Origin of waters and hydrological paths in the Colli Albani volcano, central Italy: new hints from stable and radioactive isotopes

Abstract

In the framework of a multidisciplinary project funded by the Italian Department of Civil Protection, very detailed analyses of stable (O, 2H, Sr,) and radioactive (3H) isotopes were performed for the first time in the waters circulating in the Colli Albani volcano area (central Italy) in order to: i) define their origin; ii) reconstruct their different hydrological paths; iii) deepen the knowledge of the gas-water-rock interaction processes. Waters are meteoric in origin (Fig. 1), excluding any other source (magmatic, seawater, juvenile). Almost all samples lie on the Global Meteoric Water Line (GMWL), while others draw a line with different slope due to local evaporation processes, affecting waters discharging both in the Albano and Nemi lakes. Some samples show a shift in their 18O content with respect to the GMLW, probably due to: i) exchange with gas phases and CO2-exsolution phenomena (negative shift, such as in the sample CA1 well); and ii) interaction with rocks at relatively high temperature (positive shift). It is worthy of note that waters showing positive 18O circulate in the same areas (Ciampino, Tivoli, Pomezia, Ardea) where anomalies in other geochemical parameters such as temperature, chemical composition and gas content, were found (see posters P05, P06 and P08, this session). Finally, one sample (the acidic pool of the Zolf. Pomezia) shows a 2H shift towards positive values, mainly due both to dissolution of deep H2S in a high gas/water ratio system and/or exsolution of H2S from water as free gas phase.