Chemical and isotopic characterization of groundwater and thermal waters from the Campi Flegrei caldera (southern Italy)

Abstract

We report here on the results of an extensive hydrogeochemical survey of the chemical and isotopic composition of thermal groundwater in the Campi Flegrei caldera (CFc), a restless volcanic system in the Campania Volcanic Province of Southern Italy. Our hydrogeochemical study, based on the collection of 114 groundwater samples, is the first comprehensive one since uplift, seismicity and degassing resumed in 2005, continuing today at accelerating rate. We use our results to characterize the processes that control the wide diversity of groundwater composition observed. We identify a set of key source processes governing the composition of the CFc thermal waters: i) the infiltration of meteoric water, responsible for the formation of diluted cold groundwater having bicarbonate as the prevalent anion (fresh waters), ii) mixing, upon downward infiltration in the aquifer(s), with high-temperature hydrothermal reservoir brines of probable marine origin (chloride waters), iii) the dissolution of a CO₂-He rich magmatic-hydrothermal gas, with isotopic signature (respectively δ¹³C ∼ −1 ‰ vs. PDB and ³He/⁴He ∼ 3Ra, where Ra is the atmospheric He isotope ratio) similar to the Solfatara hydrothermal gases, generating bicarbonate-rich groundwater; (iv) near-surface dilution by hot condensates, formed by partial condensation of H₂S-rich Solfatara like hydrothermal steam, generating sulphate-rich steam heated waters. In near-surface environments, upward migrating groundwater also degases, releasing poorly soluble gas compounds (He, Ar, N₂, CH₄) and becoming enriched in more soluble CO₂. The above processes can work in concert, but each dominates in specific regions of the hydrothermal system, hence causing the extreme spatial diversity of water chemistry observed within the caldera. Our conceptual model for the processes acting within the thermal groundwater system will be crucial to interpreting any temporal change in groundwater chemistry observed in the installed permanent (instrumental) groundwater-monitoring network, which is critically needed for improving geochemical volcano monitoring during current acceleration of the unrest.