The occurrence and behaviour of fluoride in the groundwater of...

Hydrological Sciences-Journal-des Sciences Hydrologiques, 42(6) December 1997 877

The occurrence and behaviour of fluoride in the groundwater of the Lower Vamsadhara River basin, India

N. SRINIVASA RAO Department of Geophysics, Andhra University, Visakhapatnam 530 003, India

Abstract Reports on the occurrence of fluoride in natural water resources and the associated health hazards due to human consumption have been made from many parts of India during the last decade. With the objective of organizing a systematic scientific programme to understand the behaviour of fluoride in natural water resources in relation to the local hydrogeological and climatic conditions and agricultural use, a typical area constituting the lower Vamsadhara River basin was chosen for a detailed study. High fluoride concentrations in the groundwater reaching a maximum of 3.4 mg l"1 were observed to be associated with weathered formations of pyroxene amphibolites and pegmatites. The groundwater in the clayey soils contained much less fluoride as compared to the sandy soils. The complex depositional pattern of these sandy and clayey soils plays an important role in the uneven spatial distribution of fluoride in the groundwater. The contribution of fluoride from geological formations is far greater than that from agriculture: the maximum yield of fluoride by superphosphate fertilizer to irrigation water is observed to be 0.34 mg l"1. The fluoride concentration is expected to be increased in the future as the groundwater is subsaturated with respect to fluorite. An inverse relationship between F and Ca and positive relationships of F with Na, HC03, P04 and electrical conductivity were observed. Best relationships were obtained in the fluoride range of 1.0-3.4 mg T1.

Présence et comportement des fluorures dans les eaux souterraines du bas bassin du Vamsadhara (Inde) Résumé Des rapports concernant la présence de fluorures dans les eaux naturelles et les risques pour la santé associés à sa consommation dans de nombreuses régions de l'Inde ont été établis au cours de la dernière décennie. Afin de comprendre le comportement des fluorures dans les eaux naturelles en fonction des caractéristiques hydrogéologiques et climatiques locales, un programme de recherche scientifique systématique a été entrepris sur le bas bassin du Vamsadhara. De très importantes concentrations en fluorures des eaux souterraines, atteignant un maximum de 3.4 mg T1, ont été observées dans des formations d'amphibolites à pyroxenes et de pegmatites altérées. Les eaux des sols argileux contiennent beaucoup moins de fluorures que les eaux des sols sableux. Les structures de déposition complexes de ces sols argileux et sableux jouent un grand rôle dans la distribution spatiale irrégulière des fluorures dans les eaux souterraines. C'est davantage des formations géologiques que de l'agriculture que proviennent les fluorures: l'apport maximum de fluorures observé provenant d'engrais superphosphatés est de 0.34 mg T'. On pense que la concentration en fluorures devrait encore augmenter à l'avenir car les eaux souterraines sont sous-saturées par rapport à la fluorine. Une corrélation négative entre F et Ca et unecorrélation positive entre F d'une part et Na, HC03, P04 et la conductivité d'autre part ont été observées. Les relations les plus étroites ont été observées dans la gamme de concentration en fluorures allant de 1.0 à 3.4 mg T1.

INTRODUCTION

Fluoride is an essential nutrient and prevents dental caries but excessive concentrations (>1.0 mg I"1) cause dental fluorosis (Handa, 1988) and even skeletal

Open for discussion until 1 June 1998

878 N. Srinivasa Rao

fluorosis (>3.0 mg 1 ). Awareness of excess fluoride consumption through water has been increasing in the developing nations in recent years. A systematic quantitative appraisal of the intake in rural populations has not been made in many underdeveloped and developing nations. In the last two decades reports on the occurrence of excess fluoride in water resources in several states of India have been published by many research workers (Chari et al., 1971; Handa, 1975; Raju et al., 1979; Sarma & Swamy, 1983; Handa, 1988; Nawlakhe & Bulusu, 1989; Gaumat et al, 1992; Rajagopalan et al., 1989). For example, at least 2.5 million people living in 8700 villages in India suffer the problem of excess fluoride intake (Nawlakhe & Bulusu, 1989). A critical evaluation of the occurrence of fluoride in the natural waters of several states and Union territories of India (Andhra Pradesh, Delhi, Gujarat, Haryana, Karnataka, Madhya Pradesh, Orissa, Punjab, Rajasthan, Madras, Uttar Pradesh and West Bengal) indicates its presence in excess of 1.5 mg l1 in the groundwater. A maximum concentration of 20 mg l"1 was reported by Handa (1988) in the groundwater of Rajasthan. The population in most of the rural districts of the country depends on groundwater for 80% of their domestic and agricultural needs in view of the insufficient surface water resources and the lack of proper management strategies. As a result, the effect of fluoride, if present in excess, has been noticed recently.

