Research Advances in the Analysis of Nitrate Pollution Sources in a Freshwater Environment Using δ15N-NO3- and δ18O-NO3. Int J Environ Res Public Health 2021;18:ijerph182211805. [PMID: 34831560 PMCID: PMC8623930 DOI: 10.3390/ijerph182211805]

Gut microbiota-bile acid crosstalk contributes to intestinal damage after nitrate exposure in Bufo gargarizans tadpoles

Bile acids (BAs) are amphipathic steroid acids whose production and diversity depend on both host and microbial metabolism. Nitrate (NO3-) is a widespread pollutant in aquatic ecosystems, which can cause rapid changes in microbial community structure and function. However, the effect of gut microbiota reshaped by nitrate‑nitrogen (NO3-N) on BAs profiles remains unclarified. To test this, intestinal targeted BAs metabolomics and fecal metagenomic sequencing were performed on Bufo gargarizans tadpoles treated with different concentrations of NO3-N. NO3-N exposure induced a reduction in the abundance of microbiota with bile acid-inducible enzymes (BAIs) and/or hydroxysteroid dehydrogenases (HSDHs), thus inhibiting the conversion of primary BAs to secondary BAs. Inhibition of BAs biotransformation decreased protective hydrophilic BAs (UDCA) and increased toxic hydrophobic BAs (CA and CDCA), which may contribute to intestinal histopathological damage. Moreover, we found that NO3-N treatment increased microbial virulence factors and decreased Glycoside hydrolases, further highlighting the deleterious risk of NO3-N. Overall, this study shed light on the complex interactions of NO3-N, gut microbiota, and BAs, and emphasized the hazardous effects of NO3-N pollution on the health of amphibians.

Tracing Nitrogen Sources and Transformation Characteristics in a Large Basin with Spatially Heterogeneous Pollution Distribution

This study used dual stable isotope analysis to examine nitrate sources and geographical distribution in the Liao River Basin (LRB), one of China's seven major river basins. During a normal hydrological season in April 2021, water samples were taken from the main streams of the Liao River (MLR), Shuangtaizi River (STR), Hun River (HR), Taizi River (TZR), and Daliao River (DLR). Monitoring results indicated that 93% of the water samples had a total nitrogen level exceeding the Class IV limit (1.5 mg/L) of the 'Environmental Quality Standards (EQS) for surface water', indicating a serious nitrogen pollution status. 71.3% of the total nitrogen on average was in the form of nitrate. The scatterplots of δD-H2O and δ18O-H2O showed that water in TZR and DLR were mainly affected by precipitation, while MLR, STR and HR were additionally impacted by evaporation and groundwater. The overall δ15N and δ18O of NO3- varied from 7.7‰ to 17.9‰ and 0.6‰ to 11.2‰, respectively. The correlations between δ15N-NO3- and δ18O-NO3-, along with attribution results from the Bayesian isotopic mixing model, indicated a predominant role of manure/sewage (MS) pollution in affecting river nitrate, accounting for 78% of total nitrate in MLR and 72% in DLR. A positive correlation between δ15N-NO3- and δ18O-NO3- in MLR indicated the occurrence of denitrification process. Overall, attribution results showed that the primary nitrate sources varied in different river systems within such a large basin, mainly due to spatially varied land use and human activities. Tailored nitrogen management strategies should be implemented to address the main anthropogenic pressures.

Toxicity threshold and ecological risk of nitrate in rivers based on endocrine-disrupting effects: A case study in the Luan River basin, China

