Combined continuous electrical tomography and very high resolution seismic surveys to assess continental and marine groundwater mixing

Salt contamination in the coastlands surrounding Venice Lagoon (Italy) and Laizhou Bay (Shandong Province, China) is generally the result of the complex relationship between fresh, continental groundwaters, paleo-saltwaters in deep aquifers, marine waters, and the seawater encroachment upstream from the river mouths. Saltwater intrusion is also enhanced by the presence of ancient, sandy, buried paleo-channels that act as preferential pathways for groundwater flow and solute transport. The characterization of these hydro-geo-morphological features, considering both the architecture and the pore water quality, is an issue of paramount importance. In this work, after a short overview of the saltwater contamination occurrence that affects the coastlands of Venice Lagoon and Laizhou Bay, we report on the preliminary results of a Very High Resolution Seismic and Continuous Electrical Tomography test carried out in very shallow water (about 1 m) in the Venice Lagoon to test a technique suitable for the better understanding of the surficial-groundwater mixing process. The combined analysis of the two data sets highlights a correlation between lithostratigraphic discontinuities and heterogeneity in the fluid filling the subsurface. The promising results obtained in the Italian site support a prompt use of this integrated methodology on the Chinese study area too.