DISSOLVED GASES IN THERMAL WATERS FROM THE SAN VICENTE GEOTHERMAL FIELD, EL SALVADOR

In many geothermal fields, the majority of hydrothermal discharges are hot springs with only few fumaroles. The geochemistry of hydrothermal gases is important to quantify the interaction of aquifers with magmatic gases and the hydrothermal alteration of the rocks forming the geothermal system. In this work, we report the results of the determination of dissolved gases in hot springs of the San Vicente Geothermal Field using the re-equilibration of dissolved gases with another inert gas phase. Concentration of dissolved gases can be obtained from a method based on Henry’s Law. The method presented is modified from the one presented by Capasso et al (1997). It consists on an equilibrium gas partition between liquid and gas phases, where dissolved gases will be partitioned from the liquid sample to an inert gas. Pure Argon gas was used as the host gas. By taking into account the solubility and partitioning coefficient of each species, we can derive their concentration of dissolved gas per litre of water as well as their partial pressures. A Quadrupolar Mass Spectrometer was used to analyze several thermal water samples from San Vicente Geothermal Field.

Data obtained from these analysis indicate that Helium and CO₂ partial pressures were higher than those found in waters in equilibrium with atmospheric air, thus pointing out to processes and interactions between geothermal gases and groundwater. This was further explored by plotting obtained values in a N₂-CO₂–O₂ ternary diagram and in a 10000He-CO₂-N₂ ternary diagram. These diagrams show that the chemical composition of the dissolved gases is a function of the degree of interactions between air contaminated recharge waters, and deep volcanic gases with CO₂ rich fluids. These samples show O₂/N₂ ratios that are equal or lower than the atmospheric air ratio, with some samples showing higher P_CO2 values. This indicates the different amounts of interaction between volcanic gases and shallow waters. On the other hand, He/N₂ ratios show low variability similar to a fumarole ratio, possibly a result of the lower reactivity of these two gases.