Abstract:
The problem of fluoride in drinking water is a major health issue in many countries. In Hard Rock (granite, gneiss) aquifers of southern India, many drinking water supplies present fluoride levels higher than threshold values (1.5 mg/L, according to WHO), beyond which, population gets affected by dental and skeletal fluorosis. Fluorine is mainly found in aqueous solution (fluoride) or in solid salt (fluorite). This fluorine is present in some minerals (micas, amphiboles) in granites, gneiss and pegmatites, and it is released in solution due to weathering of these primary minerals. In contrast, kaolinite is a sink for fluoride. Fluoride is common in semi-arid climate with crystalline igneous rocks and alkaline soils. The objective of this research project is to identify the main sources of geogenic fluoride in Hard Rock regions typical of southern India. To do so, both fresh and weathered rock samples were collected in an experimental watershed near Hyderabad (Andhra Pradesh) where high fluoride groundwater concentrations have been observed in some locations. The watershed is composed by Archean Biotite Granite (BG) and Leucocratic Granite (LG) injected by some quartz reefs, dolerite dykes arid pegmatites (BGP). Rock samples were taken from each of these main lithologies. Petrographic examination and modal analysis were first carried out and then, selected sections were chosen for detailed electron probe analysis to test the chemical composition of apatite, biotite, epidote, sphene, chlorite, and allanite minerals. In addition some weathering processes in relation with fluoride release have been identified: chloritzation of the biotite and metamictization of allanite. Another process is also inferred and may act as fl uorine sink: the formation of REE fluoro-complex in allanite only in presence of biotite. The results give the chemical composition of different minerals for each rock type. Based on the results of more than 250 probe analyses, it is found that the main fluoride-bearing minerals are in decreasing importance: apatite, sphene, biotite, allanite, epidote, and chlorite. However, according to the proportion of these minerals in the rock, the major contributors of geogenic fluoride are biotite and epidote for BG and BGP, and biotite allanite and epidote for LG.
