Geological and hydrogeological investigations for the design of a multitracer test in a major karst aquifer (Cansiglio-Cavallo, Italian Alps)

An important karst aquifer lies below the Cansiglio-Cavallo plateau, a limestone massif located on the border between the regions of Veneto and Friuli-Venezia Giulia (northeastern Italy). According to few existing hydrogeological studies, the massif hosts a mature karst system characterized by an high density of dolines and swallow holes on the surface, and groundwater circulation through very deep karst conduits. However, only few experimental data have been collected up to now concerning the actual patterns of groundwater circulation. Three main springs emerge along a 4 km long front on the southeastern margin of the massif, discharging a global mean annual flow rate of about 11.4 m3/s and giving origin to the Livenza river. These are the second biggest springs of northern Italy in terms of flow rate and represent a valuable and easily accessible resource for drinking water and for other human activities (e.g. fish farming), which need to be properly protected. 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 is being designed, employing both fluorescent tracers and heavy water. Starting from the early spring 2015, preliminary investigations have been performed on the massif and the springs for the arrangement of the tracer test (planned for late fall 2015), in collaboration with the local speleological groups: 1) new detailed information has been collected on the geologic and tectonic architecture of the plateau, in order to outline the most likely groundwater paths reaching the springs; 2) the spring discharge rates have been monitored through Doppler water velocimeters, to estimate the hydrogeologic balance of the portion of massif involved in the tracer test; 3) groundwater samples have been collected from caves, springs and rainfall before and after the main flood events, and the physico-chemical parameters of water have been continuously monitored, with the aim of identifying possible hydrochemical and isotopic affinities between the different ground- and surface waters. The preliminary investigations, presented here together with the design of the tracer test, are essential to understand the hydrogeological complexity of the Cansiglio-Cavallo massif.