|A preliminary report on fluoride content in groundwaters of Guntur area, Andhra Pradesh, India|
Charnockites (crystallines) and alluvium (unconsolidates) are the major litho-units in the area apart from granite, quartz, pegmatite and dolerite which occur as intrusives. Lithologs of the study area reveal a black cotton soil layer of about 4 m along with weathered and fractured zones 4, 10 and 30 m, respectively in the crystallines3. Varying thickness of sandy clay, loam and clay, occur as intercalations or mixed in at different levels in the unconsolidates. About 70% of the area is canal-irrigated.
Forty nine open/dug well samples (24 from crystallines and 25 from unconsolidates) analysed for the fluoride content (Table 1). Table 1 shows wide variations in fluoride (0.60 to 2.50 mg/l) in the groundwaters; and in 26.5% of the samples, it exceeds the recommended limit (1.5 mg/l) prescribed for drinking purpose. Groundwaters in the unconsolidates have higher (1.43 mg/l) fluoride content than in the groundwaters located in the crystallines (1.32 mg/l). Leaching of the fluoride bearing apatite present in the country rocks (0.13-0.70%) of the area (Table 2) contributes the fluoride to the groundwaters4. The source for fluoride in the soil (0.71 to 1.35 mg/l) appears to be the clay minerals, which promote ion-exchange of the different elements present in the soil and circulating waters during the weathering process5. Other factors such as intensity of irrigation, pH of the draining solution, alkalinity, dissolved CO2 and pCO2 in the soil enable leaching of absorbed fluoride from the clay minerals of the soil6-8.
Evapotranspiration and residence-time of water in aquifer are other factors for further enrichment of fluoride. Alkaline waters dissolve fluoride-bearing minerals under simultaneous precipitation of CaCO3 (ref. 9). A positive saturation index for CaCO3 (0.11 to 1.51) and CaF2 (2.07 to 3.57) in the study area (Table 2), suggest a precipitation of these solid phases due˙to˙high rate of evapotranspiration (1778˙mm), and a positive correlation (Figure 1b) between fluoride and total alkalinity (TA) show an increase of fluoride from the dissolution of fluoride- containing minerals with alkalinity (pH: 7.2 to 8.6)10. As observed by earlier workers7-9, the combined effect of evapotranspiration and long-time contact of the waters in the aquifer (due to low-hydraulic conductivity of the weathered zone) activate the process of dissolution.
However, the concentrations of fluoride in groundwaters are not uniform in the area (Table 1), due to the variations in (a) the presence and accessibility of fluoride-bearing minerals to water and (b) the weathering and leaching processes4-11.
Since the fluoride content in the groundwaters of some localities is greater than the safe limit, the present study recommends defluoridation of water employing Nalgonda Technique12, which is cheaper and simpler among various methods available besides educating the people about the health-implications of fluoride content.
1.ISI, Indian Standard Specification for Drinking Water, IS: 10500, 1983.
2.WHO, International Standards for Drinking Water, Geneva, 1971.
3.Prakasa Rao, J., Ph D thesis, Andhra University, Visakhapatnam, 1997, (unpublished)%0.
4.Karunakaran, C., Proceedings of the Symposium on Fluorosis, Osmania University, Hyderabad, India, 1974, vol. 33, pp. 3- 18.
5.Robinson, W. D. and Edington, G., Soil Sci., 1946, 61, 341.
6.Ramesam, V. and Rajagopalan, K., J. Geol. Soc. India, 1985, 26, 125-132.
7.Wodeyar, B. K. and Sreenivasan, G., Curr. Sci., 1996, 70, 71-73.
8.Hem, J. D., Study and Interpretation of the Chemical Characteristics of Natural Waters, India, 3rd edn, 1991, 2254, p. 263.
9.Ramamohana Rao, N. V., Suryaprakasa Rao, K. and Schuilding, R. D., Environ. Geol., 1993, 21, 84-89.
10.Sahu, N. K. and Karim, M. A., J. Geol. Soc. India, 1989, 33, 450-456.
11.Subba Rao, N., KrishnaRao, G. and John Devadas, D., J. Environ. Hydrol., 1998, 6, 1-5.
12.Nawlakhe, W. G. and Paramasivam, R., Curr. Sci., 1993, 65, 743-748.
N. SUBBA RAO
J. PRAKASA RAO
B. NAGAMALLESWARA RAO
P. NIRANJAN BABU
P. MADUSUDHANA REDDY*
D. JOHN DEVADAS
Department of Geology,
Visakhapatnam 530 003, India
*Department of Geology,
Dr B.R. Ambedkar Open University,
Jubilee Hills, Hyderabad 500 033, India
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