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Coastal lagoons are highly valued ephemeral habitats that have experienced in many cases the pressure of human activities since the development of urbanisation and economic activities within or around their boundaries. One typical intervention is dredging of canals to increase the exchange of water with the sea or for navigation purposes. In order to divert the route of large cruise liners from the historic center of Venice, Italy, the Venice Port Authority has recently proposed a project for the dredging of a new 3-km long and 10-m deep navigation canal (called Marghera-Venice Canal, MVC, in the sequel) through the shallows of the Venice Lagoon. The MVC will connect the passenger terminal located in the southwestern part of the historic center to a main channel that reaches the industrial area on the western lagoon margin. Can the new MVC facilitate saltwater intrusion below the lagoon bottom? Can the release into the lagoon of the chemicals detected in the groundwater around the industrial site be favoured by the MVC excavation? Can the depression waves generated by the ship transit (known as ship-wakes) along the MVC affect the flow and contaminant exchange between the subsurface and surficial systems? A response to these questions has been provided by the use of uncoupled and coupled density-dependent groundwater flow and transport simulators. The hydrogeological modelling has been supported by an in-depth characterization of the Venice lagoon subsurface along the MVC. Geophysical surveys, laboratory analyses on groundwater and sediment samples, in-situ measurements through piezometers and pressure sensors, and the outcome of 3D hydrodynamic and computational fluid dynamic (CFD) models have been used to set-up and calibrate the subsurface multi-model approach. The modelling results can be summarized as follows: i) the MVC has a negligible effect in relation to the propagation of the tidal regime into the subsoil; ii) the depression caused by the ship transit along the MVC is responsible of a groundwater efflux from the canal bottom into the lagoon ranging up to 100 m3 per ship; iii) ship-wakes enhance the release of anthropogenic contaminants (As, Se, Cd, Hg, Pb, Cu, Cr) from the subsoil into the lagoon over few years after the canal excavation; iv) the MVC excavation cut the over-consolidated clay layer marking the Holocene-Pleistocene limit, thus favouring a localized salt contamination of the almost fresh-water aquifers located in the uppermost part of the Pleistocene series.

Land subsidence is a widespread phenomenon, particularly relevant to transitional environments, such as wetlands, deltas, and lagoons, characterized by low elevation with respect to the mean sea level. Satellite synthetic aperture radar (SAR) interferometry offers the possibility to effectively and precisely measure land displacements for dry surfaces or anthropogenic structures, but difficulties arise in identifying long-term stable targets in natural transitional regions. In order to improve the coverage of satellite SAR interferometry in salt marshes within the Venice Lagoon (Italy), we installed a network of 57 Trihedral Corner Reflectors (TCRs). The TCRs were monitored by ENVISAT ASAR and TerraSAR-X acquisitions covering the time period from November 2006 to September 2011. The results show that the northern lagoon basin is subsiding at ~3 mm/yr and that the central and southern portions are more stable. Larger subsidence rates, up to 6 mm/yr, are measured where surficial loads, such as artificial salt marshes or embankments, rise above the lagoon bottom. The accuracy of TerraSAR-X is greater than ENVISAT due to the shorter wavelength and higher spatial resolution in relationship to the size of the TCRs. The observations obtained in the Venice Lagoon indicate that SAR interferometry using a large network of artificial reflectors is an effective and powerful methodology to monitor land subsidence in transitional environments where the loss of elevation with respect to the mean sea level can yield significant morphological changes to the natural environment. Key Points Subsidence of natural transitional environments Radar interferometry on artificial corner reflectors Environment deterioration by loss of elevation relative to the mean sea level ©2013. American Geophysical Union. All Rights Reserved.

