## 1 Introduction  
Despite the increasing investment in the expansion of open water data and the incredible amount of freely available water data provided by the different environmental agencies and institutions in charge of surface and groundwater monitoring within countries, there are still many obstacles of having available homogeneous and integrated water databases [1, 2]. The data quality depends on their own completeness, accuracy, traceability (of dataset creation and updates), contemporaney, validation, compliance (as with unified metadata standards), and understandability [3]. The availability of long-term water datasets is important to reveal important patterns, which allow trends, cycles, and pollution events to be identified [4]. The benefits ascribed to long-term water quality databases range from scientific advancement, improved government and governance (i.e., better transparency, accountability, and decision-making processes), and operational and technical efficiency (i.e., improved services, identification of tempestic solutions). With the enactment of the Italian legislation on water resources (DLSs 152/06 and related updates, [5]), in transposition of the Water Framework Directive (WFD, Dir. 2000/60/EC, [6]), monitoring of water bodies is mandatory to assess their environmental quality status. The groundwater monitoring network in Italy is not centralized and all monitoring activities (identification of networks, choice of analytical pools, and monitoring frequencies) and groundwater sample analysis are in charge of each Italian region and the related Regional Agencies for Environmental Protection (ARPAs). The ARPAs are responsible for the quantitative and qualitative monitoring of surface and groundwater. Sampling procedures,analysis methods, sampling material to be used are regulated by D.Lgs 152/2006 [5] and analyses must be performed by accredited laboratories. Data are collected in many ways and by using different protocols; the choice of sampling frequencies and analytical methods is defined by each region based on the characteristics of the territory, the resources vulnerability, pressures, impacts, and the availability of financial resources. Generally, at least two samplings are carried out per station, one in spring and one in autumn. ARPAs databases are proposed as usable and accessible tools, whose contents can be freely interrogated and downloaded. However, data organized in individual databases create the fragmentation of information and so far, a national geochemical database, which can be implemented over time according to shared rules and standards, does not exist in Italy. These actions generated and still generate a huge amount of data, which is often commented only in relation to water quality standards and relative thresholds, but that contains precious additional information that can be extracted. However, some critical issues arise when working with large heterogeneous datasets collected without shared protocols: data harmonization and cleaning, and long revision processes are required. The Po Plain selected as study area of this work is one of the largest (about 48,000 km\({}^{2}\) wide) alluvial plains in Europe and the largest of Italy, and it represents the most important groundwater reservoir of the Northern Italy because of its size, the feature of its deposits, and its possibility to recharge. The entire Po Plain and surrounding Alps and Apennines belong to five separate administrative entities called Regions; specifically Piedmont, Lombardy, Emilia Romagna, Veneto and Friuli-Venezia Giulia that individually manage and monitor their owned water resources. In this territory coexists some of the most famous and populated cities of Italy (e.g., Milan, Turin, Bologna, Venice, etc.,), huge industrial settlements, natural areas, and agricultural activities. All of these activities interact and influence surface and groundwater quality and quantity.