Fluoride is a naturally occurring element present in the Earth's crust, which is released from rocks into the soil and water. Elevated levels of fluoride have been reported in phreatic groundwater worldwide, posing significant health risks to animals, plants, and humans. This study aims to assess the fluoride content in water intended for human consumption from both public networks and groundwater sources in various regions of Tunisia. A total of 46 water samples were collected between 2020 and 2021. The fluoride levels were determined using potentiometry with a combined Fluoride perfectION™ ISE selective electrode. The analysis revealed that fluoride content ranged from 0.351 to 6.016 mg/L. Samples exceeding the regulated limit of 1.5 mg/L were predominantly found in the southern areas of the country (64.3%, averaging 1.755 mg/L), regions known by hydrogeological fluoride deposits (54.8%, averaging 1.715 mg/L), and groundwater sources (75%, averaging 2.103 mg/L). Exposure of animals and humans to such high levels of fluoride poses a serious public health concern. Consequently, the implementation of a national strategy for the monitoring and control of public water resources intended for consumption is essential in mitigating the risks associated with fluoride contamination.

  • Wide range of fluoride content observed in Tunisian water samples: 0.351–6.016 mg/L.

  • High fluoride levels in southern areas with fluoride deposits, and groundwater sources.

  • Elevated fluoride levels pose serious public health concern for animals and humans.

  • Urgent implementation of a control strategy of fluoride in water resources intended for consumption.

Fluoride is a highly abundant element in the Earth's crust. Because of its reactivity, elemental fluoride (F2) is rarely, if ever, found in nature. Instead, it commonly exists as fluoride ion (F) or as part of minerals such as fluorite (CaF2), biotite ((Mg,Fe)2Al2(K,H)(SiO4)2), cryolite (Na3(AlF6)), and fluoro-apatite (Ca10F2(PO4)6) (Subba Rao 2017). These minerals are generally insoluble in water, resulting in low concentrations of fluoride ions (F−) in surface waters. However, certain physico-chemical properties of salts and water tables, such as high temperatures, can facilitate the dissolution of fluoride-containing minerals.

Optimal fluoride concentrations in water (0.5–0.8 mg/L) have been shown to promote dental and skeletal health. Conversely, excessive fluoride exposures, often over a prolonged period can lead to a severe dental fluorosis that is considered as an early indicator and typically occurs before skeletal fluorosis (Guissouma et al. 2017; Saad et al. 2022).

Groundwater, in particular, tends to accumulate fluoride through the leaching of phosphate rocks, potentially due to the dissolution of fluorinated apatites. The solubility of these apatites increases with higher water table temperatures (above 35 °C) (Tarki et al. 2020).

The natural fluoride contamination arises mainly from the dissolution of fluoride-bearing minerals, such as fluorite, into groundwater. Besides, the industrial effluents, mining and agricultural activities contribute to this contamination through air-borne deposition of fluoride compounds, such as those emitted by aluminum smelters or phosphate fertilizer plants, settling onto soil and water sources (Bombik et al. 2020). Anthropogenic sources, including mining waste dumps, smelting and extraction processes, and residual fluids from mineral, metal, fuel, or coal extraction, contribute significantly to environmental contamination (Chunming et al. 2022).

Previous studies have identified Tunisia as a country with a significant fluoride deposit, present in various ionic forms, either free or complexed with rocks, posing a major risk to animal and human health (Ben Said et al. 2020). Utilizing a potentiometric method, high fluoride levels, exceeding the international guidelines, were detected not only in stagnant water (2.65 mg/L) in the Zaghouan governorate in Northern Tunisia (Gritli et al. 2021) but also in the sera of apparently healthy sheep-residing areas near water resources (fluoremia > 0.15 mg/L) (Hadiji et al. 2021). In the Gafsa governorate of southern Tunisia, significantly high fluoride concentrations were found in human teeth samples (6,793.1 μg/g) compared to similar examined samples from the northern region (1,068.8 μg/g). This can be attributed to chronic exposure and the utilization of highly polluted water, influenced by the geographical characteristics of the area (Ben Said et al. 2020).

