Journal of Hydrology 411 (2011) 342–354

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Journal of Hydrology

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Understanding the hydrogeology of the Venice Lagoon subsurfacewith airborne electromagnetics

P. Teatini a,b, L. Tosi a,⇑, A. Viezzoli c, L. Baradello d, M. Zecchin d, S. Silvestri e

a Institute of Marine Sciences, National Research Council, Arsenale Tesa 104, Castello 2737/F, 30122 Venezia, Italyb Department of Mathematical Methods and Models for Scientific Applications, University of Padova, Via Trieste 63, 35121 Padova, Italyc Aarhus Geophysics ApS, Hoegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmarkd National Institute of Oceanography and Experimental Geophysics, Borgo Grotta Gigante 42/c, 34010 Sgonico (TS), Italye Monitoraggi Ambientali, Rilievi, Telerilevamento (M.A.R.T.E. S.r.l.), Viale Ancona 19, 30172 Mestre-Venezia, Italy

a r t i c l e i n f o

Article history:Received 2 May 2011Received in revised form 18 September2011Accepted 14 October 2011Available online 21 October 2011This manuscript was handled by A.Bardossy, Editor-in-Chief, with theassistance of Erwin Zehe, Associate Editor

Keywords:Airborne electromagneticsGroundwater–surface water interactionFresh–saltwater mixingHydrogeology of transitional environmentsVenice Lagoon

0022-1694/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jhydrol.2011.10.017

⇑ Corresponding author. Tel.: +39 0412407949.E-mail address: luigi.tosi@ismar.cnr.it (L. Tosi).

a b s t r a c t

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 comprehensivehydrogeologic investigations. Moreover, significant mixing between continental fresh groundwater andmarine 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-scaledistribution 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 tosignificantly improve the knowledge of the hydrogeologic setting of the lagoon and nearby coastland sub-surface, irrespective of the different features characterizing the area. The environment consists of saltmarshes, mud flats, shallows, tidal channels, islands, together with reclaimed farmlands crossed by nat-ural watercourses and drainage channel networks. In particular, the AEM shows (i) the presence of freshwater (with resistivity larger than 20 X m) underneath the central part of the lagoon at depths from 10 to25 m below m.s.l., (ii) the interface between different relevant stratigraphic units, e.g., the clayey layerbounding the Holocene–Pleistocene sedimentation, and (iii) the occurrence of areas with possible sub-marine fresh groundwater discharge. Moreover, the source and inland extent of the saltwater contami-nation 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 integratedanalysis of the two data sets allows distinguishing between lithostratigraphic heterogeneity and variabil-ity of the subsurface fluids.

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1. Introduction

Understanding the hydrogeologic processes is critical for asound management of water resources in coastal areas. Here liesthe majority of human settlements and industrial production.Moreover, human pressure on the environment is constantlyincreasing and many studies predict a rising of the seawater levelin the next 50 years ranging from a few cm up to some tens of cm,depending on location (IPCC, 2007; Carbognin et al., 2010;Pardaens et al., 2010). Moreover wetlands, lagoons, and estuariesalso have unique flora and fauna depending on the groundwater–surface water interaction.

ll rights reserved.

To better understand the surface–subsurface water exchange, itis crucial to acquire information inland and within the lagoon orwetland, covering both its permanent wet areas and the tidal flats.The investigation of coastal surficial water–groundwater exchange,i.e., salt–freshwater mixing, in a consistent framework is still achallenge. Generally, inland and offshore surveys are carried outseparately and with different methodologies. Borehole electricalconductivity (EC) measurements (e.g., Kim et al., 2006), verticalelectric soundings (VES) (e.g., Choudhury and Saha, 2004; Wilsonet al., 2006), electrical resistivity tomography (ERT) (e.g., Sherifet al., 2006; de Franco et al., 2009; Nguyen et al., 2009), andtime-domain electromagnetic (TDEM) investigations (e.g., Kontarand Ozorovich, 2006) are usually performed on coastlands. Off-shore, direct measurements using seepage meters (e.g., Shinnet al., 2002), benthic chambers (e.g., Rapaglia, 2007), and naturalisotopic tracers in surface waters (e.g., Burnett and Dulaiova,

P. Teatini et al. / Journal of Hydrology 411 (2011) 342–354 343

2003; Gattacceca et al., 2009) are often used in combination withsediment resistivity profiling (e.g., Breier et al., 2005) and towedgeo-electrical array surveying (e.g., Allen and Merrick, 2007) forgeophysical investigations.

These methods have been used in the last decade to improveour knowledge of the continental–marine groundwater interactionand the subsoil deposit architecture in the Venice Lagoon, Italy. Re-gional characterization of the saltwater intrusion in coastal aqui-fers at the southern lagoon margin was carried out using VESand wellbores (Carbognin and Tosi, 2003; Carbognin et al., 2006).De Franco et al. (2009) monitored the seasonal movement of thesaltwater plume using time lapse electrical resistivity tomography.Their results indicated that seawater intrusion is characterized byseasonal fluctuation whose dynamics is sensitive to climate condi-tions. Garcia-Solsona et al. (2008) and Gattacceca et al. (2009) usedthe 226Ra and 222Rn isotopes to investigate the submarinegroundwater discharge (SGD) in the northern and southern lagoon,respectively. A significant excess of the two isotopes in the lagoonwater compared with simple steady state ternary mixing betweenAdriatic Sea water, river water inputs, and the diffusive release ofthe two isotopes from sediments, proved that SGD is an importantcomponent of the lagoon water balance. SGD was also locally mea-sured using benthic chambers (Rapaglia, 2007). The architecture ofthe lagoon deposits below tidal flats was investigated by very highresolution seismic (VHRS) surveys that pointed out the large heter-ogeneity of the subsoil setting and the occurrence of buried geo-morphological features (Tosi et al., 2009b; Zecchin et al., 2009).

