Introduction

Mining activity results in the creation of a lot of inactive/abandoned mine lands (AML). Groundwater quality and public health and safety concerns are the most common environmental issues from inactive mines in several countries with a long history of mining like India (Mhlongo et al. 2013). Due to the groundwater contamination, toxic ions are of a huge health problem noticed in inactive/abandoned mines, especially in India. Toxic elements like fluoride & nitrate are released into groundwater and can be accumulation in crops and may cause significant effect on human health (Davies 1983). Of this, the most common environmental issue related to the inactive/abandoned mines in several countries mainly human health (Sphiwe and Francis 2015). Quality of groundwater is of major concern in deciding the applicability of water quality evolution for drinking purposes, and it must be devoid of harmful elements, living & non-living organisms in groundwater, excessive mineral dissolution which may cause an impact on human health (Muralidhara Reddy et al. 2013). Geogenic and anthropogenic sources from agriculture practices and dissolution of rock and mine water pollution contaminated soil mainly affects on groundwater quality (Guo et al. 2007; Brindha and Elango 2013; Adimalla et al. 2018; Adimalla and Venkatayogi 2018; Li 2016; Wu et al. 2017; Karunaidhi et al. 2019). Freshwater has become a scanty commodity due to overuse, and water is getting contaminated. Groundwater is widely used for drinking, agriculture, and industrial purposes (Adimalla and Wu 2019; Wu and Sun 2016). Over around world application of groundwater for drinking and irrigation purpose in the arid and semiarid region is increasing considerably, and 1.5 billion people rely on groundwater for irrigation and drinking purpose (Li and Qian 2018; Adimalla et al. 2019c; Qian and Li 2011; Qian et al. 2012; Adimalla and Qian 2019b; Wu et al. 2014, 2017; Zhang et al. 2018). In India, especially in Andhra Pradesh, numerous studies have been carried out on assessment of groundwater quality for drinking and irrigation purpose, such as (Sunitha and Sudharshan Reddy 2019; Subba Rao et al. 2015, 2016; Adimalla and Wu 2019; Sudharshan Reddy et al. 2018; Muralidhara Reddy et al. 2019) and 90% of the Urban and rural areas depend purely on groundwater for drinking domestic purposes. For this water quality index (WQI), application is the very use full method to assess the suitability of water for various purposes and gives corresponding details on water quality for common people and policymakers to manage water quality. It represents the overall quality of the water in terms of index numbers and provides water quality information in a single value (Aminiyan et al. 2018). The water quality index method classified two different ways such as physicochemical and biological indices. The present attempt has been made to compute WQI based on physicochemical data. WQI is stated as a reflection and rating by the impact of various water quality parameters (Bouderbala 2017; Chaturvedi and Bassin 2010). Most of the researchers use these indices for suitability by human consumption. Hence, to evaluate the groundwater quantity and quality and providing a database are very important for planning and development of water management techniques.

Globally, for two decades, groundwater contamination by fluoride and nitrate is of major public health concern. Because of its importance, these studies have been carried out in arid and semiarid regions worldwide (Adimalla and Qian 2019b; Adimalla 2019; Wu et al. 2015; Ghadepoori et al. 2018; Jafari et al. 2014; Massoudinejad et al. 2016). Groundwater contains a huge amount of dissolved ions exceeding the permissible limit it becomes toxic. Though fluoride is required for normal mineralization of bones and formation of dental enamel in small quantities, when consumed in higher doses (> 1.5 mg/L), it causes dental fluorosis or mottled enamel and higher concentration (> 3.0 mg/L) of fluoride may cause skeletal fluorosis (Sunitha and Sudharshan Reddy 2019). The higher concentration of fluoride is of major health implication in many countries. Higher fluoride concentrations are noted in more than 23 countries in the world, particularly in parts of India, China, Central Africa, and South America (Susheela 1993; Shahriari et al. 2010). So many groundwater quality studies regarding fluoride have been taken up in arid and semiarid regions in the world (Subba Rao 2017, 2018; Adimalla and Li 2019; Adimalla 2020c). Fluoride concentration relies on various factors such as accessibility of circulating water to fluoride-bearing minerals like calcium fluoride (CaF2), apatite, or rock phosphate [Ca3F (PO4)3] and cryolite (Hem 1991; Adimalla 2020c; Adimalla et al. 2019b). The content of nitrates in groundwater is increasing in India. Higher nitrate concentrations in drinking water cause a health impact on the human body like methemoglobinemia in infants and stomach cancer in adults (Bulusu and Pande 1990). As such, the United States Environmental Protection Agency (USEPA) has demarcated a maximum contaminant level (MCL) of 10 mg/l NO3-N (50 mg/l NO3) (USEPA 2014), and also some research studies show that irrigation activates are the major cause for increasing nitrates in groundwater (Rezaei et al. 2018; Mirzaei et al. 2015; Adimalla et al. 2018). Many parts of states in India like Andhra Pradesh, Bihar, Delhi, Haryana, Himachal Pradesh, Karnataka, Kerala, Madhya Pradesh, Maharashtra, Orissa, Punjab, Tamil Nadu, Rajasthan, West Bengal, and Uttar Pradesh contain higher nitrate concentration in groundwater (more than 45 ppm) (CGWB 2013).

