1. Introduction
The sustainability and quality of groundwater are crucial, as it is relied upon by approximately 2.5 billion people globally [
1]. However, natural and human-related factors cause fluctuations in the availability and quality of groundwater [
2,
3]. The quality of groundwater has a significant role in the prevalence of various diseases. Among the various water quality parameters, fluoride is a significant contaminant that poses serious health risks and has garnered attention [
4]. Fluoride (F
−) in groundwater can be beneficial for human health within a specific range; however, when the concentration is too low or high, it can have adverse effects. Low levels of fluoride in groundwater can lead to dental caries and poor bone development, while excessive intake can cause dental fluorosis and harm to the kidneys, bones, reproductive organs, nerves, and muscles [
2]. The optimum range of F
− in drinking water can differ as a function of various factors such as environmental conditions and socio-economic factors. The World Health Organization (WHO) has specified that the safe permissible limit of F
− in drinking water is 1.5 mg/L [
5]. The primary source of intake of F
− by humans is groundwater, as it is the primary drinking water source in rural and urban areas [
6]. The natural occurrence of high fluoride concentrations in groundwater is a global health concern that is potentially affecting hundreds of millions of people, predominantly in the Global South [
7]. A large number of people in 67 countries, including India, suffer from endemic fluorosis due to excess F
− content in groundwater [
8,
9,
10,
11,
12]. In India, the F
− problem in groundwater is reported to occur in varied geological and environmental settings [
13]. In 1937, high F
− problem was first observed in Nellore District of Andhra Pradesh of India. According to UNICEF, at least 177 districts in 19 States of India were affected by excess F
− levels in groundwater [
14,
15]. It was reported that around 62 million people from these States are largely dependent on groundwater and suffer from fluorosis [
13,
16]. Further, about 60–70 million people were estimated to be at risk [
17,
18].
Geogenic origin, i.e., weathering, dissolution, and leaching of fluoride-bearing minerals into underground water through bedrock, is considered the major source of F
− in groundwater [
19,
20,
21,
22]. In addition to the geogenic factors, F
− contamination in groundwater takes place from anthropogenic sources, viz. agriculture industries using extensively high amounts of phosphatic fertilizers, industries using coal for thermal power, and discharges from industries [
10,
19,
23,
24]. Mining activities and heavy groundwater exploitations enhance the dissolution rate of fluoride [
25]. The mineral and chemical composition of bedrock is, therefore, considered one of the major factors contributing to the occurrence of F
− in groundwater. The natural path of F
− enrichment in groundwater depends largely on several factors, such as rock chemistry, solubility of fluoride-bearing minerals, temperature, pH, bedrock constituting the aquifers and their anion-exchange capacities, residence time of water in aquifers or duration of rock–water interaction, climate, well depth, and geological structures [
18,
21,
26,
27]. Numerous studies have been conducted in different geologic settings by researchers across the world to understand the linkage of F
− occurrence in groundwater vis-à-vis rock–water interaction and the geochemical processes. Earlier studies revealed that F
− has a higher affinity for sodium than calcium; hence, the sodium bicarbonate (NaHCO
3) type of water decreases with calcium ions and increases with sodium ions, and it has a neutral-to-alkaline pH, indicating favorability of chemical conditions for fluoride dissolution processes that accelerate F
− concentration in subsurface water [
28,
29,
30]. It was found that groundwater with elevated levels of F
− is generally characterized by (a) high HCO
3 alkalinity, Na
+, pH, and silica, and (b) low Ca
2+ and hardness [
18,
31,
32,
33]. Generally, a negative correlation between F
− and Ca
2+ in groundwater, including Indian groundwater, has been observed by several researchers (WHO, 2017). Cation/base-exchange (Ca
2+ for Na
+) and anion-exchange (OH
− for F
−) geochemical processes also promote an increase in F
− levels in groundwater [
31,
33,
34,
35]. Deeper wells are generally found to contain higher F
− concentrations as compared to shallow wells because of the increased solubility of minerals with an increase in temperature [
5,
36]. Further, groundwater in arid areas has comparatively higher F
− concentration than in humid areas [
37].
This study constitutes a hydrogeochemical investigation in a sedimentary formation of Gondwana Supergroup coal-bearing rocks located in central India, an area known for potential coal-mining activities. The harmful effects of F
− incidence in groundwater on the people’s health such as several incidences of dental and skeletal fluorosis have been reported in the study area [
38]. Given the large populations at risk due to high fluoride concentration in drinking water sources, there is an urgent need for a systematic and scientific investigation of F
− occurrence in groundwater [
39]. This study aims to analyze the high fluoride concentration in groundwater during pre-, post-, and mid-monsoon periods in the Barakar Formation, a particular geological formation, to understand fluoride enrichment in groundwater and geology, and to determine geochemical behavior in deep bore wells by conducting hydrogeochemical analysis.
