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The purpose of this study was to assess the levels of fluoride in drinking water and its health impact in Semema, Tigray, Ethiopia. Water samples were collected in February, March and April from three potential spring water sources, namely May Atkaru, May Sensela and May Liham. Each sample was analyzed for a variety of physicochemical parameters including fluoride using standard APHA procedures through double beam UV–Visible spectrophotometer, atomic absorption spectrophotometer and titrimetric methods. All the measured physicochemical parameters except hardness (345.78–368.35 mg/L) and alkalinity (231.3–354.6 mg/L) were recorded below the WHO permissible limit set for drinking water. The amount of fluoride in May Atkaru (4.00 mg/L) and May Sensela (3.89 mg/L) was significantly greater than the WHO permissible limit set for drinking water, 1.5 mg/L. Moreover, HQ > 1 from May Atkaru and May Sensela revealed the possibility of dental and skeletal fluorosis over extended exposure to fluoride irrespective of age and sex variations. This confirmed people in the area with mottled teeth are vulnerable to the excessive consumption of fluoride, which poses health risks. Therefore, it needs immediate interventions to minimize the debilitating effect of fluoride in drinking water by creating awareness among the community and policymakers to introduce low-cost defluoridation methods.

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  • Safe drinking water is a human right.

  • Excessive intake of fluoride causes fluorosis (dental and skeletal).

  • Children are more vulnerable to fluorosis.

  • Tigray is also vulnerable to fluorosis.

  • Defluoridation using locally available materials is cost-effective and affordable.

defluoridation, drinking spring water, fluorosis, health risk, Tigray
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High quality, safe and sufficient drinking water is essential for public health and human well-being, and thus, ensuring the availability of clean water becomes a must (Luvhimbi et al. 2022). Therefore, the supply of quality water becomes one of the essential necessities. Despite these facts, not all the water supplied to the communities is safe for drinking. Consequently, a large proportion of people lack access to safe drinking water (Godina 2004). Among these, the presence of excess fluoride is one of the well-documented problems posing health risks (Solanki et al. 2022). Frequent fluoride exposure initiates fluorosis, neurological, thyroid and osteoporosis (Sujana et al. 2009; Kashyap et al. 2021). The extent of fluorosis depends on the concentration of fluoride ingested, which can vary from dental fluorosis (1.5–4.0 mg F−/L) to crippling fluorosis (>10 mg F−/L) (Ali et al. 2018). Thus, dental and skeletal fluorosis is irreversible and no treatment exists. Hence, the possible remedy is prevention by keeping the fluoride intake within the safe limit, 1.5 mg/L (WHO 2006).

Fluoride is a persistent and non-degradable poison that accumulates in soil, plants, wildlife and human beings (Begum 2012). Fluoride can be enriched in natural water by geological and industrial processes. Fluoride-saturated wastewater is released from different manufacturing industries like semiconductor, electroplating, ceramic production, coal power and fertilizer plants (Paudyal et al. 2011; Swain et al. 2012). In some areas, foodstuffs and indoor air pollution due to the burning of coal may make significant contributions to the daily intake of fluoride (Ando et al. 2001; Yang et al. 2017).

Drinking water is often the main source of fluoride intake by humans, especially in areas where the amount of fluoride in ground and surface water is high (Tekle-Haimanot et al. 2006; Jagtap et al. 2012; Edmunds & Smedley 2013). A low amount of fluoride (about 1.0 mg/L) in food and drinking water can prevent dental carries; however, the uptake of high amounts causes severe dental and skeletal disease (Satur et al. 2010; Tiwari et al. 2023). Concentrations of fluoride in natural water are generally much higher than in rainfall, though still recorded typically in the μg/L range (Edmunds & Smedley 2013). Thus, the amount of fluoride in surface water is often below 300 μg/L. Despite this, the level of fluoride in surface water can be much higher in geothermal areas.

Many African countries, including Ethiopia, face an excess of fluoride in their drinking water (Kerdoun et al. 2022). Many lakes in the East African Rift Valley have a fluoride concentration in the order of tens to hundreds of mg/L (up to 1,980 mg/L) (Edmunds & Smedley 2005). Moreover, fluoride is found in all forms of natural water at some level of concentration. In the groundwater, the amount of fluoride depends on the nature of the rocks and the occurrence of fluoride-bearing minerals (Podgorski & Berg 2022).

The amount of fluoride in natural water depends on several contributing factors such as pH, dissolved solids, alkalinity, porosity and acidity of the soil and rocks, temperature and depth of water in the undergrounds (Firempong et al. 2013). A high amount of fluoride occurs in large and extensive geographical belts associated with sediments of marine origin in mountain areas, volcanic rocks, granitic and gneissic rocks (Vithanage & Bhattacharya 2015). Among these, the most known and documented area is the East African rift system extended from Jordan Valley through Ethiopia, Sudan, Kenya, Uganda and Tanzania (Fawell et al. 2006).

