Abstract Serious problems are faced in severalparts of the world due to the presence of highconcentration of fluoride in drinking water whichcauses dental and skeletal fluorosis to humans.Nalgonda district in Andhra Pradesh, India is onesuch region where high concentration of fluorideis present in groundwater. Since there are no ma-jor studies in the recent past, the present studywas carried out to understand the present statusof groundwater quality in Nalgonda and also toassess the possible causes for high concentrationof fluoride in groundwater. Samples from 45 wellswere collected once every 2 months and analyzedfor fluoride concentration using an ion chromato-graph. The fluoride concentration in groundwaterof this region ranged from 0.1 to 8.8 mg/l witha mean of 1.3 mg/l. About 52% of the samplescollected were suitable for human consumption.However, 18% of the samples were having lessthan the required limit of 0.6 mg/l, and 30%of the samples possessed high concentration offluoride, i.e., above 1.5 mg/l. Weathering of rocksand evaporation of groundwater are responsiblefor high fluoride concentration in groundwater of
K. Brindha · R. Rajesh · R. Murugan · L. Elango (B)Department of Geology, Anna University,Chennai 600025, Indiae-mail: [email protected]
this area apart from anthropogenic activities in-cluding irrigation which accelerates weathering ofrocks.
Groundwater is the major source of freshwateron the earth. Groundwater containing dissolvedions beyond the permissible limit is harmful andnot suitable for domestic use. Fluoride beyonddesirable amounts (0.6 to 1.5 mg/l) in ground-water is a major problem in many parts of theworld. Around 200 million people from 25 na-tions have health risks because of high fluoride ingroundwater (Ayoob and Gupta 2006). In Indiatoo, there has been an increase in incidence ofdental and skeletal fluorosis with about 62 millionpeople at risk (Andezhath et al. 1999) due to highfluoride concentration in drinking water. Dentalfluorosis is endemic in 14 states and 150,000 vil-lages in India with the problem most pronouncedin the states of Andhra Pradesh, Bihar, Gujarat,Madhya Pradesh, Punjab, Rajasthan, Tamil Nadu,and Uttar Pradesh (Pillai and Stanley 2002).Fluoride in groundwater has been studied inGuntur district (Subba Rao 2003), Varaha River
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Basin (Subba Rao 2008), Ranga Reddy district(Vijaya Kumar et al. 1991), and Nalgonda dis-trict (Ramamohana Rao et al. 1993) of AndhraPradesh, India. Earlier studies in Nalgonda district(Ramamohana Rao et al. 1993) have indicatedelevated concentration of fluoride up to 20 mg/l.After the year 1993, there are no major studieson fluoride in groundwater of Nalgonda district.Considering this factor and keeping an accountof the importance of public health, this studywas designed to understand the present status offluoride in groundwater of a part of Nalgondadistrict, Andhra Pradesh, India. Thus, this inves-tigation was carried out with the aim of assessingfluoride concentration in groundwater as a part ofNalgonda district and to understand the reasonsfor its spatial and temporal variation.
Materials and methods
Study area
The study area forms a part of Nalgonda district,Andhra Pradesh, which is located at a distance of135 km ESE of Hyderabad (Fig. 1). The south-eastern side of the study area is surrounded bythe Nagarjuna sagar reservoir and the southernside of the area is bounded by Pedda Vagu River.The northern boundary is partially along a wa-ter divide. This area experiences arid to semiaridclimate. The study area goes through hot climateduring the summer (March–May) with a temper-ature range from 30◦C to 46.5◦C, and in winter(November–January), it varies between 16◦C and29◦C. The average annual rainfall in this area is
Fig. 1 Location of the study area
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about 1,000 mm, occurring mostly during south-west monsoon (June–September). The highest el-evation in the study area is 348.1 m, and the lowestelevation is 169.5 m (Fig. 2). The drainage patternis dentritic to subdentritic in this area (Fig. 3).
Geology
The basement granitic/granitic gneisses of lateArchaen are exposed in most part of this area(Fig. 4). They are generally medium to coarsegrained. These rocks are traversed by numerousdolerite dykes and quartz veins. The Srisailamformation, the youngest member of the Cuddapahsuper group, directly overly this basement gran-ite with a distinct unconformity. This Srisailamformation is exposed in the southeastern part ofthe study area. The meta sediments of Srisailamformation include pebbly-gritty quartzite, shale,dolomitic limestone, intercalated sequence ofshale-quartzite and massive quartzites (Fig. 4).
