depends on two major factors: the raw water quality, and the treatment measures that are applied in the waterworks. Since the raw water quality develops over time, it must be determined whether the treatment measures currently used are also suitable when the raw water quality changes. For this reason, a hydrogeochemical model relevant to the drinking water quality during the treatment process was developed. By comparing the modeled results with the measured values, with the exception of chloride and sodium, all other relevant water quality parameters were consistent with one another. Therefore, the model proved to be plausible. This was also supported by the results of mass balance. The model can be used to forecast the development of drinking water quality, and can be applied as a tool to optimize the treatment measures if the raw water conditions change in the future.

Preparation of three dimensional bimetallic Cu-Ni/NiF electrodes for efficient electrochemical removal of nitrate nitrogen

It still remains a hotspot and great challenge to efficiently remove the nitrate nitrogen from high salt wastewater. Herein, a novel three dimensional porous bimetallic copper-nickel alloy electrode was fabricated with Ni foam (NiF) as substrate. The physicochemical and electrochemical characterization results showed Cu-Ni/NiF electrode possessed the smaller particle size (0.3-1.0 μm) and electrode film resistance comparing with Ni/NiF and Cu/NiF electrodes. Besides, higher double layer capacitance (C_dl) for Cu-Ni/NiF electrode indicated more electrochemical active sites could be used in the electrochemical nitrate nitrogen (NO₃^--N) removal. The electrochemical experiments showed the Cu-Ni/NiF electrode had the optimal NO₃^--N reduction ability and almost 100% NO₃^--N removal could be achieved with 30 min. All NO₃^--N removal processes were in accord with the pseudo-first-order reaction kinetics completely. The gaseous nitrogen selectivity for Cu-Ni/NiF electrode could reach 80.9% within 300 min. Stability assessment experiments indicated the Cu-Ni/NiF electrode all kept an excellent stability with Na₂SO₄ or NaCl electrolyte and the Cl^- addition could significantly improve the gaseous nitrogen selectivity. Finally, a possible removal mechanism of NO₃^--N was proposed. This work offered a direction for designing non-noble bimetallic electrodes for nitrate removal.

Modeling Groundwater Nitrate Contamination Using Artificial Neural Networks

The scope of the present study is the estimation of the concentration of nitrates (NO3−) in groundwater using artificial neural networks (ANNs) based on easily measurable in situ data. For the purpose of the current study, two feedforward neural networks were developed to determine whether including land use variables would improve the model results. In the first network, easily measurable field data were used, i.e., pH, electrical conductivity, water temperature, air temperature, and aquifer level. This model achieved a fairly good simulation based on the root mean squared error (RMSE in mg/L) and the Nash–Sutcliffe Model Efficiency (NSE) indicators (RMSE = 26.18, NSE = 0.54). In the second model, the percentages of different land uses in a radius of 1000 m from each well was included in an attempt to obtain a better description of nitrate transport in the aquifer system. When these variables were used, the performance of the model increased significantly (RMSE = 15.95, NSE = 0.70). For the development of the models, data from chemical and physical analyses of groundwater samples from wells located in the Kopaidian Plain and the wider area of the Asopos River Basin, both in Greece, were used. The simulation that the models achieved indicates that they are a potentially useful tools for the estimation of groundwater contamination by nitrates and may therefore constitute a basis for the development of groundwater management plans.

Identification and apportionment of shallow groundwater nitrate pollution in Weining Plain, northwest China, using hydrochemical indices, nitrate stable isotopes, and the new Bayesian stable isotope mixing model (MixSIAR)

Groundwater nitrate (NO₃^-) 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 NO₃^- 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 NO₃^- isotopes (δ¹⁵N-NO₃ and δ¹⁸O-NO₃), 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 NO₃^- 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 NO₃^- isotopes suggested that manure and sewage (M&S), as well as soil nitrogen (SN), were the major contributors to groundwater NO₃^-. Moreover, the results obtained from the MixSIAR model showed that the mean proportional contributions of M&S to groundwater NO₃^- 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.

Global diagnosis of nitrate pollution in groundwater and review of removal technologies

Clean water and sanitation for the world population is one of the most important challenges established by the Sustainable Development Goals of the United Nations since worldwide, one in three people do not have access to safe drinking water. Groundwater, one of the main sources of fresh water, has been considerably damaged by human activities. Nevertheless, while numerous plants are globally aimed at removing pollutants from surface waters, a much scarcer number of facilities have focused on groundwater remediation. Nowadays, there is increasing concern about the presence of nitrates (NO₃^-) in groundwaters as a consequence of the intensive use of fertilizers and other anthropogenic sources, such as sewage or industrial wastewater discharge. In this context, the selection and development of highly effective and low-cost solutions for the sustainable management of groundwater resources need to be addressed. Thus, this work collects data from the literature regarding the presence of nitrates in groundwater, and, simultaneously, it reviews the main alternatives available to remove NO₃^- from groundwater sources. A total of 292 sites have been analyzed categorized by continents, carefully discussing the possible origins of nitrate pollution. In addition, a discussion is carried out of the different technologies currently employed to treat groundwater, highlighting the progress made and the main challenges to be overcome. Finally, the review gathers the data available in the literature for nitrate treatment plants at full-scale.

