Arab J Geosci DOI 10.1007/s12517-012-0752-x
ORIGINAL PAPER
A study on the high fluoride concentration in the magnesium-rich waters of hard rock aquifer in Krishnagiri district, Tamilnadu, India
S. Manikandan & S. Chidambaram & AL. Ramanathan & M. V. Prasanna & U. Karmegam & C. Singaraja & P. Paramaguru & I. Jainab
Received: 16 February 2012 /Accepted: 31 October 2012 # Saudi Society for Geosciences 2012
Abstract Excess fluoride in groundwater affects the human Keywords Groundwater . Excess fluoride . High health and results in dental and skeletal fluorosis. Higher magnesium water . Weathering concentration of fluoride was noted in hard rock terrain of the south India, in the Krishnagiri district of Tamilnadu. The region has a complex geology ranging from ultra basic to acid Introduction igneous rocks, charnockite and gneissic rocks. Thirty-four groundwater samples were collected from this study area and Water is a prime natural resource and essential for life on analysed for major cations and anions along with fluoride. The earth. About 80 % of the diseases in the world are due to order of dominance of cations is Na+>Mg2+>Ca2+>K+ and the poor quality of drinking water (WHO 1984). The − − − 2− the anions in the following order HCO3 >Cl >NO3 >SO4 . occurrence of fluoride in groundwater is mainly due to It is found that nearly 58 % of the samples have more fluoride natural or geogenic contamination and the source of con- ranging from 1 to 3 mg/L. It is also noted that high fluoride tamination of often complex and site specific (Handa waters correspond to magnesium water types. This is due to the 1975; Saxena and Ahmad 2002). Dissolution of fluoride release of fluoride from the magnesium-bearing minerals like, ions from the fluoride bearing minerals and contributes to biotite, hornblende, etc., or weathering of apatite/hydroxyapa- higher fluoride concentrations in groundwater (Suttie tites found in charnockites. 1969; Jubb et al. 1993; Schultheiss and Godley 1995). Fluoride-rich groundwater has been associated with di- verse geological settings and lithologies. These include : : : S. Manikandan S. Chidambaram U. Karmegam C. Singaraja not only the interaction with granitic rocks (Kim and Department of Earth Sciences, Annamalai University, Jeong 2005; Valenzuela-Vasquez et al. 2006), but also Annamalai Nagar 608002, India the geothermal waters associated to basaltic rocks (Gaciri A. Ramanathan and Davies 1993; Hurtado and Gardea-Torresdey 2004). AL. Ramanathan − School of Environmental Sciences, Jawaharlal Nehru University, Groundwater is the main source of intake of F even New Delhi, India though food items like tea also contribute substantial amount of F− (Cao et al. 2000). Small concentration of M. V. Prasanna (*) − Department of Applied Geology, F is essential for normal development and strengthening School of Engineering and Science, Curtin University, of bones and for the formation of dental enamel (Bell CDT 250, 98009, Miri, Sarawak, Malaysia 1970; Fung et al. 1999; Shomar et al. 2004). Although e-mail: [email protected] F− is thought to be beneficial for tooth formation and as a P. Paramaguru mouth antibacterial, excessive fluoride may cause dental Mohamed Sathak Engineering College, as well as skeletal fluorosis and other adverse health Kilakarai, Ramanathapuram, India effects (Fordyce et al. 2007). Excessive F− intake causes dental enamel to lose its luster into either mild form of I. Jainab Department of Geology, Alagappa Government Arts College, dental fluorosis, which is characterized by white, opaque Karaikudi, India areas on tooth surface, or into severe form of dental Arab J Geosci fluorosis, which is characterized by yellowish brown to of ionic concentration with respect to space and time gives black stains on the teeth (Choubasia and Sompura 1996). a definite idea about the characterisation of hydrochemical Application of phosphatic fertilisers to the soils may also process, recharge and discharge region and region with