Tracing deep fluid source contribution to groundwater in an active seismic area (central Italy): A combined geothermometric and isotopic (δ13C) perspective

Abstract

An understanding of the origin and migration pathways of geofluids in an active seismic area is of paramount importance in terms of societal challenges such as mitigation of seismic hazards. This study investigates the relationship between the stable isotope ratio 13C/12C of deep CO2 and geothermometry in selected groundwater samples located close to extensional faults in central Italy. The temperature range is inferred from an equation based on the Na/Li ratio and chemical geothermal modelling: 113–130 °C. Globally, the calculated temperature range agrees with that resulting from deep boreholes in the Northern Apennines. An alternative method is also included to better evaluate the difference in isotopic composition between the aqueous and gaseous δ13C(CO2) at deep condition. A review of previously published data shows that monitoring of the isotopic composition of the dissolved inorganic carbon (DIC) in springs fed by meteoric origin located in seismically active areas must take into account mass and isotopic balance to correctly evaluate the component, and the role, of deep fluid during seismic events. In particular, the coupled (and corrected) isotope and geothermometrical monitoring of the springs could help to distinguish between deep gas or deep fluid contributions to shallow aquifers. The results of this study indicate that faults play a crucial role in controlling the migration of crustal fluids. In addition, they reveal that possible evaluation of potential seismic precursors mandatorily requires a long period of monthly monitoring.