Groundwater is primarily used for drinking, agricultural needs in the semiarid region of Kadapa District. Higher concentrations of fluoride and nitrate have been observed in various parts of the Kadapa District (Sunitha and Sudharshan Reddy 2019). The groundwater of this region contains more fluoride and nitrate which is exceeding the W.H.O. permissible limit compare to other ions for drinking purposes. However, comprehensive health risk assessment studies have not yet been studied in this region. However, considering this factor and keeping these views in mind of the importance of public health to understand the status of the groundwater quality in the Vemula-Vempalli region, hence the study was mainly intended to assess groundwater quality for drinking purposes using the WQI method and its health implications around inactive mines in Vemula-Vempalli region, Kadapa District, South India. This study will help in making better suitable groundwater management studies in the Vemula-Vempalli region.

Study area

The study region (408.6 km2) is located in the southwestern part of Kadapa District, A.P., and situated between latitude 14°18′00″ N; longitude 78°28′ 30″ E falls in Toposheet No. 57J/7(Fig. 1). The whole study area is characterized by quartzite–carbonate shale cycles having an aggregate thickness vary between 6 and 12 km. Barytes and Yellow ochre are important inactive mines noticed in this area; road metal quarries are also observed. Geological study area belongs to Papagni and Chitravati groups of Lower Cuddapah Supergroup (Nagaraja Rao et al. 1987). The rocks have southern dips in the northern portion becoming easterly in the midsection and gradually changing to northeasterly in the south with some local variations. The dip of the Papagni series of beds is gently ranging from 8° to 30°. The study area contains basic flows along with igneous rocks remarkably followed by bedding planes like dolerite, quartzite, dolomite, quartzite with conglomerates, and chert. The Pulivendula quartzites occur as comparatively narrow bands separating the Vempalli limestone and shale from the overlying Tadipatri shale (Fig. 2). Important geomorphic units are denudational hills, pediment and pediplain, structural hills. Paddy, groundnut, sunflower, and sweet lime are important crops. Apart from the banana plantation, seasonal fruits (Sweet lime, lemon) are also grown. Drip irrigation is applied in this region particularly for sweet lime. This area experiences arid to semiarid climate. The average annual rainfall is 600–650 mm and also experiences the effect of SW and NE monsoons. Groundwater is the only source for drinking and irrigation purposes (Geological Survey of India 2001). Temperature varies from 43.2 °C (April) to 20.4 °C (December). The depth of water level varies between 10 and 20 m (CGWB 2007, 2013). Groundwater occurs in joints and fractures with weathered zones of Papagni and Chitravathi group of rocks. Massive limestone and gulcheru quartzite formations act as good aquifers. The depth of the water table in the study area ranges from 10 to 30 m.

Fig. 1
figure 1

Location map of study area

Full size image
Fig. 2
figure 2

Geology map of the study area

Full size image

Methodology

Experimental methods, sampling, and analysis

Forty groundwater samples were collected from 27 villages around inactive mining areas in September 2018. Depth of bore wells varied between 20 m and 60 m below the ground level. Samples were collected in polyethylene bottles. pH, electrical conductivity (EC), and total dissolved solids (TDS) were analyzed during fieldwork using water analyzer 371 field kit; major cations such as calcium (Ca2+) and magnesium (Mg2+) were determined by titrimetry, while Na+ and K+ are determined by flame photometry (Systronic Model No. 128). Major anions like sulfate (SO42−) estimated using a flame photometer, chloride (Cl−) determined by titrimetry, fluoride (F−) estimated using Orion 4—star ion meter (Model: pH/ISE), and nitrate (NO3−) are determined by using spectrophotometer following APHA (2012) standard methods, and suggested precautions were taken to prevent contamination (Hem 1991; Raghunath 1987). A Global Positioning System (GPS) instrument is used for collecting samples. Geographical information system is the very effective tool for analyzing and displaying analytical data and using spatial data for decision-making in several monitoring areas like environmental and engineering fields. Spatial distribution maps were prepared by using the interpolation method in Arc GIS 10.3 software. The inverse distance weight (IDW) method is followed to understand groundwater quality. The spatial distribution of fluoride and nitrate ions in groundwater was demarcated in the maps. WQI was calculated by World Health Organization standards and Indian Standards methods, and Health Risk Assessment has been computed followed by the USEPA guidelines, respectively. The relative weight method was applied for evaluating groundwater quality in the study area (Brown et al. 1970).

