Effect of textural and hydrological properties of the soil on the residence time of infiltration water and in promoting denitrification processes in the unsaturated zone

The development of agricultural activities has led to an excessive use of fertilizers causing extensive nitrate contamination of groundwater in many areas worldwide. In Sardinia the plain of Arborea has been designated as Nitrate Vulnerable Zone (NVZ) since 2005, but other areas, such as southern Campidano, are affected by exceedings of the threshold value of 50 mg/L. Despite the reduction of nitrogen input imposed by the Nitrate Directive, no significant improvement in water quality has been observed (Pittalis et al., 2018; Biddau et al., 2019). The main objective of this research is to assess the possible processes of mitigation of nitrate contamination during the filtration process from soil to aquifer in relation to the texture and hydraulic features of unsaturated zone in two areas with different geological, hydrogeological and pedological properties. The methodology involves a multidisciplinary approach that will provide pedological, hydrological, chemical and physical characterization of the unsaturated zone, combined with the study of the aquifer. Two experimental fields sites will be equipped with soil moisture probes, tensiometers, suction cups, TDR at different depths along the thickness of the vadose zone. Sampling of soil horizons and soil water solution along the soil profile will be used for the determination of physico-chemical and hydraulic properties, as well as for the analysis of nitrogen species and environmental traces in order to define transit times in unsaturated zone. Sampling of undisturbed soil profiles will be used for subsequent column nitrate leaching experiments with the application of different nitrogen treatment cycles and controlled irrigation to determine the amount of nitrate leached after simulated events. The underlying aquifer will be properly monitored and sampled for chemical and isotopic analysis. The use of physically based unidirectional models of simulation of flow and transport of solutes in the unsaturated zone will allow to develop the future short- and long-term scenarios. The innovative aspect of this study concerns the assessment of the impact of different textures and hydraulics characteristics on nitrate leaching processes, often disregarded. Here, indeed, isotope ratios in pore water could be helpful to understand water fluxes and transit times in the unsaturated zone and define the vulnerability of the groundwater. The study will provide a tool to the Authorities to define best management strategies to protect groundwater to nitrate contamination.