HYDROGEOCHEMISTRY OF THE VOLCANIC DISTRICT IN THE TOLFA AND SABATINI MOUNTAINS IN CENTRAL ITALY

A geochemical study on 26 thermal springs and 5 samples from thermal wells (t > 20-degrees-C), 19 cold springs, 11 cold gas pools, and 14 stream samples from the Tolfa and Sabatini Mountains volcanic district (about 1300 km2) has clarified their genesis and evolution. Meteoric waters infiltrate the more permeable exposed rocks (carbonates and/or volcanites) and percolate down into two main aquifers. One is deep and located in Mesozoic anhydritic-carbonate formations, and is the regional geothermal reservoir that feeds springs that emerge at the margins of the carbonate series (either exposed or barely covered); its waters display typical bicarbonate-sulfate alkaline earth composition. The other aquifer(s) is shallower, consisting of waters that circulate primarily in volcanic deposits. The chemical characteristics of these shallow waters are heavily influenced by lithology and by a gas phase, mainly CO2, originating at depth. Temperature estimates obtained with SiO2 and gas geothermometers, which seem to be the most reliable under these conditions, indicate the widespread occurrence of deep fluids at temperatures higher than 110-degrees-C under the area covered by the Sabatini volcanites. In some areas (Manziana, Trevignano, Nepi, and Cesano) the subsurface waters reach temperatures within the range characteristic of medium to high enthalpy systems (120-160-degrees-C).