Fluoride has both beneficial and detrimental effects on human health. Concentrations of fluoride less than 1.0 mg/L in ingested water are beneficial for the rate of tooth decay, especially in children. The aims of the paper are as follows: (i) to monitor fluoride concentrations in drinking water samples (well water and tap water from the rural district of Valea Râmnicului, Romania); (ii) to study and select the optimal buffer solution and the optimal volume used in the analyses and (iii) to validate the potentiometric method for determining fluoride ions with a selective ion electrode. The values of fluoride ion concentrations in the groundwater samples and in tap water varied from 0.01 to 0.138 mg/L. The values for the hazard quotient for the studied samples varied from 0.01 to 0.13.

  • A large part of Romania's population uses water from wells as a source of drinking water.

  • Monitoring of fluoride in drinking water samples.

  • Optimization and validation of the potentiometric method for determining fluoride ions with a selective ion electrode.

  • The performed health risk assessment of fluoride exposure due to groundwater consumption is based on the estimated daily intake (EDI) and the hazard quotient (HQ).

Graphical Abstract

Graphical Abstract
Graphical Abstract

Groundwater resources are the main source of freshwater for human activities. Half of the world's population depends on groundwater resources for drinking water, irrigation, domestic and industrial use (Oki & Kanae 2006). In recent years, alongside economic development, pollutants from industry, agriculture and cities have seeped into groundwater, causing its quality to deteriorate (Sellerino et al. 2019), threatening the safety of people and nature. Both geogenic and anthropogenic origins may influence the concentrations of inorganic compounds in groundwater (Rahman et al. 2020).

Declining groundwater quality is often attributed to the increased and uncontrolled use of chemical compounds, while an ever-changing climate and an ever-growing population are also exacerbating the situation. Agricultural activities, such as the use of chemical fertilisers and manure, have been identified as major sources of surface and groundwater pollution since the early 20th century. Polluted groundwater usage can pose risks to human health; therefore, prevention and monitoring have become a necessity in order to ensure water quality.

Natural substances found in soil and rocks can dissolve in water, causing contamination. These substances are sulphate, iron, radionuclides, fluorides, manganese, chlorine and arsenic. Other substances such as decomposing materials in the soil can also leach into the groundwater, moving with it as particles.

Generally, fluoride in groundwater is contributed by the host rocks, which are naturally rich in fluoride. Because of the rock–water interaction, long residence time and evapotranspiration, the concentration of fluoride increases. Some studies indicate that the fluoride composition in groundwater increases with depth from the ground surface (Kim & Jeong 2005). The natural concentration of fluoride in groundwater depends on the geological, chemical and physical characteristics of the aquifer, the porosity and acidity of the soil and rocks, the surrounding temperature, the action of other chemical elements, depth of the aquifer and intensity of weathering (Feenstra et al. 2007).

In Romania, the main anthropogenic sources of fluoride pollution are aluminium production, coal burning, phosphorus fertilizer production, cement production, ceramics and brick-burning.

The fluoride content of water is expressed through the concentrations of free fluoride ions; these concentrations depend on the nature and origin of the water. The concentration of fluoride in groundwater in the European Union (EU) is generally low, but with large regional differences due to different geological conditions (SCHER 2011). The surface water usually has lower fluoride contents than groundwater (most often below 0.5 mg/L) and seawater (between 1.2 and 1.5 mg/L), whereas river and lake water generally have concentrations lower than 0.5 mg/L. In well water, sometimes, there are low or high concentrations of fluoride, which depend on the nature of the rocks and the appearance of fluoride-bearing minerals. High concentrations of fluoride ions can occur in groundwater, in calcium-poor aquifers and in common fluoride-bearing minerals.

Fluoride in drinking water is entirely in its ionic form (Shanthakumari et al. 2004). Natural water contains less than 0.1 ppm of fluoride ions and mineral water contains an average of 0.16–6.45 ppm. There are no systematic data regarding the concentration of fluoride in natural drinking water in EU Member States, but rudimentary data show large variations between and within countries: Ireland 0.01–5.8 mg/L, Finland 0.1–3.0 mg/L and Germany 0.1–1.1 mg/L (SCHER 2011). The recommended value of fluoride ions in drinking water by the World Health Organization (WHO) is 1 ppm (WHO 2006).

The effects of fluoride on human health

Fluoride is a beneficial microelement for human health. Concentrations of fluoride less than 1.0 mg/L in ingested water are beneficial for the rate of tooth decay, especially in children. On the other hand, due to their strong electronegativity, fluoride ions are attracted to positive calcium ions from bones and teeth. Excessive consumption or exposure to high levels of fluoride leads to pathological changes in teeth and bones, such as staining of teeth or dental fluorosis, followed by osteosclerosis (Fewtrell & Bartram 2001; Xiong et al. 2007).

