Deep fluids transported by Apennine rivers: quantification of deep CO2 emission and implications for geochemical monitoring of the seismic activity.

Central Italy is affected by a significant migration of deep CO₂ through the crust. CO₂ upraise gives rise to numerous gas emissions in the western Tyrrhenian domain where extensional deformation has dismantled the compressional structures, enabling fluid emissions through a mature set of normal faults. Conversely, the thickened crust and the abundant groundwater circulation in carbonate aquifers of the Apennine “trap” migrating deep fluids. Here, in the eastern Apennine sector, deep CO₂ dissolves in the large carbonate aquifers, while the CO₂ anomalies disappear in the easternmost Adriatic domain. This divide is reflected in seismicity patterns, with Apennine earthquakes clustering close to the degassing anomaly boundary. Significant variations in dissolved deep CO₂ were observed in some springs from large Apennine aquifers during the seismic crises of L’Aquila 2009 and Central Italy 2016-17, suggesting feedback mechanisms between CO₂ degassing and seismicity. The region is also characterised by a dense hydrological network (i.e., the Tiber River Basin, TRB) running in the different tectonic settings, with some major rivers collecting water from areas where CO₂-rich springs, sensitive to the seismic activity, are present. In this framework, a two-year geochemical survey of the major rivers of TRB was conducted aimed to explore the reliability of investigating the regional CO₂ degassing process and its relations with the seismicity by studying the river’s waters. In addition to the geological peculiarities, this area is suitable for this objective, due to the well-developed hydrometric network managed by local authorities, allowing to couple geochemical and hydrological data. More than 350 river water samples were collected from the Tiber river and its 12 main tributaries. A large geochemical dataset including major ions and dissolved inorganic carbon isotopic compositions was produced covering different hydrological periods. Results show that river waters exhibit compositions and variability resembling those of the Apennine groundwaters, allowing to identify different fluids circulating in the crust. Compositional variation remains appreciable for long distances downstream of mixing between shallow and groundwaters and between rivers with different compositions, highlighting the preservation of the geochemical information over large areas. In particular, the content of dissolved carbon in river waters and its isotopic composition shows and preserves for long distances the signature of the input of deep CO₂-rich waters. Coupling river’s geochemical and flow rate data, fluxes of dissolved deep CO₂ were computed, providing results that closely match previous estimates based on spring data, indicating minor carbon loss along rivers. These findings highlight rivers as valuable indicators of deep CO₂ flux across large areas and potentially to investigate temporal variation of the flux. This study has been also focused on the definition of ‘easily detectable parameters’ (EDP) which correlate to dissolved deep CO₂. Measuring EDP at high frequency, together with the water flow rate, could provide a tool for monitoring variations of the deep CO₂ flux to enhance a possible geochemical monitoring of the seismic activity.