Spatial distribution, source identification, and health risk assessment of fluoride in the drinking groundwater in the Sulin coal district, northern Anhui Province, China Open Access

Fluorine is the 13th most abundant element in the Earth's crust and the lightest member of the halogen group (Rafique et al. 2015; Olaka et al. 2016). The daily intake of a trace amount of F⁻ has positive health benefits, as F participates in the mineralization of bones and teeth. The World Health Organization (WHO) suggests that drinking water with a F⁻ content of 0.80–1.00 mg/L can improve the dental health of children under the age of 10 (Keshavarzi et al. 2010; Mahvi et al. 2016; Tiwari et al. 2016; Goodarzi et al. 2017). However, excessive F⁻ intake poses health risks (Brindha et al. 2016; Dehbandi et al. 2018; Yousefi et al. 2018). For example, drinking water with a F⁻ content above 1.50 mg/L over a long period can cause dental fluorosis, and if the F⁻ content exceeds 4.00 mg/L, it may lead to the development of skeletal fluorosis (Abu & Alsokhny 2004; Keshavarzi et al. 2010; Aghapour et al. 2018). According to WHO guidelines for drinking-water quality, the maximum permissible limit of F⁻ in drinking water is 1.50 mg/L (WHO 1984; Keshavarzi et al. 2010; Rafique et al. 2015; Kumar et al. 2018; Rashid et al. 2018). In China, approximately 20% of the drinking water is supplied from groundwater (WHO 1984; Zhang et al. 2016). However, compared with surface water, the F⁻ content in groundwater is higher, which can cause health risks (Rafique et al. 2015; Aghapour et al. 2018). To avoid the potential health risks of excess F⁻ intake, the Chinese government has set the maximum permissible limit of F⁻ for drinking water at 1.00 mg/L (Guo et al. 2012; He et al. 2013a, 2013b).

Several studies have demonstrated that F⁻ in groundwater mainly originates from weathering of F-bearing minerals (such as fluorite, apatite, micas, and amphibole) (Keshavarzi et al. 2010; He et al. 2013a; Brindha et al. 2016; Dehbandi et al. 2018) and anthropogenic activities (such as phosphate fertilizers and pesticides, aluminum smelting, glass and brick industries, coal combustion, and semiconductor industries) (Enalou et al. 2018; Thapa et al. 2018). Generally, an alkaline environment with low Ca²⁺ content, high Na⁺ content, and NaHCO₃-type water will increase the F⁻ content in the groundwater (Rafique et al. 2015; Olaka et al. 2016; Dehbandi et al. 2018; Enalou et al. 2018; Kumar et al. 2018). In addition, evaporation and ion exchange are two important processes that contribute to an elevated F⁻ content in groundwater (He et al. 2013b; Mondal et al. 2014; Olaka et al. 2016; Rashid et al. 2018).

The distribution, sources, and health risks of high- F⁻ groundwater in the Hetao Plain (Guo et al. 2012; He et al. 2013; Tiwari et al. 2016), Zhangye Basin (He et al. 2013a), Yuncheng Basin (Li et al. 2015, 2018), Datong Basin (Pi et al. 2015), Yanchi endorheic region (Wu et al. 2018), Taiyuan Basin (Li et al. 2007), and Wanbei Region (Yang et al. 2017) of China, which is one of 25 countries severely affected by F⁻ contamination, have been widely studied. Studies have shown that the F⁻ content in almost 73.20% of shallow groundwater samples and 82.70% of deep groundwater samples exceed China's national drinking-water limit (1.0 mg/L) in the northern Anhui Province, China, with maximum F⁻ contents in shallow and deep groundwaters of 2.85 mg/L and 3.10 mg/L, respectively (Wu et al. 2010; Wu & Qian 2011; Yang et al. 2017). Due to a lack of effective treatment and protection in the Sulin coal district of the Wanbei Region, over 3.0 million people suffer from dental fluorosis and 100,000 people have developed skeletal fluorosis (Gao et al. 2013). The quantitative source apportionment of high F⁻ groundwater is a challenge. However, factor analysis (FA) combined with a positive matrix factorization (PMF) model provide a solution for quantitative source apportionment of pollutants in soil (Guan et al. 2018), water (Rodenburg et al. 2011), and sediment (Chen et al. 2013).

