groundwater quality: focus on fluoride concentration in rural parts of

Hydrological Sciences–Journal–des Sciences Hydrologiques, 48(5) October 2003

Open for discussion until 1 April 2004

835

Groundwater quality: focus on fluoride concentration in rural parts of Guntur district, Andhra Pradesh, India

N. SUBBA RAO Hydrogeology Laboratory, Department of Geology, Andhra University, Visakhapatnam 530 003, India [email protected]

Abstract Hydrogeological investigations have been carried out in rural parts of Guntur district, Andhra Pradesh, India where agriculture is the main occupation. Granite gneisses associated with schists and charnockites are the main lithological formations, which are overlain by black cotton soils. Groundwaters are alkaline, very hard and mostly brackish. Possible sources of fluoride (F-) are weathering and leaching of F--bearing minerals under the alkaline environment. A high rate of evapo-transpiration, longer residence time of waters in the aquifer zone, intensive and long-term irrigation, and heavy use of fertilizers are the supplementary factors to further increase the F- content in the groundwaters. The investigated area has been classified into three types with reference to concentration of F- prescribed for drinking: low-F-

(<0.60 mg l-1), moderate-F- (0.60–1.20 mg l-1) and high-F- (>1.20 mg l-1). Forty-five percent of the total groundwater samples belong to the high-F- category. Dental fluorosis is noticed in the region. A groundwater management programme is suggested. Key words groundwater quality; fluoride; health; groundwater management; Andhra Pradesh, India

Qualité d’eaux souterraines: le problème de la concentration en fluorure en zones rurales du district de Guntur (Andhra Pradesh, Inde) Résumé Des recherches hydrogéologiques ont été menées dans des secteurs ruraux du district de Guntur (Andhra Pradesh, Inde), où l’agriculture est l’activité dominante. Les formations lithologiques principales sont des granito-gneiss associés à des schistes et à des charnockites, et sont couvertes de vertisols de type “black cotton”. Les eaux souterraines sont alcalines, très dures et essentiellement saumâtres. Des sources potentielles de fluorure (F-) érodent et lessivent les minéraux portant du F- dans l’environnement alcalin. Un fort taux d’évapotranspiration, des temps de résidence de l’eau dans la zone aquifère plus longs, une irrigation intensive et durable sur le long terme, et l’utilisation importante de fertilisants sont des facteurs aggravant l’augmentation de la teneur en F- dans les eaux souterraines. La zone d’étude a été décrite selon trois classes, en référence à la teneur en F- préconisée pour la consommation humaine: teneur faible (<0.60 mg l-1), modérée (0.60–1.20 mg l-1) et forte (>1.20 mg l-1). Quarante cinq pour-cent de l’ensemble des échantillons d’eau souterraine relèvent de la classe de forte teneur en F-. Des cas de fluorose dentaire ont été recensés dans la région. Nous suggérons un programme approprié de gestion des eaux souterraines. Mots clefs qualité d’eaux souterraines; fluorure; santé; gestion des eaux souterraines; Andhra Pradesh, Inde

INTRODUCTION

Fluoride (F-) is important for development of healthy teeth and bones. Low-F- water (<0.60 mg l-1) causes dental caries and high-F- water (>1.20 mg l-1) results in fluorosis (ISI, 1983). More than 23 countries in the world, including India, have problems with

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F- in the drinking water (Susheela, 1999). In India, about 62 million people in 17 states, including Andhra Pradesh, are affected with dental, skeletal and/or non-skeletal fluorosis (Susheela, 1999). Most of Andhra Pradesh falls within the highly endemic fluorosis zones. Also, the first case of endemic fluorosis in the country (India) was reported as long ago as 1937 in Prakasam district, Andhra Pradesh (Shortt et al., 1937). As the assessment of groundwater quality has not been given due importance, water-borne diseases have become common (Niranjan Babu et al., 1997; Subba Rao et al., 1999, 2002). About 80% of the diseases in the world are due to poor quality of drinking water (WHO, 1984). Guntur district (11 391 km2) in Andhra Pradesh, India (Fig. 1) has a population of 4.4 million with about 72% of the population distributed in 729 villages. Even though dental fluorosis caused by high F- content occurs in the district, studies on groundwater quality in the area are scarce, especially with reference to F- (Anonymous, 1994; Subba Rao, 1995, 1998, 2002a, 2003; Subba Rao & John Devadas, in press; Subba Rao et al., 1998b, 2002; Subba Rao & Rao, 2003). Therefore, 40 villages belonging to the three administrative units, or mandals, of Medikonduru, Phirangipuram and Muppala in Guntur district were selected for assess-ment of F- in the groundwaters. There are reports in the literature on the F--bearing minerals associated with granite gneisses and charnockites in different parts of Andhra Pradesh (Ramamohana Rao et al., 1993; Subba Rao et al., 1998b; Saxena & Ahmed, 2001; Subba Rao & John Devadas, 2003), which are similar to those of the investigated area. An attempt has been made to investigate the causes of high F- in the groundwaters and to demarcate the safe and unsafe zones with reference to F- content. Thus, this study may help in a decision-making process for groundwater management in that region.

