A little island with significant groundwater resources: hydrogeological and hydrogeochemical features of the Pianosa Island aquifer (Tuscan Archipelago, Italy)

The Pianosa Island is one of the seven islands of the Tuscan Archipelago, particularly known for its typical flat morphological structure. It is formed by Neogenic-Quaternary sedimentary rocks, mainly represented by superficial calcarenite and underlying marl and clayey marl. Despite the small extension of the island (just 10,2 km2 wide, coastal perimeter of approximately 18 km, maximum altitude of 29 m a.s.l.) and poor rainfall amount (the annual average is 480,7 mm in 1951-2002 period), the Pianosa aquifer is characterized by significant groundwater resources, which supported the presence of approximately 2,000 people at the end of Eighties. Nevertheless, the groundwater overexploitation and the land use (agricultural activity and cattle-breeding, associated to the local penal settlement activity) caused important sea-water intrusion and pollution phenomena. An improvement of such situation occurs since 1998, owing to the closing of the penal settlement and its activities. This pilot research intends to describe the hydrogeological and hydrogeochemical features of the Pianosa Island aquifer system and the groundwater quality several years after the penal settlement closing. The results of a multidisciplinary approach (hydrogeological, geochemical, isotopic) show that the groundwater recharge and circulation are substantially controlled by the hydro-structural conditions. The flat and permeable superficial calcarenite allows a high infiltration rate. The water table flow direction is generally W-E, in accordance with the dip direction of the stratigraphic contact between the calcarenite and the underlying impermeable marly-clayey rocks. However, the latter present conglomerate and sandstone intercalations, sometimes in contact (by angular unconformity) with the calcarenite, determining a general continuity in groundwater circulation, which is phreatic in the calcarenite, and confined in the conglomerate and sandstone horizons. A piezometric depression with values below the sea level has been identified in the eastern part of the island. The electric conductivity (EC) map confirms this hydrogeological structure. EC values above 1.000 μS/cm are common in almost all the groundwater analyzed. An increase in groundwater salinity is observable in the eastern part of Pianosa, where the water table depression has been recognized. In agreement with the hydro-structural and water table 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 and there are not connections with the close Elba Island. Finally, 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 a prevalently carbonate aquifer (calcarenite). 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.