HYDROLOGICAL PROCESSESHydrol. Process. 27, 2462–2474 (2013)Published online 5 June 2012 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/hyp.9247

Fluoride enrichment in aquifers of the Thar Desert: controllingfactors and its geochemical modelling

Chander Kumar Singh,1,2 Rina Kumari,2 Neha Singh,2 Javed Mallick3 and Saumitra Mukherjee2*1 Department of Natural Resources, TERI University, New Delhi 110070, India

2 Remote Sensing Applications Lab, School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India3 Faculty of Engineering, King Khalid University, Abha, Kingdom of Saudi Arabia

*CLabNeE-m

Co

Abstract:

The groundwater is the only source of drinking water in the Jaisalmer district of Rajasthan, India. The study area is a part of theThar Desert. It has low and scattered population and no industries; hence, the possibility of anthropogenic input of fluoride isalmost negligible. Thus, the enrichment of fluoride is only possible due to geochemical processes taking place in the groundwaterof the region. A total of 100 groundwater samples, 34 samples from Jaisalmer and 66 samples from the Pokharan administrativeblocks, were collected. It was observed that the concentration of fluoride ranged from 0.08mg/l to 4.56mg/l in the groundwaterof Jaisalmer and from 0.56mg/l to 6.60mg/l in the samples of the Pokharan block. The alkaline condition (average pH,7.7� 0.22 and 8.01� 0.25 in Jaisalmer and the Pokharan administrative block, respectively) in the region favours fluoritedissolution. Ion exchange, dissolution of calcite, semi-arid climate, alkaline conditions and weathering are responsible forfluoride enrichment in the groundwater of the study area. Copyright © 2012 John Wiley & Sons, Ltd.

KEY WORDS geochemical modelling; saturation index; Jaisalmer; fluoride toxicity; Thar Desert

Received 31 August 2011; Accepted 13 January 2012

INTRODUCTION

Fluorine occurs naturally in soil, water, plant and animalin trace quantities (Harrison, 2005). Fluoride is anessential element for the human body. It helps in themineralization of bones and the formation of enamel inteeth. A daily dose of 0.5 ppm is required for the properformation of enamel and bone mineralization, whichotherwise may result in formation of dental caries, lack ofenamel formation and bone fragility (Cao et al., 2000;Edmunds and Smedley, 2005; Ayenew, 2008; Singhet al., 2011). The toxicity of fluoride-containing com-pounds cannot be generalized, as it depends on itsreactivity (because it is found to react with all the metalsand noble gases except neon and helium), structureand, in the case of salts, their solubility and ability torelease fluoride ions. Higher concentration of fluoride(>1.5 ppm) in drinking water is considered as a healthhazard, and it causes chronic endemic fluorosis (Eby,2004). Fluorosis at more severe stages causes bilaterallameness and stiffness of gait (Hobbs et al., 1954; Suttie,1977; Oruc, 2008; Singh et al., 2011). Naturally occur-ring minerals such as fluorite (CaF2), cryolite (Na3AlF6),topaz, tourmaline, muscovite, biotite, hornblende andvillianmite (Handa, 1975; Pickering, 1985; Wenzel andBlum, 1992; Zhang et al., 2003; Msonda et al., 2007;Jha et al., 2010; Singh et al., 2011) are good sourcesof fluoride. Fluoride is also found in micas, where it

orrespondence to: Saumitra Mukherjee, Remote Sensing Applications, School of Environmental Sciences, Jawaharlal Nehru University,w Delhi 110067, India.ail: saumitramukherjee3@gmail.com

pyright © 2012 John Wiley & Sons, Ltd.

occurs in combination with silicates, but particularly inassociation with phosphorus as fluorapatite. The fluorideoccurrence in groundwater in an area, where anthropo-genic input is almost trifling, can be attributed to thedissolution of calcite, weathering and leaching of amphi-boles, fluorite, apatite and mica (Singh et al., 2011). Thus,the areas where such rock types have dominant highfluoride concentration can be suspected (Banks et al.,1995; Frengstad et al., 2001; Carrillo-Rivera et al., 2002).Fluoride concentration is found to be higher in waterwith high alkalinity. In groundwater, calcium ions arein excess, and under such pristine conditions, theconcentration of fluoride is controlled by CaF2, whichat normal temperatures has a solubility of about 15 ppm.Muralidharan et al. (2002) reported that Rajasthan has amaximum number of people affected by high fluoridecontent in groundwater. There is acute shortage of good-quality drinking water in the Jaisalmer district ofRajasthan, specifically in the rural areas. High fluoridein groundwater is distributed in all the 31 districts ofRajasthan, which is influenced by the regional and localgeological settings and hydrological conditions (Agrawalet al., 1997). It was observed that high fluorideconcentration is accompanied by high nitrate concen-trations in Rajasthan and a solution-evaporation andbase-exchange hypothesis was proposed to explain thegenesis of fluoride in the groundwater in Rajasthan(Handa, 1975). An anomalously high concentration offluoride (up to 16 ppm) was observed in dug/tube wellwater, around the Palri, Andor and Wan villages, in thewestern part of Sirohi district, Rajasthan (Maithani et al.,1998). Suthar et al. (2008) reported up to 4.78mg/l of

2463FLUORIDE ENRICHMENT IN AQUIFERS OF THAR DESERT

fluoride concentration in some villages of northernRajasthan. Fluoride was also reported in the sedimentsof the Ajmer district (Madhavan and Subramanian,2002). Enrichment of fluoride (4.74mg/l) in groundwaterwas reported as a result of rock water interaction in thePokharan area (Singh et al., 2011). In the present study,the hydro-geochemistry of groundwater and geologicalformations for enrichment of fluoride in groundwater hasbeen evaluated. The study examines the fluoride toxicity,its spatial distribution and possible origin in relation tothe geological features in two administrative blocks ofthe Jaisalmer district of Rajasthan. The inter-relationshipof major ions and their ionic ratio has also been studied,as it can help decipher the actual cause of fluorideenrichment in the groundwater.

