Hidrochemistry of the Guarani Aquifer System and implications for its management
DOI:
https://doi.org/10.21701/bolgeomin.123.3.006Keywords:
chemical baseline, fluorine, GAS, hydrogeochemical modellingAbstract
Within the framework of the Project for the Sustainable Management of the Guaraní aquifer system (GAS) we have studied the chemical baseline of its distribution and origin in order to build up a sound scientific understanding of the aquifer and thus be able to support its correct management. We have used chemical data obtained within this project and also in former studies. The baseline chemical facies, their distribution and the possible hydrogeochemical reactions involved were studied with the support of classical graphic tools. The hydrochemical conceptual model was checked with mass balances and inverse modeling, and the relative contribution to the balances of every thermodynamically feasible reaction was assessed. The GAS chemical baseline is quite homogeneous. Three chemical facies were found throughout the aquifer as a whole, appearing in the same order downstream from the outcropping recharge areas towards the confined sectors in the centre of the basin: facies A (Ca-HCO3 and Mg-HCO3 water), produced mainly by rainwater infiltration dissolving CO2 and carbonate minerals; facies B (Na-HCO3 water), caused mostly by facies A water undergoing cationic exchange and carbonate dissolution; and facies C (Na-HCO3-SO4-Cl to Na-Cl or Na-SO4 water), resulting from the mixing of facies B with a more saline groundwater from geological formations underlying those formally designed as being part of the SAG. Fluorine seems to be associated with this deep, saline groundwater. Under natural conditions the magnitude of the upward flows seems scarcely relevant, but uncontrolled groundwater exploitation may lead to increased flow rates and to the pollution of large aquifer areas that at the moment contain good-quality water.
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