Fig. 1 Map showing the investigated area and its features.

Groundwater quality: focus on fluoride concentration in rural parts of Guntur, India 837

THE STUDY AREA

Location and climate

The study area, lying between latitudes 16°15′–16°22′30′′N and longitudes 80°03′20′′–80°22′13′′E (Fig. 1; with a population of 0.16 million) experiences a semiarid climate. The maximum temperature is 41°C in May (summer) and the minimum temperature is 17°C in January (winter). The average annual rainfall is about 800 mm, occurring mostly (70%) during the southwest monsoon (June–September). The potential evapo-transpiration is 1770 mm (Subba Rao et al., in press).

Physiography

The region is a plain (almost flat) with a gentle slope, towards the south (Fig. 1). It has a hill with a peak altitude of 510 m a.m.s.l. on the southeast, running in a NE–SW direction with a dip towards the southeast. The soils of the region contain various proportions of clay, silt, sand, gravel and pebbles with intercalations of kankar (CaCO3concretion) and the total thickness ranges up to 12 m below ground level (b.g.l.); the black cotton soils are dominant. The area has a sub-dendritic drainage of ephemeral nature. Surface runoff occurs only during the rainy season. There is both surface and subsurface water irrigation, resulting in the recirculation of groundwater.

Geology

The geological formations in the investigated area (Fig. 1), namely granite gneisses (associated with a limited occurrence of schists) and charnockites, are Precambrian. The granite gneisses cover almost a half of the area towards the northwest and the charnockites occupy the remaining part. Dikes, and pegmatite, quartz and granite veins, which occur to a limited extent, are present in the rocks. The granite gneisses are light grey, coarse grained, consisting of orthoclase, microcline, hornblende, biotite and magnetite. The schists comprise hornblende schists, quartz-feldspar schists and mica schists. The charnockites are fine- to medium-grained and are characterized by orthoclase and plagioclase feldspars, hypersthene, diopside, apatite, ilmenite and magnetite. The rocks show NE–SW strikes with a dip of 70° SE.

Hydrogeological framework

Groundwater occurs under phreatic conditions in the low hydraulic conductivity weathered zone (shallow aquifer) and under semi-confined to confined conditions in the high hydraulic conductivity fractured zone (deep aquifer). Secondary porosity in the region leads to a development of weathered and fractured zones, which transmitsurface water into sub-surface as groundwater. The weathered zone can extend down to 15 m b.g.l., whereas the fractured zone lies between 12 and 41 m b.g.l.. The pre-monsoon (May) depth to water table is 5–16 m b.g.l. and post-monsoon (November) it is 2–8 m b.g.l., indicating a fluctuation of 3–8 m and also a significant change in annual groundwater storage. In its flow, groundwater mainly follows the topographic

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gradient. Rainfall is the main source of groundwater recharge in the investigated area (Subba Rao, 2002a).

Land use

Agriculture is the main occupation in the study area. The Indian Remote Sensing ID satellite imageries show that the agricultural area covers 91.26% of the total geographical area (Subba Rao et al., in press). Water bodies (3.44%), forest (3.04%), wasteland (1.18%) and built-up areas (1.08%) account for the remaining part.