Although having access to this huge amount of information, aclear and comprehensive image of the lagoon basin hydrogeologyis far from being achieved. Very shallow surface water (less than1 m), tidal marshes, large rivers and several reclamation canals atthe lagoon boundaries, together with the complex morpho-geo-hydrological setting have precluded an in-depth and extent inves-tigation up to date.

Airborne electromagnetics (AEM) can greatly improve the dataquality and coverage in tidal and coastal areas. The application ofAEM for groundwater monitoring and modeling has been steadilyrising in the past decade (e.g., Mullen and Kellett, 2007; Steueret al., 2009; Siemon et al., 2011), due to parallel developments ofbetter AEM systems together with processing and inversion meth-odologies. However, so far there have been extremely limited at-tempts of applying AEM to areas such as lagoons and wetlands(e.g., Fitterman and Deszcz-Pan, 1998). The most documentedstream of publications describes the use of AEM over surface waterwith the main goal of recovering bathymetric data (e.g., Vrbancichand Fullagar, 2007) rather than information about groundwaterand its interaction with surface waters.

Using the Venice Lagoon as a representative example of coast-land environments, in this work we show the capability of the Sky-TEM helicopter-borne transient EM system (Sørensen and Auken,2004) to investigate the surface water–groundwater exchange.Two areas are used as test sites. The first is located between theindustrial zone on the mainland and the Venice historical center,and the second at the southern margin of the lagoon (Fig. 1). Thetwo zones are representative of typical transitional environments,with the former characterized by wide tidal flats and the latter bysalt marshes and recently reclaimed lowlying farmlands.

After a description of the AEM methodology and a short reviewof the hydrogeologic setting of the Venice coastland, we presentand discuss the AEM outcome in the Venice area. Previous hydro-logical studies provide in situ data for both the refinement of thegeophysical data inversion and the validation of the AEM survey.The results allow a significant advance in the knowledge of thesubsoil setting, groundwater properties, and salty–fresh transitionsystems.

2. The AEM methodology

Various airborne–helicopter borne electromagnetics systemshave been applied worldwide in hydrogeophysical investigationsover the last decade with varying degrees of success. Examplesare the fixed wing transient (i.e., time domain) Tempest system(Lane et al., 2000), the frequency domain helicopter Resolve andBGR (Siemon, 2009), the time domain helicopter borne SkyTEM(Sørensen and Auken, 2004), AeroTEM (Boyko et al., 2000), andVTEM (van den Berg, 2009). These systems have different technicalspecifications and therefore can be more or less suitable for a par-ticular target.

In our study, we apply SkyTEM (Fig. 1d) as its dual moment pro-vides a bandwidth, i.e., a penetration range, from shallow to inter-mediate depths suitable for our target. The SkyTEM excellent signalto noise ratio at late times is due to the presence of the good con-ductor, i.e., the seawater and salt saturated shallow sediments, thatallows using a 12.5 Hz base frequency to reach deeper penetrationthan in usual 25 Hz base frequency set up. First and latest usablegates were 13 ls and 23 ms after current turn off, respectively.The system has a peak magnetic moment of about400,000 VA m2. Fig. 2 provides a schematic illustration of the basicprinciples of the Airborne TEM methodology. A pulsed current isinjected into a transmitter (Tx) coil suspended above the groundand carried by a helicopter or airplane. The time varying primarymagnetic field associated with this transmitted current induceseddy currents in the ground. Such eddy currents, which also havea secondary magnetic field associated with them, migrate down-ward and decay over time because of Ohmic loss. The similarityto a smoke ring has been used to describe the pattern of the in-duced currents in the subsurface. These decaying currents arelinked to a time decaying secondary magnetic field, which is re-corded in the receiver (Rx) loop as an electromagnetic force, theso called ‘‘transient’’. The transient contains information aboutthe electrical resistivity of the subsurface, and its vertical and lat-eral variations.

The recorded transients (the data) must be first processed andthen inverted to produce geoelectrical resistivity models. The dataprocessing entails three steps, as described by Auken et al. (2008).First, the data are corrected for the altitude of the frame, whichdoes not always fly parallel to the ground. SkyTEM continuouslyrecords all parameters regarding the frame movements through la-ser altimeters, GPSs, and tiltmeters. It is, therefore, possible to cal-culate and correct the effective primary magnetic field thatenergized the ground and, in theory, also the true ground clearanceof the frame that carries the Tx and Rx. However, due to the pres-ence of tree cover or over surface water like in this case, the laseraltimeters can provide erroneous readings that are corrected byautomated filters or during data inversion (Viezzoli et al., 2009).The second step consists of decoupling the data from artifactsdue to large, man-made metallic structures such as railway tracks,power lines, buried cables and pipes. In practice, this requires care-fully analyzing the transients (i.e., the decay curves) to flag anom-alous decays that cannot be due to the normal response of theground. During the transient analysis, it is crucial to have accessto all the relevant GIS information. The last part of the processingaims at increasing the signal to noise ratio to reach deeper penetra-tion while preserving lateral resolution. This is achieved using timedependent trapezoid averaging filters, which are narrow, near-sur-face and widen at depth.

After completing the processing, data are inverted. This proce-dure allows eliminating the ‘‘distorting lense’’ (Oldenburg and Li,2005) through which every AEM system ‘‘sees’’ the subsurface(the data space), and recovering instead its most likely geoelectricmodel (the model space).

