Targeting the measures for limiting nitrate pollution in groundwater by evaluation of transit time along the unsaturated zone.

Expansion of agricultural and livestock activities has resulted in an overuse of organic and chemical fertilizers applied to land, causing widespread nitrate contamination of groundwater. The EC Nitrate Directive has imposed application of measures, that have significant economic and social impact, that farmers have to comply for the mitigation of the contamination of nitrates. Despite these limitations, in most of the Nitrate Vulnerable Zones (NVZs), nitrate concentration in groundwater frequently still exceeds the limit of 50 mg/L NO3, as the adopted measures are not targeted on the basis of the local geological and hydrogeochemical features. The unsaturated zone is often the main factor controlling the movement of water and contaminant leaching from the soil to the aquifer, depending on the soil properties and infiltration rate. Natural attenuation processes in the unsaturated zone can reduce contaminant leaching into groundwater and identify their possible occurrence is a paramount challenge in optimizing agricultural practices or designing mitigation strategies. The infiltration rate determines the travel time in the vadose zone and thus the interaction time, promoting transformation processes of transported solutes. The main aim of this study is to estimate the water transit rate in the vadose zone using the stable isotope profile of water in a pilot site in Sardina (Italy). The Arborea area has been designated as a NVZ since 2005. There, soil samples were collected every 10 cm along a vertical profile and the pore water isotopic composition (18O and 2H) was analyzed together with the water content, grain size distribution and physical-chemical parameters of soils. The peak shift method was applied assuming that the seasonal effect in the isotopic composition of precipitation can be traced through a vertical isotopic profile in the soil, thus estimating the transit time rate of water percolation along the unsaturated zone. The study of percolation times can be beneficial in optimizing agricultural practices and the management of water resources, thus improving protection actions in similar contexts.