GNGTS 2015 - Atti del 34° Convegno Nazionale

150 GNGTS 2015 S essione 1.3 Evaluation of the effects induced on groundwater’s thermal state after re-injection of altered temperature water: the case study of heat transport simulation in the shallow aquifer of Turin city (NW Italy) D. Barbero 1,2 , D.A. De Luca 1 , M.G. Forno 1 , M. Lasagna 1 , A.Tissoni 2 , M. Zagami 2 1 Department of Earth Sciences, Turin University, Italy 2 Freelance Geologist Introduction. A case study of heat transport modeling in the shallow aquifer of the Turin City (NW Italy) with finite-difference computer code is here reported. A detailed geological characterization of the subsoil and the estimation of hydrogeological and thermal features of the shallow aquifer are essential tools in support of heat transport simulations for the design of open-loop system. This modeling has the purpose of evaluate the thermal plume propagation connected to the re-injection of altered thermal water in the aquifer with temperature higher or lower than the average annual value. This research focuses on the following problems: a) to study the effects induced by the discharge of water in the shallow aquifer at temperature higher than the annual average value and the consequent rise of temperature induced downstream of the same. In this case, the thermal modeling of the shallow aquifer allows to estimate the alteration on the thermal groundwater state in order to predict phenomena of “ groundwater’s thermal pollution ” with associated environmental problems; b) to study the effects related to the injection in the aquifer of water at lower temperature than the annual average value. The discharge of water at lower temperature causes a decrease of the water calorific power with a consequent loss of efficiency of the geothermal plant. From the practical point of view, the methodology here proposed can be applied in the preliminary stages of a geothermal system design, where it is convenient to assess the optimal distance between extraction and injection wells in order to avoid “ thermal feedback phenomena ” and, more generally, assessing the environmental impact related to the extension of the thermal anomaly in the subsurface. Morphological and geological setting of the Turin city. The Turin city is located in a narrow strip of the north western Po Plain, between the Alps and Turin Hill (Fig. 1A). In particular, it is placed in a marginal position, at the edge with the Turin Hill, where the current course of the Po River is located that separates a plain sector from a hill one. The plain area (altitude ranging from 200 – 350 m a.s.l.) shows a weak inclination towards east and NE (about 1‰). In detail, the morphology is slightly articulated by the presence of small embankments and depressions connected with ancient trends of Po River, characterized by small areal extension, height of few meters and mostly discontinuous areal development (Forno and Lucchesi, 2012). Nevertheless, the morphology of the plain is affected by a generalized anthropic reshaping that causes the modification of numerous natural forms and the creation of new anthropic forms. The plain does not have the typical structure of a subsiding plain, instead, it presents a sector with important recent uplift, connected to the Padane Thrust Front (TFP), that influences the geometry of the sedimentary bodies and created important variations of the hydrographic network during the Pleistocene up to the recent settling of the Po River to the north of the hill (Forno, 1982; Forno and Lucchesi, 2012). The shallow subsoil of Turin consists of Pleistocene fluvial and outwash sediments linked to Alpine watercourses forming wide fans, cut by the erosional scarps linked to the present course of the Po River and partly filled by its Holocene fluvial sediments. The distribution area of the deposits linked to Po River is restricted to the narrow band at the edge of the river bed. Overall, the fluvial and outwash deposits show a shallow thickness, comprised between 10 m, on the edge with the Turin Hills, up to 80 m towards the Alpine chain. These sediments are organized