Surface Water-Groundwater Exchange in Transitional Coastal Environments by Airborne Electromagnetics: The Venice Lagoon Example

Abstract

A comprehensive investigation of the exchange between surficial waters and groundwater in transitional environments, and hence the mixing of waters characterized by a different salt concentration, is an issue of paramount importance considering the ecological, cultural, and socio-economic relevance of transitional environments. Acquiring information, which can improve the process understanding, is often logistically challenging, and generally expensive and slow in these areas. This applies both for punctual, invasive borehole measurements as well as for ground-based non invasive geophysical surveys. Here we investigate the capability of airborne electromagnetics (AEM) to give a significant contribution to the understanding of the hydrogeology within, below and at the margin of the Venice Lagoon, Italy. The quasi-3D modelling of the SkyTEM data by the spatially constrained inversion (SCI) methodology allows to accurately distinguish several hydrogeological features, both underneath the lagoon and the farmland beside it. Hydrogeological features resolved are, for example, the extent of the saltwater intrusion in coastal aquifers and the transition between the upper salt saturated and the underlying fresher sediments below the lagoon bottom, and areas of probable submarine groundwater discharge. While the AEM data show a high degree of coherence with available ancillary information, both from wells and from other geophysical techniques such as high resolution off-shore seismic, they improve greatly the understanding of the hydrogeology in the Venice lagoon, both at large scale and in detail. The research highlights the great potential of AEM to improve significantly the hydrogeological characterization of subsurface processes in lagoons, wetlands, and deltas worldwide.