Geochemical and isotopic characterization of the shallow aquifers from the Mugello Basin (Tuscany, central Italy): Implications for assessing a monitoring network in a seismically active area

The Mugello Intermontane Basin (MIB) is located 30 km north of Florence (Tuscany, central Italy) and shows high seismicity with historical events characterized by M_w ≥ 6, e.g., on June 13, 1542 (M_w = 6.0) and June 29, 1919 (M_w = 6.4). Progresses in the identification of seismic tracers in geofluids has been made in the last decades, although reference values for a given area are necessary to assess hydrogeochemical anomalies prior to earthquakes. In this study, a detailed characterization of the chemical and isotopic composition of the natural waters discharging from MIB was performed. The aims were to (i) constrain the geochemical processes controlling the chemistry of waters and dissolved gases, (ii) assess the influence of deep-seated fluids in the shallow environment, and (iii) evaluate the suitability of geochemical parameters as reliable tracers for seismic activity. Two different types of waters were recognized, being characterized by: (A) calcium-bicarbonate (Ca-HCO₃) composition, positive Eh values (150–200 mV), slightly alkaline pH (<8.3), and an N₂-dominated dissolved gas phase; (B) sodium-bicarbonate waters (Na-HCO₃,) composition, negative Eh (< −180 mV), pH > 8.5, high contents in F, B and Li, and enrichments in dissolved CO₂ and CH₄. The chemistry of waters of group (A) is controlled by dissolution processes involving carbonate rocks, while the Na-HCO₃ waters likely result by prolonged water-silicate rock interactions and probably associated with longer circulation pathways. Argon (⁴⁰Ar/³⁶Ar) and carbon (δ¹³C in CO₂ and CH₄) isotopes indicate a predominant circulation within local aquifers by shallow fluids. Instead, helium (³He/⁴He) isotopes in dissolved gases highlighted a contribution up to 6 % by mantle/magmatic fluids probably rising through deep faults. The results obtained suggest that trace elements and the isotopic signatures of dissolved CO₂, CH₄, and He may represent reliable seismic tracers for the MIB on the basis of which a monitoring network could be deployed.