This study aimed to investigate the distribution of F⁻ concentrations, perform a quantitative source apportionment, and assess health risks associated with the consumption of groundwater in the Sulin coal district, northern Anhui Province, China, using geographic information systems, PMF, and risk-based corrective action (RBCA) models. The results of this study will contribute to the development of policies for the proper management of groundwater extraction to prevent health risks associated with excess F⁻ intake.

Sulin coal district, with an area of 1,000 km², is located in Wanbei, China, and is characterized by a subhumid monsoon climate. The study area extends from 116°15′E to 117°12′E longitude, with a latitude between 33°20′ N and 33°42′N. The annual precipitation is 750–900 mm and the mean annual evaporation is 900–1,050 mm at a mean annual temperature of approximately 14–15 °C. The area lies at an elevation of 20–40 m above sea-level and is dominated by flat terrain. The Tuo River and Hui River are the two main perennial rivers.

The land surface is covered by quaternary sediments with a thickness of 100.50 m to 771.70 m. The multilayer aquifer system in the area consists of both shallow and deep groundwater aquifers. The deep groundwater aquifers consist of sedimentary complexes and alluvial deposits with a mean thickness of 40 m, which are composed of quartz, muscovite, biotite, chlorite, and other minerals. Drinking and irrigation water for rural residents mainly comes from deep groundwater aquifers. Wheat and corn are the main crops in the study area.

In total, 30 groundwater samples were collected in the rainy season (May 2019), as shown in Figure 1. During this time, groundwater is at the abundant level period and F⁻ concentration is active (Table 1). A water level indicator (OTT PLS) was used to measure the groundwater level in open wells. Prior to collection, the groundwater at every sampling site was partially drained to access fresh groundwater. High-density polypropylene bottles used for sampling were first rinsed two or three times with distilled water and then rinsed with groundwater another two or three times. Additionally, all water samples were filtered through filter membranes with a pore size of 0.45 μm before filling the sampling bottles. The samples were usually taken at approximately 0.30 m below the groundwater. For every sampling site, three bottles (500 mL each) were collected. The pH and total dissolved solids (TDS) values were measured in the field using a portable pH meter (OHAUS ST20) and a portable TDS meter (OHAUS ST20T-B), respectively.

The concentrations of anions (Cl⁻, SO₄²⁻, NO₃⁻, and F⁻) and cations (Ca²⁺, Mg²⁺, Na⁺, and K⁺) were determined by ion chromatography (Dionex Integrion IC, Thermo Fisher, USA) with a detection limit of 0.01 mg/L. The concentrations of carbonate and bicarbonate anions were measured using acid–base titrations in an analytical laboratory with a detection limit of 0.01 mg/L. The analytical precision of the obtained ion concentrations was checked by calculating the ionic balance errors, which were generally below ±5%.

The positive matrix factorization (PMF) model is one of the receptor models recommended by the United States Environmental Protection Agency (US EPA) for source apportionment (Paatero & Tapper 1994; Paatero 1997; Liang et al. 2017; Guan et al. 2018). The PMF 5.0 program requires two input files: one for concentrations of each sample species and the other for the uncertainty values of each sample species. Before applying PMF, the dataset must follow a normal distribution and the outliers (two times or more of the variance) are ignored (Hu & Cheng 2016; Goodarzi et al. 2017; Guan et al. 2018).

Fluoride is ingested mainly through groundwater, food, breathing, and dermal absorption such as during bathing (Aghapour et al. 2018; Dehbandi et al. 2018; Emenike et al. 2018). As groundwater is the main contributor to F⁻ intake, the health risk assessment conducted in this study only evaluates the risk associated with drinking groundwater.

Considering the significant differences in health risks to people of different ages, the population was divided into the following five groups: infants (0–0.5 years old), children (0.5–10 years old), teens (11–18 years old), male adults (18–70 years old), and female adults (18–70 years old). The IR, BW, and R_fd values are listed in Table 2.