The litho units of this formation are dipping atan angle ranging from 3◦ to 5◦ towards SE. Thegeneralized stratigraphic sequence of this area isgiven below (after GSI 1995).
Cuddapah super group Massive quartziteSrisailam formation Upper shale
Late archean/lower Granite/granitic gneissproterozoic with intrusion of
dolorite dykes andquartz veins
Fig. 2 Topography of the study area
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Fig. 3 Drainage map of the study area
Fig. 4 Geology of the study area (GSI 1995)
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Fig. 5 Location of sampling wells in the study area
Sampling and instrumentation
Groundwater samples were collected during themonths of March, May, July, and September 2008from 45 sampling wells selected on the basisof well inventory survey (Fig. 5). Totally, 167groundwater samples were collected during thisstudy. The depth of the wells varied from 1.45to about 20 m below ground level. Groundwaterlevel in the wells was recorded using a waterlevel recorder, and pH of groundwater sampleswas measured in the field using a portable pHmeter. Water samples were collected in cleanpolyethylene bottles of 600-ml capacity. The sam-
pling bottles were soaked in 1:1 diluted HCl so-lution for 24 h, washed with distilled water, andwere washed again prior to each sampling withthe filtrates of the sample. In the case of borewells, water samples were collected after pump-ing the water for 10 min. In the case of openwells, water samples were collected 30 cm belowthe water level using a depth sampler. Samplescollected were transported to the laboratory andfiltered using 0.45-μm Millipore filter paper. Thefluoride concentration of groundwater sampleswas determined using Metrohm 861 advancedcompact ion chromatograph using appropriatestandards.
The minimum, maximum, mean, and other sta-tistical parameters of groundwater level, pH, andfluoride concentration measured during this studyis given in Table 1. The fluoride concentration ingroundwater of this area varied between 0.1 and8.8 mg/l. The desirable range of fluoride concen-tration in drinking water is from 0.6 to 1.2 mg/laccording to the Indian standard specifications
(BIS 1992). Thus, if the concentration of fluorideis below 0.6 and above 1.2 mg/l, the water isnot suitable for drinking purposes. However, it issuggested that the maximum limit can be ex-tended up to 1.5 mg/l (BIS 1992). Based on theconcentration of fluoride, the groundwater sam-ples obtained from the study area have been clas-sified into four groups as low (0.1 to 0.6 mg/l),medium (0.6 to 1.5 mg/l), high (1.5 to 3 mg/l), andvery high (>3 mg/l; Table 2). Figure 6 shows the
Fig. 6 Average groundwater concentration of fluoride (mg/l) during this study
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Fig. 7 Spatial variation of water level (msl in m) and fluoride concentration in groundwater (mg/l) in March 2008
average concentration of fluoride in groundwaterpresent in different sampling locations. In all the4 months of sampling, 51% of the samples werewithin the range and thus fit for drinking purpose.Of the rest of the groundwater samples, 19% ofthe samples had fluoride below 0.6 mg/l; hence, it
is unfit for drinking purpose. Two samples fromMay 2008 and one sample from September 2008had very high concentration of fluoride, i.e., above3 mg/l. Thus, on the whole, 30% of samples ofgroundwater had more than 1.5 mg/l of fluoride.It was found that the concentration of fluoride in
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49% of the total 167 groundwater samples did notfall within the desirable range of 0.6 to 1.5 mg/l offluoride.
The spatial variation in groundwater level andfluoride concentration in groundwater of this areais shown in Fig. 7. In general, it is found that thefluoride concentration in groundwater increasesalong the groundwater flow direction. That is, thegroundwater flow from west to southeastern sideof the study area and the fluoride concentrationincreases along this direction.
