The Pilato Lake (Sibillini Mts., Central Italy): first results of a study on the supposed variations of its hydrogeological conditions induced by the seismic sequence 2016-2017

The Pilato Lake has glacial origin, is located in the Sibillini Mountains, Central Italy, and is characterized by the endemism of the small crustaceous Chirocephalus marchesonii. In the context of a research agreement with the Monti Sibillini National Park, the ISPRA Geological Survey of Italy is carrying on some studies aimed to evaluate the supposed effects of the 2016-2017 Central Italy earthquakes on the hydrogeological conditions controlling the lake's evolution. The study, started in July 2018, aims primarily at the reconstruction of the geological and hydrogeological subsoil setting, beneath the valley hosting the lake. In order to define the conceptual model of groundwater circulation and, thus, to evaluate the emptying and recharge seasonal cycle of the lake, hydrogeological surveys and geophysical investigations were performed. The research program is still in progress and only preliminary results may be proposed. The hydrogeological surveys demonstrate the absence of geomorphological evidence of seismic-induced surface fractures generated by the seismic sequence 2016-2017. Consequently, the complete drying of the lake, occurred in summer 2017, was probably due to meteoclimatic reasons and/ or to not yet verified variations in the permeability characteristics of the surficial deposits and/or bedrock affecting the infiltration towards the subsoil, caused by the severe seismic shaking. The low electrical conductivity values of the lake's waters are in accordance with the prevailing origin of the lake's recharge by snow melting and direct rainfall. As concerns the first geophysical surveys in the Pilato Lake area, seismic surveys by surface wave method assessed that the maximum thickness of debris here estimated is about 12.5-14 m. As a whole, the factors responsible for the progressive lowering of the lake level are: i) the water overflow through the Fonte del Lago spring, ii) the evaporation from the lake surface and iii) the drainage through the scarcely permeable glacial deposits towards the Basal Calcareous Aquifer, hosted within the limestone at lower altitude.