Studies on the structural, petrological and mineralogical properties of the Eastern ghat ranges of India have revealed the presence of fluoride bearing minerals (Rao, 1976; Rao et al. 1979, 1980; Sriramdas & Rao, 1979). While reviewing the occurrence of fluoride in the water resources of the country, it was realized that a detailed investigation on the occurrence and behaviour of fluoride in specific geological settings would provide valuable information to design effective mitigation techniques. With this objective, the lower Vamsadhara River basin (Fig. 1) enclosed between longitude 83°50'-84°10'E and latitude 18°15'-18°38'N has been chosen for a detailed study of fluoride in groundwater. The Vamsadhara River basin is a medium size basin. The lower Vamsadhara basin, spreading over an area of 817 km2, constitutes 7.4% of the entire Vamsadhara basin. The study area is characterized by widespread alluvial cover underlain mostly by granite gneisses. Except for the eastern ghats flanking the western boundary and isolated hillocks, the study area is essentially a plain with gentle slopes. The rich alluvial cover has facilitated agricultural activity in 90% of the region. The most abundant rock type is garnetiferous granite gneiss (Padmanabhayya, 1958; Suryanarayana, 1957). The pegmatites occur as veins in the area (Padmanabhayya, 1958; Rao, personal communication). The other rocks include acid, intermediate, basic and ultrabasic charnockites (pyroxene amphibolite) of charnockite series and calc-granulites, garnet-sillimanite gneiss and garnetiferous quartzites of the khonatlite series.

The water levels in about 70% of the wells were below 2 m from the ground surface in the post-monsoon period (November 1992) and between 2 and 6 m in the pre-monsoon period (April 1993). Except for 20% of the wells whose water levels were lower than 8 m during pre-monsoon, the general water level was always within 6 m of the ground surface. Occasionally in five wells the water level rose to 0.3 m from the ground surface during the post-monsoon period. The groundwater flow is

The occurrence and behaviour of fluoride in the groundwater of a river basin in India 879

Fig. 1 Study area.

directed towards the main channel of the Vamsadhara River, following the topography; the hydraulic gradient varies from 5.7m km"1 in the upper reaches to 0.08 m km4 near the coast. The water level fluctuations between the pre- and the post-monsoon periods varied between 2 and 6 m. The area generally contains a surface layer of alluvium comprising various grades of sand and clay layers followed by weathered/fractured layers of khondalitic formations or granite gneisses. The four general hydrogeological sections representing the area are shown in Fig. 2.

The climate is dry with summer temperatures rising to 40°C and winter temperatures falling to 18°C. The area receives an average annual rainfall of 910 mm both from the southwestern and the northeastern monsoons. Apart from


Ground surface

Clay

Sand

Weathered/ Fractured formations

Sand

Clay


Sand


Clay


Fig. 2 Typical hydrogeological sections.

paddy, commercial crops like jowar, black gram, green gram, horse gram, sugar cane, groundnut, gingelly and mesta are grown in the region. The farmers are very conventional in using farmyard, pig and sheep manures as fertilizers. However, in recent years the farmers have been using fertilizers, mostly urea, super phosphate and NPK (15:15:15) and potash.