In some areas, because there are no alternatives, people in rural areas are forced to drink contaminated water. This affects their health and life quality. Children with dental fluorosis suffer from toothaches. Young people with skeletal fluorosis suffer from back pain, neck pain and stiffness in the spine.

Dental caries is a widespread condition that occurs at all ages and in both sexes. Numerous observations have shown that there is a close correlation between dental caries and the concentration of fluoride in water. Many communities add fluoride to water. Water is ‘fluoridated’ when a public water system adjusts the fluoride to a level known to prevent tooth decay. The diseases associated with the high amount of fluoride create psychological and emotional trauma for the population in disadvantaged areas.

Endemic fluorosis is a less common condition caused by fluoride in high concentrations in drinking water. The first manifestations of epidemic fluorosis appear at fluoride concentrations of above 1.5–2.0 mg/L of water, and they get deposited in the teeth leading to a condition called dental fluorosis. The symptoms of the disease are occurrence of stains on the surface of the tooth enamel and an increased friability of the teeth that acquire the appearance of teeth eaten by moths or saw teeth. At higher concentrations of fluoride (above 5 mg/L of water), it also acts on the bones, producing a significant increase in X-ray opacity, which is why the disease has been called osteosclerosis. At even higher concentrations (above 20 mg/L of water) there are changes in bone composition with an increasing amount of fluoride to the detriment of calcium. Fluorosis prophylaxis consists of demineralizing water or more precisely removing excess fluoride from water with the help of ion exchangers.

The fluoride can also affect the thyroid gland with complications especially in pregnant women. Uncontrolled hypothyroidism can raise the blood pressure during late pregnancy, increase the risk of miscarriage and affect brain development and growth rate (Kheradpisheh et al. 2018).

The high concentrations of fluoride from drinking water have been associated with great values of thyroid hormones (Kheradpisheh et al. 2018). According to data from the literature (Mohammadi et al. 2017), the age group of 51–60 years presented skeletal fluorosis 1.54 times more than people in 41–50 age range. Another study (Karimzade et al. 2014) investigated the correlation between fluoride in drinking water and children's intelligence quotient (IQ). The results show a significant linear trend for children from high fluoride drinking water region to have a lower IQ.

Daily fluoride intake (3 mg fluoride ions/day) should not be exceeded, and of course sodium fluoride in toothpaste should be replaced with calcium fluoride, which is less toxic (Veressinina et al. 2001).

Fluorosis makes people physically weak and negatively affects their family economy as many remain unemployed due to these health problems.

In order to solve the health and socio-economic problems of the population in contaminated areas, a number of measures and action programs can be taken and should be implemented urgently:

  • Medical measures—early detection and prevention of the disease through regular testing.

  • Interventions through fluoride-free drinking water supply and nutritious diet by introducing foods rich in calcium, vitamin C and vitamin D.

  • Provision of safe fluoride-free water for people living in fluorosis endemic areas. This could be achieved by locating alternative sources of safe water or by introducing remote water supply or by installing defluoridation facilities or using defluoridation units in homes. The internal fluoride filter should be provided by the government with the necessary guidance on the use and maintenance of the filter. It should be affordable and available for purchase by anyone.

Another source of consumption is rainwater harvesting which can be adopted as an alternative source of drinking water in several areas affected by fluorosis (Onipe et al. 2020). Consumers have been free of skeletal fluorosis after drinking harvested rainwater. Despite this, drinking rainwater has its own rules, such as ample space for harvesting and storing water, frequent cleaning of containers where water is harvested and which are prone to microbial contamination (Gispert et al. 2018). In addition, stored rainwater may not be available throughout the year due to seasonal changes.

Given the impact that fluoride ions have on the environment and human health, several analytical techniques have been developed for their determination in water: spectrophotometry, fluorometry, potentiometry (ISE), ion chromatography (CI), gas chromatography (GC), electrophoresis capillary area (CEH) and radioanalysis (Shyam and Kalwania 2012; Waziri et al. 2012; Perumal et al. 2013).

The aims of the paper are as follows: (i) to monitor fluoride concentrations in drinking water samples (well water and tap water from the rural district of Valea Râmnicului, Romania); (ii) to study and select the optimal buffer solution and the optimal volume used in the analyses and (iii) to validate the potentiometric method for determining fluoride ions with a selective ion electrode.