A high concentration of fluoride was first reported in Ethiopia in 1993 in the Wonji Shoa Sugar Estates, in the Ethiopian Rift Valley. Wonji Shoa and Metahara are densely populated commercial farms and agro-industrial complexes in the Rift Valley region containing excess fluoride compared with the WHO guideline (Kloos & Haimanot 1999). A high amount of fluoride was measured in wells and boreholes at the main Ethiopian Rift Valley, which suggests a hydrothermal origin for fluoride. Many lakes in the Ethiopian Rift Valley system have extremely high fluoride, and consequently, the communities of Wonji Shoa, Hawassa and Metahera are highly affected by dental and skeletal fluorosis (Haimanot et al. 1987). Moreover, the presence of fluoride in Ethiopia is well documented. However, in the current study area, Semema, Tahtay Koraro, North Western zone of Tigray, Ethiopia, which is found outside the Ethiopian Rift Valley region, a number of children and adults have practically observed dental fluorosis.

Within the scope of this problem and to promote access to safe drinking water, there is an urgent need to assess the level of fluoride concentration and other physicochemical characteristics of drinking water in the fluorosis endemic communities. In addition, the estimation of health risks associated with concentrations of fluoride in drinking water of the study area was not conducted.

Therefore, this study aimed to assess the level of fluoride in potential drinking water sources of Semema and its health risk, which is considered one of the major problems of fluorosis observed in the local communities. Moreover, this study could help to understand the prevalence of fluorine in drinking water on the scientific basis for decision and carrying out fluorosis prevention to ensure safe drinking water in the Tigray region.

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Description of the study area

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Semema is located at 14°89′51″ to 14°15′29″ N latitude and 38°18′59″ to 38°22′58″ E longitude, North Western zone of Tigray, Ethiopia. It is 332 km far away from Mekelle, the capital city of the Tigray region (Figure 1). It has a mean annual rainfall of 450 mm and a mean annual temperature of 28.5 °C, respectively. It possesses a plain topography and semi-arid climate conditions. The sampling points were purposely selected based on their potential drinking water sources and symptoms of dental fluorosis in the consumers. May Atkaru is located near the border of the Tahtay Koraro Woreda. It has an elevation of 1,745 m (a.s.l), which is comparably the hottest and deepest site. Its geological setting is semi-rift and full of highly abundant crowd rocks. May Liham is the nearest site to Semema town with an elevation of 1,958 m (a.s.l). It is highly covered by vegetation, which is suitable for agriculture. May Sensela is also located between May Atkaru and May Liham with an elevation of 1,762 m (a.s.l). Its geological setting is also a semi-rift with an abundant distribution of host rocks.
Figure 1
Location map of the study area.
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Location map of the study area.

Figure 1
Location map of the study area.
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Location map of the study area.

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Chemicals, reagents and instruments

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All the chemicals and reagents used were of analytical grade. pH meter (HANA-HI 991301, Italy), turbidity meter (AL 250T-IR, Germany), electrical conductivity meter (Jenway 4310, UK), atomic absorption spectrophotometer (AAS) (VARIAN 50B, UK), UV–Visible spectrophotometer (CE 1021, UK) and portable multi-parameter meter (9000p, China) were used in the study.

Sample collection

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A total of 27 drinking water samples were collected from three sampling sites in triplicates in February, March and April. Water samples were collected in prewashed double-caped polyethylene bottles. Before sampling, the polyethylene bottles and caps were rinsed three times with the water sample and each collected sample was preserved in a light-proof insulated box containing ice-packs. Finally, the samples were transported immediately using a portable ice box to the Laboratory of Hydro-Geochemistry, School of Earth Science of Mekelle University for physicochemical analysis.

Physicochemical analysis

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Physical water quality parameters such as pH, temperature (T), electrical conductivity (EC) and total dissolved solids (TDS) were measured in-situ using standard methods (APHA 2005). The chemical parameters such as calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+) were determined using AAS calibrated with multielement standard solutions using serial dilutions from a stock solution of 1,000 mg/L, and sample readings were taken in triplicates. The amount of chloride (Cl−), phosphate (⁠⁠), sulfate (⁠⁠) and nitrate (⁠⁠) were also analyzed in triplicates using a double beam UV–Visible spectrophotometer. The levels of fluoride (F−) in the water sample were analyzed by the SPANDS colorimetric method using the double beam UV–Visible spectrophotometer as described by APHA (2005). Moreover, total water hardness (TH), alkalinity (ALK), carbonates (⁠⁠) and bicarbonates (⁠⁠) were measured using titrimetric methods through APHA procedures (APHA 2005).

Health risk assessment

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Estimated daily intake (EDI)

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The health risk assessment was determined by estimating the exposure doses of fluoride due to the consumption of drinking water in terms of estimated daily intake (EDI) by following the generic equation (Dobrinas et al. 2022) as shown in Equation (1):
(1)
where EDI is the exposure daily intake (μg/kg/day); IR is the intake rate of water (L/day) that was assumed for children (1–3 years 14 kg, 0.95 L; 3–10 years 22 kg, 1.65 L and 10–18 years 35 kg, 2.0 L), for women (20–40 years 50 kg, 2 L; 40–70 years 60 kg, 2 L and >70 years 65 kg, 1.5 L) and for men (20–40 years 60 kg, 2 L; 40–70 years 75 kg, 2 L and >70 years 70 kg, 1.5 L); BW is the body weight (in kg) assumed for the above body weight mentioned of children, women and men, respectively; and C is the concentration of fluoride (μg/L) (IRIS 2002).

