Groundwater Nitrate Research Articles

Evaluating Groundwater Nitrate Status across the River Ythan Catchment (Scotland) following Two Decades of Nitrate Vulnerable Zone Designation

Diffuse agricultural pollution is one of the greatest challenges to achieving good chemical and ecological status of Scotland’s water bodies. The River Ythan in Aberdeenshire was designated a Nitrate Vulnerable Zone (NVZ) in the year 2000, due to the eutrophication of the Ythan Estuary and rising nitrate trends in Private Water Supply (PWS) groundwater abstractions. The third River Basin Management Plan (RBMP) for Scotland reported the Ellon groundwater body of the River Ythan catchment to be of poor chemical status as of 2021 with respect to nitrate, and forecasted groundwater recovery beyond 2027. Following two decades of NVZ designation, we investigated the drivers of groundwater nitrate across the River Ythan catchment through an analysis of long-term (2009–2018) groundwater quality monitoring data collected by the Scottish Environmental Protection Agency (SEPA) and a recent synoptic groundwater nitrate sampling survey of PWSs. Groundwater nitrate was found to remain elevated across the catchment area, and appeared to be highly sensitive to agricultural practices and meteorological forcing, indicating a high sensitivity of groundwater quality to environmental change. Further hydrogeological characterisation is recommended to better understand the effects of agricultural practices on groundwater quality, and to facilitate achievement of future RBMP goals under a changing climate.

Sources and cycling processes of nitrogen revealed by stable isotopes and hydrochemistry in a typical agricultural lake basin

Increased exposure to nitrate has been reported in many arid and semi-arid areas where groundwater is used as the drinking water source, presenting a major public health threat. Groundwater nitrate pollution is becoming serious due to intensive agricultural activities in the Nansi Lake Basin, Shandong Province, Northern China, through which the trunk line of the South-to-North Water Diversion Project passes. This study characterized potential nitrogen pollution sources by combined analysis of hydrochemical components and stable nitrate isotopes (δ15N–NO3-, δ18O–NO3-); the contribution of different sources was estimated using the SIAR model. The results show groundwater nitrate contamination in the Nansi Lake Basin is widespread both west (NSW) and east (NSE) of the lake. Manure and sewage (M&S), soil nitrogen (SN), and chemical fertilizer (CF) are potential NO3−-N sources in the study area, with the order of contribution being M&S (78.38%) > SN (15.02%) > CF (6.60%) in areas west of the lake and M&S (59.40%) > SN (30.38%) > CF (10.22%) in areas east of the lake, indicating the significant impact of human activities. The relationship between d-excess and δ15N–NO3- suggests irrigation practices increase soil moisture, resulting in more N from 15N-depleted soils leaching into the groundwater, increasing the contribution of SN. The predominant processes of the nitrogen cycle in the basin are different in the NSW and NSE areas. In the upper-middle reaches of the NSW area, nitrification dominates the nitrogen cycle due to aerobic conditions and elevated salinity; however, denitrification occurs in local downstream aquifers at depths ranging from 10 to 50 m. In contrast, SN plays a bigger role in the NSE, where the mineralization of soil organic nitrogen is the predominant process, followed by nitrification of M&S and CF nitrogen sources due to rapid groundwater flow in the carbonate and metamorphic rock aquifers.

CrowdWaterSens: An uncertainty-aware crowdsensing approach to groundwater contamination estimation

Groundwater contamination poses serious threats to public health and environmental sustainability. In this paper, we explore smart groundwater contamination sensing, which aims to accurately estimate the nitrate concentration in groundwater via a crowdsensing approach. Existing solutions often require professional groundwater collection and high-quality measurement of groundwater properties, making the data collection process time-consuming and unscalable. In this work, we leverage the approximate nitrate concentration measured by crowd sensors (i.e., participants from well-dependent communities) to accurately estimate nitrate concentration in groundwater samples. Three critical challenges exist in developing the crowdsensing-based groundwater contamination estimation solution: (i) the spatial irregularity of the crowdsensing groundwater contamination data, (ii) the hidden temporal dependency of groundwater contamination in the anthropogenic context, and (iii) the uncertainty of crowdsensing nitrate measurements from crowd sensors. To address the above challenges, we develop CrowdWaterSens, an uncertainty-aware graph neural network framework that explicitly examines the uncertainty and spatial irregularity of the crowdsensing groundwater contamination data and its relevant anthropogenic context to accurately estimate groundwater nitrate concentration. We evaluate the CrowdWaterSens framework through two real-world case studies in well-dependent communities in Northern Indiana, United States. The evaluation results not only show the effectiveness of CrowdWaterSens in accurately estimating nitrate concentration, but also demonstrate the viability of crowdsensing for community-level groundwater quality monitoring.

