## 4 Conclusions  
For the first time in Italy, this study allowed to integrate in a single and homogeneous database all the freely available geochemical groundwater data separately collected by the ARPAs of the Northern Italy (Piedmont, Lombardy, Veneto, Friuli-Venezia Giulia, and Emilia-Romagna) during 2018. The developed database permitted to describe the geochemical processes of a wide and complex aquifers system, providing a complete and unique overview on the natural and anthropic processes characterizing the entire Po plain.  
Nevertheless, the study demonstrated that the application of a rigid protocol of data management including data assembling, reformatting, correction, homogenization, and then grouping into homogeneous aquifer type (phreatic, semi-confined, and confined) allowed the comparison of groundwater data originally collected using different management and analytical protocols.  
From the geochemical point of view, mineralization and hardness are higher in the Apennine and easter Alpine sectors than in the western Alpine sector due to the prevalence of carbonate sedimentary rock in the first case, and clasts of crystalline rock in the second.  
The phreatic aquifers of Northern Italy are more exposed to contamination phenomena related to anthropic activities than confined aquifers, especially in the central area of the Po plain, where agricultural and livestock activities abound. In these contexts, water show high values of Na, K, and N, present both in the form of NO\({}_{3}\)\({}^{-}\) and NH\({}_{4}\)\({}^{+}\), which can be linked to agricultural fertilizers. It is difficult to certainly establish if water samples exceeding the NH\({}_{4}\)\({}^{+}\) threshold are due exclusively to anthropic impacts; it is more correct to affirm that farming activities cause a local intensification of natural NH\({}_{4}\)\({}^{+}\) background. Paddy fields on the border between PIE and LOM seem to be responsible for high Fe and Mn concentrations due to local anoxic conditions during the flooded period.  
Ongoing saltwater intrusion phenomena are evident in the shallow unconfined coastal aquifers along the EMR coastal areas. At the foot of the Apennine, the dissolution of evaporite formations is evident and marked by high EC values as well as Ca-SO\({}_{4}\) dominant facies, recorded in shallow and deep water samples.  
Throughout the Alps, regardless of the type of aquifer, high concentrations of Mg are found, the origin of which is identified with the dissolution of ultramafic rocks in PIE, and with the dissolution of dolomite in the stretches of the plain east of Garda lake in VEN. The presence of Na-Cl water along the Adriatic coast is associated with saltwater intrusion phenomena, while in the hinterland it is more due to the dissolution of evaporitic rocks or the upwelling of fossil water.  
In the confined aquifers of the central plain, the long residence times of groundwater allow cation exchange processes in fine sediment between Na\({}^{+}\) of relict marine water and Ca\({}^{2+}\), generating Na- HCO\({}_{3}\) water with extremely high EC. The negative redox potential, the absence of interaction with oxygenated water, and the dissolution of Fe and Mn oxide-hydroxides testified the mineralization of the abundant organic matter present in fine sediments, with consequent increase in NH\({}_{4}\)\({}^{+}\), Fe, Mn, and also As in deep groundwater.