Introduction

Groundwater contamination by fluoride is one of the serious problems in the arid and semiarid regions of the world. Particularly in India, a number of people suffer from fluorosis due to intake of high fluoride content through drinking water. Approximately, in India, the excessive fluoride in groundwater is noticed in 177 districts covering 21 states, affecting 62 million people, including 6 million children (Adimalla and Venkatayogi 2018; Ayoob and Gupta 2006).

Nearly 200 million people from 25 nations are affected by the deadly disease of fluorosis (Ali et al. 2016; Adimalla and Venkatayogi 2017). Fluorosis-affected regions are reported from China (Li et al. 2018, 2014; Wu et al. 2015), India (Narsimha and Rajitha 2018; Adimalla et al. 2018a, b, c; Narsimha 2018; Narsimha and Sudarshan 2017a, b, 2018a, b; Rao et al. 2014; Subba Rao et al. 2015), Africa (Gizaw 1996), Korea (Kim and Jeong 2005), Mexico (Diaz-Barriga et al. 1997), Kenya (Gikinju et al. 2002) and Nigeria (Gbadebo 2012). A small amount of fluoride is essential to maintain bones and formation of dental enamel (Adimalla and Venkatayogi 2017; Adimalla and Li 2018). However, prolonged intake of high fluoride in drinking water can surely cause fluorosis (Adimalla and Qian 2019a; Narsimha and Sudarshan 2017a; Li et al. 2018). In general, fluoride is released into groundwater from fluorine-bearing minerals such as fluorite, fluorapatite, biotite, apatite, muscovite, hornblende, villiaumite, tremolite, sellaite, cryolite, topaz, fluocerite, yttrofluorite, gagarinite, bastnasite, microlite, sphene, wohlerite, fluormica, epidote, amphibole, lepidolite, montmorillonite, kaolinite, pegmatite, mica, clays, villuanite, phosphorite, and some micas weathered from silicates, igneous, and sedimentary rocks, especially shale (Adimalla et al. 2018a; Ayoob and Gupta 2006; Adimalla 2018; Narsimha and Sudarshan 2017a, 2018a, b), and high rates of evaporation and low precipitation in arid and semiarid areas can also contribute to the fluoride enrichment (Adimalla and Venkatayogi 2017, 2018; Subba Rao et al. 2015). However, fluoride is an important element for human health which has certain limits for intake (Rao et al. 2017; Ali et al. 2016; Narsimha and Sudarshan 2017a, b). World Health Organization (WHO 1984) has fixed a safe limit for fluoride from 0.5 to 1.5 mg/L in drinking water. Moreover, the intake of drinking water with fluoride content less than 0.5 mg/L can cause tooth decay. Larger than 1.5 mg/L fluoride content in drinking water is risky for human consumption which leads to dental fluorosis and skeletal fluorosis when exceeds 3 mg/L (Ayoob and Gupta 2006; Rao et al. 2017; Wu and Sun 2016).

In recent years, a rapid growth of population, industrial development, intense agriculture activity, low rainfall, declining surface water resources, and climate change have caused significant stress on surface/lake water supplies especially in Andhra Pradesh, Telangana states, and other rural parts of the country. Hence, people are forced to depend on groundwater for their daily needs. Eventually, groundwater is becoming more vital water resource primarily for drinking, domestic, and other usages in Markapur provinces. Thus, dental and skeletal health problems are noticed in the Markapur region of Andhra Pradesh. It is reported that the groundwater in areas covering Santhala Moguluru, Guttala madivaram, Vemulapadu, Podili, Kanigiri, Vengayyapalem, Malakonda, Gollapalli, Pasupugallu, Pallamalla, Chandalur and Markapur villages contains fluoride concentration more than the maximum permissible limit of 1.5 mg/L in Prakasam district, Andhra Pradesh (CGWB 2013). Moreover, Prakasam district is not only known for widespread occurrence of fluorosis but also for the occurrence of a high level of fluoride (Rao et al. 2014; Subba Rao et al. 2015), and few efforts have been made to understand the geochemical processes involved in the occurrence of high fluoride concentration in the groundwater of Markapur region. For this reason, a detailed study was undertaken to understand the geochemistry of fluoride in groundwater and to find the relationship of fluoride with other water quality parameters. This study paves the way to provide baseline information on drinking water safety to researchers/scholars and decision makers for investigating local groundwater problems.

