The origin of fluoride in the granitic aquifer of Porto Alegre, Southern Brazil |
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Authors: | A P Viero C Roisenberg A Roisenberg and A Vigo |
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Institution: | (1) Departamento de Mineralogia e Petrologia, IG-UFRGS, Av. Bento Gon?alves, 9500 Bairro Agronomia, Porto Alegre, Rio Grande do Sul, CEP 91501-970, Brazil;(2) PETRBRAS, Av. Bento Gon?alves, 9500 Bairro Agronomia, Porto Alegre, Rio Grande do Sul, CEP 91501-970, Brazil;(3) Departamento de Estatística, IM-UFRGS, Av. Bento Gon?alves, 9500 Bairro Agronomia, Porto Alegre, Rio Grande do Sul, CEP 91501-970, Brazil |
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Abstract: | 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. |
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Keywords: | Geochemistry Groundwater Fluoride |
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