Fluoride and nitrogen contamination and potential health risks in the groundwater of a typical agricultural region

Over 2 billion individuals reside in regions experiencing significant water stress, a figure anticipated to rise. Furthermore, more than 1 billion people are deprived of access to clean and potable water (Tzanakakis et al. 2020). Groundwater serves as a vital resource, constituting the main supply for drinking, agricultural, industrial, and ecological uses (Jha et al. 2020; Chen et al. 2020), especially in arid and semi-arid regions, which comprise about one-third of the Earth land surface (Qian et al. 2020; Scanlon et al. 2023; Zou et al. 2024).

Understanding how hydrochemical alteration occurs in groundwater is essential for maintaining its quality (Ha et al. 2022). In hydrochemistry, anions and cations are widely regarded as crucial indicators for assessing water quality and evaluating the impact of human activities on groundwater (Gao et al. 2023). Groundwater hydrochemistry is influenced by hydrogeochemical processes, including precipitation, evaporation, weathering, adsorption and exchange reactions, and human activities. (Khan et al. 2021). The quality of groundwater has a significant impact on the health of both humans and plants (Kazi et al. 2009; Sun et al. 2024).

Groundwater with high fluoride and nitrogen levels is a global threat to drinking water safety (Andreah et al. 2023). Research indicates that more than 200 million people worldwide drink water with fluoride levels exceeding the acceptable threshold (Shaji et al. 2024). The primary source of fluoride is naturally occurring, originating from rocks and minerals that contain fluoride in their composition (Vithanage & Bhattacharya 2015; Maurya et al. 2020). Consuming groundwater with fluoride levels above the World Health Organization limit of 1.5 mg/L for drinking water can lead to widespread dental and skeletal fluorosis, affecting children's tooth development and causing leg pain, stress fractures, as well as potentially severe stomach irritation and ulcers (Chen et al. 2021; Dobrinas et al. 2022). The rise in nitrogen levels of groundwater can be linked to factors such as higher fertilizer usage, extensive irrigation, shallow groundwater tables, inadequate sanitation infrastructure, and lack of vegetative cover (Chen et al. 2016). Continuous consumption of nitrogen-rich water can lower oxygen capacity, causing methemoglobinemia in infants, and is linked to gastric cancer, multiple sclerosis, non-Hodgkin lymphoma, and thyroid enlargement (Zhang et al. 2018). In this regard, it is crucial to assess the risks associated with groundwater. The Human Health Risk Assessment (HHRA) by the United States Environmental Protection Agency (USEPA) (USEPA 2004) has been widely employed to evaluate the health risks associated with groundwater quality.

Groundwater is a vital resource for drinking, agriculture, industry, and ecosystems of the Guanzhong Plain in northwest China (Ren et al. 2021). However, high-fluoride groundwater caused the prevalence of dental fluorosis and skeletal fluorosis in the study region (Chen et al. 2021). Zhang et al. (2019a) identified that 24.61% of groundwater samples were above the nitrate limit for drinking in the plain due to agricultural activities. Due to the complex interplay of natural and anthropogenic factors, the mechanism of synchronous controls on groundwater quality and health risks is not yet fully understood.

Therefore, the main objective of this study were to (1) identify the hydrochemical properties of groundwater, (2) evaluate the groundwater quality for drinking water and irrigation purposes, and (3) identify the potential human health risks due to nitrogen and fluoride exposure in groundwater. The findings in this study would aid in gaining a comprehensive understanding of groundwater hydrochemistry and provide scientific support for decision-makers.

The Guanzhong Plain is responsible for 50% of the cultivated land in Shaanxi Province. It is known as the ‘Shaanxi Granary’ and the main food crops are wheat and maize (Zhang et al. 2023). Irrigation accounts for 60% of water use in the plain, with groundwater being the primary source, especially during dry seasons (Tang et al. 2015). However, the excessive use of chemical fertilizers in agriculture poses a great threat to groundwater quality. The fertilizer application rate in Shaanxi Province is 487.1 kg/ha (Meng-Yao et al. 2017), which is significantly higher than the mean national level 225 kg/ha (Ischinger 1979; Cai et al. 2018; Guo & Wang 2021). This indicates that excessive use of fertilizers, especially nitrogen, can lead to contamination of surface and groundwater through runoff or leaching.

A total of 60 groundwater samples were collected from the study area in April 2019. To ensure the groundwater's representativeness, wells were flushed for 15 min prior to sampling. The groundwater was preserved in 500-mL polyethylene plastic bottles. Before sampling, the bottles were rinsed at least three times with the groundwater to be tested, ensuring no excess air remained inside, and the bottles were securely sealed. The pH and total dissolved solids (TDSs) were measured in the field using portable instruments (DDBJ-350F, INESA Scientific Instrument Co., Ltd., China). Na⁺ and K⁺ were assessed using flame atomic absorption spectrophotometry (SP-3520AA, Shanghai Spectrum Instrument Co., Ltd, China), while Mg²⁺, Ca²⁺ and TH concentrations were determined through EDTA titration. ⁠, Cl⁻ and F⁻ levels were evaluated using ion chromatography (Dionex 120, Dionex, Sunnyvale, USA), was measured by potentiometric titrimetric methods. Spectrophotometry was used to measure the contents of and ⁠, and (722N, Shanghai Precision & Scientific Instrument Co., LTD, China). The samples were used for quality assurance/quality control (QA/QC) analysis. All groundwater sampling, preservation, and testing procedures are adhered to the Chinese Groundwater Quality Standards (General Administration of Quality Supervision & Quarantine of China 2017), ensuring the quality and consistency of the groundwater data and applying the ion balance method to verify the validity of the groundwater quality analysis results. The charge balance checking shows ±10%, also receivable in the study.