Application of the hydrochemistry, stable isotopes and MixSIAR model to identify nitrate sources and transformations in surface water and groundwater of an intensive agricultural karst wetland in Guilin, China

Karst water as the vital water supply source is generally suffered from NO₃^- contamination in intensive agricultural areas worldwide. Identifying NO₃^- sources and transformations is the key for understanding nitrogen pathways, and also for effectively controlling diffuse NO₃^- pollution. In this study, chemical variables and stable isotopes (δ²H-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were measured in 10 surface water (SW) samples and 13 groundwater (GW) samples collected from the Huixian karst wetland, with the application of a Bayesian stable isotope mixing model (MixSIAR) to identified NO₃^- sources and biogeochemical transformations. The results showed that the NO₃^- concentrations ranged from the below detection limit to 117 mg/L, with 30.8% of GW samples obtained from the north central part of the study area exceeding the maximum permissible limit for drinking water, and posing significant non-carcinogenic health risks for native people through drinking water pathway. Moreover, based on characteristics of the hydrochemistry and stable isotopes, different biogeochemical fates were evaluated in SW and GW: nitrification process was a dominant factor in GW, as a result of high NO₃^- levels, and this microbial process was unlikely occurred in SW associated with relatively anaerobic condition and low NO₃^- levels; however, the denitrification might not be a main process of degradation NO₃^- levels throughout the study area. The MixSIAR outputs revealed that the long-term application of synthetic NH₄⁺ fertilizer (36.6%) and soil organic nitrogen (28.0%) were the main contributors to NO₃^- pollution, followed by synthetic NO₃^- fertilizer (16.8%) and domestic sewage and manure (15.1%), whereas NO₃^- in precipitation (3.44%) played a less important role. Additionally, NO₃^- concentration was significantly influenced by agricultural activities rather than NO₃^- source's contribution between SW and GW. This work suggests that synthetic NH₄⁺ fertilizer should be the primary target for control to prevent further NO₃^- pollution of the karst groundwater.

Groundwater quality assessment of protected aquatic eco-systems in cross-border areas of Serbia and Croatia

Research results define basis for specific monitoring programs of groundwater quality in wetland eco-systems in Serbia and Croatia. The main purpose of the research was to determine the impact of nonpoint diffuse source pollution on the groundwater quality, as well as seasonal variations on the concentration levels of selected physicochemical parameters. Statistical analyses, PCA, HCA, ANOVA and t-test, encompass 18 monitored parameters in groundwater. Statistical data indicated that protected area in Serbia has a significantly higher load of pollution from agricultural activities compared to Wetlands Tompojevci. The highest load in groundwater was detected from total nitrogen, ammonia and nitrogen anions, indicating contamination of groundwater by nitrogen-based fertilizers. The results obtained within the two-year seasonal monitoring program, from 2018 to 2020, are highly essential for achieving a comprehensive database that could be used as platform for highquality groundwater management in selected protected areas with the aim of minimizing environmental pollution.

Stochastic nitrate simulation under hydraulic conductivity uncertainty of an agricultural basin aquifer at Eastern Thessaly, Greece

Lake Karla Watershed is an agricultural basin characterized by intense agricultural activities, which lead to quantitative and qualitative aquifer degradation. The exploitation of non-renewable groundwater resources and nitrate contamination are the major threats to aquifer sustainability. Groundwater resources cover mainly irrigation and domestic water needs. Hence, the simulation of groundwater resources is necessary to (a) determine the quantity available for water supply and (b) estimate probable nitrate contamination to propose subsequent remediation techniques to protect public health. Furthermore, the aquifer's heterogeneity as well as the lack of hydraulic conductivity data, in a large-scale study area, creates uncertainty regarding groundwater flow and nitrate transport simulation. Deterministic modelling approaches for spatially distributed nitrate concentration simulation could not estimate the contamination risk, since it can address only one realization of the aquifer. This study estimates the effect of hydraulic conductivity uncertainty on the simulation of groundwater nitrate concentration. The proposed framework uses Geostatistical Sequential Gaussian Simulation (SGSIM) for the generation of equally probable realizations of the spatial distribution of hydraulic conductivity. It includes the application of a modelling system based on the following inter-linked models: a rainfall-runoff model, a reservoir operation model, a lake-aquifer interaction model, a groundwater flow model, and a nitrate transport and dispersion model. The last two models simulate the multiple realizations of aquifer's groundwater flow and nitrate concentration. Furthermore, two analyses (a statistical and a threshold analysis) are employed to estimate the exceedance probability of the nitrate concentrations and their spatial extent. The results indicate that hydraulic conductivity uncertainty does affect the simulation of nitrate concentration and that nitrate concentrations will most probably exceed the thresholds in areas where groundwater is extracted for domestic use.

