Structural setting and fluid dynamics in carbonate massifs of Umbria-Marche Apennines

The aim of this research is to understand the hydrogeological mechanisms of groundwater circulation and their relationship with the structural setting in carbonate massifs of Umbria-Marche Apennines. Karst aquifers are characterized by a strong heterogeneity in their physical properties. Hence, in order to define the dynamics of fluid circulation in these environments is necessary to integrate different investigative approaches. Umbria-Marche carbonatic ridge is characterized by three hydrogeological complexes: a basal complex (Massiccio aquifer), an intermediate complex (Maiolica aquifer) and an upper complex (Scaglia aquifer). The carbonatic lithotypes covered about the 67% of the study area and only the 4.3% and 28.8% are represented by the Massiccio complex (the main aquifer of Apennine ridge) and Maiolica complex, respectively. On the contrary, the sub-basins showing higher percentage values of outcropping of Massiccio and Maiolica complexes. These results, indicate as the basal groundwater circulation is controlled exclusively by Massiccio and Maiolica complex. Analysis of fracture systems exposed in different outcrops of Massiccio and Maiolica complexes, have defined a conceptual model of structural and hydraulic properties of the main aquifers. There are two main fracture sets oriented at SW-NE (dip-direction of N115) and NNE-SSW (dip-direction of N20), that probably the main pathways for the water-circulation from infiltration zone to spring outlet. Discrete Fracture Network (DFN) models of representative geocellar volumes builted to compute fracture porosity and correspondent permeability (Kxx, Kyy, Kzz), show that the fracture porosity of Calcare Massiccio Formation is much greater than Maiolica Formation (4.3% and 1.7%, respectively), and also the permeability values result well correlated with this trend. From hydrogeological point of view, recession analysis and time series analysis (auto- and cross-correlation functions) applied at daily discharge of six karst springs highlighted the presence of two type of karst aquifers: aquifer with unimodal behaviour and aquifers with bimodal behaviour. These results indicate that Maiolica Formation is characterized by high fracturation degree and a slightly karstification, controlling the infiltration and percolation processes, whereas Calcare Massiccio Formation regulate the groundwater circulation in the deeper zones of aquifer characterized by a high karstification degree through a rather developed conduit networks.