Study area

The Markapur province is located in the central-western part of the Prakasam district (Fig. 1). The area geographically lies between the 79°10′~79°22′ north latitudes and 15°35′~15°50′ east longitudes. The vast plains of Markapur and of the adjacent areas are occupied by phyllite/slate (GSI 1993; GSI-NGRI 2006). Slate, when it is siliceous, stands out as a prominent linear ridge. The slate quarries in the study area represent minor ridges formed by siliceous slates. Mining of slate is the major commercial industry in Markapur. Among the carbonates, cherty dolomite is noticed in the south, where it trends E–W and possibly extends into the N–E direction. The carbonate and quartzite are the intercalated sequence in the Cumbum Formations. The main geomorphic units, in the study area, with reference to groundwater, are pediplain shallow, denudational hills, structural hills, and a few linear ridges. Pediplain shallow covers most of the area and is moderate to good in groundwater prospects, mainly because of secondary porosity in the form of cleavage. The groundwater prospects are poor in structural hills, denudational hills, and linear ridges. Hydrogeologically, shales and phyllites of the Markaur region under the Cuddapah Supergroup are considered as hard rocks, lacking in primary porosity. They develop secondary porosity through fracturing and weathering over a long period and become water bearing. Groundwater in shales/phyllites occurs in unconfined conditions in the weathered residuum and under semiconfined to confined conditions in the fissures, joints, bedding planes, and fractures (GSI 1993).

Fig. 1
figure 1

Location map of the groundwater samples from the Markapur region, India

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Mostly, water table aquifers occur at shallow depths, whereas semiconfined/confined aquifers at greater depths. Shallow aquifers occur within a depth of 20 mbgl (meters below ground level). The ideal sequence of the strata is weathered, semiweathered, and fractured zones. Nature and thickness of these aquifers depend on the depth of weathering, topography, and recharge conditions of the terrain and hence show wide variations in their water-yielding capacity. Groundwater in deeper aquifers occurs under semiconfined or confined conditions. The tectonic disturbance in the eastern fringe of Cuddapah Supergroup has developed deep-seated fractures in crystalline rocks and such zones form potential aquifers (GSI 1993). The deeper weathered and fractured rock aquifers are being developed by bore wells generally drilled along lineaments and at other favorable locations. The chemical composition and texture of parent rock not only determine the degree to which it can be weathered but also its reactivity and nature of the resultant product. It is observed, in the study area, that weathered clay residuum formed from argillaceous phyllites, shales, and slates generally do not yield more water. On the other hand, weathered residuum containing more quartz yields more water.

The investigated region falls under semiarid climate condition. The average annual temperature varies from 27°C in winter to 45°C in summer. The average annual rainfall is 182.9 mm. Southwest monsoon contributes 61% of the total rainfall. Winds are generally light to moderate, except during the late summer and early southwest monsoon season.

Materials and methods

Groundwater samples were collected from 123 sampling sites in 1-liter clean polyethylene bottles and labeled with sample ID starting from PDM-1 to PDM-123. The samples were analyzed for anionic and cationic constituents using standard methods APHA (1995). The pH, electrical conductivity (EC), and total dissolved solids (TDS) were analyzed on the site using pH/EC/TDS meter (Hanna HI 9811-5; Narsimha and Sudarshan 2017a). Total hardness (TH) was measured by titration method using standard hydrochloric acid and standard EDTA solution. Calcium (Ca2+) and magnesium (Mg2+) were determined titrimetrically using standard EDTA. Sodium (Na+) and potassium (K+) concentrations were determined using Flame photometer (Systronics, 130). Chloride (Cl−) was determined by standard AgNO3 titration. Bicarbonate (HCO3−) and carbonate (CO32−) were determined by titration with HCl. Sulfate (SO42−) and nitrate (NO3−) were determined by using UV–visible spectrophotometer (Spectronic, 21, BAUSCH and LOMB).