Sampling and study areas

To determine the fluoride ion concentrations from the studied samples, water samples were taken monthly from the rural district of Valea Râmnicului for a period of 7 months (November 2018–May 2019). This rural area from Romania was chosen due to the fact that here people use a lot of groundwater from wells for drinking or for domestic use. The well water was taken from four points (southern, central and northern area of the rural district, Figure 1). The tap water from the rural district was also analysed (point number 5). Samples were collected and then stored at 4 °C until analysed. Before being analysed, the samples were brought to room temperature.
Figure1

The study area. Sampling points: point 1—samples of groundwater from the southern area of the rural district of Valea Râmnicului; points 2 and 3—samples of groundwater from the central area of the rural district of Valea Râmnicului; point 4—samples of groundwater from the northern area of the rural district of Valea Râmnicului.

Figure1

The study area. Sampling points: point 1—samples of groundwater from the southern area of the rural district of Valea Râmnicului; points 2 and 3—samples of groundwater from the central area of the rural district of Valea Râmnicului; point 4—samples of groundwater from the northern area of the rural district of Valea Râmnicului.

Close modal

Reagents

Bidistilled water was used to prepare all aqueous solutions. The solutions of sodium fluoride, sodium hydroxide, as well as buffer solutions were prepared from the chemicals of the analytical grade.

Fluoride determination

Fluoride concentration was measured using the potentiometric method with a combined fluoride electrode connected to a pH/ion meter. The total ionic strength adjustment buffer solutions used were the TISAB buffer solution, the phosphate buffer solution at pH 5.5 and pH 7.6. The TISAB buffer solution consists 57 mL of glacial acetic acid and 58.5 g of dried NaCl, dissolved in 500 ml of bidistilled water, and was adjusted to pH 5.5 by 5 M NaOH. The phosphate buffer solution at pH 5.5 contains 71.64 g of Na2HPO4·12 H2O and 27.60 g of NaH2PO4·H2O diluted in a 1-L volumetric flask with bidistilled water and was adjusted to pH 5.5 by 5 M NaOH. The phosphate buffer solution of pH 7.6 contains 71.64 g of Na2HPO4·12 H2O and 27.60 g of NaH2PO4·H2O diluted in a 1-L volumetric flask with bidistilled water and was adjusted to pH 7.6 with 2N HCl.

Numerous tests were performed to draw the calibration curve until the optimal working conditions were established. The electrode was calibrated with 12 standard fluoride solutions ranging from 5 × 10−2 M to 10−7 M. These standard solutions were prepared from a stock solution (10−1 M) of sodium fluoride. Electrode potentials of standard solutions are plotted on the linear axis against their concentrations on the log axis. In the electrochemical cell was added an optimal volume of 20-mL sample and an optimal volume of 20-mL buffer solution (TISAB), determination of volumes is explained in Sections 3.1 and 3.2. The solution was stirred for homogenization and the potential of each solution was measured.

Three replicates of each water sample were analyzed.

Validation study

The optimized potentiometric method using the fluoride ion selective electrode was assessed in order to prove its appropriateness for fluoride ion measurements in groundwater and tap water samples. Validation of the method was carried out with the following characteristics: linearity, range and precision.

To determine the linearity and range, working standard solutions of different concentrations of fluoride were used covering the range between 5 × 10−2 M and 10−7 M. Analytical curves were constructed for each buffer solution (TISAB buffer solution, phosphate buffer solution at pH 5.5 and pH 7.6). Triplicate determinations of each calibration standard were performed. The linearity range was tested by the homogeneity variance test described in a previous paper (Dobrinas & Soceanu 2021).

The method precision was evaluated using 10 inter-day and 10 intra-day replicate measurements of the known concentration of the fluoride standard solution. The precision of the studied method was determined by relative standard deviation (RSD%). To obtain more representative values, simultaneous replicates at different times on groundwater and tap water samples collected every month during November 2018–May 2019 were performed in the same way as for the standard solution.

Health risk assessment

According to the Environmental Protection Agency, a human health risk assessment is the process to estimate the nature and probability of adverse health effects in humans who may be exposed to chemicals in contaminated environmental media and includes the following four steps: hazard identification, dose–response assessment, exposure assessment and risk characterization (EPA 2021). There are several routes through which humans can be exposed to contaminants such as dermal contact, inhalation and ingestion (Karami et al. 2019). Drinking water is the major contributor of fluoride exposure to human bodies, whereas water used for cooking purposes and food are minor sources (Amalraj & Pius 2013; Sawangjang et al. 2019).

In this study, risks from ingested fluoride through groundwater and tap water consumption were evaluated by estimating the exposure doses of fluoride due to the consumption of drinking water, in terms of the estimated daily intake (EDI) equation:
formula
(1)
where EDI is the exposure daily intake (μg/kg/day); IR is the intake rate of water (L/day); BW is the body weight (kg) and C is the average concentration of fluoride (μg/L) in studied samples.
To calculate the risk assessment based on the hazard quotient (HQ) we used the equation:
formula
(2)
where HQ is the hazard quotient, the ratio of the potential exposure to a substance and the level at which no adverse effects are expected; EDI is the exposure daily intake (μg/kg/day); RfD is the reference dose an estimation of daily exposure that is expected to have no significant risk of harmful effects during the lifetime of subjects; for fluorine (soluble fluoride), the value was established by the Integrated Risk Information System (IRIS 2002) (60 μg/kg/day).