OCCURRENCE OF FLUORIDE

Normally fluorine, because of its high reactivity, exists in the form of fluoride in natural waters (Leung & Hrudey, 1985). Minerals like fluorite (fluorspar), fluorapatite, fluormica (phlogopite), cryolite, epidote, topaz, phosphorite, tremolite, villuanite and certain varieties of mica contribute fluoride (Raju et al., 1979; Matthess, 1982; Hem, 1986; Handa, 1988; Gaumat et al, 1992; Gaciri & Davies, 1993). The occurrence of fluoride both in igneous and sedimentary rocks is reported to be similar (Deer et al., 1983). Sinha (1986) and Narasayya (1970) report that the fluoride content varies from 0.8 to 3.6% in apatite rocks of the Singhbhum district of Bihar, the Vizianagaram district of Andhra Pradesh and the Purulia district of West Bengal. The mining of apatite deposits of Sitaramapuram in the Vizianagaram district, the region adjacent to the present study area, has been abandoned because of the high fluorine content. Wedepohl (1978) and Gaciri & Davies (1993) report the presence of fluoride from 30 to 21 000 ppm in amphiboles present in metamorphic rocks. However, though such high values have not been reported in Indian conditions, Raju et al. (1979) report high concentration of fluoride (about 5.0 mg l"1) in the groundwater present in the amphibole formations of the Anantapur district of Andhra Pradesh. The occurrences of pyroxene amphibolite, apatites and pegmatites in the form of veins were reported by Padmanabhayya (1958) and Rao (personal communication) in the study area. The recent application of phosphatic fertilizer is also observed to have a recognizable contribution to the enrichment of fluoride in the soils and groundwater.

METHODOLOGY

A total of 250 water samples from 125 locations (Fig. 1) were collected from dug wells along with 10 surface waters in the post-monsoon (November 1992) and pre-


monsoon (April 1993) periods. In situ measurements of pH, bicarbonate, dissolved oxygen and residual chlorine were made. Samples simultaneously collected and preserved with HC1 and HgCl2 were utilized to determine iron, nitrate and phosphate. In addition electrical conductivity (EC), calcium, magnesium and chloride were estimated on the same day. F, Na, K, S04, Fe, Al, N03 and P04 were determined in the laboratory using American Standard methods (APHA, 1985). The hydrochemical facies were determined through a computer program, INTCHEM developed by the author. The facies were designated as per Back (1966) and the equilibrium calculations and the saturation indices determined through WATEQF (Plummer et al, 1976). To assess the quantitative estimate of the leachate from the soils, 20 g of the soil or rock sample were added to 200 ml distilled water and rigorously stirred for 6 h using a magnetic stirrer. After filtration the leachate was analysed for fluoride.

RESULTS AND DISCUSSION

The frequency distribution and the ranges of occurrence of fluoride (F) during the post-monsoon and the pre-monsoon periods are presented in Fig. 3. The distribution of F in groundwater during both the pre-monsoon period and the post-monsoon period are presented in Figs 4 and 5. F varied from a minimum level of "below detectable limit" to 3.4 mg l"1. In 10-20% of the samples in both the pre- and post-monsoon periods F lay below the detectable limit and almost 80% of the samples contained F below 0.75 mgi1 . Ten to fifteen percent of the samples recorded a concentration of more than 1.25 mg l"1. A maximum of 3.4 mg l"1 was observed in sample no. 194 (Mamidivalasa, Fig. 1). The concentrations of F showed similar trends in both the pre- and post-monsoon periods (Figs 4 and 5). Eight isolated pockets with concentrations ranging from 1.2 to 3.4 mg l1 (marked in Fig. 5) were observed during both periods. These high concentrations are associated with the

| | Post-monsoon (Nov, 92)

1 Pre-monsoon (Apr, 93)

cO.25 0.25-.50 0.50-0.75 0.75-1.00 1.00-1.50 1.50-2.00 2.00-3.00 3.00-3.40

F (rng/I) Fig. 3 Frequency distribution of fluoride during the post-monsoon (November 1992) and the pre-monsoon (April 1993).

882 JV. Srinivasa Rao

Fig. 4 Distribution of fluoride in the groundwater of the Lower Vamsadhara basin during the post-monsoon period (November 1992).

weathered rock formations mainly represented by pyroxene amphibolites and pegmatite veins.

The difference in F concentrations between post- and pre-monsoons is shown in Table 1. The ionic concentrations in the groundwater during the post-monsoon period (i.e. at high water table conditions) were generally less than their counterparts during the pre-monsoon period because of dilution by rainwater. In contrast, the fluoride concentrations in 30% of the samples were relatively high during the post-monsoon period. This indicates contamination of groundwater from surface pollutants. As there are no industries in this region, the contamination was probably from fertilizers (Raju et al., 1979; Jha & Jha, 1982; Handa, 1988; Gaumat el al., 1992).