cause a potential F− risk in groundwater quality (Loganathan higher ionic concentration. Hence, this study was designed et al. 2006). Fluorosis is now a worldwide problem. Reports in such a way to address major issues in determining the for high F− (>1.5 mg/L) in Mexico, suggest that its presence in hydrogeochemical processes and factors controlling fluo- groundwater is of concern mainly in the federal States ride in water chemistry. This study also attempts to high- of San Luis Potosí (Carrillo-Rivera et al. 2002); Jalisco light the relationship between fluoride in groundwater and (Hurtado and Gardea-Torresdey 2004), Sonora (Valenzuela- the geology of different aquifers along with their spatial Vasquez et al. 2006) and northernmost Baja California variations. (Soto-Rojas et al. 2004). It is also reported in nearly 20 developing countries like Argentina, USA, Algeria, Libya, Turkey, Iran, China, Australia, South Africa, Kenya, Iraq, Study area Sri Lanka, Canada, Thailand, New Zealand, Japan and India. (Mameri et al. 1998). Excessive fluoride concentra- Krishnagiri district is bounded by Vellore and Thiruvan- tions in ground water have been reported in India, where namalai districts in the East, Karnataka state in the west, 17 states are facing fluoride problem (Yadav and Khan State of Andhra Pradesh in the North and Dharmapuri 2010). Endemic fluorosis remains a challenging and exten- district in the south, covering about 5,143 km2.This sively studied national health problem in India. The high district is elevated from 300 to 1,400 m above the mean concentrations of fluoride in groundwater in India have sea level. It is located between 11° 12′N to 12° 49′N resulted due to the dissolution of fluorite, apatite, micas, latitude, 77° 27′E to 78° 38′E longitude. Eastern part of amphiboles with the OH, F group and topaz from the the district experiences hot climate and Western part has a local bedrock (Handa 1975; Chidambaram 2000). Handa contrasting cold climate. The average rainfall is 830 mm (1975) noted the general negative correlation between per annum. Cauvery River flows in the south western fluoride and calcium concentration in groundwater of boundary of the study area. Dodda Halla is the most India. important tributary of Cauvery draining the rugged terrain Many studies report on the prevalence of high fluoride in the north western part of the district. Ponnaiyar is the content in various water bodies in different parts of India major river draining the district and is ephemeral in na- (Siddiqui 1955; Jolly et al. 1973; Teotia and Teotia 1991; ture. After flowing for a short distance in an easterly Susheela 1993; Karthikeyan et al. 1996; Manivannan et direction, it again follows the district boundary before al. 2010). In India, at present, it has been estimated that entering the neighbouring Dharmapuri district (Fig. 1). fluorosis is prevalent in 17 states of India (Tables 1 and 2). The Physiographically, most of the area is occupied by flat to safe limit of fluoride in drinking water is 1.0 mg/L gently sloping land (about 300 to 1,400 m amsl) with soil (WHO 1984). The endemic fluorosis reported in different cover. The northeastern and extreme southwestern parts parts of India is chiefly of geochemical origin (Chidambaram are occupied by NW–SE running low hills and ridges et al. 2012). The Krishnagiri district contains minerals with (about 300 m to 1,400 m amsl). higher amounts of fluoride (CGWB 2009). Hence, it is im- Groundwater generally occurs under phreatic condi- portant to understand their distribution and concentration on tions in the weathered mantle and under semi-confined the groundwater of the district. The dissolution of fluoride conditions in the fractured zones at deeper levels. The from primary minerals can increase the concentration of dis- thickness of weathered zones in the district ranges from solved fluoride in groundwater, which is the only source of the less than a meter to more than 15 m. The yield of