Salt water intrusion monitoring with time lapse electrical tomography in the southern Venice Lagoon.

In this focus we present the results of an electrical resistivity tomography (ERT) time lapse experiment aimed at monitoring the dynamics of the saltwater intrusion in the coastland bounding the southern Venice Lagoon, near Chioggia (Italy). The knowledge of dynamics of the saltwater intrusion in the lagoon system is important to preserve from the salination this area where a flourishing agricultural activity is present.
The experiment was performed and funded in the framework of the CORILA (Research Consortium of the Venice Lagoon) projects and it involved Italian institutions (IDPA-CNR, ISMAR-CNR, Padova and Milan Universities) and the CEREGE institute of the CNRS. The experiment was also supported by public and administrative institutions: the Venice province and the Adige-Bacchiglione Reclamation Authority.
The experiment, operated for about one year from November 2005 to September 2006, has been carried out by means of a dedicated apparatus developed ad hoc. Some preliminary check and calibration acquisitions allowed the setup of the experimental parameters. The system acquired 10 apparent resistivity tomograms per day, five of which with high resolution by a 97.5 m long ERT line with a 2.5 m electrode spacing and five by a 300 m long and 5 m electrode spacing ERT line, suitable to monitor the intrusion process down to 50-60 m depth. Two boreholes (20 and 50 m deep) drilled near the test site identified a shallow phreatic aquifer and two confined aquifers within the depth of interest and allow to constrain the electro-stratigraphy obtained by the resistivity tomography. The shallow aquifer, located between 2 and 12 m depth below mean sea level, is the most contaminated by the salt intrusion with a minimum resistivity formation value of 0.8-1.5 ohm*m. The experimental data reveal a seasonal resistivity fluctuation corresponding to a minimum salt intrusion in the March-April period and a maximum in the dry summer season. The two deeper aquifers (the 2-nd 18-38 m depth and the 3-th greater than 44 m), characterized by a resistivity ranging from 3.0 ohm*m to 10 ohm*m, show a resistivity rise at the beginning of the summer time probably due to an increasing in the contribution of fresh water from sub-regional aquifers.
A preliminar estimate of the velocity variation of the salt intrusion front, obtained from the observational data analysis, indicate a velocity value ranging between about 10-4-10-5 cm/s.
The data collected by the experiment have been correlated with a number of environmental variables, i.e. tidal level, rainfall, depth to the groundwater table, and water levels in the watercourses adjacent to the monitoring site. In particular, a correlation is found between the resistivity in the upper 5 m shallow aquifer and the rainfalls, and between the water level in the adjacent channel (Canale di Valle) with the resistivity down to about 10 m depth. This latter relation is indicative of the draining/feeding influence exerted by the channel probably only to the first aquifer. Moreover a quite clear negative correlation with the tidal regime is observed at seasonal time scale, while at daily time scale the tidal oscillation doesnâEUR(TM)t affect the resistivity behaviour of the phreatic aquifer.

Roberto de Franco & Giancarlo Biella
Istituto per la Dinamica dei Processi Ambientali
Via Mario Bianco 9, 20131 Milano

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