Hydrogeochemical changes before and during the 2019 Benevento seismic swarm in central-southern Italy

Abstract Insights into seismic precursors have been obtained in the last decades. However, a detailed understanding of hydrogeochemical anomalies prior to earthquakes still remains the aim of many research teams worldwide. In order to investigate the earthquake-groundwater relationship, between 2018 and 2020, we performed sampling surveys coupled with continuous multiparametric monitoring in Grassano spring fed by the Matese aquifer (central-southern Apennines, Italy). Hydrogeochemical changes were observed before the onset and during the 2019 Benevento seismic sequence, including dissolved CO2 increase, pH lowering, and anomalies in major ions (i.e., Ca2+, Na+, HCO3-) that later recovered to their typical concentrations. We suggest that variations in groundwater geochemistry were induced by dilatative preparatory phases of earthquakes, typical of the extensional setting. This condition allowed the deep CO2 upwelling along tectonic discontinuities, as testified by the Cext (carbon from external sources) behaviour detected in Grassano groundwater during the 2019 year. Despite the small-intermediate magnitude of the mainshock, results highlight and confirm the occurrence of a potential pre-seismic geochemical process in the fractured carbonate aquifers, similar to the one proposed in literature for the stronger 2016-2017 Amatrice-Norcia seismic sequence.