large town’s drinking water (Manikandan et al. 2011). The variation diameter dug wells in the district, tapping the weathered mantle of crystalline rocks ranges from 100 to 500 lpm. These wells normally sustain pumping for 2 to 6 h per Table 1 Range of fluoride concentration in groundwater of India and associated severity of fluorosis (Agrawal et al. 1997) day, depending upon the local topography and character- istics of the weathered mantle (CGWB 2009). The depth Region/state Range of fluoride Severity of fluorosis to water level (DTW) during pre-monsoon ranged be- concentration (mg/L) tween 0.5 and 9.9 m bgl in the district and it ranged Northwest India 0.4−19 Severe between 2 and 9.9 m bgl during post monsoon and the Central India 0.2−10 Moderate average DTW is >5 m bgl in the entire district, except for a few isolated pockets. South India 0.2−20 Severe The regional geology of the study area is hard rock: Deccan Province 0.4−8 Moderate gneisses, granites and basic rocks, alluvium and colluviums. Arab J Geosci
Table 2 Status of fluoride level (Meenakshi and Maheshwari States Districts Range of fluoride 2006) concentration (mg/L)
Assam Karblanglong, Nagaon 0.1−18.1 Andhra Pradesh All districts Except Adilabad, Nizambad, West Godavari, 0.11−20.0 Visakhapatnam, Srikakulam, Vijzianagaram Bihar Palamu, Daltonganj, Gridh, Gaya, Rohlas, Gopalganj, 0.6−8.0 Paschim, Champaran Delhi Kanjwala, Nijafgarh, Alipu 0.4−10.0 Gujarat All district except Dang 1.58−31.0 Haryana Rewari, Faridabad, Karnal, Sonipat, Jind, Gurgaon, 0.17−24.7 Mohidergarh, Rohtak, kurukshetra, Kaithal, Bhiwani, Sirsa, Hisar Jammu and Kashmir Doda 0.05−4.21 Karnataka Dharwad, Gadag, Bellary, Belgam, Raichur, Bijapur, 0.2−18.0 Gulbarga, Chitradurga, Tumkur, Chikmagalur, Manya, Bangalore, Mysore Kerala Palghat, Allepy, Vamanapuram, Allapuzha 0.2−2.5 Maharashtra Chandrapur, Bhandara, Nagpur, Jalgaon, Bukduna, 0.11−10.2 Amravati Akola, Yavatmal, Nanded, Sholapur Madya Pradesh Shivpuri, Jabua, Mandla, Dindori, Chhindwara, 0.08−4.2 Dhar Vidhisha, Seoni, Sehore,Ralsen and Bhopal Orissa Phulbani, Koraput, Dhenkanal 0.6−5.7 Punjab Mansa, Faridcot, Bhatinda,Muktsar, Moga, Sangrur, 0.44−6.0 Freozpur, Ludhiana, Amritsar, Patila, Ropar, Jallandhar, Fatehgarh Sahib Rajasthan All the 32 districts 0.2−37.0 Tamilnadu Salem, Periyar, Dharmapuri, Coimbatore, Thiruchirapalli, 1.5−5.0 Vellore, Madurai, Virdunagar Uttar Pradesh Unnao, Agra, Meerat, Aligarh, Mathura, Raibarell, 0.12−8.9 Allahabad West Bengal Birbhum, Bhardaman, Bankura 1.5−13.0
A major portion of the Krishnagiri district consists of Methodology quartzofelspathic gneiss and followed by charnockites in the south western part of the study area (Fig. 1). The A total of 34 groundwater samples were collected from bore eastern part of the district is mainly composed of syen- wells of different depths (10–65 m) representing the major ites, granite felsite and epidote hornblende gneiss. There formations (Fig. 2). Sampling was done at definite intervals are also patches of basic, ultrabasic and mafic rocks. in a grid pattern. Each sample was collected in acid-washed Amphibolite patches are noted in the northern part of polyethylene 250-ml bottles. Samples were filtered using the district. Quartz reef is hosted by both hornblende 0.45 μm Millipore filters. The bottle was tightly capped to schist of the region and the champion gneiss (quartz- protect samples from atmospheric CO2, adequately labelled ofelspathic gneiss). The area also comprises of gold and stored at room temperature. Precaution was also taken to mineralisation with molybdenum in quartz veins (GSI avoid samples agitation during transfer to the laboratory for 2009). The epigenetic gold mineralisation is mainly con- measurement. The collected samples were analysed for F−, fined to the shear and the silicified zones in the quartz major, and minor ions by using the standard procedures sericite schist (champion gneiss) occurring in association (APHA 1995). Analysis of water samples