Water quality index method

The WQI calculations include three consecutive steps. The first step is “weight assigning,” and each of the 14 parameters has been given a weight (wi) based on its relative importance to the overall quality of drinking water as denoted in Table 1. The second step is the “calculation of relative weight” by the equation given below:

$${\text{Wi}} = {{\text{wi}} \mathord{\left/ {\vphantom {{\text{wi}} {\mathop \sum \limits_{i = 1}^{n} {\text{wi}}}}} \right. \kern-0pt} {\mathop \sum \limits_{i = 1}^{n} {\text{wi}}}}$$
(1)
Table 1 Relative weights for each parameter
Full size table

The third step is a “rating of quality (qi)” determined by the given equation

$${\text{qi}} = \left( {\frac{{\text{Ci}}}{{\text{Si}}}} \right) \times 100$$
(2)

where Ci is the concentration of each parameter in each water sample and Si is the WHO recommended value for each parameter (Kouadra and Demdoum 2019; Adimalla et al. 2018). Lastly, Wi and qi were used to determine the SIi for individual parameters, and hence, WQI can be determined by the equation given below:

$${\text{SIi}} = {\text{Wi}} \times {\text{qi}}$$
(3)
$${\text{WQI}} = \mathop \sum \limits_{{{\text{i}} = 1}}^{\text{n}} {\text{SIi}}$$
(4)

where SIi is the subindex of each parameter. Relative weight method values are presented in Table 2.

Table 2 Health risk related to fluoride ingestion in individuals.
Full size table

Health risk assessment

Health risk assessment consists of four phases like

(I) Identification of risk, (II) Assessment of Dosage, (III) Exposure valuation, and (IV) Hazard characterization are recommended US environmental protection agency (Narsimha and Rajitha 2018; Li and Qian 2018).

In this work F−, NO3− concentration in groundwater was selected as the parameter for the evolution of human health risk assessment (USEPA 2014). To determine exposure dose ADD through ingestion and potential non-carcinogenic (HQ)

$${\text{ADD}}=\frac{{\text{CPW}}\times {\text{IR}}\times {\text{ED}}\times {\text{EF}}}{{\text{ABW}}\times {\text{AET}}}$$
(5)

where ADD indicates F−/NO3− intake (mg/kg/day), CPW indicates specific groundwater pollutant (mg/L), IR denotes the rate of ingestion per year (2.5 L/day adults; 0.78 L/day for children; 0.3 L/day infants) (Narsimha and Rajitha 2018), ED indicates the duration of exposure (Years 64,12 and 0.25 for adults, children, infants, respectively) (Ahada and Suthar 2017), EC indicates the frequency of exposure (days/year, 365 for adults, children, infants), ABW denotes average body weight of the human body in kg [57.5 (adults), 18.7 (Children), and 16.9 (infants)], and AET denotes average exposure time (Days, 23,360 adults, 4380 children’s and 365 infants).

Hazard quotient appears if the pollutant exposure dose exceeds the reference dose toxicity effects may appear. This is commonly referred to as the hazard coefficient (HQ). RfD referred to as exposure dosage of F−&NO3− is 0.4 and 1.6 mg/kg/L, respectively (USEPA 2012). HQ is determined by the following equation.

$${\text{HQ}}=\frac{{\text{ADD}}}{{\text{RfD}}}$$
(6)

HQ values < 1 indicate that there is no carcinogenic effect, while HQ values > 1 is treated as the unacceptable risk of non-carcinogenic effects on human health (USEPA 2014; Li and Qian 2018).

