Seasonal salinity variations in a coastal wetland induced by complex interactions between sea, river and evapoconcentration processes

Deltaic coastal areas are often formed by a piecemeal of transitional wetland environments. These environments consist of wetland-aquifer systems, which commonly react rapidly to climate changes. Direct evaporation from surface water bodies like lakes, wetlands and lagoons will increase due to temperature increase leading to salt accumulation. To understand the hydrological exchanges between the transitional coastal wetlands of the Variconi oasis, the unconfined aquifer of the Volturno River Delta and the Tyrrenian Sea, the contributions of various processes were monitored and assessed. Physical-chemical parameters like salinity, pH, Eh and temperature were monitored during the hydrological year 2016-2017 on a monthly basis. Moreover, the use of groundwater and surface water equivalent freshwater heads, along with hydraulic conductivity tests and the geological architecture, permitted to connect the salinity variations in the wetlands with the degree of the hydraulic connection between the wetlands, the Volturno River, the Tyrrenian Sea and the coastal aquifer. The results show that the permanent wetlands fed by both the Volturno River and the Tyrrenian Sea, show a smoothed salinity peak during the summer season; while the ephemeral wetlands hydraulically disconnected from the aquifer show high salinity peaks during the summer season due to evapoconcentration processes. Spatial mapping of salinity and other measured parameters, highlighted an elevated heterogeneity of environments (from saline to freshwater environments) enclosed in the Variconi oasis. This in turn, induced a large biodiversity of both autochthonous and migratory species. Despite of this, the projected increase in salinity of these wetlands due to coastal erosion, augmented evapotranspiration rates and sea level rise, could be of serious concern for the above-mentioned biodiversity, thus monitoring and management strategies are urgently required to preserve this and other oasis in the Mediterranean area that are key sites employed as stop-over along migratory routes.