were carried in detail − − 2− 3− with amphibolite. The region is also associated with for different ions like HCO3 by titrimetry; Cl ,SO4 ,PO4 , − 2+ 2+ + + minerals like pyrite, pyrrhotite, arsenopyrite and chalco- NO3 ,H4SiO4,Ca ,Mg ,Na and K by ion chromatograph pyrite. Near Bargur, the gold mineralisation has been (Metrohm-861); F−, EC and pH were measured by Orion Ion recorded in the amphibolite band occurring within the Electrodes using standard procedures (APHA 1995) to under- gneiss (GSI 2009). This region forms a significant part stand the geochemical behaviour. The roles played by other of the southern granulite terrain of India associated with ions than those considered are less significant in the charge the metallogeny and metallic minerals with gold miner- balance calculation. The fluoride concentrations in the ground alisation in the Archean greenstone belts of Veppanapalli waters were plotted spatially by using Map info 8.5, GIS and Bargur sectors. software to identify the fluoride plume in the study area. Arab J Geosci
Fig. 1 Geology map of the study area
Fig. 2 Sampling location map of the study area Arab J Geosci
Results and discussion Compared with Na-HCO3 type groundwater, Ca-HCO3 type groundwater is known to generally contain lower fluoride The average, minimum and maximum values for different ions (Lee et al. 1997). Its hydrochemistry is characterized by are given in (Table 3). The groundwater in the study area is increased Ca2+ ion concentration with increasing total dis- generally odourless and colourless in most of the places. The solved solid due to the gradual dissolution of carbonate min- average temperature at the time of sampling varies from 25 to erals or Ca2+ bearing plagioclase in aquifer materials (Yun et 31 °C. The order of dominance of cations is Na+>Mg2+> al. 1998a and b; Kim and Chon 2001;Sujatha2003). The Na- 2+ + − − − 2− Ca >K and that of anions HCO3 >Cl >NO3 >SO4 HCO3 type groundwater are generally enriched in fluoride and Fluoride concentrations frequently are proportional to the sodium ions, due to the dissolution of silicates as well as the degree of water–rock interaction because fluoride primarily removal of Ca2+ by calcite precipitation and cation exchange originates from the geology (Gizaw 1996; Banks et al. 1995; (Chae et al. 2005;KimandJeong2005). It is interesting to Dowgiałło 2000; Frengstad et al. 2001; Carrillo-Rivera et al. note that higher F− was noted in the Mg2+-dominant water
2002). It is also reported that fluoride in groundwater is type with Mg-Na-HCO3. This may be due to the process of negatively related to anthropogenic contaminants that may hydrolysis which has resulted in the release of F− from infiltrate from the land surface (Pertti and Backman 1995; the Mg2+ bearing minerals like biotite, hornblende, etc., or
Kim and Jeong 2005). Fluorite (CaF2) has generally been weathering of apatite/hydroxyapaptites found in charnockites. considered a dominant source of groundwater fluoride, espe- Shahid Naseem et al. (2010) shows that the dissolved constit- cially in granitic terrains (Deshmukh et al. 1995;Shahand uents of groundwater, mainly Na+,K+ and Mg2+,maybe Danishwar 2003). The study area is divided into four considered as an evidence of granite–water interaction. He categories—(1) safe area (F−<1 mg/L), (2) area prone also stated that the positive correlation matrix between F- − to dental fluorosis (F 1–3 mg/L), (3) area prone to SiO2, F-Na and F-Mg reflect that flouriferous groundwater stiffness of bones (F−3–4 mg/L), and (4) area prone to originates from granitic rocks. The samples are arranged skeletal fluorosis (F− >4 mg/L). The lower limit for according to the increasing order of F− content (Table 5)and dental fluorosis has been taken as 1.0 mg/L, as inciden- they are grouped as group I (<1 mg/L), group II (1–2mg/L), ces of dental fluorosis in the country have been reported group III (2–3 mg/L), group IV (3–4 mg/L) and group V beyond this level (Chaturvedi et al. 