Hydrostructural characterization of the karst reservoir in the Umbria-Marche Apennine, Italy

In carbonate reservoir, the flow patterns are conditioned by the degree of karsification of the main conduit networks and by the lower permeability microfractured blocks. To quantify patterns and nature of discontinuities in the rock mass is the main goal in the fluid circulation researches. Indeed fractures such as joint and bedding planes control infiltration, percolation and migration of groundwater into a reservoir. In the Umbria-Marche Apennine the Lower Jurassic limestones represents the basal reservoir where the larger hydrogeological structures are locate. The stratigraphic and structural setting forces a parallel-to-the ridge circulation of groundwaters, and the ridge itself can be considered as an isolated hydrogeological system. A quantitative fractures analysis was been considerate through the whole anticline, from the gently deep SW limb, in the weakly deformed hinge zone to the subvertical and much more deformed NE external side. The prevalent joint systems are oriented N55 and N295, located in the fold limbs and in the hinge zone. Besides, N15 and N345 joint systems are predominant only in western and eastern limbs of the anticline. In the hinge zone the fractures seem to be linked to a normal faults oriented at NW and SE, in correspondence of the crestal area; possibly these shear structures are responsible both for new systems development and for pre-existent sets partial reorientation. Dip discontinuities distribution gave remarkable about water process once infiltration inside the system. The high dips strongly contributed to increase groundwater percolation velocity coming both from direct infiltration areas in the upper hydrogeological complexes. Finally, spacing and frequency fracturation analysis identify rock mass properties from a quantitative point of view; these structural parameters allowed to obtained important considerations about both mechanical and hydrogeological features of a reservoir. The Giordano karst spring drains the SW limb of the M. Nerone anticline with an average discharge of more than 0,1 m3/sec and with a catchment area of about 4 km2. A deterministic approach for the hydrostructural characterization of the spring was adopted using discharge, temperature and water conductivity. The analysis of the hydrographs show that the base flow represent only the 30% and in the recession curve the discharge is controlled by the karst conduits drainage. The recession curves show different steps with a depletion constant of about 10-3 day-1, while the water temperature and electrical conductivity show variations where prevails the conduits drainage respect the fractured matrix. The complex variability and heterogeneity of the karst reservoir seems to be controlled by the larger karst conduits, which are related to the main fracture patterns.