Analysis of Hydrochemical Characteristics and Causes of Drinking Water Sources in South China: A Case Study in Zhanjiang City

The evaluation of groundwater environmental quality and the identification of recharge sources are very important for groundwater utilization. In this study, hydrochemistry and isotope analysis methods are used to investigate the recharge sources and hydrochemical processes of groundwater in Zhanjiang City. The results show that all samples of groundwater were drawn on the left of the global meteoric water line (GMWL: δD = 8δ18O + 10) and local meteorological water lines (LMWL1: δD = 8.17δ18O + 11.74 and LMWL2: δD = 7.50δ18O + 6.18), indicating that the groundwater was mainly recharged by meteoric precipitation and influenced by the effect of evaporation. In the middle and deep confined aquifers, the isotope data depleted with the depth, indicating that there is a relatively weak hydraulic connection between them. In addition, compared with unconfined groundwater, the isotope data of confined groundwater showed relative depletion, indicating that the confined aquifer may be partially recharged from other confined aquifers. The main chemical types in the groundwater were Na*Ca-HCO3. There are three major natural hydrochemical processes controlling the source of groundwater ions: silicate weathering, carbonate dissolution, and the cation exchange reaction. In addition, the differences in physical and chemical properties between unconfined groundwater and confined groundwater are significant. Due to the differences in anthropogenic activities and land-use types, the nitrate of the unconfined groundwater exceeds the groundwater standards. Due to the geological background of Zhanjiang City, iron and manganese exceed the third standard of groundwater in confined groundwater. Due to groundwater exploitation, TDS levels in confined groundwater have been increasing. Closed groundwater extraction is not sustainable, and it is depleting ancient water reserves. This study highlights the effectiveness of hydrochemistry and isotope analysis methods for identifying the recharge area and recharge mode of groundwater, andit is significant for fully understanding groundwater hydrochemistry and scientifically managing and protecting groundwater.

Nitrogen species and microbial community coevolution along groundwater flowpath in the southwest of Poyang Lake area, China

Nitrate and ammonia overload in groundwater can lead to eutrophication of surface water in areas where surface water is recharged by groundwater. However, this process remained elusive due to the complicated groundwater N cycling, which is governed by the co-evolution of hydrogeochemical conditions and N-cycling microbial communities. Herein, this process was studied along a generalized groundwater flowpath in Ganjing Delta, Poyang Lake area, China. From groundwater recharge to the discharge area near the lake, oxidation-reduction potential (ORP), NO3–N, and NO2–N decreased progressively, while NH3–N, total organic carbon (TOC), Fe2+, sulfide, and TOC/NO3− ratio accumulated in the lakeside samples. The anthropogenic influences such as sewage and agricultural activities drove the nitrate distribution, as observed by Cl− vs. NO3−/Cl− ratio and isotopic composition of nitrate. The hydrogeochemical evolution was intimately coupled with the changes in microbial communities. Variations in microbial community structures was significantly explained by Fe2+, NH3–N, and sulfide, while TOC/NO3− controlled the distribution of predicted N cycling gene. The absence of NH3–N in groundwater upstream was accompanied by the enrichment in Acinetobacter capable of nitrification and aerobic denitrification. These two processes were also supported by Ca2+ + Mg2+ vs. HCO3− ratio and isotopic composition of NO3−. The DNRA process downstream was revealed by both the presence of DNRA-capable microbes such as Arthrobacter and the isotopic composition of NH4+ in environments with high concentrations of NH3–N, TOC/NO3−, Fe2+, and sulfide. This coupled evolution of N cycling and microbial community sheds new light on the N biogeochemical cycle in areas where surface water is recharged by groundwater.

