GNGTS 2015 - Atti del 34° Convegno Nazionale

156 GNGTS 2015 S essione 1.3 The simulations show that the thermal impact a few hundred meters downstream of the discharge is practically negligible. The extension of the thermal plume upstream (i.e. “ thermal breakthrough ”), it is clearly more limited for hydraulic reasons. Finally, it is observed that the distance between the extraction and re-injection wells is sufficient to prevent the temperature increase of the groundwater in correspondence of the extraction well: this means absence of the phenomenon of “ thermal feedback” . Second scenario. It was hypothesized a cycle of operation so structured: In winter (October to March): Q peak 15 l/s; extraction temperature water: 8 C; discharge temperature water: 3 C; ΔT 5 C. In summer (April to September): Q peak 15 l/s; extraction temperature water: 22 C; discharge temperature water: 27 C; ΔT 5 C. The two cycles are spaced from one month for stopping of the system. This hypothesis, highly conservative, assumes that the thermal plume, produced by the wells of Province of Turin Institute and San Paolo Institute, located upstream, are able to determine, in correspondence of the extraction well, a temperature of groundwater during the winter cycle equal to 8 C and during the summer cycle equal to 22 C. The simulations (Fig. 3) show the extension of the thermal after one year (Fig. 3A), after 16 months (Fig. 3B) and after two years (Fig. 3C) of operation of the plant. The thermal situation created by the operations San Paolo Institute and Province of Turin Institute wells, does not significantly change the operation of the wells P1 and P2. The simulations show a thermal bubble with increase or decrease of temperature in the aquifer of reduced size. Conclusions. The study highlights the utility of using finite-difference computer codes in support of water flow and heat transport simulations. The modeling of groundwater flow with MODFLOW and heat transport with SEAWAT code, allow to evaluate the propagation of thermal bubble and therefore the correct design of the geothermal plant (e.g optimize the distance of extraction and re-injection wells) for estimating the effects on the thermal state of groundwater. Therefore, long period simulations are useful to evaluate the environmental effects inducted by the extension of the thermal plume. The monitoring of groundwater temperature and other parameters (static level and electric conductivity of the aquifer), still ongoing, allows the future validation of the model as well as the protection of groundwater resources from groundwater’s thermal pollution. In conclusion, scenarios above proposed have shown a remarkable aptitude of the aquifer in the mitigation of thermal anomalies, as evidenced by the temperature values recorded in the monitoring piezometer and compatible with the average annual value of the temperature in the subsurface. Fig. 3 – Simulations of thermal bubble extension after one year (A), after 16 months (B) and after two years (C) of operation of the plant.