Hazard quotient (HQ)

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To estimate the potential health risks of pollutants the use of the HQ is common. It is defined as the ratio of the estimated dose of a contaminant to the reference dose, where the method is established by USEPA (Barnes & Dourson 1988). The reference dose (RfD) is an estimation of daily exposure that is expected to be without significant risk of harmful effects during the lifetime. Its value in the Integrated Risk Information System (IRIS) is 60 μg/kg/day, which comprises both the EDI from ingestion of fluoride through drinking water (50 μg/kg/day) and dietary intake of fluoride (10 μg/kg/day) (IRIS 2002) as described in Equation (2):
(2)

When the HQ is greater than 1, the estimated potential exposure exceeds the RfD and a risk of fluorosis may be assumed (Barnes & Dourson 1988).

Health risk index (HRI)

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The HRI is determined as described by Shah et al. (2012) as shown in Equation (3):
(3)
where DIR is the daily intake rate, RfDo is reference oral dose for each trace element (IRIS 2002), where human being is considered to be safe if HRI < 1 (USEPA 2004).

Statistical analysis

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A one-way analysis of variance was used to find out if there is a significant difference between the sample means at p ≤ 0.05 or 95% level of confidence. The Pearson correlation coefficient was also used to find out the relation of fluoride concentration with selected physicochemical parameters.

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Physicochemical analysis

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The levels of physicochemical parameters determined from Semema drinking water sources are summarized in Table 1.

Table 1

Mean concentration of the physicochemical parameters of Semema drinking water sources, Mean ± SD in mg/L, N = 9 (see supplemental data S1, S2, and S3)

Study area
May Atkaru
May Sensela
May Liham
MeanStandard deviationStandard error of meanMeanStandard deviationStandard error of meanMeanStandard deviationStandard error of meanWHO (2011) 
Temp. (°C) 23.82a 1.37 0.46 23.66a 1.51 0.50 23.01a 1.48 0.49 <40 
F− (mg/L) 4.00a 0.67 0.22 3.89a 0.58 0.19 0.6b 0.05 0.02 1.5 
TH (mg/L CaCO3) 368.35a 18.63 6.21 364.67a 5.41 1.80 345.78b 4.58 1.53 300 
TDS (mg/L) 703.56a 5.70 1.90 683.29b 7.02 2.34 648.58c 1.91 0.64 500 
EC (μS/cm) 997.50a,b 13.01 4.34 1040.44a 91.01 30.34 937.89b 22.37 7.46 1,000 
pH value 8.0a 0.4 0.1 7.8a 0.3 0.1 7.4b 0.2 0.1 6.5–8.5 
Na+ (mg/L) 52.90a 2.8 0.9 67.7b 3.7 1.2 52.6a 3.7 1.2 200 
K+ (mg/L) 1.03a 0.03 0.01 1.03a 0.02 0.01 0.92b 0.04 0.01  
Ca+2 (mg/L) 104.10a 2.5 0.8 113.1b 4.8 1.6 116.6b 2.4 0.8 150 
Mg+2 (mg/L) 20.80a 2.4 0.8 23.1a,b 2.1 0.7 25.2b 2.3 0.8  
Cl− (mg/L) 34.73a 2.35 0.78 37.75b 1.97 0.66 27.72c 2.35 0.78 250 
(mg/L) 14.43a 1.15 0.38 15.21a 1.66 0.55 8.20b 0.52 0.17 50 
Alkalinity (mg/L CaCO3) 274.40a 8.4 2.8 354.6b 5.4 1.8 231.3c 5.1 1.7 200 
(mg/L) 0.573a 0.064 0.021 0.462b 0.042 0.014 0.325c 0.063 0.021  
(mg/L) 326.27a 5.03 1.68 327.16a 3.86 1.29 276.67b 3.62 1.21 500 
(mg/L) 207.11a 3.55 1.18 208.33a 5.59 1.86 203.22a 6.83 2.28 250 
(mg/L) 0.007a 0.001 0.000 0.005b 0.001 0.000 0.005b 0.001 0.000 2 
Turbidity 3.76a 0.91 0.30 4.73b 0.43 0.14 2.76c 0.47 0.16 5 
Study area
May Atkaru
May Sensela
May Liham
MeanStandard deviationStandard error of meanMeanStandard deviationStandard error of meanMeanStandard deviationStandard error of meanWHO (2011) 
Temp. (°C) 23.82a 1.37 0.46 23.66a 1.51 0.50 23.01a 1.48 0.49 <40 
F− (mg/L) 4.00a 0.67 0.22 3.89a 0.58 0.19 0.6b 0.05 0.02 1.5 
TH (mg/L CaCO3) 368.35a 18.63 6.21 364.67a 5.41 1.80 345.78b 4.58 1.53 300 
TDS (mg/L) 703.56a 5.70 1.90 683.29b 7.02 2.34 648.58c 1.91 0.64 500 
EC (μS/cm) 997.50a,b 13.01 4.34 1040.44a 91.01 30.34 937.89b 22.37 7.46 1,000 
pH value 8.0a 0.4 0.1 7.8a 0.3 0.1 7.4b 0.2 0.1 6.5–8.5 
Na+ (mg/L) 52.90a 2.8 0.9 67.7b 3.7 1.2 52.6a 3.7 1.2 200 
K+ (mg/L) 1.03a 0.03 0.01 1.03a 0.02 0.01 0.92b 0.04 0.01  
Ca+2 (mg/L) 104.10a 2.5 0.8 113.1b 4.8 1.6 116.6b 2.4 0.8 150 
Mg+2 (mg/L) 20.80a 2.4 0.8 23.1a,b 2.1 0.7 25.2b 2.3 0.8  
Cl− (mg/L) 34.73a 2.35 0.78 37.75b 1.97 0.66 27.72c 2.35 0.78 250 
(mg/L) 14.43a 1.15 0.38 15.21a 1.66 0.55 8.20b 0.52 0.17 50 
Alkalinity (mg/L CaCO3) 274.40a 8.4 2.8 354.6b 5.4 1.8 231.3c 5.1 1.7 200 
(mg/L) 0.573a 0.064 0.021 0.462b 0.042 0.014 0.325c 0.063 0.021  
(mg/L) 326.27a 5.03 1.68 327.16a 3.86 1.29 276.67b 3.62 1.21 500 
(mg/L) 207.11a 3.55 1.18 208.33a 5.59 1.86 203.22a 6.83 2.28 250 
(mg/L) 0.007a 0.001 0.000 0.005b 0.001 0.000 0.005b 0.001 0.000 2 
Turbidity 3.76a 0.91 0.30 4.73b 0.43 0.14 2.76c 0.47 0.16 5 