The fluoride concentration in groundwater was determined electrochemically, using Thermo Scientific Orion Star A214 Benchtop pH/ISE meter (9609BNWP fluoride ion-selective electrode) using the USEP ion-selective electrode method. This method is applicable to the measurement of fluoride in drinking water in the concentration range of 0.1–1000 mg/L. Standard fluoride solutions (0.1–10 mg/L) were prepared from a stock solution (100 mg/L) of sodium fluoride. As per experimental requirement, 2 mL of total ionic strength adjusting buffer grade III (TISAB III) was added in 20 mL of water sample. The ion meter was calibrated for a slope of − 59.2 ± 2. The composition of TISAB solution was as follows: 58 g NaCl, 4 g of CDTA (cyclohexylene diamine tetraacetic acid), and 57 mL of glacial acetic acid per liter.

Eventually, the accuracy of all chemical analyses was verified by calculating ion-charge balance between cations (Ca2+, Mg2+, Na+, and K+) and anions (HCO3−, Cl-, SO42−, NO3−, and F−) as (cations − anions)/(cations + anions) × 100. All the 123 groundwater samples were within the accepted limit of ± 10% (Domenico and Schwartz 1990).

Results and discussion

Major ion chemistry

The analytical results of pH, electrical conductivity (EC), total dissolved solids (TDS), total hardness (TH), calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), bicarbonate (HCO3−), carbonate (CO32−), sulfate (SO42−), nitrate (NO3−), and fluoride (F−) concentrations of the groundwater samples are presented in Table 1. The groundwater is mostly alkaline in nature in the Markapur region with pH concentration ranging from 7.18 to 9.32 and with a mean of 8.36 (Tables 1 and 2). The EC concentration varies between 520 and 4400 µS/cm, with a mean of 1451.71 μS/cm. The high mean value for EC emphasizes a wide variation in ionic content among different samples, and also it is a measure of a material’s ability to conduct an electric current, and the difference of it indicates a wide variation of salts present in the groundwater. TDS concentration of groundwater is varying between 290 and 2640 mg/L, with a mean of 901.91 mg/L. Further, the TDS is classified as fresh, if it is less than 1000 mg/L; brackish, if it is in between 1000 and 10,000 mg/L; saline, if it varies from 10,000 to 100,000 mg/L; and brine, if it is more than 100,000 mg/L. Accordingly, 67% and 33% of groundwater fell under fresh and brackish category, respectively, in the present study region. The concentration of TH (as CaCO3) shows wide disparity ranging from 80 to 880 mg/L with a mean of 255.68 mg/L (Table 1).