The non-carcinogenic risk total hazard index (THI) is determined by applying Eq: 3 in which maximum permissible cutoff for non-carcinogenic THI is (USEPA 2014).

$${\text{THI}}=\sum_{i=1}^{n}\,\text{HQi}$$
(7)

Analytical accuracy/ion balance

The accuracy of geochemical analysis was assed using the relationship between the concentration of total cations such as Na+, K+, Ca2+, Mg2+ and the concentration of total anions like CO32−, HCO3−, Cl−, SO42−, NO3−, and F− denoted in mill equivalent per liter (meq/L) for each sample, and ionic balance error (IBE) was computed to collaborate the analytical accuracy by the equation shown below:

$${\text{IBE}}=\frac{\sum {\text{Cations}}-\sum {\text{Anions}}}{\sum {\text{Cations}}+\sum {\text{Anions}}}\times 100$$
(8)

The result of ionic balance error (IBE) was within the acceptable limit of ± 3% (Domenico and Schwartz 1990).

Results and discussions

The results of chemical analysis and its statistical data are denoted in Tables 3 and 4. Groundwater samples were analyzed for various ionic and non-ionic parameters to assess the quality. The results are examined in compliance with the Bureau of Indian Standards BIS (2012), and WHO (2012) drinking water standards are presented in Table 4.

Table 3 Physicochemical parameters concentrations of the study area
Full size table
Table 4 Statistical data of physicochemical parameters of groundwater
Full size table

Major ion chemistry

pH of groundwater ranges from 7.05 to 8.97, indicating that alkaline nature may be due to precipitation & dissolution process of groundwater (Wagh et al. 2019, 2020). The permissible limit of pH value for drinking water is specified as 6.5–8.5 (WHO 2012). Most of the groundwater is within the permissible limit of pH. Though higher pH could not affect human health, it may affect the taste of drinking water. Electrical conductivity (EC) ranges from 369 to 2590 µS/cm. EC of groundwater can be classified into three categories as (1) class I low salt concentration: EC < 1500 µS/cm, (2) class II average salt concentration: EC 1500–3000 µS/cm, and (3) class III high salt concentration: EC > 3000 µS/cm (Subba Rao et al. 2012). According to this classification of EC, 77.5% of the samples belong to class I (low salt concentration), and 22.5% of samples fall under class II (average salt concentration). Higher EC in groundwater may be due to evaporation and anthropogenic processes. Total dissolved solids (TDS) vary between 188 and 1340 mg/L with a mean of 1232 mg/L. Most of the samples are within the permissible limit of TDS. Generally, higher TDS values in groundwater are due to salt leaching from soils and mining and agricultural activities (Kadam et al. 2019). TH values vary from 100 to 520 mg/L in the study area. The permissible limit of TH for drinking purposes is 500 mg/L (WHO 2012; BIS 2012), and only one sample is exceeding the permissible limit. However, according to Sawyer and McCarthy (1967), TH: < 75 belong to soft category; 75–150 moderately hard; 150–300 hard; and > 300 very hard category. As per this classification 7.5% of the samples belong to the moderately hard category, 67.5% samples are of hard category and 25% of samples belong to the hard categories.

The chemistry of major ions helps to understand the hydrogeochemical process of the groundwater. In the study region, calcium (Ca2+) and magnesium (Mg2+) values in groundwater range from 8.8 to 88.5 and 9.7 to 122 mg/L, respectively. All groundwater samples are within the permissible limit of calcium (> 75 mg/L) and magnesium (> 50 mg/L) (WHO 2012; BIS 2012). Sodium (Na+) concentration ranges from 24.9 to 429 mg/L. Only two samples are exceeding the permissible limit of Na+ (> 600 mg/L) (Table 4). The high Na+ ion may be due to cation exchange and anthropogenic activities. Potassium (K+) concentration ranges between 0.780 and 12.3 mg/L (Kadam et al. 2019). Only three samples are exceeding the permissible limit of K+ (> 10 mg/L) (W.H.O 2012; BIS 2012) (Table 4). High potassium content in groundwater naturally occurs from geological sources of potassium or weathering of silicate minerals, and usage of K+ rich fertilizers (Hem 1985).

Chloride (Cl−) concentration ranges from 21.3 to 454 mg/L. Bicarbonate (HCO3−) concentration ranges between 24.4 and 366 mg/L. All groundwater samples are below the permissible limit of bicarbonate (600 mg/L) and chloride (1000 mg/L) as per WHO 2012 and BIS 2012. Sulfate (SO42−) concentration ranges from 10.684 to 224 mg/L. All groundwater samples are within the permissible limit of 400 mg/L suggested by BIS 2012.

