Luigi Tosi

Heavy metal pollution in central Venice lagoon bottom sediments: evaluation of the metal bioavailability by geochemical speciation procedure.

Bottom sediments of the central area of the Venice Lagoon (Italy) and some lagoon canals, chosen as an anaerobic reference environment, were analyzed by a sequential extraction procedure for Cr, Cu, Ni, Pb, Zn, Mn, and Fe. Bio-Available Phases (BAPs) and Non-Bio-Available Phases (NBAPs) have been determined in the Malamocco-Marghera Canal, to define the significant level of heavy metal contamination of the sediment. Relationships among metal distributions with Fe/Mn oxides/hydroxides and sulfides were determined in both environments, using statistical methods. The central lagoon area shows low pollution and a balanced influence of all the sediment components in the metal complexing processes, mostly due to the manganese and iron matrices. The inner canals are very polluted with heavy metals strongly bonded to H2S that form highly insoluble compounds, as demostrated through the correlation analysis. The authors suggest different procedures for lagoon restoration due to the different sediment behaviours, i....

Natural versus anthropogenic subsidence of Venice

We detected land displacements of Venice by Persistent Scatterer Interferometry using ERS and ENVISAT C-band and TerraSAR-X and COSMO-SkyMed X-band acquisitions over the periods 1992–2010 and 2008–2011, respectively. By reason of the larger observation period, the C-band sensors was used to quantify the long-term movements, i.e. the subsidence component primarily ascribed to natural processes. The high resolution X-band satellites reveal a high effectiveness to monitor short-time movements as those induced by human activities. Interpolation of the two datasets and removal of the C-band from the X-band map allows discriminating between the natural and anthropogenic components of the subsidence. A certain variability characterizes the natural subsidence (0.9 ± 0.7 mm/yr), mainly because of the heterogeneous nature and age of the lagoon subsoil. The 2008 displacements show that man interventions are responsible for movements ranging from −10 to 2 mm/yr. These displacements are generally local and distributed along the margins of the city islands.

Morphostratigraphic framework of the Venice Lagoon (Italy) by very shallow water VHRS surveys: Evidence of radical changes triggered by human‐induced river diversions

[1] This study is mainly based on a wide Very High Resolution Seismic (VHRS) survey that utilized an ad hoc technique designed for investigations in very shallow waters (about 1 m depth). This method allowed the acquisition of excellent images of the subsurface down to 15-20 m b.s.1. with a resolution of about 10 cm. Buried geomorphological features, such as fluvial channel-levee systems and tidal channels, were imaged for the first time in the shallows and provided new insight into the Holocene evolution of the southern lagoon basin. Furthermore, the new seismic data were used to reconstruct the morphostratigraphic framework of the Venice Lagoon. We provide an Upper Quaternary morphostratigraphic model of the Venice Lagoon and present some evidence of radical changes resulting from human-induced river diversion in the sedimentary regime and in the morphological setting of the southern basin that has occurred over the last millennium.

Peat land oxidation enhances subsidence in the Venice watershed

The southernmost part of the Venice Lagoon catchment was progressively reclaimed from marshland starting from the end of the 19th century and finishing in the late 1930s (Figure 1). As a major result, the area was turned into a fertile farmland. At present, the area is kept dry by a distributed drainage system that collects the water from a capillary network of ditches, and pumps it into the lagoon or the sea. By its very origin, this area lies below sea level and progressively sinks mainly because of bio-oxidation of the histosols (soils with high organic content) that represent a large fraction of the outcropping soil in the area. The bio-oxidation process occurs in close connection with the agricultural practices and is currently responsible for a subsidence rate of between 1.5 and 2 cm/yr.

Combining L- and X-Band SAR Interferometry to Assess Ground Displacements in Heterogeneous Coastal Environments: The Po River Delta and Venice Lagoon, Italy

From leveling to SAR-based interferometry, the monitoring of land subsidence in coastal transitional environments significantly improved. However, the simultaneous assessment of the ground movements in these peculiar environments is still challenging. This is due to the presence of relatively small built-up zones and infrastructures, e.g., coastal infrastructures, bridges, and river embankments, within large natural or rural lands, e.g., river deltas, lagoons, and farmland. In this paper we present a multi-band SAR methodology to integrate COSMO-SkyMed and ALOS-PALSAR images. The method consists of a proper combination of the very high-resolution X-band Persistent Scatterer Interferometry (PSI), which achieves high-density and precise measurements on single structures and constructed areas, with L-band Short-Baseline SAR Interferometry (SBAS), properly implemented to raise its effectiveness in retrieving information in vegetated and wet zones. The combined methodology is applied on the Po River Delta and Venice coastland, Northern Italy, using 16 ALOS-PALSAR and 31 COSMO-SkyMed images covering the period between 2007 and 2011. After a proper calibration of the single PSI and SBAS solution using available GPS records, the datasets have been combined at both the regional and local scales. The measured displacements range from ~0 mm/yr down to −35 mm/yr. The results reveal the variable pattern of the subsidence characterizing the more natural and rural environments without losing the accuracy in quantifying the sinking of urban areas and infrastructures. Moreover, they allow improving the interpretation of the natural and anthropogenic processes responsible for the ongoing subsidence.

