Tosi, Luigi

RIASSUNTO Nelle aree costiere e perilagunari del comprensorio meridionale veneziano l'approvvigionamento idrico ha risentito in più occasioni degli effetti negativi dell'intrusione di acque marine e lagunari nel sottosuolo. Per valutare l'estensione e la gravità della contaminazione salina è stato condotto un primo studio in un’area fortemente interessata dagli effetti di tale fenomeno, che si ripercuotono pesantemente nell'attività agricola, turistico-alberghiera e industriale. Lo studio è stato condotto con l'utilizzo di sondaggi elettrici verticali (S.E.V.) e con determinazioni del tenore salino dell'acqua di pozzi freatici ed artesiani, nonché dei fiumi, canali e scoli della rete idrografica. I risultati delle indagini hanno evidenziato che il settore litoraneo e quello di entroterra presentano entità e modalità di propagazione della contaminazione salina differenti. Nella fascia litoranea, costituita principalmente da sabbie di dune eoliche e di antichi cordoni litoranei, risiede una lente di acqua dolce con uno spessore di anche una decina di metri. Sottostante questa lente, la contaminazione salina può raggiungere i 70 metri di profondità. Nel settore di entroterra e di margine lagunare invece, la salinizzazione del sottosuolo è favorita dalla critica situazione altimetrica del territorio e dall'attività di bonifica idraulica. Infatti la soggiacenza del piano campagna rispetto al livello del mare, i pompaggi delle idrovore e la dispersione di acqua marina dalla rete idrografica riducono a pochi metri o addirittura annullano lo spessore dell'acqua freatica dolce superficiale rendendo la situazione particolarmente grave specialmente quando In contaminazione salina coinvolge i terreni coltivati. ABSTRACT The aim of this study is the evaluation of the salt water intrusion extent in coastal aquifers of the Southern Venetian Region. The groundwater salinity contamination was obtained by combining geophysical investigations, such as Vertical Electric Soundings (V.E.S) and Conductibility, Temperature, depth (CTD) measures of water samples, with stratigraphical data. In particular, 41 V.E.S., obtained with a maximum distance between the electrodes ranging from 300 to 2000 m, and CTD data of 17 selected water samples taken in the rivers, channels and wells, were considered. The study shows that the two dimensional extent of the saline contamination is mainly related to different geomorphology of the littoral and of the inland sectors. The littoral sector is characterized by a ground elevation up to three meters above mean sea level. The dune belt and paleolittoral strip sand formations constitute a well-developed phreatic aquifer with a fresh water body, up to 10 meters deep, floating on the saline water. The salt water contamination involves the aquifers and the aquitards below the fresh water body down to a depth of 70 meters. Reclaimed land constitutes the inland sector; because its ground level is about 2-3 meters below the mean sea level water, pumping is required to maintain the water level below land surface. The critical ground elevation, the water pumping and the marine water seepage in the rivers during high tides are the factors that seriously reduce the size of fresh water body and allow the salt water contamination of the agricultural soils. The extension of the salt water contamination of the aquifer-aquitard system is here shown by a series of profiles crossing the littoral strip and the lagoon margin. The collected data, although not sufficient to illustrate the complex interaction of the contamination factors, allowed the evaluation of the size and extent of the salt water intrusion process and constitute the base for further detailed studies.