The investigations carried out since the early '970 showed that salt water intrusion affected most of the Venetian coastal area. However, at that time, phenomena such as land subsidence and flooding (known as "acqua alta") had the priority in relation to the city protection, thus the study of the salt water intrusion process assumed a secondary role. During ,,990, the salinization process became the subject of a research project performed by the Institute of Marine Science (ISMAR CNR-National Research Council). The results revealed a severe hydrological situation, particularly critical in the southern region. Aware of saline contamination effects on the environment and the agricultural activities, mainly horticultural crops well developed around Chioggia, the Venice Province started in 1998 the "Saline Wedge Project" to understand the consequences of the salt water intrusion in the subsoil, in connection also to land subsidence, at the southern part of its territory. The knowledge acquired by the two institutions (Venice Province Authority, ISMAR-CNR) constituted the common basis for new research and monitoring projects. Several meetings were promoted to involve all the authorities and institutions responsible for land management to raise awareness of the saline contamination and its environmental and socio-economic impact, and to highlight the need of establishing appropriate monitoring networks in critical areas. In the southern sector, where the possible damages for the agricultural-horticultural activities could be economically relevant, two projects (Saline Wedge 1998-2000; ISES Project 1999-2002) were initiated in the late ,,990, involving all the authorities managing the territory. The aims of these projects were a) to increase the knowledge on the factors enhancing the salt water intrusion; b) determine the areal extent of the saline contamination; c) assess the critical conditions in relation to the economic activities, mainly of agricultural and tourist type. Once obtained a detailed hydrogeological representation, specific actions to limit the process have been planned (Brenta Project, 2003-2004). Preliminary investigations were performed to realize a mobile gate at the mouth of the Brenta-Bacchiglione river to prevent the encroachment of sea water into the river bed during high tides. More recently, scientific efforts have been devoted to better understand the process dynamics. Mathematical models have used to investigate the effects of other possible mitigation strategies (CORILA Project 3.10 2004-2007; VECTOR-Cliven Project 2007-2010). In the northern sector of the Venice territory, between the Piave river and the lagoon margin, researches aimed at improving the comprehension on the salt water intrusion phenomenon started in 2004, thorough the cooperation between the University Ca" Foscari of Venice and the University of Padua (PRIN Project "The deep aquifers of the Po Valley as regional resource: aquitard in hydrodynamics, generating of water chemically and thermally anomalous, a suitable place for the CO2 atmospheric trapping"). The combined use of adequately validated geophysical and geochemical tools conferred a multidisciplinary character to the research and allowed for the definition of the phenomenon"s superficial extension. Geophysical analysis based on Vertical Electrical Sounding (VES) and conductivity logs have been performed together with water table measurements and geochemical analysis on samples collected from selected wells. Moreover, a critical review of old geophysical data allowed for the verification of possible saline contamination changes in time. Interesting results have been obtained specifically within territory of Jesolo. In this case, considering the influence of the Piave and Sile rivers and the presence of a coastal sand dune, a map of salt water contamination was created to evaluate the agricultural use of superficial water. The effective origin of salt water found in the aquifers, its distribution, extension and its preferential way of movement within the territory were defined relating all the information acquired to the geological, geomorphological and stratigraphic characteristics of the area In addition, the presence of salt water contamination in the shallow aquifers below the city of Venice has been highlighted by recent studies developed at the IUAV University.

For the first time a comprehensive investigation has been carried out to quantify the possible effects of dredging a navigable canal on the hydrogeological system underlying a coastal lagoon. The study is focused on the Venice Lagoon, Italy, where the port authority is planning to open a new 10m deep and 3km long canal to connect the city passenger terminal to the central lagoon inlet, thus avoiding the passage of large cruise ships through the historic center of Venice. A modeling study has been developed to evaluate the short (minutes), medium (months), and long (decades) term processes of water and pollutant exchange between the shallow aquifer system and the lagoon, possibly enhanced by the canal excavation, and ship wakes. An in-depth characterization of the lagoon subsurface along the channel has supported the numerical modeling. Piezometer and sea level records, geophysical acquisitions, laboratory analyses of groundwater and sediment samples (chemical analyses and ecotoxicity testing), and the outcome of 3-D hydrodynamic and computational fluid dynamic (CFD) models have been used to set up and calibrate the subsurface multi-model approach. The numerical outcomes allow us to quantify the groundwater volume and estimate the mass of anthropogenic contaminants (As, Cd, Cu, Cr, Hg, Pb, Se) likely leaked from the nearby industrial area over the past decades, and released into the lagoon from the canal bed by the action of depression waves generated by ships. Moreover, the model outcomes help to understand the effect of the hydrogeological layering on the propagation of the tidal fluctuation and salt concentration into the shallow brackish aquifers underlying the lagoon bottom., Facultad de Ciencias Naturales y Museo, Centro de Investigaciones Geológicas