Nitrate in Groundwater in Europe

Nitrate in Groundwater in Europe

Investigating Nitrate with Other Constituents in Groundwater in Two Contrasting Tropical Highland Watersheds

Nitrate is globally the most widespread and widely studied groundwater contaminant. However, few studies have been conducted in sub-Saharan Africa, where the leaching potential is enhanced during the rainy monsoon phase. The few monitoring studies found concentrations over drinking water standards of 10 mg N-NO3− L−1 in the groundwater, the primary water supply in rural communities. Studies on nitrate movement are limited to the volcanic Ethiopian highlands. Therefore, this study aimed to evaluate the transport and fate of nitrate in groundwater and identify processes that control the concentrations. Water table height, nitrate, chloride, ammonium, reduced iron, and three other groundwater constituents were determined monthly in the groundwater in over 30 wells in two contrasting volcanic watersheds over two years in the Ethiopian highlands. The first watershed was Dangishta, with lava intrusion dikes that blocked the subsurface flow in the valley bottom. The water table remained within 3 m of the surface. The second watershed without volcanic barriers was Robit Bata. The water table dropped rapidly within three months of the end of the rain phase and disappeared except near faults. The average nitrate concentration in both watersheds was between 4 and 5 mg N-NO3− L−1. Hydrogeology influenced the transport and fate of nitrogen. In Dangishta, water was blocked by volcanic lava intrusion dikes, and residence time in the aquifer was larger than in Robit Bata. Consequently, nitrate remained high (in several wells, 10 mg N-NO3− L−1) and decreased slowly due to denitrification. In Robit Bata, the water residence time was lower, and peak concentrations were only observed in the month after fertilizer application; otherwise, it was near an average of 4 mg N-NO3− L−1. Nitrate concentrations were predicted using a multiple linear regression model. Hydrology explained the nitrate concentrations in Robit Bata. In Dangishta, biogeochemistry was also significant.

A GIS-based analysis of groundwater manganese and nitrate concentration for sustainable water supply

Groundwater is the main source of water supply in Sleman Regency. Although the water quantity is not an issue, the quality of groundwater should be understood to ensure the sustainability of the water supply. A previous study shows an increase in manganese (Mn) and nitrate (NO3) concentration in groundwater, yet the concentration distribution is unknown. Therefore, this study aims to develop a map of the spatial distribution pattern and to analyze the potential source of manganese and nitrate in groundwater in Sleman Regency in Yogyakarta Province. The method used is mapping groundwater quality using the ArcGIS application, analyzing groundwater pollution with water quality standards in the Indonesian Ministry of Health regulation number 492/Menkes/per/IV/2010 regarding drinking water quality requirements, and identifying the potential source of high pollution. Data processing uses the ArcGIS 10.8 application by performing the IDW (Inverse Distance Weighting) Interpolation technique. The result of this study has obtained a map of the distribution of groundwater quality for manganese and nitrate from 2017-2019. Manganese contamination may come from industrial and/or agricultural waste, while higher nitrate concentration might be a result of a septic tank leak and/or pesticide/fertilizer residue in paddy fields.

Identification of the hydrochemical features, genesis, water quality and potential health hazards of groundwater in Dawen River Basin, North China

Groundwater resources fulfill a key role in socioeconomic development and residential water use in the Dawen River Basin (DRB). This study sampled and tested three types of groundwater in the DRB, including pore groundwater (PGW), fissure groundwater (FGW), and karst groundwater (KGW). Groundwater in the DRB was assessed for hydrochemical characteristics, quality, and hazards to human health by nitrate contents. Results identified the dominant groundwater anion and cation to be HCO3− and Ca2+, respectively, whereas groundwater was generally weakly alkaline hard-fresh water. The groundwater contents of main chemical components during wet season exceeded those in the dry season, with the rank of groundwater types by chemical component contents being: PGW > KGW > FGW. A Durov diagram indicated groundwater in the DRB to be mixed HCO3-Ca and HCO3·SO4–Ca·Mg types. Rock weathering (silicate and carbonate rocks) was found to be the predominant process regulating groundwater chemistry of the DRB, whereas human activities also had a significant impact, particularly for high NO3− related to agricultural activities. Groundwater quality of the DRB was relatively good overall, falling into the good and moderate classification grades, with that of the PGW the worst. Long-term human direct consumption or exposure to groundwater of DRB poses certain health risks due to groundwater nitrate, with the risk for children highest. The spatial variations in quality of groundwater and nitrate health hazard in the DRB were relatively consistent, with better quality and lower health risk of PGW and FGW in the east than in the west, and that of KGW better in the center than in the east and west. The present study can provide some reference for groundwater safety and protection management in DRB and other similar areas in the world.

