The microbiome at the interface between environmental stress and animal health: an example from the most threatened vertebrate group.

Nitrate pollution and global warming are ubiquitous stressors likely to interact and affect the health and survival of wildlife, particularly aquatic ectotherms. Animal health is largely influenced by its microbiome (commensal/symbiotic microorganisms), which responds to such stressors. We used a crossed experimental design including three nitrate levels and five temperature regimes to investigate their interactive and individual effects on an aquatic ectotherm, the European common frog. We associated health biomarkers in larvae with changes in gut bacteria diversity and composition. Larvae experienced higher stress levels and lower body condition under high temperatures and nitrate exposure. Developmental rate increased with temperature but decreased with nitrate pollution. Alterations in bacteria composition but not diversity are likely to correlate with the observed outcomes in larvae health. Leucine degradation decreased at higher temperatures corroborating accelerated development, nitrate degradation increased with nitrate level corroborating reduced body condition and an increase in lysine biosynthesis may have helped larvae deal with the combined effects of both stressors. These results reinforce the importance of associating traditional health biomarkers with underlying microbiome changes. Therefore, we urge studies to investigate the effects of environmental stressors on microbiome composition and consequences for host health in a world threatened by biodiversity loss.

Pd/Cu bimetallic nano-catalyst supported on anion exchange resin (A520E) for nitrate removal from water: High property and stability.

High nitrate concentration in water can lead to eutrophication and the disruption of healthy aquatic ecosystems. Additionally, in the human digestive system, it has the potential to be reduced to nitrite, which can be damaging to people's physical health. Catalytic hydrogenation of nitrate is one of the strategies for removing nitrate from water. Using A520E resin as support, we prepared Pd/Cu nano-catalyst (Pd/Cu@A520E) according to a liquid phase reduction method. A520E could improve the transfer process of nitrate in the solution to the activity sites of Pd/Cu nanoparticles, thus increase the reaction rate of nitrate reduction. Pd/Cu bimetallic nano-particles were evenly distributed on/in the resin with a size range from 2 nm to 10 nm. The External Circulating System equipped with Venturi tube (ECSV) was designed to improve the utilization efficiency of H2 in both batch tests and long-term continuous-flow tests. Nearly 100% of nitrate removal efficiency and above 90% of N2 selectivity were achieved in both batch tests and continuous-flow tests. Coexisting Cl- and SO42- at 300 mg/L showed little impact on the property of Pd/Cu@A520E. Pd/Cu@A520E also showed high nitrate removal property and stability in continuous-flow tests of more than 800 h. NO3- was adsorbed onto the active sites (functional groups and Pd/Cu particle sites), meanwhile H2 was adsorbed onto the active sites of Pd/Cu@A520E to form Pd [H]. Then the adsorbed NO3- was reduced into N2 (main product) or NH4+ by Pd [H]. In addition, Pd/Cu@A520E showed high nitrate removal property from municipal waste water.

Comparative study of the quick action effect of multiple enzyme-based nano-emulsified oils in enhancing nitrate contamination remediation in groundwater.

Emulsified vegetable oil (EVO), as a novel green slow-releasing substrate, has performed great potential in subsurface bioremediation due to its slow release and longevity. Nevertheless, the long time it takes to initiate this process still exposed some limitations. Herein, multiple enzyme-based EVOs (EN-EVOs) were developed to enhance the quick-acting effect in nitrate-contaminated bioremediation. This study demonstrated that EN-EVOs loaded with cellulose (c-EVO) and protein enzymes (p-EVO) performed best, not only did not change the advantages of traditional EVO, but also optimized the stability and particle size to the level of 0.8-0.9 and 247.95-252.25 nm, respectively. Nitrate (NO3-N) degradation further confirmed the superiority of c-EVO in rapidly initiating degradation and achieving stable denitrification. Compared with traditional EVO, the maximum start-up efficiency and the rapid achieving stable denitrification efficiency were improved by 37.6% and 1.71 times, respectively. In such situation, the corresponding NO3-N removal efficiency, kinetics rate constant (k1), and half-life period (t1/2) reached as high as 85.39%, as quick as 1.079 d-1, and as short as 0.64 d after 30-day cultivation. Meanwhile, the rapid conversion efficiency of NO2-N was observed (k2 = 0.083 d-1). High-throughput 16S rRNA gene sequencing indicated that the quick-acting process of NO3-N reduction coupled to c-EVO was mediated by microbial reducers (e.g., Ralstonia, Gulbenkiania, and Sphingobacterium) with regulations of narG, nirS and norB genes. Microorganisms with these genes could achieve quick-acting not only by enhancing microbial activity and the synthesis and metabolism of volatile fatty acids, but also by reducing the production and accumulation of loosely bound-extracellular polymeric substances (LB-EPS). These findings advance our understanding on fast-acting of NO3-N degradation supported by c-EVO and also offer a promising direction for groundwater remediation.