Fig. 1. (a) Satellite image of the Venice Lagoon with the location of the areas monitored by SkyTEM. (b) and (c) Enlargements of the central and southern portions of thelagoon, respectively, with the trace of the AEM surveys (black lines). The location of the explorative Venezia-1 borehole (Ve1 CNR), the AEM vertical sections discussed in thepaper (a–d white lines), and the AEM–VHRS profile (e dotted line) used to integrate and complete the airborne investigation are shown. The position of the Giudecca (Gi) andSacca Sessola (Ssi) islands, Brenta River (BrR), Bacchiglione River (BaR), and Gorzone Channel (GoC) are highlighted in (b) and (c). (d) Photo of the SkyTEM acquisition in theVenice Lagoon. Tx and Rx mean transmitter and receiver, respectively.

344 P. Teatini et al. / Journal of Hydrology 411 (2011) 342–354

There are a number of different approaches for AEM data inver-sion (e.g., Brodie and Sambridge, 2006; Siemon et al., 2009). In ourcase, we applied the Spatially Constrained Inversion (SCI) tech-nique (Viezzoli et al., 2008). It consists of a fully nonlinear inver-sion with 1D forward response and 3D spatial constraints tominimize the objective functions we want. In semi-layered envi-ronments the SCI increases the resolution of the models at theupper and the lower boundaries of the system penetration range,enforcing and using a suitable degree of spatial coherency in theoutput models to stabilize the inversion. The SCI technique is per-fectly suitable in applications in which near-surface and deepinformation is important to refine the hydrogeologic model. More-over, with the SCI any prior information like electrical conductivityfrom bore logs or seismic surfaces included in the inversion of theAEM data can be extrapolated from its original location to theneighboring areas via the spatial constraints. This allows an effec-tive integration of various types of data originated with differentspatial density. From the numerical point of view, the role of theprior information is to stabilize the inversion and to extract fromthe data more robust model parameters, with finer details. Froma hydrogeologic point of view, they contribute to obtaining a morecredible result.

Viezzoli et al. (2010) presented some results obtained frominversion without any prior information. Here we add some priordata for the purpose of extracting more from the available AEM

acquisitions. We used grids of (1) bathymetry (MAG.ACQUE,2002), (2) depth to the Holocene–Pleistocene boundary, and (3)depth to the top of the first major aquifer. The two latter mapswere derived by Tosi et al. (2007a,b) and Teatini et al. (1995),respectively, through the interpolation of scattered measurements.The prior information are introduced into the inversion with therelevant uncertainties associated with them, thus reflecting thequality of the source. The effect of the prior information on the pro-cessing of the AEM data is shown at the beginning of Section 4.

Both multi (smooth) and few layers (blocky) models can beused to obtain both a robust estimation of layer properties and afull appreciation of subtle variations in the geoelectrical models.

3. Hydrogeologic background

Data from hundreds of wells drilled in the Venice mainland, lit-toral strips, and major lagoon islands during the 1950s–1960s,with the aim of supplying fresh groundwater, allowed to definethe regional lithostratigraphic setting of the Venice coastlanddown to about 350 m depth (Teatini et al., 1995).

A 30–50 m thick upper unit is characterized by phreatic, semi-confined, and locally confined aquifers. Its complex architecture isthe result of a series of heterotopic relationships among alluvial, la-goon, deltaic, and littoral deposits. Salty water seriously affects this

Fig. 2. Schematic illustration of the basic principles of the Airborne EMmethodology.

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unit along most of the Venice coastland. The salt plume intrudesirregularly inland from the nearby sea and lagoon up to 10–15 km in the northern and southern farmlands bounding the coast-al water body. The seawater intrusion is enhanced by various fac-tors: (i) the land elevation well below the mean sea level, (ii) thepresence of several buried paleo-channels crossing the lagoon mar-gin and acting as preferred pathways for groundwater flow andsolute transport, (iii) the seawater encroachment along the rivermouths, (iv) a general decrease in the freshwater river (e.g., Brenta,Bacchiglione, Adige, Gorzone) discharge, and (v) the water levels inthe drainage channels kept low by pumping stations (Rizzettoet al., 2003; Carbognin et al., 2006; de Franco et al., 2009).

The wells drilled in the past showed that six freshwater con-fined aquifers characterized by high productivity formed at regio-nal scale below the upper unit (Fig. 3b). Salty paleo-water wasfound beneath the multiaquifer system (Benvenuti et al., 1973;AGIP, 1994; Bixio et al., 1998).

Only recently, the attention has been specifically devoted to thelagoon basin where the marine-continental salt–fresh water ex-changes play a fundamental role in the hydrogeologic processesoccurring at the coastal scale. Considering the large size of the la-goon, the logistical difficulties, and high costs of direct measure-ments by well investigations in tidal environments, geophysicalsurveys have been carried out to obtain more information. Thecomplex setting of the upper unit, with the presence of buried pa-leo-channels and clinoforms within almost tabular-shaped lagoonand alluvial deposits, was revealed by shallow VHRS profiles

(Fig. 3c). The remarkably basin-scale tabular geometry of the deepaquifer/aquitard system was shown by deep multichannel seismicinvestigations (Fig. 3a).

To date, no information is available about the groundwaterquality below the lagoon bottom, in particular the depth of thetransition between the upper salty waters and the underlying freshwaters. Only a few wells drilled in Venice and in other major is-lands provide evidence of the subsurface salt existence down to30–40 m and the occurrence of fresh water below 80–100 m depth.