Probable sources of fluoride
Weathering of rocks
Apatite [Ca5(PO4)3 (F, Cl, OH)] and fluorite[CaF2] minerals are the common sources offluoride. Many granitic rocks have elevatedfluoride concentration (Murthy and Murthy 1974;Natarajan and Murthy 1974; World Health Or-ganization 1970). Granites/gneisses are the ma-jor type of rocks that occur in the study areawhich has the presence of fluoride containingminerals such as fluorite (0–3.3%), biotite (0.1–1.7%), and hornblende (0.1–1.1%) (RamamohanaRao et al. 1993). The world average of fluorideconcentration in granitic rocks was found to be810 mg/l (Wedepohl 1969), while fluoride contentof granitic rocks from Nalgonda was found to bein the range of 325 to 3,200 mg/l, with a meanof 1,440 mg/l (Ramamohana Rao et al. 1993).Thus, the granitic rocks of Nalgonda possess thehighest fluoride content than in any other parts ofthe world. Hence, the major reason for elevated
groundwater fluoride concentration in this areamust be due to weathering of rocks and rock–water interaction.
The pH of groundwater of this area variedfrom 6.3 to 9.3 with a mean of 7.5. The pHof groundwater shows slight increase with in-crease in fluoride concentration (Fig. 8). Thisindicates that the fluoride content of ground-water will vary due to the changes in alkalin-ity, i.e., carbonate and bicarbonate content. Also,the fluoride content of groundwater has an in-verse relationship with calcium and magnesiumcontent. This means that as the sum of carbon-ate and bicarbonate content of the groundwa-ter samples divided by the sum of calcium andmagnesium content increases as the fluoride con-centration in the groundwater sample increases.To demonstrate this, the groundwater samples
were differentiated into samples with fluorideconcentration above 1 mg/l and those below1 mg/l. Groundwater samples having fluorideabove 1 mg/l and below 1 mg/l were plottedagainst the sum of carbonate and bicarbonatedivided by the amount of calcium and magne-sium ions in groundwater. It was found that asthis range increases, the number of samples hav-ing fluoride concentration above 1 mg/l increases(Fig. 9). At a lower range, i.e., below 2, thenumber of samples having fluoride concentrationbelow 1 mg/l was high (n = 32) when comparedto the number of samples having fluoride concen-tration above 1 mg/l (n = 30). But, as this rangeincreases and reaches greater than 5.1, it could beseen that the number of samples with fluoride con-centration above 1 mg/l was significantly higher
Fig. 8 Plot of fluoride(mg/l) versus pH
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Fig. 9 [HCO3 + CO3]/[Ca + Mg] vs Fluoride
(n = 15) than the number of samples with fluoridebelow 1 mg/l (n = 7). This clearly shows thatthe concentration of fluoride in groundwater in-creases as the carbonate and bicarbonate contentof water increases, and it decreases along with in-crease in calcium and magnesium content. Such anobservation was also made by Ramamohana Raoet al. (1993). Overall, high pH, high carbonate plusbicarbonate, and low calcium plus magnesium ingroundwater leads to leaching of fluoride whichresults in increase in the concentration of fluoridein groundwater.
Fig. 10 Groundwater fluctuation and variation in fluorideconcentration in wells with shallow (0 to 4.5 m bgl) watertable
Groundwater level f luctuation and f luorideconcentration
Fluoride concentration in groundwater varies withthe groundwater level fluctuation. Two distincttypes of relationship between the groundwaterlevel and fluoride concentration were observed.In wells where the water table occurs at shallowdepths, that is from 0 to 4.5 m below groundlevel, the fluoride concentration was high whenthe water level was low and the fluoride concen-tration decreases with the rise in water table (caseI; Fig. 10). Relative high fluoride concentration
Fig. 11 Groundwater fluctuation and variation in fluorideconcentration in wells with deep (4.5 to 8 m bgl) water table
during the lowering of water table is because ofpossible direct evaporation of groundwater fromthe wells. The fluoride concentration was lowwhen the water level rises due to dilution by freshrainwater recharge.
In case of wells where the water level occursat depths beyond 4.5 m below ground level, thefluoride concentration varies similar to water levelfluctuation (case II; Fig. 11). That is, the concen-tration of fluoride measured in groundwater afterthe monsoonal rains (July and September) werehigher than the preceding months (March andMay). Evaporation/evapotranspiration results inprecipitation of salts on the top layers of the soil.During subsequent rains, these fluoride-rich saltsin the soil get leached from the soil along withthe percolating rain water, and it gets mixed withthe groundwater. That is, the infiltrating rainfallrecharge flushes the fluoride salts in the unsatu-
rated zone which results in increase of fluoride ingroundwater along with the raise in water table.Conceptual model of these two cases is shown inFig. 12.
