origin of fluoride in the granitic aquifer of Porto Alegre, Southern Brazil

• Published in: Environmental Geology Vol. 56; no. 8; pp. 1707 - 1719
• Main Authors: Viero, A. P, Roisenberg, C, Roisenberg, A, Vigo, A
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Berlin/Heidelberg : Springer-Verlag 01.02.2009; Springer Berlin Heidelberg; Springer; Springer Nature B.V
• Subjects: Aquifer systems; Aquifers; Calcite; Cambrian; Cenozoic; Dolomite; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Fluorides; Geochemistry; Geology; Groundwater; Hydrogeology; Hydrology. Hydrogeology; Magnesium; Mineralogy; Original Article; Physicochemical properties; Pollution, environment geology; Rocks; Silicates; Statistical analysis; Total dissolved solids; Water geochemistry; South America; Brazil; Rio Grande do Sul Brazil; Brazil, Rio Grande do Sul, Porto Alegre; Groundwater; Geochemistry; Fluoride; gneisses; fluorides; ground water; thermodynamics; calcite; aquifers; granites; hydrochemistry; lower Paleozoic; pH; fluorite; Cenozoic; fractures; Geochemistry . Groundwater; chlorides; models; salinity; calcium; dissolution; magnesium; concentration; igneous rocks; carbonates; halides; metamorphic rocks; fractured environment; Paleozoic; Cambrian; water quality; plutonic rocks; dolomite; Phanerozoic; statistical analysis
• ISSN: 0943-0105; 1866-6280; 1432-0495; 1866-6299
• DOI: 10.1007/s00254-008-1273-5

The Porto Alegre region, Southern Brazil, comprises a fractured aquifer system constituted by pre-Cambrian granitic and gneissic rocks, and a porous aquifer system formed by Cenozoic muddy-sandy to sandy sediments. A model is presented for the origin of the fluoride in the groundwater of the fractured aquifer system, based on ionic interrelations, a statistical analysis of physicochemical parameters and geochemical modeling. The fluoride is present at levels of up to 6.13 mg/L in groundwater and it arises due to the dissolution of secondary fluorite filling fractures in the granitic and gneissic rocks. The dissolution of fluorite occurs at the same time as calcite and dolomite solubilization. The statistical analysis identified three chemically distinct groups (named 1, 2 and 3) and two subgroups (1A and 1B) of groundwater in the fractured aquifer. The most significant differences between these groups are the different concentration ranges of fluoride, calcium, bicarbonate, magnesium, total dissolved solids and chloride and pH values, as well as the correlations between them. The compositional evolution of each groundwater group is governed mainly by how much high-salinity groundwater from the porous aquifer system is mixed with them and the different thermodynamic equilibrium conditions of calcite, dolomite and fluorite.