4. Results of AEM survey

Two AEM surveys were carried out in the Venice Lagoon inspring 2009. A first site is located just south-west of Venice, be-tween the historical center and the industrial area on the mainland(Fig. 1b). The second is selected at the southern margin of the la-goon, partially including the farmland bounding the water body(Fig. 1c). These zones are representative of the outer and inner la-goon environments, respectively. Similarly to the whole central ba-sin of the lagoon, the area close to Venice is characterized byalmost flat 1 m deep shallows bounded by major tidal channels.Important anthropogenic pressures, which are mainly due to thenearby industrial harbor, the large navigation channels connectingit to the Adriatic Sea, areas devoted to mollusk aquaculture, andthe city of Venice itself, affect this area. Conversely, the southernlagoon edge is characterized by a variable morphological settingwith tidal flats, salt marshes, and tidal creeks and channels. Here,the anthropogenic impact is almost negligible and related to agri-culture activities in the nearby farmlands. Nevertheless, this part ofthe lagoon margin seriously suffers from land degradation due toboth salt water intrusion (Carbognin and Tosi, 2003; Carbogninet al., 2006; de Franco et al., 2009) and relative land subsidence,i.e., the superposition of eustasy (Carbognin and Tosi, 2002; Car-bognin et al., 2009) due to climate changes and land settlementtriggered by several factors (Teatini et al., 2005; Tosi et al.,2009a; Teatini et al., 2010). We chose to apply AEM here becauseof the complex interplay between groundwater and surficial (la-goon and rivers) waters, and the availability of water quality mea-surements, VHRS, and electrical resistivity tomography (ERT) fromprevious investigations. This information is used for the validationand interpretation of the AEM data.

As mentioned in the section above, the AEM data are first in-verted without any prior information. The results obtained indicatethe parameters that the AEM could resolve in this setting. To illus-trate the effects of adding prior knowledge to the inversion prob-lem, we compare the results obtained with and without the priorinformation in one cross section of the electrical resistivity (q) to-gether with the data misfit (Fig. 4). As written above, the priorinformation used was the depths of (i) the lagoon bottom, (ii) theHolocene–Pleistocene boundary, and (iii) the top of the first majorconfined aquifer. Fig. 4a shows the results of multilayered inver-sion for reference. The middle (Fig. 4b) and lower (Fig. 4c) panelsshow the results of the few layers inversions, without and witha–priori, respectively. The main features of the resistivity sectionremain unchanged as expected. However, the input of the priorinfo, which reduced the degrees of freedom in the problem, pro-duces more consistent results, showing subtle variations (Fig. 4c)that cannot be appreciated in Fig. 4b. At the same time the resultsare, in general, characterized by lower data residual. This meansthat the prior info is not only consistent with the geophysical data,but actually adds valuable input to the numerical problem. Resultimprovement is particularly noticeable in the portion of the surveywhere the altitude was missing due to rough seas, and at the sametime the inversion suggests that the helicopter was gaining heightfast. In this case, using the depth of the Holocene–Pleistocene

Fig. 3. Geological characterization of the subsurface of the Venice coastland. (a) West–east seismic section of the Pleistocene sequence down to 1000 m depth; (b)hydrogeologic section of the multiaquifer system regionally developed below the Venice Lagoon (after Teatini et al., 1995); (c) detailed lithostratigraphy of the upper 40 m ofthe Venice Lagoon subsurface detected by VHRS (after Tosi et al., 2009b).

Fig. 4. Resistivity vertical section obtained by SKyTEM along the a–a0 alignmentshown in Fig. 1b. Comparison between the results obtained without (a and b) andwith (c) prior information together with the data misfit (red lines). ML and FL aremulti-layer and few-layer inversions, respectively. Dashed and continuous blacklines indicate the Holocene–Pleistocene boundary and the top of the first aquifer,respectively, used as a priori in the inversion procedure.

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boundary stabilizes the inversion significantly. Finally, notice howthe results do not match the prior depth completely. This is ex-pected because (a) the latter are based on sparse interpolated mea-surements, (b) there is not always direct correspondence between

lithological layers and electrical layers, and (c) there can be somemisfit between the different types of data. For all these reasons,the prior knowledge is added with a confidence value and theinversion is allowed to move it to a certain degree to fit the data.In this case, the level of confidence on the different surfaces ap-plied as a priori ranges between about 90% and 50%, from the mostsuperficial to the deepest. Models where the a priori differs signif-icantly from the inversion results (e.g., depth to the first majoraquifer versus depth to 4th layer, in few places) correspond todata-driven model parameters that are in some disagreement withthe prior data provided. This is a positive outcome, as it proves thatthe data are sensitive to the given model parameters.

All the results presented in the following sections, and thehydrogeologic interpretation built on them, are based on inversionobtained adding this prior information. We present the results ofthe AEM inversion both as horizontal average resistivity maps fora few depth intervals, and as vertical resistivity sections (from boththe few and multilayer analyses). The maps highlight the q lateralvariations and the cross-sections point out vertical changes. Thus afull perspective of the AEM capabilities, i.e., advantages and limits,to characterize the hydrogeologic setting below shallow salt-waterbasins are provided.

The misfit between real data and forward responses was belownoise level over the vast majority of the survey area.

4.1. Central lagoon

About 100 km of AEM lines were obtained in the central lagoonsouthwest of Venice (Fig. 1b). The survey is aimed at detecting thepresence of fresh groundwater underneath the lagoon saline water,identifying areas of potential submarine fresh groundwater dis-charge (SFGD) into the lagoon, and characterizing buried morpho-logical features that can act as preferential pathways forgroundwater contaminant exchange with the mainland subsurface.