STUDY AREA

The study area consists of two administrative blocks ofthe Jaisalmer district, namely Jaisalmer and Pokharan.The part of the Jaisalmer block that has been studied islocated between 70.80�E and 71.58�E longitudes andbetween 26.41�W and 27.20�W latitudes, covering anarea of approximately 3850 km2. The Pokharan block is

Figure 1. Study area wit

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located between 71.39�E and 72.09�E longitudes andbetween 26.32�W and 27.54�W latitudes, covering anarea of approximately 6000 km2. The study area alongwith land use/land cover is shown in Figure 1. Sparsevegetation consisting of species of Prosopis, Acacia,Calotropis, etc. is found in the region. The study areais situated amid the Thar Desert, where monsoon is asgood as negligible. Several palaeochannels are alsopresent throughout the area, indicating the presence ofa fluvial system in ancient times in the region.Numerous saline lakes (playas) are also present in thein the study area, which had formed due to theaccumulation of water in natural depressions, whichlater on became saline due to a high evaporation rate(Deotare et al., 2004).

General geology of Jaisalmer district

The Jaisalmer basin is dominated by Mesozoic andTertiary formations. The western and north-western partsof the district are covered by a vast blanket of youngunconsolidated deposits, including the blown sand of theThar Desert of Western Rajasthan. The geology of thestudy area is shown in Figure 2.

h Landuse/Landcover

Hydrol. Process. 27, 2462–2474 (2013)

Figure 2. Geology of study area

2464 C. K. SINGH ET AL.

The highly folded metamorphosed rocks of the Delhisuper group form the main part of the mountains andconsist of quartzite, mica schist and gneiss (Misra, 1982;Wasson et al., 1984; Sundaram and Pareek, 1995). TheAlwar group and Ajabgarh group consist mostly of calc-silicates, quartzites, grits and schistose rocks. The otherimportant lithological formations consist of a thick seriesof sedimentary rocks composed of sandstone, limestoneand shale. The Bap (Jodhpur district) and Pokharan(Jaisalmer district) beds of Upper Carboniferous agehave been exploited for groundwater. Pleistocene sandalluvium, blown sand, kankar (calcium nodules), carbon-ate beds and evaporite deposits of recent and sub-recentages are found over a large area of West and EasternRajasthan. Igneous and metamorphic rocks of lowerProterozoic age composed of slate, quartzite, phyllite,schist and gnesis are also found in the area. This igneoussuite consisting of basalt and rhyolite is overlain by thesandstone and limestone of the Marwar supergroup.The Palaeozoic Bap boulder bed along with dolomiteand minor shale overlies the Marwar rocks (Rai andSinha, 1990; Deotare et al., 1998, 2004). Carbonaterocks (limestone, marble and dolomite) are found inthis region.

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HydrogeologyThe various geological formations present in the

district can be divided into three categories, that is,consolidated, semi-consolidated and unconsolidated, asfar as occurrence and movement of groundwater areconcerned. The consolidated hydrogeological units arethe Pre-Cambrian Jalore granites and Malani rhyolite andthe Cambrian rocks of the Marwar supergroup, which areJodhpur sandstone and Chacha limestone. The semi-consolidated hydrogeological units include the Mesozoicand Tertiary formations, which are Lathi, Jaisalmer,Bhadesar, Parewar sandstone; Jaisalmer, Khuiyala andBandha limestone; and Baisakhi shale. The unconsoli-dated formations are the alluvial deposits consisting ofsand, silt and gravel. Groundwater in granites andrhyolites occur under unconfined conditions in joints,fractures and weathered mantle and is of poor yield.Depth to water table varies from 10 to 35mbgl (metersbelow ground level). Jodhpur sandstone constitutes a pooraquifer, and groundwater occurs in joints, fractures andother structurally weak zones. Depth to water level rangesfrom 5 to 25mbgl. Although the groundwater occursin all the geological formations, the Lathi sandstoneforms the most potential aquifers. Lathi sandstones are

Hydrol. Process. 27, 2462–2474 (2013)

2465FLUORIDE ENRICHMENT IN AQUIFERS OF THAR DESERT

unsaturated except for perched saturated zones. Ground-water occurs under phreatic, semi-confined and confinedconditions. The depth to water in general ranges from 2.5to 130mbgl. Depth to water table and piezometric levelsranges from over 30 to 100mbgl. Perched water levelsoccur between 6 and 30mbgl. There are generally threeaquifer zones in the depth ranges of 67 to 100m, 150m to200m and 240 to 280m, which are hydraulicallyconnected. The transmissivity of Lathi aquifers rangesfrom 100 to 2000m2/day. The deeper Lathi aquifers areunder confined conditions, and the coefficient of storageranges from 9.6� 10–4 to 1.3� 10–4 (CGWB, 2009). Theyield of wells ranges from 1000 to 3500 l/min, and theirspecific capacity ranges from 200 to 600 l/min/m.Groundwater in Jaisalmer limestone occurs under uncon-fined conditions. Depth to water table ranges from 10 to90mbgl, but it generally lies within 40 to 50mbgl. Depthof tube wells ranges from 150 to 400m, and their yieldvaries from 750 to 1670mbgl. The rock units of theBaisakhi series are generally argillaceous and do not forma potential hydrogeological unit. Depth to water tablevaries from 25 to 55mbgl. The sandstone of the Badesarand Parihar series do not form good aquifers due to thepresence of fine sandstone, shale and clays. Depth towater level in the sandstone ranges from 60 to 130mbgl.Tertiary formations, which include Sanu sandstone,Khuiala limestone and Bandha limestone, form a goodaquifer in the central part of the region. Depth to waterlevel ranges from15 to 130mbgl. The piezometric levelvaries from 40 to 80mbgl. Depth of tube well variesfrom 150 to 300m, and yield varies between 200 and1900 l/min. Quaternary formation consists of sand, gravel,kankar, silt, clay and grits, and its thickness varies from100m to 151m. Groundwater occurs under confinedconditions, and depth to water level varies from 10 to90mbgl. The discharge of wells ranges from less than 100to 1000 l/min for 2.3 to 10m drawdown (CGWB, 2009).Depth to water level is less than 40mbgl in the northernand eastern parts of the study area. In the western part, itis mostly between 40 and 60mbgl. Depth to water table isdeeper than 60mbgl (from 60 to more than 100mbgl) inthe central and southern parts of the study area.