1990;Apambireet (>4 mg/L) along with their water types for further discussion. al. 1997). The lower limit for skeletal fluorosis was It is clearly witnessed that two water types are dominant taken as 3.0 mg/L (Raju et al. 2009). Na+ and Mg2+ type, but in both the types the dominant − − Table 4 shows that most of the samples fall within the anion is HCO3 . Since HCO3 is the dominant anion the 1–3 mg/L (58 %) then followed by 3–4 mg/L concentra- process of weathering or dissolution may be the prominent tion (26.47 %). Hence, it is evident that the people of the process (Srinivasamoorthy et al. 2008; Prasanna et al. 2008). region are prone to dental fluorosis and subsequently the Enrichment of fluoride in groundwaters may also result due to stiff and brittleness of bone and joints are noted in this the process of evaporation (Jacks et al. 2005), the process of region. Even few samples of values higher than 4 mg/L evaporation is generally reflected by the dominance of Cl− ion are noted in Samalpatti and Periyajogipatti. In these in groundwater and the absence of this water type rules out the regions, few people with inability to stand straight were possibility of this process. Table 5 shows the percentage of witnessed. Only about 11 % of the samples collected were dominance of these two water types; the percentage of dom- below the permissible limit. inance of Mg2+ water type increases with increase in fluoride (i.e. >2 mg/L) , but in the range of 1–2 mg/L of fluoride ion Piper concentration Na+ dominant water type is noted. It is also interesting to note that there are representations of the two 2+ The piper plot shows two dominant water types as Mg and major water types; type A (Mg-Ca-Na-HCO3-Cl) and type B + Na type in cations (Fig. 3)andHCO3 is dominant in anions. (Mg-Na-Ca-HCO3-Cl) in both groups, group I (<1 mg/L) and
Table 3 Maximum, minimum, average of the chemical constituents present in groundwater of the study area (n034)
2+ 2+ + + − − 2+ − − Ca Mg Na K Cl HCO3 SO4 NO3 F pH EC TDS
Maximum 99.387 130.46 220.6 96.2 292.7 644.4 153.67 197.1 5.45 7.8 3,880 2,087 Minimum 31.228 21.161 31.4 16.5 49.59 195.2 10.758 21.94 0.5 6.3 703 357.5 Average 57.993 67.907 118.1 46.6 130.8 416.7 65.601 76.53 2.6 7.064 1,841 967.4083
All values are in milligrams per liter except pH; EC is in microsiemens per centimeter EC electrical conductivity, TDS total dissolved solids Arab J Geosci
Table 4 Level of fluoride con- tent in drinking water and Corresponding effects on Fluoride concentration Sample no. % of the corresponding effects on human human health (mg/L) samples health (Chaturvedi et al. 1990) safe limit < 1 5, 15, 26, 33 11.76 Dental fluorosis 1−3 2, 4, 8, 9, 12, 13, 14, 17, 18, 58.82 21–24, 27, 29–32 stiff and brittle bones/ joints 3−4 1, 6, 9, 10, 16, 19, 25, 28, 34 26.47 Deformities in knees; crippling >4 3, 7 5.88 fluorosis; bones finally paralyzed resulting in inability to walk or stand straight group V(>4 mg/L). The representation of water type A in both activity product (IAP) from the activities of uncomplexed the categories are represented in a stiff diagram and it shows ions based on the stoichiometries of minerals and other that in group I (sample nos. 26 and 33) have lesser concentra- solids in the WATEQ4F data base. The activity product tion of Mg2+ in the sample (Fig. 4a and b) but with regard to are then compared to the solubility product (KT) for the percentage it is higher. In group V (sample nos. 3 and 7) the same solid phases to the assumption that certain dissolved 2+ − quantity of Mg and HCO3 are higher and comparatively constituents in groundwater are in equilibrium with par- Ca2+ is lesser (Fig. 5a and b). Hence, it clearly shows that the ticular minerals and amorphous solids (Deutsch et al. water type with higher Mg2+ concentration have more relation 1982). Indices log (IAP/KT) were calculated to determine, to high F− groundwaters. if the water is in thermodynamic equilibrium log (IAP/KT00), Saturation indices Log (IAP/KT) was calculated by oversaturated log (IAP/KT>0) or undersaturated log WATEQ4F geochemical model for those minerals and other (IAP/KT<0) with respect to certain solid phases (Truesdell solids stored in the model data book for which the dissolved and Jones 1974). constituents are reported in the groundwater analysis. Solu- The saturation index of minerals like dolomite, magnesite bility equilibrium hypothesis were tested by computing ion and calcite were plotted against (Fig. 6) the F− concentration