The F contents in the widely used fertilizers of this region are shown in Table 2(a). The farmers in this area usually spray 150 kg of fertilizer per acre per


Fig. 5 Distribution of fluoride in the groundwater of the Lower Vamsadhara basin during the pre-monsoon period (April 1993),

year in three instalments and use 10 cm of irrigation water. Superphosphate and sufala NPK (15:15:15) introduce significant quantities of F into the water (Table 2(b)). In contrast, the consumption of urea is more or less uniform throughout the area while the other fertilizer consumption varies from place to place. This may be one of the reasons for the uneven distribution of F in the groundwater (Figs 4 and 5).

The groundwater collected from adjacent wells located close to each other in 14 villages exhibits totally different chemical properties. The water from one of the wells from each pair possesses a much higher salinity than its neighbour. A physical examination of the well locations and the associated hydrogeological conditions clearly prove that the location of one of the wells is in a clayey formation (i.e. the clay percentage in the soil is more dominant than the sand and silt) while the other


Table 1 Pre- and post-monsoon variations in F concentrations.

Range of variation in fluoride (mg 1) Percentage of samples in which: F increases (Post- to pre- F decreases (Post- to monsoon) pre-monsoon)

<0.25 0.25-0.50 0.50-0.75 0.75-1.00

25 3 1 1

30

42 16 3 1

62

No change in F cone, in 8% of samples.

Table 2 Water soluble fluoride yields from fertilizers.

Name of fertilizer (a) Fluoride yield in mg kg"1 of (b) Fluoride yield in mg l"1 of fertilizer irrigation water

Urea Superphosphate SufalaNPK (15:15:15) Potash

3.8 2750.0 1675.0

10.0

trace 0.34 0.24 trace

Table 3 Fluoride concentrations and electrical conductivity values in closely spaced wells.

Village name

Priya Agraharam Narasannapeta

Srikurmam

Kollivalasa

Miriapalle

Dappapadu

Well no.

42 42A 20 20A 185 185 A 212 212A 253 253A 254 254A

Post-monsoon F (mg I"1)

0.43 0.15 0.40 0.42 0.65 BDL 0.23 BDL

(Nov. 1992): EC CuS cm1)

577 6615 1339 1008 1333 3675 672

2068

Pre-monsoon F (mg l1)

0.74 -0.74 BDL

0.42 BDL 0.28 BDL 0.64 BDL

(Apr. 1993): EC CuS cm1)

1050 -

2584 3478

735 1590 1103 5557 1681 3659

BDL: below detection limit.

from the same pair is in a sandy formation. The fluoride concentrations in the groundwater of some well pairs are given in Table 3 along with electrical conductivity values. It is clear from Table 3 that the water from clayey soils contains very much less F compared to that from sandy soils and mostly it is below the detection limit. At Narasannapeta village (well pair 20 and 20A, Table 3), the well pair shows more or less the same F concentration during the post-monsoon period while an abrupt change was observed in the pre-monsoon period. In well no. 20A, the water reaches down into a clayey formation at about 4 m below the ground surface during the pre-monsoon period. It is expected that the F gets adsorbed in clayey soils.


The equilibrium between the quantity of substance Ct bound to an adsorbent (clay) and the quantity of this substance in solution Cw can be described by Freundlich's isothermal equation (cited by Matthess, 1982):

C, = KC'l (1)

where K and n are parameters specific to any particular substance. The relationship shows that an increase in the concentration of a solution will

raise the adsorbed quantity while a decrease in the concentration will result in desorption. Chaturvedi et al. (1988) and Nawlakhe & Bulusu (1989) have reported the removal of F from water by adsorption in clay minerals. Further, Robinson & Edington (1946) reported that the adsorbed F in clay minerals is acid soluble and water leachable. Handa (personal communication) and Burt (1993) proposed the exchange or replacement of OH ions with F due to the close similarity in their ionic radius. However, Handa is doubtful about the ion exchange that can take place in clay particles which are normally present in aquifers when the F concentrations are of the order of 1 mgl"1. An evaluation of equation (1) in consonance with the possible adsorption/ion exchange phenomena discussed above suggest that the adsorption strength is sufficiently high to inhibit significant F release into water. At Buravalli village (no. 189), the F concentration in the groundwater was 0.1 mg l"1