Table 1 Hydrochemical data from a rural part of Andhra Pradesh, India
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Table 2 Groundwater samples from a rural part of Andhra Pradesh, India exceeding the permissible limits prescribed by WHO (2004) and ISI (1993) for drinking purpose
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Among the cations, Na+ is the most dominant in the Markapur groundwater and its concentration ranged from 30 to 850 mg/L, with a mean of 205.47 mg/L (Table 1). Moreover, one and half times of groundwater samples show higher than the maximum allowable limit of 200 mg/L in the study region. The higher concentration of sodium may be derived from the dissolution of minerals and soil salts as well as the influence of anthropogenic sources (Todd 1980). The concentration of Mg2+ in the study area groundwater ranged from 40 to 520 mg/L, with a mean of 170.52 mg/L and about 58% of groundwater samples exceeded the maximum allowable limit of 150 mg/L (Table 1). The concentration of Ca2+ ranged between 10 and 360 mg/L, with an average of 85.16 mg/L recorded in the groundwater of study region which shows 1.8 times higher than the maximum allowable limit 200 mg/L for drinking purposes. Ferromagnesian minerals and artificial/human activities are the principal sources for higher content of magnesium and calcium in the groundwater (Hem 1991). The K+ content ranged from 0.5 to 88 mg/L, with a mean of 7.23 mg/L in the groundwater of the study region. K+ concentration is the least among the major cations in all the sampled groundwater with an estimated percentage mean value of 1.54%. The lower concentration of potassium is generally recorded in groundwater due to its relative greater resistance to weathering, and it is getting out of solution onto clay surfaces leading to its loss (Anim-Gyampo et al. 2018).

A high chloride concentration of groundwater is a result of leaching from the soils and also effect of domestic wastes, domestic effluents, fertilizers, leakages from septic tanks and road salt used to de-ice roads in the winter, and from natural sources such as rainfall, the dissolution of fluid inclusions, and chloride-bearing minerals, which also indicates an index of pollution (Todd 1980; Adimalla and Venkatayogi 2018; Adimalla 2018). In the present study, the concentration of chloride ranges from 25 to 940 mg/L, with a mean of 189.43 mg/L. According to the World Health Organization (WHO 2004) drinking water specifications, the permissible limit in the absence of an alternate source for chloride is 600 mg/L. The results revealed that only 2.4% of groundwater samples show higher than the 600 mg/L in the study region. The maximum allowable limit of sulfate for drinking purpose is 400 mg/L as suggested by the World Health Organization (WHO 2004). The concentration of sulfate is less than the maximum allowable limit of 400 mg/L, which reveals that all the groundwater locations are suitable for drinking purposes in the study region (Table 1). The nitrate concentration varies from 3 to 180 mg/L, with a mean value of 31.28 mg/L in the study area. Thus, the results indicated four times higher than the maximum allowable limit for nitrate in the study region. In the context of Indian scenario, groundwater containing higher than 45 mg/L of nitrate can lead to human health problems (BIS 2012). However, Adimalla et al. (2018c) categorized water based on nitrate concentration in groundwater as being low health risk when nitrate values less the 45 mg/L, high health risk when value range from 45 to 100 mg/L, very high health risk when values more than 100 mg/L. In the current study, 81%, 15%, and 4% of groundwater samples fall in low health risk, high health risk, and very high health risk category, respectively (Fig. 2). Therefore, the high concentration of nitrate in drinking water is toxic and causes blue baby syndrome/methemoglobinemia in children and gastric carcinomas (Adimalla and Qian 2019b; Adimalla 2019; Adimalla and Li 2018).

