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Venice industrial harbour acquifer simulation (italy)

MODELLING THE HYDROGEOLOGICAL IMPACT OF A CONTAINMENT CUT-OFF WALL IN THE VENICE INDUSTRIAL HARBOUR OF PORTO MARGHERA – ITALY


Project: Study of groundwater dynamics related to containment cut-off wall construction in the Venice industrial harbour of Porto Marghera - Italy


Location: Venice, Italy


Client: Venezia Nuova Consortium for Italian Ministry of Public Works – Water Board, Venice



Services: Mathematical Model Studies; Preliminary, Final and Detailed designs



Period: 2007-2011



A major source of non biodegradable organic and inorganic pollutants is represented by Porto Marghera, a port and petrochemical complex on the central-west side of the Venice lagoon. Porto Marghera is classified as "Site of National Interest" (SIN). Within the general project of decontamination activities called MISE, the Italian Ministry of Environment asked the Italian Water Authority (Magistrato alle Acque MAV) to limit the water exchange between the contaminated soils and the lagoon waters within the SIN. MAV has planned construction of a continuous 56.8 km long cut-off wall made of steel sheet piles along the entire harbour channel banks. The diaphragms are driven into the lagoon bottom down to a depth ranging between 15 and 25 m below sea level. Since such a cut-off wall constitutes an impermeable barrier to groundwater flow from the mainland toward the lagoon a study has been commissioned to Technital on expected impact of the cut-off wall on the natural flow regime of the shallowest aquifers.

To predict the impact on the hydrologic regime and mitigate the related inundation hazard in the nearby urban areas, a complex three-dimensional finite-element model has been developed and implemented over the multiaquifer system down to a depth significantly larger than the wall bottom. The model is initially calibrated against the regional piezometry and then refined locally to reproduce the groundwater volume drained along a 30-km long bank of a harbour canal already bounded. Major results from the simulations show that after the wall completion the subsurface discharge into the lagoon is successfully abated by as much as 85% relative to the preexisting rate with, however, an expected 1-m increase of the water table in the inland city of Mestre, Italy thus pointing to the need for implementing a drainage trench upstream the wall to properly reduce the raised groundwater level. The model would also indicate that the shallowest aquifers may receive underground water from a large number of old deeper boreholes abandoned in the past and not properly sealed.
From a more general perspective, the present study constitutes an important example where the reliable and sustainable design of a complex engineered structure bound to impact significantly on the surrounding environment can be much helped and improved by the use of advanced numerical models capable to capture the essential features of the underlying geohydrological processes.

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