A descriptive statistical analysis and FA were conducted using Origin 9.0 and SPSS 19.0 (IBM, USA). The source apportionment of F⁻ was carried out by PMF 5.0 (US EPA). The spatial variation of fluoride, calcium, and bicarbonate distribution was evaluated by using a spatial analyst module in ArcGIS 9.3 software (ESRI, Redlands, California, USA). An inverse distance weighting (IDW) interpolation method and the best prediction models was used for concentration zoning map of fluoride, calcium, and bicarbonate in the study area.

Descriptive data such as mean, range, and standard deviation were calculated (Table 3). The FA was then applied to obtain relationship information from the obtained data (Table 4). Finally, ArcGIS was used to determine the spatial distribution of geochemical ion contents in the study area (Figure 2).

Geochemical data for the groundwater samples are shown in Table 3. The pH values of the water samples range from 7.20 to 8.28, with a mean value of 7.88, indicating that the water body is a weakly alkaline environment. In addition, the pH values of the water samples are within the acceptable range (6.5–8.5) according to China's national standards for drinking water. Values for TDS range between 319 mg/L and 1,563 mg/L, with a mean value of 975 mg/L. The TDS values in 65.00% of the samples exceed the acceptable limit (1,000 mg/L) according to China's national standard for drinking water while, based on WHO's guidelines for drinking-water quality, the TDS values in 90.00% of the samples exceed the acceptable limit (500 mg/L). The mean cation and anion contents show the following decreasing trends: Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ and HCO₃⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻ > F⁻. The mean Na⁺, Mg²⁺, Ca²⁺, and K⁺ concentrations are 153.76 mg/L, 50.03 mg/L, 37.98 mg/L, and 0.49 mg/L, respectively. The concentrations of HCO₃⁻, SO₄²⁻, and Cl⁻ are 518.36 mg/L, 195.42 mg/L, and 71.61 mg/L, respectively.

The F⁻ concentrations in the groundwater samples range from 0.16 to 2.06 mg/L, with a mean value of 1.10 mg/L. Among these samples, 53.33% have elevated F⁻ contents that exceed China's national standards for drinking water (1.00 mg/L), and 20.00% of the samples have F⁻ contents above the WHO's recommended limit (1.50 mg/L) (WHO 1984).

Spatial variations of the concentrations of F⁻ and other geochemical ions in groundwater in the Sulin coal district are plotted in Figure 2. The groundwater in the middle and northeastern parts of the study region, including Xutong, Jougou, Wugou, Yuanyi, Tongting, Suntong, Yangliu, Taoyuan, and Zhuxianzhuang, is characterized by elevated F-content (>1.0 mg/L), as shown in Figure 2(b). Only three districts, Wugou, Jiegou, and Taoyuan, have F⁻ concentrations exceeding the WHO's recommended limit (1.50 mg/L) for drinking water.

The F⁻ concentrations of groundwater range from 0.16 to 2.06 mg/L in the rainy seasons, which were generally higher than those in the dry seasons in Table 1. This is to be expected because the stronger water–rock interaction will dissolve more F⁻ into groundwater. Figure 3 indicates that 92.86% of higher fluoride groundwater samples (>1.00 mg/L) are enriched ranging from −135 to −110 m depth. In contrast, the wells with depth lower than 120 m have groundwater with relatively low fluoride. The fact of variation in the concentration of fluoride with depth may indicate that geogenic factors play a significant role (Dehbandi et al. 2018; Ali et al. 2019).

A GIS tool was utilized to study the spatial variation of F⁻, Ca²⁺, and HCO₃⁻ concentrations in Figure 2. The GIS technique usually synthesizes available water quality data and predicts unknown data for summarizing overall water quality conditions into an easily understood format (Tiwari et al. 2016; Emenike et al. 2018). As shown in Figure 2, 48.47% of the study area has F⁻ concentrations below 1.00 mg/L; 41.65% of the study area has F⁻ concentrations in the range 1.00–1.50 mg/L; 9.88% of the study area has F⁻ concentrations above 1.50 mg/L. In other words, 51.53% of the study area shows high F⁻ groundwater concentrations (exceeding China's national permissible limit of 1.00 mg/L), posing the high health risks of dental and skeletal fluorosis.

