Tomografia elettrica di resistività per il monitoraggio dell'intrusione salina in ambiente costiero

This thesis presents the results obtained from the application of the Electric Resistivity Tomography (ERT) technique for the monitoring the saline contamination of the phreatic aquifer in a coastal environment. This work is part of the Project “Monitoring Sea-water intrusion in coastal aquifers and Testing pilot projects for its mitigation” (MoST), funded by the ItalyCroatia Interreg Program through the European Union. MoST is based on the collaboration between research institutions (University of Padua, National Research Council, University of Split) and local authorities (Consorzio di Bonifica Adige Euganeo, Region of Veneto, Croatian Water and Dunea) which are responsible for developing knowledge concerning the saltwater intrusion phenomenon in northern Adriatic coastlands, where land use is largely dedicated to agriculture. The study area is the Italian pilot site of the MoST project located in the coastal plain of Chioggia, in the proximity of the southern margin of the Venice lagoon. This territory is mostly devoted to agricultural use and lies below the sea level. It is characterized by the presence of subsurface sandy bodies linked to ancient channel systems. Such geomorphological bodies are able to store freshwater, mainly of meteoric origin and for short-time periods, because of their morphological and textural characteristics. The purpose of the project is to mitigate the saltwater intrusion by recharging of the sandy bodies for long periods, using an underground sub-irrigation system supplied by fresh water, in order to improve the agricultural productivity of the farmlands. The aim of this thesis is to support the planning of the hydraulic system and evaluate its capability to mitigate the process of saline contamination in the groundwater. More specifically, 9 ERT lines were designed in strategic positions with respect to the sub-irrigation system. They were used both for providing information on the subsoil to support the design phase of the hydraulic work, and for the subsequent phase of monitoring the hydrogeological conditions induced by its activation. The ERTs were also repeated several times, keeping the position of the electrodes fixed, to obtain time-lapse information. The geophysical survey was accompanied by the lithological analysis of continuous sediment cores and by electrical conductivity and water level measurements performed in monitoring wells located near the geo-electrical lines. This multidisciplinary approach allowed the proper calibration of the geophysical data, as well as provides the information for the interpretation of the electro-stratigraphic model of such complex hydrogeological setting. The results obtained in this work confirm the efficiency of the ERT survey in monitoring subsurface fluid migration in highly variable resistive environments when different survey techniques and data are properly combined in order to achieve the site hydro-stratigraphic model.