## 2 Material and Methods
### Hydrogeological Setting  
The cyclic alternation of vertically stacked coarse-grained strata (fluvial to coastal) and finer-grained sediments (alluvial to marine), which characterizes the Late Quaternary succession of the Po Plain, has important implication from a hydrogeological point of view [18]. The aquifer system consists of shallow, unconfined aquifers and deeper, semi-confined and confined aquifers. The main aquifer system is mostly unconfined and consists of coarse-grained sedimentary units, 30 m in thickness and made of amalgamated gravel and sand bodies [26; 36; 37; 38]. Fine-grained (silts and clays) units, with thickness ranging between about 5 and 50 m form the main aquiclude and/or aquitards [39; 40; 41]. Moving toward the coastline, some continental alluvial deposits (mostly clay and silt) are laid upon the coarse-grained sedimentary units, making the aquifer locally semi-confined, where recharge and discharge are reduced but can still occur (hydrogeological setting 3 in Figure 1). Then, as reported by [PERSON] et al. (2018) [42], downwards in the Late Quaternary succession of the Po Plain several confined aquifers are recognized within the entire hydrogeological system. Topographically high alluvial fans located at the more proximal locations of the Po Plain, at the margin of both mountain chains, are characterized by generally high permeability (up to 1-10 \(\times\) 10\({}^{-3}\) m/s of gravel and sand layers). These deposits represent the main direct recharge area of the entire hydrogeological system, including the deepest aquifers [31]. Moving toward increasingly lower and more distal portions of the alluvial plains, coarse-grained sediments are replaced by finer-grained deposits [32; 35], with hydraulic conductivities in the order of 1-10 \(\times\) 10\({}^{-5}\) m/s. Here, the hydrogeological setting is characterized by the presence of multi-layer aquifers with shallow phreatic aquifers that are generally discontinuous and poorly connected to underlying ones (Figure 1). However, despite their low topographic gradient, relatively high permeability coefficients have been documented in some areas in the western Po Plain sector [38; 42] and in the ERP coast (\"shallow coastal aquifers\" in [PERSON] et al., 2013; [43]). The higher hydraulic gradients are recorded in proximity of the Alpine and Apennine chains, in alluvial fan area (\"high plain\" in Figure 1). Typical hydraulic gradients of these areas vary from 8% to 10% in the westernmost sectors, and from 4% to 8% in other portions of the Po Plain such as the VFP [42]. Lower hydraulic gradients (normally ranging from 1% to 4%) characterize the more distal (\"low plain\" in Figure 1) aquifers along the Po River. The lowest values, ranging between 0.2 and 1%, have been reported for the central-eastern Po Plain [44; 45]. As the transition from higher to lower alluvial sectors is accompanied by the decrease in the hydraulic gradient and in grain size, this portion of the study area shows the emergence of peculiar lowland springs (fontanili), especially in the Veneto-Friulan plain and in the orographic left of the Po River. The \"fontanili line\" starts in the most western part of the Friuli plain and continues almost continuously to Piedmont; at the foothills of  
Figure 1: Geolithology of the Northern Italy (from Geological map of Italy, 1:500,000 scale, Italian Geoportal [[http://www.pcn.minambiente.it/mattm/en/](http://www.pcn.minambiente.it/mattm/en/)]([http://www.pcn.minambiente.it/mattm/en/](http://www.pcn.minambiente.it/mattm/en/)), accessed on 22 April 2022). The hydrogeological settings 1–3 are from [PERSON] et al. (2018) [42], modified. the Apennines this phenomenon is much more sporadic and occurs near Piacenza, Parma, Modena and east of Bologna [46; 47; 48]. The water level depth in shallow aquifers is highly variable in the Po Plain: minimum values of 1-5 m b.g.l. are recorded in the central sectors, whereas it may reach 10 m b.g.l. and up to 50 m b.g.l., close to Apenninic and Alpine margins, respectively.