All statistical analyses were performed using SPSS software (version 26). A Mann–Whitney U test, an efficient nonparametric method, was conducted to assess significant differences in fluoride and nitrogen concentrations between samples from the north and south banks of the Wei River. The null hypothesis assumed no variance between these two groups. This study employed ordinary kriging interpolation in ArcGIS (version 10.8.2) to examine the spatial variability of ions in groundwater.

Groundwater quality within the study area can be categorized into five distinct classes based on the WQI, WQI < 50, 50 ≤ WQI < 100, 100 ≤ WQI < 200, 200 ≤ WQI < 300, WQI ≥ 300 were regarded as (1) excellent; (2) good; (3) average; (4) fair; and (5) poor.

Table 2 summarizes hydrochemistry from sample analyses. The groundwater in the study area exhibited predominantly mild alkalinity, with pH ranging from 7.1 to 8.8, averaging at 8.1. The TDS of groundwater was from 223 to 3,930 mg/L, with an average value of 984 mg/L. This study analyzed of hydrochemistry variables for groundwater samples in the south bank of the Wei River (NB) and the south bank of the Wei River (SB). The average value of TDS in the NB was 1,507 mg/L, which was greater than the average value of 760 mg/L in the SB. In the NB, 72.2% of the samples exceeded the limit value of TDS for drinking water (Ministry of Health of the PR China 2022; WHO 2022), and only 16.7% of the samples exceeded the limit value in the SB. The higher TDS in the NB may be due to the stronger water–rock interaction and evaporation concentration (Zhang et al. 2016).

The values of fluoride in the SB ranged from 0.15 to 1.78 mg/L, averaged at 0.78 mg/L. The higher fluoride concentrations were found in the NB. The concentrations ranged from 0.76 to 4.74 mg/L, averaged at 1.71 mg/L. The results of the Mann–Whitney U test showed significant differences in fluoride concentrations in groundwater between the north and south banks of the Wei River (p = 0.000 < 0.01). Approximately 20.97% of all samples exceeded the WHO (2022) recommended reference value of 1.5 mg/L, while about 35.48% surpassed the national threshold of 1.0 mg/L (General Administration of Quality Supervision & Quarantine of China 2017). The groundwater samples in the northern part of Wugong and the northeastern part of Xingping had high fluoride levels above the safe limits (Figure 2(m)).

The average concentration of in the NB was 18.88 mg/L, which was significantly higher than that in the SB, which was 5.61 mg/L. The Mann–Whitney U test indicated significant differences in concentrations in groundwater between the north and south banks of the Wei River (p = 0.007 < 0.01). 50% of the samples in the NB, and 21.43% in the SB had high nitrate values exceeding the WHO reference value of 10 mg/L WHO (202). These groundwater samples were mainly distributed in western part of the study region (Figure 2(k)). The value of was between BDL and 0.05 mg/L in the study area, with an average of 0.003 mg/L. All the samples were below the limitation of 1.0 mg/L for WHO and Chinese standards. The concentration of in NB was 0.13 mg/L, which was lower than the concentration of 0.27 mg/L in SB. The Mann–Whitney U test indicated significant differences in concentrations in groundwater between the north and south banks of the Wei River (p = 0.022 < 0.05). A small portion of the points exceeding the standard levels were primarily located at the confluence of the Hei River and the Wei River.

The WQI serves as an effective method for evaluating groundwater quality, simplifying extensive water quality data into a singular index to reflect the overall condition of groundwater (Gazzaz et al. 2012). By giving different weights (1–5) to the groundwater quality evaluation factors, the importance of each evaluation factor to the overall groundwater quality can be reflected. TDS, TH, nitrite, and fluoride are given higher weights and are considered to be an important part of the overall water quality (Xiao et al. 2023).

When carbonates (e.g., calcite and dolomite) and sulfates (e.g., gypsum) are the dominant hydrologically acting base mineral types in the groundwater system, converge to a 1:1 trend line (Capaccioni et al. 2001). Figure 6(c) shows that almost all of the groundwater samples from NB fall above the 1:1 line, and the excess and may be due to weathering of silicate rock and cation exchange. Among (Figure 6(d)), the groundwater in the NB has more significant bicarbonate ions, indicating that the effect of calcite dissolution is secondary.

In Figure 6(e), most of the groundwater samples in the study area fall between the 1:1 and 2:1 line, indicating that dolomite dissolution is the most important source of calcium and magnesium ions.