Climate

The climatic variability of the Thar Desert, located inthe north-western part of India, is one of the most debatedtopics and has been related to the dynamics of the south-west monsoon (Bryson and Swain, 1981; Luckge et al.,2001). The climate of the region varies from semi-arid inthe east to arid and hyper-arid in the west. The variabilityof the monsoon rainfall has been linked to the expandingof the Eurasian/Tibetan snow cover, which leads tochanges in albedo and the weakening of monsoonalcirculation (Dickson, 1984; Meehl, 1994). The averageannual precipitation to evapo-transpiration ratio indicatesthat the climate of the eastern margin of the desert is semi-arid in nature (UNEP, 1991). Around 90% of the rainfallis received during the four monsoonal months of theyear (from June to September), and the winter rainfall

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constitutes less than 10% of the total rainfall. The annualprecipitation in the region varies from 450mm at theeastern margin to 100mm at the western margin. In theThar, the mean maximum temperature during the summermonths ranges from 40 �C to 45 �C, and the meanminimum temperatures during the winter months fluctuatebetween 3 �C and 10�C. The evapo-transpiration of theregion is 3–20 times higher than the precipitation,indicating a negative water balance.

METHODOLOGY

A total of 100 water samples were collected in the periodof April–May 2010 in polypropylene bottles from shallowhand pumps and tube wells, as these are the only availablewater sources for drinking and other domestic use in thearea. The study area covers two administrative blocks ofthe Jaisalmer district, namely Pokharan and Jaisalmer.Groundwater samples were analysed for fluoride ionalong with other chemical parameters. The pH, totaldissolved solid (TDS) and electrical conductivity (EC) ofthe water samples were measured onsite using portablepH, TDS and EC electrodes (HANNA). The sampleswere acidified using HNO3 (Ultrapure Merck) for cationanalysis. The samples were stored in an icebox, carried tothe laboratory and kept at 4 �C for further chemicalanalysis. Immediately after the water samples weretransported to the laboratory, the major cations were(Mg2+, Ca2+, Na+, K+) analysed using an atomicabsorption spectrometer [Thermo Fisher Scientific Mseries AAS graphite furnace (GFAAS)], and the majoranions (F–, Cl–, SO4

2–, NO3–) were analysed using an

ion chromatograph (Dionex). Bicarbonate (HCO3–) was

determined by titration method using standard proceduresas described in standard methods for the examination ofwater and wastewater (APHA, 2007). The analyticalprecision of the ions analysed was determined bycalculating the normalized ionic charge balance error,which varied within �5%. Mineralogical analysis ofsediment samples were carried out using X-ray diffraction(XRD; PANalytical X’Pert PRO). Saturation indexcalculation was done using Aquachem software.

RESULTS AND DISCUSSION

Spatial distribution of fluoride

The mean, minimum, maximum and standard deviationof various physico-chemical parameters of groundwaterfor both the Jaisalmer and Pokharan blocks that wereanalysed are given in Tables I and II. The fluorideconcentration in water samples of Jaisalmer variedbetween 0.08 and 4.56mg/l, with a mean value of1.60mg/l, and in Pokharan, it varied between 0.56 and6.60mg/l, with a mean value of 2.56mg/l. It wasobserved that 76.47% and 89.39% of the samplesexceeded the maximum desirable limit (1mg/l) set bythe Bureau of Indian Standards (BIS, 1991) in Jaisalmerand Pokharan, respectively, whereas based on the WHO

Hydrol. Process. 27, 2462–2474 (2013)