Analytical surveys are essential tools for assessing the contamination levels in water sources, characterizing their quality, and guiding the selection of appropriate treatment methods in order to safeguard animal and human health. These surveys provide valuable insights into the presence and concentration of contaminants, including fluoride, in various water resources.

The present study represents a significant contribution to the existing body of research by further investigating and examining different water sources that are extensively utilized by nomads, animals, and border troops in various regions of Tunisia. These regions encompass diverse environments, ranging from humid areas to arid and desert regions, each with distinct water characteristics and potential contamination sources.

By conducting a comprehensive analysis, this study aims to gather crucial data on the extent and nature of water contamination in these areas. The collected information will contribute to the characterization and categorization of water resource quality, allowing for a better understanding of the potential health risks associated with water consumption.

Study sites and samples

The present cross-sectional study was conducted in 2021 across 14 Tunisian governorates, encompassing five from the northern region (Tunis, Bizerte, Beja, Jendouba, and Kef), three from the central region (Sousse, Kairouan, and Monastir), and six from the southern region (Sfax, Gabes, Medenine, Kebili, Tozeur, and Tataouine). This sampling strategy was designed to ensure representation of different parts of the country, considering previously studied regions as well as variations in climate and geology (Figure 1). A total of 46 water samples were collected from various sources, including public water supply (n = 30) and groundwater resources (n = 16) (Figure 2). The depth of the visited wells and underground water sources varies from 30 to 80 meters in the northern regions and from 50 to 120 m in the south. This water is used by the entire population for sanitation, irrigation, and livestock watering. In some cases, as this is the only available water source, it is consumed by humans and animals despite the potential risks of excessive fluoride or other toxic elements, often with varying levels of awareness.
Figure 1

Map of Tunisia illustrating the water fluoride contamination levels across the visited governorates in this study.

Figure 1

Map of Tunisia illustrating the water fluoride contamination levels across the visited governorates in this study.

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Figure 2

Pictures of some water resources investigated in the present study; (a–c) Ground water sources; (d–g) Ground wells; and (h) Public water supply.

Figure 2

Pictures of some water resources investigated in the present study; (a–c) Ground water sources; (d–g) Ground wells; and (h) Public water supply.

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The samples were collected using 1-L glass bottles suitable for chemical analysis, with all relevant information recorded, such as date, region, type and availability of water, flow, richness of the source, and potential contaminations. The samples were carefully preserved in isothermal boxes and sent to the laboratory of Pharmacy and Toxicology at the National School of Veterinary Medicine in Sidi Thabet for further analysis.

Water analysis and fluoride level determination

The concentration of fluoride in the water samples was determined using the potentiometry method, employing a combined Fluoride perfectION™ type ISE (Ion Selective Electrode) (Figure 2). The analytical method was optimized and validated using five standard solutions, ranging from 10–1 to 10–6 mol/L, prepared from a stock solution of sodium fluoride (100 mg/L). This method allows for the measurement of fluoride concentrations in water solutions, with a cutoff value of 0.02 mg/L.

For the reaction mixture, an equal volume of Total Ionic Strength Adjustment Buffer (TISAB) was utilized, comprising glacial acetic acid (57 mL), sodium chloride (58 g), sodium citrate (0.3 g), and deionized water q.s.p: (500 mL). The pH of the solution was adjusted gradually to a range of 5–5.5 using a calibrated pH meter and sodium hydroxide solution (5 M). The total volume of the solution was then cooled and adjusted to 1 L with deionized water. To ensure accurate results, all manipulations were carried out using pure reagents that were free from fluoride.

Each sample was analyzed twice, and a coefficient of variation of ≤2% was considered acceptable to validate the measurements, ensuring reliable and precise data.