Nitrate, as a crucial nutrient, is consistently targeted for controlling water eutrophication globally. However, there is considerable evidence suggesting that nitrate has endocrine-disrupting potential on aquatic organisms. In this study, the sensitivity of various adverse effects to nitrate nitrogen (nitrate-N) was compared, and a toxicity threshold based on endocrine-disrupting effects was derived. The spatiotemporal variations of nitrate-N concentrations in the Luan River basin were investigated, and the associated aquatic ecological risks were evaluated using a comprehensive approach. The results showed that reproduction and development were the most sensitive endpoints to nitrate, and their distribution exhibited significant differences compared to behavior. The derived threshold based on endocrine-disrupting effects was 0.65 mgL-1, providing adequate protection for the aquatic ecosystem. In the Luan River basin, the mean nitrate-N concentrations during winter (4.4 mgL-1) were significantly higher than those observed in spring (0.7 mgL-1) and summer (1.2 mgL-1). Tributary inputs had an important influence on the spatial characteristics of nitrate-N in the mainstream, primarily due to agricultural and population-related contamination. The risk quotients (RQ) during winter, summer, and spring were evaluated as 6.7, 1.8, and 1.1, respectively, and the frequency of exposure concentrations exceeding the threshold was 100 %, 64.3 %, and 42.5 %, respectively. At the ecosystem level, nitrate posed intermediate risks to aquatic organisms during winter and summer in the Luan River basin and at the national scale in China. We suggest that nitrate pollution control should not solely focus on water eutrophication but also consider the endocrine disruptive effect on aquatic animals.

Multiple isotopes reveal the driving mechanism of high NO3- level and key processes of nitrogen cycling in the lower reaches of Yellow River

The continuous increase of nitrate (NO3-) level in rivers is a hot issue in the world. However, the driving mechanism of high NO3- level in large rivers is still lacking, which has limited the use of river water and increased the cost of water treatment. In this study, multiple isotopes and source resolution models are applied to identify the driving mechanism of high NO3- level and key processes of nitrogen cycling in the lower reaches of the Yellow River (LRYR). The major sources of NO3- were sewage and manure (SAM) in the low-flow season and soil nitrogen (SN) and chemical fertilizer (CF) in the high-flow season. Nitrification was the most key process of nitrogen cycling in the LRYR. However, in the biological removal processes, denitrification may not occur significantly. The temporal variation of contributions of NO3- sources were estimated by a source resolution model in the LRYR. The proportional contributions of SAM and CF to NO3- in the low-flow and high-flow season were 32.5%-52.3%, 44.2%-46.2% and 36.0%-40.8%, 54.9%-56.9%, respectively. The driving mechanisms of high NO3- level were unreasonable sewage discharge, intensity rainfall runoff, nitrification and lack of nitrate removal capacity. To control the NO3- concentration, targeted measures should be implemented to improve the capacity of sewage and wastewater treatment, increase the utilization efficiency of nitrogen fertilizer and construct ecological engineering. This study deepens the understanding of the driving mechanism of high nitrate level and provides a vital reference for nitrogen pollution control in rivers to other area of the world.

Review: The application of source analysis methods in tracing urban non-point source pollution: categorization, hotspots, and future prospects

The contribution of urban non-point source (NPS) pollution to surface water pollution has gradually increased, analyzing the sources of urban NPS pollution is of great significance for precisely controlling surface water pollution. A bibliometric analysis of relevant research literature from 2000 to 2021 reveals that the main methods used in the source analysis research of urban NPS pollution include the emission inventory approach, entry-exit mass balance approach, principal component analysis (PCA), positive matrix factorization (PMF) model, etc. These methods are primarily applied in three aspects: source analysis of rainfall-runoff pollution, source analysis of wet weather flow (WWF) pollution in combined sewers, and analysis of the contribution of urban NPS to the surface water pollution load. The application of source analysis methods in urban NPS pollution research has demonstrated an evolution from qualitative to quantitative, and further towards precise quantification. This progression has transitioned from predominantly relying on on-site monitoring to incorporating model simulations and employing mathematical statistical analyses for traceability. This paper reviews the principles, advantages, disadvantages, and the scope of application of these methods. It also aims to address existing problems and analyze potential future development directions, providing valuable references for subsequent related research.

Basin-wide tracking of nitrate cycling in Yangtze River through dual isotope and machine learning

The nitrate (NO3-) input has adversely affected the water quality and ecological function in the whole basin of the Yangtze River. The protection of water sources and implementation of "great protection of Yangtze River" policy require large-scale information on water contamination. In this study, dual isotope and Bayesian mixing model were used to research the transformation and sources of nitrate. Chemical fertilizers contribute 76 % of the nitrate sources in the upstream, while chemical fertilizers were also dominant in the midstream (39 %) and downstream (39 %) of Yangtze River. In addition, nitrification process occurred in the whole basin. Four machine learning models were used to relate nitrate concentrations to explanatory variables describing influence factors to predict nitrate concentrations in the whole basin of Yangtze River. The anthropogenic and natural factors, such as rainfall, GDP and population were chosen to take as predictor variables. The eXtreme Gradient Boosting (XGBoost) model for nitrate has a better predictive performance with an R2 of 0.74. The predictive models of nitrate concentrations will help identify the nitrate distribution and transport in the whole Yangtze River basin. Overall, this study represents the first basin-wide data-driven assessment of the nitrate cycling in the Yangtze River basin.