Abstract: This paper describes the geotechnical properties of the Venetian littoral. Analyses of 110 cohesive soil samples and 338 granular, taken from 30 cores, 25-30 meters in depth, were considered. The classical geotechnical classification supported by statistical analysis has pointed out that the compression index presents the best correlations with void ratio index and natural water content. Moreover, this paper shows the importance of the Atterberg limits to distinguish the cohesive soils of the late Pleistocene from those of the Holocene. Granular sediments have shown a zonation of both the grain size distribution and permeability coefficient. Resumé: Pour l'analyse des caractéristiques géotecniques du littoral venitien, qui est le sujet de ce travail, on a étudié 110 échantillons de terrains cohérents et 338 de terrains granulaires. Ces échantillons ont été prélevés de 30 carottages efféctués à la profondité moyenne de 25-30 m., et ils représentent les dernières 25.000-30.000 années de sèdiments quaternaires. En considérant la masse des donneés disponibles pour l'elaboration traditionnelle des propriétés géotechniques, une élaboration statistique a été utilisée pour obtenir des meilleures rélations de synthèse. En particulier, on a vu l'importance des limites de Attemberg pour effectuer la distinction entre les terrains cohérents du Pleistocene tardif et ceux de l'Olocene. En plus, la correlation du plus haut niveau de l'indice de compression est celle avec le pourcentage naturel d'eau et l'indice des vides. En ce qui concerne les terrains granulaires, on a verifié qu'il existe des zonations de la distribution granulometrique et du coeficient de perméabilité. Riassunto: Per l'analisi delle caratteristiche geotecniche del litorale veneziano, argomento di questo lavoro, sono stati studiati 110 campioni di terreni coesivi e 338 di terreni granulari prelevati da 30 sondaggi terebrati lungo il cordone litoraneo a profondità medie di 25-30 metri che rappresentano gli ultimi 25.000-30.000 anni di sedimentazione quaternaria. In considerazione della massa di dati disponibili all'elaborazione tradizionale delle proprietà geotecniche si è affiancata un'elaborazione statistica (analisi discriminante) che ha permesso di fornire relazioni di sintesi maggiormente interpretative. In particolare è emerso il potere selettivo dei limiti di Atterberg per distinguere i terreni coesivi tardo-pleistocenici da quelli olocenici. Inoltre si è visto che l'indice di compressione presenta le correlazioni più elevate con il contenuto naturale d'acqua e l'indice dei vuoti. Per quanto concerne i terreni granulari si sono riscontrate zonazioni della distribuzione granulometrica e del coefficiente di permeabilità.

Remote-sensing techniques represent suitable tools for geological cartography and sustainable management of natural resources. Spaceborne Synthetic Aperture Radar (SAR) systems offer the possibility, through differential SAR interferometry (DInSAR), to map surface displacements at mm to cm resolution. Spectacular results are obtained for geophysical sciences with earthquake displacement, volcano deformation, glacier dynamics and land subsidence being mapped. With regard to land subsidence, SAR interferometry exhibits complementary characteristics to the levelling surveys, because it has the capability to map large urban areas at low cost and high spatial resolution. The high precision levelling surveys, on the other hand, are used outside of the cities and to set up a reference point for the SAR subsidence values. In the case of the south-eastern Veneto region, where till to 2000 high precision levelling surveys are available only (i) around the Lagoon margin and along few lines from (ii) Venezia to Rua di Feletto (Treviso), from (iii) Mestre to Padova, (iv) form Padova to Rovigo, and (v) from Chioggia to Adria, SAR interferometry has the capability to monitor the vertical displacements of all the built-up or sparsely vegetated areas (i.e. where stable structures permit the formation of a coherent phase signal over time) not fully covered with levelling results. DInSAR makes use of two SAR images acquired from slightly different orbit configurations and at different times to exploit the phase difference of the signals. The phase signal derived from an interferometric image pair relates both to topography and line-of-sight surface movement between the acquisitions, with atmospheric phase distortions, signal noise and inaccuracy in the orbit determination as main error sources. The basic idea of DInSAR is to subtract the topography related phase (for instance simulated from a Digital Elevation Model) from the interferogram to derive a displacement map. In this study a time series of six interferometric radar images of the European Remote Sensing Satellites ERS-1 and ERS-2 from 1993 to 2000 was used. In order to generate a single subsidence map with reduced errors, the interferometric radar images were combined. In the following, displacement in the vertical direction was assumed, the land subsidence map was transformed to the Italian cartographic system with a spatial resolution of 30 m, and the pixel corresponding to the benchmark Nodale 63 (ex 24’) in Treviso (that has been already considered stable for the levelling surveys) was considered the stable reference. The high accuracy of the 1993-2000 DInSAR survey is confirmed by a quantitative validation with the 1993-2000 levelling data. For the 87 benchmarks where values from both surveying techniques are available a zero average difference of the vertical displacement velocity is prescribed and a standard deviation of 0.9 mm/year is found; the minimum and maximum differences are –3.1 mm/year and +2.5 mm/year, respectively. From this number and previous works we conclude that the accuracy of the SAR interferometric subsidence rates is on the order ±1 mm/year, which is also the expected accuracy of the levelling surveys. An example of the DInSAR capability to monitor the evolution of land subsidence in small urban area is Abano – Montegrotto Terme where clearly appears the influence of the drastic reduction of groundwater pumping used for geothermal purpose. DInSAR results match very well the available leveling data. DInSAR mapping in large cities such as Padova, Treviso, and Mestre is given. These applications indicates the potentiality of the radar satellite analysis in built-up areas characterized by a general stability, with displacement rate within the range 1 mm/year. DInSAR use looks very useful to perform detailed investigations in coastal areas too. E.g., Bibione – Lignano, Chioggia, Jesolo and Caorle reveal the presence of a significant seaward gradient in land subsidence practically impossible to be detected by other traditional monitoring techniques like geometric leveling and DGPS. Hence, the proposed methodology provides a fundamental support for coastal management projects and environmental risk analyses. SAR interferometry has been applied in other minor cities of the Veneto plain characterized by very different conditions form the displacement point of view. Castelfranco and Conegliano located close to the Alpine foothills appear quite stable; land settlement in Rovigo appears very changeable with high values (2-3 mm/year) downtown and lower rate (about 1 mm/year) in the outskirts; in Cavarzere, and Oderzo, San Donà the subsidence velocity ranges between 3 and 4 mm/year. The more critical situation is found in Adria, Taglio di Po, and Portogruaro where the sinking velocity reaches 5 mm/year in large portion of these towns. Although in 2000 the old leveling network has been extended within the framework of the ISES Project (the present overall length is about 1000 km), a detailed monitoring of several villages is impossible though leveling and DGPS. Because in these urban areas the hydrogeological risk is high due to the presence of rivers and the critical elevation, since they often lie below the mean sea level, the areally distribution of the subsidence rate has to be well known. Results shown by the present work prove that this integration in the knowledge of the subsidence process can be efficiently obtained by DInSAR. This technique represents an effective methodology for both a back-analysis during the last decade and the future monitoring in the portion of territory covered by large and small built-up areas.

