Monitoring the saltwater intrusion by time lapse electrical resistivity tomography: The Chioggia
test site (Venice Lagoon, Italy)
R. de Franco, G. Biella
Istituto per la Dinamica dei Processi Ambientali, CNR, Milano, Italy
L. Tosi, F. Rizzetto
Istituto per lo Studio della Dinamica delle Grandi Masse, CNR, Venezia, Italy
P. Teatini
Dept. of Methods and Mathematical Models for Scientific Applications, University of Padova, Padova, Italy
B. Chiozzotti, M. Giada
Morgan Srl, Venice, Italy
C. Claude, A. Mayer
CEREGE, Europôle Méditerranéen de l'Arbois, Aix en Provence, France
V. Bassan
Provincia di Venezia - Servizio Geologico, Mestre, Italy
G. Gasporetto-Stori
Consorzio di Bonifica Adige-Bacchiglione, Conselve, Italy
A novel experiment of time lapse electrical resistivity tomography (TL-ERT) aimed at monitoring the
dynamics of the saltwater intrusion in the coastland bounding the southern Venice Lagoon is presented. A
dedicated apparatus was developed and operated for about 9 months from November 2005. The system
acquired ten resistivity tomograms per day, five of which with high resolution by a 97.5 m long and 2.5 m
electrode spacing ERT line, and five by a 300 m long and 5 m electrode spacing line down to 50-60 m depth.
The stratigraphy of a 50 m deep borehole drilled in the nearby of the ERT-TL alignment outlines the presence
of a shallow phreatic aquifer in the shallower 12 m thick unit, followed by a semi-confined aquifer between
18 and 38 m depth and a locally confined aquifer down to the bottom. The shallow aquifer is the most
contaminated by the salt intrusion with a minimum value of the formation resistivity equal to 1.0 ohm m
corresponding to a salinity of 25–30 gr/l. A seasonal resistivity fluctuation is observed, with the saltwater
front that intrudes landward during the autumn-winter season and moves back seaward in spring-summer.
The first semi-confined aquifer is characterized by resistivity value of about 5 ohm m, while the confined
aquifer is less contaminated showing resistivity values greater than 7.5 ohm m. For both the two confined
aquifers the resistivity value rises at the beginning of the summer probably due to the seasonal fresh water
recharge supplied regionally from the mainland. The TL-ERT data have been correlated with a number of
environmental variables. A relationship is found between the resistivity in the upper 3-4 m of the phreatic
aquifer and the rainfalls, and between the water level in the adjacent main channel and the resistivity down
to about 10 m depth. With respect to the tidal regime, a daily/weekly correlation with resistivity changes is
not evidenced, while a significant negative correlation coefficient exists at monthly to seasonal time scale for
depths below 15 m. This experience suggests that TL-ERT is a powerful technique to perform multi-scale
contaminant monitoring at different time scales.