Note: Values in the same row not sharing the same subscript are significantly different at p < 0.05.

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Temperature, pH, EC, turbidity, hardness and total alkalinity

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Temperature is an important factor that governs the existence and the rate of biological activities to a large extent. Moreover, it supports the regulation of the metabolic activities of the water bodies. The temperature of natural water mainly depends on the origin of the water, climatic zone, season, altitude, and inflow of industrial and municipal sewage (Patil et al. 2012). Higher temperature promotes the growth of microorganisms in water, which may cause changes in taste, odor, turbidity and corrosion (Okweye 2013). The temperature of the current study was recorded in the range of 23.01–23.82 °C, which is below the WHO permissible limit set for drinking water, <40 °C.

As the human body consists of 50–60% water, the pH has an important role in regulating the body's functions. Moreover, pH below 5.3 hampers the assimilation of vitamins and minerals. The pH values of the current study ranged from 7.4 to 8, which falls within the WHO permissible limit, 6.5–8.5. Conductivity is also an important factor that directly affects the quality of water. Higher conductivity may degrade the aesthetic value of water by giving a mineral taste. The experimentally measured EC value was found in the range of 937.89–1,040.44 μS/cm. The highest value was recorded at May Sensela, with a little deviation from the WHO recommended value probably due to the agricultural practices manifested at the site. Turbidity is used to define water quality in terms of the aesthetic value of drinking water. The value of turbidity for Semema water sources varied from 2.76 to 4.73 NTU which are all below the WHO permissible limit set for drinking water, 5 NTU.

Water hardness is mainly caused by the presence of metals (calcium and magnesium), carbonate and bicarbonate species. The measured value of water hardness is found in the range of 345.78–368.35 mg/L, which falls above the WHO acceptable limits set for drinking water, <300 mg/L. Alkalinity is a measure of the capability of neutralizing acidic components due to the existence of bicarbonate ions. The mean value of total alkalinity was also found in the range of 231.3–354.6 mg/L, which is beyond the WHO acceptable limit for drinking water, 200 mg/L. The variation among each result might be due to weathering, availability of soluble minerals and geological differences.

Chloride, nitrate, phosphate, sulfate and fluoride ion

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Chloride may affect the taste of drinking water leading to health problems of hyperchloremia at exceeding concentrations. The level of chloride in the studied water sample was found in the range of 27.72–37.75 mg/L, where the highest amount of chloride was found at May Sensela. Moreover, all the results are far below the WHO permissible limit set for drinking water, 250 mg/L. Excessive nitrate negatively affects water quality and poses health risks. The levels of nitrate ion in the current study were found in the range of 8.20–15.21 mg/L which falls below the WHO permissible limit set for drinking water (50 mg/L). Though phosphate is not harmful to humans, anthropogenic activities and contact with natural minerals have a significant impact on the well-being of an ecosystem and water quality. The concentration of phosphate ions in the studied sites was found in the range of 0.005–0.007 mg/L, which is below the WHO permissible limit set for drinking water, 2 mg/L. Sulfate in water originates from natural as well as industrial effluents. However, the highest levels usually emerge from natural mineral sources (Kumar & James 2013). The major physiological effects resulting from the ingestion of large quantities of sulfates are bitter taste, laxative effect, catharsis, dehydration and gastrointestinal irritation (Uwah & Ogugbuaja 2012). The concentration of sulfate ions in the current study ranged from 203.22 to 208.33 mg/L, which falls far below the WHO recommended permissible limits of drinking water, 250 mg/L.

