Monitoring and modeling hydrological processes and freshwater recharge in salt-affected coastal farmlands near Venice

Seawater intrusion (SWI) is threatening coastal aquifers and farmland productivity worldwide. Although this phenomenon naturally occurs in coastal areas, it is intensified by anthropogenic activities such as groundwater pumping and land reclamation that cause a lowering of the hydraulic head and land subsidence. Moreover, the consequences of climate change such as sea level rise, the mean temperature increase, and the shifting of rainfall events to tropical regimes, have strong negative effects on groundwater quality and agriculture. The main outcomes are the migration of the seawater-freshwater interface inland, the deepening of the water table, and the worsening of seawater encroachment from river estuaries. Crop production is strongly affected by the spatiotemporal variability of soil and groundwater salinity as the presence of salts causes water deficit and specific ion toxicity. For these reasons, countermeasures against SWI are needed to maintain agricultural productivity and protect the freshwater resources in coastal areas. In the low-lying farmlands surrounding the southern Venice Lagoon, in northern Italy, SWI is exacerbated by land subsidence, the presence of sandy paleochannels, seawater encroachment into the river estuaries, the presence of fossil brine waters, and peat deposits. The aim of this doctoral dissertation was the characterization of an experimental agricultural field affected by SWI and located at the southern margin of the Venice Lagoon, Chioggia, Italy. This goal was achieved by analyzing a dataset collected between 2010 and 2012 to investigate the possible origins and variability of vadose zone salinity through the application of ionic ratio indexes. Secondly, the same database was investigated to identify the hydraulic property variability of the experimental area. Furthermore, this thesis presents the outcomes of an intervention established in 2021 across the farmland to mitigate saltwater contamination. This intervention involved a controlled discharge of freshwater supplied by a reclamation channel through a 200 m-long buried drainpipe installed along a well-preserved sandy paleochannel. Overall, this thesis allows the characterization of a vulnerable agricultural area from hydraulic and hydrologic points of view. Furthermore, a possible countermeasure against the expected worsening of saltwater contamination is tested and partially demonstrates its feasibility and efficacy.