Table I. Summary of physical and chemical parameters of Jaisalmer samples

Samplingpoints pH EC TDS Na K Ca Mg Cl SO4 HCO3 NO3 F HCO3/Ca Na/Ca

R 1 7.9 1800 1066 302 6 88 33 392 151 341 10 1.96 3.88 3.43R 2 7.8 2000 1063 300 9 70 31 284 147 366 60 1.92 5.23 4.29R 3 7.9 1600 832 240 11 64 23 172 127 427 5 2.04 6.67 3.75R 4 7.6 4000 2300 379 74.78 168 128.9 609 417 439 200.26 1.12 2.61 2.26R 5 8 3500 2037 623 16.81 72 29.18 595 417 524 19.84 4.56 7.27 8.64R 6 8.1 1800 972 259 10.95 74 23.1 348 93 242 79.98 2.16 3.28 3.51R 7 8 1600 810 239 14.86 66 31.89 269 112 341 9.92 2.88 5.17 3.63R 8 7.6 1400 657 67 29.33 60 53.5 70 84 524 29.76 0.4 8.73 1.12R 9 7.5 1500 896 270 20.33 50 18.51 297 122 222 50.22 1.36 4.44 5.39R 10 7.8 5200 2861 350 24.72 120 116.74 609 196 486 50.22 1.6 4.05 2.91R 11 7.4 1100 531 179 9.38 60 32.16 156 127 298 19.84 0.8 4.97 3R 12 7.8 3400 1767 583 12.9 75 34.05 652 120 610 19.84 2.16 8.13 7.77R 13 7.9 6000 4120 1143 73.85 130 72.96 1162 558 294 1000.06 1.12 2.26 8.78R 14 7.6 1000 639 189 6.65 76 30.67 257 116 305 19.84 0.84 4.01 2.5R 15 7.6 2000 1090 335 10.17 66 24.32 425 127 244 9.92 1.44 3.69 5.07R 16 7.4 3100 1790 530 9.78 112 24.32 709 192 366 29.76 0.08 3.26 4.72R 17 7.7 3000 1692 530 6.65 64 29.18 716 182 268 29.76 1.2 4.19 8.27R 18 7.8 5900 3070 950 13.69 80 97.28 1489 230 317 50.22 1.04 3.96 11.86R 19 7.4 980 449 74 2.74 50 24.32 106 19 183 79.98 0.32 3.65 1.5R 20 7.9 2000 1124 309 17.82 62 39.46 390 148 292 9.92 2.76 4.72 5R 21 7.5 1000 583 155 5.87 66 32.16 132 101 290 60.14 1.2 4.4 2.35R 22 7.8 1400 744 195 10.17 60 15.81 191 67 329 40.3 1.12 5.48 3.24R 23 7.5 8800 5206 1534 13.69 170 158.08 2233 792 549 29.76 0.12 3.22 9.01R 24 7.7 2300 1325 419 10.17 54 34.05 398 192 412 34.88 3.78 7.62 7.76R 25 8.2 3000 1997 520 12.9 108 43.78 439 220 463 360.22 1.96 4.29 4.81R 26 8.2 3100 1648 530 9.78 56 31.62 780 62 244 19.84 2.52 4.35 9.45R 27 7.6 1200 677 149 7.43 74 24.32 163 76 244 89.9 0.96 3.29 2.02R 28 7.7 1000 692 195 9.47 72 12.16 212 33 341 60.14 0.92 4.74 2.71R 29 7.9 1300 663 124 3.13 60 36.48 141 28 317 109.74 1.12 5.28 2.08R 30 7.9 2000 1102 329 7.43 72 30.67 418 106 386 9.92 2.48 5.36 4.58R 31 7.6 1000 1230 144 7.43 48 13.38 113 124 158 125.24 1.12 3.3 3.01R 32 7.8 3600 2211 670 12.9 40 48.64 631 268 439 319.92 3.12 10.96 16.72R 33 8.1 2600 1440 489 4.69 20 19.46 340 105 561 179.8 1.28 28.01 24.45R 34 8 4800 2711 801 18.38 80 133.76 1162 230 651 59.68 1.04 8.13 9.99Average 7.77 2646.47 1529 415.29 15.14 76.2 45.06 502.2 179.58 367.23 96.58 1.6 4.82 5.45Minimum 7.4 980 449 67 2.74 20 12.16 70 19 158 5 0.08 2.26 1.12Maximum 8.2 8800 5206 1534 74.78 170 158.08 2233 792 651 1000.06 4.56 28.01 24.45Standarddeviation

0.22 1786.4 1068 316 16.08 32 37.21 449 157 123 179.81 1.01 4.39 4.76

Note: Concentrations are in mg/l, except for EC (mS/cm) and pH.

2466 C. K. SINGH ET AL.

standards (2009) (1.5mg/l), 41.17% of the samples ofJaisalmer and 71.12% of the samples of Pokharanexceeded the maximum permissible limit. Higher con-centration of fluoride was observed in the north-western,north-eastern and southern parts of the Jaisalmer block(Figure 3). The concentration of fluoride in these partswas found to be more than 3mg/l. The Pokharan blockshowed higher fluoride concentration mostly in centralparts, where the concentration exceeded 5mg/l (Figure 4).The comparatively low concentration of fluoride inother parts of both administrative blocks was observedin the areas where there is a dominance of agriculture(Figure 1). The lower fluoride in these parts may beattributed to the presence of a high amount of clay. Theadsorption of fluoride is higher in clay as compared tosandy soil (Navlakhe and Bulusu, 1989; Burt, 1993).The adsorption strength of clay to fluoride is highenough to inhibit the release of fluoride in groundwater.It was observed by Madhavan and Subramanian (2002)

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in Ajmer, Rajasthan, that clay contained a high amountof fluoride, whereas sand and silt fractions containless amount of fluoride. The discrepant spatial distribu-tion of fluoride can be attributed to variety in land use,soil texture and relative abundance of fluoride-bearingminerals.

Hydro-geochemistry of fluoride

The fluoride solubility is lowest in the pH range of5–6.5 (Adriano, 1986). It was observed that thegroundwater in the study area is alkaline in all the places.The pH value in Jaisalmer ranged from 7.4 to 8.2, with anaverage value of 7.77, whereas in Pokharan, it rangedfrom7.5 to 8.6, with an average value of 8.01 (Tables Iand II). At higher pH, ionic exchange occurs betweenfluoride and hydroxyl ions (illite, mica and amphiboles),resulting in an increase of fluoride concentration ingroundwater. The presence of high concentration ofbicarbonate and sodium with high pH favours the release

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Table II. Summary of physical and chemical parameters of Pokharan block samples

