Forecasting salinization trends in a lowland aquifer: Comacchio (Italy).

Coastal aquifers salinization is an urgent problem driven by groundwater resources overexploitation and climate changes. This phenomenon is enhanced in areas lying below the sea level, like the Po river lowland (Italy). In these reclaimed lands the saltwater intrusion is usually stopped by a dense canals network that supply freshwater to agricultural lands, maintaining soil salinity at acceptable levels. 2012 was a terrible year for irrigation, since the whole Po river plain experienced a prolonged drought. Despite of this, a continuous monitoring of piezometric heads and salinity near a canal demonstrated that freshening was occurring in the first 4 m of the unconfined aquifer. The unconfined aquifer is characterized by elevated relic salinity, especially in its bottom part. A two-dimensional model, implemented via SEAWAT 4.0, was calibrated using piezometric heads and salinity depth profiles in a section perpendicular to the canal. The calibrated model was then used to forecast the behaviour of this cross section using a multiple scenario approach: increase of evapotranspiration induced by temperature raise, onset of extreme events in the temporal distribution of rainfall and managed canal dry up due to salinization of surface water. The first two scenarios have only minor influence on the aquifer salinization rate, while the third one could create serious upconing of the high salinity groundwater actually residing in the bottom of the unconfined aquifer. The scenario modelling quantified the possible future effects on groundwater salinization and could be useful to find adaptation strategies to manage the water resources of this area.