Abstract:The aim of this research was to refine the actual conceptual model related to the activation of high-altitude temporary springs within the carbonate Apennines in southern Italy. The research was carried out through geophysical, hydrogeological, hydrochemical and isotopic investigations at the Acqua dei Faggi experimental site during five hydrologic years.The research demonstrated that, in carbonate aquifers where low-permeability faults cause the aquifer system to be compartmentalized, high-altitude temporary springs may be recharged by groundwater. In such settings, neither surface water infiltration in karst systems nor perched temporary aquifers play a role of utmost importance. The rare (once or a few time a year) activation of such springs is due to the fact that groundwater unusually reach the threshold head that allows the spring to flow. The activation of the studied high-altitude temporary spring also depended on relationships between a low-permeability fault core and a karst system that locally interrupts the low-permeability barrier. In fact, when the hydraulic head did not reach the karst system, the concentrated head loss within the fault core did not allow the spring to flow, because the groundwater entirely flowed through the fault towards the downgradient compartment.

Epikarst formation in the southern Apennines (Italy) was hypothesized to be significantly influenced by diffuse rainwater infiltration through soil of pyroclastic origin. Multidisciplinary investigations were carried out at an experimental field site to test this hypothesis. At this site, other factors influencing epikarst formation can be assumed invariant. A direct relationship was observed between soil thickness and epikarst thickness. This relationship supports the hypothesis that the pyroclastic soil plays a significant role in governing epikarst evolution and thickening. The sandy loam texture and the high hydraulic conductivity of the soil allow easy rainwater infiltration, therefore causing a diffuse interaction between percolation water and the soil medium, and spatially homogeneous recharge within the aquifer system. The soil contains a large accumulation of organic carbon and considerable CO2 production results from the activity of autochthonous microorganisms belonging to different genera