Groundwater flow systems in torbiditic units: a conceptual model based on high speed train tunnels drainage between Florence and Bologna

Torbidites have been considered so far as aquitards; the occurrence of many low discharge springs, scattered around mountainous watersheds with such a lithology, has been interpreted as a confirmation of the low permeability nature of the bedrock. Northern Appenninic chain is made up mainly by oligo-miocenic torbiditic sandstones of the so-called \u201cMacigno\u201d type; the most huge outcrops belong to Marnoso Arenacea formation, an arenaceous-marly unit of Lower-Middle Miocene. Such a hydrogeological ranking of torbiditic units has received a strong change, at least relatively to Northern Appennines, as a consequence of the hydrological evidences consequent to the boring of the series of tunnels for the railway high speed connection between Florence and Bologna; Marnoso-Arenacea denoted locally a strong \u201caquifer-type\u201d behaviour with high and steady volumes of drained groundwater flowing out and the dewatering, complete or seasonal, of many springs. The presented paper deals with a conceptual model of groundwater flow systems in torbiditic catchments, taking into account: 1) the hydrogeological monitoring data to verify the environmental effects of the tunnel boring in 4 appenninic watersheds (Diaterna, Veccione, Bosso, Bagnone); 2) a hydrochemical facies, groundwater dissolved gases, isotopic analysis performed on springs and creeks in the same watersheds; 3) a tracer test to verify the hydraulic connection between creeks and tunnel outflow. During the monitoring, for a 9 year time span (1995-2004) a total of 41 springs and 21 creek sections were monitored discontinuosly from a monthly to seasonal basis (the programme is still going on). The hydrochemical data brought an important contribution to defining the partition between groundwater geologic reserve and dynamic resource flowing out from the tunnel; tracer test put in evidence the key role of creeks as discharge points of groundwater flow system. Trough a hydrogeological budget approach it has been evaluated the environmental effects of the drainage