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The SkyTEM system provides resistivity information down toabout 120–140 m depth. The average resistivity maps for the 0–5, 5–10, 30–40, and 100–120 m depth intervals show many inter-esting features and allow to describe easily the lateral variations(Fig. 5). The lowest resistivity values average 0.7 X m and are de-tected in the most superficial map. The q variability is restrictedwithin the 0.2–1.2 X m range. Spots with q � 2 X m are due tothe occurrence of surficial anthropogenic structures, e.g., the SaccaSessola island (see Fig. 1b for its location), that preclude the use ofthe low moment signal (Fig. 5a). A larger variability of q is ob-served in the 5–10 m depth interval (Fig. 5b). Patterns withq < 1 X m and q � 10 X m are detected in the central-northernand southern sectors, respectively. The cross-sections shown inFig. 6 clearly reveal the abrupt increase of q with depth and thethickness variability of the uppermost low resistivity layer. Atabout 20 m depth, a second low resistivity layer is detected by

Fig. 5. Average resistivity maps for the 0–5 (a), 5–10 (b), 30–40 (c), and 100–120 (d) msalinity values measured in some boreholes scattered in the study area are shown acco

the multi-layer model in the sector between the industrial zoneand Venice (Fig. 6a).

Between 30 and 40 m depth, a west–east large feature charac-terized by a relatively low resistivity (2 < q < 5 X m) markedly sep-arates the northern and southern sectors where q > 10 X m(Fig. 5c). Some local zones with q � 100 X m are also observed inthe southern portion. Below 40 m depth, the low resistivity areasignificantly reduces and remains well detectable just south ofthe Giudecca island and in the nearby of the Sacca Sessola island(Figs. 5d and 6a). High resistivities (q > 20 X m) uniformly charac-terize the 100–120 m depth interval and the layers down to themaximum depth of exploration.

4.2. Southern lagoon

The survey comprises about 50 km lines that extend over a por-tion of the southern lagoon and the adjacent farmland (Fig. 1c). The

depth intervals obtained by the SkyTEM system in the central lagoon sector. Therding to the well depth.

Fig. 6. Multi-layer (ML) and few-layer (FL) vertical resistivity sections obtainedfrom the AEM survey in the central lagoon along the two west–east alignments (a–a0 and b–b0) shown in Fig. 1b.

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AEM system provides data down to about 200 m depth. The aver-age resistivity maps at 0–5 (a), 5–10 (b), 30–40 (c), and 100–120 mdepth intervals are shown in Fig. 7.

In general, the q values below the lagoon bottom significantlydiffer from those in the inland sector, with an abrupt change ob-served at the lagoon-inland boundary. In the lagoon sector q in-creases from less than 1 X m in the shallower layer to about4 X m down to 100 m depth. Conversely to the values recordedin the central lagoon, here q never exceeds 10 X m down to about120–140 m (Figs. 7 and 8). Apart from the uppermost layer, q isquite homogenous within the various depth intervals. The 0–5 mdepth map shows an interesting feature characterized by verylow resistivity (q � 0.5 X m) in correspondence with tidalmarshes.

The 0.5 km wide coastland just south of the lagoon margin ischaracterized by a significant variability of the resistivity. The va-lue of q reaches the minimum (q = 1–2 X m) within the 10–20 mdepth and increases to 8–10 X m both upward at the groundsurface and downward to about 100 m depth. More inland, two

distinct layers characterized by a relatively low q value (�5 X m)are recorded in the shallowest 20 m thick unit and within the60–80 m depth interval, respectively (Fig. 7). Their landward intru-sion is highly variable with a maximum extent of about 2 km fromthe lagoon margin. In the remaining portions of the farmland sub-soil, q largely increases up to 70 X m.

5. Updated hydrogeologic model

The SkyTEM results are interpreted using the available informa-tion on the Venice Lagoon subsoil, geomorphological characteris-tics, and groundwater properties. The aim is twofold: (i) tovalidate the AEM methodology that is applied here for the firsttime in a lagoon environment, and (ii) to improve the generalunderstanding of the regional hydrogeologic setting by the AEMoutcome. The data interpretation is performed from the water/landsurface and moving downward, with the electrostratigraphic seriesthat is subdivided into three major layers: the shallower unit (Ulr)characterized by a very low resistivity, generally less than 1 X m,typical of highly-salt environments; an intermediate brackish unit(Uir) where q ranges between 1 and 10 X m; and the lower unit(Uhr) characterized by medium to high resistivity (q > 10 X m)with almost fresh groundwater.

5.1. Lagoon sector

Although relatively limited, the q variability within Ulr mainlyreflects the lagoon morphological setting. In particular, the 0–5 mdepth resistivity maps (Figs. 5a and 7a) show that the most con-ductive patterns (0.2 < q < 0.4 X m) correspond with the deepershallows (�2 m below msl) and the major channels in the areaaround Venice, and with salt marshes in the southern area. As ob-served in other wetland areas (e.g., Martinez et al., 2008), theextensive evaporation that takes place in patches of vegetationresting over the salt marshes and tidal flats probably concentratesthe salts in the sediments underneath the vegetation. Higher resis-tivity values (q � 1 X m) within Ulr occur in very shallow tidal flats,less than 0.5 m deep. In the exposed area of the Sacca Sessola is-land, where q � 2 X m, the lack of sea water likely contributes tothe q increase.