Enhancing biocathode denitrification performance with nano-Fe3O4 under polarity period reversal.

The presence of excessive concentrations of nitrate poses a threat to both the environment and human health, and the bioelectrochemical systems (BESs) are attractive green technologies for nitrate removal. However, the denitrification efficiency in the BESs is still limited by slow biofilm formation and nitrate removal. In this work, we demonstrate the efficacy of novel combination of magnetite nanoparticles (nano-Fe3O4) with the anode-cathode polarity period reversal (PPR-Fe3O4) for improving the performance of BESs. After only two-week cultivation, the highest cathodic current density (7.71 ± 1.01 A m-2) and NO3--N removal rate (8.19 ± 0.97 g m-2 d-1) reported to date were obtained in the PPR-Fe3O4 process (i.e., polarity period reversal with nano-Fe3O4 added) at applied working voltage of -0.2 and -0.5 V (vs Ag/AgCl) under bioanodic and biocathodic conditions, respectively. Compared with the polarity reversal once only process, the PPR process (i.e., polarity period reversal in the absence of nano-Fe3O4) enhanced bioelectroactivity through increasing biofilm biomass and altering microbial community structure. Nano-Fe3O4 could enhance extracellular electron transfer as a result of promoting the formation of extracellular polymers containing Fe3O4 and reducing charge transfer resistance of bioelectrodes. This work develops a novel biocathode denitrification strategy to achieve efficient nitrate removal after rapid cultivation.

Health risk analysis of nitrate in groundwater in Shanxi Province, China: A case study of the Datong Basin.

In order to identify and effectively control the impact of NO3- pollution on human health, on the basis of investigation, sampling, analysis and testing, statistical analysis software (SPSS19), groundwater pollution analysis software, Nemera comprehensive index method, correlation analysis method and human health risk assessment model are applied for analysis and research. The results indicate that the groundwater in the study area is mainly Class II water, with overall good water quality. The main influencing factors for producing Class IV are NO3-, Fe, F- and SO42-. The use of agricultural fertilizers is the main source of NO3- exceeding standards in groundwater in this area. There are significant differences in the health hazards caused by NO3- pollution in groundwater among different populations, and infants and young children are more susceptible to nitrate pollution. The division of pollution areas and high-risk groups plays an important guiding role in preventing health risks. The new achievements will help people improve their awareness of risk prevention, caring for the environment, respecting nature and implementing precise policies, promoting society to step onto the track of scientific and healthy development.

Defining the maximum nitrogen surplus in water management plans to recover nitrate polluted aquifers in Spain.

Nitrate pollution in aquifers is a global concern. Spain has developed a national strategy to recover nitrate polluted aquifers aligned with the European Union (EU) policies, specifically through the water planning under the EU Water Framework Directive. River basin management plans use PATRICAL model results to define the maximum nitrogen surplus in each polluted aquifer for the first time. The maximum nitrogen surplus allows to reach the good status in each aquifer and the model provides the number of years required. Around 30% of the aquifers in Spain is currently heavily polluted by nitrates. Model results show that 90% of these aquifers can be recovered in next 6-12 years by increasing nitrogen use efficiency and reducing nitrogen losses around 50%, which is in line with the EU Farm to Fork Strategy. The remaining aquifers require additional reductions to achieve the good status. In Spain this increase in nitrogen efficiency can be obtained with different measures including 30% reduction of current fertilization.

Occurrences of nitrate-contaminated groundwater in the piedmont aquifers: hydrogeochemical characteristics and health risks.