Both F⁻ and HCO₃⁻ show similar spatial distributions, while F⁻ and Ca²⁺ have opposite spatial distributions. In addition, F⁻ concentrations in groundwater are positively correlated with HCO₃⁻ concentrations, but negatively correlated with Ca²⁺ concentrations (Table 3). This phenomenon suggests that HCO₃⁻ and Ca²⁺ are the main geochemical factors for F-enrichment.

Factor analysis was employed to determine the relationship between F⁻ and other constituents to track the geochemical behavior of F⁻ (Keshavarzi et al. 2010; Tiwari et al. 2016; Enalou et al. 2018). The F⁻ concentration shows moderate positive relationships (>0.50) with the pH value (R = 0.63) and the HCO₃⁻ concentration (R = 0.63), suggesting that pH and HCO₃⁻ are important factors for the formation of high F⁻ groundwater. A positive correlation (R = 0.41) was observed between the pH value and the HCO₃⁻ concentration, as shown in Table 4. This behavior is due to the dissolution of CO₂, resulting in an alkaline environment in drinking-water aquifers with an elevated pH value and an increased HCO₃⁻ concentration. This indicates that an alkaline environment promotes an increase in F⁻ in drinking water (He et al. 2013b; Goodarzi et al. 2017; Raj & Shaji 2017; Ali et al. 2019). Furthermore, the F⁻ concentration has a weak correlation with the TDS value (R = 0.45), implying that a high TDS value promotes the weathering of F-bearing minerals and release of F⁻ into the groundwater (Brindha et al. 2016; Dehbandi et al. 2018). Generally, evaporation and water–rock interaction can increase the TDS value (Gibbs 1970; Raj & Shaji 2017; Li et al. 2018). As the study area is located in an arid or semi-arid zone, hot and dry weather leads to a higher evaporation rate, resulting in a high TDS values in the groundwater.

The concentration of F⁻ shows a strong negative correlation (R = −0.78) with the Ca²⁺ concentration, a negative correlation (R = −0.10) with the Mg²⁺ concentration, and a positive correlation (R = 0.25) with the Na⁺ concentration, suggesting that F⁻ mainly originates from the weathering of F-bearing minerals, such as fluorite (CaF₂), fluorapatite (Ca₅(PO₄)₃F), biotite (K(Mg,Fe)₃(AlSi₃O₁₀)(OH,F)), phlogopite (KMg₃(AlSi₃O₁₀)), and hornblende (CaNa(Mg, Fe, Al)(Si₇Al)O₂₂(OH, F)₂) (Abu Rukah & Alsokhny 2004; Li et al. 2015; Brindha et al. 2016; Thapa et al. 2018). Additionally, the correlation coefficient between F⁻ and Ca²⁺ is lower than that between F⁻ and Mg²⁺, suggesting that fluorite (CaF₂) and fluorapatite (Ca₅(PO₄)₃F) might be the major dissolved F-bearing minerals. A negative correlation (R = −0.48) between the concentrations of Ca²⁺ and Na⁺ suggests that cation exchange between Ca²⁺ in groundwater and Na⁺ in clay plays a significant role during the formation of high F⁻ groundwater (Li et al. 2015; Goodarzi et al. 2017; Enalou et al. 2018). The positive correlation (R = 0.35) between the F⁻ concentration and the NO₃⁻ concentration indicates that anthropogenic activities are another important factor in the formation of high F⁻ groundwater (Sharma & Subramanian 2008; Li et al. 2018).

The concentrations and uncertainties of F⁻ and other geochemical ions obtained from 30 groundwater samples were fed into the PMF 5.0 model. Factor number, random start seed number, and run number were optimized to 4, 20, and 10, respectively.

As shown in Figure 4, factor 1 is only predominantly loaded on NO₃⁻ (86.50%). The primary sources of NO₃⁻ in groundwater are industrial pollution, agricultural activities, urban solid waste, and coal mining (Heaton 1986; Sharma & Subramanian 2008; Enalou et al. 2018). Most of the Sulin coal district consists of farmland and coal mines. No other industry or urban solid waste processing plants were found in the study area. Moreover, based on previous studies, coal mining activities have a minor influence on drinking-water aquifers in this area (Lin et al. 2014; Yang et al. 2017; Qiu et al. 2019). In this regard, factor 1 is related to agricultural activity sources, such as untreated irrigation water, the infiltration of organic matter, synthetic fertilizers, and the runoff from surrounding farmland.