during the post-monsoon period as the groundwater is in a clayey formation. When the water level drops into the deeper sandy formation during the pre-monsoon period, it is 0.25 mg l"1. Instead of filtration of F in the top clayey formation, the increase in F in the sandy formation indicates the possible existence of F-bearing minerals. In one of the APSIDC documents it was reported that at one location the sands contain F-bearing minerals. But no such reports were available for other locations in this area.

The weathered rock samples collected from Mamidivalasa (location no. 194, Fig. 1) and Akkivalasa (location no. 148, Fig. 1) mixed in distilled water in a 1:10 ratio were observed to yield up to a maximum of 11 mg of F for 1 kg of weathered rock. Similar experiments with soil samples from eight different locations have projected to yield F in the range of 0.5-8.0 mg kg"1.

The uneven spatial distribution and unusual seasonal trend in the F concentrations of groundwater can be explained by: (a) the relative abundance of F-bearing minerals; (b) the variation in land use; (c) the variation in soil texture; and (d) the variation in the use of phosphatic fertilizers from place to place.

However, the high concentrations of F reaching 3.4 mg l"1 in groundwater are observed to be associated with various fluoride-bearing minerals (mostly fluorite and fluorapatite). To study the relative contribution of geological and fertilizer sources, soil samples at 1.5 m depth interval collected from a 4.5 m deep vertical section at Buditi junction (location shown in Fig. 1) were subjected to leachate analysis. The water-soluble F content from the different levels of the vertical section is shown in Fig. 6. The F concentrations initially decreased from 2 mg kg"1 to 1.6 mg kg"1 up to a depth of 1.5 m from the surface from where it increased up to 4.95 mg kg"1 at a


Fluoride (mg/kg)

1 2 3 4 0

1.5

I x t-0. IU o , 3.0

4*

4.5 Fig. 6 Vertical distribution of fluoride in the soil near Buditi village.

depth of 3 m. This increase was followed by a decrease to 3.8 mg kg"1 at 4.5 m depth. Though the decreasing trend in these F concentration up to a depth of 1.5 m appeared to support the fertilizer source the variations at deeper levels indicated a change to the dominance of local geological formations as the primary source. The pollution due to these fertilizers was still below the defined permissible limit of 1.0 mgl'1.

The dominant hydrochemical facies observed in this region are magnesium bicarbonate, sodium bicarbonate and sodium chloride. The median and average F concentrations in these waters are given in Table 4. Sodium bicarbonate facies are associated with the highest fluoride concentrations. Sodium bicarbonate waters are effective in releasing fluoride from fluorite mineral (Handa, 1975). The results of solubility experiments on fluorite in the sodium bicarbonate waters carried out by Rao et al. (1993) (Table 5) indicate that in the presence of sodium bicarbonate, the concentration of F can rise to 20 mg l"1. In the study area, for example, a highest F concentration of 3.4 mg l"1 was observed in the groundwater in which the Na and HC03 concentrations were 360 mg l"1 and 926 mg l"1 respectively. However, for identical ionic concentrations, Rao et al. (1993) reported the possible dissolution of 20 mg T1 of fluoride in water for a period of 30 days (Table 5). Rao et al. (1993) conducted these experiments under ideal laboratory conditions. However, in field conditions, the other existing minerals in the subsurface and other major and minor ionic constituents of groundwater may affect the dissolution characteristics of fluorite mineral. The experiments on the dissolution of fluorite mineral in sodium bi-

Table 4 Fluoride concentrations in various types waters.

Hydrochemical facies Fluoride concentration (mg l"1): Average Median

Magnesium bicarbonate water 0.50 0.40 Magnesium-sodium bicarbonate water 0.51 0.48 Sodium bicarbonate water 1.20 0.95 Sodium chloride water 0.30 0.22


Table 5 Solubility of fluorite (CaF2) in water containing sodium bicarbonate (temperature 28°C).

