Geochemistry of the high- PCO 2 waters in Logudoro, Sardinia, Italy

Abstract

A geochemical study of the high- PCO 2 waters in Logudoro, northern Sardinia, was carried out starting from regional hydrogeochemical prospecting for geothermal energy, based on the major dissolved components and some minor elements. This preliminary investigation led to the identification of five different lithologies marking the different aquifers. The high- PCO 2 waters can be divided into the less saline (TDS < 1g/l) with high tritium unit (T.U.) values and the more saline ones (TDS= 1–3.5 g/l) with T.U. values close to zero. The water-rock interaction process affecting the major components is shown to be the result of interaction between CO 2-rich waters and aluminosilicates; the process takes place at different degrees depending on the depth at which CO 2 interacts with different aquifers while migrating upward from the mantle. Consideration of the SO 4/Cl and F/Cl ratios in the solution allowed the deep ciruits of S. Martino and Abbarghente in the Oligo-Miocene volcanic rocks and S. Lucia in the carbonate-schistose-granitic basement of the Goceano Mountains to be located.