Fig. 2
figure 2

Distribution of nitrate and its health risk classification in the study area

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Fluoride geochemistry

Fluorine is the most electronegative of all elements and occurs primarily as a negatively charged ion in water (Hem 1991; Adimalla et al. 2018a, c). Intake of small concentration is beneficial for human health. However, in the present study area, the concentration of fluoride ranged from 0.4 to 5.8 mg/L with a mean of 1.98 mg/L (Tables 1 and 2), which indicates that the concentration of fluoride is not uniform in the study area. For example, the fluoride content varies from 0.4 to 1.0 mg/L in 13% of the groundwater samples, 1.1 to 1.5 mg/L in 49% of the groundwater; 1.6 to 2.9 mg/L in 23% of the groundwater; and 3.2 to 5.8 mg/L in 21% of the groundwater samples in the study region (Tables 1 and 2) and it also confirmed from the distribution map of fluoride (Fig. 3). It is clear from the distribution map of fluoride that except for extreme northern and southern parts of the study area, all other areas have excess fluoride (Fig. 3). The highest fluoride concentration is noticed in the west-central part of the area. However, fluoride can gain entry into the human body through different routes, probable transmission routes, and its health effects are shown in Fig. 4. WHO (2004) has prescribed the desirable and maximum allowable limits of fluoride as 1.0 and 1.5 mg/L, respectively, in drinking water. Moreover, United States Public Health Services (US PHS 1987) observed that the recommended limit of fluoride in the drinking water majorly relies on the climatic conditions of the region. Therefore, in the Indian context, the desirable and maximum allowable limit of fluoride in drinking water is fixed as 0.6 to 1.2 mg/L, respectively (BIS 2012). If fluoride concentration less than 0.6 mg/L can cause dental caries, while higher than 1.2 mg/L leads to fluorosis. Based on these limits, it is noticed that the fluoride concentration of groundwater in the study region is 4.83 times higher than the 1.2 mg/L (BIS 2012). However, Dissanayake (1991) categorized drinking water based on fluoride classification as being conducive to dental caries when fluoride concentration is less than 0.5 mg/L, promotes development of strong bones, when the concentration ranged between 0.5 and 1.5 mg/L, promotes dental fluorosis in children, when fluoride concentration ranged from 1.5 to 4.0 mg/L, and promotes dental and skeletal fluorosis when its concentration is greater than 4.0 mg/L, respectively. As shown in Fig. 5, fluoride classification, 1.6% and 54.4% of groundwater samples are conducive to dental caries and promote the development of strong bones and teeth category, respectively, while 39.1% and 4.9% of groundwater samples promote dental fluorosis in children and promote dental and skeletal fluorosis category, respectively (Fig. 5, Table 3).

Fig. 3
figure 3

Distribution of fluoride concentration in the groundwater of the study area

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Fig. 4
figure 4

Probable transmission routes of fluoride ingestion

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Fig. 5
figure 5

Fluoride classification in the groundwater of Markapur region, India

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Table 3 Effects of fluoride ingestion on human health Dissanayake (1991)
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Correlation of fluoride with other parameters

For understanding the relation between fluoride and other chemical parameters, correlation plots provide significant geochemical information and also help to know the controlling factors and its mechanism of fluoride enrichment in the groundwater (Adimalla et al. 2018a; Wu et al. 2019). A moderate positive correlation is observed between fluoride and pH (Fig. 6a) which indicates that the higher alkaline nature of water accelerates the enrichment of fluoride concentration and thus typically affects the concentration of fluoride in the groundwater (Narsimha and Sudarshan 2017a, b, 2018a, b; Narsimha and Rajitha 2018). A significant positive correlation is noticed between fluoride with bicarbonate (Fig. 6b) and also fluoride with sodium (Fig. 6c), which declares that the alkaline environment is the dominant controlling chemical mechanism for leaching of fluoride from the fluoride-bearing minerals in the groundwater of the study region (Adimalla et al. 2018a; Li et al. 2018; Subbarao et al. 2017; Ayoob and Gupta 2006). Moreover, as shown in Fig. 6d, the correlation of fluoride and calcium, this clearly indicates that the presence of high calcium content favored low fluoride concentration in the groundwater. It is observed that the major role of precipitation process that is a vital mechanism for enhancement of fluoride occurrence in groundwater (Adimalla et al. 2018a; Narsimha and Sudarshan 2017b, 2018a, b; Li et al. 2018) and negative relations of fluoride with magnesium (Supplementary Fig. 1) is an agreement as established by Reddy et al. 2016. A number of studies have demonstrated that the positive correlations between fluoride and both pH, bicarbonate and sodium typically accelerate the fluoride concentration in groundwater as well as an inverse relationship between fluoride and calcium, which reveals the concentration in fluorite saturated groundwater (Adimalla and Venkatayogi 2017; Adimalla et al. 2018a, b; Ayoob and Gupta 2006; Narsimha and Sudarshan 2017a, b; Narsimha and Rajitha 2018). However, Fig. 7a, b reveals that the high concentration of EC and TDS is always linked with greater fluoride concentration and the similar observations noticed in different regions (Adimalla et al. 2018a; Subba rao et al. 2015; Adimalla and Venkatayogi 2018, 2017; Narsimha 2018; Narsimha and Sudarshan 2013). Moreover, EC and TDS correlation with fluoride is not as high as pH, bicarbonate, and sodium in the study area, which divulges that a higher affinity of fluoride with pH and bicarbonate rather than EC and TDS. However, there is no significant relationship existed between fluoride and SO42−, Cl−, and K+ (Supplementary Fig 1), which revealed that the ionic concentrations added into groundwater is not from similar sources. Furthermore, it is indicated that no considerable relation exists between F− and NO3− (Fig. 8) as these elements are added to groundwater from dissimilar sources, F− from geogenic and NO3− from anthropogenic sources.