Statistical analysis

The data obtained from the study were analyzed using SPSS version 22.0 software. To evaluate the relationship between the collected data and fluoride content, the Pearson's χ2 test was utilized. A significance level (p-value) of 0.05 was chosen to determine the statistical significance of the results. Furthermore, the comparison of fluoride means was conducted using the ANOVA test.

Fluoride content in analyzed water samples

The fluoride content in the analyzed water samples varied significantly across different regions and even within localities of the same governorate. The lowest recorded value of fluoride content (0.351 mg/L) was observed in the Tabarka region in northern Tunisia, while the highest value (6.016 mg/L) was registered in water samples from the Medenine region in southern Tunisia (Table 1 and Figure 1).

Table 1

Fluoride content in the water samples of various governorates and localities

GovernoratesLocalitiesNumber of samplesFluoride content (mg/L)
Test 1Test 2Mean
Bizerte Bizerte 0.416 0.551 0.483 
Menzel Bourguiba 0.572 0.682 0.626 
0.92 0.856 0.888 
0.839 0.914 0.876 
1.008 1.069 1.0380 
Tunis Tunis  0.364 0.499 0.431 
0.793 0.882 0.837 
1.059 1.013 1.036 
Beja Beja Medjez El-Beb 0.267 0.412 0.339 
0.755 0.799 0.777 
Jendouba Tabarka Ain Drahim  0.256 0.446 0.351 
0.543 0.771 0.657 
Kef Tejerouin 1.36 1.685 1.522 
Kef 0.754 0.854 0.804 
Sousse Bouficha  0.83 0.878 0.854 
0.895 0.941 0.918 
Monastir Monastir  1.196 1.445 1.320 
Kairouan Kairouan  1.677 1.729 1.703 
Sfax Sfax  0.788 0.842 0.815 
0.812 0,942 0.877 
1.257 1.745 1.501 
1.334 1.838 1.586 
Gabes Gabes 0.558 0.543 0.550 
1.618 1.662 1.64 
2.106 2.938 2.522 
Matmata 2.888 3.259 3.073 
Medenine Medenine 5.621 6.412 6.016 
Tataouine Borj Bourguiba 2.009 2.882 2.445 
Lorzet 1.813 1.886 1.849 
Tiaret 1.796 1.949 1.872 
Bir Zar 0.254 0.653 0.245 
Borj El-Khadhra 2.841 2.996 2.918 
Ghrifa 1.812 1.921 1.886 
Kamour 0.731 0.936 0.833 
Tozeur Tozeur 1.215 1.825 1.52 
Hazoua 0.823 0.914 0.868 
Kebili Matrouha 10 3.684 3,953 3.818 
RjimMaatoug 2.156 2.851 2.053 
Mehdeth 1.488 1.599 1.543 
Kébili 0.906 0.922 0.914 
Douz 0.497 0.621 0.559 
KsarHallouf 0.882 1.249 0.565 
Jnayen 1.575 1.764 1.669 
Ain Skhouna 2.132 2.635 2.838 
GaraâtBouflija 1.580 1.873 1.726 
Jbil 0.218 0.668 0.443 
Total 46 – – – 
GovernoratesLocalitiesNumber of samplesFluoride content (mg/L)
Test 1Test 2Mean
Bizerte Bizerte 0.416 0.551 0.483 
Menzel Bourguiba 0.572 0.682 0.626 
0.92 0.856 0.888 
0.839 0.914 0.876 
1.008 1.069 1.0380 
Tunis Tunis  0.364 0.499 0.431 
0.793 0.882 0.837 
1.059 1.013 1.036 
Beja Beja Medjez El-Beb 0.267 0.412 0.339 
0.755 0.799 0.777 
Jendouba Tabarka Ain Drahim  0.256 0.446 0.351 
0.543 0.771 0.657 
Kef Tejerouin 1.36 1.685 1.522 
Kef 0.754 0.854 0.804 
Sousse Bouficha  0.83 0.878 0.854 
0.895 0.941 0.918 
Monastir Monastir  1.196 1.445 1.320 
Kairouan Kairouan  1.677 1.729 1.703 
Sfax Sfax  0.788 0.842 0.815 
0.812 0,942 0.877 
1.257 1.745 1.501 
1.334 1.838 1.586 
Gabes Gabes 0.558 0.543 0.550 
1.618 1.662 1.64 
2.106 2.938 2.522 
Matmata 2.888 3.259 3.073 
Medenine Medenine 5.621 6.412 6.016 
Tataouine Borj Bourguiba 2.009 2.882 2.445 
Lorzet 1.813 1.886 1.849 
Tiaret 1.796 1.949 1.872 
Bir Zar 0.254 0.653 0.245 
Borj El-Khadhra 2.841 2.996 2.918 
Ghrifa 1.812 1.921 1.886 
Kamour 0.731 0.936 0.833 
Tozeur Tozeur 1.215 1.825 1.52 
Hazoua 0.823 0.914 0.868 
Kebili Matrouha 10 3.684 3,953 3.818 
RjimMaatoug 2.156 2.851 2.053 
Mehdeth 1.488 1.599 1.543 
Kébili 0.906 0.922 0.914 
Douz 0.497 0.621 0.559 
KsarHallouf 0.882 1.249 0.565 
Jnayen 1.575 1.764 1.669 
Ain Skhouna 2.132 2.635 2.838 
GaraâtBouflija 1.580 1.873 1.726 
Jbil 0.218 0.668 0.443 
Total 46 – – – 

