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)

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.