Effects of an extreme flood event on an alpine karst system

The effects of an extreme storm (501 mm of rainfall in less than five days) were monitored in an alpine show cave to assess the characteristics of its aquifer unsaturated and saturated zones. The selected cave, Bossea (SW Piedmont, Italy), is an excellent test site for hydrological investigation of karst systems, because it hosts an underground karst laboratory since the late 70 s. The investigation was carried out by means of an integrated approach that combined measurements of flow discharge, physico-chemical parameters, major chemical components, and trace elements (metals, Rb, Ba, Sr and REE). The hydrology and hydrochemistry of the main underground river and two of its secondary tributaries (a drip site and a small secondary inflow) draining the unsaturated zone were monitored during the November 2016 flood, an exceptional hydrological event (estimated recurrence time of 200 years) that caused severe damages in the whole southwestern Piedmont region. The results of the 2016 monitoring were compared with those of another less extreme flood occurred in 2011 (recurrence time of 20 years). The karst system showed an impulsive response to flooding at the catchment and at the individual inflow scales, but each site exhibited a characteristic response due to the complex geological and structural setting of the cave system. The development of this karst system is, in fact, related to the lateral and vertical juxtaposition of rocks with different lithologies and mechanical properties, which form different hydrogeologic compartments. Coupling hydrograph, chemograph analysis and REE normalized patterns permitted to assess which compartments were activated during each phase of the floods. In particular, it was possible to recognize a progressive increasing contribution of the non-carbonate lithologies to flow during the peak of the flood for both the two unsaturated inflows and the main underground river, suggesting the activation of larger portions of the aquifer. However, the response of each individual unsaturated inflow is more influenced by its recharge system architecture rather than the magnitude of the meteorological event. Similar complex geological and lithological karst systems are typical of many areas characterized by orogenetic processes (Alps, Rocky Mountains, Caledonides, etc.), but despite them being relatively widespread, the hydrodynamics of these aquifers is still poorly understood. This study affirms that only long-term and well-integrated monitoring and sampling can help unravel the behavior of such complex karst systems.