Understanding the Hydrogeology of the Venice Lagoon Subsurface with Airborne Electromagnetics ⇑ P
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Journal of Hydrology 411 (2011) 342–354 Contents lists available at SciVerse ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol Understanding the hydrogeology of the Venice Lagoon subsurface with 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, Italy b Department of Mathematical Methods and Models for Scientific Applications, University of Padova, Via Trieste 63, 35121 Padova, Italy c Aarhus Geophysics ApS, Hoegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark d National Institute of Oceanography and Experimental Geophysics, Borgo Grotta Gigante 42/c, 34010 Sgonico (TS), Italy e Monitoraggi Ambientali, Rilievi, Telerilevamento (M.A.R.T.E. S.r.l.), Viale Ancona 19, 30172 Mestre-Venezia, Italy article info abstract Article history: The occurrence of alternating dry/wet conditions in transitional environments, such as wetlands, deltas, Received 2 May 2011 and lagoons, usually challenges the use of traditional direct and geophysical surveys for comprehensive Received in revised form 18 September hydrogeologic investigations. Moreover, significant mixing between continental fresh groundwater and 2011 marine salty surface waters generally takes place in these flat coastal areas. Airborne electromagnetics Accepted 14 October 2011 (AEM) is a promising tool in this respect, as it provides, in a fast and cost effective manner, large-scale Available online 21 October 2011 This manuscript was handled by A. distribution of bulk electrical conductivities that can be used profitably to develop hydrogeologic models. Bardossy, Editor-in-Chief, with the The results of a SkyTEM AEM survey in the Venice Lagoon, Italy, show the capability of this technique to assistance of Erwin Zehe, Associate Editor significantly 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 salt Keywords: marshes, mud flats, shallows, tidal channels, islands, together with reclaimed farmlands crossed by nat- Airborne electromagnetics ural watercourses and drainage channel networks. In particular, the AEM shows (i) the presence of fresh Groundwater–surface water interaction water (with resistivity larger than 20 X m) underneath the central part of the lagoon at depths from 10 to Fresh–saltwater mixing 25 m below m.s.l., (ii) the interface between different relevant stratigraphic units, e.g., the clayey layer Hydrogeology of transitional environments bounding the Holocene–Pleistocene sedimentation, and (iii) the occurrence of areas with possible sub- Venice Lagoon 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 integrated analysis of the two data sets allows distinguishing between lithostratigraphic heterogeneity and variabil- ity of the subsurface fluids. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction To better understand the surface–subsurface water exchange, it is crucial to acquire information inland and within the lagoon or Understanding the hydrogeologic processes is critical for a wetland, covering both its permanent wet areas and the tidal flats. sound management of water resources in coastal areas. Here lies The investigation of coastal surficial water–groundwater exchange, the majority of human settlements and industrial production. i.e., salt–freshwater mixing, in a consistent framework is still a Moreover, human pressure on the environment is constantly challenge. Generally, inland and offshore surveys are carried out increasing and many studies predict a rising of the seawater level separately and with different methodologies. Borehole electrical in the next 50 years ranging from a few cm up to some tens of cm, conductivity (EC) measurements (e.g., Kim et al., 2006), vertical depending on location (IPCC, 2007; Carbognin et al., 2010; electric soundings (VES) (e.g., Choudhury and Saha, 2004; Wilson Pardaens et al., 2010). Moreover wetlands, lagoons, and estuaries et al., 2006), electrical resistivity tomography (ERT) (e.g., Sherif also have unique flora and fauna depending on the groundwater– et al., 2006; de Franco et al., 2009; Nguyen et al., 2009), and surface water interaction. time-domain electromagnetic (TDEM) investigations (e.g., Kontar and Ozorovich, 2006) are usually performed on coastlands. Off- shore, direct measurements using seepage meters (e.g., Shinn et al., 2002), benthic chambers (e.g., Rapaglia, 2007), and natural ⇑ Corresponding author. Tel.: +39 0412407949. isotopic tracers in surface waters (e.g., Burnett and Dulaiova, E-mail address: [email protected] (L. Tosi). 