Groundwater nitrate (NO3-) contamination is a global concern. The distribution patterns, enrichment mechanisms, and human health risks of NO3- contaminated groundwater were investigated using 144 groundwater samples collected from domestic and irrigation wells in the piedmonts of the North China Plain (Beijing and Shijiazhuang areas). The results showed that the groundwater was neutral to weakly alkaline, and 47% of the groundwater samples had NO3- concentrations exceeding 50 mg/L, a threshold proposed by world health organization to threaten infants up to 3 months. Groundwater NO3- concentrations were generally higher in the Beijing piedmont than in the Shijiazhuang piedmont and decreased with depth in both piedmonts. High-NO3- (> 50 mg/L) groundwater was distributed sporadically spatially and mainly was of Ca-Mg-HCO3 hydrochemical facies. Stable isotopes (D and 18O) compositions and NO3-/Cl- ratios indicated that NO3- accumulation in groundwater was primarily due to use of N-fertilizers under agricultural practices, and was associated with groundwater recharge sources such as septic tank leakage and re-infiltration of reclaimed irrigation water. Water quality evaluation showed that groundwater quality was highly dependent on NO3- concentration, with entropy-weighted water quality index values increasing linearly with increasing NO3- concentrations. The potential health risk of high-NO3- groundwater was the most serious for infants in both the piedmonts. Therefore, reducing NO3- input from sources and drinking water intake is recommended to minimize the human health risk.

Cu1-Fe Dual Sites for Superior Neutral Ammonia Electrosynthesis from Nitrate.

The electrochemical nitrate reduction reaction (NO3RR) is able to convert nitrate (NO3 -) into reusable ammonia (NH3), offering a green treatment and resource utilization strategy of nitrate wastewater and ammonia synthesis. The conversion of NO3 - to NH3 undergoes water dissociation to generate active hydrogen atoms and nitrogen-containing intermediates hydrogenation tandemly. The two relay processes compete for the same active sites, especially under pH-neutral condition, resulting in the suboptimal efficiency and selectivity in the electrosynthesis of NH3 from NO3 -. Herein, we constructed a Cu1-Fe dual-site catalyst by anchoring Cu single atoms on amorphous iron oxide shell of nanoscale zero-valent iron (nZVI) for the electrochemical NO3RR, achieving an impressive NO3 - removal efficiency of 94.8 % and NH3 selectivity of 99.2 % under neutral pH and nitrate concentration of 50 mg L-1 NO3 --N conditions, greatly surpassing the performance of nZVI counterpart. This superior performance can be attributed to the synergistic effect of enhanced NO3 - adsorption on Fe sites and strengthened water activation on single-atom Cu sites, decreasing the energy barrier for the rate-determining step of *NO-to-*NOH. This work develops a novel strategy of fabricating dual-site catalysts to enhance the electrosynthesis of NH3 from NO3 -, and presents an environmentally sustainable approach for neutral nitrate wastewater treatment.

Hydrochemical evolution characteristics, controlling factors, and high nitrate hazards of shallow groundwater in a typical agricultural area of Nansi Lake Basin, North China.

Anthropogenic nitrate contamination in groundwater could not be neglected, which has been a global issue threatening public health, especially in agricultural fields where fertilizers were used intensively. The present study focused on evaluating the groundwater evolution process, quality, and associated health risks from nitrate pollution in Nansi Lake Basin (NLB), a typical intensive agricultural region of North China. For this purpose, fifty-two shallow groundwater samples were collected and analyzed major chemical parameters in June 2022. The groundwater samples are found to be mainly dominated by HCO3-Ca·Mg and SO4·Cl-Ca·Mg types. Water-rock interactions like minerals dissolution/precipitation and ion exchange were found to be the important processes influencing hydrochemistry. Nitrate content in groundwater fluctuated from 1.9 to 750.0 mg/L (average:148.7 mg/L), with about 75% of samples surprisingly exceeding the permissible limit (50 mg/L) set by the World Health Organization (WHO). Anthropogenic activities can be classified as excessive nitrogen fertilizer application, livestock manure, and industrial/domestic sewage, coupled with irrigation return flow, which brought significant hazards to human health. The calculation results of entropy weighted water quality index (EWQI) showed that about half of groundwater samples are unfit for drinking purposes. Most importantly, 88.5%, 88.5%, 73.1%, and 71.2% of the water samples had considerable NO3- health risks (HQ > 1) for infants, children, females, and males, respectively. It is suggested that the groundwater should be chemical and biological denitrification for nitrate removal before being used for drinking purposes. The findings of this work can help policymakers to solve groundwater pollution problems and ensure healthy drinking water in such intensive agricultural basins and other similar regions worldwide.

