Morpho-Sedimentary Constraints in the Groundwater Dynamics of Low-Lying Coastal Area: The Southern Margin of the Venice Lagoon, Italy

Complex freshwater-saltwater exchanges characterize most Holocene groundwater aquifersin low-lying coastal plains around the world, particularly in mechanically drained territories. Thisis due to the combination of several factors that control groundwater dynamics, including the highvariability of the Holocene coastal deposits that host the shallow aquifers and the water managementpractices. The relationships between the stratal architecture of sedimentary deposits and the verticalchanges in the salinity of the phreatic aquifer are poorly studied although they represent an issue ofprimary importance for a sustainable use of water resources and for agriculture. This research workis focused on the influence of sedimentary constraints, i.e., stratigraphic discontinuities and relatedchanges in permeability in shaping salinity stratification into the unconfined aquifer at the southernmargin of the Venice lagoon (Italy). Nine sites have been investigated by collecting sediment cores forfacies analysis and monitoring water electrical conductivity in piezometric wells. The results showthat buried channelized sandy deposits can enhance salinity mitigation of the phreatic aquifer inconjunction with precipitations and sufficient freshwater supply from nearby rivers and irrigationchannels. Our analyses also reveal that the differences in stratigraphic architecture of the upper 10 mof 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 athick, up to 5 m, paleochannel, a freshwater lens is always present in the most surficial part of thephreatic aquifer; (ii) where the subsurface is composed by fine-grained sediments of marsh andlagoon 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-saltwaterdynamics of the most surficial part of the phreatic aquifer varies more during the year, as a resultof seasonal precipitation trend. The provided characterization of saltwater dynamics representsthe basis for planning mitigation measures to improve the farmland productivity of the Venetiancoastal plains.