Using hard and soft data from direct and indirect methods to develop a model for the investigation of a metamorphic aquifer

In Italy, despite large areas of the country being covered by metamorphic rocks, the hydrogeological properties of these formations are not yet well known. The productivity of metamorphic aquifers is generally lower than the more common ones such as alluvial or carbonates. However, in some Mediterranean areas such as in Calabria region the scarcity of water resources and their considerable extension (metamorphic aquifers make up 39% of the total) determines a request for further studies either on their hydrodynamic properties and their hydraulic behaviour in order to achieve their sustainable exploitation. Interest in these metamorphic aquifers becomes ever greater if climate changes are considered. The purpose of this study is to provide the geological-structural and hydrogeological modeling of a metamorphic aquifer, through the measurement of direct and indirect data and the application of a numerical model, in a large area of the Sila Piccola, in Calabria. To recognize and characterize the geometries of the aquifer in metamorphic rocks in a complex geological setting, data on springs, wells and piezometers installed in boreholes and located at various depths were collected. These surveys were implemented by geoelectric tomography profiles and by geognostic investigations. The recognition of the geometries and above all the stratigraphic relationships between the various outcropping rocks and lithological units have been accompanied by macrostructural and meso-structural analysis to better evaluate the state of fracturing of the rock mass. The characterization of hydrodynamic properties in crystalline-metamorphic aquifers, that is constituted by granite and metamorphic rocks, is extremely complex given the lateral-vertical anisotropies. Among the main fractures there is a network of secondary connections of different order and degree which determines a continuous variation of these properties at different scales and defines the modality and direction of the groundwater flow. The MODFLOW-2005 groundwater model was used to simulate the flow phenomena in the aquifer, obtaining hydraulic conductivity values of 2.7 × 10^-6 m / s, corresponding to two orders of magnitude higher than that calculated with the slug-tests inside the slope. In summary, the mathematical model was able to estimate the equivalent permeability of the aquifer and the presence of a lateral recharge from a neighboring deep aquifer that materializes a significant water supply.