METHODOLOGY

Water samples from 40 open dugwells located in 40 villages in Guntur district, Andhra Pradesh, India (Fig. 1) were collected during the pre-monsoon (May) and post-monsoon (November) seasons of 1999. They were analysed for major ion chemistry, employing the standard methods (APHA, 1992): Hydrogen ion concentration (pH) and specific electrical conductivity (SEC) were measured, using pH and SEC meters. Total dissolved solids (TDS) were computed by multiplying the SEC by a factor (0.55–0.75), depending on the relative concentrations of ions. Total alkalinity (TA) as CaCO3, carbonate (CO −2

3 ) and bicarbonate (HCO −3 ) were estimated by titrating with

HCl. Total hardness (TH) as CaCO3 and calcium (Ca2+) were analysed titrimetrically, using standard EDTA. Magnesium (Mg2+) was computed, taking the difference between TH and Ca2+ values. Sodium (Na+) and potassium (K-) were measured by flame photometer. Chloride (Cl-) was estimated by standard AgNO3 titration. Sulphate (SO −2

4 ), nitrate (NO −3 ) and fluoride (F-) were analysed, using a spectrophotometer. All

the parameters are expressed in milligrams per litre (mg l-1) and millimoles per litre (mmol l-1) except pH (units). The analytical precision for the measurements of cations (Ca2+, Mg2+, Na+ and K-) to anions (CO −2

3 , HCO −3 , Cl-, SO −2

4 , NO −3 and F-) is

indicated by the ionic balance error, which is observed to be within the stipulated limit of ±5% (Mandel & Shiftan, 1981). An index of weathering was computed, adopting the procedure suggested by Ramesam & Rajagopalan (1985). The values of pCO2, saturation index (SI) with respect to CaCO3 and CaF2, and an index of base exchange (IBE) were computed, employing the procedures of Garrels & Christ (1965), Schoeller (1965) and Raymahashay (1988).

RESULTS AND DISCUSSION

The results of analytical parameters of both pre- and post-monsoon groundwaters are presented in Table 1. One sample, well no. 18 (Fig. 1), with extraordinarily high values of all the chemical parameters, because of its occurrence in a thick clayey horizon, was considered as a freak saline pocket. The groundwaters have pH in the range of 7.1–8.5, indicating an alkaline medium with a TA of 115–655 mg l-1, being controlled by CO −2

3

and HCO3- ions. According to the TDS classification (Fetter, 1990), the majority of the


Table 1 Summarized hydrogeochemistry of the rural parts of Guntur district, Andhra Pradesh, India.

Pre-monsoon Post-monsoon Chemical parameter Min. Max. Mean SD CV Min. Max. Mean SD CVpH (units) 7.1 8.4 7.90 0.50 6.33 7.0 8.5 7.9 0.60 7.59 TDS (mg l-1) 750 9970 1646.93 1422.36 86.364 750 10430 1796 1474.21 82.08 TA (mg l-1) 115 655 296.0 127.09 42.94 155 590 316.57 120.02 37.91 TH (mg l-1) 200 845 384.63 147.91 38.46 194 989 424.57 157.78 37.16 CH (mg l-1) 115 655 296.00 127.09 42.94 155 590 316.57 120.02 37.91 NCH (mg l-1) 85 190 88.63 20.82 23.49 39 399 108.00 37.76 34.96 Ca2+ (mg l-1) 30 120 54.75 20.83 38.05 20 100 54.20 18.02 33.25 Mg2+ (mg l-1) 26 145 60.13 25.25 41.99 35 180 70.40 27.45 38.99 Na+ (mg l-1) 95 3100 385.78 456.11 118.23 126 3440 422.15 504.07 119.41 K+ (mg l-1) 12 360 30.50 55.16 18.09 4 293 240.25 45.20 18.81 CO3

2- (mg l-1) 30 60 42.11 14.75 35.03 18 36 19.35 4.80 24.81 HCO3

- (mg l-1) 140 800 360.23 154.57 42.91 153 647 346.98 136.70 39.40 Cl- (mg l-1) 120 4680 564.33 691.59 122.55 106 4807 644.83 725.63 112.53 SO4

2- (mg l-1) 12 663 145.25 120.73 83.12 20 658 150.48 124.05 82.44 NO3

- (mg l-1) 13 56 8.63 4.72 54.69 19 72 45.78 14.98 32.72 F- (mg l-1) 0.30 1.80 1.12 0.39 34.82 0.60 2.30 1.30 0.46 35.38 SD: Standard deviation; CV: Coefficient of variation.