Fig. 3 Piper plot representation Piper Plot
Legend Legend C < 1 ppm 80 80 C 1- 2 ppm C 2 - 3 ppm C 3 - 4 ppm C > 4 ppm 60 60 C 40 40
20 C C 20 C C C C C CCC C C C CCC MgC C C SO4 C CC C C C C C 80C 80
C 60 C 60 C C C C C C C CC 40 C C 40 CC C C C CC C CC C C C C C 20 C 20 C CC C C CC CCCC CC CCCC C CC CC CCCC C C 80 60 40 20 20 40 60 80 Ca Na+K HCO3 Cl Arab J Geosci
Table 5 Dominance of fluoride ions and its relation to Mg2+-rich Groups Sample no Water type Fluoride % of dominant water type water types (in percentage) (mg/L)
2+ V (>4 mg/L) 3 Mg-Ca-Na-HCO3-Cl 5.45 100 %Mg dominant 7 Mg-Na-Ca-HCO3-Cl 4.58 2+ IV (3–4 mg/L) 10 Mg-Na-HCO3-Cl 3.88 67 % of Mg dominant water type 25 Mg-Ca-Na-HCO3-Cl 3.65
28 Mg-HCO3 3.64
6 Mg-Na-HCO3 3.59
11 Na-Mg-Ca-HCO3-Cl 3.45
16 Na-Mg-HCO3 3.37
19 Mg-Na-Ca-HCO3-Cl 3.37
1 Na-Mg-HCO3-Cl 3.26
34 Mg-Na-HCO3-Cl 3.05 2+ III (2–3 mg/L) 14 Na-Ca-Mg-HCO3-Cl 2.78 56 %Mg dominant water type 32 Mg-Na-Ca-HCO3-Cl 2.76
20 Mg-Cl-SO4-HCO3 2.67
21 Na-Mg-HCO3-Cl 2.65
8 Na-Mg-HCO3-Cl 2.61
9 Mg-Na-HCO3-Cl 2.57
22 Na-HCO3-Cl 2.49
31 Na-Ca-Mg-HCO3-Cl 2.39
4 Na-Mg-Ca-HCO3-Cl 2.38
13 Mg-Na-HCO3 2.36
24 Na-Ca-Mg-HCO3 2.36
12 Mg-Na-Ca-HCO3 2.35
23 Mg-Na-HCO3 2.32
29 Mg-Ca-HCO3-Cl 2.21
27 Mg-HCO3 2.19
18 Mg-Ca-Na-HCO3 2.15 + II (1–2 mg/L) 17 Na-Mg-HCO3-Cl 1.8 100 % Na dominant water type 30 Na-Mg-HCO3 1.69
2 Na-Mg-HCO3-Cl 1.37 2+ I(0–1 mg/L) 26 Mg-Ca-Na-HCO3-Cl 0.9 50 %Mg dominant water type 15 Na-HCO3-Cl 0.8
5 Na-Mg-HCO3-Cl 0.6
33 Mg-Na-Ca-HCO3-Cl 0.5
and it was understood that the increase of dolomite satura- minerals. Fluoride ions are adsorbed by clays in acidic solution; tion was noted with the increase of the F− concentration and however, they are desorbed in alkaline solution. Thus, an no definite trend was noted with saturation index of calcite alkaline pH is favourable for F− dissolution (Sexena and and magnesite. This indicates there is a positive relation Ahmed 2003). The groundwater samples were weakly acidic between the SI of magnesite and F− concentration. to near neutral, with pH6.3 to 7.8. In groundwaters, however, The geology of this area is chiefly composed of peninsular low or high concentrations of fluoride can occur, depending on gneiss and charnockites which consists of biotite and horn- the nature of the rocks and the occurrence of fluoride-bearing blende and apatites (Fig. 1). The F−-rich waters are character- minerals. The absence of calcium in solution allows higher ized by high concentrations of Na+ and Mg2+ along low concentrations to be stable (Edmunds and Smedley 1996). concentrations of Ca2+.LowCa2+ results from the intense High fluoride concentrations may therefore be expected in cation exchange reaction between Ca2+ and Na+ (Sarma and groundwaters from calcium-poor aquifers and in areas where − Rao 1997). High HCO3 concentrations and alkaline pH pro- fluoride-bearing minerals are common. Fluoride concentrations mote the precipitation of Ca2+ ascalcite(SarmaandRao1997), may also increase in groundwaters in which cation exchange of and all of the studied groundwaters are saturated with those sodium for calcium occurs (Edmunds and Smedley 1996). Arab J Geosci
Fig. 4 a Stiff diagram representing the major concentration of cations Fig. 5 a Stiff diagram representing the major concentration of cations and anions in (meq/L) for the sample number of 26. b Stiff diagram and anions in (meq/L) for the sample number of 3. b Stiff diagram representing the major concentration of cations and anions (in milli- representing the major concentration of cations and anions in (in equivalents per liter) for the sample number of 33 milliequivalents per liter) for the sample number of 7