Samplingpoints pH EC TDS Na K Ca Mg Cl SO4 HCO3 NO3 F HCO3/Ca Na/Ca

R 1 7.80 3800 2061 530 7.82 68 121.60 794 389 280 30.00 1.20 4.12 7.78R 2 7.90 2600 789 694 18.00 56 48.00 712 165 465 310.00 2.10 8.30 12.41R 3 8.00 7500 5296 1400 4.69 160 170.24 1170 852 976 1050.28 2.80 6.09 8.74R 4 8.10 4800 2707 875 5.47 68 58.37 921 288 781 99.82 2.16 11.46 12.84R 5 7.80 1800 1078 256 5.87 60 48.64 375 74 292 109.74 1.68 4.87 4.27R 6 7.80 3200 1686 460 7.82 52 77.82 709 196 305 29.76 1.12 5.86 8.83R 7 7.80 4800 699 239 7.82 82 22.16 237 97 366 9.92 0.96 4.46 2.92R 8 7.70 1300 701 241 7.82 52 22.16 227 77 366 9.92 0.96 7.04 4.64R 9 7.80 6600 4465 814 117.39 228 121.06 1028 427 563 799.80 2.48 2.47 3.56R 10 7.70 3500 2271 490 26.20 120 121.60 709 240 488 380.06 1.44 4.06 4.08R 11 7.60 2900 1356 255 24.86 65 26.75 106 67 513 189.72 0.56 7.89 3.92R 12 7.90 6800 3564 985 17.20 120 89.98 1501 288 423 99.82 0.88 3.52 8.19R 13 8.00 3800 2051 540 10.95 82 65.66 745 288 427 29.76 1.12 5.20 6.58R 14 8.20 2000 1302 357 4.69 66 30.67 346 52 414 169.88 1.68 6.28 5.41R 15 7.60 5000 3122 998 12.12 68 36.48 934 498 738 130.20 2.48 10.84 14.65R 16 8.00 1700 1014 270 4.69 76 32.83 252 76 414 140.12 1.04 5.45 3.55R 17 7.90 3300 1706 459 25.17 96 26.75 554 114 591 29.76 1.92 6.15 4.79R 18 8.00 5900 3369 973 9.78 120 116.74 1531 384 427 19.84 1.36 3.55 8.10R 19 7.90 2100 1231 403 6.65 67 21.89 390 183 411 29.76 2.68 6.14 6.01R 20 8.00 3000 1680 503 6.65 76 36.48 652 192 366 29.76 1.92 4.81 6.61R 21 8.00 7200 4009 1275 18.99 80 54.72 1572 552 399 99.82 3.88 4.97 15.87R 22 8.10 7600 4654 141 15.64 140 91.20 1601 600 868 280.24 6.60 6.20 10.08R 23 8.10 4700 2942 1067 24.63 66 12.16 737 201 1220 300.08 6.12 18.46 16.14R 24 8.00 2300 1354 430 9.38 40 24.32 460 144 390 50.22 3.04 9.74 10.73R 25 7.80 4200 2626 820 3.91 56 26.75 581 220 634 600.16 2.16 11.31 14.62R 26 8.10 6500 4046 1248 21.90 122 72.96 1276 518 732 450.12 2.68 5.99 10.22R 27 7.90 1600 1042 299 5.47 32 21.89 212 105 329 200.26 1.92 10.28 9.35R 28 7.90 12000 1206 1068 16.89 76 34.98 1395 145 426 179.80 3.52 5.61 14.05R 29 8.00 3600 2170 600 34.80 92 36.48 780 230 183 304.42 3.96 1.99 6.51R 30 7.90 3300 1884 560 4.69 48 72.96 780 211 305 55.18 1.44 6.34 11.64R 31 8.00 2300 1398 450 7.43 30 30.40 368 134 512 120.28 3.24 17.05 15.00R 32 7.70 3200 1815 546 16.42 52 53.50 730 201 390 19.84 2.68 7.50 10.48R 33 8.10 1300 754 159 7.82 50 42.56 184 124 170 99.82 1.52 3.41 3.19R 34 8.50 3000 1658 470 11.34 52 53.50 531 209 264 158.72 2.76 5.08 9.02R 35 8.40 5000 3118 947 25.42 125 43.78 762 374 1083 280.24 5.80 8.66 7.57R 36 8.00 2000 1313 333 20.33 60 46.21 326 96 341 259.78 2.16 5.68 5.55R 37 8.10 1490 1320 325 22.73 92 48.33 417 105 525 119.22 1.13 5.67 3.51R 38 8.10 1950 1920 309 31.06 98 29.29 379 99 422 189.21 4.74 4.31 3.16R 39 7.50 890 520 98 2.89 80 24.47 121 31 384 77.59 0.76 4.77 1.22R 40 7.90 2230 2140 319 21.02 117 29.50 423 120 443 115.11 1.87 3.76 2.72R 41 8.50 1790 1700 282 54.83 93 36.68 388 145 345 182.68 4.01 3.71 3.03R 42 8.20 1750 1530 311 38.70 91 49.01 359 129 368 304.49 2.22 4.02 3.40R 43 7.90 1730 1630 276 16.26 89 59.54 383 135 312 257.43 1.24 3.51 3.11R 44 8.20 1700 1520 253 18.82 79 43.42 393 121 321 64.70 1.58 4.06 3.20R 45 8.00 2130 1980 294 16.23 75 61.23 413 112 476 102.35 1.84 6.35 3.92R 46 8.30 1560 1460 278 28.63 98 48.52 416 86 405 175.65 2.36 4.13 2.83R 47 8.60 1890 1740 212 5.39 105 59.02 388 102 368 78.56 4.86 3.51 2.03R 48 7.60 1950 1860 215 12.45 112 68.63 412 81 418 86.56 5.63 3.73 1.92R 49 7.80 1860 1740 198 18.25 101 61.23 363 46 358 198.38 0.85 3.54 1.96R 50 8.40 960 860 158 16.53 93 53.54 305 25 316 165.84 1.23 3.39 1.70R 51 7.80 1580 1460 217 12.31 84 62.42 352 95 387 114.52 2.58 4.61 2.59R 52 8.20 1670 1570 194 56.23 88 46.30 346 112 323 74.50 3.67 3.67 2.21R 53 8.10 2280 2130 248 41.25 118 57.25 414 174 447 40.23 2.90 3.77 2.10R 54 8.60 1890 1750 232 22.31 104 42.10 316 135 282 285.95 4.22 2.72 2.23R 55 7.80 980 860 185 16.84 118 29.65 306 32 312 202.35 3.75 2.64 1.57R 56 7.90 2180 2080 231 45.26 106 63.25 388 161 386 145.84 3.56 3.63 2.17R 57 7.90 1780 1660 192 25.69 74 61.03 335 138 354 35.26 4.25 4.77 2.59R 58 8.10 1760 1650 171 24.59 94 52.65 352 82 365 46.23 2.43 3.88 1.82R 59 8.60 1180 1055 224 21.59 82 34.56 302 65 319 182.96 3.52 3.90 2.74R 60 7.80 1850 1760 195 19.65 112 39.50 323 113 337 143.65 0.65 3.01 1.74R 61 8.30 2130 2010 258 18.29 97 67.52 367 184 437 108.26 1.81 4.49 2.65R 62 7.80 1260 1100 168 11.02 88 38.06 325 65 302 38.60 1.23 3.41 1.91R 63 7.90 2110 1990 286 42.58 126 58.59 424 190 382 229.87 4.21 3.01 2.26