0022-1694/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2011.10.017 P. Teatini et al. / Journal of Hydrology 411 (2011) 342–354 343 2003; Gattacceca et al., 2009) are often used in combination with 2. The AEM methodology sediment resistivity profiling (e.g., Breier et al., 2005) and towed geo-electrical array surveying (e.g., Allen and Merrick, 2007) for Various airborne–helicopter borne electromagnetics systems geophysical investigations. have been applied worldwide in hydrogeophysical investigations These methods have been used in the last decade to improve over the last decade with varying degrees of success. Examples our knowledge of the continental–marine groundwater interaction are the fixed wing transient (i.e., time domain) Tempest system and the subsoil deposit architecture in the Venice Lagoon, Italy. Re- (Lane et al., 2000), the frequency domain helicopter Resolve and gional characterization of the saltwater intrusion in coastal aqui- BGR (Siemon, 2009), the time domain helicopter borne SkyTEM fers at the southern lagoon margin was carried out using VES (Sørensen and Auken, 2004), AeroTEM (Boyko et al., 2000), and and wellbores (Carbognin and Tosi, 2003; Carbognin et al., 2006). VTEM (van den Berg, 2009). These systems have different technical De Franco et al. (2009) monitored the seasonal movement of the specifications and therefore can be more or less suitable for a par- saltwater plume using time lapse electrical resistivity tomography. ticular target. Their results indicated that seawater intrusion is characterized by In our study, we apply SkyTEM (Fig. 1d) as its dual moment pro- seasonal fluctuation whose dynamics is sensitive to climate condi- vides a bandwidth, i.e., a penetration range, from shallow to inter- tions. Garcia-Solsona et al. (2008) and Gattacceca et al. (2009) used mediate depths suitable for our target. The SkyTEM excellent signal the 226Ra and 222Rn isotopes to investigate the submarine to noise ratio at late times is due to the presence of the good con- groundwater discharge (SGD) in the northern and southern lagoon, ductor, i.e., the seawater and salt saturated shallow sediments, that respectively. A significant excess of the two isotopes in the lagoon allows using a 12.5 Hz base frequency to reach deeper penetration water compared with simple steady state ternary mixing between than in usual 25 Hz base frequency set up. First and latest usable Adriatic Sea water, river water inputs, and the diffusive release of gates were 13 ls and 23 ms after current turn off, respectively. the two isotopes from sediments, proved that SGD is an important The system has a peak magnetic moment of about component of the lagoon water balance. SGD was also locally mea- 400,000 VA m2. Fig. 2 provides a schematic illustration of the basic sured using benthic chambers (Rapaglia, 2007). The architecture of principles of the Airborne TEM methodology. A pulsed current is the lagoon deposits below tidal flats was investigated by very high injected into a transmitter (Tx) coil suspended above the ground resolution seismic (VHRS) surveys that pointed out the large heter- and carried by a helicopter or airplane. The time varying primary ogeneity of the subsoil setting and the occurrence of buried geo- magnetic field associated with this transmitted current induces morphological features (Tosi et al., 2009b; Zecchin et al., 2009). eddy currents in the ground. Such eddy currents, which also have Although having access to this huge amount of information, a a secondary magnetic field associated with them, migrate down- clear and comprehensive image of the lagoon basin hydrogeology ward and decay over time because of Ohmic loss. The similarity is far from being achieved. Very shallow surface water (less than to a smoke ring has been used to describe the pattern of the in- 1 m), tidal marshes, large rivers and several reclamation canals at duced currents in the subsurface. These decaying currents are the lagoon boundaries, together with the complex morpho-geo- linked to a time decaying secondary magnetic field, which is re- hydrological setting have precluded an in-depth and extent inves- corded in the receiver (Rx) loop as an electromagnetic force, the tigation up to date. so called ‘‘transient’’. The transient contains information about Airborne electromagnetics (AEM) can greatly improve the data the electrical resistivity of the subsurface, and its vertical and lat- quality and coverage in tidal and coastal areas. The application of eral variations. AEM for