Intensive vegetable production results in high nitrate accumulation in deep soil profiles in China

A comprehensive understanding of the patterns and controlling factors of nitrate accumulation in intensive vegetable production is essential to solve this problem. For the first time, the national patterns and controlling factors of nitrate accumulation in soil of vegetable systems in China were analysed by compiling 1262 observations from 117 published articles. The results revealed that the nitrate accumulation at 0-100 cm, 100-200 cm, 200-300 cm, and >300 cm were 504, 390, 349, and 244 kg N ha^-1, with accumulation rates of 62, 54, 19, and 16 kg N ha^-1 yr^-1 for plastic greenhouse vegetables (PG); for open field vegetables (OF), they were 264, 217, 228, and 242 kg N ha^-1 with accumulation rates of 26, 24, 18, and 10 kg N ha^-1 yr^-1, respectively. Nitrate accumulation at 0-100 cm, 0-200 cm, and 0-400 cm accounted for 5%, 11%, and 17% of accumulated nitrogen (N) inputs for PG, and represented 4%, 9%, and 13% of accumulated N inputs for OF. Nitrogen input rates and soil pH had positive effects and soil organic carbon, water input rate, and carbon to nitrogen ratio (C/N) had negative effects on nitrate accumulation in root zone (0-100 cm soil). Nitrate accumulation in deep vadose zone (>100 cm soil) was positively correlated with N and water input rates, and was negatively correlated with soil organic carbon, C/N, and the clay content. Thus, for a given vegetable soil with relatively stable soil pH and soil clay content, reducing N and water inputs, and increasing soil organic carbon and C/N are effective measures to control nitrate accumulation.

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.

Investigation of health risks related with multipath entry of groundwater nitrate using Sobol sensitivity indicators in an urban-industrial sector of south India

In the present study, we used a variance decomposition based global sensitivity index to evaluate the sensitivity of input variables and their contribution for non-carcinogenic health risks via intake and dermal pathways. Groundwater samples were collected from an industrial sector (Tiruppur region) of south India during the month of January 2020. These samples were analysed for nitrate, which varied from 10 to 290 mg L^-1 having the mean of 87 mg L^-1. Nearly 58% of the samples surpassed the permissible limit (45 mg L^-1) defined by the World Health Organization. Total hazard index (THI) ranged from 0.29 to 8.52 for children, 0.28 to 8.26 for women, and 0.24 to 6.99 for men. The first-order effect (FOE) and second-order effect (SOE) were derived for the three different age groups using Sobol sensitivity approach. The FOE scores showed that nitrate concentration in groundwater is the most sensitive parameter followed by exposure frequency for children, men and women via oral pathway. The SOE scores showed that nitrate concentration along with ingestion rate had greater sensitiveness in the oral input model. The higher SOE was obtained for the interaction of nitrate with skin surface area for children via dermal pathway, but it was not significant for women and men. These results suggest that epidemiology due to nitrate risk should be studied taking into account of concentration of nitrate, exposure frequency, fraction of contact and body weight. Additionally, ingestion rate and skin surface area were considered for the assessment of health risks for children.

Identification of Sources and Transformations of Nitrate in the Intense Human Activity Region of North China Using a Multi-Isotope and Bayesian Model

Nitrate (NO₃^-) contamination in water is an environmental problem of widespread concern. In this study, we combined the stable isotopes of NO₃^- (δ¹⁵N and δ¹⁸O) and water (δ²H and δ¹⁸O) with a Bayesian mixing model (SIAR) to identify the sources and transformation of NO₃^- in groundwater and rivers in the Ye River basin of North China. The results showed that the mean NO₃^- concentrations in groundwater were 133.5 and 111.7 mg/L in the dry and flood seasons, respectively, which exceeded the required Chinese drinking water standards for groundwater (88.6 mg/L) (GB14848-2017). This suggests that groundwater quality has been severely impacted by human activity. Land use significantly affected the concentration of NO₃^- in the Ye River basin (p < 0.05). However, the NO₃^- concentrations in groundwater and river water had no obvious temporal variation (p > 0.05). The principal mode of nitrogen transformation for both groundwater and river water was nitrification, whereas denitrification did not significantly affect the isotopic compositions of NO₃^-. The sources of NO₃^- mainly originated from sewage and manure, soil nitrogen, and NH₄⁺ in fertilizer for groundwater and from sewage and manure for the river water. According to the SIAR model, the primary sources of nitrate found in groundwater and river were sewage and manure in the Ye River basin. The proportional contributions of sewage and manure to nitrate contamination of groundwater and river were 58% and 48% in the dry season and 49% and 54% in the flood season, respectively. Based on these results, we suggest that the local government should enhance the sewage treatment infrastructure, construct an effective waste storage system to collect manure, and pursue a scientific fertilization strategy (such as soil formula fertilization) to increase the utilization rate of nitrogen fertilizer and prevent nitrate levels from increasing further.

