COMPARISON BETWEEN VLF-EM AND RESISTIVITY ANOMALIES ASSOCIATED WITH SALT PALEO-WATERS CONTAMINATIONS IN AN ALLUVIAL AQUIFER: THE SAN RE TEST SITE (NORTHERN ITALY)

The alluvial aquifer of the Oltrepò Pavese plain sector (Po Valley, Northern Italy) is contaminated by Na-Cl paleo-waters, rising from the tertiary substratum and mixing with the shallow groundwater. This phenomenon is localized along the Vogherese Fault, a buried tectonic discontinuity. Geophysical surveys were undertaken in two separate phases to map and characterize the contaminations. The first phase involved resistivity depth soundings undertaken along a cross section of the Vogherese Fault and VLF-EM surveys carried out over 150 km2 for a rapid assessment of the distribution of saline waters, even where no wells for sampling are available. NE-SW trends of high conductivity anomalies were revealed. These trends can be correlated to the occurrence of the Vogherese Fault trace and secondary sub-parallel discontinuities along which paleo-waters uprise. San Re test site was chosen as representative of the entire study area to carry out a more detailed phase of investigations which included: four resistivity depth soundings, five 2D ERT surveys (470 m long) that were undertaken along an approximately 2600 m long profile crossing the fault zone and overlapping a significant length of VLF-EM surveys and a resistivity profiling; another ERT survey was undertaken at a transversal angle to the profile; four 3D ERT surveys and a short spread 2D ERT were undertaken to achieve a detailed investigation of the salt water plumes. The overlapping of different surveys allowed a comparison between electromagnetic and resistivity anomalies associated with the occurrence of saltwater contaminations. The detectability of VLF-EM and resistivity profiling surveys when identifying saltwater contaminations was defined through a comparison with the saltwater plumes imaging obtained by 2D and 3D ERTs. On a total of 14 saltwater contaminations (steeply-dipping and well coupled with the transmitter) with a length ranging between 8 and 195 m, 50 % have been detected as conductivity boundary, while 50 % as conductivity body by VLF-EM surveys.