Fluoride contamination has become a public threat worldwide. Excessive fluoride in drinking water causes dental and skeletal fluorosis. Consequently, WHO has set the upper limit of fluoride intake in drinking water, 1.5 mg/L. However, drinking water should maintain the minimum level of fluoride essential to develop and strengthen teeth and bones. Accordingly, the amount of fluoride recorded in May Atkaru and May Sensela were 4.00 and 3.89 mg/L, respectively. This occurrence of a relatively greater amount of fluoride might be due to the presence of bedrocks and fluoride-bearing compounds like CaF2, Na3AlF6 and Ca5 (PO4)3F (Reda 2016; Teklu 2024). This finding also proved both water sources possess a level of fluoride above the WHO acceptable limit set for drinking water. Moreover, the amount of fluoride showed an increasing trend in February, March and April, which might be due to depletion of water associated with evaporation or evapotranspiration of groundwater (Shaji et al. 2024).

The researchers proved the daily life of the community depends on the water sources of May Atkaru and May Sensela. Indeed, it is observed that local people show a yellow stain on their teeth. Similar results and symptoms are common in the Afar region (Gebreyesus 2013), Agaro town (Sisay et al. 2017) and other Ethiopian Rift Valley Zones (Edmunds 1994; Rao et al. 2002) due to geological factors, the arid climate and the Rift Valley region. Consequently, the long-term use of high fluoride in drinking water may cause dental and skeletal fluorosis in Semema. However, the amount of fluoride in May Liham (0.6 mg/L) was below the WHO permissible limit set for drinking water.

Correlation of fluoride with the analyzed physicochemical parameters

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Studies showed that the amount of fluoride in water depends on different variables (Bell et al. 1970) such as pH, the solubility of fluoride-bearing minerals, anion exchange capacity of aquifers, geological formations and contact time of water with certain geological formations (Apambire et al. 1997). The Pearson correlation was employed to examine the relationship of fluoride and other physicochemical parameters and the corresponding results are presented in Table 2. The matrix proved the existence of positive correlation within the ranges of 0.7–1.0 (high correlation), 0.4–0.6 (moderate correlation) and 0.1–0.3 (weak correlation), respectively (Soper et al. 1917).