Drinking water provision and quality at the Sahrawi refugee camps in Tindouf (Algeria) from 2006 to 2016

Drinking water provision has been a constant challenge in the Sahrawi refugee camps, located in the desert near Tindouf (Algeria). The drinking water supply system is itself divided in three zones which pump groundwater from different deep aquifers. It is equipped with reverse osmosis plants and chlorination systems for treating water. The allocation of water supplied to the Saharawi refugees for human consumption in 2016 has been estimated at between 14 and 17 L/person/day on average. This supplied water volume is below recommended standards, and also below the strategic objective of the Sahrawi government (20 L/person/day). Yet the local groundwater resources are huge in comparison with estimated consumption, and hence there is great potential for increasing the supplied volume through effecting improvements in the supply system. The physico-chemical quality of the raw and supplied water between 2006 and 2016 has been assessed according to Algerian standards for human consumption. The raw water of two zones of the supply system presents a very high conductivity and high concentrations of chloride, nitrate, fluoride, sulfate, sodium, calcium, potassium and iodide concentrations of natural origin, which may entail health risks. The treatment of water in a reverse osmosis plant greatly improves its quality and osmosed water met the standards. However, the supply of osmosed and raw water needs to be combined in Zone 1, to avoid an excessive reduction in water volume, and the supplied raw water poses a risk to the health of the refugees. The present study provides an example of a drinking water supply system under extreme drought conditions and in the political and social conditions of a refugee camp. Furthermore, it establishes a reference for supplied water allocation and quality in the Sahrawi refugee camps.

Phycoremediation of nitrogen and phosphate from wastewater using Picochlorum sp.: A tenable approach

The wastewater originates from different industrial, municipal, and agriculture processes and contains different nitrogen sources, for example, nitrate, ammonium, nitrite, and phosphate such as inorganic and organic sources. The discharge of high nitrate and phosphate to the ecosystem or nearby water bodies can cause eutrophication which disbalances the aquatic ecosystem. Furthermore, ingestion of these pollutants can cause severe toxicity and disease to humans and animals. Thus, from an environmental and social perspective, its treatment is essential with no negative impact on the ecosystem. Microalgae are fundamental, mixotrophic microorganisms that treat different wastewater and utilize nitrate and phosphate in the medium as a source of nutrients. Among them, Picochlorum sp., have the potential to remove nitrogen and phosphate from wastewater. The biomass produced by Picochlorum sp. can be a promising candidate as a sustainable feedstock for biofuel and bioproducts formation. Thus, the present review provides a brief knowledge and understanding about the concentration of nitrogen and phosphate in different wastewater, their negative impacts, and the uptake mechanism of microalgae. Furthermore, the review also provides an insight into Picochlorum sp., and the effects of different physiological and nutritional factors on their growth, wastewater treatment efficacy, and biomass for value-added products and biorefinery applications. In addition, the review is useful to understand the potential of Picochlorum sp. for a tenable wastewater treatment process.

A critical assessment of widely used techniques for nitrate source apportionment in arid and semi-arid regions

The assessment of nitrate pollution origin using stable isotope techniques is a fundamental prerequisite for the application of sustainable groundwater management plans. Although nitrate pollution is a worldwide groundwater quality problem, existing knowledge on the origin of nitrate pollution in arid and semi-arid regions is still scarce. Using the example of the Grombalia aquifer (NE Tunisia), this work summarizes the main strengths and constraints of multi-isotope techniques targeting at nitrate source identification and apportionment The results highlighted that, even in the case of well-established methodologies, like those of isotope hydrogeochemistry (δ¹⁵N_NO3, δ ¹⁸O_NO3 and δ ¹¹B) and mixing modelling for source apportionment, it is fundamental to take into account regional and local end-members to avoid biased data interpretation and to fully exploit the potential of such accurate tools.

Does It Pay Off to Explicitly Link Functional Gene Expression to Denitrification Rates in Reaction Models?

Environmental omics and molecular-biological data have been proposed to yield improved quantitative predictions of biogeochemical processes. The abundances of functional genes and transcripts relate to the number of cells and activity of microorganisms. However, whether molecular-biological data can be quantitatively linked to reaction rates remains an open question. We present an enzyme-based denitrification model that simulates concentrations of transcription factors, functional-gene transcripts, enzymes, and solutes. We calibrated the model using experimental data from a well-controlled batch experiment with the denitrifier Paracoccous denitrificans. The model accurately predicts denitrification rates and measured transcript dynamics. The relationship between simulated transcript concentrations and reaction rates exhibits strong non-linearity and hysteresis related to the faster dynamics of gene transcription and substrate consumption, relative to enzyme production and decay. Hence, assuming a unique relationship between transcript-to-gene ratios and reaction rates, as frequently suggested, may be an erroneous simplification. Comparing model results of our enzyme-based model to those of a classical Monod-type model reveals that both formulations perform equally well with respect to nitrogen species, indicating only a low benefit of integrating molecular-biological data for estimating denitrification rates. Nonetheless, the enzyme-based model is a valuable tool to improve our mechanistic understanding of the relationship between biomolecular quantities and reaction rates. Furthermore, our results highlight that both enzyme kinetics (i.e., substrate limitation and inhibition) and gene expression or enzyme dynamics are important controls on denitrification rates.

