## 3 Results and Discussion  
### Geological and Hydrogeological Settings  
The outcropping deposits in the Bologna area are characterized by the Quaternary alluvial succession of the Po River Basin, exceeding 300 m in thickness. A subdivision of the Quaternary alluvial succession of the Po River Basin in the Bologna area has been proposed by [PERSON] et al. [53] and Regione Emilia-Romagna [54]. The Quaternary alluvial sequence shows a periodic alternation of coarse-grained (gravel and sand) and fine-grained (silt and clay) deposits, their repeated alternation represents the main feature of the depositional system [53; 54]. In the study area, this arrangement is clearly recognizable: predominantly gravelly-sandy levels (characterized by variable thickness) are separated by silty-clay deposits. Beyond the backfill material, it is possible to distinguish the first level with fine granulometry, consisting of silty clays and clayy silts with thin laminations of very fine sand, followed by a second horizon composed of sands, fine sands with silt, and sandy silts arranged according to a stratigraphic succession. The third level of plastic clays and silty clays underlies the previous ones and covers the fourth stratum of medium sands with heterometric gravels. Below, consistent clays overlap sandy gravels. The lateral and vertical lithological heterogeneity has important implications regarding the variability of subsoil permeability. This alternate structure of high- and low-permeability zones can create preferential pathways for groundwater flow and solute transport [14; 32]. For this reason, the geological structure of the subsoil was subdivided based on permeability characteristics, as follows:1 Backfill (anthropogenic) materials
* Shallow aquifer
* Aquiclude 1, a low-permeability level of separation between shallow and intermediate aquifers
* Intermediate aquifer
* Aquiclude 2, a low-permeability level of separation between intermediate and deep aquifers
* Deep aquifer  
The subparallel structures that characterize the different horizons are very evident in the three-dimensional lithostritigraphic model (Figure 1), a vertical exaggeration factor was used to mark the lithological steps. The hydrogeological conceptual model shows the presence of three aquifers separated by low-permeability layers. The shallow and intermediate aquifers are relevant for the contamination issue. The shallow aquifer is composed of deposits with predominantly sandy and sandy-silty grain size. The intermediate aquifer is predominantly composed of gravelly and gravelly-sandy deposits. The shallow aquifer is characterized by variable thickness. The morphology of the limits of the intermediate aquifer is less articulated. The piezometric data collected during different campaigns allowed us to produce interpolated piezometric maps of the shallow and intermediate aquifers. The contour maps (Figure 2) illustrate the position of the excavation, which involved the removal of land attributable to shallow and intermediate aquifers for construction of the station and installation of the bypass system. Groundwater circulation in the shallow aquifer is complex in terms of flow direction (Figure 2a). The groundwater flow is oriented mainly from SE to NW. Considering hydraulic conductivity around \(10^{-7}\) m/s, we determined the velocity of groundwater flows in the shallow aquifer to be around 1-5 m/year. Note that the shallow aquifer changes its state from phreatic in some areas to fully confined elsewhere. The intermediate aquifer, which is constantly confined, shows a prevailing direction of groundwater flow from S to N (Figure 2b). Considering average hydraulic conductivity on the order of \(10^{-4}\) m/s, we estimated a groundwater flow velocity around 50-100 m/year. To provide useful information for design purposes, in terms of sizing and configuration of the remediation [15], we focused our attention on the subsoil in the pilot test area.