Monitoring the saltwater intrusion by time lapse electrical resistivity tomography: The Chioggia test site (Venice Lagoon, 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.