The elevation net for the saltwater intrusion phenomenon analysis in the coastal plain of Pisa

In coastal plains, such as that of Pisa, with particular reference to groundwater, the marine intrusion phenomenon occurs not only through the freshwater/saltwater interface in the aquifer along the shore line, but also by the rise of the salt wedge in watercourses, in case they are in hydraulic connection with the aquifer itself. The depth of the interface is primarily regulated by the hydrostatic freshwater/saltwater balance, that can be modified by anthropic exploitation of groundwater. To date, seawater intrusion is considered one of the most extensive and important processes that degrade water quality through an increase in salinity levels up to values exceeding the standard for drinking water and irrigation systems, and endangering the future use of coastal waters. Given the potential harmfulness of the phenomenon, in order to monitor its evolution, control wells are set up, within which it is possible to perform routine analysis aimed at measuring the characteristic parameters of the phenomenon itself. The essential foundation to reconstruct the freshwater/saltwater interface, and therefore also for the recognition of the phenomenon of seawater intrusion, is represented by piezometric reconstructions integrated with hydrogeochemical information that can be obtained directly from measurements made in wells and indirectly through laboratory tests carried out on samples of water. A serious limitation of the piezometric reconstructions is mostly represented by the margin of approximation with which the height of water points has been established, which in turn result in uncertainty in the relationship between piezometric and hydrometric elevation. This paper presents various methodologies, GNSS and conventional, to determine piezometers elevation, also evaluating the accuracy obtainable and the requirements in terms of time, personnel and equipment.