Out of the total 46 water samples, approximately 43.3% (20/46) exhibited fluoride levels exceeding the regulated limit of 1.5 mg/L, as specified by the national standard NT 09-14 (National Institute of Standardization and Industrial Property (NISIP) 2013). The average fluoride concentration among these samples was determined to be 1.404 mg/L (Tables 1 and 2, and Figure 1).

Table 2

Fluoride content according to bioclimatic zone, geographic region, and water type

Risk factorsCategoriesNumber of analyzed samples
Average of fluoride content (mg/L)ANOVA testaNumber of samples with fluoride content > 1.5 mg/Lb (%)
P-value
In geographic regions/Ground water resourceIn governorate/Water typesIn geographic regions/Ground water resourceIn governorate/Water types
Geographic regions and governorates North Bizerte 14 0.761 0.013* 1 (7) 0 (0) 0.334 
 Tunis     0 (0)  
 Beja     0 (0)  
 Jendouba     0 (0)  
 Kef     1 (50)  
Center Sousse 1.198  1 (25) 0 (0) 0.317 
 Monastir     0 (0)  
 Kairouan     1 (100)  
South Sfax 28 1.755  18 (64.3) 2 (50) 0.883 
 Gabes     3(75)  
 Medenine     1 (100)  
 Tataouine     7 (77.7)  
 Tozeur     1 (50)  
 Kebili     4 (50)  
Presence of fluoride depositc Yes/High 31  31 1.715 0.001* 20 (43.4) 17 (54.8) 0.000* 
No/Rare 15 15 0.762   3 (20)  
Type of water resources Ground water resource Wells water 16 2.103 0.003* 12 (75) 6 (75) 0.001* 
Natural sources     6 (75)  
 Public water supply 30 30 1.031   8 (26.6)  
Total   46 46 1.404   20 (43.4)  
Risk factorsCategoriesNumber of analyzed samples
Average of fluoride content (mg/L)ANOVA testaNumber of samples with fluoride content > 1.5 mg/Lb (%)
P-value
In geographic regions/Ground water resourceIn governorate/Water typesIn geographic regions/Ground water resourceIn governorate/Water types
Geographic regions and governorates North Bizerte 14 0.761 0.013* 1 (7) 0 (0) 0.334 
 Tunis     0 (0)  
 Beja     0 (0)  
 Jendouba     0 (0)  
 Kef     1 (50)  
Center Sousse 1.198  1 (25) 0 (0) 0.317 
 Monastir     0 (0)  
 Kairouan     1 (100)  
South Sfax 28 1.755  18 (64.3) 2 (50) 0.883 
 Gabes     3(75)  
 Medenine     1 (100)  
 Tataouine     7 (77.7)  
 Tozeur     1 (50)  
 Kebili     4 (50)  
Presence of fluoride depositc Yes/High 31  31 1.715 0.001* 20 (43.4) 17 (54.8) 0.000* 
No/Rare 15 15 0.762   3 (20)  
Type of water resources Ground water resource Wells water 16 2.103 0.003* 12 (75) 6 (75) 0.001* 
Natural sources     6 (75)  
 Public water supply 30 30 1.031   8 (26.6)  
Total   46 46 1.404   20 (43.4)  