Geochemical characteristics of phosphorus in surface sediments of two major Chinese mariculture areas: The Laizhou Bay and the coastal waters of the Zhangzi Island

Phosphorus (P) in surface sediments of the Laizhou Bay (LB) and the coastal waters around the Zhangzi Island (ZI) was analyzed. Six forms of P were separated - exchangeable or loosely sorbed P (Ads-P), aluminum-bound P (Al-P), iron-bound P (Fe-P), authigenic apatite plus CaCO3-bound P plus biogenic apatite (Ca-P), detrital apatite plus other inorganic P (De-P) and organic P (OP). The average contents of P in the LB were in the order: De-P>OP>Ca-P>Fe-P>Ads-P>Al-P; in the ZI, the corresponding order was De-P>OP>Fe-P>Ca-P>Ads-P>Al-P. Due to the high nutrient loadings from the surrounding rivers, TP contents in sediments of the LB were higher than in those of the ZI. The potential bio-available P (Ads-P and OP) accounted for 14.7% and 24.2% of TP in sediments of the LB and the ZI, respectively.

Aptitude of modern salt marshes to counteract relative sea-level rise, Venice Lagoon (Italy)

In the past 100 yr, variations in relative sea-level rise (RSLR), the increase of frequency of very high tides, and a decrease of sediment availability caused progressive morphological changes of the Venice Lagoon tidal flats. In contrast with the general erosional trend, some salt marshes in the northern lagoon preserved their main original characteristics and showed accretion and development of the tidal creek network. The evolution of one of them was sketched by the interpretation of ultrahigh-resolution aerial photographs taken from A.D. 1938 to 2006, and results were compared with RSLR rates and storm tide frequency. The long-term investigation pointed out the most significant morphological changes that occurred over the entire period (i.e., erosion of the margin, modifications of the drainage network), whereas the short-term analysis showed in detail the subsequent phases of salt marsh evolution and their relations with sea-level variations. Margin shift was mainly in agreement with RSLR trend, whereas changes of the tidal creek network also reflected frequency of very high tides. Moreover, in the past 70 yr the salt marsh demonstrated a self-renewal aptitude to counteract RSLR: even if it underwent low accretion rates, it did not disappear, probably because the remobilization of sediments eroded from the marsh front and the lagoon bottom by tides and other local hydrodynamic processes, and their accumulation on the marsh surface favored by vegetation were sufficient to offset RSLR.

Saltwater contamination in the managed low-lying farmland of the Venice coast, Italy: An assessment of vulnerability.

The original morphology and hydrogeology of many low-lying coastlands worldwide have been significantly modified over the last century through river diversion, embankment built-up, and large-scale land reclamation projects. This led to a progressive shifting of the groundwater-surficial water exchanges from naturally to anthropogenically driven. In this human-influenced hydrologic landscape, the saltwater contamination usually jeopardizes the soil productivity. In the coastland south of Venice (Italy), several well log measurements, chemical and isotope analyses have been performed over the last decade to characterize the occurrence of the salt contamination. The processing of this huge dataset highlights a permanent variously-shaped saline contamination up to 20km inland, with different conditions in relation with the various geomorphological features of the area. The results point out the important role of the land reclamation in shaping the present-day salt contamination and reveal the contribution of precipitation, river discharge, lagoon and sea water to the shallow groundwater in the various coastal sectors. Moreover, an original vulnerability map to salt contamination in relation to the farmland productivity has been developed taking into account the electrical conductivity of the upper aquifer in the worst condition, the ground elevation, and the distance from salt and fresh surface water sources. Finally, the study allows highlighting the limit of traditional investigations in monitoring saltwater contamination at the regional scale in managed Holocene coastal environments. Possible improvements are outlined.

Land subsidence monitoring service in the Lagoon of Venice

In order to provide the best knowledge of the subsidence process around the Lagoon of Venice to the authorities that manage the area, SAR-based monitoring techniques (differential SAR interferometry and the interferometric point target analysis) will be integrated with levelling and GPS surveys into an overall database and information system. In this contribution the different monitoring techniques are briefly introduced, the results presented and compared, and the integration concept explained.

Sequence stratigraphic significance of tidal channel systems in a shallow lagoon (Venice, Italy)

The Holocene succession located in the central part of the Venice Lagoon has been investigated by means of high-resolution seismic and core data, which document a full back-barrier depositional environment developed during the last 6 kyr. The studied succession, 4.5–23 m thick, consists of three seismic units bounded at the base by stratal surfaces marked by deep tidal channel incisions, recording main changes in the lagoon hydrodynamics as well as reorganizations of the tidal channel network. The recognized depositional and erosional phases in the lagoon deposits can be linked to both large-scale factors related to the Holocene sea-level rise and local factors in part due to the human impact. In particular, the lower unit is interpreted as the transgressive systems tracts of the Holocene sequence, initially characterized by the accumulation within small estuarine channels incised during the previous phase of subaerial exposure, whereas the upper units are interpreted as the highstand systems tracts, typified by the persistence of the lagoonal environment. The higher stratal surface records a hydrodynamic change related to a local transgression affecting a deltaic area placed just to the south, probably at least in part aided by human interventions. The Holocene succession accumulated in the central part of the Venice Lagoon testifies that the development of stratal surfaces in back-barrier settings, persisting during both transgressive and highstand conditions, may exhibit differences with respect to that predicted by current sequence stratigraphic models, and therefore, it is useful to improve the knowledge of these systems.