Interaction between deep and shallow groundwater systems in areas affected by Quaternary tectonics (Central Italy): a geochemical and isotope approach

Hydro- and isotope geochemistry are used to refine groundwater conceptual models in two areas of central Italy (Acque Albule Basin and Velino River Valley) affected by extensional Quaternary tectonics, where deep and shallow groundwater flow systems are interacting. The role of geology, of recent deposits filling the plains and of main tectonic features controlling groundwater flowpaths and deep-seated fluids emergences are investigated and discussed. Environmental isotopes (²H and ¹⁸O) confirm recharge in the surrounding carbonate aquifers, and meteoric origin of both shallow and deep groundwater. Major ion chemistry indicates a mixing between shallow Ca-HCO₃ groundwater from carbonate aquifers and deep Ca-HCO₃-SO₄ groundwater, characterised by higher salinity and temperature and high concentration in sulphates. Isotopic composition of dissolved sulphates (δ ³⁴S and δ ¹⁸O) and dissolved inorganic carbon (δ ¹³C), henceforth indicated as DIC, are used to verify the presence of different sources of groundwater, and to validate the mixing model suggested by the major ion analyses. Sulphate isotope composition suggests a marine origin for the groundwater characterised by elevated sulphate concentration, whose source is present in the deep buried sequences. Carbon isotope composition confirms the role of a DIC source associated to CO₂ degassing of a deep reservoir. Groundwater conceptual models are improved underlining the importance of Plio-Pleistocene sequences filling the tectonic depression. In the Acque Albule area, the travertine plateau represents a mixing stratified aquifer, where deep groundwater contribution is spread into the shallow aquifer. The alluvial–clastic–lacustrine leaky aquifer of Velino Valley enables a complete mixing of shallow and deep groundwater allowing spot-located discharge of deep groundwater along tectonic patterns and facilitating sulphate reduction in the lacustrine sediments, explaining locally the presence of H₂S.