aAnalysis of variance between mean values.

bNumber of water samples showing fluoride content above the threshold (>1.5 mg/L) indicated by the national regulations.

cZones known by hydrogeological fluoride deposits as fluorite, fluorapatite, and cryolite (Kairouan, Kef, and all southern governorates).

*Statistically significant test.

Risk factors analysis

The frequency of water samples exceeding the regulated limit of 1.5 mg/L for fluoride content varies according to the bioclimatic stage. Specifically, it is significantly higher in southern regions, with 64.3% of samples (average of 1.755 mg/L, p = 0.001) surpassing the limit, compared to the central regions where 25% of samples (average of 1.198 mg/L) exceeded the limit, and the northern regions where only 7% of samples (average of 0.761 mg/L) exceeded the limit (Table 2).

Furthermore, the frequency of samples exceeding the limit was significantly higher in areas known to have rich hydrogeological deposits of fluoride, such as fluorite and fluorapatite. In these areas, 54.8% of samples (average of 1.715 mg/L, p < 0.001) exceeded the limit, compared to other regions (Table 2).

Moreover, when considering the type of water resources, the frequency of samples exceeding the limit was notably higher in groundwater samples, with 75% of samples (average of 2.103 mg/L, p = 0.001) surpassing the limit, compared to samples from the public water supply, where 26.6% of samples (average of 1.031 mg/L) exceeded the limit (Table 2).

The present report serves as a descriptive investigation aimed at evaluating the natural fluoride contamination of water resources in Tunisia. This study complements previously reported findings in Tunisia, a region known for its abundant hydrogeological fluoride deposits. To provide a comprehensive understanding of fluoride contamination in water resources, sampling areas were selected from the northern, central, and southern parts of the country. This approach allows for the update, comparison, and monitoring of fluoride contamination in water resources.

The study revealed significant variations in fluoride content in water across different geographic regions, primarily influenced by the hydrogeological and climatic conditions that vary from north to south. Tunisian water resources exhibit a unique characteristic of scarcity, instability, and fluoride ion abundance (Gaaloul 2011; Guissouma et al. 2017). This instability can be attributed to the potential interactions between water and rocks, which are recognized as the primary source of fluoride content in groundwater from southern resources (Tarki et al. 2020).

The water resources from Gabes, Medenine, Kebili, and Tataouine governorates exhibited the highest fluoride values, consistent with the findings reported by Guissouma et al. (2017) in their study on fluoride levels in water resources across various Tunisian governorates. According to the guidelines set by the World Health Organization (WHO) and the national standard NT 09-14 (National Institute of Standardization and Industrial Property (NISIP) 2013), the permissible limit for fluoride content in water is 1.5 mg/L.

Significantly, a substantial number of samples collected from the southern regions exceeded this permissible limit. This observation can be attributed to various factors that influence fluoride concentrations, including mean annual precipitation, pH levels, and resource depth (Nizam et al. 2022).

In arid and semi-arid climates, the concentration of fluorides in residual water tends to increase due to the evaporation process commonly observed in these regions. These areas are also characterized by low mean annual precipitation, which leads to longer water retention in underground resources. As a result, mineral substances such as fluorides are released and transferred between water and rocks (Zhang et al. 2019). Moreover, higher fluoride concentrations were reported in stagnant water compared to fluvial ones (Gritli et al. 2021). This may involve the modality of fluoride air-borne deposition on surface water.

