Saltwater intrusion is a growing threat for coastal aquifers and agricultural practices in low-lying plains. Most of the farmlands located between the margin of the Southern Venice lagoon and the Northern Po delta, Italy, lie a few meters below mean sea level and are drained by a large network of artificial channels and hydraulic infrastructures to avoid frequent flooding and allow agricultural practices. This work proposes an assessment of the vulnerability to saltwater intrusion, following a new concept of the hazard status, resulting in combining the depth of the freshwater/saltwater interface and the electrical resistivity of the shallow subsoil. The sensitivity of the farmland system was assessed by using ground elevation, distance from freshwater and saltwater sources, permeability, potential runoff, land subsidence, and sea-level rise indicators. Relative weights were assigned by a pairwise comparison following the Analytic Hierarchy Process approach. The computed vulnerability map highlights that about 30% of the farmlands is under strong and extreme conditions, 28% between marginal and moderate, and 40% under negligible conditions. Results from previous vulnerability assessments are discussed in order to explain their differences in terms of hazard status conceptualization and sensitivity characterization of farmland system.

Sea water intrusion (SWI) has been widely recognized as a global problem, significantly influencing coastal aquifers, mostly through reduced water quality and agricultural production indicators. In this paper, we present the outcomes of the implementation of two independent real-time monitoring systems, planned and installed to get insights on groundwater dynamics within the adjacent coastal aquifer systems, one located in the Neretva Valley, southeastern Croatia, the other located south of the Venice lagoon, northeastern Italy. Both systems are presented with technical details and the capacity to observe, store, and transmit (Neretva site) observed values in real-time. Analysis of time series reveals the significant influence of the sea level oscillations onto the observed groundwater electrical conductivity (EC) and piezometric head values, while precipitation rate is detected as a driving mechanism for groundwater parameters in shallow geological units. The installed monitoring systems are shown to be of great importance to provide qualitative and quantitative information on the processes influencing groundwater and surface water dynamics within two coastal systems.

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.

Complex freshwater-saltwater exchanges characterize Holocene aquifers in low- lying coastal plains, such as the phreatic aquifer at the southern margin of the Venice Lagoon, object of this study. This is due to the contribution of several factors that control groundwater dynamics, including the high variability of Holocene coastal deposits and water management practices. The stratigraphic architecture of the coastal area at the southern front of the Venice Lagoon consists of sandy paleochannels and littoral ridges and fine lagoon sediments. The relationships between the sedimentary deposits and vertical changes in salinization of the phreatic aquifer are poorly studied, although they are of paramount importance for the sustainable use of water resources. This work is aimed to demonstrate the influence of morpho-sedimentary constraints (i.e. stratigraphic discontinuities) on the salt- fresh water dynamics of the salt contaminated aquifer at the southern margin of the Venice Lagoon. The adopted methodology is based on the comparison between stratigraphic logs (in terms of Facies Analyses) and Electro Conductivity profiles (in which the EC has been taken as a proxy of salinity) acquired into the aquifer. The results show that buried channelized sandy deposits can reduce the salinity of the phreatic aquiferwhen a sufficient freswater supply is simultaneously present.

Hydrostratigraphic setting and groundwater dynamics in high salinized low-lying farmlands at the southern margin of the Venice Lagoon1 Institute of Geosciences and Earth Resources, National Research Council, Padova, Italy2 Institute of Marine Sciences, National Research Council, Venice, Italy3 Department of Geosciences, University of Padova, Padova, Italy4 Regione Veneto - Soil Defence Regional Directorate, Venice, Italy5 Land Reclamation Authority Adige Euganeo, Este-Padova, Italy * Corresponding author: chiara.cavallina@igg.cnr.it Key words: salt-water intrusion, groundwater dynamics, coastal plain, VeniceThe coastal plain at the southern margin of the Venice lagoon is a low-lying territory, which is the result of river diversions, channeling, and hydraulic reclamation that took place over the last centuries. The mechanisms controlling the exchanges between surface water and groundwater progressively shifted from natural to artificial ones. Presently, most of this territory lays at ground elevation up to 3 m below the mean sea level and a complex network of drainage channels and pumping stations regulates the depth of the water table both to prevent flooding-waterlogging and to allow irrigation. Being at the lagoon margin and next to the sea, the aquifers of this area are heavily affected by salt-water intrusion that jeopardizes the farmlands productivity and specifically damages the crop yield. Focusing on the geology and stratigraphic architecture of the first 25-meters of the subsoil, in which the shallow aquifers are included, the sedimentary record consists in Pleistocene alluvial deposits and Holocene deposits. The latter show the typical transgressive wedge including prodelta, littoral and back barrier depositional environments. Specifically, the Holocene littoral sands contain the unconfined aquifer, which is almost continuous along the whole coastal strip.  Sedimentary bodies as paleo-channels and remnants of littoral ridges, commonly hosting unconfined aquifers, are present all over coastal plain. Pleistocene alluvial deposits contain the first locally confined aquifer.Several past studies have revealed the complexity of freshwater-saltwater exchanges imposed by the concomitance of various forcing factors. However, a specific study on the dynamic of saline contamination of the groundwater is still missing. We present here the preliminary results of an ongoing research aimed to characterize the groundwater dynamics and disentangle mechanisms that control groundwater salinity in the shallow aquifer system of the low-lying farmlands adjacent to the southern Venice lagoon margin and the Brenta-Bacchiglione river mouth. The results of detailed analyses from various perspectives (based on sedimentary cores, continuous hydrological and hydrogeological data, geophysical surveys) are properly integrated to describe the dynamics in the shallow aquifer system. Particular attention is payed to the hydro-morpho-stratigraphic system in relation to the mechanisms driving the freshwater-saltwater exchanges. This research is developed in the frame of the Interreg Italy-Croatia project MoST (Monitoring Sea-water intrusion in coastal aquifers and Testing pilot projects for its mitigation), which addresses to improve the quality and sustainability of the human management of water resources in coastal areas.