Seasonal Oscillation in saltwater intrusion at the Venice lagoon boundary detected by time-lapse ERT

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 (ltaly), 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 1O 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-stratig­ raphy 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 min­ imum salt intrusion in March-April and a maximum in the dry summer season. The deeper aquifers, char­ acterized 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.