4. Discussion
The results of the hydro-geochemical evaluation of groundwater, highlighted in the previous section, can be used to constrain the origin source and hydro-geochemical processes. Geographical and geological locations were major elements catalyzing the enrichment of F
− concentration in bore wells located in the study area. The main cations of groundwater were Ca
2+, Mg
2+, Na
+, and K
+, and the main anions were HCO3
−, SO42
−, Cl
−, and NO3
−. The groundwater chemistry of the micro-watershed changed in all three major flow directions, i.e., west to east, zone A, north to east, zone B, and east to southeast, zone C (
Figure 2). When comparing all three zones, it was observed that the spatial distribution of F
− (
Figure 7a,b) in the groundwater of the study area showed that bore wells with high F
− concentrations were confined to eastern part (zone C) and increased along the flow path within the zone itself. The present study shows that high F
− concentrations (>1.2 mg/L) in groundwater occurred in Muragaon, Saraitola, Pata, Kunjhemura, and Dolnara villages. The F
− concentration was more or less independent of other water-soluble components; however, remarkably, no significant correlation existed between F
− and pH. Fluoride solubility is lowest in low pH (5–6.5) [
68], while ionic exchange takes place between F
− and OH
− ions at higher pH (illite, mica), consequently increasing the F
− concentrations in groundwater [
33,
69]. The groundwater in this area was more or less alkaline with pH varying from 6.91 to 8.96; its pH mean value was 8.16 in the pre-monsoon period, indicates the alkaline characteristics of groundwater (
Table 2). Groundwater with a high pH value favors the enrichment of F
− [
16,
28,
29,
30] because fluoride (F
−) and hydroxyl (OH
−) ions have similar ionic radii, and hydroxyl ions in groundwater can displace exchangeable fluoride ions from fluoride-bearing minerals when alkaline groundwater circulates through the aquifer [
45]. F
− concentration is positively correlated with Na+ and pH, but negatively correlated with the Ca
2+, indicating that, in the aquifer, a high F
− concentration is due to the involvement of geochemical processes in increasing Na
+ and pH and decreasing Ca
2+. Therefore, the geochemical parameters Na
+, pH, and Ca
2+ can explain the geochemical processes that might have been responsible for high F
− in the groundwater of eastern part of the study area. On the other hand, groundwater types are not related to geology, whereas the source of F
− might be related to geology. The geochemical behavior of groundwater is controlled by the geochemistry of groundwater.
The study area was sedimentary-dominant aquifer; dissolution of F
− was a plausible cause for occurrence of F
− concentrationin groundwater. An increase in F
− concentration in groundwater was noticed as the Na
+ content increases (
Figure 9). The plots of F
− and lithogenic Na
+ indicated a remarkable positive correlation between F
− and lithogenic Na
+ (
Table 4 and
Figure 11b). Piper’s trilinear diagrams also showed Na
+ as the dominant cation, wherein the concentration of F
− was high. A similar research finding was proposed by [
31], who also reported that an increase in F
− content in groundwater was linked to the geochemical processes corresponding to an increase in Na
+ concentration and decrease in Ca
2+ concentration. The F
− concentration in groundwater is negatively correlated with calcium ions (Ca
2+) in groundwater. Such a relationship between Ca
2+ and F
− was also found in our research, which was supported by the significant negative correlation (−0.22 in pre-monsoon period; −0.33 in post monsoon period). Thus, due to the dominance of ion-exchange processes functioning in the studied aquifer, an increasing F
− content was shown to correlate with an increasing Na
+ content and decreasing Ca
2+. The Ca
2+ and Na
+ ion concentrations in aquifer increased from the recharge to discharge zone and flowed through the drainage direction (
Figure 2). This interpretation was also supported by the Na/Ca ratio, which was three times greater for groundwater samples with F
− > 1.2 mg/L as compared to those with F
− ≤ 1.2 mg/L (i.e., 0.42 vs. 1.16 in the pre-monsoon period and 0.46 vs. 1.32 in the post-monsoon period). This was due to ion exchange, whereby calcium ions in water may react with clay minerals to release Na
+ ions, thus increasing their concentration in groundwater [
70]. A strong correlation between high F
− and low Ca
2+ content in alkaline groundwater has also been reported by many authors (e.g., [
31]), wherein an increase in solubility of fluorine-bearing minerals with an increase in Na
+ concentration was observed [
33].
Through the saturation index analysis, it was found that all groundwater samples were undersaturated with calcite, fluorite, halite, gypsum, anhydrite, and dolomite during the pre-monsoon period. This means that these minerals could dissolve more in groundwater, which could lead to an increase in their concentration. However, during the post-monsoon period, calcite and dolomite were found to be oversaturated, which means that no further dissolution could occur, and they would precipitate as CaF
2. On the other hand, fluorite remained undersaturated due to the oversaturation of calcite, which reduced calcium activity and allowed more fluorite to dissolve, thereby increasing the F
−/Ca2+ of the solution. This finding is consistent with the study conducted by Alamry [
50].
