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Complex freshwater–saltwater exchanges characterize most Holocene groundwater aquifers in low-lying coastal plains around the world, particularly in mechanically drained territories. This is due to the combination of several factors that control groundwater dynamics, including the high variability of the Holocene coastal deposits that host the shallow aquifers and the water management practices. The relationships between the stratal architecture of sedimentary deposits and the vertical changes in the salinity of the phreatic aquifer are poorly studied although they represent an issue of primary importance for a sustainable use of water resources and for agriculture. This research work is focused on the influence of sedimentary constraints, i.e., stratigraphic discontinuities and related changes in permeability in shaping salinity stratification into the unconfined aquifer at the southern margin of the Venice lagoon (Italy). Nine sites have been investigated by collecting sediment cores for facies analysis and monitoring water electrical conductivity in piezometric wells. The results show that buried channelized sandy deposits can enhance salinity mitigation of the phreatic aquifer in conjunction with precipitations and sufficient freshwater supply from nearby rivers and irrigation channels. Our analyses also reveal that the differences in stratigraphic architecture of the upper 10 m of the subsoil determine different fresh–saltwater dynamics of the phreatic aquifer. In particular, three possible behaviors can occur: (i) where the subsurface is characterized by the presence of a thick, up to 5 m, paleochannel, a freshwater lens is always present in the most surficial part of the phreatic aquifer; (ii) where the subsurface is composed by fine-grained sediments of marsh and lagoon paleo-environment, the phreatic aquifer tends to be salt-contaminated over its entire thickness; (iii) where the subsurface contains thin, up to 2–3 m, paleochannel deposits, the fresh–saltwater dynamics of the most surficial part of the phreatic aquifer varies more during the year, as a result of seasonal precipitation trend. The provided characterization of saltwater dynamics represents the basis for planning mitigation measures to improve the farmland productivity of the Venetian coastal plains. [ABSTRACT FROM AUTHOR]

For the first time a comprehensive investigation has been carried out to quantify the possible effects of dredging a navigable canal on the hydrogeological system underlying a coastal lagoon. The study is focused on the Venice Lagoon, Italy, where the port authority is planning to open a new 10m deep and 3 km long canal to connect the city passenger terminal to the central lagoon inlet, thus avoiding the passage of large cruise ships through the historic center of Venice. A modeling study has been developed to evaluate the short (minutes), medium (months), and long (decades) term processes of water and pollutant exchange between the shallow aquifer system and the lagoon, possibly enhanced by the canal excavation, and ship wakes. An in-depth characterization of the lagoon subsurface along the channel has supported the numerical modeling. Piezometer and sea level records, geophysical acquisitions, laboratory analyses of groundwater and sediment samples (chemical analyses and ecotoxicity testing), and the outcome of 3-D hydrodynamic and computational fluid dynamic (CFD) models have been used to set up and calibrate the subsurface multi-model approach. The numerical outcomes allow us to quantify the groundwater volume and estimate the mass of anthropogenic contaminants (As, Cd, Cu, Cr, Hg, Pb, Se) likely leaked from the nearby industrial area over the past decades, and released into the lagoon from the canal bed by the action of depression waves generated by ships. Moreover, the model outcomes help to understand the effect of the hydrogeological layering on the propagation of the tidal fluctuation and salt concentration into the shallow brackish aquifers underlying the lagoon bottom. [ABSTRACT FROM AUTHOR]