This paper reports on the potential of differential SAR interferometry to map land subsidence. After a presentation of the methodology, the focus will be on feasibility demonstration and accuracy assessment. The theoretical considerations are verified with the selected cases Ruhrgebiet, Mexico City, Bologna, and Euganean Geothermal Basin, representing fast (m/year) to slow (mm/year) deformation velocities. The accuracy of the generated deformation maps and the maturity of the required processing techniques lead to the conclusion that differential SAR interferometry has a very high potential for operational mapping of land subsidence. The high reliability of the ERS satellites, with the huge data archive starting in 1991, strongly supports this application. The planned ENVISAT ASAR has the potential to maintain good data availability into the future. This will, nevertheless, strongly depend on the sensor operation strategy.

Relative sea level rise (RSLR), that has been occurred along the entire coastal areas of the Northern Adriatic Sea, includes land subsidence, both natural and man-induced, and eustacy. Their combined effect has produced relative ground settlements ranging from centimetres to meters. RSLR represents one of the geologic hazards threatening the low-lying coast. Recent progresses made in understanding these two processes are presented. Synthetic Aperture Radar (SAR) interferometry has significantly improved the knowledge of actual land subsidence. In particular, comprehensive maps of the vertical displacements occurred over the period 1992-2009 in the region between Venice and Ravenna reveal a significant spatial variability, ranging from a slight 1 to 2 mm•yr-1 uplift, to a serious subsidence of more than 15 mm•yr-1. The availability of tide gauge data in Trieste, Venice, and Ravenna allows accurate assessment and meaningful observations on sea level change. The period 1896-2006 is characterized by an average rise of 1.2±0.1 mm•yr-1. The analyses here performed show that a time series at least 50 yr long must be used to obtain statistically significant results and reliable trend, due to the 7-8 year pseudo-cyclicity, recorded at many Mediterranean coastal stations. In Venice and Ravenna the influence of land subsidence on the RSLR amounts to 57% and 85%, respectively. This percentage has been estimated in 95% at the Po Delta.

