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Causes and health risk assessment of fluorine in the Red bed groundwater and adjacent geothermal water of the Guang'an Area, Southwest China
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Abstract: Understanding the levels, causes, and sources of fluoride in groundwater is critical for public health, effective water resource management, and sustainable utilization. This study employs multivariate statistical methods, hazard quotient assessment, and geochemical analyses, such as mineral saturation index, ionic activities, and Gibbs diagrams, to investigate the hydrochemical characteristics, causes, and noncarcinogenic risks of fluoride in Red bed groundwater and geothermal water in the Guang'an area and neighboring regions. Approximately 9% of the Red bed groundwater samples contain fluoride concentrations exceeding 1 mg·L−1. The predominant water types identified are Cl-Na and HCO3-Na, primarily influenced by evapotranspiration. Low-fluoride groundwater and high-fluoride geothermal water exhibit distinct hydrochemical types HCO3-Ca and SO4-Ca, respectively, which are mainly related to the weathering of carbonate, sulfate, and fluorite-containing rocks. Correlation analysis reveals that fluoride content in Red bed groundwater is positively associated with Na+, Cl−, SO42−, and TDS (r2 = 0.45–0.64, p < 0.01), while in geothermal water, it correlates strongly with pH, K+, Ca2+, and Mg2+ (r2 = 0.52–0.80, p < 0.05). Mineral saturation indices and ionic activities indicate that ion exchange processes and the dissolution of minerals such as carbonatite and fluorite are important sources of fluoride in groundwater. The enrichment of fluorine in the Red bed groundwater is linked to evaporation, cation exchange and dissolution of fluorite, caused by the lithologic characteristics of the red bed in this area. However, it exhibits minimal correlation with the geothermal water in the adjacent area. The noncarcinogenic health risk assessment indicates that 7% (n = 5) of Red bed groundwater points exceed the fluoride safety limit for adults, while 12% (n = 8) exceed the limit for children. These findings underscore the importance of avoiding highly fluoridated red bed groundwater as a direct drinking source and enhancing groundwater monitoring to mitigate health risks associated with elevated fluoride levels.
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Key words:
- Guang'an area /
- Red bed groundwater /
- Geothermal water /
- Fluoride contamination /
- Causes /
- Health risk assessment
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Figure 1. (a) Map of Guang'an area; (b) Regional geology, distribution of sampling points and spatial distribution of fluoride in groundwater within the study area; (c) Typical geological profile in the study area.
Notes: This figure was modified from the regional geological map released by the GeoCloud Platform of the China Geological Survey and the Guang'an Hydrogeological Report by Xie et al. (1981). 1. Quaternary: Clay and gravel; 2. Suining Fm (Formation): Mudstone with sandstone, containing thin layers of gypsum; 3. Upper Shaximiao Fm: Mudstone with sandstone, containing thin layers of carbonate and gypsum; 4. Lower Shaximiao Fm: Variegated mudstone and shale with sandstone, small amounts of limestone, containing carbonate and fluorite; 5. Xintiangou Fm: Variegated mudstone and shale with limestone, containing carbonate and fluorite; 6. Ziliujing Fm: Variegated mudstone and shale with limestone, containing carbonate and fluorite; 7. Zhenzhuchong Fm: Mudstone and shale with limestone, containing carbonate and fluorite; 8. Xujiahe Fm: Sandstone with shale and thin coal seams, containing sulfide, sulfate, and carbonate; 9. Leikoupo Fm: Dolomite interspersed with shale, containing halite, sylvite, carbonate, and fluorite; 10. Jialingjiang Fm: Dolomite interspersed with shale, containing halite, sylvite, carbonate, and fluorite; 11. Feixianguan Fm: Argillaceous limestone with shale and coal seams, containing carbonate, sulfide, halite, sulfate, and fluorite; 12. Changxing Fm: Argillaceous limestone with shale, containing halite, carbonate, and fluorite; 13. Longtan Fm: Argillaceous limestone with shale, containing halite, carbonate, and fluorite; 14. Lower Permian Series: Mudstone and shales; 15. Lower Silurian Serie: Mudstone and shales; 16. Lower Ordovician Series: Dolomite and siliceous limestone; 17. Emei Mountain basalt; 18. Red bed groundwater samples of this study; 19. Geothermal water (Li et al. 2020; Xiao et al. 2011); 20. Rock samples.