Table 2

Pearson correlation coefficient of the physicochemical parameters

ParametersF- (mg/L)TH (mg/L CaCO3)TDS (mg/L)EC (μS/cm)pHNa+ (mg/L)K+ (mg/L)Ca+2 (mg/L)Mg+2 (mg/L)Cl− (mg/L) (mg/L)ALK (mg/L CaCO3) (mg/L) (mg/L) (mg/L) (mg/L)TUR (NTU)
T (°C) R 0.459* 0.118 0.378 0.655** 0.791** 0.427* 0.019 0.214 0.249 0.537** 0.450* 0.250 0.581** 0.330 0.756** 0.089 0.586** 
 p-value 0.016 0.559 0.052 0.000 0.000 0.027 0.927 0.284 0.211 0.004 0.018 0.209 0.001 0.093 0.000 0.659 0.001 
 F- (mg/L) R 1 0.550** 0.936** 0.700** 0.825** 0.519** 0.832** −0.500** −0.477* 0.877** 0.970** 0.740** 0.849** 0.975** 0.511** 0.407* 0.792** 
 p-value  0.003 0.000 0.000 0.000 0.006 0.000 0.008 0.012 0.000 0.000 0.000 0.000 0.000 0.007 0.035 0.000 
TH (mg/L CaCO3) R  1.000 0.592** 0.486* 0.2365 0.2031 0.612** −0.457* −0.094 0.642** 0.618** 0.411* 0.608** 0.608** 0.2091 0.147 0.1609 
 p-value   0.001 0.010 0.235 0.310 0.001 0.017 0.642 0.000 0.001 0.033 0.001 0.001 0.295 0.466 0.423 
TDS (mg/L) R   1.000 0.562** 0.781** 0.211 0.808** −0.703** −0.573** 0.717** 0.871** 0.487** 0.906** 0.920** 0.427* 0.593** 0.589** 
 p-value    0.002 0.000 0.291 0.000 0.000 0.002 0.000 0.000 0.010 0.000 0.000 0.026 0.001 0.001 
EC (μS/cm) R    1 0.684** 0.714** 0.424* 0.052 0.094 0.768** 0.764** 0.665** 0.573** 0.640** 0.746** −0.032 0.730** 
 p-value     0.000 0.000 0.027 0.797 0.639 0.000 0.000 0.000 0.002 0.000 0.000 0.872 0.000 
pH R     1 0.411* 0.495** −0.249 −0.216 0.718** 0.763** 0.458* 0.838** 0.718** 0.676** 0.389* 0.739** 
 p-value      0.033 0.009 0.210 0.280 0.000 0.000 0.016 0.000 0.000 0.000 0.045 0.000 
Na+(mg/L) R      1 0.284 0.330 0.117 0.739** 0.604** 0.896** 0.2185 0.512** 0.572** −0.355 0.815** 
 p-value       0.151 0.092 0.561 0.000 0.001 0.000 0.273 0.006 0.002 0.069 0.000 
K+(mg/L) R       1.000 −0.623** −0.491** 0.672** 0.826** 0.635** 0.643** 0.862** 0.1015 0.476* 0.464* 
 p-value        0.001 0.009 0.000 0.000 0.000 0.000 0.000 0.614 0.012 0.015 
Ca+2 (mg/L) R        1.000 0.748** −0.213 −0.419* −0.048 −0.573** −0.566** 0.0301 −0.699** −0.013 
 p-value         0.000 0.285 0.030 0.811 0.002 0.002 0.882 0.000 0.947 
Mg+2 (mg/L) R         1.000 −0.158 −0.386* −0.172 −0.385* −0.502** 0.0374 −0.458* −0.204 
 p-value          0.432 0.047 0.392 0.048 .008 0.853 0.016 0.307 
Cl− (mg/L) R          1 0.915** 0.852** 0.758** 0.862** 0.588** 0.084 0.779** 
 p-value           0.000 0.000 0.000 0.000 0.001 0.675 0.000 
(mg/L) R           1 0.797** 0.802** 0.946** 0.523** 0.280 0.768** 
 p-value            0.000 0.000 0.000 0.005 0.157 0.000 
ALK (mg/L CaCO3) R            1.000 0.393* 0.777** 0.432* −0.076 0.839** 
 p-value             0.042 0.000 0.025 0.708 0.000 
(mg/L) R             1 0.800** 0.583** 0.531** 0.508** 
 p-value              0.000 0.001 0.004 0.007 
(mg/L) R              1.000 0.466* 0.430* 0.768** 
 p-value               0.014 0.025 0.000 
(mg/L) R               1 0.060 0.651** 
 p-value                0.765 0.000 
(mg/L) R                1.000 0.0836 
 p-value                 0.679 
ParametersF- (mg/L)TH (mg/L CaCO3)TDS (mg/L)EC (μS/cm)pHNa+ (mg/L)K+ (mg/L)Ca+2 (mg/L)Mg+2 (mg/L)Cl− (mg/L) (mg/L)ALK (mg/L CaCO3) (mg/L) (mg/L) (mg/L) (mg/L)TUR (NTU)
T (°C) R 0.459* 0.118 0.378 0.655** 0.791** 0.427* 0.019 0.214 0.249 0.537** 0.450* 0.250 0.581** 0.330 0.756** 0.089 0.586** 
 p-value 0.016 0.559 0.052 0.000 0.000 0.027 0.927 0.284 0.211 0.004 0.018 0.209 0.001 0.093 0.000 0.659 0.001 
 F- (mg/L) R 1 0.550** 0.936** 0.700** 0.825** 0.519** 0.832** −0.500** −0.477* 0.877** 0.970** 0.740** 0.849** 0.975** 0.511** 0.407* 0.792** 
 p-value  0.003 0.000 0.000 0.000 0.006 0.000 0.008 0.012 0.000 0.000 0.000 0.000 0.000 0.007 0.035 0.000 
TH (mg/L CaCO3) R  1.000 0.592** 0.486* 0.2365 0.2031 0.612** −0.457* −0.094 0.642** 0.618** 0.411* 0.608** 0.608** 0.2091 0.147 0.1609 
 p-value   0.001 0.010 0.235 0.310 0.001 0.017 0.642 0.000 0.001 0.033 0.001 0.001 0.295 0.466 0.423 
TDS (mg/L) R   1.000 0.562** 0.781** 0.211 0.808** −0.703** −0.573** 0.717** 0.871** 0.487** 0.906** 0.920** 0.427* 0.593** 0.589** 
 p-value    0.002 0.000 0.291 0.000 0.000 0.002 0.000 0.000 0.010 0.000 0.000 0.026 0.001 0.001 
EC (μS/cm) R    1 0.684** 0.714** 0.424* 0.052 0.094 0.768** 0.764** 0.665** 0.573** 0.640** 0.746** −0.032 0.730** 
 p-value     0.000 0.000 0.027 0.797 0.639 0.000 0.000 0.000 0.002 0.000 0.000 0.872 0.000 
pH R     1 0.411* 0.495** −0.249 −0.216 0.718** 0.763** 0.458* 0.838** 0.718** 0.676** 0.389* 0.739** 
 p-value      0.033 0.009 0.210 0.280 0.000 0.000 0.016 0.000 0.000 0.000 0.045 0.000 
Na+(mg/L) R      1 0.284 0.330 0.117 0.739** 0.604** 0.896** 0.2185 0.512** 0.572** −0.355 0.815** 
 p-value       0.151 0.092 0.561 0.000 0.001 0.000 0.273 0.006 0.002 0.069 0.000 
K+(mg/L) R       1.000 −0.623** −0.491** 0.672** 0.826** 0.635** 0.643** 0.862** 0.1015 0.476* 0.464* 
 p-value        0.001 0.009 0.000 0.000 0.000 0.000 0.000 0.614 0.012 0.015 
Ca+2 (mg/L) R        1.000 0.748** −0.213 −0.419* −0.048 −0.573** −0.566** 0.0301 −0.699** −0.013 
 p-value         0.000 0.285 0.030 0.811 0.002 0.002 0.882 0.000 0.947 
Mg+2 (mg/L) R         1.000 −0.158 −0.386* −0.172 −0.385* −0.502** 0.0374 −0.458* −0.204 
 p-value          0.432 0.047 0.392 0.048 .008 0.853 0.016 0.307 
Cl− (mg/L) R          1 0.915** 0.852** 0.758** 0.862** 0.588** 0.084 0.779** 
 p-value           0.000 0.000 0.000 0.000 0.001 0.675 0.000 
(mg/L) R           1 0.797** 0.802** 0.946** 0.523** 0.280 0.768** 
 p-value            0.000 0.000 0.000 0.005 0.157 0.000 
ALK (mg/L CaCO3) R            1.000 0.393* 0.777** 0.432* −0.076 0.839** 
 p-value             0.042 0.000 0.025 0.708 0.000 
(mg/L) R             1 0.800** 0.583** 0.531** 0.508** 
 p-value              0.000 0.001 0.004 0.007 
(mg/L) R              1.000 0.466* 0.430* 0.768** 
 p-value               0.014 0.025 0.000 
(mg/L) R               1 0.060 0.651** 
 p-value                0.765 0.000 
(mg/L) R                1.000 0.0836 
 p-value                 0.679 

