Modeling the saltwater intrusion in the lowlying catchment of the southern Venice Lagoon, Italy

The coastland Surrounding the Southern Venice Lagoon, Italy, is a precarious environment Subject to both natural changes and anthropogenic pressure. One major environmental problem is the saltwater intrusion in shallow aquifers. The salt contamination is generally the result of seawater encroachment, but significant contributions call also be due to the water exchange between the bed of the major rivers and the subsurface. In fact, the reduced freshwater discharges that occur in the Brenta and Bacchiglione rivers during the dry periods allow the saltwater to flow up from the river mouths for several kilometres. Saltwater intrusion is enhanced by a land elevation well below the mean sea level and by the presence of several ancient sandy fluvial ridges and buried paleo-channels, crossing the farmland with a main direction from inland to the lagoon boundary, that can act as preferential pathways for groundwater flow and solute transport. Using as input data a geological, geophysical, hydrological data set collected around the Casetta Pumping station over 2004-2005, a numerical model has been developed to investigate the saltwater intrusion process along the margin of the southern Venice Lagoon. The model solves the coupled density dependent flow and transport equations by a highly accurate numerical approach based oil the mixed hybrid finite element (MHFE) method and a combination of MHFE with high resolution finite volumes (HRFV) for the discretization of the flow and transport equations, respectively. A set of simulations has been initially carried out to analyze the effect of the natural factors forcing the saltwater intrusion ill the coastal aquifer system. Preliminary results provide evidence of the strong influence exerted by the river freshwater that can represent an effective hydraulic barrier to salt contamination from the lagoon. By contrast the river can be a strong source of salinization for the phreatic aquifer if seawater encroaches along the final 15-20 km of the watercourses.