Evaluation of nitrate leaching processes during infiltration and transport in the unsaturated zone

The increase in intensive agriculture and animal husbandry has led to the large-scale application of organic and chemical fertilizers to the soil. This use has led to the release of various chemical pollutants into the soil, which then enter the water table through the leaching of rainwater and irrigation water. Despite the implementation of strict regulations laid down in the Nitrates Directive, groundwater quality has not significantly improved in many Nitrates Vulnerable Zones. Studies on vulnerability to aquifer contamination often focus more on the groundwater body negleting the unsaturated zone. The vadose zone is a fundamental part of the hydrological cycle, capable of storing water, providing water for vegetation, transporting solutes, and breaking down pollutants before they reach the aquifer. The study area is the Arborea plain (Sardinia), designated as an NVZ since 2005 due to heavy nitrate contamination in its aquifer. Consequently, the research aimed to monitor nitrate leaching during infiltration and transport through the unsaturated zone and evaluate any processes of natural attenuation. In the plain, an agricultural field was selected as a pilot site where an unsaturated zone monitoring system was set up to obtain information on the processes taking place through the soil profile to the saturated zone. TDR probes were installed to measure volumetric water content, while suction cups were placed at different depths to sample pore water for the analysis of ionic nitrogen species and nitrate isotopes. The experimental approach was coupled with the use of a physically based flow and transport model in the unsaturated zone FLOWS (Coppola et al., 2014, 2019; Hassan et al., 2023) for the simulation of the flow and transport of nitrate in the pilot site. Monitoring the unsaturated zone combined with models are a useful factor in assessing the vulnerability of the aquifer and contribute to a better understanding of the processes occurring in the transported solute, thereby optimizing agricultural practices and measures to protect groundwater resources.