Furthermore, samples collected from deeper aquifers (beyond 284 m) exhibited a 27% higher fluoride contamination compared to surface points. This can be attributed to the prolonged contact of alkaline water with fluoride-rich rocks, which leads to the release and transfer of fluorides into the water (Podgorski & Berg 2022). Consequently, fluorides are commonly associated with groundwater that has high pH values (>7), facilitating ionic exchanges between fluoride ions (F) and hydroxyl groups (OH) as well as bicarbonate-sodium compounds (Nizam et al. 2022).

Fluoride contamination of water resources is a global issue that extends beyond Tunisia. Similar high levels of fluoride have been reported in neighboring countries. For example, in the Algerian Sahara, districts like Adrar (3.36 mg/L) and Ouargla (4.32 mg/L) have exhibited very high fluoride concentrations in water resources (Sekkoum et al. 2012). In Libya, alarming levels of fluorides up to 10.5 mg/L were detected in groundwater wells (240 m deep) in the village of Al-Nasryah (Sweesi et al. 2022).

In other regions, fluoride concentrations have been found to be considerably below the recommended levels. In the Eastern Province of Saudi Arabia, two major cities showed fluoride contents below the optimum recommended level (<0.065 ppm) (Bakhurji & Alqahtani 2018). In India, the dissolved groundwater fluoride concentration ranged between 1.5 and 9.2 mg/L (Nizam et al. 2022). Similarly, in Argentina, fluoride levels varied from 0.01 to 2.80 mg/L in different areas (Gómez et al. 2020).

These findings demonstrate that the level of fluoride contamination in groundwater resources is influenced mainly by the hydrogeological environment such as volcanoes, soil rocks, and existing agrochemical components. These factors collectively contribute to the variation in fluoride levels found in different regions (Duggal & Sharma 2022).

Scientific research plays a crucial role in the development of techniques to treat water with high fluoride levels. Various chemical and physical tools based on ionic exchange have been explored for this purpose. However, chemical processes often pose challenges in terms of implementation, discontinuity, cost, and maintenance requirements. In light of this, the low-pressure reverse osmosis technique has been suggested as a potential solution for treating highly contaminated waters (Ly et al. 2021).

In conclusion, the findings of this study highlight the significant presence of high fluoride levels in water resources across Tunisia, particularly in the southern regions. The variations in fluoride content can be attributed to hydrogeological and climatic conditions, as well as interactions between water and rocks. These elevated fluoride levels exceed the permissible limits set by national and international standards, posing health risks to humans and animals, especially with chronic exposure. It is imperative to prioritize efforts in treating fluoride-contaminated water before consumption to mitigate these health concerns. Additionally, an awareness campaign should be initiated to discourage the consumption of untreated groundwater and promote the consumption of treated water from the safe drinking water network.

Scientific research plays a crucial role in addressing the issue of fluoride contamination in water resources. The development of effective treatment techniques is essential to ensure the availability of safe drinking water. While chemical processes have limitations in terms of implementation, cost, and maintenance, the low-pressure reverse osmosis technique emerges as a potential solution for treating highly contaminated waters. Further research and technological advancements in this area are necessary to enhance the efficiency and accessibility of water treatment methods, thereby safeguarding public health and improving the overall quality of water resources.

The authors wish to express their gratitude to all veterinary doctors for their generous contribution in sample collection.

R.S., R.H., and R.G. conceived the idea. R.H. and S.B.Y. performed the experiment. R.S., A.M., and R.H. carried out the analysis and result interpretation. R.S., M.B.S., and R.G. wrote the manuscript. All authors read and reviewed the manuscript.

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

The authors declare there is no conflict.

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),
1
11
.

Author notes

These authors contributed equally.

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