Anthropogenic nitrate in groundwater and its health risks in the view of background concentration in a semi arid area of Rajasthan, India

An increased nitrate (NO₃^-) concentration in groundwater has been a rising issue on a global scale in recent years. Different consumption mechanisms clearly illustrate the adverse effects on human health. The goal of this present study is to assess the natural and anthropogenic NO₃^- concentrations in groundwater in a semi arid area of Rajasthan and its related risks to human health in the different groups of ages such as children, males, and females. We have found that most of the samples (n = 90) were influenced by anthropogenic activities. The background level of NO₃^- had been estimated as 7.2 mg/L using a probabilistic approach. About 93% of nitrate samples exceeded the background limit, while 28% of the samples were beyond the permissible limit of 45 mg/L as per the BIS limits. The results show that the oral exposure of nitrate was very high as compare to dermal contact. With regard to the non-carcinogenic health risk, the total Hazard Index (HI_Total) values of groundwater nitrate were an average of 0.895 for males, 1.058 for females, and 1.214 for children. The nitrate health risk assessment shows that about 38%, 46%, and 49% of the samples constitute the non-carcinogenic health risk to males, females, and children, respectively. Children were found to be more prone to health risks due to the potential exposure to groundwater nitrate.

Influence of Anthropogenic Loads on Surface Water Status: A Case Study in Lithuania

Twenty-six water bodies and 10 ponds were selected for this research. Anthropogenic loads were assessed according to pollution sources in individual water catchment basins. It was determined that 50% of the tested water bodies had Ntotal values that did not correspond to the good and very good ecological status classes, and 20% of the tested water bodies had Ptotal values that did not correspond to the good and very good ecological status classes. The lake basins and ponds received the largest amounts of pollution from agricultural sources with total nitrogen at 1554.13 t/year and phosphorus at 1.94 t/year, and from meadows and pastures with total nitrogen at 9.50 t/year and phosphorus at 0.20 t/year. The highest annual load of total nitrogen for lake basins on average per year was from agricultural pollution from arable land (98.85%), and the highest total phosphorus load was also from agricultural pollution from arable land (60%).

Influence of land use and change in the proportion of electron donors required for denitrification on N2O in groundwater

Nitrate (NO₃^-) and nitrous oxide (N₂O) accumulate in groundwater in relation to human activities and pose multiple threats to the global environment (harming human health and atmospheric damage). This study focused on the evaluation of groundwater NO₃^-, N₂O, and its indirect emission factor under different land use types (agricultural land, urban land, and forest) and response mechanism of major anions to dissolved N₂O within groundwater in Dexing which has the largest copper mine in Asia. Specifically, this work used self-organizing maps (SOMs) to identify which anion conditions (NO₃^-, SO₄^2-, F^-, Cl^-) and water quality parameters were suitable for the accumulation of groundwater N₂O. Finally, we found that the shallow groundwater of agricultural land has a high concentration of NO₃^- and N₂O and the agricultural activity has a significant effect on the temporal and spatial variation of N₂O in groundwater. The result of SOMs combined with the positive correlation between N₂O and NO₃^-/SO₄^2- suggested that the electron donor required for denitrification has a significant effect on N₂O accumulation. In this respect, when an increased proportion of reduced sulfur is available as an electron donor for autotrophic denitrification, this results in lower concentrations of N₂O in groundwater. Through the comprehensive evaluation of the anion conditions and N₂O in groundwater under different land use types, this study case can help to estimate the N₂O indirect emission from groundwater, so as to constrain the global nitrogen budget.

Estimation of nitrate pollution sources and transformations in groundwater of an intensive livestock-agricultural area (Comarca Lagunera), combining major ions, stable isotopes and MixSIAR model

