Fluoride dynamics in the granitic aquifer of the Wailapally watershed, Nalgonda District, India

Abstract

High concentrations of fluoride (up to 7.6 mg/L) are a recognized feature of the Wailapally granitic aquifer of Nalgonda District, Andhra Pradesh, India. The basement rocks provide abundant sources of F in the form of amphibole, biotite, fluorite and apatite. The whole-rock concentrations of F in the aquifer are in the range 240–990 mg/kg. Calcretes from the shallow weathered horizons also contain comparably high concentrations of F (635–950 mg/kg). The concentrations of water-soluble F in the granitic rocks and the calcretes are usually low (1% of the total or less) but broadly correlate with the concentrations observed in groundwaters in the local vicinity. The water-soluble fraction of fluoride is relatively high in weathered calcretes compared to fresh calcretes.

Groundwater major-ion composition shows a well-defined trend with flow downgradient in the Wailapally aquifer, from Na–Ca–HCO₃-dominated waters in the recharge area at the upper part of the catchment, through to Na–Mg–HCO₃ and ultimately to Na–HCO₃ and Na–HCO₃–Cl types in the discharge area in the lowest part. The evolution occurs over a reach spanning some 17 km. Groundwater chemistry evolves by silicate weathering reactions, although groundwaters rapidly reach equilibrium with carbonate minerals, favouring precipitation of calcite, and ultimately dolomite in the lower parts of the watershed. This precipitation is also aided by evapotranspiration. Decreasing Ca activity downgradient leads to a dominance of fluorite-undersaturated conditions and consequently to mobilisation of F. Despite the clear downgradient evolution of major-ion chemistry, concentrations of F remain relatively uniform in the fluorite-undersaturated groundwaters, most being in the range 3.0–7.6 mg/L. The rather narrow range is attributed to a mechanism of co-precipitation with and/or adsorption to calcrete in the lower sections of the aquifer. The model may find application in other high-F groundwaters from granitic aquifers of semi-arid regions.

Introduction

The detrimental effects of long-term ingestion of high concentrations of fluoride in drinking water are well known and include physiological disorders, dental and skeletal fluorosis, thyroxine changes and kidney damage (Grandjean et al., 1992). High-fluoride groundwaters have been reported from many parts of the world, particularly in arid and semi-arid areas of India, China, Sri Lanka, Spain, Mexico and many countries in Africa, western USA and south America (Edmunds and Smedley, 2005, Ayoob and Gupta, 2006). The serious health risks associated with high F concentrations in drinking water (exceeding the WHO guideline value of 1.5 mg/L; WHO, 2004) warrant investigations of fluoride chemistry encompassing a wide spectrum of hydrochemical and geochemical analyses and appropriate methods of remediation. Granitic rocks contain a relative abundance of fluoride-rich minerals such as micas, apatite and amphiboles. Fluorite (CaF₂) is the principal fluoride mineral, mostly present as an accessory mineral in granites. Dissolution of such minerals can constitute a major source of F in groundwater (Ramesham and Rajagopalan, 1985, Abu Rukah and Alsokhny, 2004, Edmunds and Smedley, 2005, Shaji et al., 2007). High concentrations in groundwater also result from evapotranspiration which may trigger calcite precipitation and result in a reduction in the activity of Ca (e.g. Jacks et al., 2005). Several studies have noted an increase in dissolved F concentrations with increasing groundwater residence time (Nordstrom and Jenne, 1977, Apambire et al., 1997, Genxu and Guodong, 2001, Edmunds and Smedley, 2005). Relatively high F concentrations have been found in some deeply circulating groundwaters along fault lines (Kim and Jeong, 2005, Kundu et al., 2001).

This paper outlines the origin of F in groundwater in a granitic watershed located in Nalgonda District, about 70 km south of Hyderabad, India. Nalgonda District is one of the poorest and most drought-prone districts of Andhra Pradesh in southern India. The area has long been associated with high groundwater fluoride concentrations which have been reported to reach up to 20 mg/L (Rammohan Rao et al., 1993). The district has given its name to an established water defluoridation technique, the Nalgonda technique, developed in the 1970s by NEERI (National Environmental Engineering Research Institute, Nagpur, India) under a UNDP Program (Nawlakhe et al., 1975). Thousands of inhabitants with paralysing bone diseases, deformities of vertebrae, hands and legs, deformed teeth, blindness and other conditions are common manifestations of this natural hazard in the district. The first fluorosis problem in Nalgonda district was reported by Siddiqui (1968). A comprehensive study of fluoride in the granitic rocks was carried out by the Geological Survey of India (1974). This study reported fluoride concentrations up to 7 mg/L in both groundwater and stream water around Wailapally, attributed to fluorite-rich zones in the bedrock (Natarajan and Mohan Rao, 1974). Clusters of fluorite were found disseminated as grains or in vein fills in the porphyritic granite gneiss (Natarajan and Murthy, 1974).

Rammohan Rao et al. (1993) concluded that the two main factors governing fluoride in groundwater from the Nalgonda District are the presence of acid-soluble F minerals and low concentrations of Ca and Mg in rocks and soils, with high concentrations of HCO₃ in circulating groundwater. Reddy (2002) reported that the F distribution in groundwaters of the Wailapally watershed is variable and does not follow any pattern in relation to topography, gradient or weathered-zone thickness. However, neither of these studies aimed to delineate systematically the spatial distributions in fluoride across the area and to understand the origins of the fluoride. A better understanding of fluoride geochemistry in the study area is important for evaluating the contamination process more precisely.

This study uses a multifaceted approach to understand the mechanism responsible for the spatial distribution and dynamics of F across a small watershed composed of a single bedrock geological unit (granitic gneiss), based on the chemical analysis of 433 groundwater samples and a number of soil profiles, rock and calcrete samples.