Buffer volume optimization

To optimize the method, determinations were made to establish the optimal volume of buffer solution, and thus 20, 30 and 40 mL of each buffer solution were used. Each sample was measured six times, then the RSD% was calculated. Figure 2 shows the variation of the RSD% depending on the volume of buffer used.
Figure 2

RSD% variation depending on the volume of buffer solution.

Figure 2

RSD% variation depending on the volume of buffer solution.

Close modal

It can be seen that the lowest RSD% values were recorded while using 20 mL of each buffer solution. The lowest value is 1.08%, recorded in the case of the TISAB buffer. Thus, it was established that 20 mL of TISAB buffer is optimal for performing experimental determinations.

Sample volume optimization

To determine the optimal volume of sample solution, measurements were made on 20, 30 and 40 mL of sample solution added in the electrochemical cell along with 20 mL of TISAB buffer. Each sample was measured 6 times, then the RSD% was calculated.

Table 1 shows the values of the standard deviation for the three studied sample volumes. From Table 1, it can be seen that the best RSD% value was recorded for 20 mL of the sample volume and thus a 20-mL sample volume is still used.

Table 1

RSD values (%) for 20, 30 and 40 mL of sample solution

Sample volume (mL)RSD (%)
20 1.696 
30 1.815 
40 2.629 
Sample volume (mL)RSD (%)
20 1.696 
30 1.815 
40 2.629 

Linearity and range

Three calibration curves depending on the studied buffer solutions were fitted to estimate the fluoride content. Determination coefficients (R2) values obtained from linear regression analysis were satisfactory: 0.9938 for the TISAB buffer solution; 0.9860 for the phosphate buffer solution at pH 5.5 and 0.9910 for the phosphate buffer solution at pH 7.6, demonstrating that the method is linear in the studied range. The calibration curve equation obtained for fluoride ions were as follows: y = −45.167x–195.36 in case of the TISAB buffer solution; y = −54.306x–208.29 in case of the phosphate buffer solution at pH 5.5 and y = −49.334x–196.9 in case of the phosphate buffer solution at pH 7.6.

The applied test of homogeneity variance established a P = 2.56 in case of the TISAB buffer, a P = 5.27 for phosphate buffer at pH 5.5 and a P = 2.09 for phosphate buffer at pH 7.6. Also, the test established the following acceptance criterion: P < Ftab value (5.35 for n−1 = 9 free degrees). This means that no significant differences were found between the variances of the concentration range limits. The above results showed that linear calibrations for the analytical method over the calibration ranges tested (5 × 10−2 M to 10−7 M) were obtained.

Precision

The precision was successfully demonstrated by achieving RSD% of 1.08–6.74% for replicate determinations of standard solution and groundwater and tap water samples in the inter-day and intra-day precision experiment. The RSD% values for precision studies on water samples collected from November 2018 to May 2019 were between 1.75–6.27% for groundwater and 1.17–6.74% for tap water, respectively. The relatively low RSD% observed indicates the method has good precision enabling quantification of fluoride ions in groundwater and tap water samples.

Fluoride content in groundwater and tap water samples

The optimization and validation studies demonstrated that the potentiometric method using the fluoride ion selective electrode is suitable to be used for the fluoride measurements in groundwater and tap water samples.

The values of fluoride ion concentrations in the groundwater samples and in tap water from November 2018 to May 2019 varied from 0.01 to 0.138 mg/L. These values are within the maximum permitted limit of 1 mg/L (WHO 2006) and in accordance with the fluoride level for water intended for human consumption of less than 1.5 mg/L established by the Council Directive 98/83/EC of 3rd November 1998 (Council Directive 98/83/EC).

Obtained values can be compared with the concentration of fluoride ions in groundwater in different regions of Estonia, values ranging from 0 to 6 mg/L (Veressinina et al. 2001), but also with the concentration of fluoride ions in groundwater reported in other studies (0.6–1.1 mg/L) (Arancibia et al. 2004).

The wells that show the highest concentration of fluoride ions are located in the northern area of the rural district of Valea Râmnicului. One of the activities carried out in this area is agriculture, which could indicate that the source of these ions in water is associated with this activity. So, the presence of fluoride ions can be from soil contamination as a result of the use of phosphate fertilizers and pesticides leached into the aquifer (Valdez-Alegría et al. 2019). As of yet, there has been no documented or recorded health risk due to exposure (see Section 3.6). The lowest concentration is noted in the southern part of the study region in November.

