Hydrogeological investigations and a multitracer test in the Cansiglio-Cavallo karst area (Italian Alps)

The Cansiglio-Cavallo plateau is a limestone massif shared between the Italian regions of Veneto and Friuli-Venezia Giulia (northeastern Italy). The area is characterized by a high density of dolines and groundwater circulation is known to occur through very deep karst conduits. Three main springs (Molinetto, Santissima and Gorgazzo) located along a 4 km front on the southeastern margin of the massif are the main outlets of this karst aquifer, discharging a global mean annual flow rate of 11 m3/s and giving origin to the Livenza river. These are the second most important springs of the Alps in terms of flow rate and represent a valuable and easily accessible resource for drinkable water and other economic activities (e.g. fish farming). The hydrogeology of this karst area is still poorly known, also due to the common lack of groundwater flow rates suitable for dye tracer injection inside the many cave systems of the area. A dye tracing experiment in 2008 showed a connection between one of the major cave systems named Abisso del Col de la Rizza with the springs Santissima and Molinetto. Instead, the tracer injection in the other major cave system (Bus della Genziana) did not give positive results in any of the springs. In order to enhance our knowledge on the recharging areas of the springs and their hydrogeological connections with the main local caves, a cave-to-spring multitracer test was carried out in late spring 2016, by using three different fluorescent tracers. This was preceded by preparatory geological, hydrodynamic and hydrochemical investigations carried out in the area since spring 2015, which allowed collecting useful information for the setup of the tracer test. One of the underground rivers in the Bus della Genziana cave, the Abisso del Col de la Rizza cave and Fessura della Tosca cave were used as tracer injection points. Prior to injection, charcoal detectors were emplaced in the main monitoring points (caves and springs), while the three main springs were equipped with GGUN-FL fluorimeters. The hydrogeological study gave the following results: 1) new detailed information was collected on the geological and tectonic structure of the plateau; 2) the discharge rates of the three main springs were monitored and reliable h/Q rating curves were reconstructed; 3) a water budget for the entire karst area was calculated based on all available hydrological data; 4) groundwater geochemistry and the trends of physico-chemical parameters allowed understanding the discharging behavior of the system with respect to different recharge conditions; 5) the multitracer test allowed to better define the recharge areas of the three springs and the effective groundwater flow velocities in different sectors of the massif.