Anthropogenic activities
Anthropogenic activities like the application offertilizers and industrial activities like brickkilning may be some of the other factors respon-sible for elevated fluoride concentration. In thestudy area, irrigation is largely practiced, and thisis the major activity that contributes to the sourceof living for the people. Irrigation increases thesodicity of the soil. Also, the fertilizers used forirrigation purpose are expected to contribute forhigh fluoride concentration in the aquifers. It isas well possible that long-term continuous irriga-tion practice could affect the fluoride content of
Fig. 13 Average fluorideconcentration (mg/l) inirrigation and domesticwells
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groundwater. Brick kilning, which is yet anotherfactor responsible for increase in fluoride concen-tration, was found to be practiced in the studyarea. Coal, which is used by brick kiln industries,is a potential source of fluoride, and further, claytoo, used for making bricks, contains several hun-dreds of milligrams per liter of fluoride (Jha et al.2008). But, their contribution may be in negligibleamounts only, as these activities are not practicedon a large scale over the entire study area whencompared to contribution by weathering of rocks.The concentration of fluoride in groundwatersamples taken from wells in agricultural fields anddomestic wells did not show much difference. Thisdepicts that the contribution by fertilizers used inagriculture did not play a major role in increasingthe fluoride concentration in significant amountsin groundwater. Figure 13 shows that the wellsin agricultural fields and domestic areas possessfluoride concentration in similar range and do notvary much.
Groundwater management
Sustainable management of groundwater in thisarea poses many challenges besides impropersolid waste disposal, uncontrolled use of fertil-izers and pesticides, and lack of awareness ofthe public. Improving the groundwater quality inNalgonda region can be done by identifying wellswith high fluoride concentration, avoiding con-sumption of water from those wells, reducing theuse of chemical fertilizers for agriculture, adoptingorganic farming, and reducing evaporation by in-creasing vegetation cover and spreading environ-mental awareness among the public by organizingcampaigns. Defluorination of groundwater beforeusing it for consumption is essential. In Nalgondatechnique, which is used for defluorination, thechemicals such as alum and lime should be used inthe right proportion, failing which technique mayprove ineffective. Also, the generation of residualaluminum in the treated water due to complexa-tion reaction with the adsorbed fluoride has beenreported; thus, a detailed study is required toreconfirm the reported observation (Singh et al.2004). Managing the huge volume of sludge result-ing from this process is yet another difficulty. Ar-tificial rainfall recharge methods may be planned
at suitable places, which will result in reductionof fluoride in groundwater. Further, recharge ofrainwater after filtration through the existing wellscan also be planned to improve the groundwaterquality of this region.
Conclusion
The present status of groundwater in parts ofNalgonda was assessed in this study. High con-centration of fluoride in groundwater of up to8.8 mg/l was measured. About 30% of wells hadfluoride concentration above the permissible limitof 1.5 mg/l set by Indian drinking water standard.Moreover, it is also important to note that 18% ofgroundwater samples were below the prescribedconcentration (0.6 mg/l). Thus, out of 167 ground-water samples analyzed during the study, 48% ofthem had fluoride either above or below the per-missible limit. The use of groundwater for drink-ing purpose from these wells has to be restricted.The rocks of this area possess fluoride contenthigher than the world average. Weathering ofrocks and leaching of fluoride bearing mineralsare the major reasons which contribute to ele-vated concentration of fluoride in groundwater.The other important natural phenomenon thatcontributes to high fluoride is evaporation. Suit-able measures such as defluorinating the ground-water before use and recharging the groundwaterby rainwater harvesting need to be practiced toimprove the groundwater quality in this area.
Acknowledgements We would like to thank the Boardof Research in Nuclear Sciences, Department of AtomicEnergy, Government of India for the financial support.The analytical facilities created using DST-FIST and UGC-SAP grants are also acknowledged.
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