Figure 3 shows that fluoride concentrations decreased during the winter season (December–February) and increased in the spring (March–May). This shows the effects of dilution and concentration associated with the wet and dry seasons, respectively. The results indicate persistence of small fluoride concentrations in groundwater even during the wet season when dilution occurs.
Figure 3

Values of fluoride ion concentrations in studied sample (point 1—samples of groundwater from the southern area of the rural district of Valea Râmnicului; points 2 and 3—samples of groundwater from the central area of the rural district of Valea Râmnicului; point 4—samples of groundwater from the northern area of the rural district of Valea Râmnicului; point 5—tap water from the rural district of Valea Râmnicului).

Figure 3

Values of fluoride ion concentrations in studied sample (point 1—samples of groundwater from the southern area of the rural district of Valea Râmnicului; points 2 and 3—samples of groundwater from the central area of the rural district of Valea Râmnicului; point 4—samples of groundwater from the northern area of the rural district of Valea Râmnicului; point 5—tap water from the rural district of Valea Râmnicului).

Close modal

The values of the concentration of fluoride ions in the tap water samples (number 5) can be compared with the concentrations of fluoride ions reported in drinking water of other countries: Mexico: 0.3–3.7 mg/L (Ruiz-Payan et al. 2005), China: 0.76–4.51 mg/L (Xiong et al. 2007), Saudi Arabia: 0.5–0.83 mg/L (Aldrees & Al-Manea 2010) and Great Britain: 0.01–0.13 mg/L (Harrison 2005). A study conducted in the USA (Veressinina et al. 2001) showed that the optimal value of fluoride ion concentration in the temperate zone is 1.0–1.2 mg/L, the values of fluoride ion concentrations determined in the analysed samples being much lower than 1.0 mg/L.

Health risk assessment

The results of the EDI and the HQ are presented in Tables 2,34.

Different scenarios were considered in order to obtain the values for the EDI and for the HQ. These values were obtained taking into account the average daily intake of water for infants, children, women and men of different ages and different bodyweights and were calculated for different periods: spring and winter. The daily intake level of drinking water for different age groups was estimated by another survey (Amalraj & Pius 2013)