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.

Identifying nitrate sources and transformations in an agricultural watershed in Northeast China: Insights from multiple isotopes.

Accurate identification of riverine nitrate sources is required for preventing and controlling nitrogen contamination in agricultural watersheds. The water chemistry and multiple stable isotopes (δ15N-NO3, δ18O-NO3, δ2H-H2O, and δ18O-H2O) of the river water and groundwater in an agricultural watershed in China's northeast black soil region were analyzed to better understand the sources and transformations of riverine nitrogen. Results showed that nitrate is an important pollutant that affects water quality in this watershed. Affected by factors such as seasonal rainfall changes and spatial differences in land use, the nitrate concentrations in the river water showed obvious temporal and spatial variations. The riverine nitrate concentration was higher in the wet season than in the dry season, and higher downstream than upstream. The water chemistry and dual nitrate isotopes revealed that riverine nitrate came primarily from manure and sewage (M&S). Results from the SIAR model showed that it accounted for more than 40% of riverine nitrate in the dry season. The proportional contribution of M&S decreased during the wet season due to the increased contribution of chemical fertilizers and soil nitrogen induced by large amounts of rainfall. The δ2H-H2O and δ18O-H2O signatures implied that interactions occurred between the river water and groundwater. Considering the large accumulation of nitrates in the groundwater, restoring groundwater nitrate levels is essential for controlling riverine nitrate pollution. As a systematic study on the sources, migration, and transformations of nitrate/nitrogen in agricultural watersheds in black soil regions, this research can provide a scientific support for nitrate pollution management in the Xinlicheng Reservoir watershed and provide a reference for other watersheds in black soil regions in the world with similar conditions.

Woodchip bioreactors for saline leachates denitrification can mitigate agricultural impacts in mediterranean areas: The Campo de Cartagena-Mar Menor environmental issue.

Leachates from intensive agriculture containing high nitrate have been identified as a major cause of the severe eutrophication crisis that impacts Mar Menor (SE Spain), the largest hypersaline coastal lagoon in the Mediterranean basin. A best management practice for removing NO3--N is denitrifying bioreactors. This is the first study to assess the efficiency of citrus woodchips bioreactors in treating agricultural leachates that flow to the Mar Menor via surface discharges. Denitrification capacity, woodchip degradation (by weight loss), formation of potentially harmful compounds, and greenhouse gas (GHG) emissions were assessed. Three bioreactors (6 m × 0.98 m x 1.2 m) filled with citrus woodchips (3 m3 d-1 per bioreactor) through which the untreated ditch water over 1.5 years. Bioreactors were operated at 8 h, 16 h, and 24 h hydraulic residence time respectively, in each bioreactor. The main characteristics of the ditch water were: pH ≈ 7.5-8.0, electrical conductivity ≈ 5-8 dS m-1, dissolved organic carbon ≈6-10 mg L-1, and NO3--N ≈ 22-45 mg L-1. Bioreactors were highly efficient in reducing NO3--N. The average RNO3 in effluents was for the complete experimental period 8 g N m-3 d-1, 10.9 g N m-3 d-1, and 12.6 g N m-3 d-1 for 8, 16 and 24 h residence time, respectively. Nitrate reduction efficiency was modulated by seasonal changes in temperature, with an increasing efficiency in warmer periods (maximum ≈ 85-90% for all hydraulic residence time) and decreasing in colder ones (minimum ≈ 12%, 23% and 41% for hydraulic residence time 8, 16 and 24 h respectively). Woodchips degradation was greatest during the first six months (average ≈ 29% weight loss) in the material above the water level, attributable to aerobic mineralization of the organic carbon, while weight loss was ≈11% in woodchip media continuously below the water level. Dissolved organic carbon, sulfide, ammonium, and soluble phosphorus concentrations in the effluents were mostly low, although some peaks in concentrations occurred. Design consideration must be taken to avoid environmental impacts due to the occasional presence of harmful compounds in the effluents.

