Geological significance of the Perrot Spring in the Mont Avic Natural Park (NW Alps).

Alpine areas shaped in a normally fissured bedrock do not typically contain important groundwater reservoirs. In contrast, in mountainous areas a wide Quaternary cover, especially of landslide deposits, can constitute large aquifers, becoming promising for water withdrawals. A geological study of the central sector of the Chalamy Valley, a right tributary of the main Dora Baltea River (Aosta Valley), was carried out, in which the Perrot Spring is located, with the aim of providing a preliminary assessment of its hydrogeological significance. The main interest of this investigation is, as well as the high discharge of the Perrot Spring, its location within the Mont Avic Natural Park, which is a very busy area with walkers, cyclists, visitors and scholars. The geological survey showed a thick body of sandy silty glaciolacustrine sediments, consequent to the barrage of the Chalamy Valley from the glacier hosted in the main Aosta Valley. These sediments, outcropping in the north-facing slope of the Chalamy Valley are involved in significant gullies and covered by a thick landslide accumulation located in the northern slope of the Bec de Nona, formed by very heterometric sediments. A wide detachment scarp is shaped in serpentinite characterized by evident fracture systems. A preliminary hydrogeological significance for the Perrot Spring, located at the boundary between glaciolacustrine and landslide sediments, was proposed. In detail, the thick landslide cover, characterized by high permeability, represents an important aquifer with a relatively fast groundwater flow to the spring. The underlying glaciolacustrine sediments of the low band of the slope, typically with very low permeability, favour the concentration of groundwater near the boundary and the source supply.