Table 2

EDI and HQ calculated for infants and children – spring and winter

Age group for infants and childrenBW (kg)Average D.W.D.I (L)(EDI) (μg/kg/day)
(EDI) (μg/kg/day)
(HQ) calculated for March, April and May (spring)
(HQ) calculated for December, January and February (winter)
Point 1
Point 2
Point 3
Point 4
Point 5
Point 1
Point 2
Point 3
Point 4
Point 5
EDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQ
Birth–6 months 0.1 5.80 0.09 4.80 0.08 3.70 0.06 4.20 0.07 5.40 0.09 2.55 0.04 2.96 0.04 2.09 0.03 3.23 0.05 5.20 0.08 
0.25 7.25 0.12 6.00 0.10 4.62 0.07 5.25 0.08 6.75 0.11 3.18 0.05 3.70 0.06 2.61 0.04 4.03 0.06 6.50 0.10 
0.35 6.76 0.11 5.60 0.09 4.31 0.07 4.90 0.08 6.30 0.10 2.97 0.04 3.45 0.05 2.44 0.04 3.76 0.06 6.06 0.10 
6–12 months 0.45 6.52 0.10 5.40 0.09 4.16 0.06 4.72 0.07 6.07 0.10 2.86 0.04 3.33 0.05 2.35 0.03 3.63 0.06 5.85 0.09 
10 0.6 6.96 0.11 5.76 0.09 4.44 0.07 5.04 0.08 6.48 0.10 3.06 0.05 3.55 0.05 2.51 0.04 3.87 0.06 6.24 0.10 
1–3 years 12 0.8 7.73 0.12 6.40 0.10 4.93 0.08 5.60 0.09 7.20 0.12 3.40 0.05 3.95 0.06 2.79 0.04 4.30 0.07 6.93 0.11 
14 0.95 7.87 0.13 6.51 0.10 5.02 0.08 5.70 0.09 7.32 0.12 3.46 0.05 4.02 0.06 2.84 0.04 4.38 0.07 7.05 0.11 
18 1.1 7.08 0.11 5.86 0.09 4.52 0.07 5.13 0.08 6.60 0.11 3.11 0.05 3.62 0.06 2.56 0.04 3.94 0.06 6.35 0.10 
3–10 years 20 1.4 8.12 0.13 6.72 0.11 5.18 0.08 5.88 0.09 7.56 0.12 3.57 0.05 4.15 0.06 2.93 0.04 4.52 0.07 7.28 0.12 
22 1.65 8.70 0.14 7.20 0.12 5.55 0.09 6.30 0.10 8.10 0.13 3.82 0.06 4.44 0.07 3.14 0.05 4.84 0.08 7.80 0.13 
25 1.8 8.35 0.13 6.91 0.11 5.32 0.08 6.04 0.10 7.77 0.12 3.67 0.06 4.26 0.07 3.01 0.05 4.65 0.07 7.48 0.12 
10–18 years 30 7.73 0.12 6.40 0.10 4.93 0.08 5.60 0.09 7.20 0.12 3.40 0.05 3.95 0.06 2.79 0.04 4.30 0.07 6.93 0.11 
45 5.15 0.08 4.26 0.07 3.28 0.05 3.73 0.06 4.80 0.08 2.26 0.03 2.63 0.04 1.86 0.03 2.87 0.04 4.62 0.07 
Age group for infants and childrenBW (kg)Average D.W.D.I (L)(EDI) (μg/kg/day)
(EDI) (μg/kg/day)
(HQ) calculated for March, April and May (spring)
(HQ) calculated for December, January and February (winter)
Point 1
Point 2
Point 3
Point 4
Point 5
Point 1
Point 2
Point 3
Point 4
Point 5
EDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQ
Birth–6 months 0.1 5.80 0.09 4.80 0.08 3.70 0.06 4.20 0.07 5.40 0.09 2.55 0.04 2.96 0.04 2.09 0.03 3.23 0.05 5.20 0.08 
0.25 7.25 0.12 6.00 0.10 4.62 0.07 5.25 0.08 6.75 0.11 3.18 0.05 3.70 0.06 2.61 0.04 4.03 0.06 6.50 0.10 
0.35 6.76 0.11 5.60 0.09 4.31 0.07 4.90 0.08 6.30 0.10 2.97 0.04 3.45 0.05 2.44 0.04 3.76 0.06 6.06 0.10 
6–12 months 0.45 6.52 0.10 5.40 0.09 4.16 0.06 4.72 0.07 6.07 0.10 2.86 0.04 3.33 0.05 2.35 0.03 3.63 0.06 5.85 0.09 
10 0.6 6.96 0.11 5.76 0.09 4.44 0.07 5.04 0.08 6.48 0.10 3.06 0.05 3.55 0.05 2.51 0.04 3.87 0.06 6.24 0.10 
1–3 years 12 0.8 7.73 0.12 6.40 0.10 4.93 0.08 5.60 0.09 7.20 0.12 3.40 0.05 3.95 0.06 2.79 0.04 4.30 0.07 6.93 0.11 
14 0.95 7.87 0.13 6.51 0.10 5.02 0.08 5.70 0.09 7.32 0.12 3.46 0.05 4.02 0.06 2.84 0.04 4.38 0.07 7.05 0.11 
18 1.1 7.08 0.11 5.86 0.09 4.52 0.07 5.13 0.08 6.60 0.11 3.11 0.05 3.62 0.06 2.56 0.04 3.94 0.06 6.35 0.10 
3–10 years 20 1.4 8.12 0.13 6.72 0.11 5.18 0.08 5.88 0.09 7.56 0.12 3.57 0.05 4.15 0.06 2.93 0.04 4.52 0.07 7.28 0.12 
22 1.65 8.70 0.14 7.20 0.12 5.55 0.09 6.30 0.10 8.10 0.13 3.82 0.06 4.44 0.07 3.14 0.05 4.84 0.08 7.80 0.13 
25 1.8 8.35 0.13 6.91 0.11 5.32 0.08 6.04 0.10 7.77 0.12 3.67 0.06 4.26 0.07 3.01 0.05 4.65 0.07 7.48 0.12 
10–18 years 30 7.73 0.12 6.40 0.10 4.93 0.08 5.60 0.09 7.20 0.12 3.40 0.05 3.95 0.06 2.79 0.04 4.30 0.07 6.93 0.11 
45 5.15 0.08 4.26 0.07 3.28 0.05 3.73 0.06 4.80 0.08 2.26 0.03 2.63 0.04 1.86 0.03 2.87 0.04 4.62 0.07 
Table 3