Venezia, la sua laguna ed il territorio circostante sono un patrimonio artistico, storico, culturale ed ambientale tra i più noti al mondo. Il fragile equilibrio, tipico delle lagune, è per Venezia particolarmente vulnerabile; essa infatti risulta modellata da un forte contributo antropico, iniziato secoli or sono con le note deviazioni fluviali, ed è oggi esposta ad azioni antropiche di particolare rilevanza, quali quelle derivanti dalla presenza della zona industriale di Marghera che s'affaccia proprio sulla laguna. Questo delicato equilibrio è legato al particolare assetto geologico-territoriale dell'area veneziana. In questa relazione, forzatamente molto schematica in quanto sintetizza decenni di studi ed indagini a livello sia scientifico che tecnico-applicativo, si vuole indicare l'insostituibile apporto della geologia anche in un'area urbana, quale quella Veneziana. Questo "complesso urbano" è composto da una serie di centri: insulari (centro storico, Murano, Burano, Torcello, ecc.), litorali (Lido, Chioggia, Jesolo), di terraferma ed industriali (Mestre, Scorzè, Marghera, Malcontenta, Fusina, ecc.). Altra caratteristica del complesso veneziano è l'interazione dei vari centri urbani che, malgrado la dislocazione geografica, interagiscono nelle varie problematiche geoambientali con strette relazioni causa-effetto. Questa nota, frutto della collaborazione tra Enti di Ricerca, Pubbliche Amministrazioni e Liberi Professionisti, evidenzia la necessità di lavorare in modo sinergico per affrontare al meglio le complesse tematiche geologico-ambientali che si riscontrano in questo tipo di studi.

Lagoon environments are very important for groundwater modeling in costal areas, they are delicate and in rapid evolution due to global climatic changes. Airborne electromagnetics (AEM)is a very valuable methodology that can provide high density, high quality data to produce 3D hydrogeological models to depths in excess of hundred meters below surface water column. We present the results from the SkyTEM Venice lagoon survey of 2009, integrated with data from very high resolution seismic survey. The AEM data results enhance greatly the understanding of the hydrogeology and surface-groundwater interactions in the lagoon area, where indirect measurements abound but wells are missing. For example, there is clear evidence of fresh water aquifers underneath the central part of the lagoon, at depth of about 40 m. The near surface part of the AEM data compare well with seismic data, showing that main reflectors come from the interface between the superficial Late Pleistocene looser, saline water saturated sediments and the deeper, more compact and fresher Holocene sediments. There is also clear evidence of submarine groundwater discharge in the lagoon, of paleorivers, and a possible indication of gas seepage trough shallow sediments. Seismic and AEM provide complimentary datasets to discriminate between pore water salinity, lithology and gas. Seismic horizons can actually be included during inversion of AEM data, producing more robust output. AEM data from the southern part of the survey that crosses the shore line and continued also onshore allow a clear mapping of the saline water intrusion inland, and highlight the relationship between pore water salinity of the lagoon sediments and spatial distribution of salt marshes. The latter seem to act like salt sinks, increasing sediments electrical conductivity.

We report on the results of an experiment of in situ bioremediation by forced aeration, carried out in the Arsenale Vecchio shipyard dock basin of the Venice Lagoon, Italy. The experimental area is characterized by its limited size and reduced boat traffic. The forced aeration technique chosen for the oxygenation of the highly polluted bottom sediments and of the water column above does not obstruct harbor activities and is unique due to its innovative use of a system of porous pipes laid on the bottom sediments. The general recovery of the polluted shipyard dock basin resulted in the documented return of small fish to the area as an indication of a less polluted environment. Most important the bioremediation of the water body above the bottom sediment favours, with time, the formation of a clean natural sediment capping over the bottom contaminated sediments. Post-experiment control after fifteen months of aeration with no extra oxygen added, shows the presence of clean light grey sediments with no foul septic smell, deposited over the polluted bottom sediments. The experiment has indicated that oxygenation by porous pipe forced aeration could represent a cost effective and environmentally feasible way for the recovery of shallow marine and fresh water systems, in situations where other techniques may not be economically and/or environmentally feasible.

