Revised conceptual model of the Solfatara magmatic-hydrothermal system (Campi Flegrei, Italy), time changes during the last 40 years, and prediction of future scenarios

We revised the conceptual model of the Solfatara magmatic-hydrothermal system based on the results of new gas-geoindicators (Marini et al., 2022) and the available geological, volcanological, and geophysical information from surface surveys and deep geothermal wells. Using the new gas-geoindicators, we monitored the temperature and total fluid pressure over a time interval of ~40 years: (i) in the shallow reservoir (0.25–0.45 km depth), where CO equilibrates; (ii) in the intermediate reservoir (2.7–4.0 km depth), where CH4 attains equilibrium; (iii) in the deep reservoir (6.5–7.5 km depth), where H2S achieves equilibrium. From 1983 to 2022, the temperature and total fluid pressure of the shallow reservoir did not depart significantly from ~220 °C and ~25 bar, whereas remarkable, progressive increments in temperature and total fluid pressure occurred in the intermediate and deep reservoirs, with peak values of 590–620 °C and 1200–1400 bar in the intermediate reservoir and 1010–1040 °C and 3000–3200 bar in the deep reservoir, in 2020. The revised conceptual model allowed us to explain the evolution of: (a) pressurization-depressurization in the intermediate reservoir, acting as the “engine” of bradyseism, (b) time changes of total fluid pressure in the deep reservoir, working as the “on-off switch” of magmatic degassing. We also used the revised conceptual model to predict possible future scenarios in the lack of external factors.