samples (93.8%) are of brackish type with TDS greater than 1000 (1010–10430) mg l-1. The remaining samples are classified as freshwater with TDS less than 1000 (750–900) mg l-1. The TH (Ca2+ plus Mg2+) varies from 194 to 989 mg l-1, belonging to the “very hard” category (Twort et al., 1974). The relationship of TA to TH in the groundwaters suggests that the waters have both carbonate hardness (CH) and non-carbonate hardness (NCH) with the corresponding concentrations of 115–655 and 39–399 mg l-1; the former is predominant. The summarized hydrochemistry of the groundwaters is presented in Table 1. The hydrogeochemical facies is of the type: Na+ > Mg2+ > Ca2+ > Cl- > CO −2

3 + HCO −3 >

SO −24 . The groundwater in the investigated area is of meteoric origin (Subba Rao,

2002a). Based on the concentrations of F- prescribed for drinking water (ISI, 1983), the investigated area could be classified into three categories: low-F- with <0.60 mg l-1;moderate-F- with 0.60–1.20 mg l-1; and high-F- with >1.20 mg l-1 (Table 2). The spatial distribution of F- concentration in the pre-monsoon groundwaters shows that 5% of the total water samples, in isolated zones in the northern and central parts, fall within the low-F- category; 55%, in the northwestern, southwestern, central and eastern parts, are in the moderate-F- category; and the remaining 40%, in the northwestern, northeastern and southern parts, are classed as high-F- (Figs 2(a) and 3(a)). For the post-monsoon period, 55% of the total groundwater samples are in the moderate-F- category, from the northwestern, southwestern, northern, northeastern and southeastern parts (Figs 2(b) and 3(a)). The remaining water samples (45%), coming under the high-F- category, are from the eastern, western, southern and northwestern parts. The low-F-category is not represented in samples from the post-monsoon period. Comparison of chemical parameters with respect to the increasing range of F-

reveals that there is an increase in pH, TDS, TA, TH, CH, Ca2+, Mg2+, Na+, K+, CO −23 ,

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Table 2 Groundwater quality with reference to F- in the rural parts of Guntur district, Andhra Pradesh, India.

F- range (mg l-1):<0.60 0.60–1.20 >1.20 <0.60 0.60–1.20 >1.20

Chemical parameter

Pre-monsoon Post-monsoon pH (units) 7.65 7.74 8.17 No samples 7.85 8.11 TDS (mg l-1) 1260.00 1291.54 2196.44 - 1431.37 2241.67 TA (mg l-1) 214.71 281.58 469.80 - 262.71 396.31 TH (mg l-1) 255.10 311.55 496.70 - 348.95 521.55 CH (mg l-1) 214.71 281.58 469.80 - 262.71 396.31 NCH (mg l-1) 40.39 29.97 26.90 - 86.24 125.24 Ca2+ (mg l-1) 38.00 44.45 71.00 - 45.37 57.36 Mg2+ (mg l-1) 39.00 48.82 77.75 - 65.00 87.44 Na+ (mg l-1) 307.00 290.82 957.44 - 325.00 539.17 K- (mg l-1) 24.50 27.00 49.81 - 17.68 30.32 CO3

2 (mg l-1) 15.00 30.00 43.94 - 17.18 30.32 HCO3

- (mg l-1) 231.50 282.59 484.94 - 285.64 421.94 Cl- (mg l-1) 395.00 438.68 758.25 - 504.45 810.83 SO4

2- (mg l-1) 102.50 88.05 205.94 - 109.09 210.06 NO3

- (mg l-1) 48.00 34.59 42.25 - 41.68 50.78 F- (mg l-1) 0.30 0.89 1.58 - 0.90 1.69 Saturation index (SI) CaF2 2.51 3.07 3.38 - 3.07 3.33 CaCO3 0.11 0.26 1.09 - 0.40 0.78

Fig. 2 Spatial distribution of F- (a) in pre-monsoon and (b) post-monsoon groundwaters.


Fig. 3 (a) Wells classified according to range of F- (mg l-1); (b) lithogenic Na+ vs F-

and (c) TA:TH vs F-.

