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Salt contamination in the coastlands surrounding Venice Lagoon (Italy) and Laizhou Bay (Shandong Province, China) is generally the result of the complex relationship between fresh, continental groundwaters, paleo-saltwaters in deep aquifers, marine waters, and the seawater encroachment upstream from the river mouths. Saltwater intrusion is also enhanced by the presence of ancient, sandy, buried paleo-channels that act as preferential pathways for groundwater flow and solute transport. The characterization of these hydro-geo-morphological features, considering both the architecture and the pore water quality, is an issue of paramount importance. In this work, after a short overview of the saltwater contamination occurrence that affects the coastlands of Venice Lagoon and Laizhou Bay, we report on the preliminary results of a Very High Resolution Seismic and Continuous Electrical Tomography test carried out in very shallow water (about 1 m) in the Venice Lagoon to test a technique suitable for the better understanding of the surficial-groundwater mixing process. The combined analysis of the two data sets highlights a correlation between lithostratigraphic discontinuities and heterogeneity in the fluid filling the subsurface. The promising results obtained in the Italian site support a prompt use of this integrated methodology on the Chinese study area too.

The Project "Application of innovative of very high resolution seismic methodologies in shallow waters aimed at the study of the subsoil of the Venice Lagoon" allowed the characterization of the Late Pleistocene and Holocene depositional sequences with a detail, that has never been reached in the past. A boat, particularly suitable for very high resolution seismic survey in shallow water (less than 1 m deep) has been fully equipped and tested. It allowed to carried out surveys of specific areas of the Lagoon that have never been investigated in the past. Moreover a seismo-stratigraphic model of the lagoon and gulf of Venice subsoil has been performed. The seismic acquisition system allowed to investigate the subsoil down to 30-40 m deep. The new data provided significant geological features related to the Late Pleistocene and Holocene evolution of the investigated areas: the southern lagoon, the Lido and Chioggia inlets and part of the shelf of the Gulf of Venice.

Mediante un sistema di prospezione sismica ad altissima risoluzione per rilievi su bassi fondali (meno di 1 m di profondità), integrato con l'analisi di carotaggi, è stato possibile ricostruire l'assetto stratigrafico tardo Pleistocenico ed Olocenico della Laguna di Venezia fino a -20 m s.l.m. con una risoluzione di circa 10 cm ed individuare strutture geomorfologiche sepolte e subaffioranti finora sconosciute, quali incisioni fluviali, sistemi argine-canale, paleocanali lagunari ed antichi cordoni litoranei. I risultati delle indagini hanno fornito nuovi elementi utili per capire l'evoluzione olocenica del territorio ed hanno messo chiaramente in evidenza che le diversioni fluviali indotte dall'uomo nel corso dell'ultimo millennio hanno innescato drastici cambiamenti nel regime sedimentario della laguna ed ampiamente influito sulle sue modificazioni morfologiche avvenute nel corso dei secoli.

Most of the Venice Lagoon consists of shallows and their subsoil provides a window on the whole Holocene history while below the shipping channels the upper layers are lost because of the occurrence of bottom erosion and the dredging activity. Unfortunately, to take cores in shallows is really hard owing to impervious environment. In addition, the cores represent a very local situation due to the high facies variability of the lagoon and they need to be a significant number for the lateral correlated to reconstruct the geological setting. Very high resolution seismic survey overcomes these difficulty and data obtained provide a powerful tool to study the architecture of Holocene deposits. The near outcrop resolution provided by this technique allows the recognition of sedimentary structures and main stratal surfaces.

The effects of global climatic changes that occurred since the Last Glacial Maximum (LGM), are recognizable in the stratigraphic sequences from several areas in different locations around the world and, in particular, in coastal areas. Our study reports some examples from the Lagoon of Venice of the paleoclimatic change effects on the mineralogical composition and textural characteristics of clay and sand deposits, with emphasis on the stiff layer, locally called "caranto" at the Pleistocene-Holocene boundary and on cemented sand formations outcropping inside the northern lagoon and about 6 km off the southern littoral (Chioggia) in the Adriatic Sea.

The occurrence of alternating dry/wet conditions in transitional environments, such as wetlands, deltas, and lagoons, usually challenges the use of traditional direct and geophysical surveys for comprehensive hydrogeologic investigations. Moreover, significant mixing between continental fresh groundwater and marine salty surface waters generally takes place in these flat coastal areas. Airborne electromagnetics (AEM) is a promising tool in this respect, as it provides, in a fast and cost effective manner, large-scale distribution of bulk electrical conductivities that can be used profitably to develop hydrogeologic models. The results of a SkyTEM AEM survey in the Venice Lagoon, Italy, show the capability of this technique to significantly improve the knowledge of the hydrogeologic setting of the lagoon and nearby coastland subsurface, irrespective of the different features characterizing the area. The environment consists of salt marshes, mud flats, shallows, tidal channels, islands, together with reclaimed farmlands crossed by natural watercourses and drainage channel networks. In particular, the AEM shows (i) the presence of fresh water (with resistivity larger than 20 Ω m) underneath the central part of the lagoon at depths from 10 to 25. m below m.s.l., (ii) the interface between different relevant stratigraphic units, e.g., the clayey layer bounding the Holocene-Pleistocene sedimentation, and (iii) the occurrence of areas with possible submarine fresh groundwater discharge. Moreover, the source and inland extent of the saltwater contamination in the shallow coastal aquifers along the southern margin of the lagoon are clearly revealed. AEM data were complemented with very high resolution seismic (VHRS) acquisitions. The integrated analysis of the two data sets allows distinguishing between lithostratigraphic heterogeneity and variability of the subsurface fluids. © 2011 Elsevier B.V.