Nitrates in groundwater

Agriculture and livestock production accounts for nearly 80% of all the nitrogen added to the environment. Nitrate contamination in groundwater is primarily due to unrestricted usage of N-fertilizer, the unavailability of established soil and water management methods, septic tanks, and inappropriate removal of domestic wastes (Canter 1996; Adimalla 2020b). Nitrate concentration in the study region ranges from 0.143 to 269 mg/L. 62.5% of samples are above the permissible of nitrate (> 45 mg/L) (WHO 2012; BIS 2012) (Table 4). Children and young livestock are more prone to nitrate contamination in drinking water. Nitrates in groundwater cause reducing oxygen-carrying capacity in the blood; it can lead to methemoglobinemia (blue-baby syndrome) which is the deadliest effect of nitrate exposure, and there are other serious chronic health impacts like thyroid enlargement, 15 types of cancers, birth defects, hypertension, etc. (Forman et al. 1985; Wagh et al. 2019). In the different processes of the nitrogen cycle, nitrite and nitrates are produced (Ekemen Keskin 2010). NO3− is one of the predominant ions in oxygenated water because it had rapid oxidation property. Usually, nitrate found in inorganic fertilizers by this nitrate occurrence in groundwater is very low. If nitrates are found higher concentration in groundwater because the anthropogenic activates the result of leaching agricultural waste & contamination human and animal waste and leakage of septic tank (Adimalla and Qian 2019a; Karunaidhi et al. 2019; Adimalla 2020b), most of the study areas are occupied by gravelly loam and clay soils and cracking clay soils, which possess a low permeable property that makes nitrate infiltration process very slow (CGWB 2016; UDA 2007). Apart from agricultural practices, a higher concentration of nitrates is leached into groundwater in the study area may be due to pollution by inactive/abandoned mine impoundment. In the study area, inactive/abandoned mines are present nearby inhabitant’s areas apart from agricultural land areas. Due to this, maximum of the domestic waste and agriculture waste (fertilizer, pesticides, etc.) are dumped into the inactive/abandoned mining areas. Nitrogen and potassium are key elements added as fertilizer in enhancing the yielding capacity of banana and lime crops. In this region, application of nitrate and potassium fertilizers throughout crop period for banana and lime cultivation for every 2–3 weeks is 200 kg/h and 52 kg/h, respectively (Al-Harthi  and Al-Yahyal 2009). Due to heavy rainfall in the post-monsoon period, agriculture wastewater will be flown into the inactive mine impoundment. Poultry waste disposal and household/cultivate animal dung fertilizers which were used in agriculture utility are deposited in inactive mine impoundment that reach the groundwater through the infiltration and return flow of irrigation for drinking purposes (Abdesselam et al. 2016; Ako et al. 2014; Raju et al. 2009a, b; Lokesh 2013; Oenema et al. 2005; Adimalla and Taloor 2020). As the majority of the people in this region depend on agricultural practices for their livelihood, the application of fertilizers for crop yields might also be the chief contributing factor for high nitrate concentration. From the spatial distribution map (Fig. 3), it is clear that intensive farming of banana and sweet lime which are located nearby inactive mining areas might have contributed most of the nitrate concentrations in this region. Intensive agricultural practices and human and animal waste disposed of into inactive mine impoundments are the main factors responsible for higher nitrate in the study area.

