Fluorine anomalies in the groundwater of the State of São Paulo

Abstract

This work examines the data compiled from the literature about the main occurrences of fluoride (F^-) in the groundwater of the State of São Paulo. The results of this study showed that at least 208 wells had high concentrations of F^- (> 1.0 mg dm^-3), 122 of which were considered anomalous (> 1.5 mg dm^-3) and harmful when ingested in large quantities. Because of its high productivity and large-scale exploitation, the Guarani aquifer is the most studied one, but other aquifers, such as the Crystalline (Precambrian), Serra Geral and Tubarão aquifers, and the Passa Dois aquitard should also be considered important. In the past, the increase in the concentration of F^- in the aquifers was related to hydrothermal processes and tectonic events with structural controls, such as the Gondwana supercontinent breakup, which would have enable the circulation of residual magma solutions, mineralization, and remobilization of the fluorine contained in the percolated rocks. Currently, the release of F^- is caused due to the rock-water interaction under alkaline pH hydrochemical conditions, in typical sodium-bicarbonated (Na-HCO3) waters, poor in Ca²⁺, and under the influence of ionic strength, contact time, depth, temperature, ion exchange capacity, among others. In fissured aquifers of granitic composition and volcanic rocks, the minerals that can release F^- into water are biotite and amphibole (where F^- is replacing OH^-, due to the similarity of its ionic radii) and, occasionally, secondary fluorite filling fractures and apatite. In sedimentary aquifers, F^- is mainly found in clay minerals structures (illite, muscovite, smectite, vermiculite, kaolinite, among others) replacing OH^-. Therefore, the high levels of F^- in the groundwater are intrinsically associated with the characteristics and types of minerals of the aquifers.