Figure 2. Schematic map of hydrogeology and shallow groundwater level of the study area
Figure 3. Correlation coefficients of the major water parameters and elements in Red beds (n=68) and geothermal water (n=17)
(Note：** P < 0.01; * P < 0.05)
Figure 5. Gibbs plots: (a) Comparison of natural processes that define the water chemistry of water on the Gibbs plots; (b) TDS versus Na+/(Na++Ca2+); (c) TDS versus Cl−/(Cl−+HCO3−)
Figure 6. Saturation indices of minerals: (a) F− vs. SI-fluorite, (b) SI-fluorite vs. SI-calcite, (c) SI-fluorite vs. SI-gypsum, and (d) SI-fluorite vs. SI-halite (The bubble sizes represent the F− concentration)
Figure 7. The relationship between the activities of F− and Ca2+ of the groundwater in the study area
Figure 8. (a) Relationship between CAI1 and CAI2 in the groundwater sample (the bubble size corresponds to the concentration of fluoride in the sample); (b) Relationships between γ((Ca2++Mg2+)−(HCO3− + SO42−)) and γ(K+ +Na+ −Cl−) in groundwater
Figure 9. Relationships between Na/Cl and F− (a) and between NO3− and F− (b) in groundwater from the study area
Figure 11. Noncarcinogenic evaluation for fluoride concentrations in Red bed groundwater: (a) Children and (b) Adults
Table 1. Parameters for the calculation of the HRA model in this study
Parameters C IR EF ED BW AT RfD Unit mg·L−1 L·d−1 d·a−1 a kg d mg·kg−1·d−1 Adult Measured 3 120 30 57 EF×ED 0.06 Children Measured 1.5 30 6 26 EF×ED 0.06 
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Table 2. Contents of major chemical compositions and fluoride in the different types of groundwater
Sample type pH TDS Na+ K+ Ca2+ Mg2+ Cl− SO42− HCO3− NO3− F− Data source Limit 6.5–8.5 1,000 200 - - - 250 250 - 20 1.0 (GB/T 14848–2017) Red beds(n=68) Min 6.30 146 5.0 0.63 8.79 1.63 0.66 6.30 26.00 0.7 0.08 This study Max 9.19 2,520 635.0 50.40 220.00 45.80 1,030.00 520.00 511.00 186.0 3.76 Ave 7.34 482 58.0 4.60 80.97 16.38 58.58 69.47 280.35 29.3 0.58 SD 0.49 319 91.6 7.85 39.39 8.22 140.42 76.87 114.91 33.5 0.74 CV 0.07 0.66 1.6 1.71 0.49 0.50 2.40 1.11 0.41 1.14 1.27 % > Limit 4 3 4 - - - 4 3 - 46 9 Geothermal water (n=17) Min 6.45 448 2.4 1.88 226.00 43.90 1.35 335.00 91.53 - 0.10 (Li et al. 2020; Xiao et al. 2011) Max 7.69 1,512 355.0 37.30 817.00 178.00 70.89 2,114.00 317.30 - 14.71 Ave 7.14 808 41.6 14.91 556.86 112.87 18.45 1,429.56 188.35 - 7.62 SD 0.32 274 78.49 10.78 216.55 46.46 14.95 638.32 51.84 - 5.35 CV 0.04 0.34 1.89 0.72 0.39 0.41 0.81 0.45 0.28 - 0.70 % > Limit 6 18 6 - - - 0 100 - - 94 - Note: The units for ion and TDS concentrations are mg·L−1. CV is the coefficient of variation, %. pH, dimensionless.
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Table 3. F contents in the various rocks of the study area/(mg·kg−1)
Rock type Marl (n=2) Limestone (n=6) Mudstone (n=8) Carbonaceous mudstone (n=3) Siltstone (n=3) Sandstone (n=4) Basalt (n=1) All samples (n=27) Min 767 200 598 542 645 408 645 200 Max 767 476 854 767 767 476 645 854 Ave 767 365 722 671 705 432 645 593 SD - 122 80 116 61 30 - 178 Average value of continental crust (Wedepohl, 1995) 525
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Table 4. Statistics of groundwater mineral saturation indices in the study area
Mineral category Calcite
(CaCO3)Fluorite
(CaF2)Gypsum
(CaSO4·2H2O)Halite
(NaCl)Sylvite
(KCl)Geothermal water (n=17) Max 1.00 1.80 0.09 −6.72 −7.23 Min −0.27 −2.66 −0.82 −10.12 −9.65 Ave 0.40 0.78 −0.13 −8.28 −8.11 Red beds (n=68) Low F− level (n=62) Max 1.18 −1.03 −0.81 −6.66 −6.48 Min −1.88 −4.00 −3.08 −9.48 −9.90 Ave 0.03 −2.00 −1.91 −7.67 −8.33 High F− level (n=6) Max 0.38 0.24 −0.76 −4.84 −7.05 Min −0.69 −0.96 −2.00 −8.59 −8.94 Ave 0.04 −0.37 −1.55 −6.31 −7.92
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