Assessment methodology of permeability in a granitic aquifer in a pilot basin in the south of Sardinia (Italy)

Groundwater circulation in fractured hard rocks aquifer (HRA) is an essential element in the fields of water supply, environment and geotechnics. The fracture properties, as orientation, aperture, frequency, and quality, influence the circulation of water. The geometry of the fractures makes the hydrogeological environment anisotropic and heterogeneous. Hence, defining the geological model is fundamental to understand spatial variability of permeability, and to define hydrogeological features of the aquifers. The present work aimed to develop a methodology to assess the permeability on crystalline rocks. As a pilot site, the Masoni Ollastu River basin (12,5 km2) in the South of Sardinia was considered. The reconstruction of the fracture network required a multiscale characterisation using field measurements and digital photogrammetry. The fractures field survey consisted in recording the features of discontinuities. The digital photogrammetry workflow produced a high resolution DEM (5m), orthophotos, digital stereo pairs, and a lineaments map. At eleven geomechanical stations, the sets of joints were characterized and the permeability was calculated applying the cubic law for each set. Spatial distribution of fracture properties was evaluated to approach the heterogeneity of the medium. For this purpose, two different scenarios in the geological conceptual model were evaluated: the first one concerns a stratiform aquifer with a sub-horizontal and sub-vertical fissured layer in the paleo-weathering profile of the granites; the second one regards valley incision by differential erosion determined by structural features (fractures density). The layer of unconsolidated alterite above the granitic paleo-weathering profile is thin and seems to represent the deeper level where the rocks are less weathered and fractured. Probably, the significant erosion of the large part of the saprolite is related to an uplift after Pliocene time. The permeability calculated with the cubic law was in a range of 10-8 to 10-2 m/s; the highest values are conditioned by the fractures aperture in the surface. According to the geological conceptual model, density and aperture of fractures decreases with depth, thus also permeability. The applied multi-phase methodology with a multiscale approach allowed to provide a hydrogeological conceptual model in a HRA.