Groundwater hydrogeochemical changes predating and following the November 9, 2022 Mw 5.5 Adriatic offshore earthquake (central Italy)

• Geochemical changes were detected prior and after the November 2022 seismic swarm. • BET piezometer showed strong compositional changes 4 months before the event. • Metal contents sharply increased during the seismic event. • The hydrogeochemical changes were triggered by the seismic cycle. • The VAL and MAC well waters did not show any variation. We report the results from a pre- and post-seismic water monitoring carried out in the Mt. Conero area (central Italy) to evaluate the earthquake-related variations on the water hydrogeochemistry related to the November 9, 2022 Adriatic offshore seismic sequence. This latter was characterized by two main events of M w 5.5 and 5.2. The monitoring network included two wells and one piezometer located at ∼50 km from the earthquake epicentre. The wells did not show relevant changes. Contrarily, the piezometer showed an overwhelming variation in its composition and Total Dissolved Solids (TDS) since four months before the mainshock, shifting from a low-salinity (TDS < 1000 mg/L) calcium-bicarbonate facies to a high-salinity (TDS > 3500 mg/L) sodium-chloride composition. Then, composition and TDS were restored about a week after the events. These changes were accompanied by strong increases in trace elements concentrations (e.g., B, Mn), which returned to the pre-seismic values in the days following the mainshock. The strong hydrogeochemical variations recorded at the piezometer were likely related with two different seismically-induced processes linked to a mixing between shallow Ca-HCO 3 and deep Na-Cl waters, and the bedrock’s fracture unclogging. These variations are, to the best of our knowledge, among the largest ever observed before a seismic event or, at least, ever reported in the literature. These results prove hydrogeochemical monitoring for seismic surveillance can be highly effective. Besides, our work represents a further step in the development of a methodology that could potentially track geochemical changes ahead of larger, potentially dangerous earthquakes.