*Pearson correlation is significant at p ≤ 0.05.

**Pearson correlation is significant at p ≤ 0.01

View Large

The analysis of variance indicated a statistically significant difference (p ≤ 0.05) between the measured physicochemical parameters. The results of the Pearson correlation coefficient as indicated in bold (p-values) showed a significant relationship at p ≤ 0.05 or p ≤ 0.01 level of confidence. The Pearson correlation analysis of temperature (R2, 0.459), TDS (R2, 0.936), pH (R2, 0.825), EC (R2, 0.700), Na+ (R2, 0.519), K+ (R2, 0.832), TH (R2, 0.550), Cl− (R2, 0.877), (R2, 0.970), ALK (R2, 0.740), (R2, 0.849), (R2, 0.975), (R2, 0.511), (R2, 0.407) and TUR (R2, 0.792) showed a very good positive correlation with the fluoride concentration compared with the other physicochemical parameters.

However, Mg+2 (R2, −0.477) and Ca+2 (R2, −0.500) have shown a negative correlation with fluoride concentration. Other scholars also proved the presence of low levels of calcium allows stabilization of high amounts of fluoride in water sources (Fawell et al. 2006). Similar findings were also reported in Kenya (Nyanchaga & Tiffani 2003) and in India (Handa 1975) showing a negative correlation between Ca+2, Mg+2 and F−. The decrease in the amount of Ca+2 and Mg+2 contributed to enhancing the level of fluoride probably due to the complexing effect, and thus, the fluoride complexes readily formed in mineralized water (Allmann & Koritnig 1974). Anions like Cl−, ⁠, ⁠, ⁠, and also showed a positive correlation with fluoride concentration due to the simultaneous competitive effects and fluoride removal systems (Nabizadeh et al. 2015). Parameters like pH, EC and temperature also showed a positive correlation with fluoride concentration, which might be due to the presence of hot springs and highly soluble minerals (Onipe et al. 2021). Moreover, the positive correlation between pH and bicarbonate indicates an alkaline environment as the dominant controlling mechanism for the leaching of fluoride from the source rock (Shaji et al. 2024).

Health risk assessment

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Human health risk assessment is used to evaluate the nature and probability of adverse effects of fluoride exposure. Though there are several contamination routes, drinking water is the major basis of fluoride exposure to human beings (Dobrinas et al. 2022). To examine the health impacts of fluoride in human beings, different age groups (children, women and men) were considered with respect to their body weight and average water intake capacity and their corresponding EDI and HQ values are summarized in Table 3. The daily intake level of drinking water for different age groups was taken from scholarly open literature (Amalraj & Pius 2013). Accordingly, the highest EDI value (in range) was recorded in May Atkaru (children: 228.57–300.00 μg/kg/day, women: 92.31–160.00 μg/kg/day and men: 85.71–133.33 μg/kg/day) followed by May Sensela (children: 222.29–291.75 μg/kg/day, women: 89.77–155.60 μg/kg/day and men: 83.36–129.67 μg/kg/day). The existence of a higher amount of fluoride in May Atkaru and May Sensela could be due to the dissolution of fluoride ions from the bedrock of the areas. However, May Liham (children: 34.29–45.00 μg/kg/day, women: 13.85–24.00 μg/kg/day and men: 12.86–20.00 μg/kg/day) recorded comparatively smaller EDI values for all age groups which confirmed the low prevalence of fluoride in May Liham. Furthermore, children are highly exposed to more fluoride intake per kilogram of body weight than women and men, confirming that children are more vulnerable to the health effects of fluoride (Shaji et al. 2024). Similar findings were also reported in the main Ethiopia Rift Valley (Rango et al. 2012; Rango et al. 2014) and Tunisia (Guissouma et al. 2017).

