Salinization and freshening processes in the coastal aquifer of Ferrara (IT)

Salinization due to seawater intrusion is one of the major threats to coastal aquifers. Understanding origin and dynamics of the fresh-saltwater interface is a prerequisite for effective management of available water resources. This study identifies salinization and freshening processes on short-term monitoring in the coastal aquifer of Ferrara, Emilia-Romagna Region (Italy). A detailed vertical characterization of the aquifer along a flow line was performed by straddle packer multilevel sampling (MSL) technique. Water types and geochemical processes occurring in the coastal system were described. The base exchange index for application to dolomitic systems (BEXD) was used to characterize the status of the coastal aquifer. The aqueous geochemical modelling code PHREEQC was used to calculate saturation indexes of calcite and dolomite for water samples collected. Results emphasize that the salinization affects almost the entire coastal aquifer with only shallow zones of freshening in proximity of both irrigation canals and topographic heights, such as palaeodunes, which constitute recharge zone where infiltrating precipitation produces freshening process. The aquifer displays a transition zone of brackish and brackish-salt water type (Cl- from 0.3 to 10 g/l) with a thickness ranging from 2 to 5 m. Saline (Cl- from 10 to 20 g/l) to hyperhaline (Cl- >20 g/l) water type is present in the remaining thickness of the aquifer. The main water types identified in the aquifer are brackish-salt and high-very high alkaline water, and hyperhaline and high-rather extreme alkaline water, dominated by Na+ and Cl- and salinization processes. Hyperhaline water is likely due to fossil seawater, relic of Holocene transgression, rather than modern seawater intrusion. The chemical composition of groundwater in this lowland coastal aquifer is mainly controlled by dissolution of evaporite salts formed during the Holocene transgression, cation-exchange processes, and calcite and dolomite mineral dissolutio