Sample

1 2 3 4 5

Concentration of the ions (mg l1) Na+ HC03-

0 47 94

188 376

0 125 250 500

1000

Fluoride

2 days

5.6 5.8 7.0 9.5

15.0

present in solution (mg 1"

10 days

7.3 7.5 7.8

12.0 17.0

•') after

30 days

7.8 7.8 8.5

14.0 20.0

carbonate waters suggest the possible dissolution of fluorite mineral with simultaneous precipitation of CaC03. This can be explained by the following equations (Rao et al., 1993):

CaF2 + Na2C03 <=> CaC03 + 2Na + 2F (2)

CaF2 + 2NaHC03 o CaC03 + 2Na + 2F + H20 + C02 (3)

The equilibrium speciation calculations used in the present study determined the state of saturation of the water with respect to various minerals based on thermodynamic conditions and observed water quality parameters. The saturation status of 27 mineral facies were determined using "WATEQF". The potential for dissolution or precipitation of a specific mineral in an aqueous solution depends on the mineral's solubility and the composition of the aqueous solution (Burt, 1993). This potential is measured by the saturation index (SI) defined by Langmuir (1971) as:

SI = log(IAP/KT) (4)

or by the saturation percentage (SP)

SP = (IAPIKT) x 100 (5)

where IAP is the ion activity product in the aqueous solution of a given mineral and KT is the equilibrium constant (at a given temperature).

A value of 0 for the saturation index (or 100 for saturation percentage) indicates that an aqueous solution is in equilibrium with respect to the mineral. A negative value of SI (or SP less than 100) indicates sub-saturation of the solution with respect to the mineral, so that dissolution of the mineral is thermodynamically favoured. A positive value of SI (or SP greater than 100) indicates supersaturation of the solution with respect to a mineral, indicating thermodynamically favoured conditions for the precipitation of that mineral. The 57 of fluorapatite (5.1-12.5) indicated highly supersaturated conditions. The groundwater was sub-saturated with respect to fluorite (CaF2): the saturation percent (SP) of fluorite varied from 0.04 to 37 with an average of 6.6 and a median of 3.6 (Fig. 7). The SP of fluorite in 80% of the groundwater samples was less than 9. However, in weathered rock formations the SP of this mineral varied from 12 to 37 indicating the possible dissolution of CaF2 in the groundwater. The subsequent release of F into the groundwater would result in an increase of F concentrations. The dominance of sodium bicarbonate waters in weathered rock formations accelerates the dissolution of CaF2 (Table 4) to release

888 N. Srintvasa Rao

Fig. 7 Distribution of saturation percentage of CaF2.

fluoride into the groundwater in the course of time. According to equation (3), a simultaneous precipitation of CaC03 will occur. The data representing fluoride ion concentration, saturation percent of fluorite (CaF2), calcite (CaC03), dolomite (CaMg(C03)2) and aragonite (CaC03) in the groundwater located in the weathered rock formations presented in Fig. 8 clearly indicate the precipitation of CaC03 in most of the well waters while it is about to precipitate in some. According to equations (2) and (3), the reaction should go in the forward (i.e. right hand) direction till equilibrium is reached. The presence of other minerals containing F is likely to complicate the issue but the predominant feature will be the dissolution of fluorite that will control the solubility of F in groundwater.

Some of the significant relationships between F and other ions are presented in Fig. 9 (a)-(j). The inverse relationship between F and Ca and the positive

The occurrence and behaviour of fluoride in the groundwater of a river basin in India

FLUORIDE (mg/l)

i-j M r4 pJ H H N

(<fS)30¥iNi3<i3d NOtlVtMUVS

00


(a) t

2 0 0 -

§ 150-

Cal

ciu

m

o o

o

Ca = (-26.475JF +83.395

R = -0.395 •. (N = 94)

1 1.5 2 2.5 Fluoride (mg/l)

Ca = (-19.232)F +75.44

R = -0.855 (N = 14)

1.5 2 2.5

Fluoride (mg/l)

(C)

S F

S

1 o

m

800

600

400

200

n-

•

V • •

• -

.

• • • • •

r - , ~ • ••

HCO,=

". ^—-"""*"'

"t.

R = 0.46 (N = 95)

(128.065JF

.

