GNGTS 2016 - Atti del 35° Convegno Nazionale

540 GNGTS 2016 S essione 3.2 Above the Villafranchian succession a continuous cover of fluvial deposits, referred to the middle and upper Pleistocene, is found. These deposits are related to ancient trends of the Po and the Tanaro rivers and constitute a continuous succession in the Poirino Plateau. The fluvial deposits appear as a terraced fluvial succession that develops at different heights at the top of the hill ridges in the Asti Reliefs (Fig. 1d). Asti Sands and the Villafranchian succession, both hosting multi-layered aquifer systems, are the most significant water reservoirs. These reservoirs are recharged at the western side of the area by a W-E regional groundwater flow. Both aquifers drain towards East, below the Poirino Plateau, and recharged in Po River plain. Conversely, a medium-permeability shallow aquifer is hosted in the terraces fluvial deposits in the Poirino Plateau, above the multi-layered aquifer previously described (Fig. 1d). This aquifer drains westward and it is mainly fed by direct rainfall and rivers at the outlet of the valley. The water table is strongly influenced by the topography, a wide well field occurs close to the town of Cantarana. Forty-one pumping wells are located on a surface of about 3 km 2 in the Maggiore Valley and a multi-layered aquifer is hosted in the medium-permeability Asti Sands. A recent piezometric survey in the Maggiore Valley well field (see Lasagna et al. , 2014) shows that the extraction of wells activity creates a pronounced depression cone with a W to E flow direction because of the intensive groundwater pumping (Fig. 1c). The magnitude of the hydraulic gradient in the deep aquifer ranges from 1% beneath the Poirino Plateau to 10% in the Maggiore Valley well field. Borehole temperature and seismostratigraphy. The position of 5 sites where thermal data were collected is showed in Fig. 1. They are located in different hydrological contexts, namely the Poirino Plateau (P2) and Asti Reliefs (P1, P3, P8, P10). In more details, P1-P3 are vertical heat exchangers whereas P8 and P10 are inactive water wells used exclusively for groundwater level monitoring. Temperature measurements were performed using a probe equipped with Pt100 resistances with sensitivity of 0.01 °C. Wells P1 (170 m a.s.l.) and P3 (156 m a.s.l.) cross the sandy marine succession (Asti Sands) for the whole length. Well P2 (258 m a.s.l.), located in Poirino Plateau is entirely drilledwithintheVillafranchian succession. The water wells P8 and P10 are placed in the Asti Reliefs in the Maggiore Valley well field (175 m a.s.l.) at a distance of 500 m each other. Even if they do not undergo pumping activity, they are in the proximity of the cone of depression. The temperature-depth data recorded in the boreholes are showed in Fig. 2. The qualitative analysis of thermal logs clearly indicates that P8 and P10 are dominated by advection. The pattern of the temperature distribution in the two wells may be interpreted as related to a horizontal groundwater flow, occurring in the permeable sandy layers. The linear trend of other thermal profiles (P1-P3) highlights a prevailing conductive regime. The geothermal gradient ranges from 19 (P2) to 35 mK m -1 (P1) and is consistent with the value of 25 mK m -1 inferred from deep oil wells data (Pasquale et al. , 2008). Fig. 2 – Thermal logs recorded in the Maggiore Valley and surroundings (see Fig. 1 for wells location).