HCO −3 , Cl- and SO −2

4 , but a decrease in NCH and NO −3 in the pre-monsoon

groundwaters, whereas all the chemical parameters in the post-monsoon groundwaters show an increasing trend (Table 2). Sources of F- in natural waters are apatite and fluorite, besides the replacement of OH- by F- ion in mica, hornblende and soil consisting of clay minerals (Robinson & Edington, 1946; Hem, 1991). The rock types of the other parts of Andhra Pradesh, which are similar to those of the investigated area, show apatite between 0.13 and 0.70 (wt%) in the charnockites, whereas apatite, fluorite, biotite, muscovite and hornblende are in the ranges 0.20–1.20, 0.10–1.33, 0.10–3.90, 0.10–1.95 and 0.10–8.20 (wt%), respectively, in the granite gneisses (Ramamohana Rao et al., 1993; Subba Rao et al., 1998b; Saxena & Ahmed, 2001; Subba Rao & John Devadas, 2003). As a positive relationship has been established between F- content in the bulk rocks and in the associated groundwaters (Ramamohana Rao et al., 1993; Bårdsen et al., 1996; Wodeyar & Sreenivasan, 1996; Subba Rao et al., 1998a,b; Perez & Sanz, 1999; Suma Latha et al., 1999; Saxena & Ahmed, 2001; Subba Rao & John Devadas, 2003), the presence of F- in the groundwaters of the investigated area is attributed to the host rocks. The concentration of F- in the groundwaters is found to increase with an increase in Na+ (Table 2). A significant positive correlation (r = 0.87) occurs between F- and lithogenic Na+ (Fig. 3(b)). Thus, the lithogenic Na+ can be used as an index of weathering of minerals (Ramesam & Rajagopalan, 1985). The weathering caused by alternative wet and dry conditions of the semiarid climate is responsible for the

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Table 3 Chemical parameters computed on the basis of the chemical data of pre-monsoon groundwaters.

Chemical parameter

Mean Notes

pCO2 2.31 p: expressed in log of atmosphere Mg2+:Ca2+ 1.75 Ionic values expressed in mmol l-1

Na+:Ca2+ a 10.15 Ionic values expressed in mmol l-1

IBE –0.12 CA1 = Cl- – Na+ + K+: Cl- (meq l-1) –0.23 CA2 = Cl- – Na+ + K+: CO3

2-+ HCO3- (meq l-1)

leaching of F- from the minerals in the soils and rocks (Ramamohana Rao et al., 1993; Wodeyar & Srinivasan, 1996; Subba Rao et al., 1998a,b; Saxena & Ahmed, 2001; Subba Rao & John Devadas, 2003). In addition to this, intensive and long-term irrigation allows the circulating waters easy access to the weathering minerals, which contributes leachable F- to the groundwaters (Ramamohana Rao et al., 1993; Wodeyar & Sreenivasan, 1996; Subba Rao et al., 1998a,b, 2002; Saxena & Ahmed, 2001; Subba Rao & John Devadas, 2003). The thick soil cover in the study region facilitates an interaction of the soils with groundwaters and other environmental factors (Subba Rao, 2002a,b, 2003; Subba Rao et al., 1999). The F- content in the adjacent soil, consisting of clays, varies from 0.71 to 1.35 mg l-1 (Subba Rao et al., 1998b). During weathering, the clay minerals promote ion exchange among the elements present in the soils and also among those present in the circulating waters (Robinson & Edington, 1946; Hem, 1991). This is probably responsible for the formation of the sources of F- in the soils (Wenzel & Blum, 1992; Ramesam & Rajagopalan, 1995; Sahu & Karim, 1989; Subba Rao et al., 1998a,b; Subba Rao & John Devadas, 2003). The value of pCO2 (2.31) computed for the pre-monsoon groundwaters in the investigated area (Table 3) is higher than that in the atmosphere (3.50), because of both decay of organic matter and root respiration in the soil zone. This suggests an increase in pH and the subsequent supersaturation of CaCO3. The formation of kankar (CaCO3),a feature of the investigated area, suggests a long history of evaporation (Datta & Tyagi, 1996), leading to the precipitation of CaCO3. As the alkaline waters mobilize F-

from the soils/rocks, they also dissolve CaF2 with the simultaneous precipitation of CaCO3 (Ramamohana Rao et al., 1993). Therefore, the positive values of SI with respect to CaCO3 (0.11–1.09) and CaF2 (2.51–3.38; Table 2) are thought to be caused by the high rate of evapotranspiration of the investigated area and conform to the results of Handa (1975) and Subba Rao (2002a,b). The increasing SI values of CaCO3and CaF2 with an increase in F- suggest that the evaporation controls the concentration of F- in the groundwaters. The amount of TA and TH increases with the increase in F- (Table 2); F- has a positive correlation (r = 0.81) with TA:TH (Fig. 3(c)), indicating a greater relationship with TA. This is because of precipitation of TH as carbonates (Gaciri & Davis, 1993). The average concentration of CH (296 mg l-1) compared to NCH (88.63 mg l-1; Table 1), supports this view, as the CH is dominated by CO −2

