GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 3.2 539 The purpose of this study is to evaluate how borehole thermal logs can contribute to the implementation of a local groundwater flow model. As an example, we study the Maggiore Valley area, located in the Piedmont hilly region (NW Italy) (Fig. 1a), whose main well field supplies drinking water to a wide area and the population of about 100.000 people. The increasing demand of water has led to the overexploitation of the Maggiore Valley groundwater resources, over the past decades, causing progressive lowering of piezometric level, spatial reduction of the artesian zone, localized land subsidence and damage to wells. The piezometric head reduced of about 50 m from 1920 to present-day. The new hydrogeological data inferred with the thermal method can give further constraints for the implementation of groundwater numerical modeling. Geological and hydrogeological setting. The Maggiore Valley is located at the morphological edge between the Asti Reliefs to the east (130-320 m a.s.l.), and the plain of the Poirino Plateau to the west (240 to 325 m a.s.l.) (Fig. 1a). The stratigraphy of both areas consists of a succession of incoherent sediments, related to depositional environments changing with time from marine to deltaic and fluvial (Carraro Ed., 1996; Forno et al. , 2015). The Asti Sands crop out, corresponding to parallel-bedded sands of Pliocene age in the Maggiore Valley, in the lower parts of the reliefs (Figs. 1b and 1d). These sediments are usually covered by the Villafranchian succession, made up with alternations of sands and gravels with silts and clays. The Villafranchian succession continues beneath the Poirino Plateau and consists of two superimposed sedimentary complexes with a total thickness of about 100 m, which are separated by a regional unconformity (Forno et al. , 2015). All the sediments display significant deformation linked to the presence of deformation zones (Gattiglio et al. , 2016). Fig. 1 – Location of well field (A), hydrogeological complexes (B) and piezometric map (C) of the Maggiore Valley. Hydrogeological conceptual model (D): 1 Holocene fluvial deposits; 2 Pleistocene fluvial deposits; 3 Villafranchian succession (Piacenzian - Calabrian) (a: silt and clay; b: gravel and sand); 4 Asti Sands (Zanclean); 5 Lugagnano Clay (Piacenzian). Arrows indicate the groundwater flow direction (from Lasagna et al. , 2014).