Factor 2 shows strong loadings for SO₄²⁻ (85.60%) and Cl⁻ (74.40%), and moderate loadings for Ca²⁺ (44.90%), Mg²⁺ (40.00%), and Na⁺ (38.50%). The positive loading values for Na⁺, Mg²⁺, and Ca²⁺ suggest that their presence in groundwater is possibly due to weathering and dissolution of minerals (Keshavarzi et al. 2010; Kumar et al. 2018). The loading value for HCO₃⁻ (8.50%) is much lower than that for SO₄²⁻ and Cl⁻, implying that geochemical processes are dominated by evaporation (Currell et al. 2011; Goodarzi et al. 2017; Enalou et al. 2018). Therefore, factor 2 represents natural geochemical process sources such as evaporation.

Factor 3 is dominated by K⁺ (58.20%) and Ca²⁺ (42.80%). As shown in Figure 3, the loading values for all anions in the water samples are negligible and Na⁺ has the lowest loading value among the cations, suggesting cation exchange between Na⁺ in the groundwater and Ca²⁺ or Mg²⁺ in clay or amorphous silicate minerals in groundwater aquifers (He et al. 2013b; Li et al. 2015; Enalou et al. 2018). Hence, factor 3 is associated with a cation exchange source.

Factor 4 shows weights of 66.20%, 57.00%, 52.80%, and 34.20% for F⁻, HCO₃⁻, Na⁺, and pH, respectively. The high loading values for F⁻, HCO₃⁻, and Na⁺, and the low loading value for Ca²⁺ indicate that the F⁻ concentration in groundwater is controlled by the weathering of F-bearing minerals (Brahman et al. 2013; Ali et al. 2019). In the study area, rocks, such as granodiorite, porphyrite, amphibolite, and phosphorite, contain a wide variety of F-bearing minerals, including fluorite, hornblende, and phlogopite (Yang et al. 2017; Qiu & Gui 2019).

According to the factor fingerprints, the overall percent contribution from each source was computed, as shown in Figure 5. The main source of F⁻ in the groundwater samples is the weathering of F-bearing minerals, accounting for 66.20%, followed by cation exchange (16.30%), agricultural activities (13.20%), and natural geochemical processes (4.30%). Overall, geological processes, including the weathering of F-bearing minerals, cation exchange, and other geochemical processes are the predominant factors influencing the F⁻ concentration in groundwater in the Sulin coal district. Surprisingly, agricultural activities contribute approximately 13.20% to the F⁻ concentration in groundwater in the study area.

In this study, the ED and HQ of F⁻ caused by ingestion of drinking water are listed in Table 5. According to the Higher French Council for Public Health (CSHPF), the recommended maximum ED of F⁻ for adults (including males and females), children (including children and teens), and infants are 4, 0.70, and 0.40 mg/d, respectively (Hercberg 2001; Guissouma et al. 2017). As shown in Figure 6, the percentage of children whose ED exceeds the maximum ED recommended by CSHPF is the highest, reaching 86.87%, followed by teens (83.33%), male adults (30.00%), infants (13.33%), and female adults (10.00%). This observation suggests that children and teens are more vulnerable in a high F⁻ environment than other age groups.

The percentages of infants, children, teens, male adults, and female adults, whose F⁻ intake exceeds the HQ safety limit (1.00) are 20.00%, 70.00%, 6.67%, 20.00%, and 10.00%, respectively, as shown in Figures 7 and 8. The HQ value for children is the highest among all population groups, followed by infants, male adults, female adults, and teens. These results indicate that children are the most vulnerable population group and that they are more likely to suffer from health complications associated with the consumption of high F⁻ water (Guissouma et al. 2017; Emenike et al. 2018). This is possibly due to the amount of dietary F⁻ intake, which is almost twice as high for children than for adults (Battaleb et al. 2013; Aghapour et al. 2018). Interestingly, as shown in Figure 6, teens have the lowest F⁻ intake risk, which may be related to the lower intake of local drinking water (Guissouma et al. 2017).