Table 3

EDI and HQ calculated for children, women and men (Barnes & Dourson 1988; IRIS 2002; USEPA 2004; Shah et al. 2012; Rango et al. 2014)

Age group, YearBody weight (kg)Daily average drinking water intake (L)EDI (μg/kg/day)
HQ
May AtkaruMay SenselaMay LihamMay AtkaruMay SenselaMay Liham
Children 1–3 14 0.95 271.43 263.96 40.71 4.52 4.39 0.68 
3–10 22 1.65 300.00 291.75 45.00 5.00 4.86 0.75 
10–18 35 2.0 228.57 222.29 34.29 3.81 3.70 0.57 
Women 20–40 50 2 160.00 155.60 24.00 2.67 2.59 0.40 
40–70 60 2 133.33 129.67 20.00 2.22 2.16 0.33 
>70 65 1.5 92.31 89.77 13.85 1.54 1.49 0.23 
Men 20–40 60 2 133.33 129.67 20.00 2.22 2.16 0.33 
40–70 75 2 106.67 103.73 16.00 1.78 1.73 0.27 
>70 70 1.5 85.71 83.36 12.86 1.43 1.39 0.21 
Age group, YearBody weight (kg)Daily average drinking water intake (L)EDI (μg/kg/day)
HQ
May AtkaruMay SenselaMay LihamMay AtkaruMay SenselaMay Liham
Children 1–3 14 0.95 271.43 263.96 40.71 4.52 4.39 0.68 
3–10 22 1.65 300.00 291.75 45.00 5.00 4.86 0.75 
10–18 35 2.0 228.57 222.29 34.29 3.81 3.70 0.57 
Women 20–40 50 2 160.00 155.60 24.00 2.67 2.59 0.40 
40–70 60 2 133.33 129.67 20.00 2.22 2.16 0.33 
>70 65 1.5 92.31 89.77 13.85 1.54 1.49 0.23 
Men 20–40 60 2 133.33 129.67 20.00 2.22 2.16 0.33 
40–70 75 2 106.67 103.73 16.00 1.78 1.73 0.27 
>70 70 1.5 85.71 83.36 12.86 1.43 1.39 0.21 
View Large

The hazard quotient (HQ) is used to estimate the possible non-carcinogenic risk of fluorosis. The value of HQ > 1 refers to the existence of harmful non-carcinogenic health risks, while HQ < 1 indicates the adverse effects are not likely to occur, and thus can be considered to have a negligible hazard (Ashong et al. 2024). From these findings, the highest HQ values were recorded in May Atkaru (children: 3.81–5.00, women: 1.54–2.67 and men: 1.43–2.22) followed by May Sensela (children: 3.70–4.86, women: 1.49–2.59 and men: 1.39–2.16). However, lower values of HQ (≤0.75) were recorded in May Liham. This result also proved that May Atkaru and May Sensela are severely affected by fluorosis and extreme effect of fluorosis is expected in children. Similarly, other reported literature values also support that HQ > 1 creates a higher prevalence of dental and skeletal fluorosis for adults and children over continuous exposure to fluoride in drinking water (Farias et al. 2021; Kerdoun et al. 2022; Nizam et al. 2022; Ashong et al. 2024). Thus, the mean HQ > 1 from May Atkaru and May Sensela reveals the possibility of dental and skeletal fluorosis over extended exposure to fluoride irrespective of age and sex variations. This further proves the communities are vulnerable to the excessive consumption of fluoride in drinking water which poses health risks.

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Long-term exposure to high-level fluoride is a major health concern for human beings. The presence of high concentrations of fluoride in drinking water is mainly a cause of fluorosis. Thus, the amount of fluoride determined in May Atkaru (4 mg/L) and May Sensela (3.89 mg/L) water sources were far beyond the WHO permissible limit set for drinking water (1.5 mg/L). Besides, HQ > 1 for May Atkaru and May Sensela also revealed the possibility of dental and skeletal fluorosis over extended exposure to fluoride irrespective of age and sex variations. The HQ values further proved children are most vulnerable to fluoride toxicity. Therefore, from these findings, it can be concluded that the high prevalence of mottled teeth among the water consumers of the study area was a case of endemic dental fluorosis associated with drinking water from hot springs contaminated with high concentrations of fluoride. Hence, the May Atkaru and May Sensela spring water sources were unfit for human consumption. From this, the study recommends interventions to minimize the debilitating effect of fluoride in drinking water by creating awareness among the community and policymakers to introduce low-cost defluoridation methods.

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The authors are grateful for financial support from Mekelle University under the support of the MSc Thesis Grant for students and to Mr Seid Mussa for the help of statistical analysis. Moreover, the authors would like also to acknowledge the financial support of the Global Minds Programme for funding from VLIR-UOS and the Belgian Development Cooperation through the Short Research Stay to write this paper.

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

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The authors declare there is no conflict.

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