The Chlor-Alkali Index is helpful to identify cation exchange direction. The negative values indicate Na⁺ and K⁺ increase, and Mg²⁺ and Ca²⁺ decrease, while positive values suggest the reverse process (Adimalla & Li 2019). Figure 6(f) illustrates that CAI-I values range from −37.83 to 0.17, while CAI-II values vary from −1.10 to 0.05. Predominantly, the indices for the groundwater samples are negative. This negative tendency significantly contributes to explaining the observed high Na⁺ and low Ca²⁺ concentrations in the samples, as delineated in Reaction 1.

The contribution rates of ⁠, ⁠, and F⁻ to the cumulative non-carcinogenic risk (HI) of oral intake in the study area were 17.01%, 0.05%, 32.82% and 50.12%, respectively. The order of non-carcinogenic risk of each index for human body was ⁠, and the cumulative contribution rate of F and to the cumulative non-carcinogenic risk (HI) of oral intake was as high as 82.94%. It can be considered that the concentration of F⁻ and plays a controlling role in the non-carcinogenic health risk in the study area. In particular, the spatial distribution of the cumulative non-carcinogenic risk coefficient HI is similar to the spatial distribution of fluoride. This confirms the findings of Chen et al. (2021) of higher health risks associated with fluoride on the northern bank of the Wei River in the Guanzhong Plain. Higher HI and higher HQ-F⁻ are distributed near Wugong and Xingping on the north bank of the Wei River. Excessive intake of fluorine will interfere with the activity of various enzymes in the body, destroy the metabolic balance of calcium and phosphorus, and cause fluorosis of teeth, bone and joint deformation.

The study detailed analysis of groundwater in the region has unveiled critical insights, underscoring the urgency for targeted environmental and health interventions. Groundwater in the study area shows mild alkalinity. TDS averaged 1,507 mg/L in the NB, higher than the value of SB (760 mg/L). In the NB, 72.2% of samples surpassed the drinking water TDS limit, versus 16.7% in the SB. Overall, the majority of groundwater samples in the study area were characterized by the HCO₃–Ca ·Mg water type. Near Wugong County, located in the NB, the predominant water type was HCO₃·SO₄–Ca ·Na·Mg. Meanwhile, in the northwest of Huyi County in the SB, the dominant water type was HCO₃–Na–Ca.

Fluoride in groundwater primarily originates from the natural weathering of fluoride-bearing minerals such as fluorite (CaF₂). The concentration of fluoride is influenced by several factors including the interaction between water and rock, higher pH levels that enhance fluoride solubility, ion exchange processes involving calcium and sodium, and evaporation concentration in arid conditions. In areas where fluoride levels exceed standards within the research region, policies can be implemented to adopt water treatment measures and increase regular monitoring to reduce the impact of fluoride on human health. Nitrogen contamination, mainly in the form of nitrates, originates from agricultural activities, especially the use of nitrogen-based fertilizers, as well as livestock farming and wastewater discharge. The concentration of nitrogen in groundwater is controlled by factors such as excessive fertilizer application, irrigation practices that promote leaching, shallow groundwater tables, inadequate sanitation infrastructure, and the biochemical processes of nitrification and denitrification in the soil and water system. Policy improvements to address these issues could include stricter regulations on fertilizer use, enhanced wastewater treatment standards, and initiatives to restore and protect soil and water ecosystems.

The study reveals 80% of samples having a WQI below 100, indicating suitability for drinking. A significant portion of good-quality samples is found near the Qinling foothills, suggesting lateral recharge into the area. Groundwater in the SB outperforms groundwater in the NB in excellent water quality percentage (31 vs. 5.6%), likely due to the Qinling Mountains continuous supply, indicating that most groundwater in the area is above average quality, with SB having notably better quality.

This study highlights that children in a specific area face higher non-carcinogenic health risks from groundwater contaminants such as ⁠, ⁠, ⁠, and F⁻, compared to adults. The cumulative health risk (HI) for children often exceeds the USEPA safety threshold, with 88.33% of samples above the reference value, indicating significant health risks. Fluoride and ammonia nitrogen are the primary risk contributors, making up 82.94% of the cumulative HI. High-risk areas include Wugong and Xingping.

The findings of the present study can provide insights into the effect of natural factors and human activities on fluoride and nitrogen contamination in groundwater and provide a basis for further water management at a typical agricultural region.

We thank the associate editor and the reviewers for their useful feedback that improved the quality of this paper.

This research is supported by grants from the National Natural Science Foundation of China (42272289 and 42341102), the Natural Science Foundation of Shaanxi Province (2021JQ-862), Youth Talents Promotion Plan of Shaanxi Association for Science and Technology (20220711), and the Fundamental Research Funds for the Central Universities in Chang'an University (300102293205, 300102294905, 300102294906, and 300102294902).

J.Y. contributed to manuscript writing, sample collection, data analysis, result interpretation, and data generation. J.W. contributed to content review and data analysis. H.X contributed to data collection and for preparing cartography. Z.X. contributed to data collection and sample collection. Y.Z. contributed to language editing. J.C. contributed to funding acquisition and content review

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.