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

The understanding of the main hydrogeochemical processes controlling the features and behaviour of crystalline rock environments considered for the deep geological storage of spent nuclear fuel is a key issue for the improvement of our knowledge and management of these systems and for a more accurate prediction of their future evolution. In this paper, some of the most important irreversible processes (cooling, mixing and CO 2 exchange) are studied and their effects quantified by combining classical geochemical calculations and geochemical modelling. This methodology is applied to an alkaline thermal water system developed in crystalline rocks (the Caldas de Boí groundwater system, NE Spain) analogous to the deep geological environments expected for nuclear waste storage. 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 CO 2 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.