ABSTRACT

High natural fluoride (F^–) concentrations (up to 8.75 mg dm^–3) were detected in the groundwater near Salto city (São Paulo State, Brazil), on the eastern border of the Paraná Sedimentary Basin, with an average of 1.19 mg dm^–3 in a fissured aquifer and 2.01 mg dm^–3 in the sedimentary aquifer systems. The aim of the study was to evaluate granitic and sedimentary rocks (using whole rock powders at <200 mesh size), as well as biotite crystals from granites (using grains in the 35–100 mesh size fraction), as possible F^– sources in the aquifer systems. Dissolution batch tests were performed in distilled water, simulating F^– liberation in deep (Experiment I) and shallow (Experiment II) portions of aquifers and from biotite grains under the influence of high temperature, alkalinity, and salinity (Experiments III, IV, and V). In Experiment I, the F^– levels in granite leaching were very high (6.53 to 10.30 mg dm^–3) due to the small particle size fraction that increased the mineral contact area with water. The F^– concentrations (0.8 to 1.72 mg dm^–3) in the sedimentary rock leachates were compatible with the average levels found in the wells of this aquifer. The alkaline pH (time = ∞), EC (time = ∞) values, and the chemical composition of the final solutions in contact with all sedimentary rocks reflected the conditions found in the deep wells (>150 m). In Experiment II, F^– was progressively released from all rocks. Granites released more F^– (up to 8.69 mg dm^–3), which was controlled by the supersaturation of fluorite, due to Ca²⁺ from plagioclase dissolution, as well as biotite dissolution (2–5% by vol. of rocks). A good F/K value (r = 0.87) correlation implies that the F^– was released from biotite and K⁺ from biotite+K-feldspar. In sedimentary rocks, the F^– concentrations (0.12 to 0.91 mg dm^–3) are related to the low fluorine rock concentrations, and Ca²⁺ removal was controlled by calcite precipitation and/or cationic exchange of Ca²⁺ for Na⁺ in the clay minerals. With regard to biotite alteration, the influence of higher temperatures proved to be noteworthy in the process of releasing F^–, which also was more efficient under high salinity conditions (800 μS cm^–1, NaCl 0.1%) when compared with alkaline conditions (pH 8.0, NaOH 0.1%).