Chemical and isotopic characteristics of the warm and cold waters of the Luigiane Spa near Guardia Piemontese (Calabria, Italy) in a complex faulted geological framework

Abstract

Waters discharging at the Luigiane Spa come from two different hydrogeological circuits, which are chiefly hosted in the carbonate rocks and Upper Triassic evaporites of two distinct geological units, known as Verbicaro Unit and Cetraro Unit. The first unit contains a cold and relatively shallow aquifer behaving as a sort of piston-flow circuit with high flow rate, whereas the second unit encloses a warm and comparatively deep aquifer acting as a sort of well-mixed reservoir, where the circulation is slower and the rate is lower.Meteoric waters infiltrating along the Coastal Chain at similar elevations (615–670m asl on average, in spite of considerable uncertainties) recharge both aquifers and, in the first case, acquire heat from rocks through conductive transfer as a consequence of deepening along a fault system and/or crossing between different systems, as suggested by local structural geology. In particular, the warm deeper reservoir has a temperature of ∼60°C, as indicated by the chalcedony solubility and the Ca–Mg and SO4–F geothermometers, which were specifically calibrated for the peculiar water–rock-interaction (WRI) processes originating the Na–Cl–SO4 high-salinity warm waters that discharge at the Luigiane Spa. The warm deeper reservoir is probably located at depths close to 1.4km, assuming a geothermal gradient of 33°Ckm−1. The water leaving the deep reservoir discharges at the surface at 40.9±3.3°C after a relatively fast upflow and limited cooling. The upward part of both hydrogeological circuits is controlled by local low- and high-angle fault systems as well as by the tectonic window of Guardia Piemontese, where the Verbicaro Unit crops out and the Cetraro Unit approaches the surface.The reconstruction of this conceptual model has been made possible thanks to the adoption of a “local” approach integrating previously existing and new geological, hydrogeological and geochemical data and including the use of sulfur isotope data. This last technique has proven most important, as it enabled us to recognize the Upper Triassic fingerprint of dissolved sulfate, once the effects of bacterial sulfate reduction had been properly taken into account.