Geophysical monitoring of the salt wedge in the Po di Goro river (Italy)

Climate change causes global sea levels to rise and the model projections indicate a large increase of the sea level. Moreover, on a river, the rainfall deficit produces a decrease of the river flow rate and the saltwater contamination of the coastal zone in terms of intrusion and penetration in the delta system increases. The seawater intrudes in the form of a wedge along the bottom, this phenomenon, from a theoretical point of view, could be hydraulically modelled but numerical models can partly quantify these influences, but it is difficult to accurately predict the salt intrusion length. Therefore, the geophysical approach could give large contributes on the monitoring (Kiflai et al., 2022). The Po river is the biggest river in Italy and its delta is developed out of the Adriatic Sea. The Po river was affected by a dynamic advancement, which is quite simple in terms of geomorphology and kind of sediments (from sand to silt and clay). In recent decades, however, the saline wedge has assumed increasingly worrying proportions, with a progressive intrusion of water bodies to the hinterland and the 2022 was the worst one when the saline wedge in the Po Delta area intrudes several kilometres inside the delta. From a hydraulic point of view, the Ferrara territory has a flat topography and some part of it on eastern part is below the mean sea level, with value also -3.5 m a.s.l. In most of the aquifer, the water table is below water level and it is maintained by the land reclamation pumping machines that need to drain a territory that would otherwise be flooded. The research activities defined two aims: the observation of the saltwater penetration affecting the surrounding land close the Po di Goro river and the monitoring of the saline wedge. The Po di Goro river is around 50kms long and the geological and geomorphological characteristics highlight buried coastal dunes (with sand) and buried lagoon (clay and silt) deposits. Two geophysical methods were used in these activities: Frequency Domain Electromagnetic (FDEM) and electrical resistivity tomography (ERT). The FDEM method was used to detect the saline wedge in the river and the Electrical Resistivity Tomography was applied to monitor the hydrodynamic iteration between the river and the subsoil around the riverbanks. The indirect data were correlated with a “moving boat” approach with a multilevel EC probe. The ERT sections highlighted how the salty water in the river contaminated the surrounding subsoil. The FDEM data sets defined the hydrodynamic of the saltwater wedge in the river detecting the salty plume front. These results highlight the great potential of the proposed geophysical approach to monitor the saline intrusion in delta system.