Groundwater Nitrate Pollution Due to Excessive Use of N-Fertilizers in Rural Areas of Bangladesh: Pollution Status, Health Risk, Source Contribution, and Future Impacts

Groundwater Nitrate Pollution Due to Excessive Use of N-Fertilizers in Rural Areas of Bangladesh: Pollution Status, Health Risk, Source Contribution, and Future Impacts

Investigating the sustainable performance of a nanoscale zerovalent iron permeable reactive barrier for removal of nitrate, sulfide, and arsenic

Abstract The quality of groundwater resources is at catastrophic risk. The proper performance of iron nanoparticles has made a permeable reactive barrier (PRB) an alternative to conventional filtration methods. The performance of nanozerovalent iron (nZVI) PRBs is limited by particle aggregation, instability, and phase separation, even at low iron concentrations. Therefore, the precipitation of reactive materials and a decrease in the longevity of PRB are fundamental challenges. A laboratory setup is used to compare the performance of bare nZVI and xanthan gum (XG)-nZVI + Mulch PRB to simultaneously remove nitrate, sulfide, and arsenic in groundwater. nZVI (average diameter of 35–55 nm) particles are used as reactive media. The objectives are (1) to develop a method for treating nitrate, sulfide, and arsenic simultaneously in groundwater using organic mulch and XG-nZVI; and (2) to evaluate the longevity performance of the XG-nZVI + Mulch and bare nanoparticles treatment system over 10 days. The results showed that the XG-nZVI + Mulch barrier's performance for eliminating NO3-, As, and S2− was generally improved compared to the bare nZVI barriers by 5.7, 19.2, and 10.9%, respectively. Finally, despite the need for long-term sustainability assessment, XG-nZVI PRB performance is impressive, and this stability promises to improve the longevity of nanoparticles while used in PRBs.

Groundwater characterization of the eastern Minesing Wetlands in support of the endangered Hine’s emerald dragonfly (Somatochlora hineana)

The Minesing Wetlands contain Canada’s only known population of Hine’s emerald dragonfly (HED), a species listed as endangered since 2011. The HED relies on groundwater discharge areas to complete its life cycle and therefore the population is potentially sensitive to changes in groundwater. The goals of this study were to, (1) characterize groundwater discharge to the eastern portion of the Minesing Wetlands, which corresponds to the HED habitat, (2) document any significant changes in groundwater chemistry since a 1998 study, and (3) establish a comprehensive baseline water quality data set to facilitate future comparisons. Consistent with HED habitats in the USA, the chemistry of Minesing Wetlands groundwater was dominated by calcium and bicarbonate. Low groundwater chloride and nitrate concentrations indicated minimal impact from activities such as road salt application and fertilizer use. Nitrate was generally only found at the wetland margin where groundwater springs emerged from the base of the uplands. The dominant form of inorganic nitrogen in wetland groundwater was ammonium (max = 1.5 mg N/L), and soluble reactive phosphorus concentrations were also relatively high (8 to 122 µg/L). The most consistent change in wetland groundwater chemistry between the 1998 study and samples collected in 2015–2017 was that chloride concentrations in 10 of 23 wells increased over time, however the maximum mean well chloride (11.6 mg/L) remained well below the guideline for the protection of aquatic life (120 mg/L). To preserve HED habitat, potential impacts to groundwater need to be considered during land use planning for the recharge area.

