Hydrogeochemistry of groundwaters from carbonate formations with basal gypsiferous layers: an example from the Mt Catria–Mt Nerone ridge (Northern Appennines, Italy)

The Mt Catria–Mt Nerone Mesozoic calcareous ridge contains a defined, almost isolated, hydrogeological system. This system is composed of three main superimposed aquifers, separated by marly and marly-clayey aquicludes, which developed in the calcareous and marly-calcareous formations of the Umbria–Marche series. The lower water circulation occurs in the ‘Massiccio’ formation, where Ca(Mg)–HCO3 waters mix with underlying gypsum-saturated waters due to the presence of Triassic evaporites, giving rise to Ca(Mg)–HCO3–SO4 up to Ca(Mg)–SO4 water types. The mixing ratio between the two end members or the degree of gypsum dissolution depends on the depth of water circulation and depth of the top of the Triassic evaporites. Recharge areas for these water types are located at the highest elevations on the ridge, while discharge areas occur at springs along the slopes of the main reliefs as well as in two deeply eroded valleys. Waters recharged in the Mt Nerone area (north-western side of the ridge) appear to be affected by gypsum and dolomite dissolution caused by a direct interaction at depth with the Triassic evaporites, followed by calcite precipitation in a closed PCO2 system (dedolomitization). Conversely, waters recharged in the Mt Catria area (south-eastern side of the ridge) seem to be affected by gypsum-dissolution and calcite-precipitation in an open PCO2 system, which produces fracture sealing of the upper parts (120 m) of the ‘Massiccio’ formation, and a confinement of the deep waters. The consequent pressurization causes additional gypsum dissolution without calcite precipitation in a closed PCO2 system. Previous hypotheses of hydraulic continuity of the basal aquifer throughout the calcareous ridge are reconsidered here and a new interpretation based on hydrochemical data is provided.