Fluoride dynamics in the granitic aquifer of the Wailapally watershed,Nalgonda District,India |
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| Authors: | DV Reddy P Nagabhushanam BS Sukhija AGS Reddy PL Smedley |
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| Institution: | 1. Department of Biology and Environmental Science, Linnaeus University, SE-391 82 Kalmar, Sweden;2. Department of Soil and Environment, Swedish University of Agricultural Sciences, Box 7014, SE-750 07 Uppsala, Sweden;3. Division of Land and Water Resources Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden;1. Health Care Department, Metropolitan Autonomous University-Xochimilco, Mexico City, Mexico;2. Postgraduate Dental Program, School of Dentistry, National Autonomous University of Mexico, Mexico City, Mexico;3. Oral Health Branch, Ministry of Health, Mexico City, Mexico;4. Department of Epidemiology Ocular, Instituto de Oftalmología Conde de Valenciana, Mexico;5. Institute of Geology, National Autonomous University of Mexico, Mexico City, Mexico;1. University of Patras, Department of Geology, Section of Applied Geology and Geophysics, Laboratory of Hydrogeology, Rio-Patras GR 26504, Greece;2. University of Patras, Department of Geology, Section of Earth materials, Rio-Patras GR 26504, Greece;1. Department of Geology, University of Nairobi, P.O Box 30197, Nairobi, Kenya;2. Geoforschungs Zentrum, Telegrafenberg, 14473 Potsdam, Germany;3. Senckenberg Biodiversity and Climate Research Centre, Senckenberganlage 25, 60325 Frankfurt, Germany;4. Institut für Geowissenschaften, Goethe Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt, Germany;5. UFZ-Helmholtz-Centre for Environmental Research, Department of Hydrogeology, Permoserstr. 15, 04318 Leipzig, Germany |
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| 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. |
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