Seasonal oscillations in saltwater intrusion at the Venice Lagoon
boundary detected by time-lapse ERT
R. de Franco, G. Biella
Institute of Environmental Process Dynamics, CNR, Milano,
Italy
L. Tosi
Institute of Marine Sciences, CNR, Venezia,
Italy
P. Teatini, C. Rorai
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
The results of a time-lapse electrical resistivity tomography (ERT) experiment
conducted for monitoring saltwater intrusion dynamics in the coastland
bounding the southern Venice Lagoon, near Chioggia (Italy), are presented.
The survey has been carried out from November 2005 to September 2006 using a
prototype developed for this specific purpose. The system acquired 10 apparent
resistivity pseudo-sections 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 4 and 13 m
depth below mean sea level, is the most contaminated by the salt intrusion
with a minimum resistivity formation value of 0.8
ohm×m. The experimental data reveal a seasonal
resistivity fluctuation corresponding to a minimum salt intrusion in
March-April and a maximum in the dry summer season. The deeper aquifers,
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 contribution of fresh water
from sub-regional aquifers. Some preliminary results of a modeling study aimed
at understanding the factors forcing the intrusion process are presented.