Fig. 3
figure 3

Spatial distribution map of nitrate contamination areas

Full size image

Fluoride ion chemistry in groundwater

Fluoride concentration in this region varies from 0.24 to 3.0 mg/L. 60% of samples is exceeding the permissible limit of fluoride (1.5 mg/L). Based on permissible limit, fluoride ion groundwater is classified into four classes: Class I: conductive to dental caries (> 0.5), Class II: enhances strengthening of bones and teeth (1.5); Class III: dental fluorosis (1.5–4), Class IV: dental and skeletal fluorosis (4–10); and Class V: crippling fluorosis (> 10) from this classification; 40% of the samples are fall in class III category in the study region (Table 2) (WHO 2012; BIS 2012). Groundwater is the only reliable source of drinking in this region. The correlation coefficient of fluoride with other parameters was analyzed. Fluoride is showing positive correlation with pH, while the positive correlation of fluoride with Na+, K+, HCO3− reveals that alkalinity of groundwater with high bicarbonate concentration can increase fluorite solubility in this region (Fig. 4a–e). As the ionic radius of fluoride ion is almost the same as that of hydroxide ion and the two ions commonly substitute one another in the crystal structure, F− has a similar charge and about a similar range as of the hydroxide particles and form complexes with minerals of low solubility (Hem 1985). According to Brown and Roberson (1977), fluoride ions are easily replaced with OH− in many rock-forming minerals. Some mineral surfaces may be capable of absorbing anions and if such surface carried fluoride ions, they could perhaps be available for release by substitution of OH− ions from the water of high pH (alkaline). This indicates that the concentration of fluoride may be more in the waters of high pH (Hem 1985). Therefore, during chemical reaction fluoride can easily replace OH− ions in many rock-forming minerals. Fluoride solubility in groundwater differs from one rock formation to another. Alkaline pH increases fluoride dissolution activity (Hem 1985; Vikas et al. 2009), whereas in acidic medium fluoride is adsorbed in clays (Saxena and Ahmed 2003; Rizwan and Gurdeep 2013). As most of the samples are alkaline in nature and positive correlation of fluoride with pH may favor the solubility of fluorine-bearing minerals in this region, fluorite, apatite, mica, amphiboles, certain clays, and williamite have the greatest effect on the hydrogeochemistry of fluoride (Raju et al. 2009a, b; Rango et al. 2008). Overall water quality (e.g., pH, alkalinity, hardness, and ionic strength) also plays an important role by influencing mineral solubility, complexation, and sorption/exchange reactions (Apambire et al. 1997; Adimalla and Venkatayogi 2017; Meenakshi et al. 2004; Raju et al. 2009a, b). Higher fluoride in groundwater of this region may due to basic flows along with igneous formation like dolerites and carbonate minerals (Brindha et al. 2011; Brindha and Elango 2013) Hence, geogenic forces are mainly responsible for high fluoride in groundwater because of ion-exchange reaction processes, water–rock interactions, and influencing mineral solubility factors in the study area. In the chemical weathering process, dissolution of F− ions is chiefly governed by Ca2+ ion driven by thermodynamic principles. Water on reaction with varying carbonate and other minerals plays a significant role in the evolution of groundwater chemistry and hence water–rock interaction that emerges as its characteristic chemistry. Though silicate minerals do not readily interact with groundwater, however, carbonate minerals chiefly occurring in sedimentary rocks, traces in igneous and metamorphic minerals are important in the evolution of most groundwater.

Fig. 4
figure 4

Fluoride correlation with other physicochemical parameters

Full size image
$${\text{CaCO}}_{ 3} + {\text{CO}}_{{2({\text{gass}})}} + {\text{H}}_{2} {\text{O}}_{{({\text{liquid}})}} \to {\text{Ca}}^{2 + } + 2{\text{HCO}}_{3}^{ - }$$
(9)

From this, solubility of fluorite can be expressed as (Brown and Roberson 1977)

$${\text{CaCO}}_{3} + {\text{H}}^{ + } \to {\text{Ca}}^{2 + } + {\text{HCO}}_{3}^{ - }$$
(10)
$${\text{CaF}} \leftrightarrow {\text{Ca}}^{2} + 2{\text{F}}^{ - }$$
(11)

Further reaction between calcite and fluoride ions contact with groundwater media resulted in equilibrium reactions of both solid and solute species. Saturation of groundwater concerning Ca2+ and F− is given below (Prajapati et al. 2017).

$${\text{CaCO}}_{ 3} + 2{\text{F}}^{ - } + {\text{H}}^{ + } = {\text{CaF}}_{2} + {\text{HCO}}_{3}^{ - }$$
(12)

The above equations reveal that groundwater having a high percentage of fluoride when it was mostly alkaline nature or residual alkalinity nature. Now it is clear that if the pH of the groundwater is constant, fluoride activity is directly proportional to bicarbonate abundance. Presently, groundwater of the study area had mostly alkaline nature; Fig. 3a shows a slightly positive correlation between fluoride and potential hydrogen. It is clear that potential hydrogen is one of the causes to be found higher fluoride in the study area, while some samples in the study may have low fluoride concentrations because the study area under the part of the semiarid environment and solubility process of F− in groundwater vary from one rock formation to another. If pressure and temperature was under normal conditions, fluoride-bearing minerals have been sparingly soluble in water; under certain physicochemical conditions, dissolution processes are too fast (Prajapati et al. 2017; Adimalla et al. 2019d; Vikas et al. 2009; Adimalla 2020c). In the semiarid region, soil plays a major role in weathering and aqueous leaching processes that determine the amount of F− in groundwater. Extensive irrigational practices and fertilizer usages are also a cause of higher fluoride concentration in groundwater. Generally, potash fertilizer had higher content of fluoride of 13–77 mg/kg; long-term usage of potash fertilizer (52 kg per hector) throughout the year adds up to fluoride in the groundwater of the study region (Brindha et al. 2011, Brindha and Elango 2013). Groundwater fluctuations and well depth also play an important role in creating a possible environment for the dissolution process below the earth’s surface. Figure 5 represents that depth versus fluoride gives a clear idea on fluoride enhancing along with depth which indicates the source of fluoride to be fluoride minerals in the basement (Sunitha and Muralidhara Reddy 2018). From the spatial distribution map (Fig. 6), green color indicates the groundwater samples in villages that are within the standard permissible limit for drinking (< 1.5 mg/L) and red color denotes the groundwater samples in villages exceeding the permissible limits for drinking water (> 1.5 mg/L) according to WHO guidelines. North and middle portions of the study area high fluoride concentration (Fig. 6) indicate the release of fluoride due to water–rock interaction of basic flows and carbonate rocks. According to Dissanayake (1991), as per the health risk related to fluoride ingestion in human beings, 40% of samples belong to class III category (Table 2) are more prone to dental fluorosis.

