Spatial distribution, source identi ﬁ cation, and health risk assessment of ﬂ uoride in the drinking groundwater in the Sulin coal district, Northern Anhui Province, China

Previously, systematic studies of distribution, sources, and health risks of high F (cid:1) groundwater used as a drinking-water source in the Sulin coal district, northern Anhui Province of China have not been carried out. In this study, 30 groundwater samples were collected in May 2019, and the data were analyzed using geographic information system, factor analysis, positive matrix factorization, and risk-based corrective action models. The results indicated that the F (cid:1) concentration of the groundwater samples ranged from 0.16 to 2.06 mg/L, with a mean value of 1.10 mg/L. The F (cid:1) concentrations of 53.33% of the groundwater samples exceeded China ’ s maximum permissible limit for drinking water (1.00 mg/L). Quanti ﬁ cational source apportionment revealed that the weathering of F-bearing minerals is the main source (66.20%). Cation exchange (16.30%), agricultural activities (13.20%), and natural geological processes (4.30%) were the other sources of F (cid:1) . The percentages of infants, children, teens, male adults, and female adults that face health risks due to excess F (cid:1) intake were approximately 20.00%, 70.00%, 6.67%, 20.00%, and 10.00%, respectively. This research provided useful insights into the proper management of groundwater extraction to mitigate health problems associated with excessive F (cid:1) intake. in the Sulin coal district.

To avoid the potential health risks of excess F À intake, the Chinese government has set the maximum permissible limit of F À for drinking water at 1.00 mg/L (Guo et al. ; He et al. a, b).
Several studies have demonstrated that F À in groundwater mainly originates from weathering of F-bearing minerals (such as fluorite, apatite, micas, and amphibole) (Keshavarzi et  Generally, an alkaline environment with low Ca 2þ content, high Na þ content, and NaHCO 3 -type water will increase the F À content in the groundwater (Rafique et  This study aimed to investigate the distribution of F À 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 À intake.

Study area
Sulin coal district, with an area of 1,000 km 2 , is located in Wanbei, China, and is characterized by a subhumid monsoon climate. The study area extends from 116 15 0 E to 117 12 0 E longitude, with a latitude between 33 20 0 N and 33 42 0 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 domi- Drinking and irrigation water for rural residents mainly comes from deep groundwater aquifers. Wheat and corn are the main crops in the study area.

Sample collection and analysis
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 À 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. Highdensity 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. precision of the obtained ion concentrations was checked by calculating the ionic balance errors, which were generally below ±5%.

PMF model
The

Health risk assessment
Fluoride is ingested mainly through groundwater, food, breathing, and dermal absorption such as during bathing

).
As groundwater is the main contributor to F À intake, the health risk assessment conducted in this study only evaluates the risk associated with drinking groundwater.
The RBCA model was employed using Equations (1) and (2) and where ED is the estimated chronic daily exposure dose of F À through ingestion of groundwater (mg/d); ADD is the estimated daily intake of F À through ingestion of groundwater (mg/kg/d), C is the F À concentration in groundwater (mg/L), IR is the rate of groundwater ingestion (L/d), and BW is the mean body weight (kg).
The health risk of F À through ingestion of drinking groundwater can be calculated using Equation (3) where HQ is the health risk quotient, and R fd is the reference dose of F À through ingestion of groundwater (mg/kg/ d). If HQ < 1.00, the health risk of F À intake through drinking groundwater is negligible; if HQ > 1.00, the health risk of F À intake cannot be ignored. Generally, the higher the HQ value, the higher the health risk (Enalou et al. ;

Yousefi et al. ).
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.

Statistical and spatial analyses
A descriptive statistical analysis and FA were conducted using Origin 9.0 and SPSS 19.0 (IBM, USA). The source apportionment of F À 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 characterization
Geochemical data for the groundwater samples are shown in   The F À 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 À into groundwater. Figure  48.47% of the study area has F À concentrations below 1.00 mg/L; 41.65% of the study area has F À concentrations in the range 1.00-1.50 mg/L; 9.88% of the study area has F À concentrations above 1.50 mg/L. In other words, 51.53% of the study area shows high F À 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 À and HCO 3 À show similar spatial distributions, while F À and Ca 2þ have opposite spatial distributions. In addition, F À concentrations in groundwater are positively correlated with HCO 3 À concentrations, but negatively correlated with Ca 2þ concentrations (Table 3). This phenomenon suggests that HCO 3 À and Ca 2þ are the main geochemical factors for F-enrichment.

Factor analysis
Factor analysis was employed to determine the relationship between F À and other constituents to track the geochemical   Table 4. This behavior is due to the dissolution of CO 2 , resulting in an alkaline environment in drinking-water aquifers with an elevated pH value and an increased HCO 3 À concentration. This indicates that an alkaline environment promotes an increase in F À in drinking  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 À , HCO 3 À , Na þ , and pH, respectively. The high loading values for F À , HCO 3 À , and Na þ , and the low loading value for Ca 2þ indicate that the F À concentration in groundwater is controlled by the weathering of F-bearing minerals According to the factor fingerprints, the overall percent contribution from each source was computed, as shown in

Health risk evaluation
In this study, the ED and HQ of F À 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 À for adults (including males and females), children (including children and teens), and infants are 4, 0.70, and 0.40 mg/d, respectively (Hercberg ; Guissouma et al. ). 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 À environment than other age groups.
The percentages of infants, children, teens, male adults, and female adults, whose F À 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 À water (Guissouma et al. ; Emenike et al. ). This is possibly due to the amount of dietary F À intake, which is almost twice as high for children than for adults (Battaleb et al. ; Aghapour et al. ). Interestingly, as shown in Figure 6, teens have the lowest F À intake risk, which may be related to the lower intake of local drinking water (Guissouma et al. ).
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 À groundwater concentration (1.96 mg/L) in this area.
Studies have shown that the optimal F À concentration in drinking water in an area is related to its climatic condition.
The optimum F À concentration in water can be calculated by Equation (4) where D is the optimal amount of F À in groundwater (mg/L) and Tm is the maximum mean atmospheric temperature ( F).
The mean annual temperature of the Sulin coal district is 14-15 C (approximately 58.2 F). According to Equation (4), the optimum F À content in drinking water in this area is 1.05 mg/L. Therefore, managing the F À concentration in drinking water is essential to reducing the risk of fluorosis for inhabitants in regions with high F À groundwater.

CONCLUSIONS
In this study, content distribution analysis and quantificational source apportionment of F À in groundwater in the Sulin coal district, northern Anhui Province, China, were carried out. In addition, the health risk assessment of F À 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 À 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 À concentrations, which exceeded China's national standards for drinking water (1.00 mg/L) and 20.00% of the samples showed much higher F À concentrations beyond the WHO's recommended limit (1.50 mg/L). High F À 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 À concentrations revealed that 48.47% of the geographical area had F À concentrations below 1.00 mg/L, 41.65% of the geographical area had F À concentration in the range of 1.00-1.50 mg/L, and 9.88% of the geographical area had F À concentration above 1.50 mg/L.
The F À concentration in the groundwater samples had a positive relationship with pH, Na þ , and HCO 3 À , and was negatively correlated with Ca 2þ and Mg 2þ , indicating weathering of F-bearing minerals as a F À source. Quantificational source apportionment results explained that the weathering of F-bearing minerals was the main source of F À in the groundwater samples, accounting for 66.20% of the total dissolved F À , 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 À 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 À 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 À in the groundwater of this region. The optimum F À 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 À intake from drinking water.
Despite F À 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.