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Abstract
The San Giuliano low-temperature hydrothermal system is located at the foot of the Pisan Mounts in Tuscany, the most important geothermal region in Italy. Eleven water points with temperatures within the range 16-40°C and salinity between 0.6 and 2.4 g / l are distributed along the superficial contact between the Mesozoic carbonate aquifer formations outcropping on the relief and the low-permeability deposits of the Pisan plain. Hydrogeological and geophysical investigations, as well as periodic geochemical and isotopic samplings and temperature measurements of the different water-points, were carried out during the period 1988-91. These observations indicate that thermal, low-tritium, calcium-sulphate waters rise toward the surface from a reservoir made up of a Triassic evaporitic sequence through a buried fault located in the vicinity of the margins of the Pisan Mountains. The presence of a low-permeable cover, represented by Quaternary and Neogenic deposits, allows only the local upflow of thermal water and a poor flow rate at the wannest springs. These springs, located in the central part of the prospect, are used in the Spa facilities. Most of the thermal water, blocked by the impervious cover, moves laterally within the system and mixes with young, cold, meteoric water. Most of the water points represent mixtures in different proportions of the Spa water and this meteoric component. The local carbonate outcrops probably represent the main recharge areas for the system. However, assuming that a regional circulation could be present, a large amount of water, at temperatures between 70 and 80°C, as suggested by geothermometric indications, could be found at depth.