Thermal characterization of shallow aquifer in the left sector of the Lanzo Fan (Piemonte region) for geothermal applications

This research is aimed to profitably accommodate open loop systems that permit the exploitation of low enthalpy geothermal energy. A statistical analysis of the thermometric data of the shallow aquifer is performed, also, for the evaluation of the presence of a “homoeothermic surface”. Previous studies (Stringari et al., 2009) focused the attention on the thermal mapping of the left sector of Lanzo Fan shallow aquifer. The data were collected to identify the most promising areas for the installation of geothermal probes. The Lanzo Fan, extended about 300 km2, is formed by a very thick villafranchian transitional succession with a thin fluvial quaternary cover (Forno et al., 2009). The villafranchian sediments (Lower Complex of middle Pliocene and Upper Complex of lower Pleistocene) consist of prevalently strongly consolidated silt and sand, hosting productive deep aquifers. The fluvial sediments (five units of middle and upper Pleistocene - Holocene forming five terraced surfaces) consist of gravel 20-40 m thick, hosting a shallow poorly productive aquifer. Gravel sediments have a high clay tenor and a significant Fe oxides cementation, connected with the strong weathering, and thus have a low permeability. The trend of the piezometric surface follows the topographic surface, with variable hydraulic gradient between 1% in the apex of the fan (NW) and 0.1% in the low sector (SE). The water table depth varies between a few tens of meters in the high terraces, where the direction of flow is predominantly WNW-ESE, and a few meters in the other sectors, where the direction of flow is NW-SE. The statistical analysis made on spring and autumn thermometric measures (related to the year 2009), collected in environmental monitoring piezometers, allowed us to check the presence of the “homoeothermic surface”, with its depth range. Moreover it permitted to determine an average value of the temperature with its relative experimental uncertainty at the homoeothermic surface. This average value is compatible with the sinusoidal asymptotic behavior connected to seasonal temperature fluctuations found in the subsurface. The reported reading is in agreement with the solutions of the heat conduction equation in stationary boundary conditions. This method provides a statistically well-defined evaluation of the expected value of the groundwater temperature, at the homeothermic surface, and of the corresponding uncertainty.