Subsoil architecture and morphological setting shaping the saltwater intrusion in the coastal plain south of the Venice lagoon, Italy

The southern catchment of the Venice watershed (Italy) is threatened by shallow aquifer and soil salinization. The saltwater may extend inland up to 20 km from the Adriatic coastline and deepen down to some tens of meters. Here, saltwater contamination is driven by other forcing factors than excessive pumping, such as ground elevation, buried geological structures, tide encroachment along watercourses, climate and tide conditions, and drainage practices implemented in reclaimed areas. This work aims to outline a conceptual model of the saltwater contamination highlighting the mechanisms driving the saltwater-freshwater exchanges. Results show that the fresh/salt-water interface depth varies from 1 to 30 m below the ground level and exhibits a significant, mainly seasonal, time variation. The dynamics of the soil salinization process is especially sensitive to changes in river (Brenta, Bacchiglione, Adige, Gorzone) discharges, groundwater and channel levels, which are regulated by a number of pumping stations, and climate conditions. Relict geomorphological features, filled with high permeability sediments, act as preferred pathways for groundwater flow and solute transport. In fact they provide a hydraulic connection between freshwater aquifers and sea, possibly facilitating saltwater intrusion landward or, conversely, acting as reservoir of freshwater provided by precipitation, irrigation, percolation through channel beds.