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Continental and marine surficial water – groundwater interactions: the case of the southern coastland of Venice (Italy)

Abstract. Understanding the continental-marine surficial water–groundwater exchanges
in transitional coastal environments is really challenging at large scale. It
requires an image of the saltwater–freshwater relationship which is
difficult to be obtained especially in wetlands, lagoons, and marine areas.
This study is focused on the coastland of the southern Venice lagoon –
northern Po river delta (Italy), a precarious environment subject to both
natural changes and anthropogenic pressures. Here, saltwater severely affects
farmlands and aquifers. We used an airborne electromagnetics (AEM) survey
with the goal of characterizing the continental and marine surficial
water–groundwater interactions in such coastal region. The AEM survey
allowed depicting a clear image of the fresh water–saltwater occurrence in
shallow aquifers along mainland-lagoon – littoral-sea profiles, up to 20 km
long. The results reveal that continental groundwater is located in the
lagoon subsoil below a 10–20 m thick saline aquifer and extends down to
70 m depth. The whole low-lying farmland located south of the lagoon margin
is seriously affected by saltwater contamination, which occurs from a few to
about 50 m depth. The integrated analysis of AEM, seismic and borehole data
shows that buried morpho-geological structures, such as paleo-channels and
over-consolidated clay units control the saline contamination from the lagoon
and the sea into the coastal aquifer system.