Continues

2467FLUORIDE ENRICHMENT IN AQUIFERS OF THAR DESERT

Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. 27, 2462–2474 (2013)

Table II. (Continued )

Samplingpoints pH EC TDS Na K Ca Mg Cl SO4 HCO3 NO3 F HCO3/Ca Na/Ca

R 64 8.20 1910 1880 256 25.68 106 68.06 316 106 329 345.32 2.12 3.10 2.41R 65 8.10 2130 2020 258 39.87 112 63.21 392 85 354 216.59 3.24 3.16 2.31R 66 8.40 1850 1730 198 26.87 121 59.65 378 89 311 142.32 4.67 2.55 1.63Average 8.01 3009.39 1920 462.07 20.16 88.94 53.49 560.91 184.91 438.95 176.17 2.56 5.48 5.77Minimum 7.50 890.00 520 98 2.89 30 12.16 106 25 170 9.92 0.56 1.99 1.22Maximum 8.60 12000.00 5296 1412 117.39 228 170.24 1601 852 1220 1050.28 6.60 18.46 16.14Standarddeviation

0.25 2057.15 980 336.97 17.27 32.14 28.46 365.10 153.14 192.84 177.34 1.43 3.08 4.26

Note: Concentrations are in mg/l, except for EC (mS/cm) and pH.

Figure 3. Spatial distribution of fluoride in the Jaisalmer block

2468 C. K. SINGH ET AL.

of fluoride from minerals into groundwater. Thus, thealkaline nature of the groundwater favours the solubilityof fluoride-bearing minerals. In an alkaline environment,the fluoride ions are desorbed, thus enhancing thedissolution of fluoride minerals.A correlation matrix (r-value at significance level of

p< 0.05) for both the Jaisalmer and Pokharan areas wasgenerated for the water quality parameters to decipher therelationship among the various ions (Table III). Fluoride

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in the Jaisalmer block showed a good positive correlationwith pH (0.56); thus, it can be inferred that the release offluoride is due to the leaching of fluoride-bearingminerals. It also showed a positive correlation withbicarbonate (0.15) and a negative correlation withcalcium (�0.27) although it is not significant. However,it can be inferred from the correlation matrix that high-fluoride groundwater samples accomplice with lowcalcium content (�0.27) due to the low solubility of

Hydrol. Process. 27, 2462–2474 (2013)

Figure 4. Spatial distribution of fluoride in the Pokharan block

2469FLUORIDE ENRICHMENT IN AQUIFERS OF THAR DESERT

fluoride. Nevertheless, in the Pokharan block, fluoridedoes not show any significant correlation, but moderatelylow positive correlation was observed with pH (0.38) andbicarbonate (0.36). High fluoride in groundwater isgenerally associated with high concentration of bicarbon-ate ions and, in some cases, with high nitrate concentra-tion (Handa, 1975). The observed high concentration ofnitrate in a desertic terrain can be attributed to long-termaccumulation due to the leaching of soil (Walvoord et al.,2003). The anthropogenic input of nitrate can be negatedfor this region, as it has low population density andnone of the industry. Fluorine reacts immediately toform fluoride compounds, therefore making the exis-tence of free fluorine a rare possibility. Nevertheless,under favourable physico-chemical conditions andwith long residence time, it may occur in dissolvedform in groundwater (Handa, 1975). A higher concentra-tion of TDS enhances the ionic strength, leading to anincrease in the solubility of F– ions in groundwater(Perel’man, 1967).The alkaline or basic condition of groundwater favours

the enrichment of fluoride in groundwater. The hydroxylion (OH–) in groundwater with a high pH value canreplace the exchangeable fluoride of fluoride-containing

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minerals (biotite/muscovite), thus increasing the concen-tration of fluoride in groundwater.The hydroxyl ions replace fluoride from the biotite as

biotite; KMg3 AlSi3O10½ �F2 þ 2OH�

! KMg AlSi3O10½ � OH½ �2 þ 2F� (1)

In granitic- or sandstone-dominant aquifers, dissolutionof fluoride can be a possible reason for the presence offluoride in groundwater. The hydrolysis of alumino-silicates in the hard rock aquifers gives bicarbonate ion,which can enhance fluorite dissolution as shown below:

CaF2 þ 2HCO�3 ! CaCO3 þ 2F� þ H2Oþ CO2 (2)

Water with high F– concentration can form in the areaswhere alkaline (carbonate rocks) water is in contact withfluoride-bearing minerals.