Where to measure water quality ? Application to nitrogen pollution in a catchment in France.

Information on the water quality of rivers can be used to judge the effectiveness of past policies or to guide future environmental policies. Consequently, the location of water quality monitoring stations (WQMSs) plays an important role in river pollution control. In the 2000s, a literature developed on the optimization of WQMS location to identify pollution hot spots, average quality, or to minimize the detection time of a potential source of accidental pollution. This article is part of a new literature aimed at locating WQMSs in order to optimize the economic value of information (EVOI) generated by water quality monitoring networks (WQMNs). The field of study is a catchment in northeastern France where the purpose of quality measurement is to define a policy of reduction of agricultural nitrogen fertilizers in order to reach the standard of 50 mg/l of nitrate at the WQMS. Agro-hydrological and economic models estimate the net benefit of input reduction depending on the location of the WQMS on the basis of different assumptions concerning the ecological damage generated by nitrate. We show that the magnitude of the ecological damage and, consequently, the perception of the contamination generated by nitrate in water, play a decisive role on the optimal location of the WQMS, as well as on the benefit of the economic optimization of locations, compared to traditional optimization. Locating WQMSs in a way that maximizes EVOI will be more attractive for very high or very low levels of damage. However, in this context, linking damage to nitrate concentration or to concentration coupled with riparian population density alone will have little impact.

Identification and apportionment of groundwater nitrate sources in Chakari Plain (Afghanistan).

The Chakari alluvial aquifer is the primary source of water for human, animal, and irrigation applications. In this study, the geochemistry of major ions and stable isotope ratios (δ2H-H2O, δ18O-H2O, δ15N-NO3̄, and δ18O-NO3̄) of groundwater and river water samples from the Chakari Plain were analyzed to better understand characteristics of nitrate. Herein, we employed nitrate isotopic ratios and BSIMM modeling to quantify the proportional contributions of major sources of nitrate pollution in the Chakari Plain. The cross-plot diagram of δ15N-NO3̄ against δ18O-NO3̄ suggests that manure and sewage are the main source of nitrate in the plain. Nitrification is the primary biogeochemical process, whereas denitrification did not have a significant influence on biogeochemical nitrogen dynamics in the plain. The results of this study revealed that the natural attenuation of nitrate in groundwater of Chakari aquifer is negligible. The BSIMM results indicate that nitrate originated mainly from sewage and manure (S&M, 75‰), followed by soil nitrogen (SN, 13‰), and chemical fertilizers (CF, 9.5‰). Large uncertainties were shown in the UI90 values for S&M (0.6) and SN (0.47), whereas moderate uncertainty was exhibited in the UI90 value for CF (0.29). The findings provide useful insights for decision makers to verify groundwater pollution and develop a sustainable groundwater management strategy.

Assessment and validation of groundwater vulnerability to nitrate and TDS using based on a modified DRASTIC model: a case study in the Erbil Central Sub-Basin, Iraq.

In recent years, groundwater vulnerability assessment has become a crucial step in effectively protecting groundwater resources against increasing groundwater pollution in recent years. Sustainable effectual management of groundwater sources in terms of quality has become a critical factor in the development of unplanned urbanization areas, especially in regions with intensive agricultural and industrial activities in the land use/land cover (LULC) models. In this study, the GIS-based DRASTIC model was used by modified to estimate the groundwater vulnerability of porous aquifers to nitrate and total dissolved solids (TDS). The DRASTIC and the modified DRASTIC models generate four different groundwater vulnerability zones: high (33.6, 37.8%), moderate (45.9, 42.3%), low (18.7, 18.3%), and very low (1.8,1.6%). DRASTIC_LULC index map provides four different vulnerability zones: low, moderate, high, and very high, covering 0.1%, 7.6%, 83.6%, and 8.7% of the Erbil Central Sub-Basin, respectively. The most important hydrogeological factors determining the DRASTIC vulnerability obtained from sensitivity analyses are depth to the water table and impact of vadose zone parameters with average effective weight values of 23.7% and 22.6%. For validating the DRASTIC_LULC model, the water quality parameters, nitrate and TDS, have been used with an accuracy of 68% and 79%, which indicates that the validation accuracy for this model is quite high. Maps obtained as a result of this study can be used to create a baseline map for the sustainable management of groundwater quality in vulnerable areas of the Erbil Central Sub-Basin and its planning.

