## 3 Results and Discussion  
### Groundwater Dataset  
Table 1 shows the main descriptive statistics and Figure 2 the boxplots of all physicochemical parameters and ions concentrations grouped according to the aquifer type. For completeness, the descriptive statistic of the entire database is reported in Table S1 of the Supplementary Material. As can be seen in Table 1, the Kruskal-Wallis non-parametric test on the aquifer types shows a statistically significant differences for most chemical and physical parameters and these will be discussed in the following sections. On the other hand, the same test performed on warm and cold season datasets (Tables S3 and S4 in Supplementary Material) points out that the only statistically significant differences regard temperature and pH in the confined and semi-confined aquifers, while Eh and PO\({}_{4}\)\({}^{3-}\) in the phreatic aquifers. Generally, temperature values are slightly higher in the summer season and in the confined aquifers, as it is expected. Values range between 6.0 \({}^{\circ}\)C and 31.0 \({}^{\circ}\)C, with 98% of samples classified as cold water, and 2% as thermal water. As reported in Table S3 in Supplementary Material, the populations analyzed show the same median of 15.0 \({}^{\circ}\)C, and most values are between 13.7 \({}^{\circ}\)C and 16.4 \({}^{\circ}\)C in line with the Italian patterns of shallow groundwater temperatures [57]. Anomalous values above 20.0 \({}^{\circ}\)C were found in all aquifer types, regardless of the season. The differences found in most of the data are of the order of 1/10 of a degree, so effects related to seasonality or aquifer type cannot be established with certainty. The distribution of pH values highlights the presence of an effect due to the type of aquifer (Table 1). The pH values increase from acidic and neutral condition in the phreatic aquifers to mean value of 7.5 and maximum of 9.1 in the confined aquifers. The median of the phreatic aquifers (7.3) is equal to the 25 th percentile of the other population. The recorded values are typical for groundwater; neutral and slightly acid pH values are found in the phreatic and shallow aquifers due to the contribution of meteoric water (meltwater and precipitation) that infiltrates and percolates. The pH varies depending on the composition of rocks and sediments that surround the pathway of the recharge water infiltrating to the ground and varies depending on the residence time. The longer the contact time, as in the confined aquifers, the larger the effect of the rock chemistry on the composition and on the groundwater pH. Dissolution of carbonate-rich rocks (limestones and marbles) and silicate weathering result in an increase of pH. The water of the Po plain have a hardness ranging from 13 mg/L to 4950 mg/L (Table 1). In general, medium hard water prevail (32%), followed by hard water (29%), moderately hard (18%), and soft water (12%). The 7% of the samples analyzed had very high hardness values, exceeding 540 mg/L, which can be easily identified as the outliers in the distributions (Figure 2). There is a slight effect linked to the type of aquifer analyzing the different populations: in confined and semi-confined aquifers, low and medium hard water prevail, while in phreatic aquifers medium hard and hard water dominate. In phreatic aquifers located along the coastal areas of the plain, the absolute highest values of over 2000 mg/L are recorded. As can be seen in Figure 3a,b, water ranging from very soft to medium hard are found in PIE and in the western part of the plain. On the contrary, EMR water tend to be harder, with water from very to extremely hard and values that exceed 2000 mg/L along the Ravenna and Ferrara coastal areas, where the highest EC values are recorded, too (Figure 4). This different in values is related to the higher composition of carbonate rocks, sedimentary deposits, and flysch in the Appennine zone compared to the metamorphic rocks that characterize the PIE region, and volcanic rock in the north of VEN and FVG (Figure 1).