A very important issue is the quantification of the subsoil thick-ness affected by salt water and, consequently, the evaluation of thedepth below the lagoon bottom at which the fresh water occurs.Figs. 6 and 8 show that the Ulr thickness varies from about 10 to25 m. In the southern lagoon it is almost uniformly 20–25 m thick.A certain variability characterizes the central sector where thethickness usually reduces to 5–10 m and only locally increasesup to 20 m. This setting is likely related to the presence of an al-most continuous clayey layer partially confining the uppermostdeposits from the deeper ones. Sedimentological investigationsshow that the high conductivity deposits correspond well to theHolocene sedimentation and are bounded by the Holocene–Pleis-tocene limit (Fig. 9). The latter is marked by an erosional unconfor-mity with the uppermost part of the Pleistocene series generallymade by an over-consolidated stiff clayey layer (Bonardi et al.,2006; Tosi et al., 2007a,b), well known by Venetians as ‘‘caranto’’,and used as the base on which to build the foundations of the his-torical palaces of the city due to its good geotechnical properties(Donnici et al., 2011). The caranto depth increases from the innercentral lagoon toward the southern margin and the littorals (Tosiet al., 2009a). The layer is somewhere discontinuous and its lackusually corresponds to areas with the larger values of the Ulr thick-ness (Fig. 10). Holocene deposits are characterized by marine andlagoon-back barrier sediments more permeable and less com-pacted than the deeper alluvial ones. Even if locally discontinuous,

Fig. 7. Average resistivity maps for the 0–5 (a), 5–10 (b), 30–40 (c), and 100–120 (d) m depth intervals obtained by the SkyTEM system in the southern lagoon sector. Thesalinity values measured in some boreholes scattered in the study area are shown according to the well depth.

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the caranto seems to confine the saline seeps in the depositsunderneath it.

In the landward part of Fig. 6a, a 10–15 m thick low resistivitylayer (0.7 < q < 1 X m) is clearly visible below the upper Ulr at anaverage 20 m depth. The nature of this feature could be explainedby a seaward flow of industrial pollutants. It is well establishedthat the Venice industrial zone and the harbor have been the majorsource of pollutants (e.g., heavy metals, organic contaminants) forthe lagoon sediments and groundwater (Bernstein et al., 2000).

Below Ulr, the AEM survey detects a �20 m thick unit (Uir) char-acterized by a significantly higher resistivity with values typical ofaquifer systems containing brackish water. The AEM survey pointsout that the salty contamination (Uir) occurs also at large depth insome areas. A major west–east structure with an elongated-shapefigure characterized by 2 < q < 5 X m is detected west of Venicedown to 30–40 m depth (Fig. 5c). The occurrence of sandy buriedgeomorphological features, e.g., ancient rivers and lagoon chan-nels, widely distributed in the Venice Lagoon (e.g., Zecchin et al.,

2008; Zecchin et al., 2009; Rizzetto et al., 2009; Tosi et al.,2009b), suggests that this structure likely corresponds to perme-able deposits. The presence of heterogenous sedimentologicalstructures acting as a preferential pathway for fresh–salt ground-water mixing is largely reported in the literature (e.g., Rizzettoet al., 2003; Mulligan et al., 2007; Tosi et al., 2009b).

Under 40 m below msl and down to the maximum depth ofinvestigation, SkyTEM identifies the Uhr unit that generally corre-sponds to the upper portion of the confined freshwater aquifer sys-tem. Brackish waters are detected down to the maximum depth ofinvestigation right south of the Giudecca island (Figs. 5d and 6a)and in the southern lagoon sector (Figs. 7d and 8).

The depth-dependent distribution of the groundwater proper-ties described above is supported by measurements of water qual-ity available from a number of boreholes located in Venice (Zezzaand Di Sipio, 2008). A salt water content ranging from 1.5 to18 g/l (0.3 < q < 3.4 X m) was measured down to 30–40 m belowthe city (Fig. 5). The occurrence of fresh waters at depths greater

Fig. 8. Multi-layer (ML) and few-layer (FL) vertical resistivity sections obtainedfrom the AEM survey in the southern lagoon sector along the two north–southalignments (c–c0 and d–d0) shown in Fig. 1c.

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than 80 m is supported by several wells drilled for domestic use inthe lagoon islands, the mainland, and the littorals over the first halfof the past century (Serandrei Barbero, 1972). The 900 m deepexplorative Venezia-1 CNR borehole (Fig. 1b) confirmed the pres-ence of fresh water down to about 550 m depth below whichbrackish water was detected (CNR, 1972). No information is avail-able to explain the q anomaly measured at the Giudecca island. Itcould be related to a rise of deeper salty water due to past ground-water exploitation at Venice. In the southern lagoon, the presenceof higher salinity (corresponding to the Uir) down to about 120–160 m is likely connected with a tectonic line (Tosi et al., 2009a)that can act as preferential way for the upward migration of saltypaleo-waters (Di Sipio et al., 2006).

Very interestingly, SkyTEM reveals local high-resistivity occur-rences at very shallow depth in the southern portion of the centralstudy area (Fig. 5b). Significant submarine groundwater discharge(SGD) was observed in the lagoon sector north of Venice (Rapaglia,2007; Garcia-Solsona et al., 2008). Although, no direct evidence ex-

ists regarding fluid (fresh groundwater and methane) flows fromthe bed into the lagoon south of Venice, these areas potentiallyrepresent sources of submarine discharges.

AEM results are compared with the outcome of a VHRS surveyrecently carried out south of Venice. VHRS allows detecting themain architecture of the lagoon shallow subsoil, e.g., the thicknessof the Holocene deposits, the occurrence of buried paleochannels,and upward gas migration. The seismic profiles were collectedwith a boomer (the wide spectrum of the pulse ranges from400 Hz to 9000 Hz) using a mono-channel streamer with a trans-verse acquisition geometry that improves the signal-to-noise ratio(Baradello and Carcione, 2008). Fig. 9 shows a detail of an AEM anda VHRS profile carried out along the West–East alignment justsouth of Venice (e–e0, Fig. 1b). The Pleistocene–Holocene boundary(red line in the VHRS section) shows an average deepening east-ward in agreement with the Ulr bottom provided by AEM. The limitis repeatedly incised by tidal channels developed during relativelyrecent times. In this example, the Late Pleistocene tract is domi-nated by horizontal, locally high amplitude reflectors that areinterpretable as extensive peat layers and palaeosoils developedin a continental setting (Zecchin et al., 2008). Conversely, the Holo-cene sequence is dominated by channelized features linked to theactivity of tidal channels, which truncated the horizontal reflectorsreferred to subtidal flat sediments in a lagoon environment(Zecchin et al., 2009). The paleochannels often correspond with lo-cal Ulr thickenings, for example in the intervals between 700 and900 m and from 3700 to 3900 m distance in Fig. 9.

