Hydrogeologic budgeting of a fractured aquifer supported by tunnel-drained groundwater data.

The paper deals with the defining and estimating the hydrogeologic components of the runoff relatively to three Appenninic mountainous watersheds in Northern Tuscany (Italy). The closing of the groundwater budget equation and the estimate of the effective infiltration are supported by the data of groundwater drainage from tunnels bored for the High Speed railway connection between Florence and Bologna. The comparison between the surface hydrological regime of springs and creeks, the discharge monitoring of the tunnel and the effects on the environmental flows of the forced drainage, has put in evidence either a higher than expected value of the coefficient of effective infiltration, considered as the ratio between effective infiltration in the watershed and effective rainfall, or the importance of the interflow in the determination of the runoff coming out from the watershed. As a consequence of a geological, structural and tectonic control, the permeability of the local regional aquifer, so far considered a low permeability unit (Marnoso-Arenacea formation, a torbiditic layered marl and sandstone), attains an aquifer type behaviour with important consequences either for the technical obstacles to boring the tunnel or for the groundwater resources exploitation and management of the watershed. In the paper a methodology to estimate the dynamic resource from the analysis of the recession curve of creeks and springs and to derive the environmental flow of a creek, both with the availability of few discharge data, is presented. Hydrogeological considerations on local and regional groundwater flow systems are also supported by a hydrochemical analysis.