L'intrusione salina nel comprensorio lagunare veneziano. Il bacino meridionale

The salt water contamination process in the Venice watershed between the southern edge of the lagoon of Venice and the Adige River was investigated through hydrogeological and geophysical surveys and a new monitoring network of surface water and shallow groundwater (ISES Network). The results show that: the presence of saline water extends up to 20 km inland from the coast; the depth of the fresh/salt-water interface varies from 2 to 30 m below the ground level and exhibits a significant, mainly seasonal, time variation; the bottom of the contaminant plume penetrates from 10 even to 100 m depth in the subsoil. Further investigations, still in progress, are showing that the dynamics of the soil salinization process is especially sensitive to the changes in the river (Brenta, Bacchiglione, Adige, Gorzone) discharge, water levels in a complex drainage network regulated by a number of pumping stations, and climatic conditions. Other important factors are fresh-water releases for irrigation purpose and riverbed seepage. The latter is influenced by the tide dynamics which, together with river discharge, control the sea water encroachment along the river mouths. In summer 2003, for example, characterized by a severe drought, salt water flowed up the Brenta and Bacchiglione rivers for about 20 km. The saltwater intrusion has been enhanced by the ground elevation generally below the mean sea level, especially in the southern sector where land subsidence, mainly due to peat soil oxidation, has induced an elevation loss greater than 1 m over the last few decades.