Using Bayesian Belief Networks to Investigate Farmer Behavior and Policy Interventions for Improved Nitrogen Management.

Increasing farmers' adoption of sustainable nitrogen management practices is crucial for improving water quality. Yet, research to date provides ambiguous results about the most important farmer-level drivers of adoption, leaving high levels of uncertainty as to how to design policy interventions that are effective in motivating adoption. Among others, farmers' engagement in outreach or educational events is considered a promising leverage point for policy measures. This paper applies a Bayesian belief network (BBN) approach to explore the importance of drivers thought to influence adoption, run policy experiments to test the efficacy of different engagement-related interventions on increasing adoption rates, and evaluate heterogeneity of the effect of the interventions across different practices and different types of farms. The underlying data comes from a survey carried out in 2018 among farmers in the Central Valley in California. The analyses identify farm characteristics and income consistently as the most important drivers of adoption across management practices. The effect of policy measures strongly differs according to the nitrogen management practice. Innovative farmers respond better to engagement-related policy measures than more traditional farmers. Farmers with small farms show more potential for increasing engagement through policy measures than farmers with larger farms. Bayesian belief networks, in contrast to linear analysis methods, always account for the complex structure of the farm system with interdependencies among the drivers and allow for explicit predictions in new situations and various kinds of heterogeneity analyses. A methodological development is made by introducing a new validation measure for BBNs used for prediction.

Nitrate contamination and associated health risks of the Benslimane groundwater, Morocco.

Consumption of polluted water has harmful impacts on human health. This study examined the quality of groundwater in the Benslimane area for drinking purposes based on the Water quality index (WQI), Nitrate pollution index (NPI), and Total risk quotient (THQ) for different age groups. A total of 120 groundwater samples were collected for physicochemical analyses. The results showed WQI values ranging from 157.7 to 472.7 and an average of 279.4, with a total absence of water of excellent or good quality, and about 62.5% of the groundwater samples were of very poor quality for consumption. Nitrate concentrations ranged from 1 to 270 mg/L with an average of 64 mg/L, and 56.7% had values above the World Health Organization safety level of 50 mg/L. The NPI showed that 78.3% of the sampled sites showed very high pollution as a result of intense anthropogenic activities. High contamination is observed in the north and east of the region for arboriculture, grapes, maize, and vegetables as opposed to cereals. The health risk associated with nitrates, based on oral exposure, was much higher than dermal contact. The total risk quotient for both pathways was 0.02 to 6.58, 0.02 to 6.12, 0.06 to 17.06, and 0.05 to 13.35 for women, men, children, and infants, respectively. A total of 65, 63.3, 82.0, and 78.3% of groundwater samples presented a non-cancer health risk for women, men, children, and infants, respectively. Therefore, this study can help identify contaminated areas in order to track corrective safety measures to control groundwater quality in the region and improve sanitary conditions.

Reservoir NO3- pollution and chemical weathering: by dual isotopes of δ15N-NO3-, δ18O-NO3- and geochemical constraints.