The anthropogenic origin of high F
− in groundwater in the study area could be completely ruled out. There was no such activity in the study area that could be considered as potential source of fluoride inputs in groundwater as contaminant. Thus, the high concentration of fluoride in groundwater in the study area was geogenic in origin i.e., hydro-geo-chemical conditions and coal-bearing formations were responsible for the higher concentration of F
− in the bore wells in the study area. However, the process of F
− enrichment is still not well understood [
46,
71]; many authors have accepted the general principle of exchangeable fluoride (F
−) ions by hydroxyl (OH
−) ions (e.g., [
67,
72]). The geochemistry of groundwater considers factors such as adsorption, dissolution, hydrolysis, precipitation, ion-exchange, and geo-chemical processes as principle reasons that contribute to the enrichment of F
− concentration in groundwater [
71]. For example, sodium bicarbonate-rich groundwater (NaHCO
3) in weathered rock formation accelerates the rate of dissolution of fluorite (CaF
2) to release F
− into groundwater due to water–rock interaction with time, as shown in mass balance Equation (1). The presence of high bicarbonate ions (HCO
3−), sodium ions (Na
+), and pH favors the release of F
− into groundwater [
33,
73,
74], thus mobilizing F
− from fluorite (CaF
2) mineral (Equations (1) and (2));
Groundwater with high HCO
3− and Na
+ content is usually alkaline in nature [
33] and has a relatively high OH
− concentration (Equation (3)). F
−-rich minerals such as muscovite, biotite, and amphiboles have been reported in the study area, and fluoride (F
−) ion can replace hydroxyl (OH
−) ions under alkaline conditions. The reaction process of the replacement of hydroxyl (OH
−) ions by fluoride ions (F
−) from muscovite mineral [
33] is illustrated below (Equation (4)).
The concentration of F
− in the bore wells located in the study area largely depended upon factors like climate, evaporation, geology and precipitation. Such factors were also reported by Li et al. [
60]. It is widely accepted by many researchers that the enrichment of F
− in groundwater is due to persistent water–rock interaction [
51,
52,
74,
75].
F− concentration in the sedimentary aquifer of the study area had an apparent variation with well depth. The maximum F concentration of deep groundwater was 8.88 mg/L observed at the depth of 150 m, whereas F concentration was 0.41 mg/L at the depth of 70 m. Most high-F wells were observed in deep groundwater. The genesis of high F concentration may be attributed to ionic activity in deep groundwater that promotes water interaction and residence time over less deep groundwater.
Many researchers have reported that F
− occurs in alkaline environments with higher HCO
3− [
76,
77,
78] and is positively correlated with HCO
3− [
33]. There were also instances where negative correlations between F
− and HCO
3− have been reported in deeper aquifers [
56,
57]. However, in this particular study, there was a negative correlation between F
− and HCO
3−, in line with other studies that reported a negative correlation of F
− in deep-seated wells similar to our conditions. The scatter plots of F
− and K
+ showed a negative correlation, which may have been due to weathering of K-bearing minerals and/or the fixation of K
+ ions in micas and clay minerals [
34]. It is argued by many researchers that micas and clay, which often occur as pertinent minerals in rocks and contain F
− at the OH
− sites, can form the dominant source for high F
− incidence in groundwater via the process of anion exchange (OH
− for F
−) takes place especially in sedimentary terrains [
31,
33,
35]. The micas and clay minerals occurring abundantly in the lithological assemblage of the Barakar Formation could be considered the source of elevated F
− concentration in groundwater. The presence of Li
+ (although occurring in very small concentrations and having a poor positive correlation with F
−) in the high-F
− zone lends support to weathering of micas as the source of F
− at a deeper level [
44]. The absence of PO
43− in bore wells samples rules out fluoride contribution from phosphatic minerals in data from all three seasons, along with the application of phosphatic fertilizers. The absence of industrial activities and association of the high F
− zone with deeper aquifers further negate anthropogenic contamination. Because the wells are not completely cased, the collected groundwater samples represent a multiple-aquifer/aquitard system. Therefore, it is recommended to carry out depth-wise sampling of groundwater and rocks for all three seasons, which could not be conducted in the present study, to identify the subsurface horizon(s) and the constituent minerals causing the F
− problem. Hydrogeochemical investigations are needed in the adjoining areas with similar geologic setting, particularly for the aquifers in the Barakar Formation, in order to delineate unsafe zones and take mitigation measures so as to safeguard people against the danger of consuming F
−-contaminated groundwater.