Controls on the genesis of some high-fluoride groundwaters in India

India has an increasing incidence of fluorosis, dental and skeletal, with some 62 million people at risk. High fluoride groundwaters are present especially in the hard rock areas south of the Ganges valley and in the arid north-western part of the country. The phenomenon is related to groundwater with residual alkalinity ( Ca 2 + < HCO 3 - ) . Fluoride concentrations are governed by adsorption equilibria and by fluorite solubility. Evapotranspiration leads to a precipitation of calcite, a lowering of Ca activity and increase in Na/Ca ratios, and this allows an increase in F − levels. In southern India, Mg seems to be controlled by dolomite, while sepiolite and palygorskite are Mg sinks in Rajasthan but may then release F − under alkaline conditions. The latter two minerals are probably also important sources and sinks for F − in the hydroxy-positions. The increase in the extent of sodic soils as a result of irrigation is a contributing factor to the increasing incidence of fluorosis. Remedial measures including addition of gypsum and rainwater harvesting are needed even in areas where the sodicity does not cause structural problems in the soil.