GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.3 155 3rd layer: impermeable soil beneath the aquifer. Boundary conditions. Boundary conditions are used to define the water exchanges, mass or heat occurring at the interface between the volume modeled and the outside. The hydraulic (piezometric) gradient was set through boundary conditions of type 1 (Dirichlet): this condition allows to assign the hydraulic load (m) to cells/nodes of the domain (command “ Constant and General Head in MODFLOW” ). The extraction and reinjection wells were simulated with conditions of 2nd type (Neumann) through which it’s possible to assign a hydraulic flow (m/s) to cells/nodes of the domain (command “Well in MODFLOW” ). As regards the thermal features of the model, the thermal regime of the aquifer has been reproduced with a condition of constant temperature (command “ Constant concentration in SEAWAT”). This condition was set to upstream to the direction of groundwater flow. The constant value assigned at the temperature is equal to the undisturbed average temperature of the aquifer. The model no examines phenomena of recharge of the aquifer and loss through evapotranspiration. Results. After defining the hydrogeological conceptual model and heat transfer model of the area, several simulations were carried out with the aim to evaluate the effects on the thermal state of groundwater related to the propagation of thermal bubble. As a reference value of undisturbed temperature we assumed the annual average recorded in the piezometer by multiparametric probe and equal to 14 C. Two scenarios were simulated: the first concerns the thermal effects related to the use of extraction and re-injection wells (P1 and P2 in Fig. 1B); the second scenario considers the effects caused by the operation of the wells of Province of Turin Institute and the wells of the San Paolo Institute placed upstream of one’s considered in the first scenario. First scenario. It was hypothesized a cycle of operation so structured: In winter (October to March): Q peak 10 l/s; extraction temperature water: 14 C; discharge temperature water: 10 C; ΔT 4 C. In summer (April to September): Q peak 10 l/s; extraction temperature water: 14 C; discharge temperature water: 9 C; ΔT 5 C. The two cycles are spaced from one month for stopping of the system. In Fig. 2 it is shown the extension of the thermal bubble respectively at the end of the first summer cycle (Fig. 2A), after one year (Fig. 2B) and after two years (Fig. 2C) of operation of the plant. Fig. 2 – Simulations of thermal bubble extension at the end of the first summer cycle (A), after one year (B) and after two years (C) of operation of the plant.