Hydrochemical facies and fluoride concentration

The hydrochemical facies of the groundwater for bothadministrative blocks were studied, and it was observedthat the dominant water facies in Jaisalmer is of theNa-Cl-HCO3 and Na-Ca-Cl-HCO3 types, whereas inPokharan, it is of the Na-Cl-HCO3, Na-Ca-Cl-HCO3,

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Table III. Correlation matrix of physico-chemical parameters of groundwater samples

Jaisalmer pH EC TDS Na K Ca Mg Cl SO4 HCO3 NO3 F

pH 1.000EC 0.124 1.000TDS 0.115 0.983 1.000Na 0.157 0.934 0.941 1.000K 0.003 0.426 0.472 0.316 1.000Ca �0.130 0.673 0.688 0.535 0.609 1.000Mg �0.025 0.828 0.793 0.651 0.506 0.728 1.000Cl 0.066 0.949 0.927 0.956 0.274 0.604 0.756 1.000SO4 �0.046 0.844 0.884 0.845 0.544 0.737 0.700 0.798 1.000HCO3 0.281 0.485 0.430 0.421 0.155 0.264 0.505 0.368 0.378 1.000NO3 0.193 0.333 0.447 0.396 0.645 0.289 0.163 0.212 0.429 �0.025 1.000F 0.564 �0.065 �0.075 �0.010 �0.069 �0.276 �0.229 �0.124 �0.002 0.153 �0.060 1.000PokharanpH 1.000EC �0.128 1.000TDS 0.020 0.689 1.000Na �0.063 0.883 0.815 1.000K 0.135 0.055 0.265 �0.034 1.000Ca 0.103 0.285 0.589 0.220 0.621 1.000Mg �0.044 0.391 0.637 0.394 0.181 0.557 1.000Cl �0.069 0.884 0.785 0.915 0.030 0.311 0.526 1.000SO4 �0.048 0.723 0.890 0.865 0.052 0.377 0.636 0.801 1.000HCO3 �0.014 0.505 0.629 0.689 0.019 0.252 0.151 0.436 0.571 1.000NO3 0.049 0.358 0.566 0.449 0.349 0.522 0.450 0.263 0.512 0.447 1.000F 0.380 0.183 0.320 0.263 0.255 0.247 �0.045 0.193 0.208 0.366 0.165 1.000

Note: The bold values represent correlation between the parameters.

2470 C. K. SINGH ET AL.

Na-Ca-Mg-Cl-HCO3 and Na-Cl types (Figure 5a and b,Table IV). Almost all water types have sodium bicarbon-ate. It has been observed that sodium bicarbonate–typewaters are capable of releasing fluoride from fluoritemineral (Handa, 1975). In ideal (laboratory) conditions,the concentration of fluoride can go to as high as 20mg/ldue to the dissolution of fluorite mineral in sodiumbicarbonate–type water (Rao et al., 1993); however, otherminerals and ionic constituents of water may influence itsdissolution in a regional setting. The dissolution offluorite can be explained as follows:

CaF2 þ Na2CO3 ! CaCO3 þ 2Naþ 2FCaF2þ2NaHCO3 ! CaCO3þ2Naþ 2FþH2Oþ CO2

Ionic ratio to assess fluoride enrichment

The low concentration of Ca2+ reflects its precipitationas carbonate (Gaciri and Davis, 1993). Ion exchangebetween Ca2+ and Na+ due to the movement ofgroundwater in the weathering zone may also result inhigh F– associated with high Na+ and low Ca2+

concentration (Tamta, 1994). Increase in concentrationof HCO3 ions, pH and temperature can result in theprecipitation of calcite (Hounslow, 1995). A positivecorrelation between Cl– and Na+ with EC shows thatdissolution of ions from rocks is a major controllingfactor of EC. Na+/Ca2+ and HCO3

–/Ca2+ ratio was foundto be much greater than 1 in all the sampling locations inboth administrative blocks, suggesting favourable geo-chemical conditions for the fluoride dissolution process

Copyright © 2012 John Wiley & Sons, Ltd.

and indicating low calcium activity, respectively. Ahigher value of Na+/Ca2+ suggests that a high concentra-tion of sodium favours dissolution of fluoride-bearingminerals at higher pH (Shaji et al., 2007). The increasein concentration of sodium ions increases the solubilityof fluoride-containing minerals; thus, the processesresponsible for increase in sodium concentration withdecrease in calcium ion concentration have contributedthe fluoride in groundwater. In addition, the F– contentshows an increase with Na+ and decrease with Ca2+

due to the dominance of the ion-exchange process.During the ion-exchange process, calcium ions in watermay exchange with sodium ions in clay minerals(sodium-montmorillonite), thus increasing the concentra-tion of Na+ ions in groundwater (Hounslow, 1995).