Fig. 5
figure 5

Bar chart of F− versus depth

Full size image
Fig. 6
figure 6

Spatial distribution map of F− contamination areas

Full size image

Hydrogeochemical facies

Piper diagram (1944) was a very effective tool to determine the hydrogeochemical facies of groundwater. It denotes the classification of water type, identification of hydrogeochemical facies, and mixing of two samples. Piper diagram includes two triangles and one diamond-shaped area. In this, one of them shows cations and another shows anion and remaining diamond-shaped area shows both anions and cations. According to this piper diagram, groundwater of this study group is six types. (I) Ca2+–HCO3−, (II) Na+–Cl−, (III) mixed Ca2+–Na+–HCO3−,(IV) mixed Ca2+–Mg2+–Cl−, (V) Ca2+–Cl−, and (VI) Na+–HCO3− (Adimalla et al. 2019d). From the diamond-shaped triangular diagram, 57% of the samples belong to Ca–Mg–HCO3 type and remaining 42% of the samples belong to mixed Ca–Mg–Cl type reveals that carbonate rock dissolution processes dominate in the groundwater of this region, and from cation triangle zone it is clear that 50% of the sample falls in no dominant zone; another 50% samples fall in magnesium dominant zone; from triangle anionic zone, 65% of the samples fall in bicarbonate zone and 25% samples fall in no dominance zone remaining samples fall in sulfate zone (Fig. 7).

Fig. 7
figure 7

Piper diagram

Full size image

Mechanisms of groundwater chemistry

The mechanism of groundwater chemistry was prepared by Gibbs (1970) for analyzing mechanisms of groundwater chemistry. It reveals the chemical reaction between the composition of water and the controlling mechanism of the natural water chemistry variations using graphical plots. TDS versus Cl−/Cl−+HCO3− and TDS versus (Na+ + K+)/(Na+ + K++ Ca2+) were divided into three zones: precipitation, rock dominance zone, and evaporation dominance zones. Most of the groundwater samples fall in rock dominant zone, and it reveals that water–rock interaction or weathering of rock-forming minerals and also host rock weathering mechanisms predominate in this region (Fig. 8). These mechanisms are mainly responsible for high fluoride in groundwater of the study region (Adimalla 2020c; Adimalla and Venkatayogi 2017).

Fig. 8
figure 8

Mechanism controlling of groundwater chemistry plots (Gibbs 1970) a Cl−/Cl− + HCO3− b (Na+ + K+)/(N+ + K+ + Ca2+)

Full size image

Water Quality Index in groundwater (WQI)

The estimation of groundwater suitability for drinking purposes in the Vemula-Vempalli region is achieved using the relative weight water quality index method. The selection of the physicochemical parameters for calculating the index depends on several factors such as the importance of the parameter and purpose of the index method presented in Table 1 (Drinking or Irrigation) (Aminiyan et al. 2018). The systematic computation of the WQI values varies from 83.7 to 186.1 mg/L (Table 5), and it was categorized as Class I: Good, Class II: Poor, Class III: Very poor, and Class IV: Unsuitable for drinking purposes. Water quality index classification as shown in Table 6, according to this classification, 7.5% of samples are excellent, 85% good, 12.5% poor for drinking purposes (Table 6). Higher WQI in the study area may be due to extensive irrigational practices and extensive groundwater exploitation, and geogenic forces like rock weathering mineral dissolution and also anthropogenic practices are responsible for founding high WQI values in this region. The spatial distribution map of the water quality index is shown in Fig. 9; it depicts that most of the study areas fall into the good category as denoted in green color, and some eastern parts show poor water quality shown in red color in this part may cause extensive irrigational practices and extensive groundwater exploitation main responsible factors (Ewaid and Abed 2017; Abdelkader 2017).