Fig. 6
figure 6

Correlation between a fluoride and pH, b fluoride and bicarbonate, c fluoride and sodium, and d fluoride and calcium in the groundwater of Markapur, India

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Fig. 7
figure 7

Correlation between a fluoride and EC, and b fluoride and TDS in the groundwater of Markapur, India

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Fig. 8
figure 8

Correlation between fluoride and nitrate in the groundwater of Markapur, India

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Groundwater types

Piper Trilinear diagram (Piper 1953) is one of the most widely applied methods for characterizing groundwater type. Therefore, analysis data were plotted on Piper (Piper 1953) and depicted in Fig. 9. It categorized the groundwater into four types which are (1) Ca2+–mg2+–HCO3−, (2) Na+–K+–HCO3−, (3) Ca2+–mg2+–Cl−–SO42−, and (4) Na+–K+–Cl−–SO42− (Li et al. 2016a, b). Results are shown in Fig. 9 and observed that most of the groundwater samples fall in the area Ca2+–mg2+–HCO3− (54%), Na+–K+–HCO3− (34%), and Ca2+–mg2+–Cl−–SO42− (15%) which are the major water types, and few groundwater samples are plotted in Na+–K+–Cl−–SO42− (7%). However, as discussed above, fluoride is shown significant correlations with bicarbonate and sodium (Fig. 6b, c). Based on this observation, fluoride concentration is quite high in Na+–HCO3− type of water compared with Ca2+–HCO3− or mixed (Ca2+–mg2+–HCO3−)-type water (Fig. 9). Moreover, 34% of groundwater samples are found under Na+–HCO3− or mixed type (Na+–K+–HCO3−) and cause favor to fluoride dissolution in the study region. A number of previous studies suggested that the Na+–HCO3− water type, decreasing calcium concentration, increasing sodium concentration, and neutral-to-alkaline pH nature of water are more favorable for fluoride dissolution process, thereby accelerating the fluoride concentration in groundwater (Rao et al. 2017; Ali et al. 2016; Adimalla et al. 2018a, b; Narsimha and Sudarshan 2017b). Moreover, Na+–HCO3− water type, mainly deeper groundwater, is influenced by the ion exchange process (Saxena and Ahmed 2003).

Fig. 9
figure 9

Hydrogeochemical classification of the groundwater samples from the Markapur region, Prakasam district, Andhra Pradesh, India

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$${\text{CaF}}_{2} + 2{\text{NaHCO}}_{3} \to {\text{CaCO}}_{3} + 2{\text{Na}}^{ + } + 2{\text{F}}^{ - } + {\text{H}}_{2} {\text{O}} + {\text{CO}}_{2}$$
(1)

Therefore, the possible above ion exchange processes can accelerate and influence the fluoride solubility in the groundwater, which chiefly reveals that the presence of excessive Na+–HCO3− in groundwater leads to dissociation activity of fluoride and can be higher in a high alkaline condition (Saxena and Ahmed 2003; Adimalla and Venkatayogi 2017; Adimalla et al. 2018a; Li et al. 2014).

