Hallett, B.M.; (2012) The mineralogy of fluoride mobilisation to groundwater from the peninsular granite, Andhra Pradesh, India. Doctoral thesis , UCL (University College London).

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Abstract

This thesis investigates the mineralogical sources of fluoride (F) in groundwater in two gneissic granite bedrock aquifers in Andhra Pradesh, India, with the aim of developing a conceptual framework for describing the mechanisms of F release from mineralogical sources to groundwater. This included an enquiry into the spatial variation and relative importance of the mineralogical F sources, the effect of weathering and regolith development on F distribution, and the processes of waterrock interaction concerning F. The two catchments (Maheshwaram and Wailpally) are underlain by Pre-Cambrian gneissic granite, with groundwater F concentrations often exceeding the WHO guideline limit for F in drinking water (1.5 mg/l). Samples of fresh and weathered rock were collected and analysed using optical petrology, point counting, X-ray diffraction, electron microprobe and whole rock chemical analysis, the results of which were used to develop mass balances of F occurrence through the weathering profile. The availability of fluorine for release to groundwater was investigated through batch leaching experiments. Results indicate the principal F-bearing rocks to be porphyritic granites in both catchments, with apatite, biotite, fluorite, amphiboles and titanite as the principal Fbearing minerals overall. The relative availability of F for release to groundwater (as indicated by batch leaching experiments) does not directly relate to sample total F content. Weathered samples, with typically low total F content and few F-bearing minerals may provide a source of F to groundwater through the leaching of relatively mobile and water soluble F. High F leached from calcrete samples and mafic vein samples in Wailpally may be important sources of F to groundwater where present. Generalisations of F distribution and release to groundwater at both a catchment scale and local scale are represented by a series of conceptual models, and a semiquantitative analysis of F-loss through long term weathering presented.

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