Unveiling the nitrogen transport and transformation in different karst aquifers media

Karst aquifers, a significant source of drinking water worldwide, are vulnerable to anthropogenic pollutants because of their special hydrogeological conditions. However, distinguishing the nitrogen transformation process is difficult owing to the heterogeneity of karst media. Therefore, the process of nitrogen transformation in different karst aquifer media remains poorly understood. In this study, multiple stable isotopic and physicochemical indicators are combined, the sources of water and nitrogen are analysed, and the degree of openness and redox conditions of different media are discussed to elucidate the nitrate sources and N transformation in groundwater with different degrees of openness. Water isotopes indicate local meteoric groundwater origin. However, different karst aquifer media have different degrees of openness, resulting in the highest deuterium–oxygen isotope values in surface water, followed by conduit water, fissure water, and isolated cave water. Based on the evidence from isotope and physicochemical indicators, different karst aquifer media have different redox processes. Isolated cave water environments were under anoxic conditions dominated by sulfate reduction reactions. Fissure water was under temporary anoxic or sub-anoxic conditions dominated by denitrification reactions. Conduit water and karst surface water were between micro-oxygen and oxidising conditions, and nitrification was the key process. This phenomenon was observed because the diversity of karst media produces different conservation conditions and openness. The nitrate isotope and Bayesian mixing model indicate that the main sources of nitrate in karst groundwater in the study area are mostly associated with manure and sewage wastes, and a few are from soil organic nitrogen, atmospheric deposition, and chemical fertilizer. This study helps identify the migration and transformation patterns and processes of nitrogen in different karst aquifer media, providing a scientific basis for the restoration and remediation of contaminated karst groundwater.

Effective use of farmland soil samples for N and O isotopic source fingerprinting of groundwater nitrate contamination in the subsurface dammed limestone aquifer, Southern Okinawa Island, Japan

Nitrate contamination of groundwater due to improper management of fertilizer application and controls during agricultural activities is an escalating worldwide issue. Numerous studies have reported similar contamination in the Ryukyu limestone aquifer in the southern part of Okinawa Island. The study area has a unique setting where the subsurface dams hold the groundwater behind their cut-off walls, probably holding contaminated waters. This study provides updated insights on the source of contamination using stable N and O isotope ratios of dissolved nitrate (ẟ15NNO3 and ẟ18ONO3) in the aquifer waters. To propose a better reference in the source fingerprinting assessment, a special emphasis was placed on the isotopic comparison between the fertilizers and farmland soils where the respective fertilizers were applied. A total of 176 groundwater samples were collected from several spatial and monthly sampling campaigns for the purpose of source determination. Our isotopic comparison detected the enrichment of 15N of nitrate-nitrogen in the soil samples after the application of both chemical fertilizers and manure due to nitrification and volatilization. Unlike results from previous studies, the ẟ15NNO3 compositions of most groundwater samples did not fall into the range of soils mainly controlled by chemical fertilizers (from +3‰ to +5‰), but were comparable to the soils affected by mixed fertilizers including both chemical fertilizers and manure (from +7‰ to +9‰). The soils beneath untreated manure storage sites characterized by the highest ẟ15NNO3 values (from +18‰ to +20‰) affected some groundwater, which shows an even higher value (> +9‰). The contaminated waters tend to be gathered close to the subsurface dams and this tendency did not differ between two subsurface dam settings, Komesu and Giza. However, due to more impact from manure storage sites, the groundwater in the Komesu area tends to show slightly higher δ15NNO3 values than those in the Giza area. Our study demonstrated the usefulness of soil samples for precise isotopic source identification in farmland areas.