EDI and HQ calculated for women – spring and winter

Age group for womenBW (kg)Average D.W.D.I. (L)(EDI) (μg/kg/day)
(EDI) (μg/kg/day)
(HQ) calculated for March, April and May (spring)
(HQ) calculated for December, January and February (winter)
Point 1
Point 2
Point 3
Point 4
Point 5
Point 1
Point 2
Point 3
Point 4
Point 5
EDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQ
20–40 years 50 4.64 0.07 3.84 0.06 2.96 0.04 3.36 0.05 4.32 0.07 2.04 0.03 2.37 0.03 1.67 0.02 2.58 0.04 4.16 0.06 
55 4.21 0.07 3.49 0.05 2.69 0.04 3.05 0.05 3.92 0.06 1.85 0.03 2.15 0.03 1.52 0.02 2.34 0.03 3.78 0.06 
40–70 years 60 3.86 0.06 3.20 0.05 2.46 0.04 2.8 0.04 3.6 0.06 1.7 0.02 1.97 0.03 1.39 0.02 2.15 0.03 3.46 0.05 
70 3.31 0.05 2.74 0.04 2.11 0.03 2.4 0.04 3.08 0.05 1.45 0.02 1.19 0.01 1.19 0.01 1.84 0.03 2.97 0.04 
80 2.90 0.04 2.40 0.04 1.85 0.03 2.1 0.03 2.7 0.04 1.27 0.02 1.04 0.01 1.04 0.01 1.61 0.02 2.6 0.04 
Over 70 years 65 1.5 2.67 0.04 2.21 0.03 1.70 0.02 1.93 0.03 2.49 0.04 1.17 0.01 1.36 0.02 0.96 0.01 1.38 0.02 2.4 0.04 
Age group for womenBW (kg)Average D.W.D.I. (L)(EDI) (μg/kg/day)
(EDI) (μg/kg/day)
(HQ) calculated for March, April and May (spring)
(HQ) calculated for December, January and February (winter)
Point 1
Point 2
Point 3
Point 4
Point 5
Point 1
Point 2
Point 3
Point 4
Point 5
EDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQ
20–40 years 50 4.64 0.07 3.84 0.06 2.96 0.04 3.36 0.05 4.32 0.07 2.04 0.03 2.37 0.03 1.67 0.02 2.58 0.04 4.16 0.06 
55 4.21 0.07 3.49 0.05 2.69 0.04 3.05 0.05 3.92 0.06 1.85 0.03 2.15 0.03 1.52 0.02 2.34 0.03 3.78 0.06 
40–70 years 60 3.86 0.06 3.20 0.05 2.46 0.04 2.8 0.04 3.6 0.06 1.7 0.02 1.97 0.03 1.39 0.02 2.15 0.03 3.46 0.05 
70 3.31 0.05 2.74 0.04 2.11 0.03 2.4 0.04 3.08 0.05 1.45 0.02 1.19 0.01 1.19 0.01 1.84 0.03 2.97 0.04 
80 2.90 0.04 2.40 0.04 1.85 0.03 2.1 0.03 2.7 0.04 1.27 0.02 1.04 0.01 1.04 0.01 1.61 0.02 2.6 0.04 
Over 70 years 65 1.5 2.67 0.04 2.21 0.03 1.70 0.02 1.93 0.03 2.49 0.04 1.17 0.01 1.36 0.02 0.96 0.01 1.38 0.02 2.4 0.04 
Table 4

