The hydrothermal system of Bagni San Filippo (Italy): fluids circulation and CO₂ degassing

Giovanni Chiodini (1), Carlo Cardellini (2), Stefano Caliro (3), Rosario Avino (3), Marco Donnini (4), Domenico Granieri (5), Nicola Morgantini (6), Domenico Sorrenti (7) & Francesco Frondini (2)
(1) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bo- logna, via D. Creti 12, 40128 Bologna, Italy. (2) Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, via Pascoli snc, 06123 Perugia, Italy. (3) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Na- poli Osservatorio Vesuviano, via Diocleziano 328, 80124 Napoli, Italy. (4) Consiglio Nazionale delle Ricerche, Istituto di Ricerca per la Protezione Idrogeologica, via Madonna Alta 126, 06128 Perugia, Italy. (5) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, via Cesare Battisti 53, 56125 Pisa, Italy. (6) Agenzia Regionale per la Protezione Ambientale, ARPA Umbria, Via Pievaiola 207/B-3, 06132 Perugia, Italy. (7) Geotecno S.R.L., Via Domenichino 49, 20149 Milano, Italy. Corresponding author e-mail: carlo.cardellini@unipg.it

Bagni San Filippo area is characterized by the discharge of thermal waters and deeply produced CO_2-rich gases both from vents and soil diffuse degassing. The thermal waters are the results of the mixing between meteoric waters and hot fluids deriving from the condensation, at depth, of vapours uprising from a deep hydrothermal reservoir. This process gives rise to a relatively shallow thermal system at temperature close to 50°C, characterized by SO_4-rich and Cl-poor waters and elevated P_CO2 (~7 bar). Most of the incondensable gas of deep originated vapour is released as a free gas phase forming cold gas vents and localized spots of anomalous CO₂ diffuse degassing. The location and the shape of these degassing zones are strongly controlled by the main tectonic structures of the area. Through detailed soil diffuse degassing surveys and hydrogeochemical modelling, we estimate at 226-326 t d^-1 and at 965 t d^-1 the deep CO₂ emission and the amount of condensates discharged by the thermal springs, respectively. The thermal energy associated to the process results at ~ 29 MW, most of which (~ 25 MW) is associated with condensation occurring at depths greater than groundwater circulation.

CO_2- diffuse degassing, thermal waters, geothermal system.