5.2. Coastland sector

The AEM provides significant results also in the coastlandbounding the lagoon with the possibility of a continuous surveyof the hydrogeologic setting in the transitional zone. This continu-ity is generally precluded for ground-based and sea-borne geo-physical techniques due to logistical difficulties.

The 3D resistivity model obtained by SkyTEM provides animpressive picture of the salt water that intrudes from the lagoonmargin, passing underneath the rivers and canals, and extendslandward for about 2 km. In addition, AEM shows that the water-courses were recharging the aquifers with almost freshwater (i.e.,spring 2008), pushing down the salty water plume coming fromthe lagoon (Fig. 8). Conversely, the Gorzone Channel at thesouth-eastern tip of the study area acts as a salt source for thefarmland (Figs. 7 and 8). In fact, the seawater flows up the channelto a dam constructed about 5 km westward of the area monitoredby AEM.

In the mainland, an alternation of relatively fresher (Uhr) andsaltier waters (Uir) occurs down to 80 m depth below which onlyfresh waters are detected. This setting shows a strong discontinuitywith the water quality observed below the bottom of the adjacentlagoon.

Fig. 7 shows a satisfactory agreement between the salinity val-ues measured in some boreholes and the results of the AEM survey.The SkyTEM acquisition in this area superposed by a ground basedERT acquisition carried out in 2006 (de Franco et al., 2009). Despitethe time lag between the two surveys, they compare very wellidentifying the same main conductive and resistive structures.The AEM penetrates deeper than the ERT, which, in turn, displayshigher lateral resolution.

5.3. Hydrogeologic interpretation

A picture of the main hydrogeologic functioning of the surface/subsurface Venice Lagoon system clearly emerges by integratingthe AEM data and the previous available hydrogeologic, geologicaland morphological information. A representative sketch of the

Fig. 9. (a) and (b) maps of Ulr thickness (m) detected by AEM in the central and southern lagoon areas, respectively. (c) map of the depth (m below msl) of the Holocene–Pleistocene limit (modified after Tosi et al., 2007a,b).

Fig. 10. (a) AEM and (b) VHRS surveys along the alignment e–e0 sketched in Fig. 1b. The red and blue lines in (b) represent the Pleistocene–Holocene boundary and gasseepages, respectively.

P. Teatini et al. / Journal of Hydrology 411 (2011) 342–354 351

conceptual model thus obtained is provided in Fig. 11, with the twostudy sites that are characterized by similar as well as specificfeatures.

Although the main groundwater directions were alreadyknown, at least in the deeper confined aquifer system, AEM pro-vides indisputable evidence of the subsurface flows occurring inthe shallower 30–50 m thickness of the sedimentary sequence. Inthe central sector, as well as presumably along the majority ofwestward lagoon margin, groundwater flows seaward from the

mainland. This has important consequences on the necessity ofeffective mitigation strategies against inland soil and water con-tamination to protect the lagoon environment, and supports theactivities implemented by the Venice Water Authority to reducethe groundwater exchange between the polluted industrial harborof Venice and the lagoon subsurface (Paris et al., 2010). Conversely,at the southern lagoon margin, and possibly at the northern sidetoo, the groundwater moves preferentially from the lagoon sub-bottom toward the farmland. This is mainly caused by the larger

Fig. 11. A conceptualization of the hydrogeologic setting of the Venice Lagoon subsurface in the (a) central and (b) southern portions obtained by integrating the results of theAEM survey with previous available information.

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density of the saltwater and the inland piezometric head lowerthan the surface water level due to a land elevation well belowthe mean sea level and a widespread use of pumping stations tokeep lowlying lands drained.

A second aspect concerns the caranto unit. As stated above, thisover-consolidated clay layer separating the Holocene from the Ples-tocene deposits is well known in Venice for the role played in sup-porting the loads exerted by the city palaces. Here, we show forthe first time that the caranto exhibits also an important hydrogeo-logic function precluding the downward propagation of seawaters.Groundwaters with a salt concentration comparable with the mar-ine waters are encountered at a deeper depth only where this imper-meable layer lacks, generally due to natural erosion or channelexcavation. This strategic role has considerable implications forthe large projects under planning, e.g., a metro line below the lagoonbottom connecting the historical center to the mainland (Holden,2005), that may enhance the environmental hazard.

Finally, a last aspect worth focusing attention on is related to thedepth at which freshwaters are encountered below the lagoon bot-tom. Information from previous studies and restricted to the lagoonmargins, the Venice islands, and the littorals, shows the presence offreshwater at 70–80 m depth. The AEM results clearly point out that

freshwaters are located at very shallow depths, usually less than40 m, within the inner lagoon basin. The mixing layer between thesalt groundwater above the caranto and the underlying freshwatersis usually 10–20 m thick only. The AEM supports the possibility ofsignificant freshwater outflows from the lagoon bottom. Conversely,but not less surprisingly, brackish waters are detected down todepths greater than 100 m in the southern lagoon. This results re-quires to be confirmed on an area larger than that covered by the testdescribed here. In fact, borehole data collected in the past along thesouthern lagoon margin and the nearby farmland showed the pres-ence of a well-developed fresh-water aquifer system also in this partof the Venice coastland, even if the shallowest fresh-water aquiferhere is about 40 m deeper than at Venice.