The identification of nitrate (NO₃^-) sources and biogeochemical transformations is critical for understanding the different nitrogen (N) pathways, and thus, for controlling diffuse pollution in groundwater affected by livestock and agricultural activities. This study combines chemical data, including environmental isotopes (δ²H_H2O, δ¹⁸O_H2O, δ¹⁵N_NO3, and δ¹⁸O_NO3), with land use/land cover data and a Bayesian isotope mixing model, with the aim of reducing the uncertainty when estimating the contributions of different pollution sources. Sampling was taken from 53 groundwater sites in Comarca Lagunera, northern Mexico, during 2018. The results revealed that the NO₃^- (as N) concentration ranged from 0.01 to 109 mg/L, with more than 32% of the sites exceeding the safe limit for drinking water quality established by the World Health Organization (10 mg/L). Moreover, according to the groundwater flow path, different biogeochemical transformations were observed throughout the study area: microbial nitrification was dominant in the groundwater recharge areas with elevated NO₃^- concentrations; in the transition zones a mixing of different transformations, such as nitrification, denitrification, and/or volatilization, were identified, associated to moderate NO₃^- concentrations; whereas in the discharge area the main process affecting NO₃^- concentrations was denitrification, resulting in low NO₃^- concentrations. The results of the MixSIAR isotope mixing model revealed that the application of manure from concentrated animal-feeding operations (∼48%) and urban sewage (∼43%) were the primary contributors of NO₃^- pollution, whereas synthetic fertilizers (∼5%), soil organic nitrogen (∼4%), and atmospheric deposition played a less important role. Finally, an estimation of an uncertainty index (UI90) of the isotope mixing results indicated that the uncertainties associated with atmospheric deposition and NO₃^--fertilizers were the lowest (0.05 and 0.07, respectively), while those associated with manure and sewage were the highest (0.24 and 0.20, respectively).

Health risk assessment of groundwater nitrogen pollution in Songnen Plain

Access to safe drinking water is one of the fundamental human rights and an important part of healthy living. This study considered various land use methods, used geostatistical analysis, and triangular random model to explore nitrogen pollution and estimate its potential risk to human health for local populations in Songnen Plain of Northeast China and recognize parameter uncertainties. Nitrate concentrations in groundwater ranged from 0.01 to 523.45 mg/L, more than 72.35% of the samples exceeded Grade III threshold (20 mg/L of N) as per China's standard, and nitrate nitrogen content is greater than 20 mg/L accounted for around 60% of the research area, mainly distributed in the eastern and central high plain area. The nitrate-nitrogen content of groundwater in the town land was significantly higher than that of agricultural land, and the ammonia nitrogen content was conversely. The townland's risk value was two times that of agricultural land, considering different land use methods would avoid overestimating or underestimating regional risk value. Non-carcinogenic risks (HI) of two land use were above the safety level (i.e., HI > 1), suggesting that groundwater nitrate would have significant health effects on the age groups, and further threaten children. There was a wide range of fluctuations in the uncertainty of nitrogen concentration and model evaluation parameters; triangular random model was more sensitive to data changes, which could reduce the uncertainty. The contribution rate of nitrate-nitrogen concentration to risk was above 90%, which explained the need for random sampling to improve the evaluation results reliability. The findings in this paper will provide new insight for solving uncertainties in water safety management.

Groundwater pollution source identification and apportionment using PMF and PCA-APCA-MLR receptor models in a typical mixed land-use area in Southwestern China

The quality of groundwater in a region is regarded as a function of natural and anthropogenic factors. Receptor models have advantages in source identification and source apportionment by testing the physicochemical properties of receptor samples and emission sources. In our study, receptor models PMF and PCA-APCS-MLR were developed to qualitatively identify the latent sources of groundwater pollution in the study area and quantitatively evaluate the contribution of each source to groundwater quality. The performances of PMF and APCS-MLR models were compared to test their applicability on the assessment of groundwater pollution sources. Results showed that both of the models identified five sources of groundwater contamination with similar main load species of each potential source. The comparable source apportionment of species NO₂^- and NO₃^- with two models indicated the reliable source estimation for these species, whereas the contributions of sources to species Fe, Mn, Cl^-, SO₄^2- and NH₄⁺ were significantly different due to the large variability of data, difference of uncertainty analysis and algorithm of unexplained variability in the two models. R-squared value between observation and model prediction was 0.603-0.931 in PMF and 0.497-0.859 in PCA-APCS-MLR. The significant disagreement of average source contribution was detected in agricultural source and unexplained variability using PMF and PCA-APCS-MLR models. Average contributions of other sources to groundwater quality parameters had similar estimates between the two models. Higher R² and smaller proportion of unexplained variability in the PMF model suggested that PMF approach could provide more physically plausible source apportionment in the study area and a more realistic representation of groundwater pollution than solutions from PCA-APCS-MLR model. The study showed the advantages of application of multiple receptor models on achieving reliable source identification and apportionment, particularly, providing a better understanding of applicability of PMF and PCA-APCS-MLR models on the assessment of groundwater pollution sources.

High Hexavalent Chromium Concentration in Groundwater from a Deep Aquifer in the Baiyangdian Basin of the North China Plain

