Hydrochemistry of the shallow aquifer of a cultivated reclaimed marsh environment

The aim of this study is to examine the distribution of major anions and inorganic N species in the unconfined aquifer of recently reclaimed wetlands of the Po River coastal floodplain (Northern Italy). The area is intensively cultivated; water is derived by gravity from the Po River and distributed by an extended hydrological network. Most of the territory lays below the sea level, with an altitude ranging from 5 to - 11 m above sea level (a.s.l.). In these special terrains, the drainage action created a new unconfined aquifer where the depth of the water table (and the related capillary fringe) is extremely superficial. Soluble salts, accumulated in the sediments before reclamation, are the source of groundwater salinity. For this study, an agricultural field of ca. 6 ha next to Codigoro town (45°50’33’’N and 12°05’40’’E) was chosen as experimental site for the ongoing LIFE project: ZeoLIFE - Water pollution reduction and water saving using a natural zeolitite cycle (LIFE+10 ENV/IT/00321). It is characterized by clayey silty soils overlaying paleo-marsh peats and sandy paleo-channel sediments. The low permeability of the sediments and the action of subsurface drainage system (SSDS) located at about 1 m of depth, protect the deeper layers from the above agricultural activities. In the unconfined aquifer below the SSDS, waterlogged conditions and the peat mineralization induce gleying processes and accumulation of ammonium. Ten piezometers have been installed in order to sample the groundwater throughout the whole field. The results of one year of monitoring emphasize that the soil texture variability strongly affects the distribution and behaviour of the investigated chemical species. In fact, the predominant groundwater nitrogen species in clayey silt sediments is ammonium, whereas nitrate prevails in sandy layers, where water circulation favours oxygenation and nitrification. The rising and lowering of the water level in the ditches and channels surrounding the experimental field induce large fluctuations of the water table. The groundwater chemical composition is strongly influenced by the natural salinity of the soil and by inputs of fresh water for irrigation purposes through the SSDS from the lateral ditches, during summer. Thus, the hydrochemical characterization of the shallow aquifer highlights that is very challenging to infer the net input and output of agrochemicals in this kind of environment, without a statistical approach committed to minimize the soil heterogeneities.