A parsimonious model for predicting the NO3−-N concentration in shallow groundwater in intensive agricultural areas using few easily accessible indicators and small datasets based on machine learning

Groundwater nitrate poses a health risk to humans, and monitoring groundwater quality is time-consuming and expensive. To improve the efficiency and reduce the costs of monitoring groundwater nitrate, a parsimonious model was developed using easily accessible predictors and small datasets based on random forest (RF), neural network (NN), and support vector machine (SVM) algorithms. Groundwater NO3−-N concentrations of 1196 groundwater samples from intensive agricultural areas along with data on 13 predictors were obtained for the construction of prediction models. R2 and RMSE metrics were used to evaluate the models, with an emphasis on a minimal number of predictors and a low data size. The results showed that even with few readily available predictors (conductivity, EC; chemical nitrogen fertilization rate, NF; and precipitation, PPT) and a small dataset (260–700 observations), the three models achieved good precision, with R2 and RMSE values of 0.70–0.80 and 6.87–8.13, respectively. In these prediction models, EC was the primary predictor acting as an intrinsic factor, NF acted as a loading factor to supplement the blind area of EC for NO3−-N prediction, and PPT was introduced because of its accessibility to improve predictive performance. The final predictors applied in this study link the two currently dominant prediction methods by combining the high predictive accuracy of the intrinsic predictor with the accessibility of the extrinsic predictor, and the results demonstrate the practicability and applicability advantages of this model in groundwater NO3−-N prediction. This model provides technical support for the rapid and low-cost realization of the online monitoring of NO3−-N pollution in shallow groundwater in intensive agricultural areas.

Isotopic tracing of nitrogen source and interaction between surface water and groundwater of a small valley plain in the Zhangxi watershed

Identifying the mechanism for circulation of nutrient elements in the hydrological cycle is crucial for protection of water resource quality. In this study, 72 surface water and 21 groundwater samples were collected in three seasons for hydrochemical and multiple isotopic (δD-H2O, δ18O–H2O, δ15N–NO3−, and δ18O–NO3−) analysis in the Zhangxi watershed, a small valley plain in the southeastern Yangtze River Delta. The hydrochemical type in surface water and shallow groundwater was Ca–HCO3, and the δD and δ18O isotope values were similar. Thus, the hydraulic connection and its material interaction were similar. Quantitative evaluation of the mass balance model showed that shallow groundwater accounted for more than 90% of surface water in the dry season but only 33.0% in the wet season. On the average, nitrate in shallow groundwater was mainly derived from organic fertilizer, whereas nitrate in surface water was mainly sourced from soil nitrogen and chemical fertilizer carried by surface runoff. At the time scale, the nitrogen sources of surface water showed seasonal variation owing to the interaction between surface water and groundwater. The contribution of organic fertilizer in the wet season was approximately 16.2%, but in the dry season increased to 32.3% and exceeded that of chemical fertilizer (24.6%). Shallow groundwater contributed more to surface water in the dry season, and transferred more nitrogen from organic fertilizer to surface water, so that they had similar nitrate sources. At the space scale, nitrate sources of surface water differed significantly with the land use types. The NO3− and δ15N–NO3− concentrations of the surface water in the mid-valley plain and downstream were significantly higher than in the natural forest area upstream, indicating the increased contribution of organic fertilizer. Agricultural activities and the interaction between surface water and groundwater in the valley plain area have a significant impact on nitrogen pollution. Reduction in nitrogen fertilizer use and establishment of a riparian protection zone are crucial to protect the water resource quality.

Enhanced nitrate removal using in situ reactive zone with reduced graphene oxide supported nanoscale zero-valent iron.

Nitrate pollution in groundwater is becoming more serious, which is harmful to human health. The reduced graphene oxide supported nanoscale zero-valent iron (nZVI/rGO) composite prepared in this paper can effectively remove nitrate in groundwater. In situ remediation of nitrate-contaminated aquifer was also studied. The results showed that NH4+-N was the main product of NO3--N reduction, and N2 and NH3 were also produced. When the dosage of rGO/nZVI was more than 0.2g/L, there was no accumulation of intermediate NO2--N during the reaction process. NO3--N was removed by rGO/nZVI mainly through physical adsorption and reduction process with the maximum adsorbing ability of 37.44mg NO3--N/g. After the slurry of rGO/nZVI was injected into the aquifer, a stable reaction zone could be formed. NO3--N could be removed continuously within 96h at the simulated tank, and NH4+-N and NO2--N were as the main reduction products. Moreover, the concentration of TFe near the injection well increased rapidly after rGO/nZVI injection, and could be detected at the downstream end, indicating that the reaction range was large enough for NO3--N removal.