Hexavalent chromium (Cr(VI)) is known to occur naturally in shallow oxic groundwater, typically from aquifers associated with mafic and ultramafic formations, but information on the occurrence of Cr(VI) in deep groundwater from large sedimentary basins is limited. This study shows that groundwater from the Baiyangdian Lake Basin (BYB), home to the future second capital city of China, had high Cr concentration (>10 μg/L, up to 86 μg/L) in the deep aquifer (>150 m), while shallow groundwater had lower Cr concentration (<10 μg/L). Chromium occurred predominantly as Cr(VI) (>95%). Shallow groundwater was characterized by higher Mn and Fe concentrations relative to deep groundwater, likely indicating more reducing conditions. Sequential extraction experiments from aquifer sediments suggest that Cr(VI) may derive from silicate weathering and that Mn oxides in the aquifer play a major role in the formation of Cr(VI) in groundwater. Inverse correlations between Mn and Cr(VI) suggest that reductive dissolution of Mn oxides constrains Cr(VI) mobilization in the shallow groundwater, while oxic-suboxic conditions in the deep aquifer limit Mn solubility, which enhances oxidation of Cr(III) to Cr(VI) and promotes desorption of Cr(VI) under alkaline conditions. This study demonstrates the potential geogenic occurrence of high Cr(VI) concentration in deep groundwater from a nonmafic, large sedimentary basin containing Mn oxides in the aquifer sediments.

Recent advances in sensors for electrochemical analysis of nitrate in food and environmental matrices

Nitrate is one of the most common contaminants in food and the environment and mainly arises from intense human activities. Electrochemical sensors have been considered as one of the most promising analytical tools for the rapid detection of nitrate in food and environmental matrices due to their quick response, high sensitivity, ease of operation and miniaturisation, and low sample and power consumption. In this review, we summarise advances in sensors for electrochemical analysis of nitrate over the past decade. We also discuss the application of electrochemical sensing systems for the determination of nitrate in the matrices of fresh water, seawater, food, soil and particulate matter.

Application of the dual-isotope approach and Bayesian isotope mixing model to identify nitrate in groundwater of a multiple land-use area in Chengdu Plain, China

Nitrate (NO₃^-) contamination in groundwater is an environmental problem worldwide. Partitioning the pollution into its sources is the key for effective controls. In this study, NO₃^- dual isotopic compositions (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were measured in groundwater samples from 28 wells in an area with multiple land-uses, followed by the application of an isotope mixing model (SIAR) to identify the main NO₃^- sources and their biogeochemical processes. The results showed that denitrification was unlikely occur at significant rates, while nitrification was an important nitrogen transformation processes. Spatial variation of groundwater NO₃^- and its isotopic compositions was associated with the land-use types. Agricultural areas were characterized by relatively high NO₃^- concentrations and low δ¹⁵N-NO₃^- values. In contrast, industrial areas were characterized by enriched δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values. In crop field, vegetable field and poultry and livestock breading farm, the proportional contribution represented a similar pattern with highest contribution from chemical fertilizer followed by soil organic nitrogen, manure, atmospheric precipitation and sewage in order. Nitrate in groundwater in industrial areas has different pattern of the proportional contribution, in which groundwater NO₃^- is largely influenced by sewage discharge and atmospheric precipitation. We concluded that the combination of isotopic analysis together with land-use information and chemical analysis was an effective approach for assessing the source apportionment and the fate of nitrate in the aquifer in multiple land-use areas.

Quantitative identification of nitrate sources in a coastal peri-urban watershed using hydrogeochemical indicators and dual isotopes together with the statistical approaches

Surplus nutrient load and complex migration and transformation processes are the challenges for water quality management in the peri-urban coastal watershed, leading to increasing concerns worldwide. We investigated the spatio-temporal variation of hydrogeochemical parameters in surface water of Jimei Lake watershed, and distinguished the sources and transformation of nitrate-N (NO₃^--N) using dual isotopes of nitrate (δ¹⁵N and δ¹⁸O in NO₃^-) with hydrogeochemical indicators. Principal component analysis (PCA) on hydrogeochemical parameters demonstrated that surface water was seriously polluted by nutrients, especially in the southeast of the downstream. There were signs of seawater intrusion and increased wastewater discharge in the mid-lower reaches with high ammonium concentrations. Nitrification occurred throughout the monitoring period with lower δ¹⁵N and δ¹⁸O values and NO₃^- derived from mixed pollution sources. Results of Bayesian model showed that dominant NO₃^- input originated from manure and sewage (M&S, 71% and 76% in the wet and dry season, respectively) and atmospheric deposition (22% and 16%, respectively). This result implied that the controls and treatment of M&S discharges are essential to alleviate of NO₃^- pollution. The proposed method is helpful to understand the origins of NO₃^- and may be suitable to develop measures for the reducing of nitrogen loadings in the peri-urban watershed.

Stability of Sn-Pd-Kaolinite catalyst during heat treatment and nitrate reduction in continuous flow reaction

In this study, a novel and highly reactive Sn-Pd catalyst supported by environmentally benign kaolinite (Sn-Pd-kaolinite) was developed and evaluated for stability for effective nitrate (NO₃^-) reduction in batch and continuous mode. Complete NO₃^- removal with fast reduction kinetics (k = 18.16 × 10^-2 min^-1) and 71% selectivity toward N₂ were achieved by the Sn-Pd-kaolinite catalyst during batch reactions. During continuous tests, 100% NO₃^- removal and 80% N₂ was achieved for 60 h. However, NO₃^- removal efficiency gradually decreased to 80% in170 h. The catalyst was then successfully regenerated in the system by increasing H₂ flow which achieved a complete NO₃^- removal again. The metal leaching from catalyst surface was negligible (Sn 0.01% and Pd 0.006%) and the structure was stable during the continuous test, confirming that the Sn-Pd-Kaolinite catalyst had a superior reaction kinetics and operational durability.