Reservoir dams alter the nutrient composition and biogeochemical cycle. Thus, dual isotopes of δ18O-NO3- and δ15N-NO-3 and geochemical signatures were employed to study the NO3- pollution and chemical weathering in the Three Gorges Reservoir (TGR), China. This study found that the TGR dam alters the δ15N-NO3- composition and is enriched in the recharge period. Values of δ15N-NO3- varied from 4.5 to 12.9‰ with an average of 9.8‰ in the recharge period, while discharge period δ15N-NO3- ranged from 3.2 to 12.5‰, with an average of 9.3‰. δ18O-NO3- varies (1.2-11.3‰) with an average of 6.5‰ and (2.4-12.4‰) with an average of 7.5‰, in the recharge and discharge periods, respectively. Stable isotopic values sharply decreased from upstream to downstream, indicating the damming effects. δ18O-NO3- and δ15N NO3- confirm that sewage effluents, nitrification of soil organic material, and NH4+ fertilizers were the primary sources of NO3- in the reservoir. Carbonate weathering mainly provides ions to the reservoir. HCO3- + SO42- and Ca2+ + Mg2+ represent 90% of major ions in the TGR. Downstream sampling sites showed low solute concentration during the recharge period, indicating the dam effect on solute concentration. Ca-Mg-Cl-, Ca-HCO3- and Ca-Cl- were the main water types in the TGR. The average percentage of solutes contribution revealed the carbonate weathering, evaporites dissolution, silicate weathering, and atmospheric input were 51.9%, 41%, 7.8%, and 1.7% for the recharge period. In contrast, the discharge period contributed 66.4%, 29.2%, 10%, and 4.3%, respectively. TGR water is moderately suitable for irrigation, and hardness is high in drinking water. This study provides new insight into the dual isotopic approach and geochemical signatures to interpret the NO3- cycle and chemical weathering process under dam effects in the TGR. However, this isotopic application has some limitations in source identification, isotope fractionation, and transformation mechanisms of nitrate. Thus, further studies need to be done on these topics for a better undestanding.

Groundwater quality evaluation and human health risks assessment using the WQI, NPI and HQnitrate models: case of the Sfax intermediate aquifer, Sahel Tunisia.

Groundwater is a vital natural resource required to satisfy the domestic and agricultural needs. In general, human health is linked to the quality of the consumed water. For instance, long-term exposure to high nitrate levels in groundwater may cause problems. Hence, the present study was conducted to assess the nitrate contamination of groundwater as well as its related health risks for the inhabitants of the Sfax region, Sahel Tunisia. Irrigation groundwater suitability has been evaluated with sodium content (%Na), electrical conductivity (EC), magnesium hazard (MH), sodium adsorption ratio (SAR), permeability index (PI), Kelly's ratio (KR) and soluble sodium percent (SSP). The results indicate that the selected groundwater is characterized by low to moderate quality for irrigation. Furthermore, the drinking water quality index (DWQI) was assessed using potential of hydrogen (pH), total dissolved solids (TDS), magnesium (Mg2+), calcium (Ca2+), sodium (Na+), chloride (Cl-), sulfate (SO42-), potassium (K+), bicarbonate (HCO3-) and nitrate (NO3-). The results indicate that 3.63% of samples have good quality of water, while 41.82% have poor to very poor water quality and the rest (54.55%) are unfit for drinking. The nitrate pollution index (NPI) model revealed that about 42% of the samples present significant to very significant type of pollution. Based on human health risk assessment, the children are at higher risks compared to the other affected groups. The obtained results could be used as a basic document for realistic management of groundwater quality and to provide an overview for decision-making authorities to take necessary actions.

Nonlinear response of nitrate to NOx reduction in China during the COVID-19 pandemic.

In recent years, nitrate plays an increasingly important role in haze pollution and strict emission control seems ineffective in reducing nitrate pollution in China. In this study, observations of gaseous and particulate pollutants during the COVID-19 lockdown, as well as numerical modelling were integrated to explore the underlying causes of the nonlinear response of nitrate mitigation to nitric oxides (NOx) reduction. We found that, due to less NOx titration effect and the transition of ozone (O3) formation regime caused by NOx emissions reduction, a significant increase of O3 (by ∼ 69%) was observed during the lockdown period, leading to higher atmospheric oxidizing capacity and facilitating the conversion from NOx to oxidation products like nitric acid (HNO3). It is proven by the fact that 26-61% reduction of NOx emissions only lowered surface HNO3 by 2-3% in Hebi and Nanjing, eastern China. In addition, ammonia concentration in Hebi and Nanjing increased by 10% and 40% during the lockdown, respectively. Model results suggested that the increasing ammonia can promote the gas-particle partition and thus enhance the nitrate formation by up to 20%. The enhanced atmospheric oxidizing capacity together with increasing ammonia availability jointly promotes the nitrate formation, thereby partly offsetting the drop of NOx. This work sheds more lights on the side effects of a sharp NOx reduction and highlights the importance of a coordinated control strategy.