Aspetti geologici e idrogeologici del peculiare acquifero dell'Isola di Pianosa (Arcipelago Toscano)

The Pianosa Island (Tuscan Archipelago), characterized by the typical flat morphological structure, is formed by Neogenic-Quaternary sedimentary rocks. Despite its small extension and poor rainfall amount, the Pianosa aquifer is characterized by significant groundwater resources, supporting the presence of about 2,000 people in the past. Nevertheless, the groundwater overexploitation and the local land use (agricultural activity and cattle-breeding, related to the local penal settlement) caused sea-water intrusion and pollution phenomena. An improvement of such situation occurs since 1998, owing to the closing of the penal settlement. By means of a multidisciplinary approach, this research intended to describe the hydrogeological and hydrogeochemical features of the Pianosa Island aquifer system. Preliminary results show that the groundwater recharge and circulation are substantially controlled by the hydro-structural conditions. The water table flow direction is generally W-E, in conformity with the dip direction of the stratigraphic contact between the outcropping permeable calcarenite and the underlying impermeable marly-clayey rocks. However, the last present conglomerate and sandstone intercalations, sometimes in contact (by angular unconformity) with the calcarenite, determining a continuity in the groundwater circulation, which is phreatic in the calcarenite, and confined in the conglomerate and sandstone horizons. The electric conductivity (EC) map confirms this hydrogeological structure. EC values above 1.000 μS/cm are common in almost all the groundwater analyzed. Greater problems of salinity in groundwater have been observed in the eastern part of Pianosa, where piezometric depressions have been individuated. In agreement with the hydro-structural and piezometric conditions, the hydrogeochemical analyses confirm the recharge of the confined horizons (conglomerate and sandstone) by the superficial calcarenite. The isotopic data indicate that the aquifer system is recharged by the rainfall direct infiltration. The chemical analyses of most groundwater samples suggest an intermediate facies Na-Cl/Ca-HCO3, produced by the combination of the sea spray and the circulation in prevalently carbonate aquifer. Clearly Na-Cl groundwater prevails in the eastern portion of the island, evidencing the seawater intrusion in the calcarenite, also confirmed by water table conditions and isotopic data.