The chemical evolution and the origin of the groundwater, cold and thermal springs composition are analysed related, to the geochemical environment of the Poços de Caldas alkaline complex. The thermal waters origin are related to a large and deep open fracture system in three main directions: N14E, N50E and E-W. The tritium content when simultaneously analysed with the deuterium and oxygen-18 set show that thermal waters are old meteoric waters (30 - 40 years of age). On the other hand, the cold springs that circulate on the superficial levels are more recent, which is a characteristic of an acid oxidizing environment without sulfldes and greater concentration of free CO2 and Rn222. The pH increases slowjy with depth and also the HCO-3, Na+ , SO2-4 and the SiO2 content. High concentrations of fluorides follow this process. Reducing environment with an increase in the sulphide content and a decrease in radioactivity are trends in the system studied.