EDI and HQ calculated for men – spring and winter

Age group for menBW (kg)Average D.W.D.I. (L)(EDI) (μg/kg/day)
(EDI) (μg/kg/day)
(HQ) calculated for March, April and May (spring)
(HQ) calculated for December, January and February (winter)
Point 1
Point 2
Point 3
Point 4
Point 5
Point 1
Point 2
Point 3
Point 4
Point 5
EDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQ
20–40 years 60 3.86 0.06 3.2 0.05 2.46 0.04 2.8 0.04 3.6 0.06 1.7 0.02 1.97 0.03 1.39 0.02 2.15 0.03 3.46 0.05 
65 3.56 0.05 2.95 0.04 2.27 0.03 2.58 0.04 3.32 0.05 1.56 0.02 1.82 0.03 1.28 0.02 1.98 0.03 3.2 0.05 
40–70 years 75 3.09 0.05 2.56 0.04 1.97 0.03 2.24 0.03 2.88 0.04 1.36 0.02 1.58 0.02 1.11 0.01 1.72 0.02 2.77 0.04 
85 2.72 0.04 2.25 0.03 1.74 0.02 1.97 0.03 2.54 0.04 1.2 0.02 1.39 0.02 0.98 0.01 1.52 0.02 2.44 0.04 
90 2.57 0.04 2.13 0.03 1.64 0.02 1.86 0.03 2.4 0.04 1.13 0.01 1.31 0.02 0.93 0.01 1.43 0.02 2.31 0.03 
Over 70 years 70 1.5 2.48 0.04 2.05 0.03 1.58 0.02 1.8 0.03 2.31 0.03 1.09 0.01 1.27 0.02 0.89 0.01 1.38 0.02 2.22 0.03 
Age group for menBW (kg)Average D.W.D.I. (L)(EDI) (μg/kg/day)
(EDI) (μg/kg/day)
(HQ) calculated for March, April and May (spring)
(HQ) calculated for December, January and February (winter)
Point 1
Point 2
Point 3
Point 4
Point 5
Point 1
Point 2
Point 3
Point 4
Point 5
EDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQEDIHQ
20–40 years 60 3.86 0.06 3.2 0.05 2.46 0.04 2.8 0.04 3.6 0.06 1.7 0.02 1.97 0.03 1.39 0.02 2.15 0.03 3.46 0.05 
65 3.56 0.05 2.95 0.04 2.27 0.03 2.58 0.04 3.32 0.05 1.56 0.02 1.82 0.03 1.28 0.02 1.98 0.03 3.2 0.05 
40–70 years 75 3.09 0.05 2.56 0.04 1.97 0.03 2.24 0.03 2.88 0.04 1.36 0.02 1.58 0.02 1.11 0.01 1.72 0.02 2.77 0.04 
85 2.72 0.04 2.25 0.03 1.74 0.02 1.97 0.03 2.54 0.04 1.2 0.02 1.39 0.02 0.98 0.01 1.52 0.02 2.44 0.04 
90 2.57 0.04 2.13 0.03 1.64 0.02 1.86 0.03 2.4 0.04 1.13 0.01 1.31 0.02 0.93 0.01 1.43 0.02 2.31 0.03 
Over 70 years 70 1.5 2.48 0.04 2.05 0.03 1.58 0.02 1.8 0.03 2.31 0.03 1.09 0.01 1.27 0.02 0.89 0.01 1.38 0.02 2.22 0.03 

The values for the EDI are higher in spring than in winter for all age groups related with the higher concentration of fluoride for this period and with the one of the activities carried out in spring in the area: agriculture. The presence of fluoride ions can be from soil contamination as a result of the use of phosphate fertilizers and pesticides. EDI values for infants and children vary from: 3.28 to 8.70 μg/kg/day in spring and from 1.86 to 7.8 μg/kg/day in winter. For women, EDI varies from 1.7 to 4.64 μg/kg/day in spring and from 0.96 to 4.16 μg/kg/day in winter, whereas for men varies from: 1.58 to 3.86 μg/kg/day in spring and from 0.89 to 3.46 μg/kg/day in winter. Women tended to intake more fluoride per kilogram of body weight than the men because of their lower body weights, which indicates that females are exposed to more fluoride than males.

A HQ less than or equal to 1 indicates that adverse effects are not likely to occur, and thus can be considered to have negligible hazard. If the value of HQ is greater than 1, the estimated potential exposure exceeds the RfD and a risk of fluorosis may be posed (Barnes et al. 1988).

The values of the HQ for the studied samples varies from 0.01 to 0.13 demonstrating no risk of fluorosis.

Data from the literature show easy to operate and low-cost methods can reduce the fluoride concentration of drinking water such as chemical coagulation or bone char adsorption. But these methods are also deficient due to the production of large amounts of waste, high level of water hardness after chemical dosage and unpleasant water color and odor (Sawangjang et al. 2019; Akbari et al. 2021).

The present study aimed to present an optimized and validated potentiometric method for determining fluoride ions with the fluoride selective ion electrode. The proposed method was successfully applied to monitor fluoride concentrations in drinking water samples (groundwater and tap water from the rural district of Valea Râmnicului, Romania). The presented method shows acceptable and satisfactory performance for all the tested parameters.

According to the data provided by the National Institute of Statistics, in 2019, of the total resident population in Romania, i.e. 19,370,448 people, about 29.13% (5,642,304 people), was not connected to a drinking water supply network. This means that the main source of water supply used by this part of the population is water from wells. In the north-east region of the country, almost half of the population (49.9%) is not connected to a drinking water network, followed by the south-western region of Oltenia (40.7%) and by the region where the rural district of Valea Râmnicului is located, Wallachia, in the south (34.5%).

Long-term exposure to high concentrations of fluoride is a concern for the population due to possible health effects. Fluoride ions in water contribute significantly to the individuals’ exposure to this element, but it is not the only source of exposure to fluoride concentrations, especially since the introduction of fluoride into toothpaste. The values of fluoride ion concentrations determined from the analyzed samples do not exceed the WHO agreement, in which the maximum permitted limit of fluoride ion concentration in drinking water is 1 mg/L.

The performed health risk assessment of the consumption of groundwater based on the estimated HQ revealed that the HQ values were smaller than 1 for infants, children and adults; thus, the local residents of the study area were at no potential human health risk in terms of water consumed via the ingestion pathway.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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

The authors declare there is no conflict.

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