Within the Co.Ri.La. project 3.2 on Hydrodynamics and Morphology, interferometric radar images from the ERS-1/2 satellites from 1993 to 2000 have been used to generate a subsidence map of the Lagoon of Venice. The SAR interferometric land subsidence map pictures very well the different displacement rates of the various compartments of the Lagoon. The SAR interferometric displacement map is discussed in comparison to the levelling results available through the ISES project. A geocoded thematic map of the Lagoon of Venice at 30 m resolution is also presented.

Vengono riportati i risultati di uno studio mineralogico condotto sui sedimenti sabbiosi tardo quatemari sottostanti il cordone litorale veneziano compreso tra la foce del Fiume Brenta a Sud e quella del Sile a Nord. Sono stati analizzati 170 campioni indisturbati, provenienti da 18 sondaggi terebrati a carotaggio continuo fino a 25-30 m sotto il piano campagna. L'analisi mineralogica è stata eseguita su un minimo di 200 grani per campione utilizzando una microsonda elettronica (EPMA) equipaggiata con uno spettrometro a dispersione di energia (EDS). L'elaborazione dei dati analitici ha evidenziato una netta differenza di composizione mineralogica tra le sabbie di deposizione continentale tardo-pleistocenica e quelle di deposizione marino-Iagunare olocenica e la possibilità di discriminarle con la sola quantificazione dei principali componenti, quali: quarzo, feldspati, calcite, dolomite, miche e clorite. Questa marcata differenza di composizione mineralogica può quindi essere utilizzata per una migliore definizione del limite Pleistocene/Olocene, in particolare nelle situazioni in cui il caratteristico livello guida di argille sovraconsolidate, noto come caranto, è assente per erosione o per mancata formazione. L'elaborazione statistica dei dati analitici ha evidenziato inoltre che la composizione mineralogica media varia, sia con la profondità che areai mente, dal settore meridionale del litorale a quello settentrionale. Le variazioni mineralogiche verticali sono state attribuite all'evoluzione temporale degli ambienti deposizionali, principalmente legati alle variazioni climatiche degli ultimi 25.000 anni, mentre quelle areali, tra i due settori litoranei, sono dovute soprattutto alle differenze petrografiche delle aree di provenienza dei sedimenti. I sedimenti sabbiosi depositati durante l'ultima fase glaciale tardo-pleistocenica presentano una composizione mineralogica media abbastanza omogenea, mentre quelli di deposizione post-glaciale mostrano variazioni ben marcate. This paper reports the results of a mineralogical investigation on 170 sand samples from 18 boreholes, 25-30 m deep and representative of Upper Quaternary sand sediments underlying the sandy littoral bar that separates the Lagoon of Venice from the open Adriatic Sea. A minimum of 200 grains, per sample, randomly chosen, have been analyzed using an Electron Microprobe (EPMA) equipped with an Energy Dispersion Spectrometer (EDS). The statistical elaboration of the analytical data indicates that the percentages of quartz, feldspars, carbonates, mica and chlorites for the sediments of the continental depositional environments are quite different from those of marine-Iagoonal environments. The mineralogical differentiation of sand sediments and their detailed characterization can be used to more accurately define the Holocene/Pleistocene boundary, particularly in those areas of the lagoon where the limit is not represented by the overconsolidated clay layer, typical of the last Pleistocene continental sedimentation, locally known as caranto. The average mineralogical composition also shows vertical and lateral variations. Generally, vertical variations are linked to changes in depositional environment conditions mainly due to climatic changes during the past 25,000 years. The lateral variations between the northern and southern areas of the Venetian littoral are related to the petrology of the source areas of sediments, the carbonate-rich Tagliamento, Piave, Sile, Adige and Brenta Rivers basins and the silicate-rich Po River basin from north to south, respectively. This study highlights that the average mineralogical composition of sands deposited during the last stage of the Wűrmian glaciation is relatively homogeneous while sand deposited during the post-glacial Flandrian transgression, shows very distinct variations.