## 2 The Study Area  
The study area lies within the city of Milan and is part of a larger project named \"MODEL-MI\", founded by MM S.p.A. (water manager of the city of Milan, Italy). The MODEL-MI project's aim (presented in [40]) was to renovate the stratigraphy of the city of Milano according to the new information and to develop a groundwater flow model of the city, useful for the development of the Water Safety Plan for the Milan Functional Urban Area, in collaboration with the service manager and other actors involved in the water management system (i.e., the ISS--Italian National Institute of Health; the ARPA--Lombardy Regional Agency for Environmental Protection; and the Municipality of Milan). Starting from the basis of the previous project, the study area of this work was more detailed at a lower scale than the study area shown in [40] and is reported in Figure 1a. The target of the new study focuses on the evaluation of the thermal interference induced in Milan aquifers by closed-loop GSHP systems. According to MM S.p.A., the area in proximity to the drinkable wells of the Armi pumping station (NW sector of Milan) was chosen as the pilot investigation area. Close to the Armi station, a residential building named \"MIA--La Casa Italiana\" presents a geo-exchange system consisting of 60 vertical borehole heat exchangers 140 m deep (Figure 1b) and depicts a perfect case for the study of the thermal interference in the aquifer caused by geothermal systems.  
The analysis of the hydrogeological setting of the study area is essential to better evaluate the heat exchanged and the thermal interference in the Milan aquifers. Based on the fluvioglacial Plio-Pleistoceneic depositional cycles, the deposits in Milan can be divided into four hydrogeological units denoted as Aquifer Groups A, B, C, and D (from youngest to oldest) (Figure 2). These units host aquifers separated by clayey deposits. In detail, Aquifer Group A is mainly composed of gravels with sandy layers and represents the shallow unconfined aquifer. It is characterized by high values of transmissivity (often higher than \(10^{-2}\) m\({}^{2}\)/s). Aquifer Group B is constituted by sands with silty-clayey lenses, presenting a transmissivity ranging between \(10^{-3}\) and \(10^{-2}\) m\({}^{2}\)/s. Aquifer Group C is mainly composed of alternating sandy silts and clays with subordinate intercalations of  
Figure 1: (**a**) A view of the study area inside the city of Milan and (**b**) BHE layout.  
medium to fine sands and gravels, and the average transmissivity in the shallowest part is 1.1 \(\times\) 10\({}^{-2}\) m\({}^{2}\)/s versus the average of 8 \(\times\) 10\({}^{-3}\) m\({}^{2}\)/s found in the basal part [40].  
In the proximity of the MIA building, 18 extraction wells belonging to the MM Armi pumping station are present. The study focuses on the thermal perturbation occurring in Aquifer B, since most of the Armi pumping station wells are screened within that aquifer. In detail, the Armi pumping wells (Figure 3) have depths ranging from 39 m to 102 m: 8 of them are screened only in Aquifer Group B (wells n. 5, 9, 11, 12, 13, 16, 17, 18), 8 are screened in both Aquifer Group A and B (wells n. 1, 2, 3, 4, 6, 7, 10, 20), and 2 in both Aquifer Groups B and C (wells n. 8 and 19). Some wells are located straight along the hypothetical preferential direction of the thermal plume generated by MIA's GSHP system, accordingly to the groundwater flow direction, and thus present the risk of capturing water for drinking purposes at a temperature different from the undisturbed groundwater temperature. For instance, an increase in temperature could cause the development of microorganisms potentially problematic for human health.  
Figure 2: (**a**) A study area and trace of hydrogeological section; (**b**) hydrogeological section with stratigraphic logs, where A is the phreatic shallow aquifer named as Aquifer Group A; B1, B2, B3 and B4 are 4 different confined aquifers belonging to Aquifer Group B; C is the confined Aquifer Group C.