Mechanism of rock dominance

Gibbs (1970) proposed a diagram to understand the relationship of chemical components of waters from their respective aquifer lithologies. The chemical data of groundwater samples from Markapur, South India, were plotted in the Gibbs (1970) diagram (Fig. 10), using Eqs. 2 and 3, where all ionic concentrations are in meq/L.

$${\text{Gibbs ratio-I }}\left( {\text{for anions}} \right) = \frac{{Cl^{ - } }}{{(Cl^{ - } + HCO_{3}^{ - } )}}$$
(2)
$${\text{Gibbs ratio-II }}\left( {\text{for cations}} \right) = \frac{{Na^{ + } }}{{(Na^{ + } + Ca^{2 + } )}}$$
(3)
Fig. 10
figure 10

Gibbs diagram, illustrating the mechanisms controlling the chemistry of groundwater samples from Markapur, Prakasam district, Andhra Pradesh, India

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Gibbs diagram (Fig. 10) revealed that the most of the groundwater samples fall under the rock dominance, which typically originates from the weathering of the bedrocks which is a vital process to accelerate the fluoride concentration in groundwater (Li et al. 2016a, b; Adimalla and Wu 2019) and also (Fig. 10) groundwater samples end up with evaporation process of water chemistry, and finally, none of the sampling points lie in the precipitation dominance (Fig. 10). It mostly comes from igneous and sedimentary rocks in different aquifers, where fluoride-bearing minerals occur. These are the source of fluoride especially in India where longer water–rock interaction with aquifer materials and alkaline water activates leaching process in fractured zones and fluctuation zone (Kumar et al. 2014; Narsimha and Sudarshan 2013, 2017a, b, 2018a, b). Similar investigations using the Gibbs diagram such as Adimalla et al. (2018a) have found that elevated concentrations of fluoride are associated with groundwater that plots in the “rock dominance” field in a Gibbs diagram. Similarly, observations were noticed in Kolar and Tumkur Districts of Karnataka (Mamatha and Rao 2010), Chirala and Ongole of Andhra Pradesh (Subba Rao et al. 2015; Rao et al. 2017), and Chimakurthy pluton, Prakasam District, Andhra Pradesh (Reddy et al. 2016). However, it is also supported by the existing negative correlation between fluoride and nitrate (Fig. 8), which indicates that the elevated fluoride comes through the water–rock interaction, not by the anthropogenic sources.

Conclusions

Geochemical behavior and controlling factors of fluoride in groundwater of Markapur area, Andhra Pradesh, are studied and assessed. The following conclusions were drawn:

  • The fluoride concentrations varied from 0.4 to 5.8 mg/L, and it is clear that the level of fluoride is higher in 54 groundwater locations than that of recommended upper limit by WHO and by Bureau of Indian Standards.

  • Evaluation of the hydrogeochemical facies in the groundwater revealed that 54% and 34% of samples belong to the Ca2+–HCO3− or mixed Ca2+–Mg2+–HCO3−, Na+–HCO3− water types, whereas high fluoride concentration belongs to majorly Na+–HCO3− water type and relatively low concentrations in Na+–K+–Cl−–SO42− water type in Markapur region.

  • Furthermore, rock–water interaction, mixing process of aquifer, and dissolution of minerals have affected the hydrogeochemical characteristics. Therefore, the reason of high fluoride in the study region is due to rock–water interactions, deficiency of calcium, and alkaline nature of water.

  • Most of the people in the area depend on groundwater for their daily needs, without any primary treatment and actually they do not know much about the quality of groundwater and hence, people suffer from the dental and bone fluorosis in their lifetime.

  • It is recommended to avoid high fluoride groundwater and depend on alternate sources. It is suggested that rainwater harvesting is one of the ways to dilute the fluoride concentrations in groundwater and helps to avoid excessive fluoride intake in the present study region.