## 3 Results and Discussion
### Hydrochemical Facies and Major Elements  
Given that the statistically significant differences in the dataset are between aquifer types and not between seasons (cfr. the Kruskal-Wallis test results in Table 1 and Tables S3 and S4 in Supplementary Material), we decided to only show maps related to the warm season, which consists of the largest dataset. In all aquifers (Figure 5), Ca-HCO\({}_{3}\) is the dominant hydrochemical facies with 85% of water samples. The second most detected facies is Ca/Mg-HCO\({}_{3}\) (5%) followed by Na-HCO\({}_{3}\) (3%), Mg-HCO\({}_{3}\) (2%), Ca-SO\({}_{4}\) (1%), Na-Cl (1%), and Ca-Cl (1%), while the remaining water are classified as K-HCO\({}_{3}\), Ca/Mg-Cl, Mg-Cl, Ca/Mg-SO\({}_{4}\), Na-SO\({}_{4}\), and Na/K-Cl. Ca-HCO\({}_{3}\) and Ca/Mg-HCO\({}_{3}\) water are evenly distributed throughout the study area, in line with the characteristics of groundwater present in the aquifers of temperate regions as reported by various authors [42; 61; 62; 63]. Their origin is due to the dissolution of carbonate deposits [64], which are the main minerals constituting the aquifers [14]. The distribution of other hydrochemical facies is not homogeneous in the different aquifers and it is conditioned by local effects as following described. In the phreatic aquifers, the majority of samples are Ca/Mg-HCO\({}_{3}\) water, while the second most numerous group includes Ca/Mg-Cl/SO\({}_{4}\) water as shown in the Langelier-Ludwig diagram in Figure 5a. The Ca\({}^{2+}\) and HCO\({}_{3}\)\({}^{-}\) are the dominant species, while high concentrations of Mg\({}^{2+}\) are detected in PIE, linked to phoilite outcrops [65], and in the area between Garda Lake and Veneto-Friulan high plain due to the dissolution of the abundant dolomite deposits [66]. The few water samples classified as Na/K- Cl/SO\({}_{4}\) water are in the Emilian hinterland, in the high plain of the border between PIE and LOM, and along the Adriatic coast. In the Emilian hinterland, high concentrations of Na\({}^{+}\), K\({}^{+}\), and SO\({}_{4}\)\({}^{2-}\) are due to the use of fertilizers that are rich in macro (N, P, and K) and meso elements (Mg, Ca, and S) and are important in plant nutrition, processes of integration, protection, and development of crops. Close to Imola and the border between Reggio-Emilia and Modena, the concentration of sulphates and calcium increases considerably, due to the local lithologic outcrops rich in Triassic evaporite formations and Messinian gypsum [67]. In the central-western aquifers (PIE and LOM) the effect is due to the interaction between water and evaporitic rocks formed during the phases of marine transgression of the Quaternary [42; 68; 69]. It is not excluded that the increase in concentrations of Na\({}^{+}\), K\({}^{+}\), Cl\({}^{-}\), and SO\({}_{4}\)\({}^{2-}\) occurring at the border between LOM and PIE and in the central portion of the plain along the course of the Po River may be caused by anthropic activities, such as agriculture or urban wastewater treatment, being these phreatic aquifers in an agricultural context. Samples collected close to the low coastal plain of both EMR and VEN are affected by saltwater intrusion phenomena [57; 59] and groundwater salinization due to relict water [43]. EMR coastal shallow aquifers are widely impacted by the phenomena of soil and water salinization [8; 57; 58; 70; 71], also testified by the highest EC values of the database, here recorded. Figure 5: Distribution of hydrochemical facies in the (**a**) phreatic, and (**b**) semi-confined and confined aquifers with related Langelier–Ludwig diagrams. The presence of Ca-Cl or Ca/Mg-Cl water detected in the terminal portion of the Alpine chain (PIE, LOM, and EMR), could be due to processes of anaerobic decomposition of organic matter. These stations record high concentrations of HCO\({}_{3}\)\({}^{-}\), Fe, and Mn, low concentration of SO\({}_{4}\)\({}^{2-}\), and high EC values, all signals testifying processes of organic matter decomposition [72; 73].