Graphene oxide stimulated low-temperature denitrification activity of microbial communities in lake sediments by enhancing anabolism and inhibiting cellular respiration

Nitrate pollution in fresh water is becoming increasingly serious. In this study, the effects of temperature and graphene oxide materials on the potential functions of denitrification communities in lake sediments were investigated by metagenome. The addition of graphene oxide significantly affected the abundance of denitrification genes such as Nap, Nos, and enhanced the contribution of Pseudomonas, making low temperature and material addition conducive to the denitrification process. Module network implied that low temperature increased the centrality of denitrification in community functions. At low temperatures, graphene oxide enhanced community anabolism by stimulation organic carbon consumption and regulating the gene abundance in the citric acid cycle and the semi-phosphorylation Entner-Doudoroff, thus possibly stimulating extracellular polymeric substances (EPS) synthesis and secretion. In addition, graphene oxide may also regulate the transfer of reducing electrons from NADH to denitrifying enzymes by affecting the gene abundances of complex I and complex IV.

Isotopic compositions (δD, δ18O) and end-member mixing for the control interface in a complex tidal region

Identifying the mixing processes of waters and currents in tidal reach is an important aspect of environmental management to protect freshwater resources and prevent water pollution. In this study, three field investigations conducted in a typical tidal reach in August, November and the following April focused on two isotopes (δD and δ¹⁸O) and salinity. A salinity-isotope conservative mixing model was established to differentiate water flows of the important control interface (CI) from freshwater, transition zone and saltwater end-members. Results suggested that the average δD and δ¹⁸O values during the ebb and flood tides depleted from August to November, then enriched significantly in the following April and were even higher than those in August. The δD and δ¹⁸O values in the saltwater zone enriched markedly compared with those in freshwater zone and transition zone due to the stronger evaporation occurring in the saltwater zone. Based on the revised model, the average contributions of freshwater end-member, transition zone end-member and saltwater end-member in three months were, respectively, 51.50 %, 36.93 % and 11.57 %. However, the contributions of freshwater and transition zones in April end-member were equivalent (47.45 % vs 44.31 %). Meanwhile the largest contribution of saltwater end-member was 20.56 % and occurred in August. The proportions of three end-members that contributed to CI changed with different evaporation scenarios and moisture sources of precipitation. Our research provides important information that furthers our understanding of the isotopes and their applications to environmental management in estuarine regions.

Environmental health impacts and controlling measures of anthropogenic activities on groundwater quality in Southwestern Nigeria

Groundwater is the major source of drinking water in virtually all the regions of Nigeria, including the southwestern region. It is an indispensable source of drinking water that many individuals are dependent upon for daily activities in Nigeria. However, the spontaneous rise in various forms of industrialization and other anthropogenic activities of man within the southwestern region has immensely polluted these water sources. This calls for tremendous and actionable concern because of the health implications associated with the intake of contaminated water. This study aims to thoroughly disentangle the major impacts of anthropogenic activities on the quality of groundwater in the southwestern region of Nigeria through extensive reviews of literature and conceptualization of scientific and research data on the field. Unlike previous reviews, the major sources of groundwater pollution in the region were discussed extensively to set the tone for the x-raying of the subject. The study also showed major long-standing pollution cases in the region with graphical, tabular, and pictorial illustrations of some of the groundwater parameters and at the same time proposed controlling measures to enable eidetic understanding of the concepts and contribution to knowledge. In the last part of the work, we recommend improving the existing groundwater assessment techniques in Southwestern Nigeria. Regular monitoring of groundwater in Nigeria should also be encouraged to establish its quality status.