Assessment of fluoride contamination and distribution: a case study from a rural part of Andhra Pradesh, India
In total, 123 groundwater samples were collected to evaluate the suitability for drinking purpose in a rural part of Andhra Pradesh, India. The groundwater is alkaline in nature and pH varying from 7.18 to 9.32 with a mean value of 8.36. The hydrogeochemical analysis reveals that the fluoride concentration varies from 0.4 to 5.8 mg/L with a mean of 1.98 mg/L. Higher fluoride concentration is found in west-central parts of Markapur region. The villagers have been exposed to the intake of high fluoride-bearing groundwater for the prolonged period and suffering from the deadly disease fluorosis. However, with respect to groundwater chemistry, the fluoride concentration is high in Na+-HCO3−-type groundwater and low in Ca2+-HCO3−-type groundwater in the Markapur region. Data plotted in Gibbs diagram show that all groundwater samples fall under rock weathering dominance group with a trend toward the evaporation dominance category. Therefore, rock-water interaction is the primary cause of elevated fluoride in the groundwater of the study region. Furthermore, a significant positive correlation exists between F− and pH, HCO 3 - as well as negative correlation exists between F- and Ca2+ and NO3−, which supports that the alkaline nature of water is the main cause for dissolving fluoride-bearing minerals.
Fluoride contamination Groundwater Geochemical behavior Rural area Andhra Pradesh India
Electronic supplementary material
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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.
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).
Open image in new windowFig. 1
Location map of the groundwater samples from the Markapur region, India
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).