GROUNDWATER HYPERSALINIZATION IN A LOWLAND COASTAL AQUIFER (PO RIVER PLAIN, ITALY)

The study is based on chemical data related to groundwater samples collected, via multilevel sampling technique, in four piezometers located along a flow line in the alluvial plain of the Po River (Italy). Multilevel pressure head measures highlight the presence of upward vertical gradients induced by the drainage system, used to reclaim the lowlands for agricultural purposes. The drainage causes the upward transport of salts due to the presence of hypersaline groundwater in the lowest portion of the aquifer. Chemical data show high TDS values almost in the entire coastal aquifer, with only small shallow freshwater lenses close to irrigation canals and palaeodunes. The aquifer structure and the paleogeography of the region, together with Br/Cl ratio, suggest that the hyperhalinity comes from solution of evaporite salts formed in inter-basins during the Holocene transgression, rather than from modern seawater intrusion or mixing processes with brines as confirmed by the relationships between the major ions of the groundwater samples, the seawater and the freshwater end-members. The chemical composition of groundwater in this lowland coastal aquifer appears to be controlled by several water-sediment interactions.