Los sistemas geotermales del Pirineo central. I. Caracteres geoquímicos y fisicoquímicos de los manantiales termales

Abstract

The compositional characters of selected hot springs from the Panticosa, Benasque, Arties, Caldas de Bohí, Luchon and Cauterets geothermal systems in the Central Pyrenees have been studied. All the springs have the same global geochemical features (high pH, low total dissolved solids, very low Mg concentrations, Na as the dominant cation, and absence of a dominant anion) and belong to the same granite-related alkaline thermal waters group. Chemical composition of the springs is similar to each other, and to that found in other Pyrenean geothermal systems. A more detailed comparison shows, however, that Panticosa and Benasque systems have the lowest sodium and potassium concentrations found in any water belonging to the group of alkaline waters.
Differences in pH values among the springs are related to emergence temperature as it occurs in the Pyrenean geothermal systems. According to the high pH of these waters, speciation calculations at emergence temperature show a very low CO₂ partial pressure, dissolved silica partially ionized as H₃Si0₄^-, and Al(OH)₄^- as the dominant aluminium species.
Saturation indexes show that almost al1 the waters are highly oversaturated with quartz (close to equilibrium with chalcedony), potassium feldspar, and albite, which are the minerals equilibrated at depth with the solutions. Furthermore, all waters are in equilibrium with kaolinite and ca1cite at emergence conditions, showing the existence of a reequilibrium process during their ascent to the surface. This process happens to be one of the most important in the geochemical evo1ution of these alkaline systems.