Table 5 WQI at individual sampling station
Full size table
Table 6 Water quality classification based on WQI value
Full size table
Fig. 9
figure 9

Spatial distribution map of water quality Index

Full size image

Human Health Risk Assessment (HHRA) of F− and NO3−

An innovative method developed by USEPA (United Environment Protection Agency) has designed as a modern method to assess human health risk assessment has vide applicability (Adimalla 2020a, b; Adimalla et al. 2019a). The evolution of non-carcinogenic risks due to fluoride and nitrate in groundwater is of greater concern in many countries (Narsimha and Rajitha 2018; Nadia et al. 2015). Continuous consumption of contaminated drinking water affects human health. Fluoride and nitrate contamination in drinking water is of greater public health concern in many regions of the world. These ions are regarded as a non-carcinogenic risk to human health for adults, children, and infants. Eventually, excessive concentration of F− and NO3− in drinking water in the study region poses adverse impact on human health, for which health risk assessment has been employed in the study area. Hence, this study aims in the assessment of human health risk considering drinking water as the main passage. To identify this problem, hazard quotient (HQ) is an essential tool to determine non-carcinogenic health risk by computing-related hazard index (HI) as depicted in Table 7. The higher concentration of fluoride and nitrate ions is observed in Vemula-Vempalli region. HQ values of fluoride in this region range between 0.15 and 1.92 (adults), 0.16 and 1.95 (children), and 0.12 and 1.49 (infants) with an average values of 0.78, 0.79, and 0.60, respectively (Table 7). From the USEPA 2014 guidelines, 30%, 30%, 12.5% of the groundwater are > 1 (Table 8) and may cause adverse health impacts on adults, children, and infants, respectively. HQ values of nitrates in the study area range from 0.003 to 6.46 (adults), 0.003 to 6.55 (children), and 0.003 to 4.99 (infants) with the average value of 1.31, 1.32, and 1.01, respectively (Table  7). 70%, 70%, and 37.5% of the groundwater samples are above the acceptable limit > 1 (Table  8) and may cause a health risk of adults, children, and infants, respectively. According to the USEPA 2014 guidelines, THI values must not be above > 1. If the THI values are above > 1, that shows non-carcinogenic effects on human beings. From the results, it is clear that THI values in this region range from 0.41 to 7.28 (adults), 0.41 to 7.38 (children), and 0.31 to 5.62 (infants) with the average values of 2.06, 2.09, and 1.59, respectively (Table 8). Figure 10 denotes comparison of THI in the studied groups as denoted, the THI in each of the three groups was greater than 1, which has to be taken care of and proper precautionary measures.

Table 7 Computation of hazard quotient for adults, children, and infants
Full size table
Table 8 Assessment of non-carcinogenic risk of adult, children, and infants based on total hazard index
Full size table
Fig. 10
figure 10

Comparison of THI in the studied groups with THI Standard

Full size image

Conclusions

Groundwater is the prime source for drinking and irrigation in the Vemula-Vempalli region, Kadapa District, South India. The study mainly focused on exploring the groundwater quality in the view of drinking purposes.

  • Groundwater is alkaline and hard. Most of the parameters are below the permissible limit of BIS and W.H.O. limits except fluoride and nitrate. Groundwater is not potable in terms of nitrate and fluoride. Based on the WQI classification for drinking purposes, 12.5% of samples are of poor quality for drinking purposes and unhealthy for the domestic purpose in this region. It may be due to a higher amount of hardness and extensive agricultural practices in this region.

  • Around 60% of the groundwater is above the permissible limit (1.5 mg/L) of fluoride in groundwater of the study area. Geogenic forces like water–rock interaction of carbonate rocks and basic flows (Dolerites), mineral dissolution from carbonate rocks, extensive usages of fertilizers, and alkalinity of groundwater may cause higher fluoride concentration in this region, and it has Ca2+–Mg2+–HCO3− water type and rock dominance was favoring the dissolution of fluoride in groundwater and suitable defluoridation techniques are needed to purify this water. Higher nitrate concentrations may be due to extensive irrigation practices runoff from agricultural fields to inactive mine impoundments, and the usage of intensive potash-type fertilizers causes higher nitrate in groundwater. Hence, the source of fluoride and nitrate is geogenic and anthropogenic sources, respectively.

  • Total hazard index values reveals that children are more prone to the risk of fluoride and nitrate than adults.

  • This study will help the policy-makers to adopt proper remedial management techniques like defluoridation, nitrate removal setup (reverse osmosis process and ion exchange), and rainwater harvesting in the study area.