Groundwater (GW) and surface waters (SW) salinization is affecting coastal aquifers, and this phenomenon, exacerbated by climate change (CC), is altering water cycle in transitional coastal environments. To compare GW fluxes and salinity origins, two coastal unconfined aquifers of Italy, the lower Po River lowland and Volturno River, were selected. A density-dependent numerical model was realized (SEAWAT4.2) with the same grid resolution (200x200m) and time steps (monthly, 2010-2020), to enhance the comparison. The models allowed to quantify GW flow directions changes and salinity evolution and to compare GW and saline fluxes within the aquifers. Both models were used to underpin SW-GW interactions and the CC impact. Each model was calibrated versus GW heads observations and high-resolution salinity profiles with good model performance.
The Po River aquifer simulation highlighted a salinity increase in the deeper aquifer layers due to increased upward GW fluxes triggered by decreased recharge rates and a “Polder” like situation. Shallow layers experienced both increasing and decreasing salinization trends depending on irrigation. The salinization of the drainage network is increasing during the modelled period, despite the seasonal and interannual variability.
The Volturno aquifer simulation highlighted an increasing GW salinization pattern due to seawater intrusion from the Volturno riverbed, induced by the decreased discharge rate. This salinization mechanism is complicated by salt leaching from peaty and silty-clay lenses deposited during the Late Holocene, when the coastal area was an inner bay.
The model budget intercomparison suggested that the classical mechanism of seawater wedge intrusion from the coastline is limited to the first km inland in both aquifers. While large inland portions of the model domains are characterized by high salinity due to remnant paleo seawater in aquitards, driving the GW salinity evolution and the salinity exchange with SW.

Upward saline groundwater (GW) seepage is provoking surface water (SW) canals to be salinized in a large, reclaimed area of Po River lowland (Italy), particularly in tile drained agricultural fields embodying shortcuts among SW and shallow GW bodies. To identify salinization causes, a continuous monitoring network of SW channels, saturated and vadose zone (VZ) was established in two adjacent agricultural fields: the A1 field plot, mildly saline cultivated with maize and crossed by a paleochannel; and the A2 field plot, saline, uncultivated and covered by salt tolerant weeds.
The VZ continuous monitoring allowed to identify capillary rise as major driver of soil salinity in A1 field; in A2 additional salt is released by roots decomposition after the mow. These findings were integrated with remote sensing data on vegetation health (SAVI) and water requirement (NDMI).
Piezometers and drainage ditch continuous monitoring allowed to identify SW-GW relationships and saline sources in the aquifer/aquitard lenses.
Frequency domain analysis highlighted internal salinity dynamics, such as increased porewater salinity after mowing salt tolerant vegetation that increased temporary the EC up to 20 mS/cm.
Finally, measurements of water discharge and salt concentration at the outlet enabled a reliable estimation of salt fluxes from tile drained agricultural fields, showing that 70% of the total annual salt export (21±1.9 t/ha/yr) occurred during sub irrigation periods.
The upward saline flux from GW, together with the presence of halophilic vegetation fragments that slowly release salts into porewater and help to maintain elevated concentrations in GW and SW, pose a serious threat for the SW resources that are used for irrigation in these reclaimed lands, especially considering the ongoing climatic change that are already stressing the Po River lowland.