2+ − group of minerals or the higher Mg might be due to pH and F the weathering of enstatite in charnockites and the F− due to the dissolution of apatite/hydroxyapatite in char- pH of the groundwater samples in the study area ranges nockites. It is interesting to note that the higher fluoride from 6.3 to 7.8. There are higher concentrations of fluoride concentration is noted in the charnockite region and the at alkaline pH (Fig. 7) and this is mainly due to the process contact between the peninsular gneiss and other acidic of dissolution and weathering of rock forming minerals like intrusions. The higher Mg-F associations are noted in the hornblende, biotite, etc. In these regions with hornblende biotite gneiss and biotite undergoes weathering to vermicu- lite. The dissolution takes place, where water with dissolved − HCO3 in the form of H2CO3 acts on the mineral and − − − releases OH ,F, cation and HCO3 ÀÁ þ 2þ ðÞNa Ca ; MgFe Al2Si6O20ðÞOH ; F þ H2O ð1Þ 2ÀÁ3 4 ! 2þ 2þ ðÞþ þ Mg Fe 3Al3Si5O20 OH OH F þ þ þ 2þ þ 2þ þ Na Ca Mg HCO 3 In this scenario, the Ca2+ released from the dissolution can combine with F− and can result in a lesser concen- tration in solution. Hence, the water with higher fluoride concentration might have been derived from source rock of lesser Ca2+ content and probably higher Na+.The higher concentrations of Mg2+ may be due to weathering of the Mg2+-rich minerals like micas or amphiboles Fig. 6 Saturation index Arab J Geosci
Fig. 7 Relation between pH 8 and fluoride 7.8
7.6
7.4 Dissolution or hydrolysis of the F bearing minerals 7.2 Both the process of dissolution and ion echange may be active 7 contributing to the higher F pH
6.8
Ion exchange process by the 6.6 uptake of OH in water and releases F increasing the H+ reducing the pH of the solution 6.4
6.2
6 0123456 Fluoride concentration (mg/L) groundwaters of acid igneous rocks (Shahid Naseem et sorption of OH− to the surface of clay minerals and the al. 2010). release of the adsorbed F− At higher pH hydrolysis takes place, where OH− is re- R F þ HOH ! R OH þ HF ð2Þ leased into the system with more cations and fluoride. The − + release of fluoride in the natural system is mainly due to the Moreover, groundwaters with high HCO3 and Na con- process of ion exchange or due to the process of selective tent are usually alkaline and have relative high OH− content,
Fig. 8 Spatial distribution of fluoride (in mg/L) Arab J Geosci
Table 6 Range of fluo- 2008) as a result of silicate mineral hydrolysis. Chidambaram ride concentration (in Fluoride Area Area % 2 et al. (2012) found that the occurrence of groundwater with milligrams per liter) in concentration (km ) − + – groundwater (mg/L) high HCO3 and Na concentration due to water rock interactions is also one of the important reasons for fluoride 0−1.5 151.69 2.95 % release from the aquifer matrix into groundwater. This 1.5−2.0 2393.2 46.5 % higher Na+ may be exchanged for the H present in the clay/ 2.0−2.5 2370.5 46.59 % weathered matrix in the host rock thereby increasing the 2.5−3.0 227.77 4.43 % pH (Manivannan et al. 2011) and resulting in the dominance of Mg2+-rich water type. so the OH− can replace the exchangeable F− of fluoride- − Spatial distribution of fluoride bearing minerals, increasing the F content in groundwater. The reactions are basically as follows: The occurrence of F− in groundwater is mainly due to natural Muscovite: or geogenic contamination and the source of contamination often unknown (Handa 1975; Saxena and Ahmad 2002). The KAl ½ AlSi O F þ 2OH ¼ Al ½ AlSi O ½ OH þ 2F þ Kþ 2 3 10 2 2 3 10 2 water–rock interaction assessment can be used as a tool to ð3Þ assess the geogenic input of fluoride from fluoride minerals in the deep groundwater aquifers (Saxena and Ahmed 2001). Biotite: The study area is chiefly composed of epidote hornblende