Hidroquímica das águas termais e não termais da região da caldeira vulcânica de Poços de Caldas - MG

Abstract

This paper presents a hydrochemical study of the thermal and non-thermal waters of the Poços de Caldas volcanic caldera region through a survey and characterization of the sources, evaluation of water quality considering physical-chemical analyses. Hydrochemical characterization can be obtained through ionic balance and analytical error calculation, Piper diagram, Base Exchange index (BEI) calculation and ionic ratio calculation. The waters were classified according to the content of their main cations and anions. The points studied are located in areas where Pedra Branca syenite, tinguaite and nepheline syenite and foiaite lithologies occur. More than 64% of the points studied have high pH values, above 7.4. The thermal water group showed higher values for temperature, pH, conductivity, turbidity and total dissolved solids (TDS) and lower values for oxidoreduction potential (ORP). The analyses showed high concentrations of sodium, bicarbonate and sulphate in most of the samples. The hydrochemical study showed a predominance of sodium bicarbonate waters and some points as sodium chloride waters. Most of the points had a negative base exchange index (BTE) due to the higher concentration of Na+ and K+ compared to chloride. The classification according to temperature was mostly classified as mesothermal and secondarily as thermal and hypothermal. The classification according to the order of the main cations resulted in rNa+ > rCa2+ > rMg2+ in all the samples, and according to the order of the main anions most of the samples showed rHCO3- > rSO42- > rCl-, with some points having the order rCl- > rHCO3- > rSO42-, typical of aquifers in crystalline rocks. The chemical composition of the waters of the studied sources has characteristics attributed to the host lithotype, cation exchange processes, chemical weathering and longer water/rock contact time, probably due to the hydrolysis of orthoclases and sanidines (sodium feldspar) and incongruent dissolution of nepheline silicates.