+328.885

1 1.5 2 2.5

Fluoride (mg/l) 1.5 2 2.5

F l u o r i d e (mg/l)

(e)

500

„ 400

•§• 300

E .2 200 XI o <f> 100

0

Na = (38.331 )F

: • !

: • •

• l i t ;••

- 96.075

.

R = 0.24 (N = 89)

1 1.5 2 2.5 Fluoride (mg/l)

~ 300 :

ra E £ 200-=J

0 100-

0 -

/ ^

.^^ ^-^

°" ^ ^ " ^ • V ^ < ^ R = 0.886

> - " f " " (N = 14) • Na = ( 1 2 1 . 9 2 9 ) F - 5 9 . 1 4 7

2 2.5 3 F l u o r i d e (mg/l)

(g)

(1/

m

hosp

hate

(m

u.

6

• i -

4 -

3

2-

1 -

0-

i f .

a

...

s.

•

r~*^T

PO4 =

1

(0.457)F

R = 0.26 (N = 95)

+ 2.754

1 1.5 2 2.5

Fluoride (mg/l)

E.C. = (140.291)F +866.054 R = 0.21 (N = 78)

CD %

1 1.5 2 2.5

Fluoride (mg/l)

^ 4-B)

ê 3 -(U

•*-> re X 2 -

£ t-a.

o -

a D H

R = 0.39 (N = 16)

PO 4 = (0.340)F + 2 . 9 3 4

2 2.5

Fluoride (mg/l)

1500-

,0

*§. 1000-

u

UJ 500-

0-

E.C. = (370.791 )F + 4 3 9 . 2 4 9

" .^^.^^^^

R = 0.76 (N = 17)

1.5 2 2.5 3

Fluoride (mg/!)

N = No. of data points

R = Correlation coefficient

Fig. 9 Relationships between fluoride and other ions.


relationship between F and HC03 and between F and Na are in agreement with equations (2) and (3). The relationships are given for two ranges of F concentrations (i.e. 0.05^3.4 mg l1 and 1.0-3.4 mg l1). From Fig. 9, it is clear that better relationships of F with Ca, Na, HC03, P04 and EC are established in the fluoride range of 1.0-3.4 mg l"1. These higher concentrations of F are associated with weathered rock formations which contain considerable quantities of fluorite and fluorapatite minerals. However, a high scatter is observed in all relationships where the fluoride concentrations are less than 1 mg l1. These low concentrations might have been derived from different sources.

SUMMARY AND CONCLUSION

The fluoride concentrations in the groundwater varied from "below detection limit" to 3.4 mg T1. The higher concentrations of fluoride (> 1 mg l"1) in the groundwater were associated with weathered formations of pyroxene amphibolites and pegmatites. The uneven spatial distribution of fluoride in the groundwater may be attributed to: (a) the relative abundance of fluoride bearing minerals; (b) the variation in land use; (c) the variation in soil texture; and (d) the variation in the use of phosphatic fertilizers from place to place. Though phosphatic fertilizers yield significant quantities of fluoride to irrigation water, the pollution due to these fertilizers was still below the defined permissible limit of 1.0 mg 1"1. The dominance of natural geological formations over the fertilizers was also observed to contribute fluoride to the groundwater. The groundwater in the clayey soils contained much less fluoride as compared to the sandy soils. The adsorption/ion exchange of fluoride in clayey soils may be responsible for such low concentrations. Since sodium carbonate waters are more effective in releasing fluoride from fluorite mineral, high concentrations of fluoride were observed in these waters. Further increases of fluoride in the groundwater are expected as the groundwater is undersaturated with respect to fluorite. Fluoride inversely related with Ca and positively related with Na, HC03, EC and P04. Best relationships were obtained in the fluoride range of 1.0-3.4 mg l"1.

Acknowledgements The author is grateful to B. K. Handa (formerly of Central Ground Water Board, Chandigarh) for a critical evaluation of this paper. The financial support received from the Department of Science and Technology, New Delhi, is gratefully acknowledged.

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Pradesh. Indian J. Nutr. Diet. 8, 5-8. Chaturvedi, A. K., Pathak, K. C. & Singh, V. N. (1988) Fluoride removal from water by adsorption on china clay.

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