3 and HCO −3 ions, whereas the

NCH is dominated by Cl-, SO −24 and NO −

3 ions. The solubility of CaF2 increases with the increase in TA in the groundwaters (Ramamohana Rao et al., 1993; Saxena &


Ahmed, 2001), according to the following reactions:

CaF2 + CO −23 → CaCO3 + 2F − (1)

CaF2 + 2HCO −3 → CaCO3 + 2F − + H2O + CO2 (2)

Thus, a positive relationship of TA with F- (Table 2) suggests the dissolution of F--bearing minerals in the groundwaters. In these reactions, the concentration of dissolved ionic species and the pH of the water play important roles. The presence of CaCO3 also favours the dissociation of F- from F--containing minerals. These reactions are as follows:

CaCO3 + H+ + 2F − → CaF2 + HCO −3 (3)

CaF2 → Ca2+ + 2F − (4) The pH of the circulating waters also shows an increasing relationship with the increase in F- (Table 2), further supporting the leaching of F- from the F--bearing minerals, as is also reported by Wodeyar & Sreenivasan (1996) and Subba Rao et al.(1998b). The residence times of waters with the aquifer materials also significantly regulate the F- concentrations in the groundwaters (Ramamohana Rao et al., 1993; Wodeyar & Sreenivasan, 1996; Subba Rao et al., 1998a,b, 1999; Saxena & Ahmed, 2001). For instance, the groundwaters in the investigated area are associated with the weathered zone of low hydraulic conductivity. The clay materials reduce the hydraulic conductivity to some extent. The occurrence of intrusive bodies creates groundwater barriers (Karanth et al., 1992; Subba Rao, 1992; Singh & Jamal, 2002). With the increase in residence time between the waters and aquifer materials due to the weathered zone and intrusive bodies, the dissolution of minerals, including that of F--bearing minerals, and ion exchange activity between OH- and F- also increase, thus contributing to the concentration of F- in the groundwaters. As the precipitation of CaCO3 results in a decline of Ca2+, the latter has a lower concentration than Mg2+ and Na+ (Table 3). Further, the exchange of Ca2+ for Na+ ions in waters (Hem, 1991) also reduces the concentration of Ca2+. Negative values (–0.12 to –0.23) of IBE computed in the pre-monsoon groundwaters in the investigated area suggest the enrichment of Na+, because the wells in the investigated area are associated with clay minerals. Thus, because of weathering, precipitation and ion exchange, the concentrations of Na+ in the groundwaters are higher than those of Mg2+ and Ca2+. The resistance to weathering of K+ and its fixation in the clay minerals causes low concentrations of K+ among the cations in the groundwaters of the area (Table 1). Probably, these factors suggest that the F- increases with increases in Na+, Mg2+, Ca2+

and K+ in this same order (Table 2). Generally, a high rate of evapotranspiration during the dry pre-monsoon climate causes a low freshwater exchange and results in precipitation of salts, including F--rich salts, temporarily in the top layers of the soil. These salts act as a semi-permanent reservoir of easily soluble fluorine, after the monsoon. In the subsequent monsoon, the infiltrating waters leach these soils, with high concentrations of salts, resulting in greater salinity (TDS) and addition of F- (Table 2), which conforms with the observations of Schuiling et al. (1988), Gupta (1991), Gupta et al. (1993), Venkateswara Rao et al. (1996) and Subba Rao et al. (2002). A higher concentration of

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TDS enhances the ionic strength, leading to an increase in the solubility of CaF2 in the groundwater (Perel’man, 1977). Hence, the concentration of F- is observed to be greater in the post-monsoon groundwaters than in pre-monsoon groundwaters; along with high TDS, whereas the concentration of NCH decreases in the pre-monsoon groundwaters, but increases in the post-monsoon ones with increasing F-

concentrations (Table 2). This suggests that the higher concentrations of Cl-, SO −24 and

