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.