Hydrogeochemistry and Hydrogeology
Geothermometry and Hydrogeochemistry of thermal waters from Poços de Caldas, state of Minas Gerais, Brasil
Thermal water springs occur at several locations with different geological settings on the earth. The city of Poços de Caldas is located in the north of an alkaline-volcanic complex probably representing a deeply eroded caldera. The magmatic rocks have an age of around 70 Ma. The occurrence of hydrothermal springs reflects the elevated heat-flow in this region (~38 °C/km), which is most likely due to the heat production of radioactive decay of Uranium and Thorium. The magmatic rocks show high concentrations of these elements and were locally exploited (1997 closed Osamu Utsumi uranium mine, Poços de Caldas).
Several artesian thermal and non-thermal springs in the city area of Poços de Caldas (portuguese for “hot springs”) are used for Spa and health treatment. The character of these waters is very different. They can be divided into two groups with respect to their hydrogeochemical properties, one group is shallow groundwater and the other group represents deeply arising thermal waters.
It is possible to predict temperature and depth of the thermal-water formation with the aid of chemical Geothermometers. These Geothermometers (e.g. Quartz-, Na/K-, K-Mg-Geothermometer) rely on the temperature dependency of the dissolution or exchange reactions between water and rock. The understanding of the hydrogeochemical genesis of the thermal water is essential for the choice of the Geothermometer and its significance.
Example of a chemical Geothermometer:
The Na/K-Geothermometer is the most used and declared as the most reliable of the cation-Geothermometer group. It is based on the temperature depending cation exchange reaction between albite and K-Feldspar (equation 2). The Na/K ratio decreases with increasing fluid temperature and vice versa.
(eq. 2) K+ + Na-Feldspar = Na+ + K-Feldspar
With the equilibrium constant Keq for the exchange reaction expressed as (equation 3):
(eq. 3) Keq = [Na + ]/[K+]
The equilibrium constant Keq is temperature dependent and can be approximated by using the Van´t Hoff equation (equation 4)
Where a and b coefficients are inferred from linear regression between log Keq and 1/T.
Amounts of rare earth elements (REE´s) are analyzed in order to get a better understanding of the thermal water formation. The REE´s offer the possibility to preserve a “finger print” of the aquifer host rock, what gives further valuable insight into the thermal water formation.
Influences of the upper earth mantle as potential heat source of the hydrothermal system can be identified by the amount and isotope ratios of dissolved noble gases. Especially the ratio of 3 He (“mantle helium”) to 4He (“radiogenic helium”) can give information about mass and/or heat flow from the upper mantle to the crust through deep open fault structures.