Saturation index and effect of climate

Another major process responsible for fluorideenrichment can be attributed to evaporation. Highertemperatures are known to increase both the solubilityand the rate of dissolution of most rock minerals (Hem,1985). The saturation index was calculated for ground-water samples of the Jaisalmer and Pokharan adminis-trative blocks. It was observed that nearly all thesamples of the Jaisalmer block are oversaturated withrespect to calcite (except three samples); however, allexcept one sample is undersaturated with respect tofluorite (Figure 6a). In the Pokharan block, all thesamples are oversaturated with respect to calcite, butsome of the samples are oversaturated with respect to

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Figure 5. (a) Piper plot for Jaisalmer. (b) Piper plot for Pokharan

Table IV. Hydrochemical facies and number of samples in eachtype for Jaisalmer and Pokharan

Water type No. of samples

JaisalmerNa-Ca-Cl-HCO3 7Na-Ca-HCO3-Cl 1Na-Mg-Ca-Cl-SO4 1Na-Cl-SO4-HCO3 1Mg-Ca-Na-HCO3 1Na-Cl-HCO3 10Na-Mg-Cl-HCO3 2Na-Ca-Mg-HCO3-Cl-SO4 1Na-Cl-NO3 1Na-Cl 5Na-Ca-Mg-Cl-HCO3 1Na-Ca-Mg-HCO3-Cl 1Na-Mg-Ca-HCO3-Cl 1Na-Ca-Cl-SO4-HCO3 1PokharanNa-Mg-Cl-SO4 1Na-Mg-Cl-HCO3 3Na-Ca-HCO3-Cl 2Na-Cl 9Na-Cl-HCO3 16Na-Cl-HCO3-SO4 1Na-Ca-Cl-HCO3 13Na-Cl-SO4-HCO3 1Na-Mg-Ca-Cl-HCO3-SO4 1Na-Mg-Ca-Cl-HCO3 6Na-Ca-Mg-Cl-HCO3 11Na-Ca-Cl 1Na-Mg-Cl 1

2471FLUORIDE ENRICHMENT IN AQUIFERS OF THAR DESERT

fluorite; however, most of them are undersaturated withfluorite (Figure 6b). Undersaturation reflects the charac-ter of water from a formation with insufficient amount ofthe mineral for solution or short residence time. Thestudy area lies in a semi-arid environment; thetemperature in the summer is very high, and rainfall isvery little, so due to evaporation, the groundwaterbecomes oversaturated with calcite, thus precipitatingcalcite, which in turn reduces calcium and thus promotesdissolution of fluorite. Oversaturation reflects that thegroundwater discharging from an aquifer contains ampleamount of the mineral with sufficient resident time toreach equilibrium. After oversaturation, the concentra-

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tion of Ca2+ ions overrides the solubility limit offluorite, as fluorite dissolution is suppressed by acommon ion effect and the correlation between thetwo ions (Ca2+ and F-) becomes negative (Handa, 1975).The fluoride concentration in groundwater is mainlygoverned by oversaturation of calcite due to evaporationand calcium ion-exchange process. The high rate ofevaporation in semi-arid conditions of the study areamight enhance the calcite precipitation in an alkalineenvironment of groundwater, thus creating a deficiencyof calcium ions and creating a harmonious environmentfor dissolution of fluorite.

Mineralogy

X-ray diffraction was carried out for 50 sedimentsamples. The XRD diffractogram was interpreted usingX’Pert HighScore Plus (Table V), and it was foundthat most of the samples indicated the presence offluorite and calcite along with other major mineralssuch as quartz (Figure 7). The mineral compositionpoints towards the granitic nature of rocks, and it hasbeen observed that these rocks serve as the genesis offluoride-rich groundwater (Fordyce et al., 2007). TheNa/Na +Cl ratio is supportive to differentiate water ofdifferent origins (Hounslow, 1995). The averageNa/Na +Cl ratios of the groundwater in Jaisalmer andPokharan are 0.469 and 0.441, respectively, indicating

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Figure 6. (a) Saturation index for the Jaisalmer block. (b) Saturation index for the Pokharan block

Table V. Major minerals identified using X’Pert HighScore Plus

Reference code Compound name Chemical formula

05-0586 Calcite, syn CaCO3

35-0816 Fluorite, syn CaF233-1161 silica SiO2

2472 C. K. SINGH ET AL.

the presence of albite. The normative mineral compos-ition indicates a high proportion of albite in the graniticrocks. The high Na concentration imparts a bicarbonate-type character to groundwater, which permits higherfluoride concentration as and when equilibrium isreached with fluorite. Sedimentary rocks have a fluorineconcentration ranging from 200 ppm in CaCO3 up to1000 ppm in shale (Frencken, 1992). Fluorine is presentas fluorite in carbonate sedimentary rocks. Clasticsediments have higher fluorine concentrations, as thefluorine is concentrated in micas and illites in the clayfractions; however, high fluorine concentrations may alsobe found in phosphate beds of sedimentary rocks(Frencken, 1992).

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CONCLUSION

Groundwater is the only source of drinking water in thispart of Western Rajasthan. The concentration of fluoridewas well above the maximum permissible limit. Theanthropogenic input of fluoride in this desertic terrain is notan obvious possibility, as the area is not inhabited heavilyand there is no industrial setup in this region. Thus, highconcentration of fluoride is geogenic in origin; that is,hydro-geological conditions are responsible for higherfluoride concentration in groundwater. The granite in thearea contains abundant fluoride-bearing minerals, andduring weathering, fluoride can leach and dissolve theaquifers. The climate coupledwith geochemical processes isfound to be a main controlling factor for higher concentra-tion of fluoride. The high Na+/Ca2+ and HCO3

–/Ca2+ ratioalong with high TDS also substantiates enrichment offluoride in groundwater. The fluoride in the study area ismoderately mobile (coefficient of aqueous migration); thus,during weathering, it leaches out from granitic rocks anddissolves in groundwater. Ion exchange, dissolution, hightemperature, alkaline conditions and weathering areresponsible for fluoride enrichment in groundwater.

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Figure 7. X-ray diffractogram of a sediment sample

2473FLUORIDE ENRICHMENT IN AQUIFERS OF THAR DESERT

ACKNOWLEDGEMENTS

The author (Chander Kumar Singh) thanks Satpal MittalTrust for providing financial support to the carrying outof the research work. The author also acknowledgesDr. Manoj Kumar Singh of AIRF, JNU, for carrying outthe XRD analysis. The suggestions of the anonymousreviewers, which have improved the standard of themanuscript, are duly acknowledged.

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