The Pianosa Island (10.2 km2) is one of the seven islands of the Tuscan Archipelago (Italy), known for its flat morphological structure. It is formed of Neogenic-Quaternary sedimentary rocks, mainly represented by calcarenite, and underlying marl and clayey marl. Despite its small extension and relatively low rainfall amount (500 mm/year, approximately), the Pianosa Island include an interesting aquifer characterized by significant groundwater resources that at present is little expoited.
The main goal of this work was the development of a groundwater flow numerical model based on a conceptual model obtained by a multi-tool and integrated approach. In particular, stratigraphic and geophysical data were elaborated to define the geometry of the aquifer system; piezometric surveys (12 campaigns from 2015 to 2018) and hydraulic tests were performed for the hydrodynamic and hydraulic characterization; and, geochemical data (physical-chemical parameters, chemical compounds concentrations and isotopic ratio) were produced on groundwater and rainfall samples (respectively collected seasonally from wells and monthly from a rain gauge) for characterizing the water inputs and components involved into the aquifer, including sea water. Recently, a specific lysimeter was moreover installed to quantify and characterize in geochemical terms the effective infiltration water.
All data processing allowed to obtain a reliable conceptual model and identify some processes that affect groundwater, as the marine intrusion. It was also possible to estimate the main input and output components of the aquifer
Based on the conceptual model, a steady-state flow model was developed by MODFLOW code and Visual MODFLOW as graphical user interface. The implementation step involved the spatial discretization of the domain, the attribution of hydraulic parameters to each cell and the assignation of initial head and boundary conditions. The model implemented was then calibrated with measured data using a manual “trial and error” approach. On the whole, the groundwater model confirms the hydogeological conceptual model and identifies some knowledge gaps which need further investigation.
The calibrated groundwater flow model let us also to simulate forecasting scenarios with different rate of groundwater exploitation and different value of effective infiltration. The results indicate that the variable that mostly intensifies the marine intrusion phenomenon is a reduction of the effective infiltration, thus confirming that the aquifer system of the Pianosa Island is strongly sensitive to rainfall regime and climate change.