NO −3 along with F- reach the groundwater after the monsoon. The higher

concentrations of Cl-, SO −24 and NO −

3 with F- in the post-monsoon groundwaters is attributed to the heavy use of fertilizers for higher crop yields (Subba Rao & John Devadas, in press; Subba Rao, 2002a; Subba Rao et al., 2002), further supporting the role of leaching and dissolving activities in the investigated area. As a result, 45% of the total groundwater samples from the post-monsoon period are above the threshold limit of F- (1.20 mg l-1), compared to the only 40% of those from the pre-monsoon period (Fig. 3(a)).

GROUNDWATER MANAGEMENT

Incidences of the harmful effects of F- on health are on the increase, due not only to the widespread occurrence of F--bearing minerals in the Earth’s crust, but also to the impacts of environmental factors and human activities (Handa, 1975; Ramamohana Rao et al., 1993; Wodeyar & Sreenivasan, 1996; Subba Rao et al., 1998a,b, 2002; Suma Lath et al., 1999; Susheela, 1999; Subba Rao & John Devadas, in press, 2003; Subba Rao, 2002a; Subba Rao & Rao, 2003). Therefore, a sustainable management plan on fluorosis is long overdue and is required for the study area. For formulating a sound management plan in the area, the following recommendations are made for the supply of high-quality groundwater with safe concentrations of F-:– High F-concentration waters should be discouraged through regulations such as the

provision of protected water-supply schemes and the control of groundwater usage for drinking purposes through effective monitoring.

– Foods rich in calcium and phosphorous are recommended, as the rate of accumulation of F- in the human body decreases when these are consumed (WHO, 1984; Deshmukh & Chakravarti, 1995).

– The adoption of an activated alumina adsorption technique is recommended for defluoridation, to reduce the F- content in water (Susheela, 1999) where TH exceeds TA in the groundwaters.

– Drip irrigation should be preferred over traditional irrigation practices, such as flooding, to prevent weathering and leaching and to reduce water consumption and evaporation.

– An ecologically suitable vegetative cover would reduce the evapotranspiration from the soil, increase the rainfall and thus help to reduce the otherwise increasing concentration of salts.

– Judicious use of fertilizers to obtain higher crop yields should be based on crop requirements and soil characteristics.

– Recharging the underground aquifer through the rainwater harvesting at appropriate locations can reduce the F- content significantly through dilution (Eenadu Daily Newspaper, 2001).


– In the process of planning, an assessment of groundwater quality should be given due importance before consent is given for land development, to prevent groundwater contamination and spread of water-borne diseases.

– Environmental awareness of the health implications of F- should be emphasized through education of the public and community participation.

CONCLUSIONS

The present investigation on groundwater quality with reference to F- concentration in rural parts of Guntur district, Andhra Pradesh, India indicates that the groundwaters are alkaline, of medium to very hard category and mostly brackish. Weathering and leaching of F--bearing minerals under the alkaline environment account for the enrichment of F- in the groundwaters. A high rate of evapotranspiration, long-term contact of waters in the weathered zone by virtue of its low hydraulic conductivity and stagnation of water in the aquifer zone caused by intrusive bodies, intensive and long-term irrigation, and heavy use of fertilizers are the supplementary factors to further increase the F- content in the groundwaters. In 45% of the total groundwater samples, the F- content is higher than the maximum permissible limit (1.20 mg l-1). A management programme is needed at the community level to reduce the excess F-

intake. This study is confined to a small area; however, the approach is applicable to other locations with the same problem. In order to enable sustainable development of groundwater resources, it is necessary to delineate the safe and unsafe zones with reference to F- content.

Acknowledgements The author is thankful to Dr D. John Devadas, Research Fellow-DST, Department of Geology, Andhra University, Visakhapatnam, India for his help in the field as well as in the Laboratory work. Financial support of the author by the Department of Science and Technology (DST), Government of India, New Delhi under the major research project (ESS/72/023/98) is gratefully acknowledged. The author also thanks Professor A. Bårdsen, University of Bergen, Norway and an anonymous referee for their valuable suggestions in improving the manuscript.

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Received 20 March 2002; accepted 19 July 2003

groundwater quality: focus on fluoride concentration in rural parts of

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