Spatial-temporal difference between nitrate in groundwater and nitrogen in soil based on geostatistical analysis

The study of temporal and spatial variations of nitrate in groundwater under different soil nitrogen environments is helpful to the security of groundwater resources in agricultural areas. In this paper, based on 320 groups of soil and groundwater samples collected at the same time, geostatistical analysis and multiple regression analysis were comprehensively used to conduct the evaluation of nitrogen contents in both groundwater and soil. From May to August, as the nitrification of groundwater is dominant, the average concentration of nitrate nitrogen is 34.80 mg/L; The variation of soil ammonia nitrogen and nitrate nitrogen is moderate from May to July, and the variation coefficient decreased sharply and then increased in August. There is a high correlation between the nitrate nitrogen in groundwater and soil in July, and there is a high correlation between the nitrate nitrogen in groundwater and ammonium nitrogen in soil in August and nitrate nitrogen in soil in July. From May to August, the area of low groundwater nitrate nitrogen in 0–5 mg/L and 5–10 mg/L decreased from 10.97% to 0, and the proportion of high-value area (greater than 70 mg/L) increased from 21.19% to 27.29%. Nitrate nitrogen is the main factor affecting the quality of groundwater. The correlation analysis of nitrate nitrogen in groundwater, nitrate nitrogen in soil and ammonium nitrogen shows that they have a certain period of delay. The areas with high concentration of nitrate in groundwater are mainly concentrated in the western part of the study area, which has a high consistency with the high value areas of soil nitrate distribution from July to August, and a high difference with the spatial position of soil ammonia nitrogen distribution in August.

Response of microbial taxonomic and nitrogen functional attributes to elevated nitrate in suburban groundwater

Anthropogenic nitrogen (N) input has led to elevated levels of nitrate nitrogen (NO3−-N) in the groundwater. However, insights into the responses of the microbial community and its N metabolic functionality to elevated NO3−-N in suburban groundwater are still limited. Here, we explored the microbial taxonomy, N metabolic attributes, and their responses to NO3−-N pollution in groundwaters from Chaobai River catchment (CR) and Huai River catchment (HR) in Beijing, China. Results showed that average NO3−-N and NH4+-N concentrations in CR groundwater were 1.7 and 3.0 folds of those in HR. NO3−-N was the dominant nitrogen specie both in HR and CR groundwater (over 80 %). Significantly different structures and compositions of the microbial communities and N cycling gene profiles were found between CR groundwater and HR groundwater (p < 0.05), with CR groundwater harboring significantly lower microbial richness and abundance of N metabolic genes. However, denitrification was the dominant microbial N cycling process in both CR and HR groundwater. Strong associations among NO3−-N, NH4+-N, microbial taxonomic, and N functional attributes were found (p < 0.05), suggesting denitrifiers and Candidatus_Brocadia might serve as potential featured biomarkers for the elevated NO3−-N and NH4+-N concentration in groundwater. Path analysis further revealed the significant effect of NO3−-N on the overall microbial N functionality and microbial denitrification (p < 0.05). Collectively, our results provide field evidence that elevated levels of NO3−-N and NH4+-N under different hydrogeologic conditions had a significant effect on the microbial taxonomic and N functional attributes in groundwater, with potential implications for improving sustainable N management and risk assessment of groundwater.

Defining the Extent of Mine-Influenced Groundwater in a Mineralized and Agricultural Area Using Multiple Lines of Evidence

Mine-influenced groundwater (MIW) at a former copper mine site has affected an alluvial aquifer that also contains groundwater with solutes characteristic of natural mineralization and agricultural activities. Many of the solutes are common to at least two of three potential sources, and some solutes (e.g., sulfate and uranium) are common to all three. This precludes the conventional use of one, or several, characteristic solutes to identify the extent of MIW. So, we used five independent lines of evidence (LOE) to define the extent of MIW in the alluvial aquifer. The five LOE are multivariate statistical analysis of an extensive groundwater analyte suite, groundwater sulfur isotope (δ34SSO4) data, probabilistic calculations of groundwater velocity and flow distance, isotopic (3H/3He) estimates of groundwater age, and extent of nitrate in groundwater originating from agricultural activities.