Monitoring of physical-chemical parameter depth profile to assess sea water intrusion phenomena in a coastal multi-layer aquifer

Seawater intrusion is a global phenomenon occurring in many coastal aquifers. The excessive and uncontrolled withdrawal of groundwater and/or reduction in recharge to aquifers decrease the freshwater hydraulic head and can result in the saline front advancing inland toward abstraction boreholes. Sea-level rise due to the climate change can exacerbate these effects. Old saline groundwater related to eustatic effects resulting from climate change during the last post-glacial period can also occur in coastal aquifers. According to the Ghyben-Herzberg principle, the depth of fresh-saline groundwater interface is mainly controlled by density and, in turn, by salinity. However, aquifer geometries and intrinsic heterogeneity of the geological medium, can affect the fresh-saline groundwater interface position and the response times to the forcing that control the salinization processes. Therefore, the knowledge of the response dynamics of the aquifer conditioning the position of the interface are essential to design countermeasures to compensate the salinization processes. Results of the monitoring of electric conductivity, temperature, pH and Eh log profile at about 30 m deep boreholes in the highly anthropized coastal plain of Muravera, in south-eastern Sardinia (Italy), are presented. Since the early fifties, in the plain area the natural hydrodynamic equilibrium between groundwater, surface-water, and seawater has been deeply modified by the construction of dams across the Flumendosa river, embankments, and the development of agriculture, tourism, and aquaculture activities along the coast. Moreover, abandoned branches of the river have been salinized by a fishpond that created a direct opening to the sea. According to a geological–depositional model based on sequential stratigraphy, the geometry of the aquifers in the Muravera coastal plain has been defined integrating stratigraphic, geophysical, geochemical, and isotopic data. A complex multilayer aquifer, mostly phreatic and locally confined, has been recognized. Results of the monitoring campaigns showed that the position of the fresh-saline groundwater interface along the plain cannot be explained by the Ghyben-Herzberg model. In the north area of the Muravera Plain, where the semi-confined condition of the aquifer occurs, the position of the interface doesn’t change significantly. Moreover, the lowering of pH as conductivity increases suggests high residence time of saline groundwater in the aquifer and interaction with marshes sediments. In the central sector of the plain, in unconfined conditions, the deepening of the interface as the piezometric head increases occurs and has been related to the higher transmissivity of the aquifer and a recharge rate coming from the Flumendosa river during extreme rainfall events. The multilayered aquifer geometry and the relationship between surface waters and groundwater have been recognized as responsible for the recharge rate of the aquifer and for the relative position of the freshwater–saltwater interface.