As can be seen from Figure 8, infants and male adults with HQs > 1.00 live mostly in Wugou, Taoyuan, and Xutong; teens and female adults with HQs > 1.00 are located in Wugou. Except for Qinan, Linhuan, and Qidong, the HQ of children in the rest of the Sulin coal district is above 1.00 (Figure 7(c)).

The population in Wugou has the highest ED and HQ values, indicating that the risk of developing dental fluorosis is highest. This result agrees with the high F⁻ groundwater concentration (1.96 mg/L) in this area.

The mean annual temperature of the Sulin coal district is 14–15 °C (approximately 58.2 °F). According to Equation (4), the optimum F⁻ content in drinking water in this area is 1.05 mg/L. Therefore, managing the F⁻ concentration in drinking water is essential to reducing the risk of fluorosis for inhabitants in regions with high F⁻ groundwater.

In this study, content distribution analysis and quantificational source apportionment of F⁻ in groundwater in the Sulin coal district, northern Anhui Province, China, were carried out. In addition, the health risk assessment of F⁻ exposure was evaluated for individuals in different groups, such as infants, children, teens, male adults, and female adults. Results can be summarized as follows.

The F⁻ concentrations in the groundwater samples ranged from 0.16 to 2.06 mg/L, with a mean value of 1.10 mg/L. Among the samples, 53.33% had elevated F⁻ concentrations, which exceeded China's national standards for drinking water (1.00 mg/L) and 20.00% of the samples showed much higher F⁻ concentrations beyond the WHO's recommended limit (1.50 mg/L). High F⁻ groundwater (>1.00 mg/L) was mostly located in the central and the northeastern parts of the study region, including Xutong, Jougou, Wugou, Yuanyi, Tongting, Suntong, Yangliu, Taoyuan, and Zhuxianzhuang. Spatial variations of F⁻ concentrations revealed that 48.47% of the geographical area had F⁻ concentrations below 1.00 mg/L, 41.65% of the geographical area had F⁻ concentration in the range of 1.00–1.50 mg/L, and 9.88% of the geographical area had F⁻ concentration above 1.50 mg/L.

The F⁻ concentration in the groundwater samples had a positive relationship with pH, Na⁺, and HCO₃⁻, and was negatively correlated with Ca²⁺ and Mg²⁺, indicating weathering of F-bearing minerals as a F⁻ source. Quantificational source apportionment results explained that the weathering of F-bearing minerals was the main source of F⁻ in the groundwater samples, accounting for 66.20% of the total dissolved F⁻, followed by cation exchange (16.30%), agricultural activities (13.20%), and natural geochemical processes (4.30%).

The percentages of infants, children, teens, male adults, and female adults, whose F⁻ intake exceeded the HQ safety limit (1.00) were 20.00%, 70.00%, 6.67%, 20.00%, and 10.00%, respectively. With the highest HQ value, children were the most vulnerable age group in high F⁻ regions. Apart from Qinan, Linhuan, and Qidong, children in the rest of the Sulin coal district were more likely to develop fluorosis. In addition, infants and male adults in Wugou, Taoyuan, and Xutong, as well as teen and female adults in Wugou, were at high risk of developing fluorosis. Wugou had the highest ED and HQ values due to the high level of F⁻ in the groundwater of this region. The optimum F⁻ content in the groundwater of the study region was calculated to be 1.05 mg/L, which provided a reference for local water management authorities to reduce fluorosis caused by excess F⁻ intake from drinking water.

Despite F⁻ enriched groundwater in the study area, most residents are not aware of the risks of fluorosis from drinking. Our findings are limited to assisting the informed management of groundwater resources for drinking within the study area. Overall, it is highly suggested for all parts of the geochemical cycle that efforts should be made to find the pathways, mobilization mechanisms, and reduction measures for fluoride that should be taken in the future in efforts to improve public safety.

The authors gratefully acknowledge the financial support provided by the Open Fund of State Key Laboratory of Groundwater Protection and Utilization by Coal Mining (no. SHJT-17-42.17), the Ecological restoration project in Lengshuijiang Antimony Mine area (no. LCG2020009), and Fundamental Research Funds for the Central Universities of China (no. 3142018011). We would like to thank Editage (www.editage.cn) for English language editing.

The authors declare that they have no conflict of interest.

All relevant data are included in the paper or its Supplementary Information.