Secondary processes determining the pH of alkaline waters in crystalline rock systems

One of the most remarkable results obtained in this study is related to the high pH-buffering capacity exhibited by these alkaline-thermal waters during the mixing processes, with colder and shallower waters, taking place during the uprising of the hot waters from the deep reservoir. Even for high mixing proportions of cold water (up to 60%), the final pH remains as alkaline as in the thermal and deep end member, although the rest of the physicochemical parameters (including temperature) change consistently with mixing. Another important secondary pH-determining process detected in this study is the conductive cooling of the thermal waters suffered during their rise to surface conditions, which is clearly responsible for a pH increase which enhances the typical alkaline character of these waters. Finally, the existence of external inputs of edaphic or biogenic CO2 to the groundwater system, producing a pH decrease in some of the springs, has been proven to play a key role in their behaviour and evolution.