## 2 Materials and Methods
### Conceptual Site Model Reconstruction  
The reconstruction of the conceptual site model started from the geometries of aquifers that were provided by previous works and were improved through the analysis of 110 stratigraphic logs, which have been used to obtain an accurate representation of the hydrogeological structure of the study area (Figure 2b). The collected stratigraphic information has been compared with data from previous studies [30; 35], with the topographic data of the Digital Terrain Model (DTM) [36; 37] and with the geological map that was elaborated for the CARGproject (Cartografia Geologica e geotematica) by the Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA) [32]. For this study, eleven cross-sections have been prepared, six with a NS direction (1-6) and five are WE oriented (A-E), as shown in Figure 2b. Section 4 and Section C are shown in Figure 3.  
What is possible to understand from the hydrogeological sections is that the Grandate area is a valley that is carved in the rocky substratum of Wurm glaciers into which the glacial and alluvial deposits have accumulated. This formation is generally considered semipermeable because there are very few wells with screens in this unit and they are only in the most superficial and fractured part of the rock. Moving southward, the bedrock deepens, covered by two aquifers: the first, generally phreatic, corresponds to the gravelly-sandy complex, while the second, confined, is composed by the fluvioglacial Riss/Mindel deposits and the Villafranchian ones and is found more than 100 m below ground level. The first aquifer is separated from the deepest one by the glacial Riss/Mindel deposits that are about 30 m below the ground level. In the southern part of Grandate, the rocky substratum and the Villafranchian deposits rise again. The valley in the substratum tends in fact to close in this zone where the separation level between the first and the second aquifer also disappears. The aquifers here are thus in contact and reduce their thickness, making the outflow section southward thinner. From Section 4 and Section C (Figure 3), it is possible to notice what was previously explained: the Grandate plain is like a big bucket, surrounded by morainic hills, where water flows from the topographically more elevated zones, infiltrates in the plain, and hardly discharges southward. In fact, for the uplift of the glacial Riss/Mindel deposits and the approaching of the morainic arcs, the outflow section shrinks in the southern zone of the plain, making it difficult for groundwater to flow downgradient. Figure 2: **(a) Digital Elevation Model of the study area, principal hydrographic network, and model domain (red square); (b) stratigraphic logs (yellow dots), and realized cross-section traces (in black Section 4 and Section C). **  
In November 2015, a piezometric survey over 31 wells/piezometers and 5 hydrometers along Seveso River was carried out. The data that has been collected are not numerous because unfortunately in the past, the area has never been characterized from the hydrogeological point of view and there are not many monitoring points. The piezometric map (Figure 4) confirms the conceptual site model reconstruction: the first aquifer, though presenting a moderate hydraulic conductivity, hosts a groundwater body that is sustained by finer sediments, which confine the second aquifer. Part of this groundwater flows toward the lake (as happens in some similar systems [38]) and part thereof flows southward through the Seveso River depression aquifer. Most of the groundwater, however, flows from the borders of the morainic hills that surround the plain toward the center of the plain and, with difficulty, discharges southward where the flow section shrinks. The low gradient (<0.5%) that is registered in the plain (Figure 4) is a consequence of what was previously explained, because this hydrogeological structure has been recognized in many cases as a stagnation area.