Best management practices from agricultural economics: Mitigating air, soil and water pollution

Often the several stakeholders involved in the agricultural sector place a greater emphasis on the negative externalities from farming production rather than on the solutions and approaches to mitigate, namely impacts from pollution. The scientific literature, in certain circumstances, follows this tendency leaving a vast chasm of enormous potential left to be explored. It is important to contribute towards the reduction of this gap, highlighting the best management practices implemented across the agricultural sector around the world, specifically to make them more visible and give incentive to the several agents in adopting and spreading their use. In this way, the main objective is to stress the best management practices presented by the global scientific literature from the farming sector. To achieve this objective methodology based on bibliometric analysis-factor-analysis-literature survey approach was considered, applied to 150 documents obtained from the Web of Science (core collection) related with the following topics: best management practice; agricultural economics; air, soil and water pollution. As main insights, it is worth referring the best management practices to deal with problems from agricultural production, such as, for example, the use of agricultural residues as feedstock for renewable energies. With regard to sustainable development in the agricultural sector, concepts such as "sustainable remediation" have their place. On the other hand, the agricultural and environmental policies and the agricultural costs associated with the several farming practices also play a determinant role here. Finally, only fraction of the scientific documents analysed (16 papers) belong to the group of studies related to policies, showing that there are potential subjects to be addressed here in future studies related with these topics. The same happens for cost-benefit analyses (24 documents).

Spatio-temporal variations of shallow and deep well groundwater nitrate concentrations along the Indus River floodplain aquifer in Pakistan

Excessive use of nitrogenous fertilizers and their improper management in agriculture causes nitrate contamination of surface and groundwater resources. This study was conducted along the seasonally flooded alluvial agricultural area of Indus River Basin to determine the spatial and temporal dynamics of nitrate concentrations in the groundwater along the river. Total of 112 samples were collected from shallow (30-40 ft) and deep groundwater (120-150 ft) wells at seven sites, 25 km apart from each other and covered an area of 170 km along the river, during four sampling campaigns between October 2016 to May 2017 i.e. in start, mid and end of dry season. The study period covered the whole agricultural cycle including the wet summer season with no agricultural activities under flooding and the sampling sites were always less than 2 km from the river bank. Nitrate concentrations of shallow wells were 15-54 and 20-45 mg L^-1 during the start and middle of dry season, respectively. However, at the end of the dry season, the highest nitrate concentrations of 35-75 mg L^-1 were recorded and 70% of these samples contained nitrate concentrations above the permissible limit 50 mg L^-1. Similar seasonal patterns of nitrate concentrations were observed in deep wells, however, δ¹⁸O data suggested lower recharge in deep well than shallow wells. The results illustrated that high nitrate concentrations in shallow wells were associated with high δ¹⁸O values indicating that the quantity of evaporated water infiltrated from the floodplain, possibly from distribution channels, along with the nitrate polluting shallow wells more than the deep wells. At the end of the dry season, nitrate concentrations exceeded the permissible limits in both shallow and deep wells, which possibly happened due to the horizontal movement of groundwater along with the nitrate mixing during vertical seepage of river water to the aquifers.

Nitrate reduction on surface of Pd/Sn catalysts supported by coal fly ash-derived zeolites

In this study, we synthesized four zeolites (i.e., Zeolite-X&A9, -X&A&HS12, -X&HS15, -X&HS18) from coal fly ash (CFA), and evaluated their potential for use as support materials to fabricate novel Pd-Sn bimetallic catalysts for reactive and selective reduction of NO₃^- to N₂. The successive transformation of zeolite (Na-A and Na-X to hydroxy sodalite (HS)) was observed with increasing crystallization time from 9 to 18 h, which resulted in different degrees of crystallinity, morphology, BET surface area, and pore volume. Compared to other monometallic and bimetallic catalysts, Pd-Sn/Zeolite-X&HS15 (crystallization time = 15 h) showed remarkable nitrate removal (100%) with the highest kinetic rate constant (k = 0.055 min^-1, K' = 0.219 min^-1 g_cat^-1, K'' = 2.922 L min^-1 g_Pd^-1) and N₂ selectivity (88.1%). These results can be attributed to high surface area and stability of each of the zeolite phases (i.e., Na-X and HS). The reaction mechanism was elucidated by Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses, demonstrating the presence of Pd°, Sn°, and Sn²⁺ and the uniform distribution of proximate Pd-Sn ensembles on the surface. These results suggest new promising strategies for applying industrial solid waste-derived zeolites to the synthesis of novel bimetallic catalysts to ensure efficient and economical denitrification of wastewater.