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A recent research has provided a high resolution map of the present land subsidence of the Venice coastland (Italy). The map, which refers to the 1992-2002 decade and covers an area of about 100x100 km2, has been obtained by the innovative "Subsidence Integrated Monitoring System" (SIMS) that efficiently merges the different displacement measurements obtained by spirit leveling, differential and continuous Global Positing System (GPS), Synthetic Aperture Radar (SAR)-based interferometry. The investigation points out a significant spatial variability of the ground vertical movements, both at regional and local scales, and displacement rates ranging from a slight (1-2 mm/yr) uplift to a serious subsidence of more than 10 mm/yr. Tectonics, differential consolidation of the Pleistocene and Holocene deposits, and human activities, such as groundwater withdrawals, land reclamation of marshes and swamp areas, and farmland conversion into urban areas, superimpose to produce the observed displacements. In this work we have distinguished the displacement components on the basis of the depth of their occurrence. Deep causes, acting at a depth generally greater than 400-600 m below m.s.l., refer to downward movements of the pre-Quaternary basement and land uplift (up to 2 mm/yr) most likely related to neo-tectonic activity connected with the Alpine thrust belts and a NW-SE fault system. The displacement factors located in the medium depth interval, i.e. between 400 and 50 m below m.s.l., are of both natural and anthropogenic origin. The former refers to the Medium-Late Pleistocene deposits that exhibit a larger cumulative thickness of clayey compressible layers at the lagoon extremities with respect to the central lagoon area where stiffer sandy formations prevail. Land subsidence due to aquifer exploitation mainly occurs in the north-eastern sector of the coastland where thousands of active wells are located. In a 10-15 km wide coastal strip thickness, texture, and sedimentation environment of the Holocene deposits play a significant role in controlling shallow land subsidence. Other factors that contribute in increasing land sinking at a smaller areal extent are the salinization of clay deposits due to saltwater intrusion and biochemical oxidation of outcropping peat soils enhanced by. Even the load of houses and structures after the conversion of farmland into urbanized areas cause local shallow compaction.

Descrizione dell'assetto geologico, stratigrafico e geomorfologico della Laguna di Venezia e sua evoluzione a partire dal Pleistocene.

Descrizione dell'assetto geologico, stratigrafico e geomorfologico del territorio compreso nel Foglio 128-Venezia alla scala 1:50.000 e della sua evoluzione a partire dal Pleistocene.

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.