6. Conclusions

On account of high data quality, capability of monitoring largeterritories in short times, and cost effectiveness, airborne electro-magnetics is a very useful method for surveying large and/orhardly accessible areas in order to support hydrogeologic investi-gations. The method has been widely applied in urban and rural

P. Teatini et al. / Journal of Hydrology 411 (2011) 342–354 353

areas over the last decade, whereas applications over surfacewaters are scarce. Our study has been aimed at understandingthe capability of the AEM system to improve the knowledge onthe hydrogeologic setting in transitional, shallow water coastalenvironments.

The knowledge of marine-continental water exchanges in la-goons and wetlands all over the world is of paramount importancefor understanding the continental–marine surficial–groundwaterinteractions occurring in these unique ecological environments.The test has been carried out in the Venice Lagoon which is a coast-al wetland particularly complex from the hydrogeologic point ofview. The results show that the AEM technique is capable of pro-viding useful outcomes improving the hydrogeologic knowledge,even though it has limitations. The SkyTEM system efficiently iden-tifies the transition between salt and fresh-saturated sedimentsbelow the lagoon and in the surrounding farmland. An extremebenefit for the comprehension of the processes is obtained byacquiring data at a very large scale, over both water and land with-out discontinuity. The SkyTEM low base frequency, the inversion ofthe transmitter height, and the use of SCI allow obtaining deeppenetration and spatially coherent models. Specifically, for theVenice Lagoon the fresh water is found just at shallow depths,i.e., 30–40 m and locally less than 10 m below the lagoon bottom.An unexpected result is also the lack of fresh waters in the confinedaquifer system below the lagoon in southern basin. In addition,AEM clearly points out the role exerted by the rivers boundingthe lagoon margin in mitigating the salt contamination plume thatintrudes from the lagoon bottom. Finally, AEM data suggest thatthe clayey unit between the marine Holocene and continentalPleistocene deposits likely prevents the vertical penetration ofthe lagoon water into the underlying aquifer system.

On the other hand, the geoelectrical model by AEM is unable tocompletely resolve the clay–silty-sand layered structures typical ofsedimentary basins if the electrical contrast between the layers isnot large enough or the layers are too thin. This is the case of theVenice multiaquifer system where all the different lithotypes aredistributed in the permeable/impermeable hydrogeologic units,of course in various percentages. The combination of SkyTEM andVHRS surveys proved to be an excellent integrated system to dis-tinguish the variability between the subsoil architecture and thesubsurface fluid characteristics.

AEM data need an accurate processing and inversion to achievethe best possible depth of penetration without degrading the lat-eral resolution, and to prevent possible artifacts caused byinfrastructures.

In conclusion, the above results show the capability of AEM toprovide 3D large-scale views of subsurface water and soil charac-teristics. From a general perspective, this has a strong hydrogeo-logic relevance to define conceptual models (e.g., characteristicgeometries, boundary conditions, major natural and anthropogenicforcing factors) representative of the actual subsurface system.Quantitative data can be used by hydrogeologists and modelersfor process interpretation and calibration of density dependentflow and transport numerical models. AEM appears as a promisingand powerful tool for the hydrogeologic characterization of largefresh–salt transitional environments such as wetlands, lagoonsand deltas. Regarding Venice, it clearly appears that a comprehen-sive AEM survey of the whole lagoon could provide an in-depthunderstanding of the hydrogeologic setting impossible to acquirewith other more traditional ground-based and water-borneinvestigations.

One major challenge for future research on seawater–ground-water interaction is the detection of seasonal and long-term mod-ifications in large complex settings such as lagoons and wetlands.In fact, the response to climate change and human activities of salt-water intrusion is an important topic for an effective management

of lowlying coastal areas. An issue still open from our study is thepossibility and feasibility of repetitive AEM surveys to detect salt-water plume changes. This work shows that the AEM outcomes arecharacterized by vertical resolution of a few meters in the shallowsubsoil (i.e., in the upper 30–50 m depth) and a few decameters inthe horizontal direction. Time-lapse ground-based investigationscarried out in the Venice area showed that the seasonal fluctua-tions of the saltwater intrusion in the coastland bounding the la-goon are characterized by these orders of magnitude (de Francoet al., 2009). This poses doubts about the possibility to monitorthese small scale variations of groundwater quality with AEM.Things could be different in other settings, where the seasonal sig-natures are larger. In the Venice area, however, AEM could be ap-plied to detect changes occurring at larger spatial scale over bothlong-time, e.g., climate-change driven, and short-time intervals,e.g., due to sudden variations in the rate of groundwater extrac-tion/injection in wells.

Acknowledgments

The SkyTEM survey has been supported by SkyTEM Aps, AarhusGeophysics ApS and M.A.R.TE. S.r.l. VHRS survey has been carriedout with the financial support of the Water Authority of Venicein the framework of the CORILA Sub-activity ‘‘Investigation of theHolocene subsoil of the Venice Lagoon’’ (SHALLOWS Project). Datainterpretation and analyses have been developed within the Re-search Programme ‘‘GEO-RISKS – Geological, morphological andhydrological processes: monitoring, modeling and impact in thenorth-eastern Italy’’, WP4, funded by the University of Padova,Italy. The constructive comments of the reviewers and the helpfulsuggestions of Jane Frankenfield (ISMAR, National Research Coun-cil) are gratefully acknowledged for significantly helping to im-prove the original paper.

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