Hydrostratigraphy of the late Messinian-Quaternary basins in southern Piedmont (northwestern Italy).

An integrated stratigraphic approach has been adopted to define the 3D hydrostratigraphic regional framework of the fresh-water aquifers of the Savigliano and Alessandria Basins, in southern Piedmont. In the present work, the Aquifer Groups represents the “fundamental” hydrostratigraphic units, since they physically correspond to synthems, that have been recognized both in the subsurface and outcrops. Each Aquifer Group is characterized by Hydrogeologic Units (HU) defined on the basis of lithofacies associations that display homogeneous hydrogeologic properties (e.g. permeability ratio, storage coefficient). Four main typologies of Hydrogeologic Units have been distinguished, on the basis of their hydrogeologic characteristics: HU I) unconfined aquifers, HU II) multi-layered “discontinuous” aquifers, HU III) multi-layered “continuous” aquifers, HU IV) aquitards/aquicludes. It has been assumed in this work that, due to its peculiar stratigraphic architecture (i.e. internal stratal organization) each Aquifer Group has a distinctive response to the regional groundwater flow system. In this sense, each main type of Hydrogeologic Unit will present a peculiar and distinctive instance in each single Acquifer Goup. Consequently, the Aquifers Groups can be subdivided into lower rank hydrostratigraphic units, here defined as “Synthemic Hydrogeologic Units”. These units, although encompassed in the four main typologies of Hydrogeologic Units, do inherit the peculiar architectural properties of the Aquifer Group they belong to. In this sense, each main type of Hydrogeologic Unit will present its own instance within the different Aquifer Groups. Following these criteria, nineteen Synthemic Hydrogeologic Units, grouped into seven Aquifer Groups were recognized within the Savigliano and Alessandria Basins. The reconstruction of the hydrostratigraphic model allowed to portray, for the first time, the geometry and architecture of the Aquifer Groups of the whole upper Messinian- Quaternary successions in southern Piedmont and to reconstruct the regional distribution and characteristics of potentially exploitable aquifers. Moreover, the use of stratigraphic constraints (i.e. distribution of depositional systems and lithofacies associations) led to the reconstruction of the geometry of the fresh-salt water interface, also in those areas with scarce well data, and to better estimate the maximum thickness of deep freshaquifers that, up to date, are still unexploited. The hydrostratigraphic reconstruction here proposed constitutes a forward step with respect to the previous knowledge on Piedmont subsurface hydrogeology, and cast the bases for future development of water exploitation and water flow modeling.