Monitoring and modelling hydrothermal fluid emission at La Solfatara (Phlegrean Fields, Italy). An interdisciplinary approach to the study of diffuse degassing

Discharge of hydrothermal fluids at La Solfatara is one of the many pieces of evidence testifying to the level of activity and the potential danger characterizing the volcanic complex of the Phlegrean Fields. Recent measurements of diffuse degassing shed light on the unexpected magnitude of this process and prompted further research to improve our understanding of the hydrothermal system feeding the surface phenomenon at La Solfatara. In this work, we first present new data on CO2 diffuse degassing, that were collected during a campaign in July 2000, and confirm previous measurements of the degassing rate. These data, together with other information on subsurface structures and conditions, provided the necessary constraints to perform physical modelling of the hydrothermal fluid circulation at La Solfatara. Numerical simulations describe the ascent and evolution of a hot, multi-phase and multi-component fluid, composed of water and carbon dioxide, through a homogeneous, porous medium. When certain system properties and conditions are used this model reproduces some of the main features which characterize the natural system, including the energy budget associated with the ascent and condensation of hot fluids, and the development of a single-phase gas region whose existence was inferred from independent chemical data. Further simulations investigated the effects associated with changes in rock properties and fluid injection rates. Results showed that an increase in the permeability at shallow depth may have an important influence on the system conditions and fluid discharge. They imply that surface phenomena, accompanied by very high temperatures at shallow depths may survive long after deep feeding has ceased.