The foothill aquifer system of the Piedmont Alpine zone: geology, hydrogeology and groundwater chemistry

Many foothill zones in Italy contain aquifer systems of strategic interest for water supplying, especially for drinking purposes (Doveri et al., 2016). The hydrogeological features in these environments generally promote the infiltration of both local rainfall and stream water originating in mountain catchments. On the other hand, because of their linkage with mountain areas, these systems are characterized by significant sensitivity towards the meteo-climatic variations and changes. Hence, a high level of knowledge of the foothill aquifers should be reached for addressing the water management issues by real technical approaches and management actions. This work focus on the aquifer system extending in the foothill plain located in the Piedmont region (NW Italy), between the Western Alps and the Torino Hill (Piana et al. 2017). This area is of scientific interest, since it represents the hydrogeological "transition zone" between the Western Po Plain and the southern Piedmont Plain aquifer systems (Irace et al., 2010). Furthermore, the interest towards these aquifers is tied to the reliance on their groundwater resource for drinking water supply, which is performed by SMAT (Società Metropolitana Acque Torino). In close cooperation with SMAT, which also co-founded the project, and taking also into account previous studies (De Luca et al., 2014; De Luca and Osella, 2014) and datasets from monitoring activities institutionally performed by the Environmental Protection Agency of Piedmont Region (ARPA Piemonte), the study examined the geological, hydrogeological and hydraulic-hydrodynamic features of the aquifer, as well as the chemistry of groundwater. Furthermore, some preliminary isotopic analyses were performed. The aquifer recharge estimated through a hydrologic balance approach by Brussolo et al. (2018 - in this conference) was also taken into account. This comprehensive approach steered the definition of the aquifer system conceptual model, comprising the kind of rocks hosting groundwater and their hydraulic properties, the arrangement of groundwater flow, the seasonal evolution of groundwater quantity and the chemical quality of groundwater. Furthermore, the statistical analysis performed on datasets from monitoring stations highlighted some trends over decades. The main results can be summarized as follows: - the system consists of a multilayer that has a phreatic aquifer overlying a succession of impermeable and permeable layers, in which groundwater flow generally occurs in confined to semi-confined conditions; -the phreatic aquifer has thicknesses between 30 and 80 m and it is hosted in gravel, sandy gravel and pebbles of fluvial and fluvioglacial origin (Pleistocene-Holocene deposits). The hydraulic conductivity ranges from 5 * 10-4 m/s to 5 * 10-3 m/s; -the confined to semi-confined sub-system mainly develops within the Pliocene continental deposits, which consist of coarse-grained sands and gravels, alternating to peat-rich clayey and silty deposits. The average hydraulic conductivity of this system ranges from 4*10-5 to 4*10-4 m/s; -the aquifer system is fed by infiltration of local rainfall, river seepage and transfers of groundwater that originate in upland zones. The outputs of groundwater is for exploitation by wells and for drainage operated by rivers, in the lower part of the plain. Generally, the groundwater flow of the phreatic aquifer is separated from that of the confined to semi-confined sub-system. Nevertheless, over wide zones the shallow aquifer and the uppermost permeable layers of the deeper system result in continuity (or separated by weak thicknesses of clayey deposits), thus making possible the mixing of groundwater between the two sub-systems. The main groundwater flow paths occur from northwest to southeast and from west to east; -groundwater are mainly of the Ca(Mg)-HCO3 type with a relatively low salinity for both unconfined and confined aquifers. Nevertheless, the phreatic aquifer hosts groundwater with a relatively higher salinity and, in some zones, characterized by increasing of Cl and SO4 contents. Hence, a certain impact from anthropic activities seems to be possible. In this terms the confined to semi-confined sub-system appears effectively more protected, even though the higher salinity and Cl contents observed in a few deeper wells suggest the possibility of connection with the shallower aquifer.