Hydrogeochemical modeling of fluoride contents in groundwater in outcrop area of Guarani Aquifer System, southern Brazil

Abstract

Groundwater quality problems with high fluoride contents were studied in an outcrop area on a portion of Guarani Aquifer System (GAS), in the state of Rio Grande do Sul, southern Brazil. The methodology was based on the evaluation of tectonic structures lineaments and geochemical modeling of 13 groundwater physical-chemical data with anomalies of fluoride from tube wells. The lineaments trace and speciation code PHREEQC 3.3.5 used suggest the occurrence of distinct mineralogical contributions, in which the precipitation of carbonate minerals from the mineral structure of the GAS sandstone and cation exchange play a fundamental role in the hydrogeochemistry of fluoride. The results were analyzed by the main transects, which allowed the definition of characteristics of mineral dissolution and precipitation in the environment. The halite and sylvite showed dissolution tendencies along the traced transects, towards the mineral equilibrium. The fluorite mineral presented undersaturation with tendencies to mineral equilibrium and, possibly, precipitation in the most salinized waters. The applied methodology allowed the extraction of information for a better understanding of the hydrogeochemical dynamics of salinized waters and their water/rock interactions in outcrop area of the GAS, constituting a material to support the research of anomalous fluoride concentrations in groundwater and the practices of management of this important aquifer system.

Keywords

• Groundwater
• Geochemical modeling
• Fluoride